Effect of Thermal Cycling on the Microstructure of Nuclear Grade Dissimilar Metal Weld

Abstract As the crucial structure in the pressure boundary of the steam generator in the high-temperature gas-cooled reactor (HTGR), it is essential to investigate the microstructure of T22/800H dissimilar metal weld (DMW) under thermal cycling. This is especially critical in the context of rapid temperature fluctuations under extreme conditions, which are likely to inflict irreparable damage on the material’s microstructure, thus posing a severe threat to the service life of the DMW. T22 and 800H base metal (BM) were welded by tungsten inert gas (TIG) welding, filling with UNS N06082 inserts. To investigate the impact of thermal cycling under extreme conditions on the microstructure of DWM, and to accelerate the evolution of microscopic phenomena, simulated thermal cycling tests were carried out at 500°C with a holding time of 1 hour per cycle and 300 cycles at two ramp-up/down rates of 3°C/min and 8°C/min between room temperature and 500°C, respectively, much more serious than the temperature evolution in start-ups and shutdowns during the operation of HTGR. Characterization of the microstructure of three typical failure-prone areas of T22/800H DMWs, namely the T22 base material (BM), the T22 heat-affected zone (HAZ) and the T22/WM fusion zone by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), compared with that of the as-welded. It is obtained that limited cavities formed at the interface of T22 BM and the weld fusion zone (FZ), due to the prominent difference of thermal expansion, which contributes to the creep and fatigue, especially in the thermal cycling. Some cavities were also found in the fine-grained heat-affected zone (FGHAZ) of T22, ranging from 100 to 700nm in size. It is predicted that the cavities in FGHAZ were resulted from stress relief without preheat and post-weld heat treatment during the manufacturing of T22/800H DMWs. In addition, a mount of cavities, less than 100nm in size, were observed in the T22 BM during the thermal cycling test at 8°C/min; meanwhile, there were also some cavities, ranging from 100 to 300nm, found in the coarse-grained heat-affected zone (CGHAZ). It is supposed that the differences of thermal-physical properties, such as thermal expansion and hardness, make sense for the small cavities under thermal cycling. Sharp notches resulted from oxidization and thermal stress were found at the interfaces between T22 BM and weld metal, except for specimens without simulated thermal cycling.