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PHYSICAL METALLURGY OF STEELS

Asok Joardcr

ScientistNational Metallurgical Laboratory

Jamshedpur 831 007

INTRODUCTION

Steel is an alloy of iron and carbon and with or without one or

more than One of the alloying elements such as silicon, molybdenum,

tungsten, chromium, nickel, vanadium, manganese etc. In addition to

the above it contains trace amount of sulfur and phosphorus. Around

90% of the total amount of metals used by men for various applicationsare ferrous alloys which Includes mainly steel and cast iron. The

reasons of its wide spread applications are mainly as follows :

(1) It is cheaper

(2) Availability of iron bearing minerals in nature

(3) Iron has the ability to be. alloyed with many metals

(4) A wide range of properties are possible to obtain by controlling

its microstructure through a very easy heat treatment method

The extensive range of mechanical properties could be obtained

from 200 MPa to as high as 2000 MPa by introducing alloyingelements and heat treatment.

In addition to the above steel can also be used in variousenvironments, at. elevated temperatures and aL sub zero temperatures

without impairing its toughness, corrosion resistance and strength.

Therefore, the study of steel is still required because it is most

widely used metallic materialS even today In a large variety of

applications, both simple and sophisticated. In the present lecture

notes a various possibilities of microstructures in steels are discussed

briefly in the following sections.

MICRO-CONSTITUENTS IN STEELS

The three allolroic forms of iron are (a) delta, ferrite (8), (b)

austenite (y) and (c) alpha ferrite (a). These three forms have been

named in decreasing order of transformation temperature. Ferrite in

steel observed at higher temperature has been designated as delta

ferrite (6) and at lower temperature designated as alpha (a). The

crystal structure of both 8 and a is body centered cubic. On the other

hand austenite is face centered cubic form of iron and has been named

as austenite (y) . Auslenite in pure iron exists between 910 to 1400°C.

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Cementite (FeaC) or iron carbide : Iron and carbon form an inter-

metallic compound, which is very hard and brittle, and represented by

the formula Fe3C. The crystal structure of cementite is orthorhombic.

Many different kinds of ferrite are found to form in steels. Some

of the commonly observed ferrite microstructures are described

below :

Grain boundary allotriomorphs : Allotriomorphic ferrite crystals

nucleate at prior austenite grain boundary [Fig.1(a)] and tends to grow

along the austenite boundaries at a rate faster than in the direction

normal to the boundary plane.

Widmanstatten Ferrite (WF) WF are plates or wedgeshaped crystals

which develop into the interior of a matrix grain from the vicinity of

the matrix grain boundaries (Fig.1(b)).

Acicular Ferrite (AF) : The latest known micro-constituent in steels

and very frequently observed in weld deposit. The term acicular means

shaped and pointed like a needle. Acicular Ferrite is defined as a

highly substructured, non-equiaxed phase that forms on continuous

cooling by a mixed diffusion and shear mode of transformation at a

temperature slightly higher than the upper bainite transformation. AF

is different form bainitic ferrite with respect to prior austenite grain

boundary network which is completely eliminated in AF. This

structure is very fine grained.

Pearlite : It is a mixture of two phases i.e. ferrite and carbide: in which

each micro-constituent takes shape like lamella. Pearlite is eutectoid

reaction product of ferrite and carbide which is formed by diffusionalmechanism (Fig. 1 c).

Bainite : The microstructural definition of bainite is non-lamellar

eutectoid reaction product of ferrite and carbide. In steel bainite is

formed below the nose of the time - temperature - transformation

(TIT) diagram and upto the martensite start temperature. Two

classical variants of bainite are upper bainite which is formed at higher

transformdtion temperature and lower bainite which is formed above

the martensite start temperature.

Martensite : The martensitic transformation is a solid state phase

transformation that occurs by the co-operative movement of atoms by ashear mechanism without involving any diffusion of atoms i.e.

transformation occurs without any change in the concentration. The

crystal structure of martensite in steels is body centered tegragonal

and degree of tetragonality depends on the percentage of carbon in

steels.

THE IRON-CARBON DIAGRAM :

The micro-constituents in steels are discussed in the previous section.

Alloys of iron and carbon will be discussed in details with the help of a

phase diagram known as iron-carbon diagram. It is not a complete

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diagram in that it is constructed only for carbon concentration of upto

6.67%, the composition of Fe3C, or cementite. The iron-carbon

diagram containing carbon concentration higher than 6.67% has very

little commercial application. This diagram is not a true equilibrium

diagram because cementite is not an equilibrium phase and under

proper conditions cementite decomposes to graphite. The iron-carbon

equilibrium diatram is shown in Fig.2.

Above 1540°C iron remains in the liquid state and below this

temperature it starts Lo solidify to a body centered cubic lattice,

known as delta ferrite (8). The iron carbon diagram shows three

horizontal lines each representing a reaction point involving three

phases and occurring at constant temperature. These reaction points

are :

(a) Peritectic point at 0.17% C and 1493°C temperature

(0.09%C) + liquid (0.53%C) (0.17%C)

(b) Eutectic point at 4.3% carbon at 1130°C

Liquid (4.3%C) (2.11%C) + Fe3C (6.67%C)

(c) Eutectoid point at 0.8%C at 723°C temperature

y (0.8%C) (0.02%C) + Fe3C (6.67%)C

Steels containing less than 0.8% C are called hypoeutectoid

steels, and those containing more than 0.8% C are calledhypereutectoid steels. The iron-carbon alloys above 2% C are generally

termed as cast iron. In general cast irons are alloys of iron, carbon and

silicon. Details of the cast irons will be discussed later in a separatelecture.

Better understanding of the iron-carbon diagram is possible

considering a steel containing 0.25% C. This can be obtained by

drawing a line between 0 and 0.5%C. Above around 1520°C steel is in

liquid form. As the temperature decreases solidification starts with the

formation of 5 ferrite. It. continues to form until the temperature

reaches to 1493°C. AL this temperature periteclic reaction occurs. As

the cooling continues more austenite will form until a temperature

(1480°C) is reached at which the solidification is completed. Below

this temperature the entire mass will be composed of austenite (y), fccform of iron. On further cooling austenite phase does not change until

temperature reaches to 815°C. At this temperature ferrite starts to

form and continues to form until a temperature of 723°C is reached. At

this temperature the remaining austenite disappears, and transforms

to a eutectoid mixture of ferrite (a) and Fe3C, cementite, called

pearlite. Ferrite that forms prior to the eutectoid point is called

proeutectoid ferrite. Similarly, cementite that forms prior to the

eutectoid point is called proeutectoid cementite. Ferrite and

cementite present in the pearlite Is called eutectoid ferrite andcementite respectively. On further cooling of this alloy, a small amount

of cementite will precipitate from the errite because of the

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decreasing solubilty of carbon in iron wth the decreasing

temperature. The microstructure of the steel at 0.25% C composition

consists of ferrite and pearlite.

The microstructure of the other alloys in the range of 0 to 2% C

can be interpreted in a similar manner, the microstructure ofeutectoid steel is entirely pearlite. However, alloys with more than

eutectoid composition exhibit proeutectoid cementite in place of

proeutectoid ferrite. It is clear from the iron-carbon diagram that iron

containing upto 2.11% carbon can be heated to a one phase structure,

while the alloys containing more than 2.11% C always exhibit a two

phase -y + Fe3C eutectic structure.

