13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- layer. These results suggest that the depth of diffusion layer is quite shallow for stainless steel. In our previous works, plasma nitriding was carried out using a same equipment using chromium molybdenum steel (JIS SCM435) [17]. The thickness of hardened layer was about 0.1 - 0.3 mm with ~10µm compound layer for each material. The hardened layer is obviously thicker than that of present study. The difference indicates that the passivation film reforms at the stainless surface during the plasma nitriding process though the passivation film is removed intermittently by hydrogen etching effect [26, 27]. The reformation of the passivation film hinders nitrogen diffusion, and this results in a thinner diffusion layer. The thickness of the hardened layer and the value of the hardness near the surface are important parameters that influence the site of fatigue crack initiation, as discussed in detail in the following sections. Figure 3. Distribution of Vickers-hardness in the nitrided specimens. 3.2. S-N curves The fatigue tests to failure were carried out in air up to N= 108 cycles. Figure 4 shows the results of fatigue tests for as-received, N15 and N25 specimens. The S-N curve of the as-received specimens (represented by open square mark in Fig. 4) presented a horizontal asymptote shape, which is typical shape for low strength material [28, 29], with a fatigue limit up to N = 108 cycles. The fatigue limit σw defined as the horizontal line was about 500 MPa. The shape of two S-N curves of plasma nitrided specimens, represented by open triangle and circle marks in Fig. 4, showed also horizontal asymptote shape with a fatigue limit up to N = 108 cycles, that were similar to the S-N curve of the as-received specimens. The fatigue limit σw defined as the horizontal line was about 600 MPa for the N15 and 550 MPa for the N25 specimens, respectively. The fatigue limit of the N25 specimen with thicker hardened layer was slightly lower than that of the N15 specimen with relatively thin hardened layer as well as the fatigue strength in the life regime N < 106 cycles. Increased fatigue strength after nitriding is a well-known phenomenon for various steels. The increase is caused by the increase of surface hardness and the formation of compressive residual stresses in the surface layer during the nitriding process. The high hardness layer hinders the fatigue crack initiation. The compressive residual stress is superimposed to the external load and leads to a reduction of the effective stress in tension. Since only a tensile stress produces fatigue cracks and contributes to crack propagation, a reduction of the effective stress in tension increases the fatigue
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