Dynamic recrystallization of A2 and M2 tool steels

Continuous hot torsion tests were performed on a cold work tool steel (A2) and a molybdenum high speed steel (M2) from 900 to 1150°C at strain rates of 0.1, 1 and 4 s−1 The flow curves assume the classical shape of dynamic recrystallization (DRX) — rising to a peak, σp then decreasing towards a steady state stress σss The critical stress σc for the commencement of DRX is determined as the inflection at the end of the second linear portion of the graph of strain hardening rate θ (dσ/dε) versus flow stress σ. The effective completion of DRX 99% is taken at the inflection of the flow curve on its descent after the peak as it attains steady state. The rate of DRX is described by an Avrami function while the time, tss, for 99% DRX depends on σss in a sinh–Arrhenius equation. The recrystallized steady state grain size Ds increases with decrease in both σss and the Zener-Hollomon compensated strain rate Z. The presence of carbide particles stimulates DRX nucleation with the result that the M2 steel experiences earlier critical strain εc and faster DRX kinetics because of its greater volume fraction of carbides and smaller grain size.