Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (-500 degreesC) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.