Mechanism of Embrittlement and De-Embrittlement for 2.25Cr-1Mo Steel

The constant embrittlement curve for constant segregation concentration on grain boundary of impurity element P and relationship between equilibrium grain boundary segregation concentration and operation time for 2.25Cr1Mo steel were derived based on the theory of equilibrium grain boundary segregation. The mechanism of step-cooling test and mechanism of de-embrittlement for 2.25Cr-1Mo steel were explained. The segregation rate will increase but equilibrium grain boundary segregation concentration of impurity element P will decrease as temperature increases in the range of temper embrittlement temperature. There is one critical temperature of embrittlement corresponding to each embrittlement degree. When the further heat treating temperature is higher than critical temperature, the heat treating will become a de-embrittlement process; otherwise, it will be an embrittlement process. The critical temperature of embrittlement will shift to the direction of low temperature as further embrittlement. As a result,some stages of step-cooling test would change into a de-embrittlement process. The grain boundary desegregation function of impurity element P was deduced based on the theory of element diffusion, and the theoretical calculation and experimental results show that the further embrittlement or de-embrittlement mechanism can be interpreted qualitatively and quantitatively by combining the theory of equilibrium grain boundary segregation with constant embrittlement curve.