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41
Engineering Alloys
(Ferrous and Non-Ferrous) UNIT 2 ENGINEERING ALLOYS (FERROUS
AND NON-FERROUS)
Structure
2.1 Introduction
Objectives
2.2 Production of Iron and Steel
2.3 Casting of Ingots
2.4 Continuous Casting
2.5 Steels
2.6 Heat Treatment of Steel
2.7 Hardenability of Steel
2.8 Tempering
2.9 Special Treatments
2.10 Surface Hardening
2.11 Heat Treating Equipment
2.12 Alloy Steels
2.13 Cast Iron
2.14 Non-ferrous Materials
2.15 Aluminium
2.16 Copper and its Production
2.17 Copper Alloys
2.18 Magnesium and its Alloys
2.19 Titanium Alloys
2.20 Bearing Materials
2.21 Alloys for Cutting Tools
2.22 Summary
2.23 Key Words
2.24 Answers to SAQs
2.1 INTRODUCTION
Out of solid materials used in engineering practice metals, plastic and ceramics are very
common. Then metals may be used in their elemental forms like aluminium, copper and
titanium. When a metallic element has additives much smaller in quantity than base
element, the resulting material is called an alloy of base element. Out of all metallic
elements it is iron whose alloys are used in largest quantity. All such alloys in which iron
forms the base are grouped as ferrous material. The other alloys are grouped as
non-ferrous materials.
Ferrous materials, both metal and alloys have iron as their base and due to wide range of
their properties are most useful for use in engineering machines and structures. Owing to
the advents in steel technology and casting technique ferrous metals are cast, shaped and
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Engineering Materials machined in various shapes and sizes. Several standard shapes of sections are variable
commercially which make the job of designer and constructor much easy. They are used
for making trusses, bridges, ships and boilers. For such construction standard section and
sheets of plats of steel are available. The other machine parts like shafts, gears, bearings,
pulleys and bodies of machines can be made in steel through forming, cutting or casting
processes or combination thereof. Metal cutting tools, dies, punches, jigs and fixtures are
also made in ferrous metal. One of the largest consumer of steel is automobile industry.
Despite the modern trend of making light cars nearly 60% of weight of car is still due to
steel and an average passenger car contains about 500 kgf of steel in India. Perhaps in
countries like USA where cars of bigger size are in use this weight could be as high as 800
kgf/car.
The first human effort in the direction of making tools was based upon meteoritic iron
obtained from meteorite that had struck the earth. This happened more than 3000 BC. In
India the well known Ashoka Column in Delhi was constructed more than 4000 years ago.
The blast furnace was invented in 1340 AD and then it became possible to produce large
quantities of iron and steel. The future trend is to replace steel by plastics in many
machines and equipment. This target has been achieved in a number of home appliances.
The demand for steel is level since 1965. Cost fluctuations in most metals have been
controlled. The same is true for steel whose cost is increasing at constant rate since early
eighties. The comparative price of various metals with piece of gold at 1000 is given in
Table 2.1.
Table 2.1 : Approximate Comparative Prices of Various Metals
with Gold Piece of 100 as Base (per Weight)
Steel 0.0476
Aluminium 0.2078
Copper 0.3140
Magnesium 0.3528
Zinc 0.1840
Gold 1000
Lead 0.075
Nickel 1.5151
Tin 1.0823
Titanium 1.1363
Silver 15.1515
Objectives
After studying this unit, you should be able to
• know how iron and steel are produced,
• know what are different classifications and applications of steel,
• understand how different types of steel are formed as alloy,
• identify different constituents of steel and their effects on properties,
• know how steel can be treated,
• understand an alloy steel and effects of alloying on properties,
• distinguish between steel and cast iron and properties and uses of cast iron,
• know about alloys of copper, aluminium and their properties and uses,
• identify bearing materials, and
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Engineering Alloys
(Ferrous and Non-Ferrous) • identify creep resistant materials.
2.2 PRODUCTION OF IRON AND STEEL
Reviewing the principles of the iron and steel making processes, beginning with new
materials is taken up first. This knowledge is essential to an understanding of the quality
and characteristics of the steels produced by different processes.
2.2.1 Raw Materials
The three basic materials used in iron and steel making are iron or, limestone and coke.
Iron does not occur in a free state in nature, yet it is one of the most abundant elements,
making up about 5 percent of the earth’s crust in the form of various ores. The principal
iron ores are taconite (a black flintlike rock), hematite (an iron oxide mineral) and
limonite (an iron oxide containing water). After it is mined, the iron ore is crushed into
fine particles, the impurities are removed by various means (such as magnetic separation),
and it is formed into pallets, balls, or briquettes using binders and water. Typically, pellets
are about 65 percent pure iron and 25 mm in diameter. The concentrated iron ore is
referred to as beneficiated. Some iron-rich ores are used directly without palletising.
Coke is obtained from special grades of bituminous coal, which are heated in vertical coke
ovens to temperatures of 1150oC and cooled with water in quenching towers. Coke has
several functions in steel making. One is to generate the high level of heat required for
chemical reactions to take place in iron making. Second, it produces carbon monoxide (a
reducing gas) which is then used to reduce iron oxide to iron. The chemical by-products of
coke are used in making plastics and chemical compounds. Coke oven gases are used as
fuel for plant operations, and power generations.
The function of limestone (calcium carbonates) is to remove impurities from the molten
iron. The limestone reacts chemically with impurities, action as a flux which causes the
impurities to melt at a low temperature. The limestone combines with the impurities and
forms a slag, which is light and floats over the molten metal. Slag is subsequently
removed. Dolmite (an ore of calcium magnesium carbonate) is also used as a flux. The
slag is later used for making cement, fertilizers, glass, building materials, rock wool
insulation, and road ballast.
2.2.2 Iron Making
The three raw materials are charged into blast furnace by carrying them to the top of and
dumping into the furnace. The principle of this furnace was developed in Central Europe,
and the first furnace began operating in 1621. The first steel plant in India begins its
operation in the early part of twentieth century. The blast furnace is basically a large steel
cylinder lined with refractory (heat-resistant) bricks and has the height of about a ten-
storey building.
The charge mixture is melted in a reaction at 1650oC with air preheated to about 1100oC
and blasted into the furnace (hence the term blast furnace) through nozzles (or tuyeres).
Although a number of reaction may take place, basically the reaction of iron oxide with
carbon produces carbon monoxide, which in turn reacts with the iron oxide, reducing it to
iron. Preheating the incoming air is necessary because the burning coke along does not
produce sufficiently high temperature for the reactions to occur.
The molten metal accumulates at the bottom of the blast furnace, while the impurities float
to the top of the metal. At intervals of four or five hours, the molten metal is tapped, into
ladle cars. Each ladle car can hold as much as 160 tons of molten iron. The molten metal
at this stage has a typical composition of 4 percent carbon, 1.5 percent silicon, 1 percent
manganese, 0.04 percent sulphur, and 0.4 percent phosphorous, with the rest being pure
iron. The molten metal is referred to as pig iron. Use of the world pig comes from the
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Engineering Materials early practice of pouring molten iron into small sand molds, arranged like a litter of small
pigs around a main channel. The solidified metal is called pig and is used in making iron
and steels. The blast furnace is shown in Figure 2.1.
Figure 2.1 : Blast Furnace
2.2.3 Steel Making
Steel was first produced in China and Japan in about 600-800 AD. The process is
essentially one of refining the pig iron obtained from the blast furnace. The refining of pig
iron consists of reduction of the percentage of manganese, silicon, carbon and other
elements, and control of its composition by the addition of various elements. The molten
metal from the blast furnace is transported into one of three types of furnace. The steel
making furnaces are open hearth, electric, or basic oxygen. The name open hearth derives
from the shallow heart shape that is open directly to the flames that melt the metal.
Developed in the 1860s, the open-hearth furnace is being replaced by electric furnace and
by the basic-oxygen process. These newer methods are more efficient and produce better
quality steels.
The electric furnace was first introduced in 1906. The source of heat is a continuous
electric arc formed between the electrodes and the charged metal (Figure 2.2).
Temperature as high as 1925oC are generated in this type of furnace. There are usually
three graphites electrodes in direct arc electric furnace, and they can be as large as
750 mm in diameter and 1.5 to 2.5 m in length. Their height in the furnace can be adjusted
depending on the amount of metal present and water of the electrodes.
Figure 2.2 : Direct Arc Electric Furnace
Hopper
380C
4800C
12050C
16500C
Molten Slag Molten Iron
Reduction Zone
Combustion Zone
Fusion Zone
Heat Absorption Zone
Gas
Iron Ground Slag
Tuyere
Hot Blast
Coke Ore
Lime Stone
Coke + Ore + Lime Stone
Power Leads
Carbon Electrodes
Door
Slag
Metal Rammed Hearth
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Engineering Alloys
(Ferrous and Non-Ferrous) Steel scrap and a small amount of carbon and limestone are dropped into the electric
furnace through the open roof. Electric furnaces can also use 100 percent scrap as its
charge. The roof is then closed and the electrodes are lowered. Power is turned on, and
within a period of about two hours the metal melts. The current is shut off, the electrodes
are raised, the furnace is titled, and the molten metal is poured into a ladle, which is a
receptacle used for transferring and pouring molten metal. Electric-furnace capacities
range from 60 to 90 tons of steel per day. The quality of steel produced is better than that
of open-hearth or basic-oxygen process.
Figure 2.3 : Indirect Arc Electric Furnace
The induction type electric furnace (Figure 2.4) is used for smaller quantities. The metal
is placed in crucible, made of refractory material and surrounded with a copper coil
through which alternating current is passed. The induced current in the charge melts the
metal. These furnaces are also used for re-melting metal for casting.
Figure 2.4 : Induction Type Electric Furnace
The basic-oxygen furnace (BOF) is the newest and fastest steel making process.
Typically, 200 tons of molten pig iron and 90 tons of scrap are charged into a refractory
lined barrel shaped vessel called converter [Figure 2.3(a)]. Pure oxygen is then blown into
the furnace for about 20 minutes under a pressure of about 1250 kPa, through a water-
cooled lance, as shown in Figure 2.5(b). Fluxing agents, such as burnt lime are added
through a chute.
Electrodes
Trunion
Roller Metal
Crucible
Copper Induction
Coil
Refractory Cement
Molten Metal
(c) Tapping the (d) Pouring the
Lance
(b) Blowing with
(a) (i) Charging Scrap
into Furnace
(ii) Charging
Molten
Iron
(iii) Addition of
Burnt Lime
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Engineering Materials
Figure 2.5 : Basic Oxygen Process of Steel Making Illustrated through Various Operations
The vigorous agitation by the oxygen refines the molten metal through an oxidation
process in which iron oxide is first produced. The oxide reacts with the carbon in the
molten metal, producing carbon monoxide and carbon oxide. The iron oxide is reduced to
iron. The lance is retracted and the furnace is tapped by tilting it. The opening in the vessel
is so provided that the slag still floats on the top of the molten metal as seen in Figure
2.5(c). The slag is then removed by tilting the furnace in the opposite direction.
The BOF process is capable of refining 250 tons of steel in 35 to 50 minutes. Most BOF
steels, which are of better quality then open-hearth furnace steels and have low impurity
levels, are processes into plates, sheets, and various structural shapes, such as I-beams and
channels.
Steel may also be melted in induction furnaces from which the air has been removed,
similar to the one shown in Figure 2.4. The vacuum melting produces high quality steels
because the process removes gaseous impurities from the molten metal.
2.3 CASTING OF INGOTS
After the molten steel has been poured from the steel making furnace it has to be converted
in solid shapes called ingot. The ingot is further processed by rolling it into shapes, casting
it into semi-finished forms, or forging. For eliminating the need for ingot the shaping
process is being rapidly replaced by continuous casting, thus improving efficiency. The
molten metal is poured (teemed) from the ladle into ingot moulds in which the metal
solidifies. Moulds are usually made of cupola iron or blast-furnace iron, with 3.5 percent
carbon, and are tapered in order to facilitate the removal of the solid metal. The bottoms of
the moulds may be closed or open; if open, the mould are placed on a flat surface. The
taper may be such that the big end is down.
The cooled ingots are removed (stripped) from the moulds and lowered into soaking pits,
where they are reheated to a uniform temperature of about 1200oC for subsequent
processing by rolling. Ingots may be square, rectangular, or round in cross-section, and
their weights ranges from a few hundred kgf of 40 tons.
Certain reactions take place during the solidification of an ingot, which in turn have
important influences on the quality of the steel. For example, significant amounts of
oxygen and other gases can dissolve in the molten metal during steel making. However,
much of these gases is rejected during solidification of the metal because the solubility
limit of gases in the metal decrease sharply as its temperature decreases. The rejected
oxygen combines with carbon, forming carbon monoxide, which causes porosity in the
solidified ingot.
Three types of steel are produced depending on the amount of gas evolved during
solidification. These types are : killed, semi-killed, and rimmed.
Killed Steel
Killed steel is usually deoxidized steel; from which oxygen has been removed and
porosity eliminated. In the de-oxidation process the dissolved oxygen in the molten
metal is made to react with elements such as aluminium, silicon, manganese, and
vanadium that are added to the melt. These elements have an affinity for oxygen and
form metallic oxides. If aluminium is used, the product is called aluminium-killed
steel. The term killed comes from the fact that the steel lies quietly after being
poured into the mould. The oxides in the slag. A fully killed steel is thus free of any
porosity and blowholes caused by gases. The chemical and mechanical properties of
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Engineering Alloys
(Ferrous and Non-Ferrous) killed steel are relatively uniform throughout the ingot. However, because of metal
shrinkage during solidification, an ingot of this type develops a pipe at the top. It is
also called shrinkage cavity and has the appearance of a funnel-like shape. This
pipe can comprise a substantial portion of the ingot and has to be removed.
Semi-killed Steel
Semi-killed steel is partially deoxidized steel. It contains some porosity, generally in
the upper section of the ingot, but has little or no pipe; thus scrap is reduced.
Although piping in semi-killed steels is less, it is compensated for by the presence of
porosity in that region. Semi-killed steels are economical for deoxidation process is
quite costly.
Rimmed Steel
Rimmed steel, generally having a low carbon content (less than 0.15 percent), have
the evolved gases only partially killed or controlled by the addition of elements such
as aluminium. The gases form blowholes along the outer iron of the ingot – hence
the term rimmed. Blowholes are generally not objectionable unless they break
through the outer skin of the ingot. Rimmed steels have little or not piping, and have
a ductile skin with good surface finish. The blowholes may break through the skin if
they are not controlled properly. Impurities and inclusion tend to segregate toward
the centre of the ingot. Thus, products made from this steel may be defective and
should be inspected.
Refining
The properties and manufacturing characteristics of ferrous alloys are adversely
affected by the amount of impurities, inclusions, and other elements present. The
removal of impurities is known as refining, much of which is done in melting
furnaces or ladles, with the addition of various elements. The cleaner steels having
improved and more uniform properties and consistency of composition are
increasingly being demanded. Refining is particularly important in producing high-
grade steels and alloys for high-performance and critical applications, such as in
aircraft. Moreover, warranty periods on several machine parts such as shafts,
camshafts, crankshafts for diesel trucks, and other similar parts can be increased
significantly using higher-quality steels.
The trend in steel making is for secondary refining in ladles and vacuum chambers.
New methods of ladle refining (injection refining) generally consist in melting and
processing in a vacuum. Several methods of heating and re-melting have been
introduced for their efficiency and cleanliness. These are normally used in controlled
atmosphere. Some methods are : electron-beam melting, vacuum-arc re-melting,
argon-oxygen decarburisation, and vacuum are double-electrode
re-melting.
2.4 CONTINUOUS CASTING
The traditional method of casting ingots is a batch process. Each ingot is stripped from its
mould after solidification and processed individually. Additionally the defects like piping
and micro-structural and chemical variations are present throughout the ingot. These
problems are alleviated by continuous casting process, which produce better quality
steels.
Continuous or strand casting was first developed for casting non-ferrous metal strip. The
process is now used for steel production, with major efficiency and productivity
improvements and significant cost reduction. A system for continuous casting is shown in
Figure 2.6. The molten metal in the ladle is cleaned and nitrogen gas through it is blown
for five to ten minutes to equlise the temperature. The metal is then poured into a
refractory-lines intermediate pouring vessel called tundish where impurities are skimmed
off. The tundish can hold as much as three tons of metal. The molten metal vessels through
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Engineering Materials water-cooled copper moulds and begins to solidify as it travels downward along a part
supported by rollers.
Before starting the casting process, a solid starter, or dummy, bar is inserted into the
bottom of the mould. The molten metal is then poured and freezes onto the dummy bar
(Figure 2.6). The bar is withdrawn at the same rate the metal is poured. The cooling rate is
such that the metal develops a solidified skin to support itself during its travel downward
at speed maintained at 25 mm/s. The shell thickness at the exit end of the mould is about
12 to 18 mm. Additional cooling is provided by water sprays along the travel path of the
solidifying metal. The moulds are generally coated with graphite or similar solid lubricants
to reduce friction and adhesion at the mould walls. Vibration of moulds may further reduce
friction and adhesion tendency.
The continuously cast metal may be cut into desired lengths by shearing or touch cutting,
or it may be fed directly into a rolling mill for further reduction in thickness.
Whether the steel is obtained in form of ingots, stationary moulds or in form of slab from
continuous casting process, it is converted into blooms, billets and slabs. The subsequent
hot rolled products are described as follows :
Blooms
Beams and angle sections, rails, bars of different sections.
Billets
Wire nails and wire mesh, pipes and tubes.
Slabs
Plates, strips and further cold rolled for reducing thickness.
Figure 2.6 : Continuous Casting Process of Steel Schematically
SAQ 1
(a) Distinguish between an elemental metal and alloy.
Starting Dummy
Oxygen Lance
(For Cutting)
Pinch Rolls
Catch Basin
Air Gap
Argon Platform
Electric Furnace
Tundish
Oil
Cooling Water
X – Ray Transmitter
Molten Metal
Solidified Metal
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Engineering Alloys
(Ferrous and Non-Ferrous) (b) What are the raw materials used in blast furnace in iron making process?
(c) What are electric heating processes for making steel?
(d) Describe BOF and its advantages.
(e) Distinguish between killed and semi-killed steels.
(f) List steel products obtained from blooms, billets and slabs.
2.5 STEELS
Steel perhaps is the oldest material of construction which has weathered and history and
not only maintained its foremost place in industrial application but also enhanced it
greatly. The enhancement of its position in industrial world is mainly due to its capacity to
be produced in several alloyed varieties and response to various heat treatments. The
cutting properties of sharp edges of steel was recognised long back when man began to use
swords and knives made in steel. The very same properties of this material have been
exploited to create cutting tools which wear very little and such machine parts as gears,
shafts, bearing, etc. The steel is now being used in every conceivable engineering structure.
Machine bodies, railways and rail road rolling stocks, ships, bridges and boilers are a few
examples. Several forms of steel are now available commercially and each is produced to
serve some specific purpose. The requirements of steel very largely and more often than
one involve consideration of their tensile strength, impact strength and hardness. Table 2.2
describes some applications and requirements of some steels.
Table 2.2 : Typical Carbon Steels and their Applications
Requirements % Composition Application
C Mn Si P S
Axles, shafts,
small gears
Availability in form of bar stock. Good
strength in bending and torsion. Heat
treatable for improvement of surface
resistance.
0.4 0.8 0.1 0.05 0.05
Helical
springs
Availability in rod form, this is obtained
through rolling. Good ductility for
coiling. Good response to heat treatment
for spring properties.
