1. Chapter No-02
Steels and Alloy of steels
(Iron-carbon alloys containing appreciable concentration of other alloying
elements; contain ≤ 2% C)
1Prof.Ghadage M.M.
2. 2
Ferrous Metals - Iron and Steel
Pure iron is soft and ductile to be of much practical use.
BUT when carbon is added, useful set of alloys are produced.
They are known as carbon steel.
The amount of carbon will determine the hardness of the steel.
The carbon amount ranges from 0.1% to 4%.
Prof.Ghadage M.M.
3. Classification of steels
Depending upon %C:
Low carbon steel
Medium carbon steel
High carbon steel
These 3types are further sub-classified as;
Plain carbon steel: Contain only residual concentrations of impurities other
than carbon and a little manganese
Alloy steel: More alloying elements are intentionally added in specific
concentrations
3Prof.Ghadage M.M.
7. Mild steels
% C = 0.15-0.25
Microstructure consists of about 25% pearlite in a
ferrite matrix
Properties:
High strength, low ductility as compared to
conventional low carbon steels (0.1% C)
Excellent weldability
Y.S. = 300-350MPa, U.T.S = 400-450MPa,
%elongation = 26-30
HAZ near the weld attains a temperature above A3 and
becomes austenite. When the welding is complete this
region cools more rapidly than in air cooling, due to
self-quenching
If carbon content does not exceed 0.25% the
hardenability is low for non-martensitic products to
form in HAZ
If martensite forms, its hardness is less that 45Rc
Applications: Ship hulls, boilers, oil pipelines, I
beams, H beams, angles, channels, grills, building bars
etc
Weathering steels: Adding phosphorous and copper to
mild steels to improve the resistance to atmospheric
corrosion 7Prof.Ghadage M.M.
8. Medium carbon steels
Also known as machinery steels
% C = 0.3 – 0.6
Properties:
Intermediate to low and high carbon steels
Medium hard, Not so ductile and malleable, medium tough, slightly difficult to
machine, weld and harden
Difficult to cold work and hence hot worked
Least expensive
Applications: Bolts, axles, springs, wires, wheel spokes, rods, hammers, lock
washers, crankpin, turbine rotors, railway rails, railway tyres, cylinder liners
etc
8Prof.Ghadage M.M.
9. High carbon steels
Also called as tool steels
% C = 0.6 – 2%
Properties:
Hard, wear resistant, brittle, difficult to machine, difficult to weld and can be
hardened by heat treatment
Can be cold worked
Applications: Knives, Chisels, cutting tools, forging dies, punches, hammers,
springs, clips, clutch discs, drills, leaf springs, razer blades, balls and races for
ball bearings, mandrels, cutters, reamers etc
9Prof.Ghadage M.M.
10. Properties and uses of alloying elements
Sulphur:
Combines with iron and forms FeS (hard and brittle)
FeS has low melting point and hence solidifies last; appears at grain boundaries
During hot working, cracks develop during working (hot short)
Thus amount of S to be restricted to 0.05% and more than 5 times of S, Mn to
be added
MnS is not so hard and brittle as FeS
MnS has higher melting point than FeS
Thus Mn addition reduces brittleness and hot shortness
Hence, some amount of Mn is always present in any steel
FeS and MnS promote chip formation and hence improves machinabilty
10Prof.Ghadage M.M.
11. Alloy Steels
Carbon content less than 1.2%
Alloying element of Steel :-
Mn-Manganese
Si-Silicon
Cu-Copper
Cr-Chromium
Ni-nickel
Molybdenum
Cobalt
vanadium
Example :-
•Stainless steel
•High speed steel
11Prof.Ghadage M.M.
12. Effect of alloying element on properties of alloy steel:-
1. The maximum ultimate tensile strength increased
2. Thick section are available with high hardness
3. More controlled quenching
4. Improve impact resistance
5. Improve corrosion resistance
6. Improve high temperature performance
7. Improve machinability
8. Improve high or low temperature stability
9. Better were resistance
12Prof.Ghadage M.M.
13. Effect of individual alloying element on
properties of alloy steel:-
13Prof.Ghadage M.M.
