Effect of microstructure on the temper embrittlement of Cr-Mo-V steels

The effect of transformation product on the temper embrittlement susceptibility of a Cr-Mo-V steel doped with P and Sn has been investigated at different strength levels. Results show that at low strength levels (< 10R _C ) embrittlement susceptibilities of tempered bainite and ferrite-pearlite structures are comparable to each other, but lower relative to tempered martensite. The lower susceptibility of tempered bainite relative to tempered martensite obtains up to about 40R _C , above which the susceptibilities for the two structures are similar. Variation of embrittlement susceptibility with microstructure is completely consistent with the degree of grain boundary segregation of P and Sn. The segregation is smaller in bainite than in martensite at a given strength level and increases with increasing strength level for a given structure. When compared at the same degree of embrittlement (i.e., same shift in FATT), the amount of grain boundary segregate and the extent of intergranular fracture are lower in tempered bainite compared to martensite indicating that embrittlement of interfaces other than prior austenite boundaries might be an important factor in the embrittlement of bainite structures.