1.0 0.6 0.3 0.05 0.05
Automative
bodies and
panels
Availability in form of thin sheets which
can be pressed into accurate shapes. Good
ductility and low yield strength.
0.0
8
0.3 − 0.05 0.05
Ship hulls Availability in thick plate forms. Good
strength to withstand stresses during
forming and service, particularly good
ductility at low temperatures. Good
weldability since most ships have welded
structure.
0.1
8
0.8 0.1 0.05 0.05
The carbon content of steel plays an important role in deciding its properties. If no carbon
is present in iron, it crystallises in form of ferrite which is bcc soft material and very
ductile. Pure iron without impurities is perhaps used only in laboratories and may be as
costly as gold. Pure iron is though but not very strong. With addition of carbon, increasing
amounts of cementite (Fe3C) crystallise in the structure. Cementite being hard reduces
ductility considerably. Table 2.3 would illustrate how ductility (measured as % elongation)
of steel is reduced with increasing amount of carbon. When carbon is as much as 6.67% in
iron, the entire structure crystallises as cementite and is not at all usable commercially
because it neither has ductility nor is machinable.
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Engineering Materials Table 2.3 : Ductility of Plain Carbon Steel
Carbon Content
(%)
% Elongation
Pure iron 42
0.2 37
0.4 31
0.6 22
0.8 17
1.2 3
The carbon content in steel serves to classify steel according to its application. Table 2.4
describes the application of different iron-carbon alloys. Figure 2.7 illustrates the effect of
carbon percentage on tensile strength of steels mainly. The tensile strength of pure iron
(0% C) is around 250 N/mm2 which increases to about 850 N/mm
2 at a carbon percentage
of 0.8. It has already been pointed out that a very low carbon content the entire metal is
made up of ferrite grains which are soft and ductile. As the percentage of carbon increases
more and more material is made up of pearlite, a substance that appears to have colour of
mother-of-pearl, hence the name. At 0.4% C, pearlite appears to be almost half the area
viewed through microscope. At this level of carbon the tensile strength is about 540
N/mm2. At 0.8% C almost total area consists of pearlite when a strength of 850 N/mm
2 is
reached.
Table 2.4 : Carbon Percentage in Plain Carbon Steels and Application
Range of Carbon
(%)
Application
0.1 – 0.8 General engineering
purposes
0.0 – 1.2 Wear resistance steel
1.3 – 2.2 Not used normally
2.4 – 4.2 Cast iron, casting
2.5.1 Plain Carbon Steel and Applications
Some applications of carbon steels have been described already in Table 2.2. Here some of
the applications will again be described after emphasising the manner in which plain
carbon steels are classified. Plain carbon steels are those which contain carbon as principal
alloying element. These steels may also contain small amounts of such impurities as
manganese, sulphur, phosphorous, silicon and nickel. The sulphur and phosphorous are
mainly undesirable impurities and attempts are make to keep them at as low level as
possible. Their levels beyond 0.05% are not permissible.
According to carbon percentage (or microscope structure) the steel is divided into three
groups.
Eutectoid Steels
Such steels contain ideally 0.83% of carbon and have entirely lamellar peralite
structure. In practice fully pearlite structure appears in all steels containing about
0.8% carbon. Moreover many alloying elements influence the carbon contents of
eutectoid steels. For instance, Mn to the extent of 1% reduces carbon in the
eutectoid to 0.7%.
Hypo-Eutectoid Steels
These steels contain carbon between 0.08% to just below 0.83%. They contain
grains of ferrite together with grains of pearlite. The strength increases with
increasing carbon content due to increasing proportion of strong pearlite formed but
ductility decreases proportionally.
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Engineering Alloys
(Ferrous and Non-Ferrous) Hyper-Eutectoid Steels
These steels contain carbon significantly in excess of 0.8%. The structure contains
pearlite and cementite. Cementite forms along the grain boundaries of pearlite as an
inter-granular network and increase brittleness.
With respect to range of carbon content the plain carbon steels are divided into following
groups. The carbon percentages may overlap in many cases.
Dead Mild Steels
The carbon range for this steel is between 0.07% to 0.15%. These steels are highly
ductile and hence can withstand large amount of plastic deformation through cold
working. Solid drawn tubes are made out of deal mild steels.
Mild Steels
The carbon percentage for mild steels very between 0.15 to 0.25. This steel does not
harden appreciably when quenched. It has very good weldability and is commonest
of the steels used for structural purposes.
Medium Carbon Steels
These steels contain carbon between 0.25 and 0.55%. These steels respond to
suitable types of heat treatments.
High Carbon Steels
The carbon content in these steels vary between 0.55% to 0.9%. Very high
hardnesses can be achieved through heat treatment. They develop extreme resistance
to wear and hence are good for tooling applications.
In yet another classification steels containing upto 0.3% C are known as low carbon steels
thus including both dead and mild steel. Those containing carbon between 0.3 and 0.6%
are medium carbon steels and those containing carbon in excess of 0.6% carbon are high
carbon steel thus including carbon tool steels.
Table 2.5 describes various applications of plain carbon steels, whereas Table 2.6
describes plain steels for several applications.
Table 2.5 : Some Applications of Plain Carbon Steels
Steel % Carbon Applications
Dead mild steel 0.07 – 0.15 Rivets, nails, tin plates, stamping,
chains, seam welded pipes, automative
body sheets, other materials subject to
drawing and pressing.
Mild steel 0.10 – 0.20
0.20 – 0.30
Structures, rolled steel sections, drop
forgings, screws, case hardening
purposes.
Machine structures, shafting and
forging, gear.
Medium carbon steel 0.30 – 0.40
0.40 – 0.50
0.50 – 0.60
Shafting, axles, crane hooks,
connecting rods, general purpose
forgings.
Axles, gear, shafts, rotors, tyres, skip
wheels, crank shafts.
Rails, loco tyres, wire ropes.
High carbon steel 0.60 – 0.70 Drop hammers, dies, saws, screw
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Engineering Materials
0.70 – 0.80
0.80 – 0.90
drivers.
Hammers, anvils, wrenches, leaf
springs, cable wires, band saws, large
dies for cold presses.
Shear blades, punches, rock drills, cold
chisels.
Tool steels 0.90 – 1.10
1.10 – 1.40
Drills, knives, taps, picks, screwing
dies, axes.
Razors, files, broaches, boring and
finishing tools, such machine parts
where wear resistance is required, ball
bearings.
Table 2.6 : Plain Carbon Steels for Different Applications
Sl. No. Application Properties Steel
1 Nails, rivets,
stampings
High ductility, low
strength
Low C (AISI 1010)
2 Beams, rolled
sections, reinforcing
bars, pipes boiler
plates, bolts
High ductility, low
strength, toughness
Low C (1020)
3 Shafts and gears Heat treatable for
good strength and
ductility
Med C (1030)
4 Crank shaft, bolts,
connecting rod,
machine component
Heat treatable for
good strength and
ductility
Med C (1040)
5 Lock washers, valve
springs
Toughness High C (1060)
6 Wrenches, dies,
anvils
High toughness and
hardness
High C (1070)
7 Chisels, hammers,
shear
Retaining sharp edges High C (1080)
8 Cutters, tools, taps,
hacksaw blades,
springs
Hardness, toughness,
heat treatable
High C (1090) tool
steel
SAQ 2
(a) What are distinguish features of eutectoid, hypoeutectoid and hypereutectoid
steels?
(b) How are plain carbon steels classified as low, medium and high carbon
steels?
(c) Describe uses of low, medium and high carbon steels.
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Engineering Alloys
(Ferrous and Non-Ferrous) 2.5.2 Iron-Carbon System Phase Diagram
Iron and carbon make a series of alloys which include a number of steels and cast iron.
Steels are the alloys which contain upto 1.2% of carbon while cast irons contain carbon
within range of 2.3% to 4.2%. Alloys with carbon greater than 4.6% have poor properties
and hence not used.
The carbon atom is smaller than the iron atom (the diameters being 1.54 o
A and 2.56 o
A
respectively) and dissolves interstitially in iron. Pure molten iron solidifies at 1539oC in
bcc structure. At 1400oC, on further cooling this structure changes into fcc and again at
910oC back to bcc. These three structures in reverse order (i.e. form room temperature)
are named as α, γ and δ.
The α-iron is ferromagnetic but loses its ferromagnetism when heated above 770oC (curic
temperature). The non-magnetic α-iron was earlier known as β-iron until X-ray studies
showed that structures of α − and δ − irons are same. The γ-iron is the structure of closest
packing while α − and β − irons are not. There will be abrupt changes of dimensions at
transitions α to γ and γ − β. Figure 2.7 depicts the cooling curve of pure iron.
Figure 2.7 : Cooling Curve of Pure Iron with Steady Rate of Heat Loss
Alloys in iron-carbon systems also undergo complex structural changes which play an
important role in deciding their characteristics. Changes that occur in iron-carbon system
are illustrated in iron-carbon phase diagram of Figure 2.8. Strictly speaking the diagram
refers to the iron-iron carbide system but still the phase relationship can be expressed in
terms of carbon percentage, hence the name.
Iron-carbon diagram is divided in various phase fields characterised by the existence of
one or mixture of two phases. The liquids above which only molten metal in liquid state
can exist is identified as ABCD. Iron and dissolved carbon exist in the liquid state. The
solidus below which iron-carbon exist in solid state is identified as AEPGCH. Between
these two lies there exist mixtures of solid and liquid.
α and γ phases can dissolve carbon and the solubility changes with temperatures. The solid
solution of carbon in α-iron is called ferrite while the solid solution of C in γ-iron is
known as austenite. Ferrite and austenite are also designated as α and γ respectively. the
solubility of C in austenite is upto 0.2% while ferrite can dissolve C only upto 0.025%.
The C solubility in austenite changes along GK in austenite and along LN in ferrite.
Beyond point C on solidus, the compound Fe3C which is cementite, separates. Cementite
is hard and brittle whereas ferrite on the extreme left of phase diagram is soft and ductile.
Austenite also transforms into cementite along the ling GK and into ferrite along IK.
0
500
1000
1500
Liquid
1539
1400
δ (bcc)
γ (fcc)
910 β (Non-magnetic α)
770
α (bcc)
Time
54
Engineering Materials The line IK and GK are respectively designated as A3 and Acm. Austenite is unstable below
the lines A3 and Acm if carbon content is less than 0.8%. Austenite begins to transform into
ferrite on cooling and gets enriched in carbon along line A3 until point K is reached.
Similarly for carbon content between 0.8 and 2.0% iron carbide will precipitate and
carbon in austenite will vary until point K (carbon 0.8%) is reached. At point K austenite
will transform into pearlite. Pearlilte is an intimate mixture of ferrite and cementite (Fe3C)
having a characteristics lamellar structures composed of alternate platelets of ferrite and
cementite. The transformation reaction at K wherein a single solid phase splits into two
phases is termed eutectoid. The equation is written as
(Austenite) (Ferrite) (Cementite)
Solid phase ( ) Solid phase ( ) Solid phase ( )A B C→ +
Apparently at point K three phases exist (P = 3) and three are two components (C = 2),
hence using Gibb’s phase rule the degree of freedom, F can be calculated.
Gibb’s rule at constant pressure,
P + F = C + 1
F = 2 – 3 + 1 = 0
Thus the eutectoid point like eutectoid point is non-variant.
Slightly above 110oC at point C on solidus, the eutectoid transformation occurs. The liquid
state at C contains 4.3% C and this liquid begins to transform into two solid phase. One
phase is called Ledeburite which is eutectic mixture of austenite and cementite while the
other phase is cementite. On further cooling eutectic austenite transforms gradually into
cementite, changing composition along GK until it changes into pearlite and cementite at
eutectoid point K.
Peritectic
Peritectic transformation occurs in steel having carbon upto 0.55%. This
transformation occurs at point P and is characterised by transformation of liquid
and solid phase into a single solid phase. For example, the transformation occurring
at P is represented by the following equation,
Liquid (0.5% C) + δ iron (0.08% C) → γ iron (0.18% C)
Explanation of Reading Iron-Carbon Phase Diagram
Assume that in liquid state mixture of iron and carbon contains 0.4% carbon and is
represented by point 1 in Figure 2.8. The line xx shows the path of cooling.
As cooling begins the freezing starts in intersection point 2 of xx and liquidus ABC
at 1510oC. At a temperature of 1470
oC at point 3 the liquid is solidified completely
into solid austenite. This phase gradually cools until point 4 on line IK is reached.
The precipitation of proeutectoid ferrite begins and the carbon content of austenite
varies along line A3. The composition of ferrite varies along the line IL. In the region
enclosed between IL and IK he percentage of austenite and ferrite can be determined
by using lever rule. The remaining austenite transforms into peralite (88% ferrite
and 12% cementite) at eutectoid temperature of 723oC. The % weight of (ferrite +
cementite) can be determined by lever rule along line LKM. Further cooling will
effect no change in microstructure since carbon percentage in ferrite practically
remains constant of 0.008. The microstructure is built by pearlite matrix embedded
with ferrite crystals.
Taking example of alloys with 3% carbon and considering its cooling from a point
above liquids along yy. The composition of course is in the region of cast iron. The
austenite separates from liquid and its content will increase along the solidus PG
and the amount of liquid reduces with composition varying along liquidus BC.
When the temperature of 1130oC is reached, the mixture will consists of austenite
containing 2% carbon and liquid of eutectic composition.
55
Engineering Alloys
(Ferrous and Non-Ferrous) The liquid will solidify at constant temperature into ledeburite – a mixture of
austenite and cementite. On further cooling eutectic austenite decomposes to
cementite along the line GK. At temperature of 723oC (eutectoid). The remaining
austenite will transform to pearlite. Below the temperature of 723oC all ledeburite
will be transformed into a mixture of pearlite and cementite.
Transformation Reactions
Several transformation were described in above paragraphs while explaining how to
read iron-carbon diagram. These transformation are summed up afresh here.
Eutectoid Reaction
Eutectoid reaction takes place when austenite containing 0.77% C
decomposes into ferrite and cementite at 723oC (point K in the phase diagram
of Figure 2.8).
o
0.77% C
3723 C
Solid austenite ( ) Ferrite ( ) Cementite (Fe C)γ → α +
Solid phase splits into two solids phases. Steel containing carbon between
0.008 and 0.8% is called hypoeutectoid and those containing between 0.8
and 2.0% carbon are hypereutectoid.
Eutectic Reaction
Eutectic reaction occurs when liquid solution containing 4.3% C
transformations into austenite (γ) and cementite (Fe3C) at 1130oC. Point C in
Figure 2.8 is eutectic point.
o
4.3% C
31130 C
Liquid Austenite ( ) Cementite (Fe C)L → γ +
Liquid phase transforms into two solid phases.
Figure 2.8 : Iron-Carbon Phase Diagram
Peritectic Reaction
Peritectic reaction occurs when a liquid and a solid phase freeze to form a
solid phase. In iron-carbon system peritectic reaction takes place when alloy
containing 0.55% C and containing liquid and solid δ iron transforms at
1495oC into solid austenite (γ).
o(1495 C)(0.99% C)(0.55% C) (0.17% C)
Liquid -Iron Solid austenite+ δ →
56
Engineering Materials 2.5.3 Time Temperature Transformations
The phase diagram of iron-carbon system evokes much interest for engineers to use the
information for useful purposes of increasing strength and eliminating locked in stresses by
retaining or avoiding a particular phase. However, important aspect of such a diagram to
keep in mind is that it represents equilibrium cooling. Such equilibrium cooling is neither
obtained in practice nor it is conducive to develop desired strength and structure in a
particular steel. Increasing cooling rates reduces the transformation temperature as was
highlighted and may result information of metastable phase. As an example very high
cooling rate of iron-carbon system in steel range causes development of a metastable phase
called martensite. This phase does not appear in Figure 2.8. The steel and cast iron
normally carry some alloying elements, though in varying amounts. These alloying
elements have great deal of influence on precipitation of all the phases and this is also not
represented by phase diagram of Figure 2.8.
The metastable phase marteniste has been introduced here and other phases like pearlite
ferrite, and cementite were mentioned earlier. More about their structure and properties
will be discussed now but before that transformation curves are described.
Experimental determination of isothermal transformation curves goes as under. A number
of samples of the size of a 50 paise coin are austenised just above the temperature of
723oC (eutectoid temperature). The samples are rapidly cooled in a salt bath maintained at
a temperature slightly below 723oC. After allowing various time intervals the specimens
are taken out one by one from the salt both and quenched into water at room temperature.
The resulting microstructure is then examined at room temperature. The experiment is
repeated with isothermal transformation of eutectoid steel (both regions of hypo- and
hypereutectoid steel) at progressively reducing temperatures. If temperature of
transformation is plotted as function of time of transformation the resulting curve as
shown schematically in Figure 2.9 is called isothermal transformation (IT) curve, or
temperature-time-transformation curve (TTT). This is also known as Bain curve after the
metallurgist who first introduced the idea of S curve because of its shape.
Figure 2.9 : Time Temperature Transformation Diagram for Plain Carbon Steel
The line marked Ps shows the beginning of transformation of austenite into pearlite and the
line Pf represents the completion of such transformation. Figures 2.10 and 2.11 show the
TTT diagrams respectively for 0.8% C steel and 0.3% steel. The difference between the
two can be noted. Even the fastest cooling rate will not be able to mess the nose of the S
curve and hence 100% martensite will not be retained at room temperature in structure of
steel. The important feature of TTT curve is its bending backwards at nose. Below the nose
the austenite transforms into Bainite. Whether it is pearlite or bainite, at a temperature
called Ms the transformation into a transition phase Martenstite takes place.
Both binite and martensite may be retained in steel by controlling the rate of cooling. If
cooling is sufficiently fast so that nose is avoided then austenite transforms into bainite. If
the part is now held at this temperature for sufficiently long time the bainite is stabilised.
Unlike, pearlite, in bainite the cementite is in particle form, distributed uniformly in the
Eutectoid Temp
Coarse Pearlite
Fine Pearlite
Bainite
Pf Ps
PS
Pf
Time
Martensite
Tem
pera
ture
(0 C
)
57
Engineering Alloys
(Ferrous and Non-Ferrous) matrix of ferrite. Bainite is harder, stronger and tougher than pearlite. Bainitic steel is
more ductile than pearlitic steel for some level of hardness.
Figure 2.10 : TTT Diagrams for 0.8% C Steel with 0.76 Mn
If heated steel is cooled sufficiently fast the nose of Ms temperature, martensite is formed.
Its transformation is complete at temperature Mf. Martensite has C dissolved in Fe
whereby its bcc structure changes into body centered tetragonal (bct) structure and is
marked by high hardness because of :
(a) distortion of iron lattice,
(b) very fine size of martensite plates, and
(c) high density of dislocations associated with twining.
Figure 2.11 : TTT Diagrams for 0.3% C Hypoeutectoid Steel
Figure 2.12 schematically shows the cooling pattern to produce different phases in steel.
Heat treatments given to steel will be governed by the cooling rates producing final phases
with pearlite, bainite or martensite. Many refinements are possible with control of cooling
rate and soaking time.
4sec 0-5min 1min 4min 1 hr 15 hr
MF
Ms Martensite
γ
γ Ps
PF
Bs BF Bainite
Fine Pearlite
Coarse Pearlite
727
550
230
110
Tem
pera
ture
4sec 0.5min 4min 1 hr 15 hr
MF
Ms Martensite
Ps
PF
Bs BF
Ferrite+Pearlite
727
340
210
Tem
pera
ture
(0 C
)
Ferrite + Pearlite + Bainite +
Time
230
110
727
1 2 3 4
Ms
MF
I II III
1sec 2sec 4min 2hr
Time
Tem
pera
ture
(0 C
)
58
Engineering Materials
Figure 2.12 : TTT Curves for Steel and Different Cooling Rates
At this stage, before taking up different methods of heat treatments in any detail, it will be
worthwhile to first describe formation and characteristics of different phases.
