Fracture Cause of Shaft Seal Steam Return Pipe of Small Steam Turbine in Supercritical Unit
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摘要: 某超临界机组小汽轮机轴封回汽管道运行8.1×104 h时发生断裂。采用宏观形貌观察、金相检验、硬度检测、力学性能试验等方法分析了该回汽管道断裂的原因。结果表明:回汽管道所用TP304H奥氏体不锈钢的抗晶间腐蚀能力差,且在冷弯后未进行固溶处理,造成弯头处位错密度和残余应力增加,从而加速了含铬化合物的析出,形成了晶间贫铬区;在腐蚀环境及残余应力等因素综合作用下,管道内外表面萌生晶间腐蚀裂纹,裂纹沿贫铬区被弱化的晶界不断扩展,最终导致回汽管道发生晶间腐蚀断裂。Abstract: The shaft seal steam return pipe of a small turbine in a supercritical unit fractured after operation for 8.1×104 h. The fracture cause of the steam return pipe was analyzed by macromorphology observation, metallographic examination, hardness test and mechanical property test, etc. The results show that the intergranular corrosion resistance of TP304H austenitic stainless steel used for the steam return pipe was poor. The pipe was not treated by solid solution after cold processing, resulting in the increase of dislocation densities and residual stresses at the elbow, which accelerated the precipitation of chromium containing compounds, finally leading to intergranular chromium-depleted zones. Under the combined action of corrosion environment and residual stresses, intergranular corrosion cracks were initiated on the inner and outer surfaces of the pipe, and expanded along the weakened grain boundaries in the chromium-depleted zone, eventually leading to intergranular corrosion fracture of the steam return pipe.
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