Long-time isothermal temper embrittlement in Ni-Cr-Mo-V steels

Abstract

Four commercial purity Ni-Cr-Mo-V steels of closely comparable bulk chemistry and grain size, but tempered to various strength levels, were embrittled by exposure at 600°, 750°, and 850°F for times up to 35,000 hr. Maximum temper embrittlement occurred at 850°F in all steels. Severe cases of embrittlement resulted in a marked decrease in tensile ductility and an intergranular tensile fracture. Auger electron emission analysis showed that P, Sn, Ni, and Cr were segregated at prior austenite boundaries in the steels exposed to 750° and 850°F. Increased segregation of phorphorus and tin was always accompanied by increased segregation of nickel and chromium. The severity of grain boundary segregation increased with increasing values of fracture transition temperature. Despite comparable bulk chemistry and grain size, the degree of segregation was different in different steels. Under exposure conditions causing severe embrittlement, the FATT values displayed a strong dependence on the strength level of the steel. In a giyen steel, while the composition and morphology of carbides at austenite boundaries were the same as in the matrix, the density and size of carbides were much higher at the austenite boundaries. The preference of these boundaries as fracture sites would seem to arise from two considerations, namely, a high degree of impurity and alloy element segregation and the fact that the density and size of carbides at these boundaries is higher than that in the matrix.