Modelling the Material Behaviour of Metastable Stainless Steels

Metastable austenitic stainless steels are designed to be thermodynamically unstable such that deformation even at room temperatures can bring about a change in the phase of face centred cubic austenite to either hexagonal close packed martensite and/or to body centred cubic martensite. This solid state phase change is a function of the strain path, strain, strain rate and temperature. A set of carefully designed experiments is conducted in the pure states of shear, compression and tension under varying temperature and strain rate. The fraction of the growing martensite phase is recorded with the help of a magnetic sensor attached to the mechanical tester. These data are then cast in a general form of constitutive equation and the transformation equations are derived from Olson-Cohen type functions. The set of equations, thus generated are further used to define an equivalent plastic stress and strain that are used to do finite element calculations for large strain forming of the metal sheets. The accuracy of the constitutive equation is validated by experiments.