Enhancing torsion fatigue behaviour of a martensitic stainless steel by generating gradient nanograined layer via surface mechanical grinding treatment

A gradient nanograined (GNG) surface layer was formed on a martensitic stainless steel bar sample by means of the surface mechanical grinding treatment (SMGT). The average grain size is similar to 25 nm on the topmost surface layer and increases gradually with increasing depth. The torsion fatigue strength is elevated by 38% with the GNG surface layer compared with the original material. An additional 8% increment in fatigue strength is achieved after a post-annealing treatment of the SMGT sample. By analysing the microstructure, hardness, surface roughness and residual stress distribution in the SMGT samples, we believe that the enhanced fatigue resistances originate from the GNG structure with a hard surface layer and a high structural homogeneity.