Abstract: During the maintenance of a coal-fired boiler in a power plant, it was found that there were bulges in many positions of the front wall water wall. The causes of bulge were determined by means of macroscopic analysis, chemical composition analysis, tensile test, hardness test, metallographic and scale layer analysis. The results show that the local overtemperature of the water wall tube of the boiler front wall is caused by the scaling of the inner wall, which accelerates the spheroidization rate of the microstructure of the material and leads to the decrease of the strength performance. With the passage of time, plastic deformation occurs at the over-temperature part under high temperature and high pressure service environment to form bulges.