HOT WORKABILITY OF M2 TYPE HIGH-SPEED STEEL

The hot workability of an M2 type HSS in cast, forged and rolled condition has been studied by means of a torsion test and metallography. Continuous tests were used to determine the temperature and strain-rate dependence of the flow stress and temperature dependence of the strain to fracture, while multistage tests were used to determine the extent of interpass softening. Temperature and strain dependence of the flow stress is described by a relation of the form: Z = epsilon.. e(Q)HW/RT) = A . e(beta.sigma) in which the activation energy for hot working (Q(HW)) is found to be temperature dependent. The hot ductility of the cast steel is not only lower but is also little affected by temperature in comparison to worked steels, in spite of the fact that critical strain for dynamic recrystallization is smaller, and the extent of metadynamic and static recrystallization during interpass interval is more extensive in the former. This is related to a large volume fraction of carbides, which give rise to a high stored energy and enhanced recrystallization on the one hand, and to the suppression of recrystallization within the continuous network of carbides on the other hand. Carbides give rise to an easy nucleation, growth and coalescence of cracks.