Iron Iron carbide equilibrium diagram, phases involved, transformations in Fe-Fe3C diagram, critical temperatures,

1. IRON-IRON CARBIDE
EQUILIBRIUM DIAGRAM

2. Introduction
• Plain Carbon Steels
• Alloy of iron and carbon which contain carbon ranging from 0.008 to
2.%
• no minimum percentage of other alloying elements is mentioned
• Alloy steels
• In steels some of the other elements are intentionally added to steels
to increase some of the required properties, these steels are called
Alloy steels
• Alloying elements are manganese, silicon, boron, chromium,
vanadium and nickel
• The properties of plain carbon steels and alloy
steels can be discussed with the help of Fe-C
diagram.

3. Iron carbon equilibrium Diagram
Various phases
existing in the
phase Diagram
α- Ferrite
γ- Austenite
δ- Ferrite
Cementite
(Fe3C)

4. α- Ferrite

5.  Almost a pure Iron
 Essential solid solution of carbon in low
temperature BCC iron
 The solubility of carbon in α-iron at room
temperature is 0.008% and increases with
increase in temperature to about 0.025 at 7270C
 Relatively soft and ductile phase having hardness
about 80 BHN
 Can be extensively cold worked without cracking
 Strongly ferromagnetic upto 7680C and becomes
paramagnetic at 7680C during heating
7270C
Curie Temperature
The temperature (7680C) at which α-ferrite
becomes paramagnetic is called curie temperature
α- Ferrite

6. γ- Austenite

7. γ- Austenite
 The name “Austenite” was given in honour of
Sir Austin, who was one of the first
metallographer to study its properties
 Interstitial solid solution of carbon in FCC γ-
iron
 Can dissolve upto 2.0% carbon at 11470C
 Stable only above 7270C
 Soft, ductile, malleable, and paramagnetic
phase
 Can be extensively worked at the
temperatures of its existence
7270C
11470C

8. δ- Ferrite

9. δ- Ferrite
 Interstitial solid solution of carbon in high
temperature BCC δ- iron
 Similar to α- ferrite except its occurrence
temperature

10. Cementite (Fe3C) or Iron Carbide or Carbide

11. Cementite (Fe3C) or Iron Carbide or Carbide
 It is an intermetallic compound of iron and carbon
with a fixed carbon content of 6.67% by weight
 It has complex orthorhombic crystal structure with
12 iron atoms and 4 carbon atoms in a unit cell
 Extremely hard and brittle phase having hardness
900 BHN
 It is ferromagnetic upto 2100C and paramagnetic
above this temperature

12. Transformations in Fe-Fe3C Diagram
• Peritectic transformation
• Eutectic transformation
• Eutectoid transformation

13. Peritectic Transformation is
a reaction of a liquid phase
and solid phase to form a
second solid phase during
cooling
In iron carbon system:
δ- ferrite of 0.1% C is added
with liquid of 0.55% C at the
temperature 1493oC to get
γ- Austenite
Peritectic Transformation
Peritectic transformation

14. Applying Lever Rule
Peritectic
Transformation
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝛿 =
0.55 − 0.18
0.55 − 0.1
× 100
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝛿 = 82.2%
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝐿 =
0.18 − 0.1
0.55 − 0.1
× 100
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝐿 = 17.8%
Commercial heat treatment processes are not
done at this temperature
Amount of δ- ferrite and Liquid at the point of Peritectic transformation

15. Eutectoid Transformation is
a decomposition of a solid
into two solids
In iron carbon system:
γ- Austenite of 0.8% C is
decomposed into α-ferrite
of 0.025% C and cementite
of 6.67%C
Eutectoid transformation
The eutectoid mixture of
ferrite and cementite is
called pearlite
Alternate lamellae of ferrite
and cementite is obtained.
Properties based upon
interlamellar distance

16. Microstructure of Pearlite
The two phases of pearlite are clearly visible in the micrograph. These phases
are ferrite and cementite. The ferrite appears white, and is laminated against
the cementite which appears grey.

