Ductile-brittle transition temperature shift controlled by grain boundary decohesion and thermally activated energy in Ni-Cr steels

Abstract

Abstract Temper embrittlement induced by segregation of metalloid solutes to grain boundary (GB) was evaluated by a shift of the ductile-brittle transition temperature (DBTT). DBTT was found to be linearly correlated with the amount of metalloid on the GB (X gb) for both dynamic and static displacement rates (dδ/dt) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δe p) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope (α) of DBTT vs. X gb was found to be linearly dependent on Δe p regardless of the segregated solutes. In high hardness steels, the slope is independent of dδ/dt, while in medium hardness steels the α is dependent on dδ/dt. An Arrhenius plot of dδ/dt vs. the reciprocal DBTT was used to drive the thermal activation energy (E act), which represents a barrier to plasticity. It was found that E act correlates to a reduction in the GB fracture surface energy. The E act depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.