Evaluation of Factors Affecting Cyclic Softening Behaviors of Mod. 9Cr-1Mo Steel

Tensile and low cycle fatigue tests were conducted at RT-600 °C and cyclic softening behavior during low cycle fatigue was evaluated with temperature, strain range, heat treatment, aging, and welding. Yield stress decreased with temperature and was the highest for two step heat treatment and almost the same with welding. Elongation decreased to 400 °C, was minimum at 400 °C, and increased above 400 °C. Elongation was the lowest for91B of two step heat treatment. Cyclic softening was not much different with temperature to 500 °C but was high at 600 °C. Cyclic softening was almost the same with strain range at RT but increased with strain range at 300-600 °C. Cyclic softening decreased with welding and was not much different with aging. Amount of cyclic softening increased with temperature, was the lowest for weldment, and increased with yield stress. The decrease of dislocation density by cell structure formation and the increase of lath widthwas important for cyclic softening to 500 °C and recovery was important for cyclic softening at 600 °C.