TEMPERING EFFECTS ON HIGH-TEMPERATURE BRITTLE INTERGRANULAR FRACTURE IN MN-MO-NI STEEL

The influence of tempering on high temperature brittle intergranular f racture has been examined in the simulated heat affected zone microstr uctures of three Mn-Mo-Ni steel alloys, namely, the base alloy and pho sphorus doped or phosphorus-sulphur doped base alloy. Crack growth beh aviour was determined as afunction of stress intensity at 550 and 450- degrees-C in high vacuum conditions,for material as quenched and after tempering at 615-degrees-C. The resulting fracture surfaces were exam ined in detail and analysis of crack tip and general grain boundary ch emistry was performed using scanning Auger electron spectroscopy. Micr ostructural and rheological changes induced by tempering were evaluate d via electron microscopy and hardness tests. At 550-degrees-C, temper ing 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 susce ptible to high temperature brittle intergranular fracture in the as qu enched state. The main factor governing this behaviour was the absence of stress induced sulphur segregation in tempered alloys. At 450-degr ees-C, crack growth was dominated by intergranular microvoid coalescen ce, even in the as quenched state. The effects of tempering were simil ar, except on the alloy doped with phosphorus. In this case, the combi nation of grain boundary segregation and heavy carbide precipitation e ncouraged the nucleation of microvoids in the tempered alloy, producin g higher crack growth rates via intergranular microvoid coalescence. ( C) 1993 The Institute of Materials.