Structural refinement in austenitic stainless steel by submerged friction stir processing

In the current study, surface modification of austenitic stainless steel, SS316L, was done using friction stir processing. The strain rates during processing were varied over a wide range by using two different tool rotational speeds of 388 rpm and 1800 rpm. The processing was done at two different conditions of: (i) ambient cooling and (ii) submerging in liquid bath maintained at 0 °C. Friction stir processing resulted in significant grain refinement from 22 μm for the as-received alloy to 0.67 μm for the sample processed at 388 rpm under submerged cooling condition. A relationship between the final recrystallized grain size and Zener-Holloman parameter was developed as Z=72.84/d. Deformation texture for processed specimens showed strong strain rate and temperature dependence. Sample processed at high temperature and low strain-rate showed copper-type texture while low temperature and high strain-rate promoted brass-type texture. Hardness of processed specimens showed an increase while elastic modulus decreased after processing which is explained by texture evolution during processing. Tribological properties of processed specimens were evaluated using reciprocating wear tests and slurry erosion tests. The processed samples showed significant improvement in the wear and erosion resistance and were found to have strong correlation with the flow work given as hardness to elastic modulus ratio.