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 embritlement (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 aneq uilibrium-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, therm al history (tempera ture, time and impurity segregation characteristics) and material properties (micro structure, hardness and strength) are critically assessed and compared. Interms of chemistry it is shown that the extent of grain boundary segregation, viz. phosphorus monolayers, exhibits very con sistent and small datascatter 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 RT E effects can be adequately expressed in terms of (I) grain sizebulk phosphorus trends and/or (2) a grain boundary phosphorus factor.