If an alloy with 3% carbon is allowed to cool from its liquid state

austenile starts lo form at 1300°C. Austenite that forms from liquid

prior to the formation of eutectic is called proeutectic austenite. On

further cooling formation of austenite continues until the temperature

reaches to 1130°C. At this temperature liquid eutectic reaches to1.3%C. This liquid eutectic transforms to austenite and cementite,

called ledeburite. Proeutectic austenite contains 2.11% carbon, on

further cooling from 1130°C to 723°C, will precipitate cementite,

because of decreasing solubility of carbon in austenite. Cemen tite

precipitates from austenite prior to eutectoid reaction is called

proeutectoid cementite. The proeutectoid cementite has been formed

from proeutectic austenite, therefore, this cementite can bedesignated as procut.ectic cementite. The austenite, of eutectoid

composition will transform to pearlite.

The phase transformation so far discussed on iron-carbon

diagram, the foundation on which all heat treatment of steels is based.

This diagram describes the temperature-composition regions where

various phases in steels are stable. However, this diagram does not

convey how various arrangements of phases or microstructures are

produced by austenite transformation due to different rates of cooling.

Similarly this diagram does not convey the austenite formation rate

due to different rate of heating. Therefore, some ideas arc necessary to

know Ole lime required for Hie Iranslimnalion of austenite at different

cooling rates. Initially a basic concept of the transformation of

austenite can be obtained at constant temperature, called isothermal

transformation (IT) or time-temperature-transformation (TTT). Fig.3

shows the - DT diagram of 0.89% carbon and 0.29% manganese steel.The microstructure that is formed by the transformation of austenite

at subcritical temperature is dependent upon the temperature of

transformation. If the transformation is occurred just below the

critical temperature, 723°C (AC1), coarse pearlite is formed. On

further decreasing the isothermal transformation temperature down

to about 550°C, will produce fine lamellar structure. However, below

about 550°C, the ferrite and carbide mixture does not show lamellar

form but instead, the eutectoid mixture exhibits a non-lamellar

mixture, termed as bainite. Lamellar form of eutectoid mixture,

pearlite is formed by diffusional mechanism while the formation

mechanism of bainite is partly controlled by diffusional mechanism

and partly controlled by diffusionless shear mechanism. When

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austenite transforms below 200°C, face centered cubic structure is

completely transformed to body centered tetragonal structure of

ferrite. Transformation at lower temperature is occurred by

diffusionless shear mechanism and the transformation product is

called martensite.

Commercial application of '1 1̀'1' diagram is limited to special

cases only. In almost all cases, steel is heated into the austenitic range

and continuously cooled to room temperature. Therefore, the concept

of continuous cooling transformation diagrams to predictmicrostructural development is very important. The difference

between isothermal and continuous cooling transformation diagrams

can be clearly understood form Fig.4 for an eutectoid steel. It is clear

from the Fig.4 that although curves indicating beginning and end of

transformation resemble similar curves of the IT diagrams, they have

shifted to lower temperatures and longer times. Fig.5 shows differenttransformation products due to different. cooling rates. On Increasing

the cooling rates, the transformation products varies from coarsepearlite-fine pearlite to finally fully martensite. The minimum cooling

rates at which y transforms to completely martensite is called critical

cooling rate.

The iron-carbon, IT or CCT diagram so far discussed only

applicable to plain carbon steels. IIowever, the y-transformation

behaviour changes significantly with the addition of alloying elements.

THE EFFECT OF ALLOYING ELEMENTS

Alloying elements can influence in two ways :

(a) By expanding y-field; these elements are called

y-stabiliser i.e. C, N, Ni Mn etc.

(b) By contracting y-field; these elements are ferrite stabiliser.

i.e. Cr, Mo, W, V etc.

Irig.6 shows how tlic iron-carbon equilibrium diagram changes

its shape with the influence of alloying elements. The effect of

different alloying elements on Fe-C diagram is discussed below :

Type 1 (Open y-field)

The important alloying elements are nickel, manganese andcobalt. Ni and Mn, if added in sufficiently high amount, completely

eliminate bcc, a-iron as shown in Fig.6.

Type 2 (Expanded y-field)

Carbon and nitrogen are the most important element.

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Type 3 (Close rfield)

Alloying elements in this group are silicon, aluminium,phosphorus, titanium, vanadium, molybdenum, chromium. These

elements encourages the formation of bcc iron, y-area contracts to a

small closed loop.

Type 4 (Contracted rfield)

Here y-loop is strongly contracted but is accompanied by

compound formation. The alloying elements in this group are, boron,

tantalum, molybdenum and zirconium.

Alloying elements also change the transformation temperatureand euteetold composition of iron-carbon diagram. Fig.7 shows the

effect of alloying elements on eutectoid temperature and eutectoid

composition.

Similarly alloying elements influence strongly IT and CCT

diagram i.e. most of the common alloying elements shift IT diagram

towards right side, thus increase the hardenability. The effect of

alloying elements on isothermal transformation and continuous cooling

transformation diagrams are shown in Figs. 8 and 9 respectively.

Hardenability is defined as susceptibility to hardening by rapid cooling

or the property in ferrous alloys that determines the depth and

distribution of hardness produced by quenching. In another way

hardenability can be "as the capacity of a steel to transform partially or

completely from austenite to some percentage of martensite at a given

depth when cooled under some given condition".

HEAT TREATMENT

Heat treatment can be defined as the heating or cooling of steels

in solid state to obtain a desirable mechanical properties by producing

different microstructures during heating or cooling cycles. A large

variety of microstructures could be obtained in steels by heat

treatment. The iron-carbon, IT and CCT diagrams which have been

discussed earlier are the foundation on which entire heat treatment

processes are based.

Hardening : Hardening consists of heating the steels to a sufficiently

high temperature and time to produce austenitic structure and then

quenching it to room temperature to obtain a fully martensite

structure. Therefore, hardening process can be divided into two steps:

Formation of Austenite and Quenching of Austenite.

Formation of Austenift : The transformation from ferrite-carbide to

austenite is accomplished by the dissolution of carbide particles. This

process is a diffusion controlled which is dependent on time and

temperature. Another factor which influences the time required for

austenitising is the composition of steels. In a steel, if appreciable

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amount of alloying elements are present which are carbide former i.e.

strong affinity to carbon than iron, then a carbide phase will not be

simply Fe3C, but a complex alloy carbide, the dissolution rate of alloy

carbide is slow therefore, diffusivity of the carbide former at that

temperature will be considered rate controlling. The time required to

austenite formation and carbide dissolution depends on the original

ferrite-carbide matrix as shown in Fig.10. For conventional hardening

treatment, hypoeutectoid steel is generally heated about 30°C above

AC3 temperature, to transform all ferrite and carbide into austenite. In

hypereutectoid steel, the austenitising temperature is chosen between

AC1 and ACm.

Quenching : Quenching is the process of cooling the steels in

controlled manner to obtain a fully martensite structure. The common

quenchants are, brine, water, oil, salt, air etc. Generally, water is used

for plain carbon steels while in the case of alloy steels oil, salt and air

are used as quenchant.

