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Steel
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Metal Alloys
Most engineering metallic materials are alloys.Elemental metals are generally very soft and not very
usable.
Metals are alloyed to enhance their properties, such as
strength,
hardness or
corrosion resistance,
and to create new properties, such as
superconductivity and
Engineering metal alloys can be broadly divided into
Ferrous alloys and
Non-ferrous alloys
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Metal
Non-ferrousFerrous
Carbon Low Alloy High Alloy
Cast ironsSteels
Low-C
Medium-C
High-C
Tool (Mo,V,W,Cr,Ni)
Stainless (Cr, Ni)
High-
strengthlow-alloy
Grey iron
Nodular iron
White ironMalleable iron
Alloy cast irons
Classes of Metals
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Steel
Structural framing
Roofing / Cladding
Interior products
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Steel-making
Since the mid-1800s, a number of processeshave been developed for refining pig iron into
steel
Today, the two most important processes are Bessemer converter
Basic oxygen furnace(BOF)
Electric furnace Both are used to produce carbon and alloy
steels
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Making Steel
(lowering the carbon and removing impurities)
In 1856, H. Bessemerpatented the converter for
purifying pig iron.
Hot air is forced through
the molten metal in a
pear-shaped vessel
Si, Mn, C and other
impurities are oxidized
and removed as slag.
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The Bessemer converter
Much smaller furnace
More impurities removed
(oxidised)
Calculated amount of carbon
added to make steel!, ifrequired
Poured into molds to form
ingots Entire cycle time (tap-to-tap time)
takes 25 to 30 min
http://upload.wikimedia.org/wikipedia/commons/7/7c/Bessemer_Converter_Sheffield.jpghttp://upload.wikimedia.org/wikipedia/commons/f/f1/Bessemer_Converter_(PSF).jpg7/30/2019 Steel & CostIron Making
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Basic Oxygen Furnace (BOF)
Accounts for 70% of steel production in U.S Adaptation of the Bessemer converter
Bessemer process used airblown up through
the molten pig iron to burn off impurities
BOF uses pure oxygen
Typical BOF vessel is 5 m inside diameter &
can process 150 to 200 tons per heat
Entire cycle time (tap-to-tap time) takes 45 min
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Basic Oxygen Furnace
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Basic Oxygen Furnace - Stages
(1) Charging
(2) Pig iron(3) Blowing(4) Tapping(5) Pouring
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Electric Arc Furnace
Accounts for 30% of steel production in U.S.
Scrap iron and scrap steel are primary raw materials
Capacities commonly range between 10 and 100
tons per heat Complete melting requires about2 hr; tap-to-tap
time is 4 hr
Usually associated with production of alloy steels,tool steels, and stainless steels
Noted forbetter quality steel but higher cost per ton,
compared to BOF
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Electric Arc Furnace
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The whole spectrum of steel products!
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Alloy Designation
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Carbon Steels and Low Alloy Steels
Alloy Designation Alloy Designation AISI: American Iron and Steel Institute
SAE: Society of Automotive Engineers
ASTM: American Society for Testing and Materials
UNS: Unified Numbering System
AISI Grade X1X2X3X4
Older, butstill widely
used
Primaryalloying
elements
Carboncontent
10, 11, 12 plain C steel13 Mn steel
2x Ni steel, x=%Ni3x Ni-Cr Steel, x=%Ni+Cr4x Mo Steel, x=%Mo5x Cr steels, x=%Cr6x Cr-V Steels, x=%Cr+V7x W-Cr Steels, x=%W+C
9x Si-Mn Steels, x=%Si+Mn
X1X2
eg. 15 = 0.15%C
5195 =?
1040
Fe-0.4%C
2520Fe-5%Ni-0.2%C
Fe-1%Cr-0.95%C
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What is a steel and alloy of?
Iron (Fe) and Carbon (C)
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Plain Carbon Steels
An alloy of Fe & C whose properties depends only
upon the %age of Carbon present in it.
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Plain Carbon Steel vs. Alloy Steel
3 Classifications
Low Carbon Steel
Medium Carbon Steel
High Carbon Steel
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Plain Carbon Steels: General Properties
Yield strength: 300MPa (mild steels) - 700MPa (high C steels) Tensile strength: 400-1000 MPa
Ductility: EL% 15-30
Youngs modulus: 210 MPa.
Divided into low (
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Low Carbon Steel
Carbon < 0.3wt% Used wherever soft,
deformable materials
are needed
E.g., structural sections,rivets, nails, wire, pipe.
