天然气场站异径接头开裂失效原因分析

Analysis of Cracking Failure Causes of Reducing Joint in a Natural Gas Distribution Station

对某一天然气分输场站内在役开裂失效异径接头的化学成分、金相、力学性能、宏观形貌、微观结构进行了综合分析,结果表明:该异径接头基体化学成分符合GB 6479-2000标准对20钢材质要求;壁厚分布较均匀,未见明显塑性变形;裂纹起源于小头环焊缝内表面的焊根处,具有较密集的贝纹线分布,是典型的疲劳断口;裂纹起源于内表面且较平直,在试样断口附近的焊根处出现微裂纹萌生。该异径接头基体-20℃冲击功符合GB 6479-2000标准对20钢低温冲击性能要求,但单个试样冲击功远低于小头焊缝及热影响区试验值。

The chemical composition,metallographic structure,mechanical properties,macro morphology and microstructure of a in-service cracking failure reducing joint in a natural gas distribution station were analyzed comprehensively.The results show that:the matrix chemical composition of the reducing joint meets the requirements of the standard GB 6479-2000 for 20 steel material;its wall thickness is uniform,and no obvious plastic deformation is found;the crack originates from the weld root of the inner surface of the circumferential weld of the small diameter end,there is dense distribution of shellfish lines,which is a typical fatigue fracture;the crack originates on the inner surface and is more straight,and the initiation of micro cracks occurs at the weld root near the fracture surface of the specimen.The impact energy of the reducing joint matrix at-20℃meets the requirements of the standard GB 6479-2000 for the low temperature impact performance of the 20 steel,but the impact energy of the single specimen is much lower than the experimental values of the small diameter end weld and the heat affected zone.