Strengthening Mechanism of Low Cycle Fatigue Resistance for Cr-Mo Tempered Martensite Steel at 350 °C

In the process of fatigue loading, the segregation of solid solution elements makes dislocations continuously pile-up in ferrite lath to form subgrain, which improves the fatigue properties. In this study, the low cycle fatigue properties of two kinds of Cr-Mo tempered martensitic steels with different grain sizes were investigated at 350 °C. Then, the fracture morphology and the subgrain of ferrite lath were analyzed. The results show that the low cycle fatigue life of 90H steel is significantly higher than that of 80H steel, and a large number of secondary cracks are found in the fatigue fracture. The reason is that the segregation of solid solution atoms affects the dislocations movement during low cycle fatigue. And the finer lath subgrain structure of 90H steel makes it easier to occur in grain. Thus, the evolution process of subgrain structures is changed, and the crack propagation is hindered, resulting in a large number of secondary cracks, which improves the fatigue life. In addition, the segregation effect on fatigue life decreased with the strain amplitude increasing.