14. Properties and uses of alloying elements
Phosphorous:
Dissolves in ferrite and forms a solid solution
Increase tensile strength and hardness
If solubility is exceeded, Fe3P is formed which is hard and brittle
Thus, amount of phosphorous is kept below 0.05%
Phosphorous reduces solubility of carbon in ferrite and thus rejects carbon
adjacent areas forming banded structures (Alternate pearlite and ferrite layers);
easy crack propagations; hence banded structures are not desirable
14Prof.Ghadage M.M.
15. Properties and uses of alloying elements
Silicon:
Dissolves in ferrite and forms a solid solution
Increases strength, hardness and toughness without loss of ductility
Strong deoxidiser
Upto 5% Si, produces magnetically soft materials for transformer, motor and
generator cores; Less eddy current losses due to high electrical resistivity
Steels with 2% Si, 1%Mn and %C between 0.5 to 0.7 are suited for
manufacturing leaf springs, coiled springs, chisels, punches [Heating : 840-930ᵒ
C, holding and oil quenching followed by tempering @ 400-550ᵒ C]
Higher amount of Si (say 8% or more) are never added, because cementite from
steel decompose into graphite and ferrite which spoil the properties of steel
15Prof.Ghadage M.M.
16. Properties and uses of alloying elements
Manganese:
Either less than 2% or more than 10% because Mn content between 2-10%
induces brittleness
Dissolves in ferrite and increases yield strength, tensile strength, toughness and
hardness
Least expensive and hence added to all structural steels for strengthening
Enhances response to heat treatment
Normalizing improves impact property of manganese steels
Combines with S and forms MnS and reduces detrimental effects of FeS
Improves machinability and hence added to free cutting steels upto maximum
1.6%
Applications:
Low carbon steels with Mn content 1.65-1.9%: Rails, gears, axles, connecting
rods, crankshafts, bolts, nuts, studs, steering levers, aircraft fittings and gun
barrels
16Prof.Ghadage M.M.
17. Properties and uses of alloying elements
Hadfield steel:
1-1.2 % C, 12-14% Mn
Extremely tough, wear resistant and non-magnetic on suitable heat treatment
(Heating: 1000ᵒ C, holding and quenching in water)
Mn is austenitic stabilizer and with high amount Mn, critical temperature is
sufficiently lower, so that by rapid cooling austenitic structure can be obtained at
room temperature
Applications: Jaw plates for stone crusher, frogs in rail road tracks, dredge
bucket and power shovel teeth
17Prof.Ghadage M.M.
18. Properties and uses of alloying elements
Nickel:
Dissolves in ferrite and increases tensile strength, hardness and toughness
without decreasing ductility
Added upto 5% to increase tensile strength and toughness
Austenitic stabilizer: High addition of Ni makes steel austenitic at room
temperature. Such steels are soft, ductile, malleable and non-magnetic
Increases corrosion and oxidation resistance if added in excess of 5%
Increases impact resistance of steels at low temperature
Increases hardenability of steels
Reduces coefficient of thermal expansion:
Invar:- 36% Ni, 0.2%C and 0.5%Mn; Elinvar:- 36%Ni, 12%Cr and W; Ni-
span:- 42%Ni, 5.5%Cr and 2.5%Ti. All these three alloys have zero coefficient
of thermal expansion in the temperature range of 0-100ᵒ C
These three alloys can be used for surveyor’s tape, gauges, watch parts etc
Applications: Steels with 2-3% Ni are used in large forgings, castings and
structural components which cannot conveniently quenched, locomotive boilers,
bolts, railway axles and bridge structures 18Prof.Ghadage M.M.
19. Properties and uses of alloying elements
Chromium:
Increases hardenability
Forms carbides and increases hardness and wear resistance of
steels
Increases corrosion and oxidation resistance when added in
substantial amount
Increases service life and performance of steels
May cause temper embrittlement
Surface markings (Chrome lines) may be formed
Applications:
Composition and heat treatment for gears, jaws of wrenches,
machine gun barrels, axles and shafts: 0.35% C, 0.5% Cr;
Heating:870ᵒ C, holding and oil/water quenching followed by
low temperature tempering ]
Composition and heat treatment for springs and compressed air
tools: 0.5% C, 1.5% Cr; Heating: 840ᵒ C, holding and oil
quenching followed by tempering @ 300ᵒ C
Composition and heat treatment for twist drills, hacksaw blades,
knives, hammers: 0.9% C, 1% Cr; Heating: 810ᵒ C, holding and
oil quenching followed by tempering @ 250-300ᵒ C]
Composition and heat treatment for ball bearing: 0.95-10% C,
1.3-1.6% Cr; Heating:840ᵒ C, holding and Oil quenching
followed by tempering @ 150-160ᵒ C [long time; spherodising to
improve machinability]
Medium chromium and high chromium steels find applications in
cutting tools, dies, stainless steels, heat resisting steels
19Prof.Ghadage M.M.
