Research On The Formation Mechanism Of Flat Spots In The Flash-butt Welding Joint Of Rail Steel

Rail welded joints usually produces in the rail welding factory, the assessment of the quality of rail welded joints by drop test. According to the rules in the GB Standards, a hammer head weight1000kg fall freely, hite on the rail welded joints, if at least25welded joint can guarante not fracture after two consecutive hammer, it is be considered qualified. We can see that the drop test is very strict on the quality of rail, generally speaking, if the rail welded joints produced in the rail welding factory can pass the drop test, quality can be guaranteed.Statistical studies have found that unqualified rail welded joint in the drop test, usually contains defects.This research first study on the organization and toughness of rail welded joint, and then on the composition, distribution regularity and spatial form of flat spots. Creatively put forward that separated the preheating stage and the flashing stage of the rail flash-butt welding, from the entire flash-butt welded process, research the two stages, mainly study on the oxide and inclusion which are formed in the two stages, through the analysis of these given the formation mechanism of flat spots.In this paper, the main research results are as follows:1. The used U71Mn rail material is lamellar pearlite microstructure, weld zone appears softening phenomenon caused by high temperature carbon burnt, and the value of hardness greatly lower than the parent metal.Joint is After heat treatment pearlite colony is relatively small, the fracture toughness improved.But for the defects like flat spots which is very difficult to find through ultrasonic flaw detection, once defects like flat spots appeared in the weld zone and heat affected zone are larger, even through normalizing treatment after welding, joint in drop test is still not standard table.2. The flat spots which appeared on the surface of the fracture face contain Silicate inclusions which are messy without rules, having a irregular flat spatial form structure.In flash heating process, the two docking rail surface layer can be flow of liquid metal, lead to Mn and Si elements get gather and be oxidized, Fe element into the surrounding collective, forming a silicate inclusions.3. Generally, there are two main factors about the formation of silicate inclusions in the weld zone:(1) the content in the formation of the melted metal layer;(2) inclusions formed in the surface area, having been covered by the blasting area layers which consist of liquid metal,become deeper inclusions.These inclusion formed in the deep are hard to squeeze will be left on the rail welded joints. Because the silicate inclusions have different the thermal expansion coefficient with the parent metal substrate, and silicate inclusions are embedded in the substrate, in the process of cooling, tiny cracks can formed on the interface between silicate inclusions and the surrounding metal.The crack in the drop test always cause rail cracking and influence the property of welded joint.