Fatigue limit of chemical heat treated specimens and effect of shot peening

The fatigue limits of smooth specimens in 40Cr, 30CrMo, 20CrMnTi and 20Cr steels after carbonitriding, carburising, nitriding and shot peening treatments were investigated quantitatively, and the compressive residual stress fields (CRSFs) were determined by X-ray diffraction technique with step by step electrolytic polishing. The experimental results indicate that the fatigue crack initiation sites (or fatigue crack sources) might be located in one of the following three regions: at the surface, within the hardened layer, and beneath the hardened layer, where the fatigue crack initiates depends on the surface strengthening technology and the residual stress distribution along surface layer. The highest fatigue limit will be obtained when the crack initiates beneath the hardened layer. A judgement that considers the crack initiation site for optimum condition of surface strengthening technique to evaluate the surface chemical heat treatments has been proposed. The strengthening technology can be considered to Optimum if Z(s)>Z(o) (Z(o) is the depth of compressive residual stress field and Z(s) is the distance of fatigue crack initiation site from surface); otherwise, the strengthening technology is not good and should be modified or a further shot peening may be needed while 0 <= Z(s)<Z(o).