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Medium Carbon Steels
Carbon = 0.3 - 0.6wt%
Used where higher strength is
required
E.g., gears, shafts, axles, rods,
etc.
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High Carbon Steels
Carbon = 0.6 - 1.2wt%
used where high hardness
is required
Eg. hammers, chisels, drill,
springs.
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Mild steel panelsfor easy shaping
Medium-carbon steelchassis for strength and
toughness
high-carbon steelsprings
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Alloy Steel
Alloy steel may be defined as one whose
characteristics properties are due to some
elements other than Carbon. Although all
Plain-Carbon steels contain moderate
amounts of Mn & Si, but they are not
considered alloy steels because the principalfunction of Mn & Si is to act as de-oxidizer
during steel manufacturing process.
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Why alloying is necessary?
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Why alloying is necessary?
Many purposes, some of the most important are:-
increase hardenability,
reduce danger of warpage
improve strength & toughness at high & low temperatures,
resist grain growth at elevated temperature,
improve wear, corrosion, fatigue & creep resistance.
improve machine-ability,
improve magnetic properties
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Alloying Elements used in Steel
Nickel (Ni) (2xxx)
2% to 5%
Increases toughness
Increases impact resistance
12% to 20% with low amounts of C possessgreat corrosion / scaling resistance
universal grain refinerin alloy steels
unfortunately is a powerful graphitiser.
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Alloying Elements used in Steel
Chromium (Cr) (5xxx)
Usually < 2%
increases hardenability and strength
5 % Cr steels used for making forging dies
typically used in combination with Ni and Mo
10.5% < Cr < 27% = stainless steel
used for corrosion resistance
Improves non-scaling properties
Causes grain growth
Reduces toughness
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Alloying Elements used in Steel
Molybdenum (Mo) (4xxx)
Usually < 0.3%
increases hardenability and strength
Mo-carbides help increase creep resistance at
elevated temps improves the tensile strength & sp. heat resistance
has favourable influence on the welding properties.
Steel with higher contents tend to be difficult to forge
typical application is hot working tools
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Alloying Elements used in SteelManganese (Mn)
acts as de-oxidizerduring steel manufacturing
combines with sulfur (MnS) to prevent brittleness& improves machining
>1%
increases hardenability
improves strength, wear resistance of steel 11% to 14%
increases hardness
good ductility
high strain hardening capacity excellent wear resistance
Ideal forimpact resisting tools
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Alloying Elements used in Steel
Vanadium (V)
Usually 0.03% to 0.25%
stabilities martensite and increases hardenability.
induces resistance to softening at high temperatures once
the steel is hardened
increases hot hardness properties in High Speed & Tool
steels by increasing cutting properties.
increases strengthwithout loss of ductility
Like Nickel it restrains grain growth
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Alloying Elements used in Steel
Tungsten (W)
increases hot hardness
used as cutting tool steels
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Alloying Elements used in Steel
Sulfur (S) (11xx)
Imparts brittleness
Improves machining Some free-machining steels contain
0.08% to 0.15% S
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Alloying Elements used in Steel
Boron (B) (14xx)
for low carbon steels, can drastically increase
hardenability
improves machinablity and cold forming capacity
Aluminum (Al)
deoxidizer
0.95% to 1.30% produce Al-nitrides during nitriding
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Alloying Elements used in Steel
Copper (Cu)
0.10% to 0.50%
increases corrosion resistance
Reduces surface quality and hot-working ability
used in low carbon sheet steel and structural
steels
Silicon (Si)
About 2%
increases strength without loss of ductility
enhances magnetic properties
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Alloy Steel
> 1.65% Mn, > 0.60% Si, or >0.60% Cu
Most common alloy elements:
Chromium, nickel, molybdenum, vanadium,
tungsten, cobalt, boron, and copper.
