某高速铁路钢轨断裂失效分析

Failure analysis of a high speed railway rail

某高速铁路采用U71MnG热轧钢轨,在常规质量检查时发现钢轨断裂。文中运用金相显微镜及扫描电子显微镜等手段对钢轨断口的宏微观形貌、断裂处材料的显微组织和夹杂物成分进行了观察和分析,从无缝焊接等工艺角度查找钢轨断裂失效的原因。结果表明,钢轨采用闪光焊对接工艺过程中,由于顶锻距离和顶锻力控制不当,使焊接末期产生的SiO2-MnO硅酸盐夹杂及孔洞未被挤出而留在焊缝中形成裂纹萌生的核心,未被挤出的液态金属释放热量降低了焊缝及热影响区的冷却速度,导致焊缝周围先析铁素体以网状形式大量析出,网状铁素体降低了材料的强韧性,成为裂纹快速扩展的通道。钢轨在使用中受到来自列车行驶过程中所施加的交变应力和自身重力作用下,容易在焊缝中形状不规则的硅酸盐夹杂物尖端和孔洞造成应力集中,诱发微裂纹的产生。在交变应力持续作用下,微裂纹在晶界遭受弱化且强韧性降低的焊缝中进行沿晶快速扩展,最终导致钢轨发生断裂。

U71 MnG hot-rolled rail was used in a high-speed railway,rail fracture was found in routine quality inspection. In this paper,the macro-micro morphology,microstructure and inclusion composition of fracture material were analyzed by means of metallurgical microscope and scanning electron microscope,and the failure cause of rail fracture was found out from the perspective of seamless welding and other processes. The results showed that the SiO2-MnO silicate inclusions and holes produced at the end of welding were not extruded and forced to remain in the weld to form the core of crack initiation due to the improper control of upset distance and upset force in the butt welding process of flash welding. The heat released by the unextruded liquid metal reduced the cooling rate of the weld and the heat-affected zone,leading to a large amount of ferritic precipitation in the form of network around the weld. The network ferrite reduced the strength and toughness of the material and became a channel for the rapid crack propagation. The rail was affected by the alternating stress and gravity exerted by the train during its running,the irregular shape of the silicate inclusion tip and the holes in the weld tended to cause stress concentration and induced the generation of microcracks. Under the continuous action of alternating stress,the microcracks propagated rapidly along the grain boundary in the weld where the grain boundary was weakened and the strength and toughness were reduced,and finally the rail fracture occurred.