A new view on the temperature-time dependence of temper embrittlement

Abstract

It is well known that grain boundary segregation of certain impurities like P, S, Sb and others gives rise to temper embrittlement of low-alloy steels. It is important to understand the mechanism of temper embrittlement in some detail so as to predict the embrittlement for low-alloy steels. It is often assumed in the treatments of temper embrittlement that the impurity segregation to grain boundaries is brought about by an equilibrium segregation mechanism [1-4]. In 1950, Jaffe and Buffum [5], using standard Charpy specimens with a V-notch, evaluated the degree of embrittlement, as determined by the shift in ductile-brittle transition temperature (DBTT), of a certain heat of SAE 3140 steel with composition shown in Table I. The material was austenitized for 1 h at 900 °C, water-quenched to give 100% martensite with a prior austenite grain size of ASTM 8 (22/~m), then tempered for l h at 675°C and again waterquenched. This heat-treatment was known to give a low DBTT of -83 °C and thus this became its unembrittled reference state. The material was then given isothermal holding treatments for between 10 min and 200 h at certain temperatures in the temperature range 375-650 °C. The shift in DBTT from the reference state gave the measure of embrittlement and, when plotted as constant embrittlement curves as a function of time at temperature, gave the T t embrittlement diagram shown in Fig. 1. In a more detailed study of the double nose present in Fig. 1, Buffum and Jaffe [6] discovered that the DBTT in the embrittled state was microstructure-dependent to the extent of 2 °C per Rockwell C hardness number increase, i.e. the DBTT increased as the material became harder. The DBTT shown in Fig. 1 were corrected to constant hardness and the T t diagram, as shown in Fig. 2 [7], exhibited only one nose. This T-t embrittlement diagram stands for that component at constant microstructure and grain size. In practice, most commercial structural steels may not undergo the toughening treatment around 650 °C after quenching and before tempering in view of the requirement of strength. Therefore, we