2.5.4 Pearlite
The tensile strength of pure iron (0.00% C) is around 250 N/mm2 which increases to about
850 N/mm2 at a carbon percentage of 0.8. At very low carbon percentage the entire metal
is made up of ferrite grains which are soft and ductile. With increasing carbon percentage
more and more material is made up of pearlite, a substance that appears to have colour of
mother-of-pearl, hence the name. At 0.4% carbon pearlite occupies almost half the area
viewed through microscope. At this level of carbon the tensile strength is about 540
N/mm2. At 0.8% carbon almost total area consists of pearlite with a strength of 850
N/mm2.
Examination of pearlite at higher magnification (X 1500) reveals that it has a laminated
structure. It is a composite consisting of alternate layers of ferrite and cementite.
Cementite provides the strength while ferrite retains ductility of steel. At carbon content
above 0.8% the quantity of Fe3C begins to increase and steel begins to lose its ductility.
Figure 2.13 shows relationship between pearlite and carbon content. Outside the region of
Figure 2.13, within the range of carbon between 0.9 and 1.2% pearlite structure still exists
but grains are surrounded by cementite network. These cementite networks not being parts
of grains are referred to as free cementite and are the cause of brittleness in steel. In
general the strength of steel does not increase due to the presence of free cementite, later it
may reduce below the level of 0.8% carbon steel.
Since pearlite is an important constituent of steel is control becomes important if
properties of steel are to be controlled. It will be worthwhile to understand how it is
formed. With reference to Figure 2.7 three forms of iron were identified. These forms are
referred to as allotropic forms of iron and at each transformation a thermal arrest is
experienced.
Figure 2.13 : Relationship between Pearlite % and Carbon Content
During the cooling of pure iron the transformation from γ-iron (fcc) to α-iron (bcc) occurs
at a unique temperature of 910oC. However, carbon is added to the iron to form steel this
transformation takes place over a range of temperatures. The limits of this range are
known as the critical points and the higher temperature at which transformation starts
during cooling is designated A3. The lower point is designated as A1. It was stated in
0.0 0.2 0.4 0.6 0.8 1.0 1.2
50
100
Ferrite + Pearlite Fe3 C + Pearlite
Carbon Content (%)
Are
a o
f M
icro
gra
ph O
ccupie
d
By P
earlite
59
Engineering Alloys
(Ferrous and Non-Ferrous) Section 2.7 that the lower temperature A1 remains constant at 723oC irrespective of the
carbon content while the higher temperature A3 gradually reduces as carbon percentage
increases (Figure 2.8). At a carbon percentage of 0.8, A3 and A1 are equal to 723oC. If
carbon percentage further increases, A3 also increased. The equilibrium diagram will be
obtained if A3 and A1 temperatures are plotted against carbon percentage as seen in
Figure 2.14. The detailed phase diagram has already been presented in Figure 2.8. This
diagram shows how γ-iron which is austenite is transformed into pearlite.
Figure 2.14 : Critical Points for Carbon Steel
It may be noted that carbon solubility is different in austenite than in ferrite. The fcc
(γ-iron) can hold as much as 1.7% carbon in solution at 1130oC. Within the range of
carbon under discussion the structure is entirely austenite at this temperature and all the
carbon is in solution. The carbon is dissolved interstitially or in other words, a carbon
atom does not replace an iron atom to form part of the cube (Figure 2.15(a)). When the
lattice is arranged to have a bcc structure the carbon atoms is ejected from its site
(Figure 2.15(b)) because space is insufficient. This is the reaction that takes place at A3.
Figure 2.15 : Interstitial Solution of Carbon
The sequence of events between temperatures A2 and A1 is illustrated in Figure 2.16 during
which austenite transforms into pearlite. The bcc ferrite grains nucleate at the boundaries
of austenite grains. As the temperature reduces the ferrite covers more and more are in
austenite grain which reduces in size. The separation of ferrite from austenite will result in
rejection of carbon which will be absorbed by remaining austenite. Thus the remaining
austenite gets richer in carbon until the amount of carbon reaches 0.8% causing
precipitation of layers of cementite in the austenite region. Thus, the austenite transforms
into pearlite consisting of layers of cementite and ferrite. Thus transformation of austenite
Ferrite + Pearlite Pearlite + Cementite
Austenite + Cementite
Cem
A
A1
A1
A1
A1
A1
A1
723
910
A3
A3
A3
A3
0.8
Carbon Content (%)
Tem
pera
ture
( 0
C )
Cementite
Carbon atom in Space between Iron Atom
Insufficient Space Between Iron Atom to Accommodate Carbon Atom
60
Engineering Materials is similar to eutectic reaction but since it occurs in the solid state it is termed eutectoid
transformation. This eutectoid reaction (or breaking of solid phase austenite into ferrite
and cementite as already described in Section 2.9) takes place at temperature of 723oC
which is termed A1. For this reason pearlite is also often referred as eutectoid.
Figure 2.16 : Transformation of Austenite in 0.4% C Steel, as Cooling
occurs from Molten State above A3 Temperature
2.5.5 Martensite
Carbon atoms cannot move fast through the lattice between temperatures of A3 and A1.
The formation of pearlite and ferrite depends upon allowing sufficient time for the
movement of carbon atoms. This means that such transformation is possible only under
equilibrium condition. It may be realised that the edge of fcc cell is 23% greater than that
of bcc cell. If a carbon atom is trapped between two iron atoms, they are kept apart and
are not able to take up the position in bcc cell. Carbon atoms in martensite occupy the
position on edge of unit cell between two iron atoms and thus elongate the edge and cause
distortion. Figure 2.17 compares austenite, ferrite and martensite unit cells at (a), (b) and
(c) respectively. At (d) the size of edge of martensite unit cell on which the carbon atom
size is compared with other edge on which there is no carbon atom. As a result a distorted
lattice is obtained. It may be emphasised that such distortions resulting into regions of high
strain energy, would impede the movement of dislocations whereby the material would lose
its ductility or increase its hardness. This trapping of carbon atom between atoms of Fe
occurs when the steel is not allowed to cool in equilibrium condition or in other words it is
suddenly cooled from A3 to room temperature or to about 300oC. This process of sudden
cooling from A3 is called quenching which may be achieved by plunging a heated steel (A3
temperatures) into water or some other quenching medium. During this process the carbon
does not have sufficient time to diffuse or carbon does not occur below a temperature
300oC hence a quench treatment in which temperatures is suddenly dropped from A3 to
300oC is sufficient to create permanent hardening in steel. If the resulting structure is
examined under microscope it appears to be needle like, often referred to as acicular, and
is called martensite.
Figure 2.17 : Comparison of Unit Cells of Austenite, Ferrite and Martensite
Also Dimension are Compared at (d)
Different carbon contents will naturally result in different hardness. At low carbon levels
there is very little change in hardness. The noticeable change in hardness is achieved when
carbon content is at least 0.4% and reaches a maximum at eutectoid composition of 0.8%.
The hardness that can be achieved through martensite transformation at 0.4 and 0.8% C
are respectively 255 VHN and 530 VHN. Also the formation of martensite depends upon
the cooling rate. As the carbon content increases the critical cooling rate becomes slower.
Fe Atom C Atom
Fe
Fe
C
a
c
C Atom Fe Atom
61
Engineering Alloys
(Ferrous and Non-Ferrous) The critical cooling rates for plain carbon steels vary between 400oC/sec and 500oC/sec. It
means that it is easier to harden steel containing 0.8% C than the one containing 0.4% C
(Also refer to Figure 2.12). Quenching in water results in fastest cooling rate and
corresponds to critical cooling rate for 0.35% carbon. This is not good for hardening.
Figure 2.18 shows the critical cooling curves for some plain carbon steels.
Figure 2.18 : Critical Cooling Rate for Different Carbon Content
Example 2.1
A 0.4% C hypoeutectoid plain carbon steel is slowly cooled from 1540oC to
(i) slightly above 723oC and (ii) slightly below 723
oC.
Calculate the weight percent
(a) austenite present in the steel,
(b) ferrite present in the steel in case (i),
(c) proeutectoid ferrite prevent in the steel, and
(d) eutectoid ferrite and eutectoid cementite % present in the steel in
case (ii).
Solution
Refer to Figure 2.8. Point 1 above the liquidus represents the state of liquid steel.
The cooling occurs along the line xx and an equilibrium cooling is assumed.
Freezing begins at point 2 which is intersection of liquidus and line xx. Temperature
at 2 is 1510oC. The steel solidifies completely at point 3 where temperature is
1471oC. The whole alloy is now composed of austenite (γ-phase) as indicated by
first of Figure 2.19. No change occurs until point 4 on line A3 is reached. At this
point the precipitation of ferrite begins out of solid austenite. Further cooling
increases the amount of ferrite and austenite decreases. The amount of austenite
varies along IK. The composition of ferrite varies along the line IL.
Calculation of % content will be made by lever rule.
The amount of austenite slightly above 723oC is calculated from the line LK itself.
i.e. taking LK as tie line.
(a) Weight % of austenite 5 0.4 0.025
0.8 0.025
L
LK
−= =
−
0.375
0.484 or 48.4%0.775
= =
. . . (i)
Weight % of ferrite 5 0.8 0.4
0.8 0.025
K
LK
−= =
−
0.4
0.516 or 51.6%0.775
= = .
. . (ii)
Time
0.4% 0.6%
C = 0.8%
Tem
pera
ture
(0C
)
62
Engineering Materials (b) Weight % of proeutectoid ferrite slightly below 723oC is same as that
slightly above, i.e. 48.4%. . . . (iii)
For calculating eutectoid ferrite, the weight of carbide will have to be
subtracted form total mass of ferrite and cementite. Just below
isothermal line LKM ferrite and pearlite are present and lever arm will
extend upto ordinate representing 6.67% C.
Weight % of total (ferrite + cementite) just below 723oC
6.67 0.4 6.37
6.67 0.025 6.645
−= =
−
0.96 or 96%=
Weight % of Fe3C just below 723oC
0.4 0.025 0.375
6.67 0.025 6.645
−= =
−
0.0564 or 5.64%=
Weight % of eutectoid cementite = total ferrite – proeutectoid ferrite
96 51.6 44.4%= − =
Weight % of eutectoid cementite (by difference)
100 48.4 5.64 44.4 1.56%= − − − = . . .
(iv)
Example 2.2
A hypoeutetoid steel which was cooled slowly from γ-state to room temperature was
found to contain 10% eutectoid ferrite. Assume no change in structure occurred on
cooling from just below the eutectoid temperature to room temperature. Calculate
the carbon content of steel.
Solution
Refer to phase diagram of Figure 2.8 and let the vertical line xx cross the isotherm
at 5 such that 5 is at a distance x′ from temperatures axis. Then by lever rule
% total ferrite 6.67 6.67
6.67 0.025 6.645
x x′ ′− −= =
−
% proeutectoid ferrite 0.80 0.80
0.80 0.025 0.775
x x′ ′− −= =
−
% eutectoid ferrite = % total ferrite – % proeutectoid ferrite
or 10 6.67 0.80
100 6.645 0.775
x x′ ′− −= =
0.51,498 5.169 0.775 5.316 6.645x x′ ′= = − +
0.51,645 5.87 x′=
∴ 0.51645
0.088%3.87
x′ = =
The steel has 0.088% C.
Example 2.3
Heat treatments as mentioned below are given to thin steel strips for which TTT
diagram is as shown in Figure 2.19. What will be the resulting structure of steel in
each case.
63
Engineering Alloys
(Ferrous and Non-Ferrous) Treatments
(a) Water quench to room temperature.
(b) Hot quench in molten salt to 690oC hold for 2 hours and water quench.
(c) Hot quench to 610oC and hold 3 minutes, water quench.
(d) Hot quench to 580oC, hold for 25 minutes, water quench.
(e) Hot quench to 450oC, hold for 1 hour, water quench.
(f) Hot quench to 300oC, hole for 30 minutes, water quench.
(g) Hot quench to 300oC, hold for 5 hours, water quench.
Figure 2.19 : Cooling Scheme of Example 2.3
Solution
From Figure 2.19 the final structures of steel can be determined
(a) Martensite
(b) Coarse pearlite
(c) Fine pearlite
(d) 50% fine pearlite and 50% martensite
(e) Bainite
(f) 50% fine bainite and 50% martensite
(g) Fine bainite.
SAQ 3
(a) Describe cooling curve for pure iron. Will this curve change in presence of
impurity.
(b) Explain eutectoid and peritectic transformation by the help of Fe-C phase
diagram.
(c) Describe the following phases in iron-carbon phase diagram. Pearlite, ferrite,
cementite, austentite and ledeburite.
(d) What is an S-curve? What are its other names?
(e) What are allotropic forms of iron? Correlates these forms with temperatures
of iron cooling form molten state.
(f) What is martensite and how is it formed? Explain using unit cell structure.
1 10 102 10
3 10
4
200
300
400
500
600
700
Time (sec)
(a)
(d)
(f)
(g)
Ms
300 0 C
4500C
580
680 0 C
690 0 C
(e) Bainite
Pearlite
(c)
TTT Curves
Tem
pera
ture
( 0
C )
64
Engineering Materials 2.6 HEAT TREATMENT OF STEEL
A range of properties may be produced in steels because the structure of various phases of
microstructure depend upon the rate of cooling. Some aspects in this connection have
already been explained. In this section specific treatments will be outlined. All heat
treatment processes consist of three main steps.
(a) The heating of metal to predetermined heat treating temperature.
(b) The soaking of the metal at that temperature until the structure becomes
uniform throughout the section.
(c) The cooling of the metal at some pre-determined rate such as well cause the
formation of desirable structure.
The heat treatments are normally applied to hypo-eutectoid carbon steels. These are :
annealing, normalising and hardening. The temperature to which heating is done in all
three cases is about 50oC above A3 temperature as indicated in Figure 2.20.
Figure 2.20 : Heat Treatment Range for Carbon Steels
The heat treatment of other steels will be discussed in specific section where these steels
are described.
2.6.1 Annealing
It is a heat treatment basically to soften the steels. The heating and cooling both at
controlled rate are performed in a furnace. Hypoeutectoid steels are heated above the
upper critical temperature (A3 line) while hypereutectoid steels are heated only above the
lower critical temperature (A1 line). The cooling is so done that γ-to-α transformation goes
to completion to each temperature. The resulting structure consists of large grains of
ferrite with coarse pearlite in which thick plates of ferrite and carbide are present. This is
softest possible structure and is ideal point for starting mechanical working of steel. Low
yield strength and tensile strength are associated with this treatment.
Different purpose which are achieved through annealing are listed below :
(a) the relief of all internal stresses within the metal.
(b) the production of uniform grain structure throughout the metal.
(c) the softening of the metal.
The annealing processes are classified either as full annealing or process annealing. Full
annealing is essentially the process described above. However, cooling may be done in
furnace, in ashes of sand or in specially built cooling pits lines with refractory and covered
with refractory lid. If heated to too high a temperature or soaked for too long a time the
austenitic phase undergoes grain growth resulting into coarse peralite grains. Such a
structure is termed “overheated” and exhibits low mechanical strength.
0.2 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6
600
700
800
900
1000
A1
Stress Relief Spheroidizing
738 0 C
A3
Normalizing
Hardening
Annealing
Full Annealing
Acm
Composition, % C
Tem
pera
ture
(0
C)
65
Engineering Alloys
(Ferrous and Non-Ferrous) One main problem to overcome during annealing process is the decarburisation and
oxidation on the surface. Packing the steel into special boxes are used a neutral
atmosphere in the furnace may overcome this problem. For example, low carbon steel
parts could be packed into boxes filled with sand, line, ground mica or cast iron swarf
while higher carbon components are usually packed into charcoal and other carbonaceous
materials.
Full annealing is not usually desirables as it results into considerable loss of mechanical
strength. Further it is too slow and costly process.
Process annealing, also known as commercial annealing or referred as stress relieving is
performed by heating steel to a predetermined temperature which is below the A1
temperature. The metal is air-cooled or quenched in a suitable pickling bath. Mild steels
(or hypoeutectoid steel containing less than 0.3% C) after having undergone the
mechanical treatment are softened by this process by heating to a temperature between
550oC and 650
oC. The distorted grains of ferrites in steel are fully recrystallised by the
process. The pearlite grains are not affected by process annealing so that the structure
consists of stress free ferrite matrix with distorted pearlite.
2.6.2 Normalising
Normalising is used as a finishing treatment for carbon steels giving higher strength than
annealing. There is no serious loss of ductility also. In this process the heating and soaking
is same as in full annealing but part is allowed to cool in air so that cooling are much
faster. The finer grains are produced because there is lesser time available for them to
grow. The finer grain structure increases the yield and ultimate strengths, hardness and
impact strength. The ductility is, however, slightly reduced.
The effect of small grain size is more noticeable in impact strength than in tensile strength
at low carbon content, say 0.2%. The tensile strength improves only by 5% while impact
strength may improve by as much as 20% by normalising than by annealing. At higher
carbon content, say 0.4% C it is only the impact strength but also the tensile strength that
improves marked by about 15%.
Normalising often applied to castings and forgings is stress relieving process. It increases
strength of medium carbon steel to some extent. When applied to low carbon steel it
improves machinability.
Alloy steels in which the austenite is very stable can be normalised to produce hard
martensitic structure. Cooling in air produces high rate of cooling which can decompose
the austenitic structures in such steels and martensite is produced. This increases the
hardness to a great extent. Table 2.7 describes the hardness obtainable in various
normalised steels. Table 2.8 describes variation of annealing and normalising temperature
and resulting hardness for different carbon contents of plain carbon steels.
Table 2.7 : Hardness of Annealed and Normalised Carbon Steels
Hardness BHN
Structural Steel
Conditions
Commercial
Iron Low C Medium C High C
Total Steel
Annealed 80 – 100 125 160 185 220
Normalised 90 – 100 140 190 230 270
Table 2.8 : Variation of Annealing and Normalising Temperature and Resulting
Hardness for Different Carbon Contents
Sl. No. C % Annealing
Temperature oC
Normalising
Temperature oC
Hardness
after
Annealing
(BHN)
Hardness
after
Normalising
(BHN)
1 0.18 – 0.22 860 – 900 900 – 925 110 – 149 120 – 160
66
Engineering Materials 2 0.23 – 0.28 850 – 890 890 – 910 130 – 180 140 – 190
3 0.29 – 0.38 840 – 880 880 – 900 140 – 206 150 – 220
4 0.39 – 0.55 820 – 870 840 – 870 150 – 217 180 – 230
5 0.56 – 0.80 790 – 840 810 – 840 160 – 230 210 – 270
6 0.81 – 0.99 790 – 830 810 – 840 170 – 230 260 – 300
2.6.3 Hardening
If a steel part is heated 30-50oC above A3 temperature complete austenising is permitted
by soaking at that temperature and then cooled suddenly (quenched), the breakdown of
austenite is suppressed. The new phase that forms is martensite in which all the dissolved
carbon is held in form of body centered tetragonal structure shown in
Figure 2.21. Martensite is only metastable phase and may be tempering. It is extremely
hard and brittle and has a characteristic acicular appearance when examined under
microscope under high magnification. In the steels upto eutectoid composition, the
martensite formed by this drastic quenching operation contains all the carbon that was
contained in austenite. In higher eutectoid steels, however, some carbon is converted into
carbide particles also.
