Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann. Formerly a Research Fellow at the Department of Materials Science, University of Pennsylvania, Philadelphia, PA 19174.