Evolution of the second phases in the weld seams of 2.25Cr-Mo-0.25V steel in different heat treatment states

The evolution characteristics of the second phases in the weld seams of 2.25Cr-Mo-0.25 V steel in three states - welded, post-weld heat-treated and creep states - during manufacturing and service were evaluated by calculations and observations using optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction spectrometry. The results showed that the second phase was M3C in the as-welded state. The types and quantities of the precipitated phases increased greatly after post-weld heat treatment (PWHT), and M3C, M2C, M7C3, M23C6 and MAT were the main precipitated phases. During PWHT, the amount of M7C3 decreased, the amounts of M23C6 and MX increased, and the sizes of the second phases increased. The precipitates coarsened after long-term creep, and M7C3, M6C, M23C6, M2C, and MX"were the main precipitate phases in this state. The size of the MATphase remained relatively stable during long-term creep, which contributed greatly to the creep resistance.