The effects of composition and carbide precipitation on temper embrittlement of 2.25 Cr-1 Mo steel: Part I. Effects of P and Sn

Temper embrittlement of 2.25 Cr-1 Mo steel doped with P and Sn was studied systematically. Carbide extraction by electrolysis, X-ray diffraction, transmission (replica) electron microscopy, chemical analysis of the matrix, and scanning Auger microprobe analysis were conducted to determine the effect of carbide precipitation and subsequent variation of the Mo concentration in solution on the segregation of P. These analyses were correlated with the ductile-to-brittle transition temperature (measured by use of a slow-bend test), as well as hardness measurements and fractographic information obtained by scanning electron microscopy. The results indicate that the principal role of Mo is to suppress embrittlement by scavenging of P, presumably by a Mo-P compound formation, thereby diminishing P segregation. However, due to the stronger interaction between Mo and C, Mo is precipitated in an M₂C carbide during tempering or aging, and the matrix is depleted of Mo. The P thereby released segregates at a rate consistent with the rate of M₂C precipitation. At a Mo concentration >0.7 pct the beneficial effect of Mo is decreased due to enhanced M₂C precipitation, the content of Mo in solution remaining essentially constant. The M₂C is formed at the expense of Cr-rich M₇C₃; this results in more Cr in solution, thereby permitting more Cr-P cosegregation, and embrittlement increases. Tin was found not to produce temper embrittlement in this steel when present at concentrations up to 0.04 pct.