某输气管线用国产A105钢法兰盘在-16℃条件下的输气试压过程中出现贯穿裂纹造成失效;利用光学显微镜、扫描电子显微镜、材料试验机等对失效法兰盘的化学成分、显微组织、力学性能以及裂纹的形成、扩展进行了分析,以找出失效原因.结果表明:失效法兰盘材料组织粗大,局部区域有出现魏氏组织倾向,材料塑性、冲击韧性较差,满足不了标准要求;裂纹起源于法兰盘颈部斜坡面与平台交界应力集中处的加工刀痕根部,在焊接残余应力和管线压力的共同作用下,裂纹沿脆性组织扩展,最终造成法兰盘开裂失效.
A through-wall crack occurred in a domestic A105 flange used in gas pipeline during the gas pressure testing at -16 ℃. The chemical composition and mechanical properties as well as formation and propagation of crack were analyzed by means of optical microscopy, scanning electron microscopy and material property tester. The results show that the elongation and impact toughness of the failed flange were decreased as a result of coarse grained structure and formation of Widrnanstatten microstructure in local region. The machining line with no smooth bottom between the cone surface and tube surface in flange structure, which could cause stress concentration, became a source of crack. The crack propagated along brittle microstructure under the pipe pressure and welding residual stress which caused the flange cracking failure ultimately.
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