17. Applying Lever Rule to find out
amount of ferrite and cementite in
pearlite at room temperature
Amount of α- ferrite and cementite at the point of eutectoid transformation
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 α =
6.67 − 0.8
6.67 − 0.025
× 100
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓α = 88.33%
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐𝑒𝑚𝑒𝑛𝑡𝑖𝑡𝑒 =
0.8 − 0.025
6.67 − 0.008
× 100
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐𝑒𝑚𝑒𝑛𝑡𝑖𝑡𝑒 = 11.67%

18. In eutectic transformation liquid
transforms into mixture of two solids
In iron carbon system:
Liquid transforms to eutectic mixture of
austenite (2%C) and cementite (6.67%C)
Eutectic transformation
The eutectic mixture of austenite and
cementite is called Ledeburite
The amount of pearlite and cementite in
transformed Ledeburite at room
temperature according to phase rule
𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝𝑒𝑎𝑟𝑙𝑖𝑡𝑒 =
6.67 − 4.3
6.67 − 0.8
× 100
= 40.4%
𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐𝑒𝑚𝑒𝑛𝑡𝑖𝑡𝑒 =
4.3 − 0.8
6.67 − 0.8
× 100
= 59.60%
Cementite phase having carbon content 6.67% due
to which cementite is hard and brittle and pearlite
is slightly and therefore, transformed Ledeburite is
hard and brittle
The mixture of pearlite and cementite at room temperature is called
transformed ledeburite

19. Critical Temperatures
 These are temperatures at which transformations in solid state takes places
A0- Curie Temperature of Cementite
A1- Lower Critical Temperature
A2- Curie Temperature of Ferrite
A3- Upper Critical Temperature for Hypoeutectoid Steels
Acm- Upper Critical Temperature for Hypereutectoid Steels
A4

20. A0 - Curie Temperature of Cementite
2100C Cementite becomes paramagnetic

21. A1 – Lower Critical Temperature
7270C Pearlite starts transforming to austenite

22. A2 – Curie Temperature of Ferrite
7680C Ferrite becomes paramagnetic

23. A3 – Upper Critical Temperature for Hypoeutectoid Steels
7270C- 9100C Completion of ferrite to austenite transformation

24. Acm – Upper Critical Temperature for Hypereutectoid Steels
7270C- 11470C Completion of cementite to austenite transformation

25. A4
14000C- 14930C Completion of austenite to δ- ferrite transformation

26. Proeutectoid Ferrite
Proeutectoid Ferrite Region
α- Ferrite which is separated
before eutectoid transformation
Eutectoid Point

27. Hypoeutectoid Steels
o Contains carbon from 0.008 to 0.8%
o As the carbon increases, the amount of Proeutectoid ferrite decreases and pearlite
increases
o For 0.8% C, the amount of Proeutectoid ferrite becomes 0% and pearlite becomes
100%.
o For 0.008% C, the amount of α is
100% and for 0.8%C, the amount
of pearlite is 100%.

28. Microstructures of Hypoeutectoid Steels
Ferrite -
White in colour
Pearlite –
Dark or Lamellar
At 100X

29. Proeutectoid Cementite
Eutectoid Point
Proeutectoid Cementite Region
Fe3C which is separated before
eutectoid transformation

30. Hypereutectoid Steels
Hypereutectoid steel contains free cementite 0% for 0.8% C steel and increases
linearly with increasing carbon reaching to Maximum (20.4%) for 2.0% carbon steel.
• In hypereutectoid steel, the microstructure at
room temperature contains proeutectoid cementite
and pearlite.
• The main difference here with hypoeutectoid steel
structure is that, a continuous network of
cementite is obtained, which separates each
pearlite colony.
Microstructure of Hypereutectoid Steel

31. Classification and Applications of Steels
Criterions for Classifications
Criterions for classification
Amount of Carbon
Amount of Alloying elements and Carbon
Amount of Deoxidation
Grain Coarsening Characteristics
Method of Manufacture
Method of Hardening
Form and Use

32. Amount of Carbon
Type of Steel Weight % of Carbon
Low Carbon Steels 0.008 – 0.3
Medium Carbon Steels 0.3 – 0.6
High Carbon Steels 0.6 – 2.0

33. Low Carbon Steels
• 0.008 – 0.3 % Carbon
• Soft, Ductile, Malleable, Tough, Machinable, Weldable
• Can not be hardened by heat treatment
• They are good for cold working processes (Rolling, Press working, Tinning, Galvanizing)
• Good for fabrication work
Applications
Rivets
Wires
Nails
( Low Carbon Steel Zinc Plated)
Screws Mild Steel Welding Electrodes