Figure 2.21 : The Unit Cell of Distorted Body-centered Tetragonal Lattice of Martensite
The hardness of martensite is dependent upon the percentage of carbon present in
structure. Figure 2.22 illustrates how this hardness varies with carbon content. It can be
seen that hardness of plain carbon steels increases rapidly until the eutectoid composition
is reached. After this composition the hardness increases very normally. The fact is that
the hardest martensite is formed at eutectoid composition while hardness remains at this
level in hypereutectoid steels. The slight increase in hardness of hypereutectoid steel is due
to formation of carbide particles which are hard and brittle.
Figure 2.22 : Variation of Hardness of Martensite with Carbon Content of Steel
a
a
c
C - Atom
Fe Atom
800
600
400
200
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
% Carbon
Hardness as Annealed
Hardness as Quenched
% Hardness Increase After Quenching
Brinell
Hard
ness
67
Engineering Alloys
(Ferrous and Non-Ferrous) 2.6.4 Cooling Rate and Quenching Media
Rate of cooling plays an important role in determining the final structure after quenching
in case of eutectoid steel. Figure 2.23 schematically illustrates four different structures
that are obtained cooling rates when austenised steel is cooled in four different media.
Figure 2.23 : Mircostructure Resulting from Different Cooling Rates
Applied to Austenitised Samples of Eutectoid Steel
Only the drastic water quench produces a fully martensite structure. When quenched in oil
the austensite transforms into very fine pearlite. Fine pearlite also results if the austenised
eutectoid steel is air-cooled. However, if allowed to cool in furnace coarse pearlite is
formed. These effects have already been considered under the headings of annealing and
normalising. However, it may be mentioned that very fine pearlite structure that is
obtained form oil quenching is named primary troostite. Table 2.9 describes the effects of
various cooling media on mechanical properties of eutectoid steels.
Table 2.9 : Mechanical Properties of Eutectoid Steels
after Cooling in Different Quenching Media
Cooling
Media
Structure UTS
(N/mm2)
Y. S.
(N/mm2)
Hardness
(Rc)
Elongation %
(50 mm g. L)
Water Martensite 1700 − 65 Low
Oil Troostite 1100 550 35 5
Air Fine pearlite 850 270 25 8
Furnace Coarse pearlite 520 140 15 12
Water is the cheapest quench media for plain carbon steels. However, while using water
care must be exercised that water is properly agitated during this treatment otherwise air
bubbles may be trapped on the surface and insulate the spots from which heat flow will be
delayed. Such spots will develop softness. Salt water or brine is more severe quenching
media as it removes heat faster. However, in this case the steel must be thoroughly cleaned
after quenching otherwise the surface begins to rust. For very low carbon steels hydroxide
solutions are often used instead of brine. If slower cooling rates are desired, the steel may
be quenched in oil with high flash points. Various grades of quenching oil are available.
High carbon steels are invariably quenched in oil since water quenching will develop
cracks in such steels.
2.7 HARDENABILITY OF STEEL
When a piece of metal is quenched its loss of heat is determined by several factors but ore
face can be easily understood that the loss will not be uniform from total volume. That
part of the metal which is in direct contact with quenching media will lost heat faster than
Heat
Ferrite and
Water Quench
Oil Quench
Cementite
Air Cool
Furnace Cool
0.83%
Austenite and Cementite
Ferr
ite
Austenite
Eutectoid Martensite (Black)
Cementite (White)
Very Fine
Pearlite
Fine Pearlite
Coarse Pearlite
68
Engineering Materials the inner side. This will bring thickness or diameter of the part into focus to play an
important role in hardening. Figure 2.24 shows the cooling curves for surface, for material
just below the surface and the core. The cooling rate inside the material is governed by
thermal conductivity of the steel. It is also the function of the thermal gradient existing
within the piece to be hardened. There is always a possibility that at the inner core the rate
of cooling is less than the critical rate resulting into unhardened material there.
Figure 2.24 : Depth of Hardening in Steel
The depth of hardened layer is the measure of hardenability. A good hardenability will
mean even thicker sections are uniformly hardened. On the other hand a poor hardenability
will produce a soft core inside the piece of steel. The hardenability is dependent upon
following factors :
(a) The steel composition plays an important role. The steels with high carbon
content harden to a greater depth than steels with lower carbon for the former
have lower critical cooling rate. Alloying element such as chromium improve
the hardenability.
(b) The quenching medium.
(c) The section dimensions.
The standards describes the steel hardenability by stating the ruling section which is
maximum thickness which can be used and still achieve the stated properties throughout
the section. Size of thickness also plays an important role in terms of distortion that may
occur in the material. Variations in the rate of cooling within the component can lead to
different amount of contraction at different points across the section. This differential
contraction will result into the distortion of the component.
2.7.1 Measurement of Hardenability
Two testing methods have been developed to measure the hardenability of steels. The first,
known as cylinder series test, gives a single value of hardenability. The value is stated in
terms of percentage of martensite at the centre when quenched in a certain manner. The
second, known as Jominy and quench test results into a curve.
2.7.2 Cylinder Series Test
A series of round bars of different diameters are austenised and quenched in oil or water.
The bars are long enough so that the cooling of section at the middle of the length is not
affected by the ends. After hardening each bar is cut into half and hardness measured at
various points along a diameter. The graph between hardness and distance from the centre
is then prepared (Figure 2.25). From this graph the diameter at which 50% of the structure
is martensite is determined. When this graph the diameter is plotted against bar diameter, it
becomes possible to determine the bar diameter in which 50% martensite would form at
the centre. This is called critical diameter for that quenching medium. Since the rate of
cooling is less for an oil quench than for water quench, the critical diameter, of any steel
Tem
pera
ture
Cooling Curves
Tem
pera
ture
Tem
pera
ture
Hardened Region
69
Engineering Alloys
(Ferrous and Non-Ferrous) will be less for oil quenching than for two water quenching. Severity of quench is an index
that quantitatively defines the quenching condition. This index denoted by H is defined as
following ratio.
Heat transfer coefficient between steel and fluid
Thermal conductivity of steelH =
Figure 2.25 : Variation of Hardness with Depth in Water-quenched Cylindrical Bars of
(a) Plain Carbon Steel, (b) 1% Cr-V Alloy Steel
Naturally, when H → ∞ it represents the severest condition of quench, meaning that
surface of steel immediately reaches the temperature of the quenching medium. The critical
diameter for such an ideal and unrealisable condition is called ideal critical diameter. For
infinite H-value the critical diameter and ideal critical diameter will be same. For other H
values the critical diameter will be smaller. Figure 2.26 shows the relationship between
critical and ideal critical diameters for various H-values. Table 2.10 describes relative
values of H that can be obtained in various quench media under different condition with
value of one for still water as base.
Figure 2.26 : Relation between Critical Diameter, Ideal Critical Diameter and Severity of Quench
Table 2.10 : Relative Quench Severities
Severity of Quench Agitation of
Quenching
Medium
Movement of
Pieces Air Oil Water Brine
None None 0.02 0.3 1.0 2.2
None Moderate − 0.4 – 0.6 1.5 – 3.0 −
None Violent − 0.6 – 0.8 3.0 – 6.0 7.5
Violentor spray − − 1.0 – 1.7 6.0 – 12.0 −
2.7.3 Jominy Test
More convenient laboratory test for hardenability is Jominy test. A standard specimen of
steel (Figure 2.27) is austenised in normal manner. The lower end of the specimen is then
quenched by a standard jet of water, resulting into a varying rate of cooling.
50 25 0 25 50 200
300
400
500
600
700
25 mm
D.P
.N.
75 mm
125 mm
50 25 0 25 50 200
300
400
500
600
700 25 mm
75 mm
D.P
.N.
125 mm
0 25 50 70 100 125 150 175 200
0
25
50
75
100
125
150 3 2 1
0.3
0.1
0.02
Quench Severity ∞
Ideal Critical Diameter (mm)
Critical D
iam
ete
r (m
m)
Thin End Cools Slowly
70
Engineering Materials
Figure 2.27 : The Jominy Test of Hardenability
The rate of cooling at the jet end is about 300oC/sec while that at the other end is about
3oC/sec. This varying cooling rate produces a wised range of hardness along the length of
Jominy specimen. A flat portion is ground along the length and hardness measured at
various points. The plot of hardness along the length gives Jominy index of hardenability.
The best use of the Jominy curve (Figure 2.28) is made by drawing a horizontal line
corresponding to hardness of the semi-martensite zone. The hardness of semi-martensite
zone is described in Table 2.11. The point where this line needs the Jominy curve
determines the distance from the quenched end which can be inserted in Figure 2.29 to
determine the diameter for a particular steel which will be fully hardened in water or oil.
Figure 2.28 : Hardenability Curves Plotted from End Quench Test Data
(a) For Shallow; and (b) For Deep Hardening Steel
0 3 6 9 12 15 18
10
20
30
40
50
60
336 105 55 42 28 16.5 13.5 10
1.5 4.5 7.5 10.5 13.5 16.5 19.5
Distance From Quenched End (mm)
Hard
ness,
RC
a
b
0 6 12 18 24 30 36
20
40
60
80
100
Dia
mete
r of
Work
pie
ce (
mm
)
42
120
140
Water
Oil
71
Engineering Alloys
(Ferrous and Non-Ferrous)
Figure 2.29 : Determining the Diameter of Fully Hardened Articles
according to the Distance from the Quenched End
Table 2.11 : Relationship between the Hardness of the
Semi-Martensite Zone and the Carbon Content
Hardness of the Semi-martensite (Rc) Carbon-content %
Carbon Steel Alloy Steel
0.08 – 0.17 − 25
0.13 – 0.22 25 30
0.23 – 0.27 30 35
0.28 – 0.32 35 40
0.33 – 0.42 40 45
0.43 – 0.52 44 50
0.53 – 0.62 50 55
2.8 TEMPERING
The hardening treatment given to steel increases the hardness but introduces internal
stresses because of different cooling rates. The internal stresses are also created because of
transformation from austenite to martensite. Tempering treatment aims at reducing these
stresses. The treatment consists in heating the hardened component to between 200oC and
600oC and holding it at that temperature for a predetermined period of time and then
cooling slowly to room temperature. Since martensite itself is metastable phase, structural
changes induced by tempering proceed fairly rapidly. All structures resulting from
tempering are termed martensite. The changes occurring during various temperature
ranges are described below :
100o – 220
oC
Very little change occurs in the micro-structure. However, this heating helps remove
considerable amount of internal stresses. The stress relieving treatment is given
when maximum hardness is desirable and brittleness is not a problem. The strain is
relieved because of removal of carbon atoms from their trapped positions.
240o – 400oC
In this range martensite decomposes rapidly into emulsified form of pearlite known
as secondary troostite. This material is very fine in nature and hence provides good
shock resistance. The fine edge tools are tempered in this range but more precisely
within 270oC-300
oC.
400o – 550
oC
The precipitate troostite begins to coalesce forming a coarser from of globular
pearlite known as sorbite. It may be recalled both troostite and sorbite are now
preferably called tempered martensite. This treatment is desirable in such
components as beams, springs and axles.
600o – 700oC
72
Engineering Materials Heating hardened steel in this range causes spheroidisation, the structure being
known as spheroidite. This structure is formed because of further coalescence of the
carbide within the alloy. Spheroidised steels show fairly good machinability since
the hard carbide particles are embedded in the soft ferrite matrix and consequently
do not have to be cut by the cutting tool. If the spheroidised steel is heated to just
above its lower critical temperature the pearlite present will alter to austenite and
cooling to room temperature will yield a structure of lamellar pearlite plus pro-
eutectoid ferrite or cementite depending upon carbon content.
Judging the temperature of tempering by colour appearance is a tradition which is helpful
on shop floors. However, for accuracy the exact temperature measurement are to be
preferred. Table 2.12 describes the colour appearance and temperature in connection with
several tools.
Table 2.12 : Tempering Temperature and Colours of Tools
Tool Temperature oC Colour
Planning tools 230 Paste straw
Milling currents 240 Dark straw
Taps and dies 250 Brown
Punches, drill bits 260 Purplish-brown
Press tools 270 Purple
Cold chisels 280 Dark Purple
Wood saws, springs 300 Blue
Changes in Mechanical Properties with Tempering
Tempering improves the ductility and toughness of quenched steel while decreases
hardness. Figure 2.30 illustrates how these properties are influenced by tempering.
The tempering temperature is so chosen that it results in the desired combination of
the properties. Some steel show drop in impact values in the certain tempering
temperatures range. Third drop is an indication of brittleness and such range should
be avoided. For mild steel this brittleness (termed blue brittleness) occurs at a
tempering temperature of about 300oC.
Figure 2.30 : Tempering Diagram of Property Chart for Water Quenched 26 mm Diameter
Bars of Eu-12 Steel (Scale for Stress Only)
2.9 SPECIAL TREATMENTS
In cases of large sections where the water quenching will most likely produces cracks
special treatments are used for hardening. The cracks on the surface are produced because
0 100 200 300 400 500 600 700
250
400
550
700
850
1000
1150
1300
Tempering Temperature (0 C)
Izod,
N –
m,
Brinell
Hard
ness
Str
ess,
N/m
m2,
Ductilit
y,
%
Elong BHN
Y.P. 0.1 % P.S.
U.T.S
Reduction Of Area
Izod
73
Engineering Alloys
(Ferrous and Non-Ferrous) the skin cools faster and changes into martensite while the inner core cools slowly and
transforms later accompanied by dilation. This dilation causes outer skin to crack. To
avoid this type of cracking special treatments have been developed. Before they are
described it will be worth-while to revise Section 2.8 wherein the isothermal
transformation was described.
2.9.1 Isothermal Transformation
Austenite is not usually converted into martensite instantaneously but the process
continues for sometime. Different steels take different time for full transformation and the
time depends upon the temperatures from where cooling is begun. How to obtain
isothermal transformation diagram was discussed in detail in Section 2.8. The selected
specimen is austenised and then quenched in liquid bath held at temperature to be
investigated. The specimen is held for a different length of time in the bath and then
quenched in the water. The resulting structure can be studied under microscope or any
other associated property like hardness may be studied. It is seen that definite times are
required for the initiation and completion of the transformation and these times vary with
the temperature. The progress of transformation, say for 10%, 50% or 90% may also be
found.
Figure 2.31 which is Figure 2.30, reproduced with more details, illustrates complete
transformation diagram of eutectoid steel. As the transformation temperature is lowered
from A1 temperature to about 550oC, the nucleation and completion time decreases and
pearlte and lamellae become finer. From about 550oC to 250
oC, the nucleation and
completion time increase. The transformation product here is termed bainie which is
composed of two equilibrium phases that are ferrite and cementite. The time of minimum
nucleation is identified as the nose or knee. Below about 250oC the transformation
product is martensite. This forms almost instantaneously, but the amount formed depends
upon the temperature. The upper and lower limits of martensite transformation
temperatures are termed MS and MF temperatures. These diagrams are also known as
Time-Temperature-Transformation or TTT diagrams and were discussed earlier. They
are also referred to as S-curve because of their shape.
Figure 2.31 : Isothermal Transformation Diagram for a Eutectoid Steel.
Structures Present after 105 Seconds are given on the Right-Hand Side
1 101
102
103
104
105
0
100
200
300
400
500
600
700
Austenite
Austenite
Pearlite Course Pearlite
Fine Pearlite
Upper Bainite
Martensite +
Lower Bainite
Bainite
Martensite + Austenite Martensite
100 %
50%
0
Mf
Insta
nta
neous
Tra
nsfo
rmation t
o
Mart
ensite
MS
Time (sec)
Tra
nsfo
rmation T
em
pera
ture
( 0
C)
Start of Transformation
Finish of Transformation
50 % Transformation
+
74
Engineering Materials 2.9.2 Austempering
The component to be hardened is first austenised and then quenched into a lead or salt bath
held at just above the martensite transformation temperature. The component is held in the
bath until the bainite transformation is completed. It is then removed from the bath and
cooled in air to room temperature. The bainite so produced is somewhat softer than
martensite of same carbon content and distortion is minimum. Also the austempered steel
has improved shock resistance and low notch sensitivity. The process of austempering is
depicted in Figure 2.32. Austempering is often limited to section thickness of 20 mm.
Austempering is applicable to a few plain carbon steel and requires facility of molten salt
bath. This may be regarded a disadvantages over quenching and tempering.
Figure 2.32 : Austempering Shown on the TTT Curve
2.9.3 Martempering
The piece to be hardened is fully austenised and then quenched into a lead of salt bath held
at a temperature just above the at which martensite would begin to form. It is kept at this
temperature until its temperature becomes uniform throughout (i.e. outside and inside
temperatures do not remain different) and is then water quenched to form complete
martensite structure and bainite formation is prevented. This process successfully
separates the cooling contraction from the austenite-martensite expansions and thus
prevents quench cracking in large articles. The process of martempering is shown in
Figure 2.33.
Figure 2.33 : Martempering Shown on the TTT Curve
The steel can be tempered to low temperatures to further refine the structure.
Table 2.13 describes a few properties obtained from quenchtemper, austemper and
martemper treatments.
Table 2.13 : Some Mechanical Properties of 0.95 C, 0.40 Mn Steel at 20oC
after Different Treatment
Heat Treatment Hardness
(Rc)
Impact
(J)
Elongation %
(on 25 min gl)
Water quench, Temper 53.0 8.22 0
Water quench, Temper 52.5 9.60 0
Martemper, Temper 53.0 19.20 0
Martemper, Temper 52.8 16.44 0
Austemper 52.0 30.83 11
Time (Log Scale)
Tem
pera
ture
M
MS
Surf
ace
Centre
Time (Log Scale)
Tem
pera
ture
Surf
ace Centre
75
Engineering Alloys
(Ferrous and Non-Ferrous) Austemper 52.5 27.40 8
2.10 SURFACE HARDENING
In many situations surface hardening instead of through hardening only is sufficient to
serve the purpose. Gears are examples. Surface hardening is achieved through case
carburising, nitriding or induction heating. Steels containing 0.1 to 0.25% C are best
suited for case carburising. Good combination of tough core (lesser hardness) and high
surface hardness is achieved by case carburising of nickel steel. Case hardness of 60 RC
with a core hardness of 33 to 38 RC gives best results in case of gears. The case hardness
is due to residual compressive stress introduced on the surface by penetration of C
and N2.
Surface hardening is classified into two types :
(a) Without addition of any element from outside but only transforming outer
layer to martensite. This could be achieved by heating the surface by gas
flame or causing magnetic induction so that complete austenite
transformation occurs on surface. On quenching martensite and retained
austenite form on surface while on the inner side peralite-ferrite is the main
phase.
(b) The second method is called case hardening in which C and/or N2 are
introduced in the surface layer. In carburising the part is surrounded by
material or atmosphere rich in C and on heating this C is released and
absorbed in steel. Recently case carburising is more effectively performed by
heating steel part in the atmosphere or natural gas, coke oven gas, butane or
propane or the valatised form of liquid hydrocarbons like terpenes and
benzene. Volatilised form of alcohol and glycols or ketones are also used. In
these cases the thickness of hardened layer is proportional to root of the time
of treatment in hour.
Liquid carburising consists in dipping the part in fused mixture of chlorides, carbonates
and cyanides. Baths maintained at 840oC to 900
oC produce a case depth of 0.075 to
0.75 mm. 0.5 to 3.0 mm case depth is attainable if bath is maintained at 900 to 950oC.
Plain carbon steel and low alloy steel can be carburised in liquid bath.
Nitriding of steel surface is the absorption of N2 in the surface. Nascent N2 for this
purpose is obtained from ammonia. Molten cyanide (sodium cyanide) bath maintained at
560oC is quite effective in nitriding particularly if thin case is desired. Plain C steel are not
good for nitriding because iron nitride so formed is very brittle. Steels alloyed with Al and
Cr and Ni, Cu, Si and Mn are better nitrided than plain C steel.
