Introduction to Basic Manufacturing Processes and ... - always yours

Heat Treatment 133
steels, in iron carbon equilibrium diagram. This range is also known as critical range. Over
heating for too long at a high temperature may lead to excessive oxidation or decarburization
of the surface. Oxidation may manifest itself in the form of piece of scale which may be driven
into the surface at the work piece if it is going to be forged. If steel is heated, well above the
upper critical temperature, large austenite grains form. In other words steel develops
undesirable coarse grains structure if cooled slowly to room temperature and it lacks both in
ductility and resistance to shock.
Molten Iron
Austenite
1505
E
Austenite 0.5% C
E
1340
D
Austenite
Ferrite
D
Temperature °C
Pearlite
Ferrite
800
721
C
B
A
A
B
C
Time
Time
Fig. 8.5 Heating and cooling curve of steel
8.6 IRON-CARBON EQUILIBRIUM DIAGRAM
Fig. 8.6 shows, the Fe-C equilibrium diagram in which various structure (obtained during
heating and cooling), phases and microscopic constituents of various kinds of steel and cast
iron are depicted. The main structures, significance of various lines and critical points are
discussed as under.
8.6.1 Structures in Fe-C-diagram
The main microscopic constituents of iron and steel are as follows:
1. Austenite
2. Ferrite
3. Cementite
4. Pearlite