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

The temperature dependencies of true strain-hardening and plastic-in stability properties are investigated for austenitic stainless steels; including annealed 304, 316 316LN, and 20% cold-worked 316LN, at test temperatures from -150 to 450 degreesC. In both annealed and cold-worked conditions, strength decreases with increasing temperature, while ductility peaks below room temperature and is least at about 400 degreesC. 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. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.