Modes of fracture in temper embrittled steels

Abstract

Metallographie and fractographic observations in a low carbon 3.5% Ni-1.7% Cr steel doped with antimony or phosphorus show that temper embrittlement can involve low energy fractures along grain boundaries other than prior austenitic grain boundaries. Examples given here are: 1. (1) the boundaries of blocky ferrite and upper bainite formed during austenitic decomposition, 2. (2) the grain boundaries of the equiaxed ferrite formed by complete recrystallization below the eutectoid temperature. These observations show that the occurrence of prior austenite grain boundary fracture, which has been considered an essential and almost classical aspect of temper embrittlement, is actually not an essential feature of the problem. Prior austenite grain boundary fracture can be made to occur in the present low carbon steel (when temper-embrittled) if these boundaries are sufficiently well preserved during austenite decomposition, e.g. by the very high quenching rates achieved near a free surface, or due to the presence of sufficient phosphorus, which segregates in the austenite phase and retards the nucleation of ferrite at austenite grain boundaries.