Impact of cooling rate on microstructural development and mechanical characteristics of CGHAZ in long-service 15Cr1Mo1V steel post-repair welding

A thermal simulation test for welding was performed on a 15Cr1Mo1V reheated steam pipeline that had been in service for around 130,000 hours in a power plant. Microstructural characterization and hardness evaluations were conducted on the CGHAZ of the welded joint, and the SH-CCT (Super Heat-Continuous Cooling Transformation) curve for this specific region was systematically developed. Experimental results indicate that within a cooling rate range of 0.1–0.3 °C/s, the microstructural composition of the CGHAZ is predominantly composed of ferrite, pearlite, and a small fraction of bainite. As the cooling rate increases to 0.5 °C/s, there is a noticeable augmentation in bainite content accompanied by a concomitant reduction in ferrite. Cooling rates between 1–5 °C/s yield a microstructure dominated by granular bainite, whereas lath bainite becomes the primary constituent when cooling rates exceed 7 °C/s. At an elevated cooling rate of 60 °C/s, the microstructure is characterized by lath martensite as the major phase, with residual bainite present as a secondary component. Additionally, micro-Vickers hardness measurements of the 15Cr1Mo1V steel exhibit a positive correlation with increasing cooling rates. Specifically, the presence of martensite at 60 °C/s drives the hardness value to reach 402 HV0.2, reflecting the strengthening effect of the martensitic transformation.