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

The present paper attempts to assess the many aspects which are involv ed in the old, interesting and somewhat perplexing grain boundary segr egation phenomenon commonly known as reverse temper embrittlement (RTE ). The actual mechanisms involving grain boundary failure, the mechani cs of impurity segregation at grain boundaries and the speciality of g rain boundaries are discussed at length. The thermodynamics of impurit y segregation, which is treated as an equilibrium-type process, togeth er 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 portrayi ng the effects of reverse temper embrittlement in terms of bulk chemis try, grain boundary chemistry, thermal history (temperature, time and impurity segregation characteristics) and material properties (microst ructure, hardness and strength) are critically assessed and compared. In terms of chemistry it is shown that the extent of grain boundary se gregation, viz. phosphorus monolayers, exhibits very consistent and sm all data scatter trends. Consideration is given to other aspects such as thermal history, together with impurity diffusion and material prop erties such as hardness, tensile strength and grain size, and it is es tablished that RTE effects can be adequately expressed in terms of (1) grain size-bulk phosphorus trends and/or (2) a grain boundary phospho rus factor.