Tempering of steel : Tempering is defined as the heating of hardened

steels in the temperature range of 150 to 700°C to improve the

mechanical properties. As hardened microstructures are usually a

mixture of austenite and martensite. Both the microconstituents are

unstable and changes very slowly at room temperature. Martensite is,

although a very hard phase also very brittle phase. The major objective

of tempering is reduction of brittleness or increasing the toughness of

a hardened steels. The as-hardened martensite is a complex structure

i.e. supersaturated solid solution of carbon in body centred tetragonal

crystal lattice. The tempering of plain carbon steels can be divided

into the following stages :

First stage of Tempering : The temperature range for this stage is

between 100 to 250°C. Epsilon (e ) carbide is formed in this stage and

carbon content of martensite is also decreases.

Second stage of Tempering : The temperature range for this stage is

between 200 to 300°C. Retained austenite transforms to bainite or

ferrite and carbide in this stage.

Third stage of Tempering : The temperature range for this stage is

from 100°C to 350°C. c-carbides dissolve and picks up additional

carbon from martensite and transforms into very fine plate or rod of

cementite (Fe3C).

Fourth stage of Tempering : The temperature range for this stage is

from 300°C to 700°C. Cementite particles starts coarsening between

300 to 400°C, while spheroidisation of cementite particles starts on

further increasing temperature up to 700°C. The changes of hardness

with tempering temperature of plain carbon steel is shown in Fig.11

and hardness as a function of time at three tempering temperatures is

shown in Fig.12.

Tempering of alloy steels : The alloying elements commonly used Insteels are Cr, Mo, W, V, Ni, Si etc. The tempering behaviour of alloy

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Highly alloyed

austenite

(FeCr)23C6

lower alloyausenite

(MoW)6C

lower alloy

austenite

Highly alloyed ndissolvedmartensite arbides

c - carbide MoW)6C and

Fe3C 4C3

(Fe,Cr)23C6

(MoW)6C

(Secondary

Hardening)

200

425

540

martensite differs significantly from plain carbon martensite. The

alloying elements delay the softening either by entering into the

carbide or by segregating at the carbide/ferrite interfaces.

The changes upon tempering of alloy steels can be described in

tabular form

Tempering tructural Components

Temp. °C

To martensite

on cooling or

Ferrite and

carbide on

Isothermal

holding

(Secondary

Hardening

Growth of

carbides

No change

The changes of hardness with tempering temperatures of alloy steel

martensite is shown in Fig.13.

Tempered Martensite Embrittlement

When martensite is tempered in the temperature range of 250

to 400°C, the loss in impact toughness is observed as shown in Fig.14.

In high purity steels, this phenomena is occured due to thetransformation of retained austenite to thin plates or cementite on

tempering in the range of 250 to 400°C. This loss in toughness

observed after tempering is more pronounced if phosphorus

segregates at the prior austenite grain boundaries in the as quenched

martensite. The interaction between phosphorus and cementite is

responsible for this loss in toughness. The appearance of fracture

surface is intergranular type.

Temper Embrittlement (TE) : When many steels tempered in the

range of 500 - 650°C or very slowly heated or cooled in thistemperature range loss in toughness occurs. The presence of temper

embrittlement is detected by measuring impact transition

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temperature. Impact transition temperature increases significantly as

shown in Fig.6.

The important impurities such as antimony, phosphorus, tin,

arsenic, bismuth play a greater role in temper embrittlement. Very

small amount of these elements even in the order of 100 ppm (0.01%)

or less is also detrimental. Plain carbon steels are not susceptible to

TE if manganese content is kept below 0.5%. Alloying elements such

as Ni, Cr, Mn are very harmful in this regard. Therefore, interaction of

impurities and alloying elements is responsible for segregation of

impurities at the prior austenite grain boundaries. Molybdenum

reduces the segregation of impurity elements and suppresses

embrittlement by scavenging phosphorus through the formation of 'a.

Mo-P compound.

Martempering : Martempering or interrupted quenching process

consists of holding the austenitised steel above the Ms temperature

usually by quenching in a salt bath and then air cooling to roomtemperature. The schematic diagram of martempering process is

shown in Fig.16. Because of interrupted cooling of austenitedistortion, residual stresses, and cracking are almost eliminated by

this treatment. The final microstructure obtained after martempering

is martensite.

Austempering : Austempering is a process in which austenitised steel

is isothermally hold in molten salt bath at a temperature above Ms.

The final microstructure obtained after austempering is bainite. The

heat treatment cycle of austempering is shown in Fig.17.

Annealing : Annealing is a heat treatment process and can be definedas the heating a steel to the single phase austenite field and then

slowly cooling to room temperature. The cooling rate may be of the

order of 10-2°C/second and this can be accomplished by furnace

cooling.

Spheroidizing Annealing : Spheroidizing annealing treatment is given

to a steel to obtain a structure which consist of uniform dispersion of

carbides in the form of spheroids in ferrite matrix. The heat treatment

cycle for spheroidizing annealing treatment is shown in Fig.18.

(a) Long time isothermal holding below AC1 temperature i.e.

subcritical annealing

(b) Repeated heating and cooling of steels above or below the ACi

temperature

(c) Heating just above AC1 temperature and then slow cooling.

The rate of spheroidisation depends on the initial

microstructure and alloying elements in steels.

Normalising : Normalising process is defined as cooling the steel in air

from austenitising temperature. Very fine grained ferrite-carbide

structure is obtained after normalising treatment. The heat treatment

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cycle for normalising is shown in Fig.19. The austenitisingtemperature for normalising is same as that for annealing.

Function of alloying elements, : Alloying elements are added in plain

carbon steels to modify mechanical properties of plain carbon steels.

Effects of alloying elements on iron-carbon, TTT and CCT diagrams

have already discussed. Effect of alloying elements on the softeningduring tempering have also been discussed. Here, effects of alloying

elements on mechanical properties will be discussed. Common

alloying elements in steels are : C, Si, Mn, Ni,Cr, Mo, V, W etc. Alloyingelements are used in steels due to the following reason :

1. To improve hardenability

2. To improve resistance to softening

3. To improve resistance to corrosion and oxidation

4. To improve high temperature properties

5. To improve resistance to abrasion

6. To strengthen steels.

Nickel steels : Extensively used for engineering purposes up to about

5%. It improves strength and toughness. It is used in larger amount in

stainless and maraging steels. Nickel does not form carbides. Presence

of nickel in steel makes iron carbide less stable. It reduces the co-

efficient of thermal expansion.

Manganese : It increases strength and toughness when added 1.5 to

2%. Manganese is widely used for deoxidation and desulphurisation:

Manganese is not a strong carbide former but it has a very powerful

effect to stabilize carbide.

If manganese content is increased to between 11 to 14% andcarbon 1 to 1.2%, a very tough and wear resistant steel is obtained

known as Hadfield steel.

Chromium : Chromium improves hardenability, strength and wear

resistance. Chromium is carbide stabilizer and forms hard carbides,

Cr7C3 or Cr23C6. These carbides are harder than ordinary cementite,

Fe3 C. The main disadvantage in the use of chromium is its tendency

to promote grain growth and temper embrittlement. With 1% carbon

and 1.4% chromium, steel is used to manufacture ball bearings. Higher

percentage of chromium is used in stainless steels.

