Temper embrittlement in 12%Cr martensitic steel

The minimum in impact toughness ofmartensitic 12% Cr steels, classically associated with the maximum hardening between 475 and 500°C, is shifted towards 550°C in a commercial heat containing phosphorus, while the brittle mode of failure becomes predominantly intergranular. This phenomenon is shown to be typical temper embrittlement induced by the segregation of phosphorus and chromium to the former austenitic grain boundaries, the fragility being amplified by the relatively high strength of the steel. The kinetics of phosphorus segregation are studied by Auger electron spectroscopy. The temperature dependence of equilibrium segregation is shown to’ be consistent with the theory of synergistic ternary segregation, which is discussed here with particular emphasis on the effect of high alloying element concentrations. In particular, the high Cr content is responsible for the segregation (and thus the embrittlement) remaining significant at much higher temperatures (e.g. 700°C) than in low-alloy steels. Therefore embrittlement cannot be completely alleviated by a reversion treatment without crossing AC ₁ unless the phosphorus concentration is kept to a minimum or molybdenum additions are made which counteract the effect of phosphorus.