Microstructure Evolution and Mechanical Properties of AISI 316H Austenitic Stainless Steel Processed by Warm Multi-Pass ECAP

Abstract

AISI 316H austenitic stainless steel was subjected to equal channel angular pressing (ECAP) at 350°C up to eight passes and the resultant mechanical properties were compared with those obtained after cold rolling as representative for a cold worked state. A high density of thin parallel slip bands was observed inside coarse grains at the beginning of ECAP. Intense micro shear banding took place when pressing was continued by adding more passes. While no martensitic transformation was detected in the microstructure deformation twinning identified to occur at late passes of ECAP. The final microstructure after 8 passes is characterized by a tri-modal grain size distribution with equiaxed ultrafine grains of 176 nm and regions with larger grains of 217–1376 nm surrounded by shear bands containing nano-crystalline grains. Nano twins, 8 nm on average wide, were observed inside the nanocrystalline austenite grains. The observed microstructural features were explained by the stacking fault energy and temperature range for martensitic transformation of the studied material. In terms of mechanical properties, the processed material displayed a combination of very high yield strength exceeding 1550 MPa (close to four times the initial value) and good ductility with deformation εf = 11.6% at failure.