Identification for the optimal working parameters of as-extruded 42CrMo high-strength steel from a large range of strain, strain rate and temperature

The intrinsic workability of as-extruded 42CrMo high-strength steel was investigated by using processing maps constructed from the stress–strain data generated by hot compression tests with a height reduction of 60% over a range of temperatures 1123–1348 K and strain rates 0.01–10 s−1. As the true strain was −0.3, −0.5, −0.7, and −0.9, respectively, the responses of strain rate sensitivity (m-value), power dissipation efficiency (η-value) and instability parameter (ξ-value) to temperature and strain rate were evaluated. By the superimposition of power dissipation and instability maps, the stable and unstable regions were clarified clearly, and furthermore in the stable area the regions with higher power dissipation efficiency (≥0.2) were identified and recommended. As the true strain is −0.3, −0.5, −0.7, and −0.9, respectively, three domains were identified at each strain. The whole area of the domains increased with increasing true strain from −0.3 to −0.7, while it decreased with increasing true strain from −0.7 to −0.9. On the basis of determination for domains with dynamic recovery (DRV) or dynamic recrystallization (DRX) microstructural evolution, the full DRX regions were identified in the processing map at true strain −0.5 to −0.9, and the full DRV regions at true strain −0.2 to −0.9. Then not only the domains optimized were validated by DRX refined microstructures without any wedge crack, but also the regimes of flow instabilities were validated by the microstructures involving intercrystalline cavity, dynamic strain aging (DSA), deformation twinning. The optimal working parameters identified by processing maps and validated by microstructure observations contribute to designing the reasonable hot forming process of as-extruded 42CrMo high-strength steel without resorting to expensive and time-consuming trial-and-error methods.