Effects of V and Carbides on the Temper Embrittlement of the 2.25Cr-1Mo Steel

The characteristics of temper embrittlement and carbides precipitated in 2.25Cr-1Mo and 2.25Cr-1Mo-V steels which were aged up to 50,000h at 454∼515°C were investigated. The temper embrittlement susceptibility was evaluated as the shift of 54 Joule transition temperature (vTr₅₄) and 50% fracture appearance transition temperature (FATT₅₀) by the Charpy v-notch impact tests. The shift of vTr₅₄ and FATT₅₀ in the 2.25Cr-1Mo steel rapidly increased with the aging time up to 10,000h and then slowly saturated. Peak ΔvTr₅₄ and ΔFATT₅₀ in the 2.25Cr-1Mo-V steel were obtained up to 30,000h aging at 454°C. The distribution of phosphorus in the 2.25Cr-1Mo-V steel and 2.25Cr-1Mo steel after aging treatment was different. Phosphorus in the conventional 2.25Cr-1Mo steel was mainly observed at the prior austenite grain boundary, however, phosphorus in the 2.25Cr-1Mo-V steel was observed at the prior austenite grain boundary and the interfaces between carbide and matrix. The type and composition of carbides were changed to the stable Mo-rich ones, the carbide of M₆C type was manifestly precipitated in both steels, and the new type of M₄C₃ carbides in the 2.25Cr-1Mo-V steel were finely distributed within grain and the atomic ratio of M₄C₃ was changed from (Fe_0.08Cr_0.21Mo_2.61V_1.10)C₃ in the PWHT state to (Fe_0.22Cr_0.20Mo_2.78V_0.80)C₃ with aging.