Microstructure and Low-Temperature Toughness of Reheated Coarse-Grained Heat-Affected Zone of 06Ni9DR Steel

The microstructure and low-temperature impact toughness of the reheated coarse-grained heat-affected zone (CGHAZ) of 06Ni9DR steel were investigated by using optical microscopy (OM), scanning electron microscopy (SEM), backscattered electron diffraction (EBSD) and welding thermal simulation techniques. The results show that both the impact energy and hardness of the reheated CGHAZ increase first and then decrease with the increase in the peak temperature of the second thermal cycle. The intercritically reheated CGHAZ (IRCGHAZ) has the highest impact energy and hardness, and the subcritically reheated CGHAZ (SRCGHAZ) has the lowest impact energy and hardness. The impact energy of the reheated CGHAZ is higher than that of CGHAZ, but much lower than that of the base metal. In addition, the hardness of the reheated CGHAZ is higher than that of the base metal. This indicates that the embrittlement of the overall reheated CGHAZ occurs. The SRCGHAZ becomes the local brittle zone. The SRCGHAZ embrittlement is due to the small number of effective large-angle grain boundaries and the coarse grain caused by "structure heredity".