Mod. 9Cr–1Mo steel is considered for cladding of sodium fast reactors and pressure vessel of very high
temperature reactor because this material had higher thermal conductivity, lower thermal expansion coefficients, and
low void swelling[1-3]. Void swelling limited the use of austenitic steels for fuel cladding and other in-core
applications.
Low cycle fatigue is one of the most important damage mechanisms that limit operation of next generation
reactor during 60 years. Low cycle fatigue is induced by temperature gradient during start-up and cool-down of
nuclear reactor. Cyclic stress of Mod. 9Cr-1Mo steel which has tempered martensite microstructure is softened
during low cycle fatigue[4-6]. Cyclic softening during low cycle fatigue is affected by microstructural change and
experimental condition because cyclic softening is caused by the decrease of dislocation density and the amount and
distribution of precipitation[7-9]. Cyclic softening is an important factor to determine allowable stress and design
reactor safely. Cyclic softening behavior should be evaluated with operating condition (temperature and strain
range) and metallurgical change (heat treatment, welding and aging) to assess integrity of structure until 60 years. In this study, cyclic softening behaviors are evaluated with metallurgical and experimental change