为研究某厂联箱与支管运行1．4×105 h后发生开裂的原因，采用宏观渗透及光学显微镜、扫描电镜、能谱仪、显微硬度计等测试方法对失效管进行检测．研究表明：焊后存在焊接缺陷，大量的裂纹垂直于焊缝，加速裂纹的扩展，改变应力分布状态；部分夹杂物沿直线分布，使材料受力具有方向敏感性，导致材料开裂失效；断口表面主要是Fe的氧化物，焊件长期过热，外表面形成不均匀氧化层，产生应力集中，致使焊缝从管壁外侧开裂；母材硬度超标．优化焊接工艺方案，焊前预热焊后回火，消除管件外壁的氧化层及减少设备停机次数均能有效防止同类焊接结构开裂失效．%This paper is to investigate the causes of cracking in a header and branch pipe after 140 000 hours in service. The failed pipeline was examined by using macroscopic permeability, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and microhardness tests. It was found that many cracks were perpendicular to the welding line and it was believed that the welding defects accelerated the propagation of the cracks and changed the stress distribution status. It was also found that a number of inclusions distributed along a straight line, which made the material sensitive to directional stresses and resulted in the cracks. The fractured surface was mainly covered by oxides of Fe. The pipe was overheated for long time and the uneven oxidation layer was therefore formed on the outer surface so that stress concentration occurred and the crack was initiated on the outside surface. The hardness of the parent metal was higher than the expected value. To prevent the cracking occurring in such welded pipe, the welding process needs to be optimized and few factors need to be taken into account, such as preheating before welding and tempering after welding, eliminating the oxide layer of outside the pipe wall and reducing the frequency of downtime.
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