某等径无缝三通在试压投产过程中支管侧发生开裂，采用形貌、显微组织观察，无损检测，化学成分、力学性能测试等方法分析了其开裂原因。结果表明：该失效三通的开裂位置在支管的环焊缝上，环焊缝是在三通制造阶段产生的，不符合等径无缝三通的生产质量要求；开裂的主要原因是环焊缝上存在严重的未焊透缺陷，随着试压压力增大，未焊透缺陷处起裂并扩展直至穿透剩余壁厚。

Cracking occurred on the branch pipe side of an equal diameter seamless tee during the process of pressure testing and putting into production. The cracking reason was analyzed by morphology and microstructure observation, nondestructive testing, and chemical composition and mechanical property tests. The results show that the cracking location of the failed tee was on the girth weld of the branch pipe, and the girth weld was produced in the manufacturing stage of the tee, which did not meet the production quality requirements of the equal diameter seamless tee. The main reason for cracking was that there were serious incomplete penetration defects in the girth welds. With increasing test pressure, cracks were initiated at the incomplete penetration defects and expanded until penetrating the remaining wall thickness.