Carbide evolution and service life of simulated post weld heat treated 2.25Cr–1Mo steel

To estimate the service life of the reactor internals made of 2.25Cr–1Mo steel in the high temperature gas-cooled reactor pebble-bed module demonstration plant, creep rupture test was performed on the simulated post weld heat treated steel at 773 K over a range of stress levels 300–200 MPa. It has been shown that the rupture time is 126–5532 h, correspondingly. Then, the service life under stress of 161.2 MPa at 648 K was calculated to be so long as 210,374,853 h (≈24,015 years). Four types of carbides M3C, M2C, M7C3 and M23C6 existed in both the steel plate and the post-rupture specimens. As the rupture time increases, the weight percentages of M23C6 and M2C that are stable in thermodynamics in the post-rupture specimens increase while those of metastable M3C and M7C3 decrease on the whole. During the long term creep exposure, the deformation promotes the precipitation of M3C, M2C and M23C6, according to the experimental results with the deformed and undeformed parts of the post-rupture specimens, but its influence on the weight percentage of M7C3 seems rather complicated.