Role of molybdenum in phosphorus-induced temper embrittlement of 12%Cr martensitic stainless steel

Abstract

Molybdenum considerably reduced reversible temper embrittlement (RTE) of 12%Cr martensitic stainless steels by scavenging P in the matrix, which decreased P segregation to the grain boundaries. Contrary to the case of many low-alloy steels, Mo additions as large as 1 wt-% were still beneficial, due to the lower C content which allowed more Mo to remain in solid solution in the materials studied. The thermodynamics of the co segregation and precipitation processes of P, Mo, and Cr were analysed quantitatively and shown to be in good agreement with previously proposed models. In particular, the Mo-P and Cr-P chemical interactions were shown to be primarily responsible for the segregation of Mo and Cr, which means that their intrinsic segregation energies in Fe are very small. The segregation of P being too small to drive a segregation of Mo in the materials studied, the embrittling potency of P was unaffected by Mo additions and the impact transition temperature was a unique linear function of the intergranular P content, independent of the nominal Mo content. The embrittling potency of P was similar to that in lowalloy steels (6K/at.-%P), but since the transition temperature of a virtually P segregation-free condition is much higher in 12%Cr than in low-alloy martensitic steels, RTE is potentially more dangerous in the former materials.