Effect of strain paths and residual delta ferrite on the failure of cold rolled austenitic stainless steels, type 304L

Abstract

This study was carried out to assess the effect of strain paths and residual delta ferrite on failure characteristics of austenitic stainless steels cold rolled to 20% reduction. Optical metallography was carried out to determine grain size and quantify residual delta ferrite. Mechanical tensile tests to failure along three orthogonal directions were carried out on annealed and 20% cold rolled samples to study the effects of strain paths on mechanical properties of the material. Post-mortem scanning electron microscopy was used to study fracture surface and cross-sectional views of the failed specimens. The yield strength of material cold rolled to 20% reduction increased relative to annealed material along the rolling and transverse directions by twice as much along the normal direction. The increase in the yield strength occurred at the expense of ductility which decreased by about half in all directions. It also emerged that material loaded along rolling and transverse directions showed a gradual failure (from rupture strength), while that loaded along the normal direction exhibited a rapid failure. Correlation between ferrite morphologies on the three orthogonal planes and failure characteristics on fracture surfaces were established. The scanning electron microscopic micrographs suggest that materials loaded along transverse and rolling directions failed with characteristic features of pure ductile failure, while specimen loaded along normal direction showed mixed features of ductile and brittle failure.