About the role of residual stresses and surface work hardening on fatigue the deltaKth of a low-alloy nitrided and shot peened steel

Nitriding and shot peening are surface treatments widely used to improve the fatigue strength of mechanical and structural components. Both treatments enhance the mechanical properties of the surface layer of material: nitriding mainly by means of chemical transformations and formation of a very hard case, shot peening mainly due to compressive residual stress field in the sub-surface layer of material. The combined application of nitriding and shot peening has not been adequately investigated in literature and the known data do not allow to define a general criterion to assess if and when shot peening application after nitriding can be useful. In this paper the effect of nitriding plus shot peening on fatigue strength of a low-alloy steel is investigated by means of experimental tests carried out on specimens containing a micro-hole, acting as a pre-crack. To analyse the role played by shot peening induced residual stresses, a series of specimens was heat-treated after shot peening, being the aim the partial removal of residual stresses without strongly modifying the mechanical properties of the surface layer of material. After a critical discussion of the results in terms of residual stress, micro-hardness trend and fatigue strength, an original fracture mechanics based approach is proposed to predict the threshold value of the stress intensity factor of nitrided and shot peened steels. The results are in good agreement with the experimental evidence.