Formation of Nanostructure in AISI 316L Austenitic Stainless Steel by Friction Stir Processing

Abstract

Friction stir processing (FSP) was used to modify the microstructure in 316L austenitic stainless steel. FSP was successfully applied on 6 mm thick plate using a 16 mm diameter cylindrical WC-3%Co tool with rotating and traverse speeds of 315 rpm and 31.5 mm/min, respectively. Grain refinement, microstructural control and the related recrystallization mechanisms were characterized using optical microscopy and FE-SEM. The results indicated that the nanograins with 50-200 nm in size have been achieved from base material with grain sizes of 30-40 μm. The restoration mechanism responsible for the formation of nanostructure is likely to be discontinuous dynamic recrystallization (DDRX). Decreasing the grain size during FSP led to considerable improvement of surface mechanical properties such as twofold increase in wear resistance and threefold increase in surface hardness. There was a continuous decrease in hardness from the nanograin sized stir zone with 510 HV towards the base material with 180 HV.