Molybdenum : One of the main uses of molybdenum in low alloy steel isto reduce the tendency of temper brittleless. Molybdenum is a stable

carbide former, it forms Mo3C, Fe3Mo3C and Fe21Mo2C6. Creep

resistance is improved because of the stability of the carbides.

Tungsten : Tungsten forms a very hard and brittle carbides W2C, WC

and a double carbide Fe4W2C. Tungsten is mainly used in tool and die

steels. Tungsten is also used in heat resistant steels in which it

improves creep strength. With suitable heat treatment tungsten

bearing steels develop high temperature i.e. hot hardness up to 600°C.

It is extensively used in high speed steels.

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Vanadium : Vanadium is a strong and stable carbide former. It forms,

VC or V4C3 type carbide. It is also a very important grain refiner. It

stabilises martensite and bainite on heat treatment. It also inducesresistance to softening at high temperatures. The combine effect of Cr,

Mo, and V in steels will be discussed in brief.

To obtain required properties at higher service temperature for

long time proper alloying element addition and heat treatment are

very important. This means secondary hardening reactions must be

used to achieve required mechanical properties at higher service

temperatures.

Addition of chromium only can not impart to optimummechanical properties. Chromium containing steels retard softening

during aging or long term service, but it is less effective beyond 500°C

as shown in Fig.20. Secondary hardening by molybdenum and

vanadium additions markedly increases the tempering resistance as

shown in Fig.2 1

The MoC2 carbide can dissolve both chromium and vanadium.

Chromium causes Mo2C less stable, whereas vanadium stabilises Mo2C.

However, too much vanadium cannot be used in steels due to its low

solubility and higher austenitising temperature is required for

dissolving vanadium carbide. Therefore, steels containing Cr, Mo and

V are widely used in power plant components.

CLASSIFICATION OF STEELS

This means a systematic arrangement or division of steels into

groups on the basis of some common characteristics. Hereclassification of steel has been described on the basis of chemical

composition. Plain arbon steels are classified on the basis of the carbon

content. According to the carbon content it can be classified as low

carbon, medium carbon and high carbon steels. Low carbon steels

contain carbon up to 0.25% C with manganese up to 1.5% Mn.

Medium carbon steels contain carbon ranges from 0.25 to 0.60% and

manganese from 0.60 to 1.65%. High carbon steels contain carbon

ranges from 0.6 to 1.25 with manganese ranging from 0.30 to0.90 . In addition to the above, ultra high carbon steels contain

'carbon ranging from 1.25 to 2.0%C.

High strength low alloy steels

These are commonly used in oil and gas line pipe, ships, off-

shore structures, automobiles, highway equipment and pressure

vessels etc. The chemical composition of HSLA steels are; Carbon

(0.05 to 0.25%), manganese content up to 2.0%. Small amount ofother alloying elements like chromium, nickel, molybdenum, copper,

nitrogen, vanadium, niobium, titanium and zirconium are used in

various combinations. Mcro alloyed steels, with very small additions ofalloying elements such as niobium, vanadium and or titanium for

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•

refinement of grain size and or precipitation hardening are widely

used in recent years.

Low alloy steels

Total alloy content in the low alloy steels can range from 2% to

about 5%, in medium alloy steels, total alloy content is in the range of

5 to 10% and high alloy steels contain total alloy content more than

10%.

Low alloy steels can be classified as nickel steels, nickel-

chromium steels, molybdenum steels,. chromium-molybdenum,chromium-molybdenum-vanadium steels etc.

Therefore, carbon and low alloy steels groups comprise a largenumber of steels that differ in chemical composition, strength, heat

treatment, corrosion resistance and weldability. The classification of

steels given in Fig.22.

STAINLESS STEELS

Stainless steels are a large family of specialised steels in which

the first and the most important property is the corrosion resistance.

Therefore, stainless steels are widely used in various industries where

corrosion and oxidation resistance in operating environment is

required. Stainless steels are also used widely for high temperature

creep resisting or heat resisting applications.

Classification of stainless steels

Stainless steels can be classified into three major categories :

(a) Austenitic

(b) Ferritic

(c) Martensitic

Austenitic stainless steels contain 18-25% Cr with 8-20% Ni and

low carbon (0.1% C maximum). The crystal structure of austenitic

stainless steels is FCC. The structure of 18% Cr steel can be made fully

austenitic by the addition of 8% Ni. Corrosion resistance of 18% Cr -

8% Ni is superior to that of the ferritic or martensitic steels.

Ferritic stainless steels contain 15-30% Cr and low carbon, with

some molybdenum, niobium or titanium. Nickel is not added in ferritic-

stainless steel. The crystal structure of this type of stainless steel is

BCC and they remain ferritic over the whole solid state temperature

range.

Martensitic stainless steels contain 12-17% Cr and 0.1 to 1.0%

C. These steels are austenitic in the temperature range of 950 to1000°C but transform to martensite on cooling.

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Phase diagrams for stainless steels

Fig.23 shows Fe-Cr phase diagram, chromium is ferrite stabiliser

and tends to stabilize a-iron and its high temperature counterpart.

which merge to form closed y loop. Beyond 16% chromium there will

be no austenite in binary Fe-Cr alloys. From Fe-Cr diagram it is

observed that a-phase forms only at chromium contents higher than

17%, a-phase can form in the temperature range between (510 to

1000°C). At lower temperature rate of precipitation is very slow,

a—phase precipitation is accelerated by cold working or in presence of

ferrite forming elements such as silicon, niobium and molybdenum.

Elements dissolve in a-phase and extend the temperature range over

which it is stable a-phase is a hard iron-chromium compound. In

addition molybdenum can lead to the formation of iron-chromium-

molybdenum compound, Chi-phase, which is stable over a wider rangethan a-phase.

In ferrite steels, molybdenum, a strong ferrite former, is added

to increase corrosion resistance, whilst niobium and titanium are

added to stabilize the steels against intergranular corrosion by

lowering the carbon and nitrogen dissolved in the steel.

The martensitic grade of stainless steels can be understood with

the help of Fe-12% Cr-C diagram as shown in Fig.24. Chromium

restricts the y-field of the Fe-C diagram. Eutectoid composition

decreases to 0.7% carbon.

When FCC nickel is added to iron, as discussed earlier, only

austenite can be obtained at all temperature, the effect of increasing

Ni content on Fe-Cr alloys are shown in Figs. 25 & 26. These diagrams

clearly indicate that higher Ni contents would tend to give a more

stable austenite.

The development of a-phase

Austenitic stainless steels containing high proportion of ferrite

forming elements, when pass through the temperature range from

600-900°C a-phase is expected to form. a-phase is an extremely

brittle constituent. Therefore, stainless steel will suffer a substantial

loss of ductility when heated in the a-phase forming temperature

range.

