某火电厂超超临界机组锅炉用T91钢蒸汽采样管焊接接头在使用约4 200 h后出现了开裂, 通过宏观检验、显微组织分析、拉伸试验、拉伸断口分析以及硬度检测等对开裂的原因进行了分析。结果表明: 在焊接接头靠近母材的热影响区中, 马氏体组织出现典型的索氏体化, 粗大的析出相(Laves相和M23C6相)在晶界上呈链状分布, 导致该区域的强度和硬度降低, 并引起晶界弱化, 在高温和多向应力的共同作用下该区发生开裂。

Crack was found on welded joint of a T91 steel steam sampling tube in power plant ultra-supercritical unit boiler after using about 4 200 h. The cracking causes were analyzed through macroscopic inspection, microstructure analysis, tensile test, fracture analysis and micro-hardness testing. The results show that the martensitic structure of the heat affected zone near base metal was obviously decomposed into sorbite, and the coarse Laves phase and M23C6 phase distributed on the grain boundaries continuously. These changes in microstructure not only resulted in the decrease of strength and hardness of the area, but also brought about grain boundary weakening. Under the combined action of high temperature and multiaxial stress, the welded joint eventually generated type IV crack failure.