Evolution of the residual stress state after surface mechanical attrition treatment and low temperature plasma nitriding
SMAT (Surface Mechanical Attrition Treatment) and plasma nitriding are two surface treatments that enhance the mechanical properties of surface layers by inducing superficial nanostructures and the formation of hard phases, respectively [1-2]. The idea of this work is to combine SMAT with nitriding: SMAT provides a superficial nanostructure with nanosized grains and a high dislocation density and thus creates a large volume fraction of grain boundaries and defects; during subsequent nitriding these "fast" diffusion pathways enhance atomic nitrogen diffusion. Compressive residual stresses, nanocrystalline and nitrided layers as well as work hardening are the main effects induced at or near the surface by this duplex treatment. On the one hand, just like shot-peening SMAT generates an in-depth residual stress gradient, resulting from the incompatibility of the plastic strains after treatment. On the other hand, plasma nitriding generates residual stress due to the formation of S-phase, called also expanded austenite due to the increase of the interplanar spacing. So, an effective measurement of the stress gradient is an important factor in order to bring a better understanding of this duplex treatment and to assess fatigue and wear properties of materials. Therefore, in the present work, in-depth residual stresses were measured using a combined system of blind-hole drilling and digital speckle pattern interferometry [3]. It is a sensitive optical measurement technique that responds to the displacement field between two states of the material: before (initial) and after (deformed) the drilling. This displacement is caused by the local stress relaxation of the entire surface of the specimen. The resulting images, obtained by phase shifting, made it possible to get the displacement field for each increment, and to calculate the complete stress profile of each treated sample. In this paper the Design of Experiment (DoE) technique was used to examine the effect and the interaction of duplex treatment parameters such as the temperature of plasma nitriding, the SMAT intensity (two levels) and a polishing step (after SMAT and before nitriding) on the residual stress state of a 316L ASTM F138 alloy.