Surface Small Crack Growth Behavior of Type 304 Stainless Steel in Low-Cycle Fatigue at Elevated Temperature

Surface small crack growth behavior of Type 304 stainless steel during low cycle fatigue under fast-fast and slow-fast cyclings was investigated at a temperature of 873 K by using the smooth specimens which had the different grain sizes. It was shown that the crack, which had already grown up to a few grain size, predominantly propagated with strain cycling, and that it was very important for the safety assessment of the components in service to detect the crack of a few grain sizes. It was also shown that small crack growth rate showed the minimum when they arrived at the grain boundaries. Above behavior resulted from that the grain boundaries temporarily impeded the small crack growth. The crack length below which the grain boundaries notably affected the small crack growth rate was also given as the function of relative length to the average grain size. Furthermore, the small crack growth rate was compared with the macroscopic crack growth one. In fast-fast cycling, the small crack growth rate was about ten times as large as the macroscopic crack growth one, where its length was comparable to the grain size. Based on the results thus obtained, the application limit of macroscopic crack growth law to the surface small crack growth was discussed. The application limit proportionally increased with the grain size, and it was about ten times average grain size in both fast-fast and slow-fast cyclings.