34. 10mm thickness Low Carbon Steel Sheet
SA283GrC for the boiler and ship
Gears
Valves
Connecting Rod
Low Carbon Steels
Fan blades

35. Low Carbon Steels
Railway Axels
Fish Plates
Tubes for bicycles
Tubes for automobiles

36. Medium Carbon Steels or Machinery Steels
• 0.3 – 0.6 % Carbon
• These steels have intermediate properties to those of low carbon and high carbon steels
• They are medium hard not so ductile and malleable, medium tough, slightly difficult to
machine, weld and harden.
• They require high cooling rates for hardening and hardness produced after hardening is not
so high.
• The depth of hardening is also less and hence they are shallow hardening type
• They are difficult to cold work and hence hot worked.
Applications
Bolts
Lock Washers
Hammers

37. Large Forging Dies
Turbine rotors
Springs
Medium Carbon Steels

38. Medium Carbon Steels
Wires Railway rails
Railway tyres

39. High Carbon Steels or Tool Steels
• They are also called as Tool steels.
• They are hard, wear resistant, brittle, difficult to machine, difficult to weld,
and can be hardened by heat treatment.
• The hardness produced after hardening is high.
• The depth of hardening is also high i.e. hardenability is more as compared
to medium carbon steels.
• These steels cannot be cold worked and hence are hot worked.
High Carbon Steel Machine
Screw Hex Dies Punches
Clips

40. Clutch Disc
shear blades, drills, leaf springs, music wires,
knives, razor blades, balls and races for ball
bearings, mandrels, cutters, files, wire
drawing dies, reamers, and metal cutting
saws.
Car bumpers
Vice Jaws

41. On the basis of Alloying elements and Carbon
On the basis of Alloying elements
1. Low alloy steels- Contain alloying elements less than 10%
2. High alloy steels- Contain alloying elements more than 10%
On the basis of Alloying elements and Carbon
Carbon Content Total content of alloying elements
Low (< 0.3%)
Medium (0.3 – 0.6%)
High (>0.6%)
Low (<10%)
High (>10%)
1. Low carbon low alloy steels
2. Low carbon high alloy steels
3. Medium carbon low alloy steels
4. Medium carbon high alloy steels
5. High carbon low alloy steels
6. High carbon high alloy steels

42. Classification of Steels On the basis of Deoxidation
1.Rimmed Steels
2.Killed Steels
3.Semi-killed steels

43. Rimmed Steels
• This layer of the ingot i.e. rim is of
less carbon, more purity, free from
blow holes and free from
segregation of impurities
• Entrapped gases forms blow holes
which eliminates during subsequent
working operation
• Used for deep drawing and forming
operations

44. • Killed Steels
• Dissolved oxygen in the melt is
completely removed by the addition of
strong deoxidizing agents like Al, Si, or
Mn
• They rapidly combine with Dissolved
oxygen
• So oxygen is removed
• This steel shows more pipe because
of absence of oxygen
• These steels are used for components
which have to be forged, carburized,
or heat treated

45. Semi-killed Steels
• Part of dissolved oxygen is removed by the
addition of deoxidisers
• Blow holes are formed by the evolution of CO
compensate for the part of the shrinkage hence
pipe is less
• Used for sheets, plates, structural shapes etc

46. Classification of Steels On the basis of Grain
Coarsening characteristics
Coarse Grained Steels
 coarsen rapidly with temperature
 Rimmed steel behave coarse grained
Fine Grained Steels
 Do not coarsen much upto a definite temperature
 Aluminium killed or alloy steel behave fine grained

47. Classification of Steels On the basis of method of
manufacturing
o Basic Open Hearth
o Electric Furnace
o Basic Oxygen Process
o Acid Open Hearth
o Acid Bessemer
This method of classification does not throw light on composition
or mechanical properties of steels

48. Classification on the basis of Depth of Hardening
 Non-hardenable Steels
o Contains less carbon and almost no alloying element.
o Suitable for fabrication by cold working and welding.
o Applications similar to those of low carbon steels.
 Shallow Hardenable Steels
o They are medium carbon with or without alloying elements
o Intermediate to those of non-hardening and deep hardening steels.
o It gets hardened only at the surface
o Used for gears, camshafts and such other applications.
 Deep Hardenable Steels
o Contains more carbon and alloying elements.
o Used where depth of hardening required is more or through hardening is
necessary.
o Applications similar to those of high carbon steels.

