An experimental investigation of the behaviour of steels over large temperature and strain rate ranges

During forging and machining manufacturing processes, the material is subject to large strains at high strain rates which provoke local heating and microstructural changes. Modelling of these phenomena requires precise knowledge of the stress–strain constitutive equations for a large range of strains, strain rates and temperatures. An experimental study of the rheology of both hyper- and hypo-eutectoid steels (with different microstructures) over a temperature range from 20 °C to 1000 °C and with strain rates from 10 − 2 to 10 5 s − 1 has been undertaken. These tests were performed in compression on cylindrical specimens and in shear using hat-shaped specimens. Both a GLEEBLE 3500 thermomechanical testing machine and a Split-Hopkinson Pressure Bar apparatus were used. From these tests, three deformation domains have been identified as a function of the material behaviour and of the changes in the deformed microstructure. Each domain was characterized by its behaviour, including the competition between hardening and softening, strain rate sensitivity on the flow stress and the softening phenomenon (i.e. recrystallisation or recovery, etc.). Finally, based on thermodynamical considerations, the conditions of thermoplastic instability (i.e. shear bands, twinning, heterogeneities, etc.) and microstructural changes are highlighted using process maps of the dissipated power repartition.