Evaluation of fatigue crack growth behavior of type 347 stainless steels under Thermo-mechanical load conditions

Nuclear power plant facility is operated in a hot and highly pressurized environment for a long period of time, it is necessary to study the relevant fracture factors such as the strength, fracture toughness, fatigue, and to establish suitable resources after careful research. Localized thermal mechanical fatigues occur in the pressurizer surge line due to a thermal stratification and, consequently, the thermal stripping phenomena is caused by the flux characteristics of the hot liquid and cool liquid in a piping system. AISI type 347 stainless steel(SS), an Nb-stabilized austenitic stainless steel, is the material used for the Korea Standard Nuclear Power Plant (KSNPP) pressurizer surge line. The code for fatigue characterization is included in section XI of the ASME Boiler and Pressure Vessel Code. The code provides the methodology for a flaw evaluation based on the Paris' law Fatigue Crack Growth Rate (FCGR) equation and a reference fatigue crack growth curve for austenitic stainless steels. In this study, the isothermal FCGR test including the near-threshold area and the thermo-mechanical FCGR test were performed for two types of 347. The objective of this study was to evaluate the effects of the distribution and contents of precipitates on the fatigue crack growth behavior and the results compared with the ASME reference curve