Cyclic stress response and microcrack initiation mechanism of modified 9Cr‐1Mo steel under low cycle fatigue at room temperature and 350°C
Shouwen Shi, Jianpeng Cui, Haiyan Li, Gang Chen, Qiang Lin, Xu Chen
Abstract
Abstract To investigate the effect of temperature on microcracking mechanisms of modified 9Cr‐1Mo steel, low cycle fatigue tests were performed at 350°C and room temperature (RT). At RT, the cyclic stress response at low strain amplitude (<0.5%) is controlled by friction stress while dominated by back stress at high strain amplitude. By comparison, the disappearance of sub‐grain boundaries induced by dislocation annihilation occurs at 350°C, which contributes to the decrease in back stress, resulting in more significant effects of back stress compared to RT. As a result of the accumulated fatigue damage, the distance between two parallel extrusions at 350°C is larger than that at RT, which is ascribed to the disappearance of low angle sub‐grain boundaries due to dislocation annihilation at 350°C. Due to the disappearance of small‐angle grain boundaries at 350°C, microcracks initiate along high‐angle grain boundaries at 350°C instead of along low‐angle grain boundaries at RT.