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High Strength Low Alloy Steels
Low alloy = alloying elements
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Tool Steels
A class of (usually) highly alloyed steels
designed foruse as industrial cutting tools,
dies, and molds
To perform in these applications, they must
possess
high strength, hardness, hot hardness, wear
resistance, and toughness under impact
Tool steels are heat treated
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AISI Classification of Tools Steels
T, M High-speed tool steels - cutting tools in machining
H Hot-working tool steels - hot-working dies for
forging, extrusion, and die-casting
D Cold-work tool steels - cold working dies for
sheet metal press-working, cold extrusion, andforging
W Water-hardening tool steels
S Shock-resistant tool steels - tools needing high
toughness, as in sheet metal punching and
bending
P Mold steels - molds for molding plastics and rubber
T l St l
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Tool Steels Carbon tool steels: 0.8~1.2%C
High alloy tool steels are oftenalloyed with Mo, V, W, Cr
and/or Ni
E.g., HSS, W-Cr-V (18-4-1)
Yield strength: 1000-1500+
MPa
Tensile strength: up to
2000MPa Ductility: EL% 5-15
Youngs modulus: 200 MPa
(alloying generally reduces
Youngs Modulus)
T l St l
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Tool Steels
Uses
Used where extremehardness is required.
Ductility/toughness
usually sacrificed Eg. Moulds and dies,
saws, cutting tools,
punches
Stainless Steel (SS)
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Stainless Steel (SS)Highly alloyed steels designed for corrosion resistance
Principal alloying element is chromium, usually greater than
11.5% Cr forms a thin impervious oxide film that protects surface
from corrosion
Nickel (Ni) is another alloying ingredient in certain SS to
increase corrosion protection
Carbon is used to strengthen and harden SS, but high C
content reduces corrosion protection since chromium carbide
forms to reduce available free Cr, therefore Carbon content is
kept very low - < 0.1% to avoid Cr3C2 formation
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Properties of Stainless Steels
In addition to corrosion resistance, stainlesssteels are noted for theircombination of
strength and ductility
While desirable in many applications, these
properties generally make SS difficult to
work in manufacturing
Significantly more expensive than plain C or
low alloy steels
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Types of Stainless Steel
Classified according to the predominant
phase present at ambient temperature:
1. Austenitic stainless - typical composition
18% Cr and 8% Ni
2. Ferritic stainless - about 11.5% to 27%
Cr, low C (0.25% max), and no Ni
3. Martensitic stainless - as much as 18%
Cr but no Ni, higher C content (0.15-0.75%) than ferritic stainless
Stainless Steels Typical Mechanical
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Stainless Steels - Typical Mechanical
Properties
Yield strength : 200-1600 MPa Tensile strength : 300-1800MPa
Ductility : EL% 2-20
Youngs modulus:~170 MPa (alloyingreduces Youngs Modulus)
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Cast irons Abraham Darbys Ironbridge
Ductile iron used
in drain grids
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Overview of cast iron Iron with 1.7 to 4.5% carbon and 0.5 to 3% silicon
Lower melting point and more fluid than steel (better
castability)
Low cost material usually produced by sand casting
A wide range ofproperties, depending on composition &
cooling rate
Strength
Hardness
Thermal conductivity
Damping capacity
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Composition of Cast Iron
A typical cast iron contains
1.7 to 4.5% carbon,
0.5 to 3.0% silicon,
less than 1.0% manganese, and
less than 0.2% sulfur.
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Production of cast iron Raw material
Pig iron,
scrap steel,
limestone and
carbon (coke) Cupola
Electric arc furnace
Electric induction furnace Usually sand cast, but can be gravity die
cast in reusable graphite moulds
finished by machining
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Cupola Melting of Gray Cast Iron
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Dependence of Types of cast iron
Various types of cast iron can be produced,
depending on the
Chemical composition,
Cooling rate, and
the type andamount of Inoculants used.
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Effect of cooling rate Slow cooling favours the formation ofgraphite &
low hardness
Rapid cooling promotes carbides with high
hardness
Thick sections cool slowly, while thin sections cool
quickly
Sand moulds cool slowly, but metal chills can be
used to increase cooling rate & promote white iron
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Types of cast iron Gray cast iron - carbon as graphite
White cast iron - carbides, often alloyed
Ductile cast iron
nodular, spheroidal graphite
Malleable cast iron
Graphite nodules are irregular clusters / tempered graphite
Compacted graphite cast iron CG or Vermicular Iron
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Microstructures of cast iron
Nodular iron
aFe and
graphite spheres
Gray iron
aFe and
graphite flakes
White iron
cementite and
pearlite
Malleable iron
aFe and
tempered
graphite flakes
low melting point, castable, cheap; however, can be brittle.
