Role of molybdenum in phosphorus-induced temper embrittlement

The influence of Mo on reversible temper embrittlement(RTE)of a low-alloy 2¼Cr-lNi steel with variableP and Mo contents was studied by establishing the relationships among the Charpy impact transition temperature shifts, the nominal P and Mo concentrations, and their intergranular concentrations as measured by Auger electron spectroscopy. Segregation was also studied in ternary Fe-Mo-P alloys.

Embrittlement in the steels exhibits a deep minimum for a nominal Mo content of 0.7 wt-%,while at 1.1 wt-% it resumes its value in the Mo-free steel. The de-embrittling action of Mo has two distinct origins. When dissolved in the ferrite matrix, Mo decreases the segregation of P atoms by tying them up in the grain interior. When segregated at the boundaries, Mo counteracts the embrittling effect of segregated P atoms. The former effect is dominant in low-P steels and ternary Fe-Mo-P alloys, while the latter operates in higher-P steels where the segregation of Mo is enhanced by that of P due to the strongly attractive Mo-P interaction. Both beneficial effects of Mo are limited by its affinity for carbon, which causes Mo to be absorbed in the carbides when the aging time or temperature or the nominal Mo content are increased.