49. Classification on the basis of form
 Cast Steel
o Iron alloy with carbon upto 1.7%
o Used in the casting process
o Castings are heat treated to achieve specified properties and
machined to required dimensions
 Wrought Steels
o Undergoes two operations
1. Poured into ingots
2. Metal is reheated and hot rolled into the finished form

50. Classification on the basis of Applications
 Boiler Steels
 Case hardening steels
 Corrosion and heat resistant steels
 Deep drawing steels
 Electrical steels
 Free cutting steels
 Machinery steels
 Structural steels
 Tool steels

51. Designation of Steels
Criteria of
Designation
Method of
Manufacture
Chemical
composition
Heat treatment
Mechanical
properties
Quality
Majority of
Specification
The knowledge of the specification of the steel helps to select a proper type of steel
as per the service requirement

52. Indian Standard Designation System
 Adopted by INDIAN STANDARD INSTITUTION in 1960
 Revised in 1971 into two parts
• Designation of steel based on LETTER SYMBOLS
• Designation of steel based on NUMERALS
 Again revised in 1974
Designation on the basis of Mechanical Properties
 Based on the tensile or yield strength
 Symbol Fe is used to designate minimum Tensile Strength in N/mm2
 Symbol FeE is used to designate minimum yield strength in N/mm2
 Symbol St is used to designate minimum Tensile Strength in Kg/mm2
 Symbol StE is used to designate minimum Yield Strength in Kg/mm2
 Sometimes special characteristics are used after the above letters

53. Designation on the basis of Mechanical Properties
Fe 410 K Killed steels with minimum tensile strength of 410
N/mm2
St 42 Steel with minimum tensile strength of 42
kg/mm2
Fe E 270 Steel with minimum yield strength of 270 N/mm2
Examples

54. Designation on the basis of Chemical Composition
1. Numerical figure which indicates 100 times the average percentage of
carbon
2. Letter ‘C’ for carbon steels and letter ‘T’ for tool steels
3. Figure indicating 10 times the average percentage of ‘Mn’ content
C 20 Steel with average carbon of 0.2%
C 40 Steel with average carbon of 0.4%
25 C 5 Steel with average carbon of 0.25 and Mn 0.5%`
80 T 11 Plain carbon tool steel with average 0.8% carbon and
1.1% Mn
Examples
Designation of Plain Carbon Steels

55. Designation of Unalloyed Free Cutting Steels
The designation consist of
1. A figure indicating 100 times the average percentage carbon.
2. Letter ‘C’
3. Figure indicting 10 times the average percentage of Mn.
4. Symbol 'S(Sulphur)', 'Se (Selenium)', 'Te (Tellurium) ' or ' Pb (Lead)' depending
on the element present which makes the steel free cutting followed by the figure
indicating 100 times the percentage content of the element.
5. In the case of the phosphorized steels the symbol P shall be included
6. Symbol indicating special characteristics covering the method of deoxidation,
surface condition and heat treatment
25C12S14 free cutting steel with 0.25% carbon, 1.2% Mn, and 0.14% S
35C10S14K free cutting steel with 0.35% C, 1% Mn, and 0.14% S, killed quality
Examples

56. Designation of Alloy Steels
 Low Alloy Steels
 High Alloy Steels
 Alloy Tool Steels
 Free Cutting Alloyed Steels
 Low Alloy Steels
 High Alloy Steels
 Alloy Tool Steels
 Free Cutting Alloyed Steels

