Effect of Austenite-to-Ferrite Phase Transformation at Grain Boundaries on PWHT Cracking Susceptibility in CGHAZ of T23 Steel

The post-weld heat treatment (PWHT) cracking susceptibility of a coarse grain heat-affected zone (CGHAZ) in SA213-T23 (2.25Cr-1.6W steel) that was used for boiler tubes employed in thermal power plants was investigated using a Gleeble thermal cycle simulator. The PWHT cracking susceptibility test was performed at 650 °C, 700 °C, and 750 °C, and it can be judged that the lower the reduction of the area, the more susceptible it is to PWHT cracking. The results of the test also showed higher cracking susceptibility at 650 °C and 700 °C, which mostly involved intergranular fracture, while at 750 °C, transgranular fracture was exhibited. Therefore, the PWHT cracking susceptibility is considered to be closely related to grain boundary. The microstructure of the simulated CGHAZ and PWHT at 650 °C, 700 °C, and 750 °C was observed after etching with nital and alkaline sodium picrate etchants. Alkaline sodium picrate-etched microstructures showed a white band at the grain boundary at 650 °C, 700 °C, and 750 °C, which did not appear in nital etching. An analysis of the white band using an electron probe micro-analyzer (EPMA), TEM, and nanoindentation revealed that it was intergranular ferrite depleted with C, W, and Cr as compared with that in the matrix. Based on these results, we investigated the mechanism of intergranular ferrite formation during PWHT and its effect on PWHT cracking susceptibility at 650 °C, 700 °C, and 750 °C