Effect of Shot Peening Coverage on Microstructure and Mechanical Properties of the Plasma Nitrided AISI 316l Stainless Steel

Abstract

In this study, surface nanocrystallization produced by air-blast shot peening is used as a pretreatment step to enhance the kinetics of plasma nitriding. Hot-rolled plates of AISI 316L grade stainless steel samples were subjected to air-blast shot peening. Peening was carried out at different peening coverage, from conventional to severe; to assess the effect of coverage on surface nanocrystallization and subsequent diffusion. This was followed by plasma nitriding in the temperature range of 300-500 °C for the duration of 4 hours. Microstructural characterization was carried out by using scanning electron microscope, X-Ray diffractometer, transmission electron microscope, while mechanical properties of the treated layer was analysed by using microhardness and nanoindentation testers. Peening process lead to the formation of deformation induced martensite (DIM), and its fraction was found to increase with the peening coverage. Depth of the nanostructured layer and surface microhardness also increased with the increase in coverage. Severe shot peening generated about 500 μm thick gradient nanostructured (GNS) layer at the peened surface. The hot-rolled parent austenitic microstructure having grain size in the range of 40–80 μm was refined to dislocation cell- type martensite of cell size in the range of 100–140 nm. Nucleation of DIM was not limited to shear band intersections. Martensite units found to nucleate at multiple locations in the austenite; like parallel to shear bands, within the shearbands, across the shearbands, at the grain boundary, etc. Nitriding temperature of 300 °C was found insufficient to produce continuous nitride layer in spite of the severe peening pre-treatment; while nitriding at 500 °C resulted in the precipitation of the chromium nitride phase. Nitriding of un-peened and severe peened samples at 400 °C did not show chromium nitride precipitation. Significant improvement in nitride layer thickness was obtained at this temperature; from less than 1 μm for the un-peened sample to about 50 μm for the severe peened sample. Transmission electron microscopy affirmed that the nitride layer in the severe peened nitrided sample was largely martensitic in nature. Synergetic effect of selection ofright nitriding temperature, surface nanocrystallization, drastic increase in the number of dislocations and other defects, deformation induced transformation of austenite to martensite, etc. are opined to be reasons for enhanced diffusion kinetics. Hardness of the treated surface evaluated by microhardness method showed that the surface hardness increase with increase in the peening coverage and a similar trend was observed upon nitriding as well. These results were found to be in accordance with the results of scratch testing. Nanoindentation and nano-scale wear tests of the base metal, severe peened and severe peened-nitrided sample (nitrided at 400 °C) affirmed the improvement in the properties after the duplex treatment of shot peening and plasma nitriding.