Study of the Reverse Martensitic Transformation Using Non-Destructive Electromagnetic and Materials Characterization Techniques

摘要

The deformation-induced phase transformation hardening phenomenon is normally observed in austenitic stainless steels of the AISI 300 series. The amount of hardening is limited by the structural stability of the material. In fact, during the cold forming process, low nickel austenitic stainless steels can undergo deformation-induced phase transformation of fcc gamma-austenite to bcc alpha'-martensite (DIM). On the other hand, by heat treatments, the austenitic structure can be recovered. The reversion of martensite to austenite is studied on a set of austenitic stainless steel specimens (AISI 316). These had been submitted to 63% deformation at a temperature of (-70°C), followed by isochronic, isothermal heat treatments at temperatures between 200 and 900°C. The reversion of martensite in the specimens is studied by optical and electron microscopy, saturation magnetization and electromagnetic non-destructive (ND) methods (conductivity measurements, Feritscope, and permeability evaluation by eddy currents (EC)). A new calibration curve for the assessment of α' phase with Feritscope is presented. The results obtained with the different techniques are compared and discussed.