Abstract: In commissioning process of the platen superheater of supercritical Wshape flame boiler in a power plant, the tube cracking accident occurred repeatedly. The original tube sample, the cracked tube sample and substances similar to oxide skin were inspected by several methods, which include the macroscopic inspection, the chemical composition analysis, the tensile test, the metallographic analysis, the scanning electron microscope and the energy spectrum analysis. Combining with inspection result obtained by endoscope after the boiler shutdown, it was found that the main reason of pipe cracking is that a mass of original or subsequent oxide skin fell off and blocked the throttle orifices and pipe of the header in inlet of the platen superheater, which causes that the medium flow in the pipe reduced, the cooling effect of steam for the pipe became lower; and the local parts of pipe was over temperature, and its hightemperature creep strength decreased.

Key words: supercritical boiler, tube of superheater, commissioning, substances similar to oxide skin, tube cracking