20. Properties and uses of alloying elements
Tungsten:
Increases hardenability
Forms carbides and increases hardness and wear resistance of steels
Resistance to tempering (Secondary hardening)
Refines grain size, and carbide prevents grain coarsening
Reduces tendency of decarburisation
Molybdenum:
Reduces temper embrittlement (added upto 0.5%)
Properties similar to W
Resistance to grain coarsening and decarburization is less as compared to W
20Prof.Ghadage M.M.
21. Properties and uses of alloying elements
Vanadium:
Excellent resistance to grain coarsening
Improves fatigue and creep resistance; hence used in leaf and coil springs,
heavy duty axles, gears, pinions, valves etc
Strong deoxidiser
Excellent wear resistance and resistance to tempering
HSS : 1% V
Super HSS: 5% V
Titanium
Strong carbide former
High wear resistance with no loss of toughness
Prevents grain coarsening
Cobalt
Neutral element
Only element reducing hardenability of steels
Resistance to tempering
Applications: Permanent magnets, Cemented carbide cutting tools
21Prof.Ghadage M.M.
22. Properties and uses of alloying elements
Aluminum:
Powerful deoxidiser
Prevents grain coarsening
Boron:
0.001-0.003% B increases hardenability of medium carbon steels
Reduces grain size but does not prevent grain coarsening
Improves machinability
Boron diffused steels have high surface hardness, wear resistance and
corrosion resistance
Boron diffused surfaces of hot forging dies considerably increase service life
Used for control rods in nuclear reactors
22Prof.Ghadage M.M.
24. Free cutting steels
Can be machined and cut with fast speeds, because of their high machinability
and hence named free cutting steels
Also known as resulphurized grade steels
Both extremely hard and extremely soft materials are difficult to machine
Low carbon steels containing 0.6% S, 0.12% P, Mn: 5-8times amount S
Mn + S = MnS Favors chip formation and breaking; Increases strength
and hardness
P + Fe = Fe3P Favors chip formation
High carbon steels containing 0.35% Pb
Pb is insoluble and appears as microscopic globules in steel
Favors chip formation with less resistance to tip of tool
Pb improves machinability without affecting normal temperature ductility and
toughness
24Prof.Ghadage M.M.
25. High strength low alloy (HLSA) OR Micro-
alloyed steels
% C: 0.07-0.13% with small (< 0.5%) additions of Ti, V, Nb and Al
Properties:
High strength to weight ratio than conventional steels of identical carbon
content
Good ductility, malleability, formability, toughness and weldability
Y. S. = 400-700MPa, U. T. S. = 500-800MPa, % Elongation = 18-25
Superior properties because of ultrafine grain size, solid solution strengthening
of ferrite, precipitation of carbides and nitrides and martensitic or bainitic
transformation which are likely to occur in these steels due to increased
hardenability
Applications: Oil and gas pipelines, Automotive (pressed chassis and
reinforcement parts, beams or welded tubes), construction and farm machinery,
industrial equipment, storage tanks, mine and railroad cars, barges and
dredges, lawn mowers, and passenger car components, bridges, power
transmission towers, light poles, lifting and handling equipment (cranes, fork
lifts, platforms, warehouse shelves, lifts)
25Prof.Ghadage M.M.
26. Maraging steels
Composition: 0.03% C, 18-25% Ni, 3-5%Mo, 3-8% Co and 0.2-1.6% Ti, with
small amounts of Al
Formed by martensite transformation (comparatively soft because of low
carbon content) + Cold working (as desired) + Aging @ 500ᵒ C
During aging, strain induced precipiatation hardening occurs due to the
precipitation of Ni3TiAl and Ni3Mo phases
Y. S. upto 1800 MPa with excellent fracture toughness
Good weldability
Expensive
Applications: Rocket casing and other aerospace applications, pressure vessels,
injection moulds and dies
26Prof.Ghadage M.M.
