An investigation of the embrittlement in X20CrMoV12.1 power plant steel after long-term service exposure at elevated temperature

X20CrMoV12.1 alloy steel used as main steam pipes in power plants exhibits embrittlement at room temperature after long-term service exposure at 550 °C and the embrittlement can be partly removed by retempering. Microstructural observation shows that the carbides in the exposed pipe coarsened heavily with enriching alloying elements of Cr and Mo from the matrix during long-term service exposure, so the coarsened carbides reduce both precipitation and solid solution strengthening mechanisms. The degradation of the material is mainly related to carbide coarsening. AES investigations show that the embrittlement is mostly due to the segregation of phosphorus to the prior austenite grain boundaries, and plenty of coarsened carbides residing on the grain boundaries result in this embrittlement tendency as well.