重载铁路低温焊接铝热焊接头断裂失效研究

Study on Fracture Failure of Low Temperature Welded Joint of Aluminothermic Welding in Heavy Haul Railway

2017年12月下旬至2018年1月上旬,大准线60kg/mU75V淬火轨铝热焊接头连续发生多处同一形式断裂,且该断裂接头均是2017年11月完成的焊接。通过选取两处典型断口进行宏观形貌观察、微观断口分析、能谱分析、化学成分分析、金相组织分析、力学性能分析及硬度分布分析等手段,对折断铝热焊接头进行失效原因分析。通过分析,断裂铝热焊接头断裂方式和裂纹源位置相同,均为解理脆性断裂,裂纹均起始于轨腰与轨底的过渡区表层。该位置属于焊接区,存在比较密集的夹渣、孔洞等焊接缺陷。断裂原因主要是2017年大准线秋季集中修时间安排较晚,预热时间不足和保温措施不够造成焊缝处钢轨表面预热不彻底和热能散发速度快,使得钢轨表面和内部的温度不均匀,最终导致钢轨轨腰和轨头尺寸相差较大的部位内部溶液凝固时收缩不一致,最终在焊接区形成密集的夹渣、孔洞等焊接缺陷。

From the late December 2017 to the early January 2018,the aluminothermic welded joints of 60kg / mu75v quenched rail of Dazhong alignment were broken in the same form continuously,and the broken joints were all welded in November 2017.In this paper,two typical fracture surfaces were selected for macro morphology observation,micro fracture analysis,energy spectrum analysis,chemical composition analysis,metallographic structure analysis,mechanical property analysis,hardness distribution analysis and other means to analyze the failure causes of the broken thermit welded joints.Through the analysis,the fracture mode of the joint is the same as that of the crack source,which is cleavage brittle fracture.The cracks start from the surface of the transition zone between the rail waist and the rail bottom,which belongs to the welding zone,and there are relatively dense slag inclusions and holes and other welding defects.The main reason for the fracture is the late arrangement of centralized repair time in the autumn of 2017,insufficient preheating time and insufficient insulation measures,resulting in incomplete preheating of the rail surface at the weld joint and rapid heat emission,resulting in uneven temperature on the rail surface and inside,resulting in inconsistent shrinkage of the internal solution at the parts with large difference in rail waist and rail head size,and finally in the shape of the welding area There are many welding defects such as slag inclusion and holes.