27. TRIP steels
Stands for Transformation Induced Plasticity
Composition: 0.25% C, 2% Mn, 2% Si, 8% Ni, 9% Cr and 4% Mo
Composition is so adjusted that Ms temperature is below room temperature
and Md is above room temperature (Md = highest temperature upto which
deformation of austenite can induced martensite)
Steel is first heavily deformed above Md, where no transformation occurs
Deformation produces the right degree of metastability so that a small plastic
strain at the tip of the crack is sufficient to induce the austenite to transform to
martensite
Plastic zone size is enlarged, so that more work is done during crack growth
Y. S. = 1400MPa with excellent fracture toughness
Expensive and hence used for specialized applications
27Prof.Ghadage M.M.
28. Rail steels
Structural parts used by railways such as rails, wheels, axles, are either forged,
or hot rolled and have carbon of 0.5-0.65%
Higher level of carbon combines with about 1% Mn shifts eutectoid
composition sufficiently to a yield a mostly pearlitic structure
Lowering of transformation temperature by Mn results in fine pearlite
Weight loss due to wear of rail steels decreases with increasing hardness of the
steel and decreasing interlamellar spacing of pearlite
Hadfield steel:
Used where there is exceptionally high rate of wear in rails
0.75 – 0.9% C, 12-14% Mn
Steel is austenitic in structure and high rate of work hardening
28Prof.Ghadage M.M.
29. Spring steel
Carbon content: 0.5-0.65%
Spring properties: High resilience
Quenched and tempered to get a yield strength of about 1500MPa
Role of alloying elements in spring steels:
Increase hardenability
Presence of Si in 55Si2Mn90 spring steel serves the purpose of retarding the
softening during tempering, so that stresses are relieved are without much loss
in hardness and strength
Vanadium in the 50Cr1V23 steel prevents grain coarsening during
austenitizing and improves the toughness of steel. A fine grain size and
prevention of decarburization during heat treatment ensure a good fatigue
strength
29Prof.Ghadage M.M.
30. Ni-Cr-Mo low alloy steels
Ni increases toughness of ferrite; Cr increases hardenability, strength and wear
resistance but at the expense of toughness. Thus for structural alloys Ni/Cr
should be about 2.5
To reduce temper embrittleness induced due to Ni-Cr, 0.25% Mo is added
Well known Ni-Cr-Mo low alloy steel is AISI 4340
For same ductility and toughness, low alloy steel possess superior strength
Conversely for same strength, the low alloy steel would have larger ductility
and toughness
High hardenability implies slower cooling rates and hence less residual
stresses
High hardenability makes welding difficult in case of AISI 4340
30Prof.Ghadage M.M.
32. Properties/Requirements
Hardenability
Rates the steel on the probability of hardening during cracking
Depth of hardening: Higher the alloying elements, higher is the depth of hardening
Resistance to decarburisation:
Ability to resist loss of carbon at the surface during hardening
Loss of carbon leads to softening and cracking
Red hardness
Capacity to withstand hardness at high temperatures
HSS have high red hardness as compared to other tool steels
Wear resistance
Removal of surface area of a material by abrasion, erosion, adhesion and other processes can
cause wear and tear of the material
Abrasion: Removal of material by action of hard, sharp particles or projections on sliding
surface
Erosion: Progressive loss of material from surface by mechanical action of fluid on surface
Adhesive wear: Wear caused by action of relatively smooth surfaces sliding together
Toughness
Must absorb sufficient energy and resist breaking
Should be rigid and there should be no plastic deformation
Machinability:
Ease of machining
Specific alloying elements to be added to improve machinability
32Prof.Ghadage M.M.
33. Types of tool steels
Tool steels
Cold
work tool
steels
Water
hardening
(W-series)
Oil
hardening
(O-series)
Air
hardening
(A-series)
High
carbon
high
chromium
(D-series)
Hot work
tool steels
(H-series)
High
speed tool
steels
Special
purpose
tool steels
33Prof.Ghadage M.M.
