超超临界锅炉水冷壁开裂原因分析

通对某超超临界锅炉在安装期间发现裂纹的水冷壁鳍片和管子进行化学成分、机械性能、示波冲击、显微硬度、金相组织、断口形貌试验等分析,得出鳍片部位存在折叠和微裂纹等轧制原始缺陷。诱发鳍片与管子焊接过程中产生裂纹并扩展的主要原因是鳍片硬度高、脆性大,而且存在原始缺陷;次因之一是鳍片与管子的焊接质量差、氧气切割时产生热应力,在切开的尖锐的直角缺口部位产生较大的应力集中,导致鳍片在缺口处开裂并扩展;次因之二是水冷壁管屏在生产工地到安装工地的吊装及运输过程中在二次应力的作用下产生多次振动和冲击,在裂纹的启裂部位产生弯矩和扭矩,导致裂纹迅速开裂并扩展。

Analysis on the Causes of Water-wall Cracking of Ultra-supercritical Boilers

Multiple cracks were found on the water wall of an ultra-supercritical boiler during installation.An analysis was carried out on the cracked water-wall tubes and fins in terms of chemical composition,mechanical performance,instrumented impact,micro-hardness,optical microstructure and fracture appearance,and such rolling genetic defects as fold and micro fissures were found in the fin area.It was concluded that the high hardness and brittleness of the fins and their internal defects are the main causes for the cracks and their propagation induced in the process of fin and tube welding.The poor welding quality,thermal stress induced by oxygen cutting,and high stress concentration in the location of sharp right-angle gap are the second causes for cracking and propagation of fins;The moment and the torque produced at the fissure ends by repeated vibration and impact during lifting,handling and transporting process of the water-wall tubes are another secondary causes for the rapid cracking and propagation of fissures.