Peak strength, strain hardening and dynamic restoration of A2 and M2 tool steels in hot deformation

Hot torsion continuous tests were performed on a cold work tool steel (A2) and a high speed steel (M2) in the temperature range of 900–1150°C at strain rates of 0.1, 1 and 4 s−1. The stress–strain curves, up to the peak, were analyzed for the dependence on temperature and strain rate, of strength, strain hardening, and dynamic recovery and recrystalization. The θ–σ curves (where θ is the strain hardening rate, dσ/dε) display a linear decline from a common origin, due to dynamic recovery (DRV), gradually changing to a second linear segment as a result of the formation of subgrains, and then a rapid decline, at the onset of dynamic recrystalization (DRX), to θ=0 at peak stress. Work hardening was related to the Kocks–Mecking model and the average activation enthalpy so determined compared quite well with the activation energy derived from the hyperbolic sine analysis of peak stress. The M2 steel has a significantly greater volume fraction of carbides, particularly the harder types, than the A2 steel and consequently the M2 is stronger and starts to recrystalize sooner due to earlier nucleation around the carbide particles.