34. Water hardening tool steels
Composition: % C = 0.6-1.4%
Properties:
Used when maintenance of sharp cutting edges and wear resistance are more
important that shock resistance
Poor hardenability, thus hardened by water, and hence known as water
hardening steels
Applications: Blanking dies, threading dies, tube drawing dies, drills, forming
tools, hammers, chisels, wood working tools, shear blades, knives and razors
Drawbacks:
Poor red hardness and strength
More distortions
Shallow hardening type
More tendency of oxidation, decarburisation and grain coarsening
To eliminate this drawbacks, small amount of Cr, V and Mo are added
34Prof.Ghadage M.M.
35. Oil hardening tool steels
Also known as oil hardening non-shrinkage (OHNS) steels
Composition: 1% C, 0.95% Mn, 0.5% W, 0.75% Cr, 0.2% V and small
amounts of Mo
Better hardenability than water hardened steels and can be hardened by oil
quenching
Less expensive than other tool steels
Distortion during hardening is less and hence called as oil hardening non-
shrinkage tool steels
Applications: Blanking and forming dies, shear blades, master tools, cutting
tools and gauges
35Prof.Ghadage M.M.
36. Air hardening tool steels
Contain alloying elements like Mn, Cr, Mo and W.
Total alloying elements is > 5%
Properties:
High hardenability
Less distortions
High wear resistance and good depth of hardening
Applications: Thread rolling and slitting dies, drawing dies, intricate die
shapes, gauges and punches
36Prof.Ghadage M.M.
37. High carbon high chromium (HCHC) steels
High hardenability and hence can be hardened by oil or air quenching
Less distortions
Composition: % C > 1.5 and some grades contain % C > 2, % Cr = 12, with
some other alloying elements like W, Mo, and V
Thus, amount of complex alloy carbides is more which increases hardness and
wear resistance of steels, but these are difficult to machine
Maintain hardness upto 550ᵒ C due to presence of alloy carbides
Applications: Drawing dies, blanking dies, forming dies, coining dies, thread
rolling dies, trimming dies, bushings, shear blades, punching, cold forming
rolls, cutting tools, gauges etc
Oil hardening, air hardening and HCHC show less distortion during hardening
and hence are called as non-deforming or non-shrinkage tool steels
37Prof.Ghadage M.M.
38. Hot work tool steels
Composition: % C = 0.35-0.65 with alloying elements varying from low to
high content
Properties:
Good strength, toughness, hardness and wear resistance at elevated
temperatures
Excellent resistance to tempering at elevated temperature
Depending upon principal alloying elements, classified as;
Chromium type tool steels:
Composition: % C = 0.35-0.55, 3-7% Cr, with small amounts of W, Mo and V
Properties: High ductility, toughness and resistance to splitting
Applications: Aluminium and Magnesium die casting dies, extrusion dies,
forging dies, mandrels and hot sheers
Tungsten type tool steels:
Composition: % C = 0.3-0.5, 2-12% Cr, 9-18% W
Properties: Excellent red hardness and resistance to wear at elevated
temperature
Applications: Dummy blocks, hot extrusion dies for brass, nickel and steel,
forging dies and hot punches
38Prof.Ghadage M.M.
39. Hot work tool steels
Molybdenum type tool steels
Composition: % C = 0.55-0.65, 14-20% alloying elements like Mo, Cr, V and
W
Properties: Intermediate properties
Applications: Used when compromise in resistance to high temperature and
toughness is required
Dummy block and its use 39Prof.Ghadage M.M.
40. High speed steels (HSS)
Properties:
Maintain high hardness upto temperature of about of 550ᵒ C and hence cane be
used for cutting metals at high speeds
High wear resistance and cutting ability.
Classification depending upon principal alloying elements:
T-type HSS [18:4:1 steels/ Tungsten steels]:
Composition: 0.7% C, 18% W, 4% Cr, 1% V
M-type HSS [Molybdenum steels]:
Composition: 0.85% C, 6% W, 5% Mo, 4% Cr, 2% V
Properties: Low cost compared to T-type; Difficult to heat treat because of
more tendency of oxidation, decarburization and grain growth as compared to
T-type steels
W, Mo, Cr and V are carbide formers and hence increase red hardness, wear
resistance and cutting ability at high temperatures
V increases resistance to grain coarsening. Super high speed steel contains 5%
V
Applications: Drills, taps, milling cutters, saw blades, lathe tools, punches,
drawing dies and wood working tools etc
40Prof.Ghadage M.M.
