The evaluation of high-stress creep ductility for 316 stainless steel at 550 °C by extrapolation of constitutive equations derived for lower stress levels

Abstract

The CDM-based constitutive equations for the creep of 316 Stainless Steel at 550 °C are reviewed. During creep tests carried out under these conditions, it has been observed that as time elapses inelastic straining takes place due to time independent plasticity and to creep. It has been recognised that at high stress levels the time dependent plastic strain accumulated during constant load creep tests forms a major part of the inelastic strain and dominates over the creep strain. Hence, due to the plastic strain the true stress level is not constant during the test. The time independent plastic strain has been evaluated using a stress–strain curve obtained at a high strain rate, and the creep strains have been evaluated for the relevant stress history by integration of the constitutive equations. Minimum creep rates and lifetimes have been extrapolated from low stresses to higher stresses using linear stress versus logarithmic plots. In this way, the creep strain–time history, the minimum creep rates, lifetimes and ductilities have been evaluated. In the stress range 325–450 MPa a lower shelf ductility of 1.1% has been found. The model is also shown to predict the isochronous rupture locus determined from multi-axial test data obtained from a range of different sources.