Stepwise S-N Curve and Internal Fracture of Austenitic Stainless Steel under High Temperature and High Cycle Fatigue Conditions

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In order to clarify the relationship between stepwise S-N curve and fracture mode for austenitic stainless steel under high temperature and high cycle fatigue conditions, rotating bending fatigue tests were carried out on stainless steel SUS321-B at 700°C. Under high stress amplitudes and short fatigue life region in the stepwise S-N curve, surface fracture occurred where the crack initiation site was located at the specimen surface, but internal fracture occurred under low stress amplitudes and long fatigue life region. Two types of fracture modes were recognized in the internal fracture depending on the position of the crack initiation sites, that is, being located at the subsurface of the specimen and the interior of the specimen where fish-eye pattern was observed. At the crack initiation sites, multiple inclusions composed of oxides and carbonitride were recognized by EPMA analysis. As the results of the observation of change in deflection at the loading point during fatigue test, it was recognized that the specimen hardened remarkably at the early stage of the fatigue test but a tendency of softening appeared after large number of stress cycles such as 10<font size="-1">7</font> cycles. It was considered that characteristic of the stepwise S-N curve at 700°C was brought by changes in deformation behavior and fracture mode. As for the result that surface fracture did not occur under long life region, an effect of oxidized surface layer on preventing for a fatigue crack from initiating at the specimen surface was pointed out, but any evidence to support this opinion was not obtained.
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Stepwise S-N Curve and Internal Fracture of Austenitic Stainless Steel under High Temperature and High Cycle Fatigue Conditions

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