On the effect of molybdenum in the embrittlement of phosphorus-doped iron

Abstract In an effort to clarify the effects of molybdenum, which greatly retards the kinetics of temper embrittlement of alloy steels, the incidence of intergranular fracture and the segregation of phosphorus and other elements to grain boundaries were studied in a series of Fe-Mo-P alloys by means of fracture experiments in a high-resolution scanning Auger microprobe. Decarburization was necessary to produce intergranular fracture in these alloys, which originally contained residual carbon. This element was the principal determinant of the tendency for intergranular fracture. There was no evidence of scavenging or retardation of diffusion of phosphorus by molybdenum. The results were analyzed to determine the existence and importance of synergistic cosegregation of molybdenum and phosphorus. The evidence based on the mean values of the grain boundary concentrations was ambiguous and inconclusive. However, consistent support for the coseregation model could be produced by use of a spectrum of segregation energies, based on the broad spectrum of grain boundary concentrations observed experimentally and presumed to result from the spectrum of grain boundary structures. It is proposed that this kind of analysis is physically more realistic than one which employs mean values. Using this model the attractive interaction between molybdenum and phosphorus in iron was deduced to be weaker than heretofore believed; this finding is consistent, however, with other recent work. The interaction is too weak to influence the segregation of the phosphorus, but it should promote the segregation of molybdenum, absent the effects of other elements like sulphur and carbon. The interaction between molybdenum and carbon appears to be the dominant factor which influences grain boundary cohesion in these alloys.