Effect of cryogenic temperatures and processing parameters on gradient structure of a stainless steel treated by ultrasonic surface mechanical attrition treatment

The beneficial effect of using cryogenic temperatures for the ultrasonic surface mechanical attrition treatment (SMAT) is depicted in the case of the 304 L austenitic stainless steel. The cryogenic temperatures induce an additional driving force for the formation of martensite at depth where the effect of shot peening tends to vanish, raising the subsurface hardness by 20 to 30%. The maximum amount of alpha' martensite was always located within the subsurface. This is explained by different mechanisms operating at different stages during the SMAT process: (i) higher shear stresses at the subsurface due to the Hertzian contact stress, (ii) local heating due to the shot impacts and (iii) surface martensite destabilization when the microstructure enters the nanometric range. Consequently, the gradient structure formed by the SMAT superimposes different natures of gradients in terms of grain size and martensitic fraction. The steel being harder to deform at cryogenic temperatures, a significant decrease of the apparent surface roughness (down to 44%) is also obtained. An appropriate selection of processing parameters allows to tailor the surface towards a broad range of hardness and roughness values: 425 to 560 HV and 1.6 to 3.3 mu m, respectively.