Evolution of the second phases in the weld seams of 2.25Cr-Mo-0.25 V 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 M${}_3$C in the as-welded state. The types and quantities of the precipitated phases increased greatly after post-weld heat treatment (PWHT), and M${}_3$C, M${}_2$C, M${}_7$C${}_3$, M${}_{23}$C${}_6$ and MX were the main precipitated phases. During PWHT, the amount of M${}_7$C${}_3$ decreased, the amounts of M${}_{23}$C${}_6$ and MX increased, and the sizes of the second phases increased. The precipitates coarsened after long-term creep, and M${}_7$C${}_3$, M${}_6$C, M${}_{23}$C${}_6$, M${}_2$C, and MX were the main precipitate phases in this state. The size of the MX phase remained relatively stable during long-term creep, which contributed greatly to the creep resistance.