超超临界机组低温过热器水平管焊接接头开裂原因分析

Analysis of Cracking Reasons for the Welded Joints of Horizontal Pipes in the Low-Temperature Superheater of Ultra-Supercritical Units

以某1000 MW超超临界机组低温过热器进口泄漏水平管为研究对象,采用宏观检测、化学成分分析、裂纹低倍金相检测、微观金相检测、扫描电镜形貌观察、力学性能检测等方法对低温过热器水平管泄漏失效原因进行分析。结果表明:裂纹从焊接热影响区的管子外壁粗晶区启裂,裂纹沿晶在粗晶区扩展,裂纹内部覆盖大量氧化物,在三晶粒交界处存在楔形孔洞;裂纹尖端附近存在大量孔洞,部分孔洞与孔洞呈链状分布,判断裂纹性质为蠕变裂纹。提出焊接工艺优化建议,供避免低温过热器蠕变开裂参考。

Taking the inlet leaky horizontal pipe of the low-temperature superheater in a 1000 MW ultra-supercritical unit as the research subject,chemical composition analysis,low-magnification metallographic examination of cracks,microscopic metallographic examination,scanning electron microscopy observation,and mechanical property testing were conducted to analyze the causes of leakage failure in the horizontal pipe of the low-temperature superheater.The results indicate that the cracks initiate from the coarse grain area on the outer wall of the pipe in the welding heat-affected zone,propagate intergranularly in the coarse grain area,and are covered with a large amount of oxides inside.Wedge-shaped holes exist at the junction of three grains.Additionally,numerous holes are present near the crack tips,and some of them are chain-like distributed.Based on these characteristics,the nature of the leaky cracks in the horizontal pipe of the low-temperature superheater is creep cracks.Suggestions for optimizing the welding process are proposed to prevent creep cracking in the low-temperature superheater.