Carbonitiriding, nitrocarburising or gas cyaninding is a process similar to gas carburising
in which ammonia is also added to carburising atmosphere. This process produces better
hardened case than carburising.
2.11 HEAT TREATING EQUIPMENT
The main equipment for heat treatment is furnace. There are two major types of furnaces –
batch and continuous. The furnace selection requires consideration of several points
particularly for the reason that they consume a good amount of energy. The efficient use of
energy in furnaces and hence their design and use of proper insulation need consideration.
The man power requirement for operation, the initial and cost and convenience of
maintenance and repair are other important considerations in selecting furnace for heat
treatment. Since temperature control and temperature cycle control are very important in
heat treatment, electronic and computer controlled furnace are taking precedence over
older type.
76
Engineering Materials Heating in furnace is done by burning oil or gas or by electrical resistance or inductance
heating. Gas or oil furnace have distinct disadvantage over electrical furnaces because the
former often introduces products of combustion in the heating space thus affecting the part
to be treated. The electrical heating, on the other hand, has slower start-up and is not easy
to control.
2.11.1 Batch Furnaces
An insulated, chamber for placing the job, the heating system and a door or several doors
for placing the job in place are the requirement of these furnaces. The parts to be heat
treated are loaded and unloaded in individual batches. A furnace which is easy to use,
simple to construct in several sizes and having versatility to accommodate several size is a
box furnace which could be horizontal rectangular. Many times a flat platform on wheels
is used to carry the parts in the furnace.
A pit furnace is made in form of a vertical pit below the ground level. The parts to be
treated are lowered in the pit. Long parts like rods, bars, tuning, shafts, etc. can be
suspended in the space of the pit furnace. These parts are susceptible to distortion if placed
in horizontal position in the box furnace.
A bell furnace does not have bottom and is lowered on the stack of parts to be treated.
The furnace chamber could be round or rectangular.
In elevator furnace the parts to be treated are placed on the rolling platform which is
rolled in the proper position and lifted to the heating chamber of the furnace. By placing a
quenching tank directly below the surface the savings on space and quenching time are
made.
2.11.2 Continuous Furnace
Parts to be treated are placed on some sort of conveyor which move into the furnaces
according to programmed heating cooling cycle. The time for loading-unloading is greatly
saved and handling of job is reduced. For high production rate and better control of heat
treating cycle these furnaces are most suitable.
2.11.3 Salt Bath Furnaces
Salts baths used for heating ensure good control uniform temperature and high heating
rates are compared to air or gas. The molten salts or metals have higher conductivity than
air or gas. The salt may be heated from outside if it is non-conducting or by passing a low
voltage alternating current between electrodes placed in the salt. Direct current is not used
because it is likely to cause electrolysis of salt. Among metals, lead is commonly used.
Wide range of temperature may be obtained from such baths.
2.11.4 Induction Heating
Alternating current through induction coil surrounding the part to be treated induces eddy
current through the part which is thus heated. The advantage of such heating is that no gas
or liquid source for heating is used and coil can be shaped to surround the part
geometrically. Figure 2.34 shows example of induction coils, which normally are made in
copper or copper based alloy which are water cooled. The coil is also designed for
quenching the part after heating. For surface hardening requiring local heating induction
heating is suitable.
Slideway to be Surface Hardened
Travel
Shaped Coils
Induction Coils
Parts to be
Cooling Water
77
Engineering Alloys
(Ferrous and Non-Ferrous)
Figure 2.34 : Coils for Induction Heating
Furnace Atmospheres
If the heating is not through salt bath, then the job to be treated is subject to varying
atmosphere which could be atmospheric air or any one of several gases. Surface
oxidation, tarnishing and decarburisation are the problems which the metals face.
Oxygen can cause oxidation, rusting and scaling. Carbon dioxide may cause
decarburisation depending upon its concentration in furnace atmosphere. Thin blue
film, is formed on the surface in the presence of water vapour. Bluing of surface is
done for improving surface appearance. Nitrogen provides a neutral atmosphere.
Vacuum furnaces often used for small and accurately finished parts provide
complete safety from effects of atmosphere.
SAQ 4
(a) What are different heat treatment given to steel?
(b) Differentiate between annealing and process annealing.
(c) What is quenching? Why should quenched steel be tempered?
(d) Do you think the term martempering is misnomer? Suggest a better term.
(e) Differentiate between austempering and martempering.
(f) Describe different methods of surface hardening. Give examples of surface
hardened parts.
(g) Describe how heating is done for heat treatment.
2.12 ALLOY STEELS
Carbon steels in their commercial forms always contain certain amounts of other elements.
Many of these elements enter the steel from the ores and it is difficult to remove them
during the process of steel making. All commercial steels contain varying amounts of Mn,
Si, S and P and frequently varying amounts of such elements in Cr, Ni, Mo and V. If
alloying elements other than carbon are present only in small amounts (e.g. Mn upto 0.8%,
Si upto 0.3%, etc.) then the steel is usually called low alloy steel or plain carbon steel.
Sulphur and phosphorus when more than 0.05% of either is present, tend to make steel
brittle, so that during steel making these elements are reduced to at least this value. Si has
little effect on strength and ductility if less than 0.2% is present. As the content is rasied to
0.4% the strength is raised without effecting ductility, but above 0.4% Si, the ductility is
impaired. Si is added as deoxidiser and that part which does not make silicon dioxide
remains in steel as impurity.
Mn is another alloying element which is present in most steels. If it exists in solid solutin
in the ferrite it has a strengthening effect. It may also exist in forms of Mn3C which forms
part of the pearlite of MnS. Upto 1% of Mn has strengthening effects on steel and its
78
Engineering Materials presence in excess of 1.5% induces brittleness in steel. Excess Mn is added to melt during
steel making to bring its level to desired value. It also acts as a deoxidiser.
Intentional addition of many other elements modifies the structure of steel and hence
improves its properties. Steels to which such intentional additions have been made
(including those steel which contain Mn in excess of 1% or Si in excess of 0.3%) are
known as alloy steels. One particular effect of alloying is that it enables martensite to be
produced with low rates of cooling and permits larger sections to be hardened than is
possible with plain carbon steel.
The important elements that are used to alloy with steel in varying quantities are Ni, Cr,
Mo, W, Mn and Si. The bcc metals like Cr, W and Mo when alloyed with steel tend to
form carbides which reduce the proportion of Fe3C in the structure. On the other hand the
fcc elements like Ni, Al, Cu and Zr do not form carbides. Mn which has three allotropic
complex structures also forms carbide.
Several advantages in terms of improved mechanical properties and corrosion resistance
are obtained by adding one or several alloying elements.
The various advantages of alloy steel are :
(a) Higher hardness, strength and toughness on surface and over bigger
cross-section.
(b) Better hardenability and retention of hardness at higher temperature (good for
creep and cutting tools).
(c) Higher resistance against corrosion and oxidation.
The alloying elements affect the properties of plain C steel in four ways :
(a) By strengthening ferrite while forming a solid solution. The strengthening
effects of various alloying elements are in this order : Cr, W, V, Mo, Ni, Mn
and Si.
(b) By forming carbides which are harder and stronger. Carbides of Cr and V are
hardest and strongest against wear particularly during tempering. High alloy
tool steel use this effect.
(c) Ni and Mn lower the austenite formation temperature while other alloying
elements raise this temperature. Most elements shift eutectoid composition to
lower C percentage.
(d) Most elements shift the isothermal transformation curve (TTT) to lower
temperature, thus lowering the critical cooling rate. Mn, Ni, Cr and Mo are
prominently effective in this respect.
2.12.1 Effect of Individual Alloying Elements
Sulfur
Sulfur is not a desirable element in steel because in interferes with hot rolling and
forging resulting in hot-shortness or hot embrittlement. Sulfur however, is helpful
in developing free cutting nature. Thus sulfur upto 0.33% is added in free cutting
steel. Otherwise, sulfur is restricted to 0.05% in open hearth or BOF steel and to
0.025% in electric furnace steel.
Phosphorous
It produces cold shortness which reduces impact strength at low temperature. So its
percentage is generally restricted to level of sulfur. It is helpful in free cutting steels
and is added upto 0.12%. It also improves resistance to corrosion.
Silicon
79
Engineering Alloys
(Ferrous and Non-Ferrous) Silicon is present in all steels but is added upto 5% in steels used as laminates in
transformers, motors and generators. For providing toughness it is an important
constituent in steel used for spring, chisels and punches. It has a good effect in steel
that it combines with free O2 and form SiO2 and increases strength and soundness of
steel casting (upto 0.5%).
Manganese
1.2 to 1.4% of Mn produces extremely tough, wear resistant and non-magnetic steel
called Hadfield steel. It is important ingredient of free cutting steel upto 1.6%. Mn
combines with S, forming MnS. For this purpose Mn must be 3 to
8 times the S. Mn is effective in increasing hardness and hardenability.
Nickel
It is good in increasing hardness, strength and toughness while maintaining ductility.
0.5% of Ni is good for parts subjected to impact loads at room and very low
temperatures. Higher amounts of Ni help improve the corrosion resistance in
presence of Cr as in stainless steel. Nickel in steel results in good mechanical
properties after annealing and normalising and hence large forgings, castings and
structural parts are made in Ni-steel.
Chromium
Chromium is common alloying element in tool steels, stainless steel, corrosion
resistant steel (4% Cr). It forms carbide and generally improves hardness, wear and
oxidation resistant at elevated temperature. It improves hardeanbility of thicker
sections.
Molybdenum
Molybdenum is commonly present in high speed tool steel, carburising steel and
heat resisting steel. It forms carbide having high wear resistance and retaining
strength at high temperatures. Mo generally increases hardeability and helps
improve the effects of other alloying elements like Mn, Ni and Cr.
Tungsten
It is important ingredient of tool steel and heat resisting steel and generally has same
effects as Mo but 2 to 3% W has same effect as 1% Mo.
Vanadium
Like Mo, V has inhibiting influence on grain growth at high temperature. V carbide
possesses highest hardness and water resistance. It improves fatigue resistance. It is
important constituent of tool steel and may be added to carburising steel.
Hardeability is markedly increased due to V.
Titanium
Addition of Ti in stainless steel does not permit precipitation of Cr carbide since Ti
is stronger carbide former and fixes are carbon.
Cobalt
It imparts magnetic property to high C steel. In the presence of Cr, Co does not
permit scale formation at high temperature by increasing corrosion resistance.
Copper
Atmospheric corrosion resistance of steel is increased by addition of 0.1 to 0.6%
copper.
Aluminium
80
Engineering Materials Aluminium in percentage of 1 to 3 in nitriding steels is added to improve the
hardness by way of forming Al nitride. 0.01 to 0.06% Al added during solidification
produces fine grained steel castings.
Boron
Very small percentage (like 0.001 to 0.005) of B is effective in increasing hardness,
particularly in surface hardening boriding treatment.
Lead
Less than 0.35% Pb improves machinability.
The effects of alloying element in respect of various desired effects are summarized below
:
(a) Hardenability – Si, Mn, Ni, Cr, Mo, W, B
(b) Toughness – Si, Ni
(c) High temperature strength – Cr, Mo, W
(d) Corrosion resistance – Cr, Mo, W
(e) Wear resistance – Cr, Mo, W, V
(f) Low temperature impact strength – Ni
(g) Atmospheric corrosion resistance – Cu
(h) Machinability – S, P, Pb
(i) Fatigue strength – V
(j) Surface hardening – Al
2.12.2 Some Important Alloy Steels
Structural Steels
Low alloy steels are used for structural purposes. Such steels are required to
possess high yield stress, good ductility and high fatigue resistance. The high yield
stresses result in direct weight saving in part of the structure.
A typical low alloy structural steel will have following composition :
C – 0.12%, Mn – 0.75%m Si – 0.25%, Cu – 0.3%
This alloy steel has a yield strength of 350 N/mm2 and about 15% elongation after
hot rolling. The presence of Cu improves corrosion resistance while Mn and Si
improve weldability by preventing weld embrittlement.
Small amounts of Ni, Cr and V added to these steels may improve the yield strength
to 625 N/mm2 if used in quenched and tempered conditions. Additions of Ni, Cr and
V do not effect the weldability.
Stainless Steels
Stainless steel are particularly known for their resistance to corrosion. This
resistance is obtained because of formation of protective oxide layer which spreads
all over the surface. This layer does not allow the surrounding atmosphere to further
react with the steel which retains its luster and appearance. The oxide layer on the
stainless steel surface is formed by the oxide of Cr when it is present in large
proportions. This oxide film is impervious to both metal ions and atmospheric
oxygen. Improved corrosion resistance is obtained with increasing percentage of Cr,
provided that Cr is in solid solution and not combined as carbide. The corrosion
resistance is further enhanced by addition of certain amounts of nickel. According to
structures obtainable at room temperatures the stainless steels are subdivided into
three groups.
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Engineering Alloys
(Ferrous and Non-Ferrous) Ferrite stainless steels contain only chromium as alloying element in addition to
small percentage of carbon. The carbon varies between 0.05% to 0.15%, while Cr
varies between 13% to 30%. This alloy contains only α-phase at all temperatures.
Some Cr precipitates in form of carbides along with ferritic grains at room
temperature. This alloy is very ductile and used where outstanding formability in
complicated shapes is required. Many deep drawn objects are produced from ferritic
stainless steel. This material possesses excellent resistance to corrosion.
When alloy steel contain at least 24% Cr and Ni together but not less than 8% of
either element, the γ-phase is retained on cooling at normal rates. At very low
cooling rates the α-phase may separate fully. Austenitc phase is obtained when
quenched from upper critical temperature. The commonest of these steels contain
18% Cr, 8% Ni and 0.1% C. It is called 18 : 8 steel. Austenitic steel is used for
construction of chemical plants, decorative purposes and household utensils.
Neither of above two groups is heat-treatable. If steel contains Cr and Ni in such
proportions that it has a γ-phase at high temperature and an α-phase on cooling at
normal rates, it can be quenched to give a martensitic structure. Such
heat-treatable steels are known as martensitic steels even when not in heat treated
conditions. For developing martensitic these steels are oil-quenched from above
upper-critical temperature. Three types of martensite steels are available
commercially. These are :
(a) 0.07% − 0.1%, C 13% Cr,
(b) 0.2% − 0.4% C, 13% Cr, and
(c) 0.1% C, 18% Cr, 2% Ni.
These steels are used for turbine blades, surgical instruments, springs, ball bearings,
pump shafts, aircraft fittings, etc.
While martensitic steel can be heat treated to obtain high strengths, the strength of
ferritic and austenitic steel can be improved only by mechanical working. Various
precipitation hardening stainless steels have also been developed.
At high temperatures somewhere between 500 and 700oC, the stainless steels lose
their resistance to corrosion. This happens mainly because the chromium has a
tendency to separate from solid solution and precipitate in form of carbides at grain
boundaries. This makes welding of the stainless steels difficult and causes what is
known as weld decay. If welded part is reheated to a temperature of about 900-
1000oC the carbides are re-dissolved and can be converted into stable solid solution
on quenching.
High Carbon Tool Steels
Tools are implements that are used to shape, deform or cut other materials. They
are largely made in steel, though other alloys have also been developed. The
common tool steels contain C, W, Cr, Mo, V, Mn, Si in the range of 0.6 to 1.0%.
They have hardness and wear resistance. For shock resistance C is restricted to
0.5%. W and Mo between 2 to 18% provide high temperature strength. V between
0.1 to 2% enhances hardenability while Si adds to toughness.
Though the tool and die steels are not produced in as large amount as other steels
are, yet they are industrially very important. A variety of steel differing widely in
composition and treatment is used for varying purposes. They are used in such
operations as cutting, shearing, forming and rolling. These operations require
adequate hardness, strength, toughness, wear resistance and heat resistance. For
many purposes near-eutecoid and hyper-eutectoid steels have been used for metal
cutting but these plain carbon steels have tendency to loose hardness through
tempering when rise in temperature occurs during cutting. To overcome this
problem high steep tool steel have been developed. The 18.4.1 type of high steel
82
Engineering Materials contains 18% W, 4% Cr and 1% V. These steels retain sufficient hardness due to
carbide formation which is a complex compound Fe4W2C. A tough matrix is
provided by Cr. These steel may retain hardness upto a temperature of 500oC.
When 5-12% of cobalt is also added, in addition, the hardness through a secondary
hardening process is increased at temperature around 600oC.
High Duty Tool and Die Steels
High duty punching tools or dies are made out of many different steels of fairly high
alloying contents. High-carbon high-chromium die steel contains 2% C,
0.3% Mn and 12% Cr. Though extremely hard and unmachinable in its hardened
form, small and large carbide particles get dispersed in ferritic matrix on annealing
from 800oC. In annealed state this steel can be machined with difficulty. This is an
important requirement because tools and dies have to be machined before they are
hardened for final use.
Magnetic Alloy Steels
These steels are divided into two groups. Those which retain their magnetism and
those which do not. The steels that retain their magnetism are termed hard
magnetic steels. The other group is magnetically soft.
1% plain carbon steel in its fully hardened condition was the earliest permanent
magnetic material. Later developments occurred when W, Cr and cobalt were added
as alloying elements. The most useful of permanent magnetic steels contain high
proportions of Ni, Co, and Al, with small amount of W. Alnico is a good example
which contains 10% Al, 18% Ni, 12% Co, 6% Cu, Rest Fe.
Magnetically soft materials are required to demagnetise quickly. In earlier days soft
iron was used as a soft magnetic material but later iron-silicon alloys containing
upto 4.5% Si were developed. However, modern high duty soft magnetic materials
are iron-nickel alloys such as Permalloy which contains
78% Ni. Another soft magnetic material is mumetal containing 75% Ni. These
alloys are used for transformer cores and as shield material for submarine cables.
Alloy steels find a wide range of application and a few of them are described in
Table 2.14.
Table 2.14 : Application of Alloy Steels
Sl. No. Application Desired Properties Composition (%)
1 Rail steel Strength, ductility, impact and
fatigue strength
C – 0.4 to 0.6 Mn and
Cr – upto 1
2 Spring steel (tension
compression, torsion)
Good elongation high elastic limit
(20 to 30%, 1200-1400 MPa).
Good surface finish for fatigue
strength
(a) C – 0.6, Mn – 0.9,
Si – 2.0
(b) C – 0.5, Mn – 0.8,
Cr – 1.0, V – 0.15
(c) C – 0.5, Ni – 0.9,
Cr – 0.5, Ni – 0.6,
Mo – 0.2
3 Structural steel (bridges,
building, cars, gears,
clutches, shafts)
High strength, toughness, high
temperature strength, corrosion
resistance
Wide range of alloy steels
containing several alloying
elements
4 Weldable steel for
welded structures
Weldability, high resistance to
atmospheric corrosion, resistance
to brittle fracture
C – 0.15 to 0.3 with some
Cu and V
5 Concrete reinforcing
steel
Bend 90o – 180o, tor-steel with
ribs for greater surface area.
