Abstract
This research was undertaken to clarify effect of phosphorus and boron on cryogenic mechanical properties of a sensitized 17Cr-12.5Ni-2Mo-0.05Nb-0.2N steel. The steel was sensitized at 923 993 K for 270 720 ks and was investigated at low temperature, especially at 4 K. An addition of either phosphorus or boron affected precipitation at grain boundaries during sensitization. Phosphorous increased the grain boundary precipitates. Boron suppressed the phosphorous effect. Both elements, however, did not show clear effect on tensile properties at 4 K of the sensitized steel. In a high phosphorus alloy, Fe2Mo other than CrNbN, M23C6, Cr2N and Nb (CN) precipitated and led to intergranular fracture. Therefore a deterioration of fracture toughness at 4 K for sensitized steel became much evident with an increase in phosphorous content. On the otherhand, boron addition prevented intergranular failure and resulted in high fracture toughness for the sensitized one.
Segregated phosphorus atoms seem to promote nucleation of precipitation and weaken an interfacial binding of precipitates, especially Fe2Mo. On this assumption, effect of phosphorus and boron addition on mechanical properties at 4 K could be explained.
