
204 Engineering Materials Technology
For a 1.2 per cent hypereutectoid steel (as in Figure 11.6) at just above
723°C:
Mass of cementite x (1.20 - 0.80) = mass of austenite x (6.67 - 1.2)
mass of cementite _ 6.67 - 1.2
mass of austenite 1.2 - 0.80
= 13.7
Thus at this temperature about 93 per cent is cementite and 7 per cent
austenite. The austenite forms pearlite at 723°C and thus about 7 per cent will
become pearlite. There will be little further change on cooling to room
temperature and thus the composition of a 1.2 per cent hypereutectoid steel
will be 93 per cent cementite and 7 per cent pearlite.
The effect of carbon content
Ferrite is