Martensitic transformation in a high purity austenitic steel during low cycle torsion fatigue test

Plastic deformation of austenite decreases the energy required to transform the initial microstructure into a biphasic austenitic-martensitic one. Consequently, austenitic steels can transform above Ms temperature when plastically strained, giving complex biphasic microstructures with potentially interesting mechanical properties. However, the threshold for the strain induced martensitic deformation is not clearly defined, even under monotonic loading, since the conventional Md temperature refers to a given amount of martensite formed under monotonic plastic strain of a given magnitude. The present paper deals with the cyclic behaviour of the 17-13 high purity (HP) steel under symmetrical low cycle torsion loading (LCTF) at room temperature and for two different sequences of applied strain amplitudes. Due to the presence of a radial strain gradient, observations of sample sections allow to conclude about the threshold of the cumulative strain at 20 °C required for the fatigue induced martensitic transformation (Md-th). The analysis of the macroscopic response of the alloy with respect to LCTF gives important information about the effect of the strain amplitude on the y→α’ transformation. Finally, the transformation mechanisms under monotonic and cyclic torsion are discussed