Chromium carbide precipitation

When austenitic stainless steels of 18-8 composition (18% Cr -

8% Ni) is heated within the temperature range of 500 to 600°C, for

any appreciable length of time, chromium rich carbide will precipitate

along the grain boundaries. Chromium rich carbides are expected toform in unstabilised steels. The carbide precipitation process is called

"Sensitization". Within the sensitisation range, carbon atoms diffuse to

1)-13

8/13/2019 D1-36


the grain boundaries combine with chromium atoms to form

chromium carbide. As a result of chromium carbide precipitation, the

area adjacent to the grain boundaries generally deplete of chromium

content. The low chromium content area become anodic to the bulk of

the metal. Therefore, these areas are preferentially attacked by the

corrosive media. Sensitisation can be avoided in following manner.

Very low carbon steels i.e. below 0.03% is recommended to

avoid carbide precipitation.

Stabilisation of stainless steel with niobium or titanium is not

always completely effective under corrosive conditions. If a stabilized

steel is heated to above 1100°C for titanium bearing steels and above

1300°C for niobium bearing steels, carbides of titanium or niobium

goes into solution and consequently, chromium carbide precipitation

may take place along the grain boundaries if the steel is cooled slowlyor held for long time within the carbide precipitation range.

Therefore, the degree of chromium carbide precipitation can be

minimised effectively by reducing the carbon content in steels.

Steels for Thermal Power Plants

0.5 Cr-0.5 Mo-0.25V. 2.25Cr-1Mo. 1Cr-lMo-0.25V and 1Cr-

0.5Mo steels are widely used in thermal power plants. Carbon content

of this group of steels are from 0.1 to 0.3%. A large variety of

microstructures could be obtained by heat treatment. The heat

treatment cycles of the four steels are discussed here.

1Cr-lMo-0.25V

lCr-lMo-0.25V steels are most widely used for powergenerating plant components. such as bolts. steam turbine casingshigh pressure (HP) and intermediate pressure (IP) turbine i c -_ors etc.This steel is heat treated to obtain predominantly upper '_Dainiticmicrostructures with fine dispersion of vanadium carbides. Rotorforging are austenitised either at 1010°C or 955°C followed by eitherair cooled or oil quenched. In UK. the rotors are oil quenched fromaustenitising temperature to achieve better toughness while .n USA,

the rotors are air cooled from austenitising temperature :2 achievebetter creep strength. Air cooled or oil quenched rotors a_re thentempered in the temperature range of 650 to 700°C. Co sinuouscooling transformation for this steel is shown in Fig.27. Tthaperingbehaviour of :rotor steel is shown in Fig.28. The as hea: -_reatedmicrostructure consists predominantly Fe3C either plate -.:vpe or

rounded and fine spheroidal vanadium carbide with very little amountof Mo2C.

Tensile properties of Cr-Mo-V rotor steel austenitized at two.afferenttemperatures is given below :

D- 14

8/13/2019 D1-36


Austenitizing Temperature : 1010°C

Ultimate Tensile Strength ((MPa) : 940Yield Strength MPa) : 780

Elongation (%) : 13

Reduction of Area (%) : 24.8

Austenitizing Temperature : 955°C

Ultimate Tensile Strength (MPa) : 775Yield Strength MPa) : 620

1% Cr-0.5%Mo Steel and 2.25Cr-0.1Mo Steel

This steel is extensively used in power plant for superheater

tubes and headers. The steel is austenitised in the temperature range

of 900 to 950°C followed by either air cooled or furnace cooled. Air

cooled steel is then tempered in the range of 650 to 730°C. The

continuous cooling transformation diagram for the steels are shown inFig.29. Depending upon the cooling rate a large variety of

microstructures such as, ferrite-pearlite, ferrite-bainite or bainite

could be obtained.

The steels so far discussed are mainly used upto 550°C.

However, for increasing the operating temperature upto 650°C. new

materials i.e. high chromium ferritic steels have been developed for

advanced power plants. The base composition of new rotor steels is as

follows :

C 0.1-0.15%, Si : 0.03-0.05%

Mn : 0.5%; Ni : 0.5-0.8%Cr : 10-11%; Mo : 0.15-1%; W: 2%

V : 0.2%; Nb : 0.08%: N : 0.025%

and other alloying elements like. cobalt about 3% and boron about

0.015% may also present in We steel. The austenitising temperature

for this steel is in the temperatures ranges of 1000 to 1050°C followed

by air cooling from austenitising. Carbon content of this steel is

maintained very low level i.e. 0.12 - 0.13% to minimise the carbide

formation. The steel is then tempered in the temperature range of

700 to 720°C.

For boiler tubes 9Cr-0.1Mo or 12Cr-0.1Mo steels exhibit

excellent high temperature properties. The chemical composition for

boiler tubes are 9-12% Cr, 0.05% - 0.2%C, 0.1-0.3%V, 0.03%-ONb. Mo+ 0.5W > 1.0%.

The steel is austenitized in the range of 1050 to 1150°C

followed by either air cooled or oil quenched. Then tempered in the

temperature range of 750 to 800°C and then air cooled.

For casing, valves, turbine blades and bolts etc., 9- 12% Cr,

0.15%C, 0.1% Ni, 0.2% Mo. 2% W. 0.2% V, 0.08% Nb, 0.025% N and

0.015% B steel could be used. This steel have higher carbon content

D-15

8/13/2019 D1-36


than the steels for boiler tubes. It is not possible to cover all grades of

steels in a single lecture. Some selected references were given in

Bibliography for further study,

BIBLIOGRAPHY

1. Structure and Properties of Alloys : Robert MBrick, Robert B.

Gardon and Arthur Philips.

2. Alloying Elements in Steels : Edgar C. Bain and Harold W. Paxton.

3. Materials Science and Technology : Vol.7, Constitution and

Properties of Steels: Volume Editor: F.B.Pickering; Edited by

R.W.Cahn, P.Haasen and E.J.Kramer.

4. Physical Metallurgy of Iron and Steel : Rajendra Kumar.

5. Engineering Metallurgy : Part-I; Applied Physical Metallurgy :

Raymond A.Higgins.

6. Principles of Heat Treatment of Steel : George Krauss.

7. Heat Treatment of Ferrous Alloys : Charlie R. Brooks.

8. Physical Metallurgy and the Design of Steels - F.B. Pickering

9. The Physical Metallurgy of Steels : W.C. Leslie.

10. Steels : Microstructure and Properties : R.W.K. Honey Combe.

11. Steels : Metallurgy and Applications : D.T.Llewellyn.

8/13/2019 D1-36


;a) Gran boundary allotr omorphs of Ferrite,

0.29 C, reacted 13 Sec, at 750°C, 1000 X;

(b) Primary side plates of Ferrite; 0.29 C,

reacted 90 Sec at 725°C, 2000 X, c) Pear-

lite in a 0.8 C steel, slowly cooled from

austenite (annealed,.

D-1

8/13/2019 D1-36


( 1 , 3 4 1 1

200

: R O N . G R A P H I T E E O U I L I O R I U MION- Fe3 0 E a u l u a n i u m

S O U R C E : B A S E D O N M E T A L S H A N D B O O K , 0 1 1 1E D I T I O N . V O L . 8 , " M E T A L L O G R A P H Y ' 5 T R U C T U E S ,A N D P A S E D I A G R A M S " ( A F T E R J O I I N C H I P M A N ) .

FIGURE

0 5 . 0 5 . 0 . 5 . 0 . 5 . 0 . 5 0T E M P E R A T U R E . F

lion-co:bon equilibrium diagram (after Metal Progress Data book © A 7 7 11 Society of Metals 19771.

