Abstract: By observing the macroscopic and microscopic morphology of cracks,metallography,hardness,scanning electron microscopy, energy spectrum, and joint structure analysis, the failure reasons of welding seam cracks in the installation of the water-cooled wall of a 1 000 MW ultra supercritical unit boiler are studied. The results show that the gap at the root of the weld, the protrusion at the root of the weld, and their hardness exceed the standard, coarse weld microstructure and low melting point carbide phase segregation, weaken grain boundaries, and there are microcracks in the weld seam, which can be determined as welding hot cracks based on the analysis of the oxide phase inside the microcracks. Under the action of high temperature and high pressure stress during operation, crack propagation ultimately leads to cracking failure, and welding hot cracks are the main cause of this failure.

Key words: 15CrMoG steel, water wall tubes steel, butt welds, intergranular crack, hot crack