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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 (Fe0.08Cr0.21Mo2.61V1.10)C3 in the PWHT state to (Fe0.22Cr0.20Mo2.78V0.80)C3 with aging.

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