SOME ALLOY ELEMENT EFFECTS ON PHOSPHORUS-INDUCED TEMPER EMBRITTLEMENT OF IRON AND STEEL

Some alloy element effects on P-induced embrittlement in a model NiCr steel, a NiCrMoV rotor steel, and in Fe-Cr and Fe-Mn alloys were studied. It was found that Mn segregates on its own and can cause an intrinsic lowering of cohesion at grain boundaries. Synergistic co-segregation of Mn+P has been clearly demonstrated in decarburized Fe-Mn-P alloys. A C-P repulsive interaction in grain boundary segregation was found in all experimental alloys. After decarburization, pure iron (containing 1 ppm P, 1 ppm S and a few ppm O) fractured in an intergranular mode at 133 K; binary alloys (Fe-P, Fe-Mn, Fe-Cr) fractured by a mainly-cleavage mode before decarburization, but the fracture mode changed to intergranular after decarburization. These facts show that C is a very important strengthener for grain boundary cohesion. It was found in both the grain boundary and surface segregation studies of Fe-Cr-P alloys that co-segregation of Cr+P is minimal and can be ignored. The grain boundary strengths of Fe-P and Fe-Cr-P alloys gradually converged as C decreased. Therefore, the main effect of Cr additions in low-Cr (up to 2%) alloy steels is to decrease the solubility and activity of carbon, thus diminishing the beneficial effect of carbon on grain boundary cohesion. Embrittlement equations for P- and Sn-induced temper embrittlement of the NiCr and NiCrMoV rotor-type steels were developed empirically, based on the derivation of Takayama et al and modified for the more complicated conditions where two impurities (P and Sn) and microstructure (bainite vs martensite) can be taken into account. The effect of bainite vs. martensite on FATT was explored and was found to be independent of temper embrittlement; it could be rationalized in terms of carbide size and distribution. It was found that multistage, delayed embrittlement is characteristic of Mo-containing steels and is related with Mo-rich carbide precipitation and/or transformations