Elongation = 16%,
UTS = 500-650 MPa,
Y. S. = 35 MPa.
Elongation = 13%,
C – 0.3 to 0.4,
Mn – 0.5 to 0.8
C – 0.45 to 0.6,
Mn – 0.7 to 1.1
83
Engineering Alloys
(Ferrous and Non-Ferrous) UTS = 600 MPa,
Y. S. = 350 MPa
6 High speed steel for
cutting tools
Resist temperature upto
550-600oC. Cutting tools
requiring high hardness at
working temperature 18 : 4 : 1
steel and
6 : 5 : 4 : 2 steel
C – 0.8, W – 18,
Cr – 4, V – 1,
C – 0.8, W – 6,
Mo – 5, Cr – 4,
V – 2
7 Creep resisting steel Application in pipelines upto
400-550oC. Other parts upto
550oC
Mo – 0.4 to 6
V – 0.25 to 1.0
Cr – upto 6.0, C – low
8 Hadfield Mn steel,
excavating and crushing
m/c. rail road crossing,
oil well, cement, mining
industries. Used as
casting and hot rolled
Resistance to abrasion and shock,
high toughness strength and
ductility
C – 1 to 1.4,
Si – 0.3 to 1.0,
Mn – 10 to 14
9 High strength low alloy
steel (HSLA) for
automotive parts
High strength/weight ratio.
Balanced properties such as
toughness, fatigue strength,
weldability and formability
C – 0.07 to 1.3, Ti, V, Al,
Co less than 0.5
10 Ball bearing steel Rolling element, inner and outer
races. High hardness
61-65 Rc high fatigue strength
C – 0.9 to 1.1,
Cr – 0.6 to 1.6,
Mn – 0.2 to 0.4
2.12.3 Heat Treatment of Stainless Steels
Stainless steels, like other steels, respond to heat treatments over wide range. They are
subjected to one or more of following heat treatments.
Stress Relief
Stress relieving treatment removes undesirable residual tensile stresses that are
induced in the material as consequence of mechanical working.
Stress relieving of stainless steel is carried out by heating it to 370oC and then
cooling at a low rate. This treatment also reduces susceptibility to corrosion.
However, resistance against corrosion cracking is improved by stress relieving
at 770oC.
Annealing
Since ferritic stainless steels are prone to form patches of transformation products
particularly after welding it is subjected to annealing treatment. The treatment is
carried out by heating ferritic steel to 770oC and then cooling it in furnace or air.
This treatment will stress relieve and homogenise the structure.
Solution Annealing
This treatment is given to austenitic stainless steel. When this steel is heated to
1000oC to 1120oC the austenite acts as a powerful solvent for chromium carbide
and makes a homogenous structure. This structure is retained by quenching heated
steel in water, oil or air depending upon the thickness of the section. Air is best
quenching media for thin sections. The process is also referred to as quench
annealing.
Hardening and Tempering
This is a treatment very much similar to one for plain carbon steel but only meant
for martensitic stainless steel. The steel is heated to a temperature of 950 to 1050oC
and quenched in air or oil resulting in formation of hard marteniste. The quenched
steel is tempered between 100 and 700oC depending upon the required hardness.
Stabilising Treatment
84
Engineering Materials This is special treatment for stabilised grade of auetenitic steel. The steel is heated
to 870 to 900oC and held for 2 to 4 hours. It is then quenched in air, oil or water
which causes precipitation of titanium or columbium carbide. These carbides do not
permit precipitation of chromium carbide during service life.
Post Weld Treatment
The welding induces undesirable residual stresses and reduces corrosion resistance.
Stress relieving, annealing or solution annealing are the treatments recommended for
weldments before putting them to use. Weld decay is a common welding defect in
stainless steel which is caused by precipitation of carbide of chromium in the weld
region and HAZ. The result of this precipitation is great susceptibility to
intercycstalline cracking when a corrosive media comes in contacts.
2.12.4 Heat Treatment of Tool Steels
Heat treatment in case of tool steels in an important step before actually using the tool.
Most properties are achieved after heat treatment. There are certain special but necessary
precautions to be observed in heat treating tool steels. Such precautions may be clear
understanding of austenising temperature which is affected very much by addition of
alloying elements.
Normalising and Annealing
Normalising becomes necessary in case of forged tools. Forging induces undesirable
residual stresses, coarse grains and non-uniform structure. These defects are
removed by normalising treatment. The normalised tools are machined to exact
dimensions. Such machining increases the hardness because of strain hardening.
The intermediate annealing from just below A1 temperature is applied upon the tool
to remove the effects of strain hardening.
The tools steels have lower thermal conductivity and if the tool is heated at a faster
rate distortion and cracking may occur. To avoid such distortion and cracking the
tool before any treatment is preheated slowly to temperature between 600 to 800oC.
The tool is held at the preheat temperature sufficiently long to allow whole body to
come to uniform temperature and then heat to required temperature for any
treatment.
Austenising
Following preheat the tool is austenised by heating to correct austenising
temperature to obtain fine grained austenite in which carbides are dissolved.
Quenching
The austenised tool is quenched in brine, water, oil, air or molten salt bath
depending upon the hardenability and thickness of cross-section. The correct rate of
cooling will decide the quenching media and cooling faster than the correct rate will
result in under stresses.
The distortion of tools in more conveniently avoided by martempering or
austempering.
Tempering and Stablisation
Tempering of tool steel is again an important step in heat treatment. The quenching
of such steels causes the existence of untempered martensite, retained austenite and
carbide in the structure. This happens because of finer changes in composition do
not permit to establish correct austenising temperature. Tempering besides removing
these structural deficiencies also help induce secondary hardness. Figure 2.35 shows
the presence of secondary hardness manifested by increased hardness at certain
tempering temperature in case of high alloy steels. Both medium and high alloy steel
decrease in hardness respectively upto about 460oC and 500
oC and thereafter they
85
Engineering Alloys
(Ferrous and Non-Ferrous) increase in hardness. High alloy steels show greatest increase in hardness with
increasing tempering temperature.
Figure 2.35 : Tempering Characteristics of Tool Steels
After tempering initially the treatment is repeated two or three time in
multi-tempering practice. Such treatment stabilises the microstructure by reducing
the amount of retained austenite. Yet another method of stabilization is to cool the
tempered steel to subzero temperature by placing it in solid carbon dioxide
(– 75oC) or dipping it in liquid nitrogen (– 196
oC).
Normalised quenched, tempered and stabilised tool steel possesses maximum
hardness, wear resistance and dimensional stability.
2.13 CAST IRON
Cast iron is an important alloy of Fe and C are largely used in industry for its convenience
to casting in intricate and good mechanical properties. Steels could also be cast but
process is often costlier. Figure 2.8 clearly shows the region of cast iron in equilibrium
diagram. The diagram shows that there is a eutectoid at 4.3% (point C) which has
solidification temperature of 1135oC. Alloys within carbon range of 2.3% to 4.2% are
sometimes referred to as hypo-eutectic irons, since their carbon content is below the
eutectic composition. Although, the melting points of cast irons are much higher than
several non-ferrous alloys, yet they are within the reach of simple melting furnaces, several
of which are commercially available. This is one reason as to why cast irons are so
popular as structural material.
To understand various phases that are present in a cast iron one may consider cooling of a
typical Fe – C mixture from melt, say one having y % C (Figure 2.8). The first material to
solidify is austenite which is a solution of carbon in fcc iron. The line PG will give the
amount of C in solution in austenite during solidification. This amount is always less than
the average percentage in the melt, which means carbon is rejected out of austenite while
the liquid phase is enriched in carbon. The last liquid to solidify has the eutectic
composition, i.e. 4.3%. The eutectic contains the austenite and carbon.
If the liquid is cooled slowly maintaining near equilibrium conditions, the carbon solidified
as flakes of graphite in a matrix of soft ferrite, in pearlite or in a mixture of ferrite and
pearlite. If liquid is cooled rapidly then ferrite is suppressed and pearlite and cementite
precipitate.
Casting is a process in which molten is poured in a mould and on solidifying the casting of
the shape of mould is obtained. Cast iron, as already stated, in a good material for casting.
General properties of cast iron are :
(a) Cheap material.
(b) Lower melting point (1200oC) as compared to steel (1380-1500
oC).
300 400 500 600
1
2
3
4
Tempering Temperature ( 0 C)
Hard
ness
86
Engineering Materials (c) Good casting properties, e.g. high fluidity, low shrinkage, sound casting, ease
of production in large number.
(d) Good in compression but CI with ductility are also available.
(e) CI is machinable is most cases.
(f) Abrasion resistance is remarkably high.
(g) Very important property of CI is its damping characteristics which isolates
vibration and makes it good material for foundation and housing.
(h) Alloy CI may be good against corrosion.
2.13.1 Contents of Cast Iron
Cast Iron (CI) is prepared form melting pig iron in electric furnace or in cupola furnace.
Electric furnace gives better quality.
CI contains different elements in addition to Fe. The carbon content of CI is more than
2%. Si varies between 0.5 to 3.0%. It is very important because it controls the form of C
in CI. S content in CI varies between 0.06 to 0.12% and is largely present as FeS which
tends to melt at comparatively low temperature causing hot shortness. Mn inhibits
formation of FeS.
Though P increases fluidity of CI – a property helpful for pouring – it has to be restricted
to 0.1 to 0.3% because it reduces toughness. P is present in the form of FeP.
Mn in CI varies from 0.1 to 1.0% through such a small Mn does not affect properties of
CI it certainly helps improve upon hot shortness by taking care of S.
Several other alloying elements like Ni, Cr, Mo, Mg, Cu and V may be added to CI to
obtain several desirable properties.
2.13.2 Classification of Cast Iron
CI containing C in form of cementite is called white cast iron. Microstructure of such CI
consists of pearlite, cementite and ledeburite. If C content is less than 4.3% it is
hypoeutectic CI and if C is greater than 4.3% it is hypereutectic CI. White cast iron has
high hardness and wear resistance and is very difficult to machine. It can be ground,
though. Hardness of white CI varies between 300-500 BHN and UTS between
140-180 MPa. White CI is normally sand cast to produce such parts as pump liners, mill
liners, grinding balls, etc.
Cast iron containing carbon in form of graphite flakes dispersed in matrix of ferrite or
pearlite is classified as gray cast iron. The name is derived from the fact that a fracture
surface appears gray. Gray cast iron differs in percentage of Si from white cast iron while
C percentage is almost same. The liquid alloy of suitable composition is cooled slowly in
sand mould be decompose Fe3C into Fe and C out of which C is precipitated as graphite
flakes. Addition of Si, Al or Ni accelerates graphitesation. The graphite flakes vary in
length from 0.01 to 1.0 mm. The flakes provide an easy passage to cracks thus not
allowing softer microstructure to deform plastically. Larger flakes reduce strength and
ductility. The best properties of gray cast iron are obtained with flakes distributed and
oriented randomly. Inoculant agents such as metallic Al, Ca, Ti, Zr and SiC and CaSi
when added in small amount, cause formation of smaller graphite flakes and random
distribution and orientation.
Gray cast iron is basically brittle with hardness varying between 149 to 320 BHN and
UTS of 150 to 400 MPa. Different properties are obtained by varying cooling rate and
quantity of inoculant agents. It has excellent fluidity, high damping capacity and
machinability. If gray CI is repeatedly heated in service to about 400oC suffers from
permanent expansion called growth. Associated with dimensional changes are less of
ductility and strength as a result of growth. When locally heated to about 550oC several
times this material develops what are called fire cracks resulting into failure.
87
Engineering Alloys
(Ferrous and Non-Ferrous) High strength gray cast iron is obtained by addition of strong inoculating agent like CaSi
to liquid metal before casting process. UTS in the range of 250 to 400 MPa is obtained.
This cast iron is called Meehanite iron and can be toughened by oil quenching treatment
to a UTS of 520 MPa.
If graphite in cast iron is present in form of nodules or spheroids in the matrix of pearlite
or ferrite the material is called nodular cast iron. This cast iron has marked ductility
giving product the advantage of steel, and process the advantage of cast iron. It is
basically a gray cast iron in which C varies between 3.2 to 4.1%, Si between 1.0 to 2.8%
while S and P are restricted to 0.03 to 0.1% respectively. Ni and Mg are added as alloying
elements. Crank shafts, metal working rolls, punch and sheet metal dies and gears are
made out of nodular CI. The defects like growth and fire cracks are not found in this
class of iron. This makes it suitable for furnace doors, sand casting and steam plants. It
also possesses good corrosion resistance making it useful in chemical plants, petroleum
industry and marine applications.
White CI containing 2.0 to 3.0% C, 0.9 to 1.65% Si, < 0.18% S and P, some Mn and
< 0.01% Bi and B can be heat treated for 50 hours to several days to produce temper
carbon in the matrix of ferrite or pearlite imparting malleability to CI. This class is known
as malleable cast iron and can have as high as 100 MPa of UTS and 14% elongation. Due
to such properties as strength, ductility, machinability and wear resistance and
convenience of casting in various shapes, malleable CI is largely used for automotive parts
such as crank and cam shafts, steering brackets, shaft brackets, brake carriers and also in
electrical industry as switch gear parts, fittings for high and low voltage transmissions and
distribution system for railway electrification.
Addition of alloying elements such as Ni and Cr provide shock and impact resistance along
with corrosion and heat resistance of cast iron. These are called alloyed CI 3 to
5% Ni and 1 to 3% Cr produce Ni-hard CI with hardness upto 650 BHN and modified
Ni-hard CI with impact and fatigue resistance is produced by adding 4.8% Ni and
4.15% Cr. Ni-resist CI with 14 to 36% Ni and 1 to 5% Cr is alloy CI having good
corrosion and heat resistance.
Most castings in CI must be stress relieved at 400-500oC because CI has a property to
relieve locked in stresses after sometime. CI can be annealed by heating to 800-900oC to
improve machinability. Cast iron can be quenched in oil to improve hardness. Such
quenching treatment is often followed by heating to 300oC and cooling slowly
(Table 2.15).
2.13.3 Heat Treatment of Cast Iron
Castings in iron are often heat treated for improving mechanical properties and
microstructure. The treatments given to cast iron are described briefly here.
Stress Relieving
Internal stress in cast material is very common because every casting undergoes
cooling which is non-uniform. After certain time period has passed, these internal
stresses tend to be relieved almost spontaneously. Such self-relieving of stresses
may cause changes in dimensions which may not be permitted for working of parts
of machine. Therefore, it is imperative that cast material (parts) must be stress
relieved before bringing such parts in service. Cast iron is stress relieved by heating
it to a temperature in the range of 400-500oC and kept at this temperature for a few
hours. The cooling is done slowly or as per the rate for a particular structure. Stress
relieving of cast iron is referred to as seasoning of casting.
Annealing
Annealing of cast iron is heating it to a temperature between 800 and 900oC and
cooling slowly. This process decomposes iron carbide into ferrite and graphite and
machinability is improved. This may be necessary for such parts that require
machining.
Quenching and Tempering
88
Engineering Materials Cast iron pearlite structure may be heated to lower critical temperature and then
quenched to effect very rapid cooling. This treatment causes the precipitation of
hard martensite phase and casting is thus capable of providing high wear resistance.
The casting after quenching treatment may be further heated to 300oC and cooled
slowly to restore original toughness.
Surface Hardening
Many applications of cast iron necessitate high surface hardness. This may be
achieved by surface hardening treatments such as nitriding and induction hardening.
Table 2.15 : Typical Mechanical Properties and Applications of Cast Iron
Cast Iron Composition
wt. %
Condition Structure UTS
MPa
YS
MPa
Elongation
(%)
Typical Application
Ferrite 3.4 C,
2.2 Si,
0.7 Mn
Annealed Ferrite matrix 180 − − Cylinder blocks and
head clutch plates
Pearlite 3.2 C,
2.0 Si,
0.7 Mn
As-cast Pearlite matrix 250 − − Truck and tractor
cylinder blocks, gear
box
Gray Cast
Iron
Pearlite 3.3 C,
2.2 Si,
0.7 Mn
As-cast Pearlite matrix 290 − − Diesel engine castings
Ferrite 2.2 C,
1.2 Si,
0.75 Mn
Annealed Temper
carbon and
ferrite
345 224 10 General engineering
service machinability
Pearlite 2.4 C,
1.4 Si,
0.75 Mn
Annealed Temper
carbon and
pearlite
440 310 8 General service with
dimensional tolerance
Melleable
Cast Iron
Martensitic 2.4 C,
1.4 Si,
0.75 Mn
Quenched
and
Tempered
Tempered
martenstie
620 438 2 High strength parts,
connecting rods, yokes
for universal joints
Ferrite 3.5 C,
2.2 Si
Annealed Ferritic 415 275 10 Pressure casting as
valve and pump bodies
Pearlite 3.5 C, 2.2 Si
As-cast Ferritic pearlite
550 380 6 Crank shaft, gears and rollers
Ductile
Cast Iron
Martensitic 3.5 C,
2.2 Si,
Quenched
and
Tempered
Martensitic 830 620 2 Pinions, gears, rollers
and slides
SAQ 5
(a) What is an alloy? Give the range of composition of alloying elements.
(b) State effects of following alloying elements in steel. Tungsten, nickel,
chromium, vanadium and cobalt.
(c) Which elements will improve the following properties of alloy steels?
Hardenability, toughness, machinability, corrosion and wear resistance,
fatigue strength.
(d) What is stainless steel? Mention those properties which distinguish stainless
steel from plain carbon steel.
(e) Describe heat treatments for tool steel.
(f) Classify cast iron. What are Ni-hard and Ni-resist cast irons?
2.14 NON-FERROUS MATERIALS
Modern technology has been highly dependent upon non-ferrous and alloys for in certain
cases they present the advantages of high strength and low weight and for certain other
cases they surpass the mechanical strength of ferrous metals. In certain cases the
non-ferrous meals like copper and aluminium alloys have no alternative in wide range of
steel. Electrical conduction and aircraft bodies are examples. A jet turbine engine is a good
example of application of these materials. A typical engine of this type contains 38%
89
Engineering Alloys
(Ferrous and Non-Ferrous) titanium, 12% chromium, 37% nickel, 6% cobalt, 5% aluminium, 1% niobium and 0.02%
tantalum. Though steel is the largest consumed metal, good amounts of
non-ferrous metals are coming into demand for mechanical, electrical, elevated
temperature and corrosion resistance. Typically aluminium alloys are used for cooking
utensils, aircraft bodies and as building materials, copper is used as electrical conductor in
electrical machines and power transmissions, copper alloys are also used at tubing
wherever good thermal conductivity is desired. And there are several other examples.
In Table 2.16 the prices of several metals was compared with gold price as base at 1000.
Comparing these prices one must be careful that the densities of various metals and their
alloy will vary widely. The application of material generally can be seen in the machine
and structure as its volume and not as weight. Table 2.8 compares the prices on weight
basis. Table 2.16 compares the prices on the basis of both weight and volume.
Table 2.16 : Comparison of Prices of Various Non-ferrous Alloys
Price Metal
Per Volume Per Weight
Mo alloys 3.3 – 4.170 6.24 – 7.8
Ti alloys 0.33 – 0.660 1.37 – 3.71
Cu alloys 0.083 – 0.166 0.30 – 0.36
Zn alloys 0.025 – 0.115 0.67 – 0.158
Stainless steel 0.055 – 0.150 0.082 – 0.37
Mg alloys 0.032 – 0.640 0.31 – 0.74
Al alloys 0.032 – 0.048 2.65 – 3.97
Low alloy steel 0.023 0.097
Gray cast iron 0.020 0.050
Carbon steels 0.0167 0.041
Gold – 1000 (per weight and per volume)
2.15 ALUMINIUM
Aluminium was first produced in 1825. Presently it is produced in quantity second only to
steel. It is the most abundant metallic element on the crust of the earth easily comprising
about 8% of the crust. Bauxite, an hydrous oxide of aluminium and several other oxides,
is the principal ore of aluminium. Aluminium is extracted from its ore mainly through
electrolytic process. The ore is first washed off to remove clay and dirt, the ore is crushed
into powder and treated with hot sodium hydroxide (caustic soda) to remove impurities.
