Surface Mechanical Attrition Treatment effect on gas nitriding treatment of a martensitic steel

Abstract : SMAT and gas nitriding are two surface treatments that enhance the mechanical properties of surface layers by inducing superficial nanostructure and the formation of hard phases, respectively. The objective of this work is to study the association of SMAT with gas 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. More precisely, we wish to investigate and characterize the microstructure evolution of X37CrMoV5-1 (Z38CDV5) martensitic steel subjected to surface mechanical attrition treatment followed by gas nitriding. The treated samples have been characterized using cross-sectional optical micrography, micro and nano-indentation and X-ray diffraction. The results show that performing SMAT before nitriding significantly improves nitrogen diffusion into the substrate, leading to a 20% increase in the thickness of the nitrided layer compared with the nitriding treatment alone. The residual stress profiles of each treated sample were measured using blind-hole drilling method: deeper compressive residual stresses are generated after SMAT followed by nitriding compared to nitriding only.