Temperature dependence of strain hardening and plastic instability behaviors in austenitic stainless steels

The temperature dependencies of true strain-hardening and plastic-instability properties are investigated for austenitic stainless steels; including annealed 304, 316, 316LN, and 20% cold-worked 316LN, at test temperatures from -150 to 450 °C. In both annealed and cold-worked conditions, strength decreases with increasing temperature, while ductility peaks below room temperature and is least at about 400 °C. At room temperature or below, the strain-hardening behavior exhibits two stages consisting of a rapid decrease for small strains and an increase-decrease cycle before plastic instability occurs. At higher temperatures the strain-hardening rate decreases monotonically with strain. The characteristics of these strain-hardening behaviors are explained by changes in deformation microstructure. Transmission electron microscopy (TEM) of the deformed 316LN steel shows that twins, stacking faults, and/or martensite laths, along with dislocations, are formed at subzero temperatures, and dislocation-dominant microstructures at elevated temperatures. It is also shown that the average strain-hardening rate during necking to failure is almost equal to the true stress at the onset of necking. This stress is called the plastic instability stress (PIS). Cold-worked specimens fail by prompt necking at yield when the yield stress exceeds the PIS of annealed material, indicating that the PIS is independent of prior cold work.