REVERSE TEMPER EMBRITTLEMENT - A COMMON SOURCE OF PERPLEXITY IN LOW-ALLOY STEELS

The present paper attempts to assess the many aspects which are involved in the old, interesting and somewhat perplexing grain boundary segregation phenomenon commonly known as reverse temper embrittlement (RTE). The actual mechanisms involving grain boundary failure, the mechanics of impurity segregation at grain boundaries and the speciality of grain boundaries are discussed at length. The thermodynamics of impurity segregation, which is treated as an equilibrium-type process, together with the segregation kinetics, which are required for an estimation of the extent of grain boundary impurity segregation in terms of time and temperature, are also considered. The various methods of portraying the effects of reverse temper embrittlement in terms of bulk chemistry, grain boundary chemistry, thermal history (temperature, time and impurity segregation characteristics) and material properties (microstructure, hardness and strength) are critically assessed and compared. In terms of chemistry it is shown that the extent of grain boundary segregation, viz. phosphorus monolayers, exhibits very consistent and small data scatter trends. Consideration is given to other aspects such as thermal history, together with impurity diffusion and material properties such as hardness, tensile strength and grain size, and it is established that RTE effects can be adequately expressed in terms of (1) grain size-bulk phosphorus trends and/or (2) a grain boundary phosphorus factor.