液化天然气槽车真空失效研究

Failure research of T-joint at liquefied natural gas tanker

液化天然气槽车贮罐在低温下长期服役,贮罐封头处丁字焊缝热影响区容易产生裂纹,导致漏气。本文模拟实际使用环境,设计液氮浸泡实验,借助于X射线衍射,光学显微镜,扫描电镜,系统分析了贮罐焊缝模拟样品热影响区在液氮中浸泡前后显微结构的变化。低温环境下,焊缝热影响区发生马氏体相变,且随着焊接内应力的增加和低温保温时间的延长,马氏体相变量增加。与实际失效样品对比可知,槽车贮罐的真空失效是由于低温使用过程中丁字焊缝热影响区内马氏体的增加而导致。笔者认为,降低焊接热影响区的应力可以有效降低槽车贮罐真空失效的发生几率。

Cracks would generate in the heat-affected zone（ HAZ） of T-joint and cause leakage when the liquefied natural gas（ LNG） tank was used in low temperature environment for a long time. The low temperature experiments were designed by simulating the service environment of liquefied natural gas（ LNG） tank. The microstructure changes of welded joint HAZ of simulated samples was analyzed using X-ray diffraction, optic microscopy, and scanning electron microscopy. The results show that martensitic transformation is induced in the HAZ by the low temperature（- 196℃） environment. With the increase of the residual stress and the time of immersion in liquid nitrogen,the volume fraction of martensite increases. Comparing with actual failure sample,the vacuum failure of the LNG tank is caused by the increase of martensite at the heat-affected zone of T-joint during the service process.Therefore,reducing stress in the HAZ is beneficial for decreasing the vacuum failure probability of the LNG tank.