Alumina (the oxide of aluminium) extracted from this solution is dissolved in molten
sodium fluroride and aluminium fluoride bath at 940-980oC. This mixture is then
subjected to direct current electrolysis by passing direct current between carbon anode and
cathode. The metallic aluminium forms in liquid state and sinks to bottom of the cell. This
liquid aluminium is tapped off from time to time. The aluminium so obtained is 99.5 to
99.9% pure with iron and silicon as the major impurities.
Aluminium, then is taken to large refractory lined furnaces for refining before casting. The
chlorine gas is used as purging agent to remove the dissolved hydrogen gas, and the liquid
metal surface is skimmed off to remove oxidized metal. The molten metal is then cast into
ingots for remelting or rolling.
2.15.1 Wrought Aluminium Alloys
Sheets and extrusion ingots are cast through semi-continuous direct chill method. The
sheet ingots are scalped wherein about 12 mm of ingot surface is removed. The scappled
ingots are preheated to homogenise the structure by heating to a high temperature and
90
Engineering Materials soaking there for 10-24 hours. The preheating is done at a temperature below the lowest
melting point of the constituents. The ingots are then hot rolled to about 75 mm thickness
in 4 high reversed rolling stand. Thereafter the rolled sheet is further reheated to the same
temperature and further hot rolled to 18 mm to 25 mm thickness. Further thickness
reduction may be achieved through cold rolling. The products obtained this way are termed
wrought alloys and normally are inform of sheet, plate, rod, wire and extruded sections.
The wrought alloys are identified by a four digit code out of which the first digit signifies
the aluminium purity (if pure aluminium) or the major alloying element. The second digit
indicates the modification of alloy. The third and fourth digits indicate the minimum
amount of aluminium in the alloy. The first digit indicates following :
(a) Aluminium is pure no alloying element.
(b) Alloying element copper but magnesium is also added.
(c) Alloying element manganese.
(d) Alloying element silicon.
(e) Alloying element magnesium.
(f) Main alloying elements are magnesium and silicon.
(g) Main alloying elements are zinc, magnesium and copper.
2.15.2 Aluminium Cast Alloys
Aluminium alloys the cast by any one of the following processes.
Sand Casting is the simplest and most versatile process small castings, complex castings
with intricate cores, large castings and structural castings are produced by sand casting
with equal case.
In permanent mould casting a metallic mould is used which may be gravity6 filled or
rotated for centrifugal action. The castings from permanent mould are fine grained as
compared to sand cast products. In die casting maximum rate of production is achieved.
The molten metal is forced into die which is split but sufficiently strong to withstand
pressure. One important characteristic of die casting is close tolerance in parts. Fine
grained structure and automation of process are other advantages.
Aluminium casting alloys need such element for alloying which will not only impart
mechanical strength but will also increase fluidity and feeding ability. Therefore their
chemical composition must differ from wrought alloys. Silicon is the most preferred
alloying element in aluminium cast alloys for its improves fluidity and feeding ability as
well as its mechanical strength. Normal silicon content varies between 5 to 12%.
Magnesium in the range of 0.3 to 1% provides strength mainly through precipitation. Mg,
Zn, Sn, Ti are also added sometimes.
2.15.3 Properties of Aluminium Alloys
Among the various properties of aluminium alloys following are notable :
(a) low density (2.7 gm/cc)
(b) high electrical and thermal conductivity, only next to Cu
(c) good resistance to atmospheric, water and seawater corrosion
(d) good machinability, formability and castability
(e) maintains good light reflectivity
(f) non-toxic, non-magnetic and non-sparking.
Aluminium is a soft but weak material whose strength is increased by strain hardening and
several heat treatments. Aluminium is used as a matrix in several fibre reinforced
composites. Al2O3 an oxide of Al is very hand and strong and can be dispersed in the
matrix of Al by powder metallurgy to produce SAP (sintered aluminium product). Other
91
Engineering Alloys
(Ferrous and Non-Ferrous) reinforcing elements used in softer aluminium matrix are boron whiskers, stainless steel
fibres and whiskers of Al3Ni.
Alminium alloys are divisible in three groups :
(a) cast Al alloys
(b) wrought Al alloys
(c) aluminium composite reinforced with fibres or particles.
Cast Al Alloys
Low melting temperature, insolubility to gases except H2 and good surface finish
are characteristics of these alloys. Important drawback of cast aluminium alloys is
their shrinkage after solidification and hence careful mould design is called for.
Mechanical properties are inferior to wrought alloys except in creep. Alloys can be
sand cast gravity die cast, and cold chamber pressure die cast. Si, Cu, Mg and Sn
increase fluidity when casting thin sections. Mechanical properties of cast Al alloys
is improved by adding Cu which induces age hardening to impart hardness and
stability upto 250oC. Alloys used for die casting are : 380 (Al, Si, 3.5 Cu) and 413
(Al, 11.5 Si). Alloys preferred for permanent mould casting are : 332 (Al, 9 Si, 3
Cu, 1 Mg) and 319 (Al, 6 SI, 4 Cu). Y-alloy containing 4% Cu and 2% Ni retain
strength at high temperatures. It is used for piston and cylinders of IC engines.
Wrought Al Alloys
Wrought aluminium alloys are obtained by addition of Mn and Mg. The Al-Mn and
Al-Mg alloys cannot be heat treated. Al-Mn alloy combines high ductility with
excellent corrosion resistance. Beverage cans, cooking utensils and roofing sheets
are made in Al-Mn alloy.
Al alloy that responds to heat treatment by age hardening are Al-Cu, Al-Cu-Mg and
Al-Mg-Si. Some Al alloys, their composition and applications are described in
Table 2.17. Duralumin is one such alloy which contain 4% Cu and small amounts
of Mg, Mn and Si. After heat treatment this alloy develops a UTS of 450-550 MPa
and finds use in aircraft structures.
Apart from cast and wrought alloys the greater tonnage (about 85%) of Al is used
in commercially pure form in which impurities are less than 1%. Al extrusions,
tube, rods, wire, electrical conductors, chemical process equipment, foils and many
architectural fittings are made in commercially pure Al. The properties of
aluminium are described in Table 2.18.
Table 2.17 : Some Aluminium Alloys – Properties and Applications
Alloy
Designation Composition (%)
UTS/Elongation (%)
N/mm2 Characteristics and Applications
EC – O 99.5 Al (mini) 75/50 Ductile, high electrical conductivity
3003 – O
3003 – H16
98.8 Al, 1.2 Mn
98.8 Al, 1.2 Mn
130/140
190/140
Good formability and corrosion
resistance weldable, storage and
utensils
2024 – T4 93 Al, 4.5 Cu, 1.5 Mg,
0.5 Si, 0.5 Mn
500/19 High strength, aircraft parts,
bridges, rivers
5056 – H18
5056 – O
94.6 Al, 5.2 Mg, 0.3 Mn
94.6 Al, 5.2 Mg, 0.3 Mn
450/10
300/35
Good corrosion resistance to sea
water good finish when buffed or
anodized, marine parts, cooking
utensils, bus bodies
6061 – T6
6061 – O
98 Al, 1 Mg, 0.6 Si, 0.4 Cu
98 Al, 1 Mg, 0.6 Si, 0.4 Cu
320/17 Good corrosion resistance and
formability, general structure,
anodized articles, marine and
transport parts
7075 – T6
7075 - O
90 Al, 5.5 Zn, 2.5 Mg,
1.7 Cu, 0.3 Cr
90 Al, 5.5 Zn, 2.5 Mg,
600/11
240/16
High strength and corrosion
resistance, aircraft parts, bridges
92
Engineering Materials 1.7 Cu, 0.3 Cr
Table 2.18 : Typical Properties of Aluminium
Sl. No. Property Value
1 Purity % 99.5 Al, 0.25 Si, 0.25 Fe
2 Melting point oC 660
3 Sp. Gravity 2.70
4 Tensile strength, N/mm2
O – Temper
H – 18 Temper
72
135
5 Elongation %
O – Temper
H – 18 Temper
60
17
6 Hardness BHN
O – Temper
H – 18 Temper
19
35
7 Electrical conductivity* % IACS
O – Temper
H – 18 Temper
62
61
8 Thermal conductivity
J/m2/oC/s at 25oC
234
9 Corrosion resistance Very good in rural marine and
industrial, atmosphere
* Compare with copper, 62% of copper electrical conductivity.
In Table 2.17 aluminium alloys have been assigned certain temper like O-Temper and H-
18 Temper. The temper designation indicates the condition and heat treatment of any given
alloy. Generally the temper designation must follow the alloy designation and separated by
a dash. For example, the alloys in Table 2.17 must be described as 3003-O, 2004-T4. The
temper designations are described below. There are four basic tempers :
(a) F – As fabricated
(b) O – Annealed
(c) H – Strain hardened
(d) T – Heat treated
H is always followed by two or more digits. The first digit indicates basic operations while
the following digit stands for the final degree of strain hardening.
H1 – only strain hardened
H2 – strain hardened and partial annealed
H3 – strain hardened followed by stabilization
The second digit stands for amount for cold work. The digit 8 represents fully cold worked
or full hard. The digit of 4 means half hard and 2 means quarter hard. Thus, H18 means
full hard by strain hardening only.
T designation is followed by numbers 2 to 9. Their meanings are :
T2 – Annealed (only for castings)
T3 – Solution heat treated and then cold worked
T4 – Solution heat treated and naturally aged to stable condition
T5 – Artificial ageing after any one of the following : Elevated temperature, rapid
cool fabrication such as casting or extrusion
T6 – Solution heat treated and fabricated
T7 – Solution heat treated and stabillised
T8 – Solution heat treated, cold worked and then artificially aged
T9 – Solution heat treated, artificially aged and then cold worked.
93
Engineering Alloys
(Ferrous and Non-Ferrous) 2.15.4 Age-hardening of Aluminium Alloys
In certain alloys precipitation from a single phase may occur. The precipitate phase may
be in form of find sub-microscopic particles distributed both around the grain boundaries
and throughout the grains. In certain alloys of Al-Cu, Mg-Si and Be-Cu such phases
precipitate after suitable heat treatment. These precipitated phases have strengthening
effects of the alloys. This hardening of alloys is termed age hardening or precipitation
hardening.
Here the process of age-hardening will be described with particular reference to aluminium
alloys containing 4% Cu. Figure 2.36 depicts the equilibrium diagram of
Al-Cu system. It is seen that the solubility of Cu in α-phase solid solution decreases
steadily and quite considerably with decrease in temperature. At temperature
corresponding to point 3, copper forms copper aluminide (CuAl2) which is deposited as
coarse particles in and around the grains of α-solid solution. CuAl2 is extremely hard and
brittle. If the alloy is now reheated to about 550oC, between the points 2 and 3, CuAl2 is
reabsorbed in α-solid solution resulting into single-phase alloy. If alloy from this state is
quenched to room temperature, there is insufficient time for CuAl2 to form and Cu atoms
are now held in a super-saturated solid solution within the aluminium.
Figure 2.36 : The Aluminium-rich Portion of the Copper Aluminium Equilibrium Diagram Showing
the Mechanism of Precipitation Hardening for a 4% Copper Alloy, Over Aging Causes a
Coalescence of the CuAl2 Particles and Consequent Loss of Strength in the Alloy
When this alloy is allowed to stay at room temperature for five to seven days, the strength
improves significantly because of slow precipitation of find submicroscopic particles.
These particles are almost uniformly distributed around the grains. The time of this
precipitation may be reduced to a few hours by heating the quenched alloy to 120oC. This
is known as artificial age-hardening. Closed control of both time and temperature is
essential in precipitation hardening for this purpose. Salt baths at constant temperatures
are used 4% Cu aluminium alloy is most suitable for this type of treatment. However, this
alloy loses its corrosion resistance in hardened state and must be protected by cladding.
Age-hardening alloys containing Si and Mg behave in a similar manner. However, the
submicroscopic particles that provide strengthening are made of magnesium silicide
(Mg2Si). Thus the age-hardening effect of CuAl2 is reinforced by Mg2Si.
2.16 COPPER AND ITS PRODUCTION
Copper is marked by a host of good engineering properties. The foremost is its good
electrical conductivity and bulk of copper is used as electrical conductor. It also has a high
thermal conductivity and coupled with its resistance to corrosion it is largely used as heat
exchanger tubes particularly under circumstances when corrosive atmosphere exists. Its
medium tensile strength and ease of fabrication are added advantages in its industrial
application.
0 2 4 6 8 10 % Copper
Cu Al2
10
20
30
40
50
600
66
3
2 a + Liquid
Re-Heat Slow Cool
Quench Overage
Age
Tem
pera
ture
(0
C)
1 Slow Cool
As Cost
94
Engineering Materials Copper is extracted from its sulfide ore. Such ores also contain sulfides of iron. Low grade
ore is converted into sulfide concentrate which is smelted in reverberatory furnace to
produce a mixture of sulfides of iron and copper, called mate. The slag is separated from
matte. The copper sulfides is then chemically converted into impure or blister copper of
98% purity, by blowing air through the matte. The iron sulfides is oxidized and converted
in slag. The blister copper is then transferred to refining furnace where most of impurities
are converted into slag and removed. This fire refined cooper is called tough pitch copper
and is further refined electrolytically to produce 99.95% pure copper called electrolytic
tough pitch (ETP) copper.
ETP copper is used for production of wire, rod plate and strip. These products serve
several industrial purposes. But ETP copper contains 0.04% oxygen which forms
interdendritic Cu2O when copper is cast. If copper is heated to a temperature of 400oC in
the atmosphere of hydrogen, then hydrogen reacts with densritic Cu2O and produces
steam. These H2O molecules being large in size do not diffuse readily and cluster around
grain boundaries thus causing internal holes. This phenomenon is called hydrogen
embrittlement. The methods of avoiding hydrogen embrittlement are adding phosphorous
in the alloy copper and thus allowing P2O5 to form. The other method is to cast ETP
copper under a controlled reducing atmosphere to produce copper which is oxygen free
high conductivity (OFHC) copper.
2.17 COPPER ALLOYS
Several alloys of copper are used in industry for varying purposes. Copper forms alloys
with zinc (the brasses), tin (the bronzes), with tin and phosphorous (the phospher bronzes),
aluminium (the aluminium bronzes) and with nickel (the cupronickels).
2.17.1 The Brasses
70/30 brass also known as cartridge brass contains 70% Cu and 30% Zn. It is used for
cartridge cases, condenser tubes, sheet fabrication and for general purposes. Its ultimate
tensile strength varies between 350 and 600 N/mm2. It is soft and ductile in and annealed
form can withstand severe cold working.
60/40 brass of Muntz metal contains 60% Cu and 40% Zn. Its UTS varies between 400
and 850 N/mm2. It is suitable for hot working operations as well as for casting. Many cast
valves and marine fittings are made out of this brass. Addition of 2% Pb improves its
machinability.
Small additions of Fe, Al, Sn, Mn and Ni to 60/40 brass improves its strength
considerably. Marine propellers and shafts, pump rods, autoclaves, switch gears and high
strength fittings are made out of these brasses.
Brazing alloys are essentially the brasses of 50/50 composition with small additions of Sn,
Mn and Al. These brasses are hard and brittle.
2.17.2 The Bronzes
The coinage bronze used for making coins in earlier days contains 95% Cu, 4% Sn and
1 % Zn. The Zn acts as a deoxidiser. This alloy is soft and ductile.
Admiralty gun metal contains 88% Cu, 10% Sn and 2% Zn. This bronze is normally cast
to produce steam and water fittings and bearings. The addition of Pb improves the
pressure tightness of the alloy.
Phosphor bronzes are commonly used in manufacture of bearings, hard drawn wires and
bronze springs. In addition to tin they contain small percentage of phosphorous as alloying
element. 0.2% P forms Cu3P which is a hard compound. It acts as deoxidiser and improves
fluidity.
Copper aluminium alloys posses high strength with good resistance to fatigue, corrosion
and abrasion and are golden in colour. Aluminium can dissolve in copper to the extent of
9% and greater content than this induces brittleness. Wrought alloys which are good for
95
Engineering Alloys
(Ferrous and Non-Ferrous) hot and cold working applications contain 5 to 7% Al. Casting alloys contain 10% Al.
Small percentage of Fe, Ni and Mn are added to casting alloys to make them more easily
heat treatable. Aluminium bronze is well known for its colour and often called Imitation
gold. Al bronze compares well with the strength of steel.
Bronzes in general are known for the following characteristics :
(a) costlier than brass,
(b) better corrosion resistance,
(c) stronger than brass, and
(d) bearing material.
2.17.3 Copper-Nickel Alloys
Complete solubility occurs between copper and nickel. All alloys have similar
microstructure and can be cold or hot worked.
Cupro-nickel also known as German silver is extremely malleable and ductile. It is good
against corrosion due to salt water. Condenser tubes are main parts made out of this alloy.
It is also used for coinage. 70/30, 80/20 and 75/25 alloys are very common.
Monel metal is essentially 70% Ni and 30% Cu with small amounts of iron and other
elements. Alloy is well known for its high strength and corrosion resistance. This alloy is
largely used for chemical and food processing plants. It also finds great use as turbine
blades, valves corrosion resistance bolts, screws and nails. It is known for its
characteristics silver luster.
Tables 2.19 and 2.20 respectively describe Brasses and Bronzes with their applications.
Table 2.19 : Composition, Properties and Applications of Brasses
Gliding metal (95 Cu, 5 Zn) High ductility and corrosion resistance, coins,
medals, gold platings
Red brass (85 Cu, 15 Zn) Good corrosion resistance, workability, heat
exchanger tube, radiator cores
Cartridge brass (70 Cu, 30 Zn) Good strength and ductility, rivets, springs,
automotive radiator cores
Yellow brass (65 Cu, 35 Zn) Screws, rivets reflectors, plumbing accessories,
automotive radiator cores
Muntz metal (60 Cu, 40 Zn) Soundness and good machinability, condenser
tubes, architectural work
Leaded red brass (85 Cu, 5 Zn, 5 Sn, 5
Pb)
Fair strength, soundness and good machining in
cast state, pressure valves, pipe fittings, pump
fittings
Leaded commercial bronze
(89 Cu, 9.25 Zn, 1.75 Pb)
Screws, screw machine parts, electrical connectors,
builder’s applications
Admiralty brass (71 Cu, 28 Zn, 1 Sn) Condenser, evaporator and heat exchanger tubes,
marine applications
High leaded brass (64 Cu, 33 Zn, 2 Pb) Flat products, gears, wheels
Table 2.20 : Composition and Applications of a Few Bronzes
Phosphor bronze (94.8 Cu, 5 Sn, 0.2 P) Bolts, electric contracts, spring, bearing
Phosphor bronze (89.8 Cu, 10 Sn, 0.2 P) Such applications where high strength and
resistance to salt water is desired, bushing and gears
Gun metal (88 Cu, 10 Sn, 2 Zn) Sand cast, sued under heavy pressure such as gears
and bearings
Aluminium bronze
(88 Cu, 10.5 Al, 3.5 Fe )
High UTS
Beryllium bronze (98 Cu, 1.7 Be, 0.3
Co)
Very high mechanical strength, springs, used
against fatigue, wear and corrosion
(UTS – 1200 MPa)
96
Engineering Materials 2.17.4 Copper-Beryllium Alloys
Copper-beryllium alloys contain between 0.6 to 2% Be and 0.2 to 2.5% Co. These alloys
can be precipitation hardened and cold worked to develop a tensile strength as high as
1460 MPa. This is the highest strength among the copper alloys. Cu-Be alloys are used as
tools requiring high hardness and non-sparking characteristics for the chemical industry.
