Module 4: Solidification and casting and finishing operations
Lecture 36: Heat treatment
Isothermal decomposition of austenite
Let us consider isothermal decomposition of austenite. Steel in the austenitic condition is rapidly quenched to a particular temperature and then allowed to transform at that temperature. Depending on the quenching temperature, different phases can be formed. The figure 36.1 shows isothermal transformation diagram for a eutectoid plain carbon steel showing formation of different phases.
It must be emphasized that very slow cooling of steel from austenitic region will produce ferrite and cementite. However such small cooling rates are not practically kept.
Figure 36.1
Isothermal transformations for an eutectoid plain carbon steel
In the figure the lines a,b,c,d,e,f and g indicates the cooling rates. MS and M90 are the temperatures to begin and 90% completion of martensitic transformation. Line a denotes a very fast cooling rate which will transform all austentine into martensite. The cooling rate and the type of transformation are given in the table
Line
Type of trnaformation from austenite
a
All martensite
b
All coarse pearlite
c
All fine pearlite
d
Approximately 50% fine pearlite and 50% martensite
e
All upper bainite
f
Approximately 50% lower bainite and 50% martensite
