Tempering effects on high temperature brittle intergranular fracture in Mn–Mo–Ni steel

Abstract

The influence of tempering on high temperature brittle intergranular fracture has been examined in the simulated heat affected zone microstructures of three Mn–Mo–Ni steel alloys, namely, the base alloy and phosphorus doped or phosphorus–sulphur doped base alloy. Crack growth behaviour was determined as a function of stress intensity at 550 and 450°C in high vacuum conditionsfor material as quenched and after tempering at 615°C. The resulting fracture surfaces were examined in detail and analysis of crack tip and general grain boundary chemistry was performed using scanning Auger electron spectroscopy. Microstructural and rheological changes induced by tempering were evaluated via electron microscopy and hardness tests. At 550°C, tempering largely eliminated high temperature brittle intergranular fracture, leaving intergranular microvoid coalescence as the dominant fracture mode. Crack growth rates were attenuated by tempering in alloys susceptible to high temperature brittle intergranular fracture in the as quenched state. The main factor governing this behaviour was the absence of stress induced sulphur segregation in tempered alloys. At 450°C, crack growth was dominated by intergranular microvoid coalescence, even in the as quenched state. The effects of tempering were similar, except on the alloy doped with phosphorus. In this case, the combination of grain boundary segregation and heavy carbide precipitation encouraged the nucleation of microvoids in the tempered alloy, producing higher crack growth rates via intergranular microvoid coalescence.

MST/1784