41. High speed steels (HSS)
M-series drill bits with titanium
coating
Drawing diesMilling cutter
Taps
Wood working tools
41Prof.Ghadage M.M.
43. Stainless steel
Have high corrosion resistance and hence they do not corrode in most of the
usual environment conditions; hence called stainless steels
Exhibits extraordinary corrosion resistance due to formation of a very thin
layer hydrous chromium oxide is formed on the surface
Composition of the alloy varies from alloy to alloy and with treatment of alloy
such as rolling, pickling and heating; and thus corrosion resistance also varies
For sufficient corrosion resistance, minimum Cr content in solid solution form
should be greater than 12%
When Cr added to steel, it first combines with carbon and form complex
chromium carbides and remaining goes in solid solution form
Since the Cr chromium going with carbon is 17times the amount of carbon, the
Cr is solid solution form will be:
Cr in solid solution form = Total Cr – 17 x %C
Higher the Cr is solid solution form and lesser the amount of carbides, the
corrosion resistance is more
In addition to Cr, many other elements like Ni, Mn, Mo, Ti, Nb, Ta etc are
added to improve the properties
43Prof.Ghadage M.M.
44. Stainless steel
Properties of stainless steel:
Corrosion resistant
High ductility and formability
Good mechanical properties at low and high temperatures
High resistance to scaling and oxidation at elevated temperatures
Good weldability
Good machinability
Good creep resistance
Excellent surface finish and appearance
Types of stainless steel:
Martensitic stainless steel
Ferritic stainless steel
Austenitic stainless steel
Precipitation hardened stainless steel
44Prof.Ghadage M.M.
45. Martensitic stainless steel (Group A)
Amount of Cr in solid solution form is
less than 13% i.e.
% Cr – (17 x % C) < 13
Shows austenitic phase at high
temperature and hence can be hardened
by martensitic transformation. Thus,
called as martensitic stainless steel
Properties: Hard, wear resistant,
corrosion resistant and magnetic in
nature
Typical mechanical properties in
hardened condition;
Y. S. = 1200MPa, U. T. S. = 1300 MPa,
% elongation = 5
Composition of AISI 410: 12-14% Cr,
< 0.15 %C
Applications: Springs, ball bearings,
valves, razors and razor blades, surgical
instruments, cutting tools, cutlery items
etc
Surgical instruments
45Prof.Ghadage M.M.
46. Ferritic stainless steel (Group B)
Amount of Cr in solid solution form is greater than 13% i.e.
% Cr – (17 x % C) > 13
Cr is ferrite stabiliser and at 12.5% Cr austenitic phase disappears, thus steels
containing more than 13% Cr show only ferrite from room temperature to high
temperature and are called ferritic stainless steel
Cannot be hardened by martensitic transformation
Properties: High corrosion and oxidation resistance as compared to group A,
soft, ductile, malleable and magnetic in nature, low cost (absence of Ni), good
formability
Typical mechanical properties in annealed condition;
Y. S. = 350MPa, U. T. S. = 550 MPa, % elongation = 30
Composition of AISI 430: 14-18% Cr, < 0.12% C, with small amounts of Mo,
V
Applications: Vessels in chemical and food industries, pressure vessels,
furnace parts, heaters, heat exchangers, juice carrying pipes in sugar industries,
restaurant equipments, pots and pans etc
46Prof.Ghadage M.M.
47. Austenitic stainless steel (Group C)
Includes at least 24% of total of Cr, Ni and Mn
Ni and Mn are austenitic stabilizers and hence these steels contain austenite at
room temperature and called as austenitic stainless steel
Composition of AISI 202: 17-19% Cr, 4-6% Ni, 7-10% Mn, < 0.15% C,
0.25% N
Properties: Soft, ductile, malleable (more than group B), non-magnetic,
excellent cold forming strength, high temperature strength, high coefficient of
thermal expansion, low thermal conductivity, high corrosion resistance (more
than group A and B, because of high amount of nickel and chromium)
Applications: Engine manifolds, food and chemical plants, tubular
exchangers, utensils, wrist watches, sanitary fittings etc
47Prof.Ghadage M.M.