EFFECT OF STRESS AND STRAIN ON MARTENSITIC TRANSFORMATION IN A Fe-Ni-Mo-C ALLOY WITH A HIGH Ms TEMPERATURE

The interactions between stress and martensitic transformation are studied for a low alloyed steel (60NCD11) with a Ms temperature of 230°C. For tensile tests carried out at temperatures just above Ms, the transformation doesn't exhibit the stress-assisted behavior expected for athermal martensitic transformation. In contrast, for "static" tests, Ms is enhanced even at low stresses, but this enhancement is cooling-rate dependant. A comparison has been made with an Fe-Ni-C-Cr alloy obtained by adding Nickel to the 60NCD11 steel and an Fe-20 Ni - 0.5 C alloy. For these two alloys Ms is below room temperature. A classical scheme with stress-assisted martensite and strain-induced martensite is observed for the Fe - 20 Ni - 0.5 C alloy but an intermediate behaviour is obtained for the Fe-Ni-C-Cr alloy.