Thermal Simulation Analysis of Microstructure and Hardness for CrMoV with PWHT in Type IV Region

Welded components of CrMoV steam pipe exhibit a pernicious form of type IV cracking after long-term service at elevated temperature. To investigate the cracking mechanism, the type IV microstructure and hardness were characterized after thermal simulation of post-weld heat treatment. Below 1098 K (825 °C), loss of carbon from the pearlite region was apparent, and the work zone exhibited a slightly lower hardness than the parent material because of a minor amount of austenite transformation. In addition, for peak temperatures above 1133 K (860 °C), additional transformation into austenite occurred and was followed by retransformation into ferrite upon further increasing the temperature. The pearlite formed at 1173 K to 1223 K (900 °C to 950 °C) resulted in an increase of the volume fraction of pearlite and microstructural refinement, which yielded a remarkable increase of hardness in the work zone. For the peak temperature of 1573 K (1300 °C), previous austenite grains were coarsened and alloy carbides were dissolved in the austenite, which significantly hardened the work zone.