1 . 4 0 0

1 . 3 0 0

1 , 2 0 0

1 , 1 0 0

1 , 0 0 0

2,600

L I Q U I D + G R A P H I T EF e 3 C 2 ,400

L I Q U I D U S( C A L C U L A T E D ) /

— .= - 1 2 , 2 0 0

4 . 3 0 % .000-

/L I Q U I D + F 1 :3 C

\ 0 . 1 7 %— D E L T A

I R O N

A U S T E N I T C( G A M M A : R O N )

( 2 . 7 2 3 )

— S O L I D U S

L I Q U I D • A U S T E N I T E

2 . 0 0 %

L I Q U I D U S

(2 098)

1 , 1 5 4

912

1,674) •

IA

900 —FERRITE -I.

----AUSTEN:TE ./

CURIE TE:.:P.

( . 1 0 0 7 7 0V'' • •

1 , 4 1 8 ) . 6 8 %

7

7 3 3( 1 , 3 6 0 ) — 1 , 4 0 0

1 . 6 0 01 . 5 3 8

— ( 2 ,8 0 0 )L I Q U I D + D E L T A IRON

0 . 5 3 %1 , 4 9 51 . 5 0 00 . 0 0 %

D E L T A I R O N +I A U S T E N I T E1 , 3 9 4

(2,5411

( 2 , 1 0 9 )1 . 1 4 8

1,800A U S T E N I T E • F e 3 C

— 1 , 6 0 0

— 2.800

L I Q U I D

7 0 2 2 1 0 . 0 2 1 7 7

F E R R I T E( A L P H A I R O N )E G O '

7 2 7

— 1 , 2 0 0

— 1 , 0 0 0 .6 3 0 1

: 0 0

330

2 3 0 ,

' c o

0F e

8/13/2019 D1-36


011N -293

Re 31OlIN 170

At 5

•

1300

1200

1100

000

900

CO O

700

— 100200

I

t A ns-,...\.„ ....,\ % /

1

'C

• 0 10Ac

00"700

Avslenihr (E)itth1t )

1225F

G OO

011N 300

At 4 1 SOTHERMAL TRANSFORMATION DIAGRAM

OF

EUTECT010 CARBON STEEL

C - 0.09% Mit - 0.29%

AusItnilizet1 al 1625•1; Gran Sits 45

Photerricrogh..phs aonely 121.00

At 42

01111 401500

750*F

Mv•suis

(1.kwatiO

81111 415

500

G O O — -A

Re 44 400

EJ DAN 555

at 56

300

olin 570

r Re 50

Z:r1S;#4• ` r :•/.Ms

100 200

50%1.1.111TENS17E

300

WlTEflSlTE

100

0

J. • •• .?: 0i s •-• • • t• _,

DI Iti 682

. .1 :••e''. .'.1 .: Re 66

• .t .1 • ,...:

'''• _L:::

:HO UR D A Y EEK — 0MIN.

I

1 0 02 o 3 o'' o' 0 6

TIME — SECOND S

Fig.3:sothermal tranformation diagram of eutectoid

carbon steel : C-0.89 , Mn — 0.29 .

D— 19

8/13/2019 D1-36


70 E ND-QUE NC H HA RDE NA B ILIT YA

I L0

0

ILI 4Z

/ki

X r.,.,

.,•Q

' 2 , . .E ,1

a) / 0 .5 ; ;.J .5 .0 ', 2.i

5 .40

I-- .: : :STANCE FROM CUENCHED END - INCHES

1400 — //, t

------k-4 . 4 );....... i......... 01

1200

1000

300

600

400

200

- - - &

- Nxr/

\ / -S s -4.7\//(

z .)r \ \/

\ \\ \\

\

AUSTENITE -•MAIITF_NSITI: \" ,\

...1.111•111•t•• ;it'd

•

":•••11stlat rt•.1,1•1••

11.61:ND

--7..=--':::;Olituj Iraiir.lOrtmillon (Iowa:::

. : • . n . . 1 1 o r t n a l trarinfOrin.11,011 clincrxri— -- e'_ ohm, ( ;1 .101t) :

• • •• • • • • ; • : 111 ' . i f lf 111 :111 ,1,1011 1,1t i t t • o i :11• ;k _ _

0rune :;(2cond:‘,

Fig. 4 . Relationship of CT (heavy lines) and IT (light

lines) diagrams of eutectoid steel. Also shown are

Jominy end-quench specimen and four cooling rates

from different positions on the specimen superimposed

on the CT diagram.

8/13/2019 D1-36


Ao,

Ae,

5 5 4 -- END-QUENCH HARDENABILITY TEST

5 0

c c

1 4 0 0

1 2 0 0

1 0 0 0

\\\. \ '

800

\\\'

8 '''.:5". I '1--::'s,;;,AUsid --11 3 P NtT"',,••,'-,',••••

.... .• . / II // //:;N;;; .

;/, ••:Ir ;, // •/ / :.01• .. / • : :\

400 `

\,."` AUSTENIT --MARTENSITEN

. . .-.>,.., ....\A,-

ii)

- - . . . , ? . ,2 0 0 artonsdo nttonnilo, Forrilo nitonsila artnnsitn, F0f1110

andUninno oullo And Dninlo nd thunilo

o o l in g t ra n s l o r m a l i o n d i a g r a msothormal transformation diagram

C o o l in g c u r v e s-o f + + $ ft Transformation during cooling

1

6 0 0

Tmpaue—

F

0 E G E N D

•( r )(r) 404- 1 lz y ,a ,,,.,---,$ .7 30— /

/ 0 0.5 . 0 /.5 2.0 2.5 3.0i i

, / I S T A N C E F R O M Q U E N C H E D E N D - I N C H E S

— 7 —\,\\\\\\\c<<:.....\\.44ss4.c....,> _ 1 =7,

. . . . . - <,0 0 , ? N \\\\\\ " '

, .

:\, , ,6 7 —:-/. . c40' _ : F i E A F q . 1 1 -

.\ \ ,, \ __,, , / Z....../.: - .,\ _ _ . . . .\--'s- N '' "

.... • • • •

2 0 1 0 zTime - Seconds

1 0 3 0'

Fig. 5 CT diagram (heavy lines) for 4140 steel. Also shown are

Jominy end-quench data and IT diagram (light lines).

8/13/2019 D1-36


Ty Pe 1 y pe 3

I i ( j . 6 : r r recto or oiloy tic) oil i liNse d tuirmo

a- 22

8/13/2019 D1-36


1

'

15 1-----=: m„

i

/

M.

I

z0m

U.0.0

z• ill

o 0.7(

rr.

n. 0.60

O0.50

t-f 0 0.40n.

O 0.30U

1 3 0.20

0.10

Ui0

1 0 2 4 6A LLO Y IN G E LE M E N T P E R C E N T

effect 0 alloyinz eloments on the eulccloiLi composition.__ •

1 2 00

Ti W

u60

0 1000

o r :, ')O0

Ui

600

;6 0

CENT

50°0 4 2 •

ALLOYING ELEMENT PER

. Hu. 7. he effect of alloying elements on 'die culecinici iemperature.

