焦炭塔底部进料管线弯头焊缝失效分析

Failure Analysis of Weld Cracks of Elbow on Bottom Feed Line of Coke Drum

介绍了焦炭塔底部进料管线腐蚀开裂情况,对开裂部位进行了外观检查。弯头断裂失效部位发生在环焊缝附近区域,且沿周向开裂,裂纹平直,而裂纹扩展方向与焊接断面基本一致。对开裂部位弯头取样,进行化学组成及金相热应力分析和硬度测试,对腐蚀产物进行化学组成分析。根据生产工艺、介质组成和分析结果确定了失效的主要原因:随着装置原料劣质化,经过10 a运行后,管线高温硫腐蚀加剧,降低了管线的承载力;由于焊缝处存在异种钢的焊接,所使用的309不锈钢导热系数较321不锈钢略小,加之前者的外壁温度又低于后者,使309钢产生较大的轴向拉伸引力,导致焊缝处存在热应力、焊接残余拉应力,在高温硫腐蚀和异种钢焊接的共同作用下导致了焊缝开裂。

The corrosion cracking of the bottom feed line of the coke drum was briefly described.The inspection found that the cracking failure occurred near the area of ring weld and promulgated circumferentially.The cracking was straight and flat,and promulgation direction was consistent with weld cross surface.By chemical composition analysis,thermal stress analysis and hardness testing of samples from elbow,the main causes of cracking were studied.The results showed that the high-temperature sulfur corrosion was the main culprit,which was resulted from processing of low-quality high-sulfur feedstock.After 10 years＇ operation,the pipeline bearing strength was reduced due to high-temperature sulfur corrosion.As different steels were welded,the heat conductivity coefficient of 309 stainless steel used was lower than that of 321 stainless steel and the temperature of external wall of 309 stainless steel was lower than that of 321 stainless steel,greater axial tensile stress was produced.The cracking propagation was accelerated under the combination effect of the thermal stress,welding residual tensile stress and the high-temperature sulfur corrosion,resulting in weld cracking.