Scanning electron microscopic observations are made at the
fracture surface of the Charpy impact samples to identify the
mode of fracture as a function of tempering time, Fig. 6. In the
2 h tempered specimens, ductile fracture mechanism is dominant
as shown in Fig. 6(a). This dimple like morphology clearly
indicates a ductile failure mode and the progress of damage
following a void nucleation, growth and coalescence process.
However, the fracture surface of the specimens tempered at 24 h
and 48 h shows presence of quasi cleavage features along with
dimples, which suggests a mixed mode fracture behaviour at
higher tempering time, Fig. 6(b). The dimple size is also coarser
than that in the case of 2 h tempered specimens.