1. Development of Structures in Iron– Carbon Alloys
MME 293: Lecture 9
Development of Structures in Iron– Carbon Alloys
Department of MME
BUET, Dhaka

2. Today’s Topics  The critical temperature lines
 Microstructural development during slow cooling of steels
 Classification of steels
 Effect of carbon and other alloying elements on structure and properties of steels
Reference:
1. SH Avner, Introduction to Physical Metallurgy, 2nd Ed., Ch. 7, pp
2. WD Callister, Jr. Materials Science and Engineering An Introduction,

3. Iron – Iron Carbide Phase Diagram
Temperature, °C
wt. % carbon L
austenite + L
L + cementite
austenite
austenite + ferrite
austenite + cementite
ferrite
cemenite
ferrite + cementite Fe
Fe3C

4. The Critical Temperature Lines
Temperature, °C
wt. % carbon
0.80
2.00
1130
910
723
Upper Critical Temperatures
where transformation of austenite begins during cooling
A3  g-a allotropic transformation
Acm  precipitation of Fe3C from g
Upper-critical Temperature Line A3
Acm g
g+Fe3C
Lower Critical Temperatures
where austenite transformation ends during cooling by forming pearlite isothermally
g+a A1
A3,1
Lower-critical Temperature Line
For a particular alloy, both upper- and lower-critical temperatures change during heating and cooling.
Critical temperature line goes down on cooling, while it goes up on heating.
The variation in temperature depends on cooling and heating rate. For slower rates, the two temperatures approach each other.
It is found that in actual practice the critical line on heating and the critical line on cooling are not occur at same temperature.
The critical line on heating is always higher than the critical line on cooling. The former is denoted by Ac and the later is denoted by Ar. 
A for arret (means arrest), C for chauffage (means heating), R for Refroidissement (means cooling), e for equilibrium.
If extremely slow rates of heating or cooling are employed then critical temperatures are nearly equal i.e. Ac1 = Ar1 = Ae1
a+P
P+Fe3C
Steel portion of Fe-Fe3C diagram
(d-zone omitted)

5. Slow Cooling of Steels Eutectoid steels (0.80 % C)
Temperature, °C
wt. % carbon
0.80
2.00
1130
910
723
Austenite
Ferrite + Pearlite
Pearlite + Cementite
Austenite + Ferrite
When an eutectoid steel is cooled slowly, all austenite grains transforms into the eutectoid mixture pearlite, a lamellar or layered structure of ferrite and cementite. 
The layers of alternating phases in pearlite are formed in the same way as layered structure of eutectic is formed: by redistribution C atoms between ferrite and cementite by atomic diffusion. 

6. Slow Cooling of Steels Eutectoid steels (0.80 % C)
Austenite (0.80 % C) cannot change into ferrite (0.025 % C) until some of its carbon atoms come out of solution.
Therefore the first attempt of transformation is the precipitation of carbon atoms out of austenite to form plates of cementite (6.67 % C).
Pearlite microstructure containing alternate layers of ferrite (white) and cementite (black)
In the areas immediately adjacent to cementite, the iron is depleted of carbon, and the atoms rearrange themselves to form ferrite.
Thus, thin layers of ferrite are formed on each sides of cementite plate. This process continues until all austenite change into pearlite.

7. Slow Cooling of Steels Hypoeutectoid steels (< 0.80 % C)
Temperature, °C
wt. % carbon
0.80
2.00
1130
910
723
Austenite
Ferrite + Pearlite
Pearlite + Cementite
Austenite + Ferrite
Allotropic transformation of austenite (FCC) to ferrite (BCC) starts at A3 line at the grain boundaries of austenite. 
0.20
Since ferrite can dissolve very little carbon, the extra carbon comes out of solution and the remaining austenite becomes richer in carbon.
The carbon content of g gradually moves down and to the right along A3 line. A3
Proeutectoid Ferrite   A1
At 723 C (A1 line), the remaining g grains containing % carbon undergoes eutectoid reaction and forms pearlite.

8. Slow Cooling of Steels Hypereutectoid steels (> 0.80 % C)
The Acm line is a solvus line along which carbon solubility of austenite decreases with T.
Below Acm, the extra carbon comes out of g along the grain boundary as Fe3C precipitates. 
Temperature, °C
wt. % carbon
0.80
2.00
1130
910
723
Austenite
Ferrite + Pearlite
Pearlite + Cementite +
Cementite 
1.0
Acm
Proeutectoid Cementite
During their growth, Fe3C combine each other to form one single grain (just like joining two soap bubbles).  
A3,1
As Fe3C precipitates, carbon content of the remaining g gradually moves down and to the left along Acm line.
At 723 C (A3,1 line), the remaining g containing 0.80 % carbon undergoes eutectoid reaction and forms pearlite.

9. Slow Cooling of Steels
Variation in steel structures with carbon content
Dead soft steel (C0.01%)
Ferrite grains only
Low carbon steel (C0.1%)
Mostly ferrite with a few pearlite
Medium carbon steel (C0.4%)
Almost equal ferrite and pearlite
Eutectoid steel (C0.8%)
Pearlite grains only
Hypereutectoid steel (C0.9%)
Pearlite grains surrounded by cementite network

10. Problem
For an Fe-0.35C alloy at a temperature just below the eutectoid, determine the following:
[a] The fraction of ferrite and cementite phases
[b] The fraction of ferrite and pearlite
[c] The fraction of eutectoid ferrite

11. Solution
[a] Apply lever rule for a tie line that extends all the way across the Ferrite + Cementite phase field. Then,
WF = 100 (6.67 – 0.35) / (6.67 – 0.025) = 95 %
WFe3C = 100 – 95 = 5 %
[b] Use tie line that extends only up to the Ferrite + Pearlite phase field. Then,
WF = 100 (0.80 – 0.35) / (0.80 – 0.025) = 58 %
WP = 100 – 58 = 42 %
[c] The amount of ferrite calculated in [a] is the total (i.e., proeutectoid plus eutectoid) ferrite content, while that calculated in [b] is the proeutectoid ferrite content only. Thus,
The eutectoid ferrite content = 95 – 58 = 37 %.

12. Classification of Steels
Steelmaking process
basic oxygen process, electric process, etc.
De-oxidation practice
rimmed, capped, semi-killed, killed
Product form
wire, bar, plate, sheet, strip, tubing, or structural shapes
Finishing method
cold drawn, cold rolled, hot rolled, extruded, etc.
Application
tool steels, bearing steels, spring steels, etc.
Metallography
hypoeutectoid, eutectoid, hypereutectoid
Composition
plain carbon, low alloy, high alloy, high-strength-low-alloy (HSLA)

13. Classification of Steels
Based on carbon content
Dead soft (0.03–0.10 % C) Steel
wires, rivets, chain, sheet, strip, welded pipe
Mild (0.10–0.35 % C) Steel
rolled plate, structural shapes, gears, forgings
Medium carbon (0.35–0.65 % C) Steel
connecting rods, crane hooks, shafts, axles, gears, rotors, rails
High carbon (0.65–2.0 % C) Steel
screw drivers, saws, drills, dies, hammers, wrenches, punches, chisels

14. The Influence Of Other Alloying Elements
Additions of other alloying elements (Cr, Ni, Ti, etc.) bring about rather dramatic changes in the binary iron–iron carbide phase diagram.
One of the important changes is the shift in position of the eutectoid with respect to temperature and to carbon concentration.
Thus, other alloy additions alter not only the temperature of the eutectoid reaction but also the relative fractions of pearlite and the proeutectoid phase that form.