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Gray Cast Iron
R M t i l d f G C t I
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Raw Materials used for Gray Cast Irons
Iron Sources
Iron Scrap
Internal Returns
Machined Chip Briquettes
External Purchased Scrap Steel Scrap
Pig Iron
Coke
Graphite and Silicon Carbide Ferro-silicon and Ferro-manganese
P ti f G t i
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Excellent compressive strength (compressive
strength is typically 3-4 times tensile strength),
Excellent machinability (graphite acts to break up the
chips and lubricate contact surfaces), Excellent wear resistance (graphite flakes self-
lubricate), and
Outstanding sound and vibration-damping capacity(graphite flakes absorb transmitted energy).
Properties of Gray cast irons
Properties of Gra cast irons
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Good corrosion resistance and the enhanced fluidity
due to high silicon contents
Thermal conductivity high
The formation of the lower-density graphitereduces
the amount of shrinkage, making possible the
production of more complex iron castings
The pointed edges of the flakesact as preexisting
notches or crack initiation sites, giving the material a
characteristic brittle nature resulting low impact
resistance
Ductility is low (0.6%)
Properties of Gray cast irons
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The size, shape, and distributionof the graphite flakes
have a considerable effect on the overall properties ofgray cast iron.
For maximum strength, small, uniformly distributed
flakes are preferred.
Properties of Gray cast irons
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Applications of Gray cast irons
Engines Cylinder blocks, liners,
Transmission housing
Brake drums, clutch plates
Pressure pipe fittings, Machinery beds
Furnace parts, ingot and glass moulds
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Ductile or SG iron
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Ductile or SG iron
Also known as spheroidal graphite (SG),
and nodular graphite iron
Inoculation with Ce or Mg or bothcauses graphite to form as spherulites,
rather than flakes
Farbetter ductility than gray cast iron
P d ti f SG i
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Production of SG iron Composition similar to gray cast iron except
for higher purity
Melt is added to inoculant (Mg) in ladle.
Magnesium as wire, ingots or pellets is
added to ladle before adding hot iron
Mg vapour rises through melt, removing
sulphur.
P d ti f SG i
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Production of SG iron
Prior to solidification, graphite forms assmooth-surface spheres.
This addition is known as a nodulizing,
and the product becomes ductile or
nodular cast iron
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Properties of SG iron good ductility, high strength
Strength higher than gray cast iron
Ductility up to 6% as cast or 20% annealed
Low cost
Simple manufacturing process makes complex shapes
Machineability better than steel
Good toughness, wear resistance,
low-melting-point castability, up to a 10% weight reduction compared to steel
makes ductile iron an attractive engineering material
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Applications of SG iron
Automotive industry 55% of ductile iron in
USA
Crankshafts, front wheel spindle supports,steering knuckles, disc brake callipers
Pipe and pipe fittings (joined by welding)
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Malleable iron
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Malleable iron Produced by heat treatment ofwhite cast iron
Graphite nodules are irregular clusters
Similar properties to ductile iron
starting white iron structure restricts the sizeand thickness of malleable iron products such
that most weigh less than 5 kg
depending on the type of heat treatment,
various types of malleable iron can be
produced
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Properties of Malleable iron
Similar to ductile iron
Good shock resistance
Good ductility
Good machineability
corrosion resistance
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Applications of Malleable iron Similar applications to ductile iron
Malleable iron is better for thinner castings
Ductile iron better for thicker castings >40mm
Vehicle components
Power trains, frames, suspensions and wheels
Steering components, transmission and
differential parts, connecting rods
Railway components Pipe fittings
products such as door keys, gear wheel, and crank
levers
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White cast iron
White cast iron
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White cast iron White fracture surface
No graphite, because carbon forms Fe3C or more complexcarbides. Features promoting the formation of cementite
over graphite are:
low Carbon equivalent (1.8 to 3.6% carbon),
0.5 to 1.9% Silicon,
0.25 to 0.8% Manganese, &
rapid cooling
very hard, brittle & abrasion
resistant
Often alloyed
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Uses ofWhite cast iron
Products such as
gates,
fences,
parts of stove are manufactured by
using white cast iron.
In addition it is also used tomanufacture malleable cast iron
Fracto graphs of cast irons
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Fracto-graphs of cast irons
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Thanks
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Welding
Weldability of cast iron is low and
depends on
the material type,
thickness,
complexity of the casting, and
on whether machinability is
important
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Weldability
White cast iron - not weldable
Small attachments only
Grey cast iron - low weldability
Welding largely restricted to salvage and
repair
Ductile and malleable irons - goodweldability (inferior to structural
steel) Welding increasingly used during
manufacture
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