57. Designation of Low Alloy Steels
The designation consist of
1. A figure indicating 100 times the average percentage carbon.
2. Chemical symbol for alloying elements, each followed by the figure for its
average percentage content multiplied by a factor as given
Element Multiplying Factor
Cr, Co, Ni, Mn, Si and W 4
AI, Be, V, Pb, Cu, Nb, Ti, Ta, Zr and Mo 10
P, S, N 100
NOTE 1 - The figure after multiplying shall be rounded off to the nearest integer.
NOTE 2 - Symbol ‘Mn' for manganese shall be included in case manganese content is equal to or greater than 1 percent,
NOTE 3 - The chemical symbols and their figures are listed in the designation in the order of decreasing content.
3. Symbol indicating special characteristics covering degree of purity hardenability
weldability guarantee, elevated temperature properties, surface condition,
surface finish and heat treatment
25Cr4Mo2G Steel with guaranteed hardenability and having 0.25% carbon, 1% Cr, and 0.25% Mo
40Ni8Cr8V2 Hot rolled steel with average 0.4% carbon, 2% Cr, 2% Ni, and 0.2% V

58. Designation of High Alloy Steels
The designation consist of
1. Letter ‘X’
2. A figure indicating 100 times the average percentage carbon.
3. Chemical symbol for alloying elements, each followed by the figure for its
average percentage content
4. Symbol indicating special characteristics covering degree of purity hardenability
weldability guarantee, elevated temperature properties, surface condition,
surface finish and heat treatment
25Cr4Mo2G
Steel with guaranteed hardenability and having 0.25% carbon, 1% Cr,
and 0.25% Mo
40Ni8Cr8V2 Hot rolled steel with average 0.4% carbon, 2% Cr, 2% Ni, and 0.2% V

59. Designation of Alloy Tool Steels
The designation is same as that of low and high alloy steels, except that the symbol
Tis included in the beginning
XT75W18Cr4v1 High alloy tool steel with 0.75% C, 18% W, 4% Cr and 1% V
XT98W6Mo5Cr4V1 High alloy tool steel with 0.98% C, 6% W, 5% Mo, 4%Cr and 1% V

60. Designation of Free Cutting Alloy Steels
The designation is same as that of low and high alloy steels, except that depending
on the percentage of S, Se, Te, and Zr, the symbol of the element present followed
by the figure indicating 100 times its content
X15Cr25Ni15S40
High Alloy free cutting steel with 0.15%C, 25%Ni, 15%Ni, and
0.40% S
X12Cr18Ni3S25
High alloy free cutting steel with 0.12% C, 18%Cr, 3% Ni and
0.25% S

61. AISI/ SAE Designation of Steel
• Designation is based on the chemical composition of steel
• Designation consists of 5 numerical digits
First Digit (from left)
Indicates type of steel
1 Carbon steels
2 Nickel steels
3 Ni – Cr steels
4 Molybdenum steels
5 Chromium steels
6 Cr – V steels
7 Tungsten steels
8 Ni – Cr – Mo steels
9 Si – Mn steels
Second Digit
Indicates the concentration of major
alloying element in percentiles
Last two or three Digit
Indicates 100 times the weigh percentage of carbon
Difference between AISI and SAE Designation
In AISI designation system, in addition to the numbers,
it includes a letter prefix to indicate the manufacturing
process of that steel. Absence of the letter prefix implies
that such a steel is predominantly open hearth.
SAE designation system does not use a letter prefix.
A Basic open hearth alloy steel
B Acid Bessemer steel
C Basic open heart carbon steel
D Acid open hearth carbon steel
E Electric furnace steel

62. AISI/ SAE Designation of System
1040
1- indicates plain carbon steel
40- indicates 0.40% C
2440A
2- indicates nickel steel
4- indicates 4% Ni (approximate)
40- indicates 0.4% C
A- indicates basic open hearth
manufacturing process
9260
9- indicates Si - Mn steels
2 – indicates 2% Si
60- indicates 0.6% C

63. British Standard Designation System
• It is well known by ‘En’ series
• En stands for emergency number
• Developed during the emergency period of world war- II (1942)
• En number of a steel has no correlation with the composition or mechanical properties
of the steel
En No C Mn Ni Cr Mo Others
8 0.35/0.45 0.60/1.0 - - - S- 0.06
P- 0.06
Si- 0.05/0.35
9 0.50/0.60 0.50/0.80 - - - S- 0.06
P- 0.06
Si- 0.05/0.35
24 0.35/0.45 0.45/0.70 1.30/1.80 0.90/1.40 0.20/0.35 Si- 0.10/0.35