D- 23

8/13/2019 D1-36


LL,___7001

00 500 c-

‘cc 4 OOFF

w- 7( 300L

200}-

(Li100

0r

600alrbonScries

Se et / C X . 1 1 4

1- Iiypo.111. 011 1 . 1 1 )I fuleclod 005 0303- Hyper-la 0.30

1 03 4

10 10 1 0 502 3 10 4

10

700

600 /

U 500(-

tif 401

1 — 300

a 200k

• 100

Chearrom S;r75hich CJeda,

Sled :Cr C en1 00 1.13 030

z 026 117 0.30

2

2

0

3 41 0 0 1 0

TIME IN SECS-

Pa:. 8Comparison chart showing influence of chromiumon the isothermal

transformation curve. Left, beginning curves ; right,

ending curves. (After Davenport)

0.51 1010

BEGIN:NG

102

103

1 04

1 05

ENDING

0-5 1 0 02

BEGINING IME IN SECS. NDING

F-tG. g

Comparison chart showing influence of carbon on the isothermaltransformation curve. Left, beginning curves ; right,

ending, curves. (After Davenport)

D-24

8/13/2019 D1-36


5 1020 010020050010002000 .: .2 .5

AIR COOL

OIL QUENCH

WATER QUENCH

5 :0 0 0 00 150 200 300

• 0 0 00150200300

1 1 A 1 1 DI A NI C T C R m m

Fig. 9 CT diagram for plain carbon steel containing 0.38 C and 0.70 Mn.

Transformation and microstructures urn plotted us a function of bar diameter.

A

If 1,1:1

•+:1):

C PER VINCOLING RATE AT 700 C

E ..=;,1, 5

E-F 1

i0 0 0 00:50200393 500

20 0 00 50200300500 m OILmm WATER

-m 0.2 .5 2

I IW A T E R i

`QUENCH j i AIR COOLt

r' OIL t--r; --r l_ QUENCH __.. ._ Ausleni le 11')...

0°; START

: .--............_1 MM

linallig...:-.•1 ----- -...1111=1 .......o i R Z Z 7 2 1 4 3

i i i . _ _ _1..---Piscr. .., . 5 ' . 1laVv.—4 ,7 W W .1 1.,, . . _ .razz-.------. ---...vgrx4....

• ,i. i4 . -..m T FINISH

W r

..mig . .am

, miiiim a il etei

401:111 1 1 1 1 1 1 1 1 1 1 1 1 1 • 1 1 1 1 M I I I A I I } t

1 1 1 1 .NM

}----;

1 1 1 1 1 1 1 1 1 1 1 • 10 1 = 1 1. „ „ .I _L.Manensi le , 1 • 1 1 •1 1 1 1 1 1 1 1 1 1 M 1 1 1 1 1 1 1 • 1 1 1 1 1 1 1 1 1 1 1 1 • 1 •1 1 1 1 1 1 1 • 1 1 1 1

I

:1

•

I •• .. . I:

; I

TRANSFORMATION TEMPERATURE.-C

900i

800 A

700-

600

5 0 0 0 90 00 00 1000 2000 frrl

A:a

Fig.9 CT diagram for on alloy steel with 0.40% C, 1.50% Ni, 1.20% Cr and

0.30% Mo, plotted as a function of bar diameter. Steel was austenitized at 850°C

(1562 °F); previous treatment: rolling, then softening at 650 °C (1202 'F).

D- 25

50 0,

400L

,

300"M

M.

2

:00,

100050020010050 20 10 5

8/13/2019 D1-36


C o m p i l e d f ro n t c a : aLuerssen and Greene (108:1Engel (112)

"i Fletcher and Cohen (100)Bain (113)L i n d se y an d P .0 ( 1 1 .1 )

• Gr at is an d L am o n t ( 1 1 5 )Wellauer (1:5)

A n d O w n D a m

1

70 1-—

60

- 14000000001000

.100-

Tempering temperature, F

200 :100 400 500 000 :60tempurature, C

•. . ? . .

40

7̀:to

I

:."

AN- 2 0.0quenched

100

4 2 3 K3 7 3 K

031L As quenched473K

4 800

HceD

700

•

0.25 .0 .0 G

Tempering time (hours)

. Hardness as a function of time at three

tempering temperatures for martensitc in an Fe-1.22C

alloy.

U 800ai

lo" 780

o.

,2 ). 7601f

0.5 y

740 Pearlite

72 01

820

840

99.5% 7

\

\\\ Homogeneous 7

\

\\ 7with carbon\\

\mhomoseneity \

\ \\ .\

7+ res idual Fe 3 C

N

10 00 000 0.000

Time, sec

FIG. 10. Graph showing time for austenitization of a plain carbon eutectoid steel,

starting with a pearlitic structure obtained by normalizing from 875ce. The first

curve (0.5c70 -0 represents the first visible evidence of austenite; the second curve

(99.5% -0 represents the Jisappearanee of pearlite, although some residual carbides.

remain undissolved. The third line, dashed, represents the approximate tune-

temperature limits for solution of all residual carbide; and the last dashed curve

represents the probable attainment of homogeneity in the austenite. (Meld.)

Fig . 11 ecrease in IICirCirlOf...; with increasing temper-

ing temperature for steels of various carbon contents.

Reference numbers after investigators are from list in

Grossmann and Bain.

D — 2 6

8/13/2019 D1-36


04006008001000 1200

400 00 00 000Tempering temperature, F

Q. Softening, with increasing tenl:ering temperature,i).•It) to 0.•If; ,̀; C steels as influenced hromium content.

Temperature, C

300 00 00

5% Mo

02C0

(„„•,„.1irsi

1200

of

Carbon steel/

47

30

20

1 0

0.3 C

700000060

6

5

5

4

'3 4

0ct

30

25

20

1 5

eiliperwurt,I00200300400500600700

Dr—

- "------.

i

i

1

/2.9% Cr

1

I% Cr/

Carbon steel

1

11 .

0.40 to 0.45%C


Fig.13 6 Softening, with increasing tempering temperature, ofquenched steels as influenced by molybdenum content.

D- 27

8/13/2019 D1-36


1 2 0000 00 00 00 000

i 1

. . ., i , i t

4

A0 1 .M

, - r . - - - - - -1 A „ . . t o : *x.......- " . 4 0 % C —

— \ s " ‹'.., - n o t c hAlk

C h a r p y

I . . 0 . 5 0 % C7Y T ; / .i

p•

s . . ; .i I zod

i-- --I.--


Fig. (q Impact toughness as a function of tempering

temperature of hardened, low-alloy, medium-carbon

steels.

1 0

8

60

40

20

IzoVnchCpa om empaueb

Testing tempera:ore. F

-300 200 100 00

-200 -160 -120 -80 --10 080

Testing temberature. C

Energy abscrted

nembrittIe.c

Isothermy er battled

• Furnace-ccPRO embrittIed

Fig. is hift in impact transition curve to higher

temperature as a result of TE produced in SAE 3140

steel by isothermal holding and furnace cooling

through the critical range.

1 2 0

100

80

Egab o

60

4 0

2 0

0 1

••

sr .._Ar

1

D — 2 8

8/13/2019 D1-36


Ac 3

Ac,

a >

E

ITempered martensi te

Time -- -4-

Fig. 1G Schematic diagram of martempering heat

treatment cycle superimposed on an IT diagram for a

medium-carbon steel.

