13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Fig. 1 shows S-N diagrams of plain bar specimens for 12Cr stainless steel and Ti-6Al-4V alloy in steam containing industrial gases such as H2S and CO2[24]. Reduction of corrosion fatigue strength in 12 Cr stainless steel is 87.5% in steam containing 20ppm H2S gas, while the reduction of corrosion fatigue strength in Ti-6Al-4V alloy is only 4%. The SO2 gas effect in humid air was as same as that of H2S gas in humid air. The CO2 gas effect in steam was not remarkable as compared with that of H2S gas in steam. In the results of fatigue crack propagation tests with frequency of 30 Hz for 12 Cr plate specimen the effect of 20ppm H2S gas containing in humid air on corrosion fatigue crack propagation rate of 12 Cr stainless steel was scarcely noticeable. Therefore it can be concluded that the major cause of the reduction in corrosion fatigue strength of 12Cr stainless steel is acceleration of corrosion fatigue crack initiation by H2S gas in steam and humid air. In fact fracture surface observations revealed that crack initiated at a corrosion pit with about 10μm depth and propagated in association with intergranular fracture as shown in Fig.2. Figure1. Influence of industrial gas in steam on fatigue strength of SUS410J1and Ti-6Al-4V[Ebara[24]]. Rotating bending, 60Hz, R=－1 Figure 2. Corrosion fatigue fracture surface of 12 Cr stainless steel in 20ppm H2S and 4000ppm CO2 in steam[Ebara[24]]. 171.6MPa, 2.36X107cycles a) Corrosion pit at initiation area b) 2mm from initiation
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