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