Fig. 11. Schematic diagram of austempering heat

treatment cycle superimposed on an IT diagram for a

medium-carbon steel.

D-29

8/13/2019 D1-36


Ferrite

and

arbide

Time

Fig.16 Schematic heat treatment cycle for spheroidiz-

ing an air-hardening steel. Martensite forms first and

then is tempered close to the Ac, to produce a spheroid-

ized structure. (Ref 5.5)

Heating oolingcycle ycle

Norma:ize

Time ime

F i c . - _ , 1 9 Schematic time-temperature cycles for nor-

rr.clizing and full annealing. The slower cooling of

annealing results in higher temperature transforma-

tion to ferrite and pearlite and coarser microstructures

than does normalizing. (Courtesy of M. D. Geib, Col-

orado School of Mines, Golden, Colo.)

8/13/2019 D1-36


STEEL co. t

19

2 4 0

2 1 .22

2

N c 4 0

Z 400

.)tX

1 .5 5 0 i 8

T (zotLO:-.) Iv)

600'

500

c ncr,4 4 1

400

ccSTEEL 4, 4, 4 : 1

5 48

6 98

500 - 92

a 1 60

H ) 6 7 8

T (20• LOC r) i3-3

/C OI,Zt3

450500550603 650700750lh G 1 7€1.PZRATU9E

Fig.2 4 Effect of chromium on the tempering characteristics of 0.3 C, 1°/ 3fr:

steels.

4G0450500550600650

ih 0r tExPERATuRE

Fig.210_ Effect of molybdenum on the tempering characteristics of 0.4 C, 1

lan,1 Cr steels.

400 :: 00 50 .: t:0.44 0, ;1;

Fig.21 hEffect of vanadium on the tempering characteristics of 0.4 C, 7./..tln,

1 Cr steels.

D.- 31

400

100 )0

:G O 50

8/13/2019 D1-36


F e r ro us a l lo y s

Classification by

commercial name

or application

Classification

by structure

steel

Alloys without

eutectic(<2% Con Fe-C

diagram)

Rain carbon

stee l

Low-carbon

steel

(<0.2% C)

Medium-carbon

steel

(0.2-0.5% C)

High-carbon

steel

(>0.5% C)

Low-alloy

steel

s 8% alloying

elements

Ferri tic

Ferritic-pearlitic

Pearlitic

Marlensitic

Bainitic...1

High-alloy

stool

> t)°;) alloying

oleinonlsAustonitic

(;ui iesi

t 1 ; 1 n I

I lea;

rosislant

Wear

resistant

Procihit

hardened

Austenitic-

lorritic

Duplox

strucluro

Elg.22 Classification of steels. Source: D.M. Siefanescu, Lniversity of Alabama, Tuscaloosa

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0 0 0 0 00F e

t . % ChromiumFra.23 inary Fe-Cr phase diagram showing the closed y loop as body-centered

chromium and body-centered iron form ferrite solid solutions.

Liquid

a+(CrFe) C and (CrFe)7 3

1

Fe+ .5 .0

12?; Cr

Carbon

1.5

FIG. 1.Pseudo-binary phase diagram of iron plus 12% Cr, with varying per-

centages of carbon. Tie lines in two-phase fields do not show compositions of the two

phases or relative proportions. The crosshatched areas are three-phase fields.. A.

second carbide may exist at the higher carbon contents,'

D 33

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0.5

w t . % C

0F e , 18Cr , 4Ni

1.0Fe , 18Cr, 8Ni

wt. % C

13

1600

5

1200

800

a+ y-

Tmaue C

, ///ct+7+Carbide

M

- Carbide (̀<0', '0,/%// // 400

at'

Liquid

7+Carbide

D'ul f2 and 2 Pseudo-hinary phase diagrams of Fe + 18% Cr -F 4% Ni versus

varying carbon content (12.•) and loo -F I8% Cr -F i versus varying carbon

content ( . The carbide in both cases is (CrPe),C, at least, up to 0.5% C. Notehow increasing nickel content stabilizes austenite, restricting the high-temperature •5

and room-temperature a. The three-phase fields are crosshatched. Phase composi-

tions and relative proportions cannot, be obtained from tie lines in two- or three-phase

fields in these diagrams.

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900-

„Ferrite

Range

cofolin Range of cooling

rates rates betweenfor rim rim and core

of of rotor bodyrotorbody

//ARange ofcoolingrates forcore ofrotor bod

Lowerbainite•..

300—

Martensite

900

10 1000000 000TIME,s

Idealized continuous-cooling transformation diagram for 1%Cr—IVIo—V rotor forgings

100000

33)

30)

3 70

r

IaCn H 2 ? ) —

2 2 0

30 06 37 38

• 10 1 (Tin •R. t in h)

353 -

340

320

5 2 0

3 : 3 —

.330

0

2 7 0 —

2 40 —

Tempering

2 5 4 — ymbol temperature

7F

24 00 —

220

2 2 0 —

•

00

X

A

6 10001 0 5 5

1:(0)

1 1 7 01200123 01 2 5 013 0013 4 0

5 405706006 1 563065066560070572 5

210--

200

100

180 I

29 30 31 2 3 4 35

Tempering parameter. T(20 • 109 1)

Fig.28 Tempering behavior of Cr-Mo-V rotor steel is

a- 35

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8:300 'Fai0 Trariskomationstar::

Trx;formation1500

1 76 0I i

Composition1 . V : 7 0

C - 0 10

13307.1,1 -0 .;

C 18

MO - 0 93

Grain size 4

F -

P - Pearlite0 - Dainite

AusteniteTr - T ra c e a m ount s

74,

1 50300

CPH 335

800

760

600

S O O

1130

1600 -

900 -

457 70F 757 (• P) 91553 306 25E1

1 1 1 I 111 III 1 139

0.1 0 00

9 75

8 1 5

705

55)

595

5 43

480

425

370

3:5

263

20 5

150

200 TrFAlicros:ructure 9,

100 •0.0i

1 5 : 4 200 • F/h

305

O Tra ns form a t ion mans• Tra ns form a t ion s t ops

Composition

C -017Mn -0 41

S- 0 . 7 1Cr - 42 ?MO 0 4

Grainsize. 7

F - FerriteP - Pearlite0 - ElamiteA - AustemteTr - T ra c e a m ount s

160

Trpam

Tmpauv•C

Tim e f rom 1750 ' F ( 955

A,4 uS :e n ite

1100

6.000

1700

1:co

4000

S O O

800

700

200 0

NO .•- 150

70F

1ii 1 1

2 C0- ictosttuchoe 5F57 30P 707 3

5 5 9 58 Tr8 30P

0 01 1 0 30

Time from 1750'F 7955 'C).

(a)

1700

1600

1500

1400

500

430

8:5

750

: CS

- 633

- S43

-1430

--I 425

1 3 7::

253

-1235

9 2 5

370

1

Tmpaue C

Fig. ontinuous cooling transformation diagrams for (o) 21/4Cr-1Mo steel

austenitized ot 955 °C (1750 °F) and (b) 11/..Cr-V,Mo steel oustenitized at 910 °C

(1675 3F) (Ref 17).

D1-36

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