Segregation mechanisms of temper embrittlement

Grain boundary segregations of impurities like P, S, Sb, Sn and others are the origin of temper embrittlement in low alloy steels. Till now it has been assumed that segregation kinetics is determined by bulk diffusion. But some recent experimental results and a more sophisticated account for diffusion behaviour of substitutional solutes in ferromagnetic iron show such an interpretation to be invalid at least for the embrittlement by phosphorus segregations. In contrast one has to start from the formation of a miscibility gap below 600°C within the grain boundary due to weak repulsive interaction between phosphorus and carbon. The growth of the P-rich phase by grain boundary diffusion should be rate determining to develop embrittlement. For energetical reasons the P-rich phase is favourably formed at prior austenite boundaries and may reach their dimensions. The growth kinetics of this phase is described by one-dimensional diffusion at grain boundaries. This model is successfully applied to experimental results of Möller et al. [Archs Eisenhütt. 55, 543 (1984)]. It allows an unique interpretation of many observed details connected with segregation induced embrittlement.