These alloys are very useful for making springs, gears, valves and diaphragms for their
excellent corrosion resistance, fatigue properties and strength. These alloys, however, are
costlier.
2.18 MAGNESIUM AND ITS ALLOYS
Magnesium is a light metal with density of 1.74 g/cm3. Magnesium is much costlier than
aluminium (density 2.74 g/cm3) with which it compares for lightness. Magnesium in its
molten state burns readily, hence it is difficult to cast the alloys of magnesium. Magnesium
alloys have low corrosion resistance and show poor fatigue and creep behaviour. Their h.
c. p. structure does not permit to deform readily at room temperature since only three slip
systems exist in h. c. p. at room temperature. The best advantage that magnesium alloys
offer is that of low density and many aircraft parts are made in these alloys.
Al when added to Mg in the range of 3 to 10% with small amounts of Zn and Mn increases
strength, hardness and castability. Addition of Mn (1.2%) with small amount of C does not
increase strength but improves corrosion resistance. Mg-Al-Zn alloys have good
mechanical strength and corrosion resistance. These alloys are good casting material and
generally used at high temperature like 250oC. Extrusions and forgings for general
purpose are made in these alloys are used in aircraft, automotive, radio and instrument
industries.
Some magnesium alloys are described in Table 2.21 along with their applications.
Table 2.21 : Magnesium Alloys
Composition (%) Condition UTS
N/mm2
YS
N/mm2
Elongation
%
Application
Mg, 3 Al, 1 Zn, 0.2 Mn Annealed 228 − 11 Air borne cargo
equipment
Mg, 2 Th, 0.8 Mn T8 228 198 6 Missile and
aircraft sheets
upto 427oC
Wrought
alloys
Mg, 6 Zn, 0.5 Zr T5 310 235 5 Highly stressed
aerospace uses,
extrusions,
forgings
MG, 6 Zn, 3 Al, 0.15
Mn
As-cast 179 76 4 Sand casting
requiring good
room temperature
strength
Cast
alloys
Mg, 3 Re, 3 Zn, 0.7 Zr T6
T5
235
138
110
97
3
2
Pressure tight and
permanent mould
castings used at
150-260oC
2.19 TITANIUM ALLOYS
Pure titanium is a strong ductile and light weight metal. It is very strong, highly resistant
to corrosion of all types but has the drawback that it readily reacts with common gases at
around 300oC. It reacts readily with C, O2, N2 and theses elements cause embrittlement of
Ti. It melts at 1725oC, has a UTS of 600-800 MPa and precent elongation of 25%.
Ti 6 Al 4 V alloy develops a UTS of 1300 MPa and had good creep, fatigue and oxdiaton
resistance. Aero engines gas turbine blades and other parts of engine and components of
97
Engineering Alloys
(Ferrous and Non-Ferrous) air frame are made of this alloy. Ti 5 Al 2.5 Sn is also a strong alloy (900 MPa UTS)
which is used in aircraft engine components at 470 to 500oC.
2.20 BEARING MATERIALS
In general it can be said that a good bearing material should posses following
characteristics :
(a) it should be strong enough to sustain bearing load.
(b) it should not heat rapidly.
(c) it should show a small coefficient of friction.
(d) it should wear less, having long service life.
(e) it should work in foundary.
Generally it is expected that the journal and bearing would be made of dissimilar materials
although there are examples where same materials for journals and bearings have been
used. When the two parts are made in the same material the friction and hence the wear are
high.
Cast iron has been used as bearing material with steel shafts in several solutions.
However, the various non-ferrous bearing alloys are now being used largely as bearing
material because they satisfy the conditions outlined above more satisfactory.
Bronzes, babbitts and copper-lead alloys are the important bearing materials that are
widely used in service. Certain copper zinc alloys, that is brasses, have been used as
bearing materials, but only to limited extent. Since brass in general is chapter, it has
replaced bronze in several light duty bearings.
2.20.1 Bearing Bronzes
Bearing bronzes are the copper-tin alloys with small additions of other constitutions.
Under conditions of heavy load and severe service conditions, bronzes are especially of
great advantages. They possess a high resistance to impact loading and, therefore, are
particularly used in locomotive and rolling mills bearings. However, they get heated up
fast as compared to other bearing materials, such as babbitts. Bronze lined bearings are
easily removed and finished bushings are generally available in stocks. A few of the
bronzes that are widely used are described in Table 2.22.
Table 2.22 : Bearing Bronzes
Mechanical Properties Bronze and
SAE Number Composition (%) UTS
N/mm2
YS
N/mm2
Elongation
%
Application
Leaded gun
metal, 63
Cu 86-89, Sn 9-11,
Pb 1-2.5, P 0.25
max. impurities 0.5 max.
200 80 10
Bushing
Phosphor
bronze, 64
Cu 78.5-81.5, Sn 9-11,
Pb 0.05-0.25, Zn 0.75
max. impurities 0.25 max.
167 80 8
Heavy loads
Bronze backing
for lined
bearings 66
Cu 83-86, Sn 4.5-6.0,
Pb 8-10, Zn 2.0,
impurities 0.25 max.
167 80 8
Bronze backed
bearings
Semi-plastic
bronze, 67
Cu, 76.5-99.5, Sn 5-7,
Pb 14.5-17.5, Zn 4.0 max.,
Sb 0.4 max., Fe 0.4 max.,
impurities 10.0 max.
133 − 10
Soft and good
antifriction
properties
2.20.2 Babbitts
The alloys of tin, copper, lead and antimony are called babbitts. The tin provides the
hardness and compressive strength of babbitts, copper makes them tough, antimony
prevents shrinkage while lead contributes to ductility. Bearing liners are extensively made
in babbitts for their better antifriction properties than bronzes.
98
Engineering Materials When Babbitt is backed up a solid metal of high compressive strength it gives good service
under high speeds, heavy pressure, impact loads and vibrations. The backing material
could be bronze or steel. A thin layer of high-tin Babbitt thoroughly fused to a tinned
bronze or a steel shell has exceptional load carrying capacity and impact strength. In case
of cast iron bearings the Babbitt in anchored in place by dovetail slots or drilled holes,
because Babbitt does not fuse with cast iron. Babbitt bearing linings of dependable
strength and life are made by pouring molten material into bearing, allowing to solidify
and fuse thoroughly and then machining to finished sizes. While the melting point of
Babbitt varies between 180 to 245oC, depending upon composition, the pouring should be
done when metal is in fully fluid state. For example, SAE 10 babbitt has a melting point of
223oC, it should not be poured below 440oC.
Some Babbitt materials are described in Table 2.23.
Table 2.23 : Babbitts (White Bearing Metals)
SAE No. Composition (%) Applications
10 Sn 90; Cu 4-5; Pb 0.35 max.; Fe
0.08 max.; As 0.1 max.;
Bi 0.08 max.
Thin liner on bronze backing
11 Sn 86; Cu 5-56; Sb 6-7.5;
Pb 0.35 max.; Fe 0.08 max.; As
0.1 max.; Bi 0.08 max.
Hard Babbitt good for heavy
pressures
12 Sn 59.5; Cu 2.25-3.75;
Sb 9.5-11.5; Pb 26.0 max.;
Fe 0.08, Bi 0.08 max.
Cheap Babbitt good for large
bearings under moderate loads
13 Sn 4.5-5.5; Cu 0.5 max.;
Sb 9.25-10.75 max.;
Pb 86.0 max.; As 0.2 max.
Cheap Babbitt for large bearing
under light load
2.20.3 Copper-Lead Alloys
Copper-lead alloys, containing a large percentage of lead have found a considerable use as
bearing material lately. Straight, copper-lead alloys of this type have only half the strength
of regular bearing bronzes. They are particularly advantageous over Babbitt at high
temperature as they can retain their tensile strength at such temperature. Most babbitts
have low melting point and lose particularly all tensile strength at about 200oC. Typical
copper-lead alloys contain about 75% copper and 25% lead and melt at 980oC. The room
temperature tensile strength of copper-lead alloy is about 73 MPa and reduces to about 33
MPa at about 200oC.
2.20.4 Other Bearing Materials
An extensively hard wood of great density, known as lignum vitae, has been used for
bearing applications. With water as lubricant and cooling medium its antifriction
properties and wear are comparable with those of bearing metals. Lignum vitae has been
used with satisfactory results particularly in cases of step brings of vertical water turbine;
paper mill machinery, marine service and even roll neck bearings of rolling mills.
More recently, in such cases where use of water as lubricant is necessary, especially if
sand and grit are present soft vulcanised rubber bearings have been used. A soft, tough,
resilient rubber acts as a yielding support, permitting grit to pass through the bearing
without scoring the shaft or the rubber. Longitudinal grooves in the rubber lining allow
free passage of the cooling water with any foreign matter present. With feathered edges
these grooves are also very effective in forming passages in the front of which the
supporting pressure is built up in the fluid film. These bearings have coefficient of friction
which compares well with roller bearings and pressure of 4.0 to 5.5 MPa may be carried if
journal is very smooth and load is applied after it has attained a peripheral speed of 150
m/min. The cooling water temperature in case of rubber bearings must always be below
boiling point. In some cases rubber bearings have been found to give as much as ten times
the service as bearings of lignum vitae or metals.
99
Engineering Alloys
(Ferrous and Non-Ferrous) Rubber bearings have been successfully used in centrifugal and deep well pumps, and
washers and several other applications where water must be used as lubricant. The
resilience and cushioning properties of rubber may be exploited in reducing vibrations of
high speed shafts.
Synthetic and neutral composite materials, plastic and reinforced plastic are also being
used as bearing materials now-a-days. However, their characteristics are not well
established as yet. Powder metallurgy bushing permits oil to penetrate into the materials
because of its porosity and is good for its antifriction properties.
Bearings are frequently ball-indented in order to provide small basins for the storage of
lubricant while the journal is at rest. This supplies some lubricant during starting. The
bearing walls may some time be indented and filled with graphite to provide lubricating
effect at the start.
2.21 ALLOYS FOR CUTTING TOOLS
Apart from tool steels described in Unit 11, may alloys which contain wholly
non-ferrous elements have been developed. Such alloys behave better than tool steel in
many respects and are widely used in industry. These alloys are mainly divided into two
groups : stellites and cemented carbides.
Stellite
Stellite is an alloy of Co (40-60%), Cr (25-35%), W (4-25%) and C (1-3%). It is a
cast alloy containing C, Cr, and W in Cobalt matrix. Its main characteristic is low
coefficient of friction and it possesses high hardness, red hardness, high wear and
corrosion resistance. Desired size and shape is achieved by casting and no heat
treatment is required. They are mainly used for cutting tools and can cut steel at
twice the cutting speed of HSS stellite can be used to cut all types of materials like
steels, as high speed steels because they are cast but perform better than HSS with
higher life.
Satellites are used for cutting hard die faces, can surfaces wear plates and crushes.
The hardness varies between 40 to 60 RC and they retain their hardness upto high
temperature because they do not undergo phase changes.
Cemented Carbide
These are small pieces with cutting edges and mechanically jointed or brazed to tool
shank. Cemented Carbide tool tips are produced by process of powder metallurgy
by sintering the powder carbides of W, Ta, Ti in Co powder. The contents are 40-
95% WC, 3-30% Co, 0-30% TaC and TiC and hardness of tips is in excess of 65
RC compared to 60 RC of stellite. High hardness, high compressive strength at high
temperatures are the main characteristics.
Cermets
Cermets are the variation of cemented carbides when the carbides of W and Ti are
solidified in the softer matrix of Co and Ni to obtain high hardness, resistance to
oxidation and thermal shock and resistance to high temperature abrasion.
Ceramic Tools
Aluminium oxide (Al2O3) is pressed and sintered in a powder metallurgy technique
in various shapes of cutting edges which are fastened to mechanical shanks. The
hardness of this ceramic tool is above 65 RC and has chemical inertness and high
resistance to wear. Ceramic tools are made in small pieces of various geometrical
shapes and can be disposed off when not usable.
100
Engineering Materials 2.22 SUMMARY
Engineering alloys can conveniently be subdivided into two types : ferrous and
non-ferrous. Ferrous alloys have iron as their principal base metal, whereas non-ferrous
alloys have a principal metal other than iron. The steels, which are ferrous alloys, are by
far the most important metal alloys mainly because of their relatively low cost and wide
range of mechanical properties. The mechanical properties of carbon steels can be varied
considerably by cold working and annealing. When the carbon content of steels is
increased to about 0.3%, they can be heat-treated by quenching and tempering to produce
high strength with reasonable ductility. Alloying elements such as nickel, chromium, and
molybdenum are added to plain-carbon steels to produce low-alloy steels. Low-alloy steels
have good combination of high strength and toughness and are used extensively in the
automotive industry for uses such as gears, shafts, and axles.
Aluminium alloys are the most important of the non-ferrous alloys mainly because of their
lightness, workability, corrosion resistance, and relatively low cost. Unalloyed copper is
used extensively because of its high electrical conductivity, corrosion resistance,
workability, and relatively low cost. Copper is alloyed with zinc to from a series of brass
alloys which have higher strength than unalloyed copper. Bronzes are other series of alloys
when Cu is alloyed with tin or aluminium.
Stainless steels are important ferrous alloys because of their corrosion resistance in
oxidising environments. To make a stainless steel “stainless”, it must contain at least
12% Cr.
Cast irons are other industrially important family of ferrous alloys. They are low in cost
and have special properties such as good castability, wear resistance, and durability. Grey
cast iron has high machinability and vibration damping capacity due to the graphite flakes
in its structure. White and iron, yet another variety having carbon in cementite form and is
harder.
Other non-ferrous alloys briefly discussed in this unit are magnesium, titanium, and nickel
alloys. Magnesium alloys are exceptionally light and have aerospace applications and are
used in radio and instrument industry. Titanium alloys are expensive but have a
combination of strength and lightness not available from any other metal alloy system and
so are used extensively for aircraft structural parts. Nickel alloys have high corrosion and
oxidation resistance and are therefore commonly used in the oil and chemical process
industries. Nickel when alloyed with chromium and cobalt forms the basis for the nickel-
base superalloys which are necessary for gas turbines in jet aircraft and some electric-
power generating equipment.
In this unit, we have discussed to a limited extent the structure, properties, and
applications of some of the important engineering alloys. However, it must be pointed out
that may important alloys have been left out due to the limited scope of this unit.
2.23 KEY WORDS
Austenite (γγγγ Phase in Fe-Fe3C : An intersitial solid solution of carbon in FCC iron;
Phase Diagram) the maximum solid solubility of carbon in
austenite is 2.0%.
Austenitising : Heating a steel into the austenite temperature range
so that its structure becomes austenite. The
austenitising temperature will vary depending on
the composition of the steel.
αααα Ferrite (αααα Phase in the : An intersitial solid solution of carbon in BCC
Fe-Fe3C Phase diagram) iron; maximum solid solubility of carbon in BCC
iron is 0.02%.
101
Engineering Alloys
(Ferrous and Non-Ferrous) Pearlite : A mixtue of a ferrite and cementite (Fe3O) phases
in parallel plates (lamellar stsructure) produced by
the eutectoid decomposition of austenite.
Eutectoid Ferrite : A ferrite which forms during the eutectoid
decomposition of austenite.
Eutectoid Cementite (Fe3C) : Cementite which forms during the eutectoid
decomposition of austenite; the cementite in
pearlite.
Eutetoid (Plain-carbon Steel) : A steel with 0.8% C.
Hypoeutectoid : A steel with less than 0.8% C.
(Plain-carbon steel)
Hypereutectoid : A steel with 0.8 to 2.0% C.
(Plain-carbon Steel)
Proeutectoid Ferrite : A ferrite which forms by the decomposition of
austenite at temperature above the eutectoid
temperature.
Proeutectoid Cementite (Fe3C) : Cementite which forms by decomposition of
austenite at temperature above the eutectoid
temperature.
Maenstie : A supersaturated interstitial solid solution of
carbon in body-centered tetragonal iron.
Bainite : A mixture of a ferrite and very small particles of
Fe3C particles produced by the decomposition of
austenite; a non-lamellar eutectoid decomposition
product of austenite.
Spheroidite : A mixture of particles of cementite (Fe3C) in an a
ferrite matrix.
Isothermal Transformation : A time-temperature-transformation diagram which
(IT) Diagram indicates the time for a phase to decompose into
other phases isothermally at different
temperatures.
Continuous-cooling : A time-temperature-transformation diagram which
Transformation (CCT) Diagram indicates the time for a phase to decompose into
other phases continuously at different rates of
cooling.
Martempering (Marquenching) : A quenching process whereby a steel in the
austenitic condition is hot-quenched in a liquid
(salt) bath at above the Ms temperature, held for a
time interval short enough to prevent the austenite
from transforming, and then allowed to cool slowly
to room temperature. After this treatment the steel
will be in the martensitic condition, but the
interrupted quench allows stresses in the steel to be
relieved.
Austempering : A quenching process whereby a steel in the
austenitic condition is quenched in a hot liquid
(salt) bath at a temperature just above the Ms of
the steel, held in the bath until the austenite of the
102
Engineering Materials steel is fully transformed, and then cooled to room
temperature. With this process a plain-carbon
eutectoid steel can be produced in the fully bainitic
condition.
Ms : The temperature at which the austenite in a steel
starts to transform to martensite.
Mf : The temperature at which the austenite in a steel
finishes transforming to martensite.
Tempering (of a Steel) : The process of reheating a quenched steel to
increase its toughness and ductility. In this process
martensite is transformed into tempered martensite.
Plain-carbon Steel : An iron-carbon alloy with 0.02 to 2% C. All
commercial plain-carbon steels contain about 0.3 to
0.9% manganese along with sulfur, phosphorus,
and silicon impurities.
Hardenability : The ease of forming martensity in a steel upon
quenching from the austentic condition. A highly
hardenable steel is one which will form martensite
throughout in thick sections. Hardenability should
not be confused with hardness. Hardness is the
resistance of a material to penetration. The
hardenability of a steel is mainly a function of its
composition and grain size.
Jominy Hardenability Test : A test in which a 1 inch (2.54 cm) – diameter bar
by 4 inch (10.2 cm) line is austenitised and then
water-quenched at one end. Hardness is measured
along the side of the bar up to about 2.5 inch
(6.35 cm) from the quenched end. A plot called the
Jominy hardeability curve is made by plotting the
hardness of the bar against the distance from the
quenched end.
White Cast Irons : Iron-carbon-silicon alloys with 1.8-3.6% C and
0.5-1.9% Si. White cast irons contain large
amounts of iron carbide which make them hard and
brittle.
Gray Cast Irons : Iron-carbon-silicon alloys with 2.5-4.0% C and
1.0-3.0% Si. Grey cast irons contain large amounts
of carbon in the form of graphite flakes. They are
easy to machine and have good wear resistance.
Ductile Cast Irons : Iron-carbon-silicon alloys with 3.0-4.0% C and
1.8-2.8% Si. Ductile cast irons contain large
amounts of carbon in the form of graphite nodules
(spheres) instead of flakes as (about 0.05%) before
the liquid cast iron is poured enables the nodules to
form. Ductile irons are in general more ductile than
gray cast irons.
Malleable Cast Irons : Iron-carbon-silicon alloys with 2.0-2.6% C and
1.1-1.6% Si. Melleable cast irons are first cast as
white cast irons and then are heat-treated at about
940oC (1720
oF) and held about 3 to 20- h. The iron
103
Engineering Alloys
(Ferrous and Non-Ferrous) carbide in the white iron is decomposed into
irregularly shaped nodules or graphite.
2.24 ANSWERS TO SAQs
Please refer preceding text for answers of all the SAQs.