Nanostructure Formation in Austenitic Stainless Steel

Abstract

Samples of 316LVM stainless steels were hydrostatically extruded in a multi-step process to a total true strain of 1,84 and subsequently annealed at different temperatures. The structural changes occurring as a result of HE and annealing were observed using a transmission electron microscope. The microhardness of the samples was measured using a load of 200g. The results show that hydrostatic extrusion results in a uniform microstructure characterized by a high density of nano-twins. Subsequent annealing at 600°C produces a partial transformation to a nano structure of average grain size 54 nm. At an annealing temperature of 700°C a fullyrecrystallized uniform microstructure consisting of 68 nm diameter nano-grains was formed. It should be noted that 700°C is much lower than that required to recrystallize a micro-grained alloy of the same composition. The microstructural changes which occurred during annealing have a significant effect on the mechanical properties. The microhardness after HE increased following annealing at 500°C. However, annealing at 800°C resulted in a drop in microhardness, indicating the occurrence of grain growth.