Research On Formation Mechanism Of Flat Spots And Micro-crack Defects In Rail Welded Joints

In this paper,combined with the overall performance test of the rail flash butt welding head,the focus of the study is on the flat spot that affect the joint performance.The relationship between the performance of different Mn content rail flash-butt welding joint and flat spot was compared and analyzed.the composition and content of non-mentallic inclusions in flat spot were statistically analyzed,systematic study of flat spot properties of rail flash welding.The mechanism of the the formation of flat spot in flash butt welding was studied with the aid of difital photography of high-speed photography and infrared imagers.Based on the observation of blasting characteristics of liquid flash lintels on rail flash butt welding,a flat spot formation model was established,and the welding model was used to verify the model.At the same time,the formation reasons and formation process of micro-cracks defects at the track bottom subsurface are generally analyzed.The effects of different Mn content of the steel rail parent metal and manganese sulfide inclusions on the flat spot and micro-crack defects of the rail flash butt welding head were studied.The main research work and achievements of this article are as follows.(1)The statistical results of fractures and flat spots on static bending and drop weight of U71 Mn and U75 V rail flash butt welding joints show that the U71 Mn rail fracture rate and the average flat spot area at the fracture are significantly higher than U75 V rails.There are obvious differences between the flat spot of the drop weight or the static bending fracture of the rail flash butt welding head and the surrounding matrix.The macroscopic features of the flat spot are darker and flatter.Spot-like or sheet-like non-metallic inclusions can be seen under the scanning electron microscope,dimples can be seen on the surface of the plaque.Non-metallic inclusions are mainly manganese-containing silicates.The most abundant flat spot non-metallic inclusions are Mn element accounting for 40%,followed by the O element,the third is the Si element.There is a significant linear relationship between the Mn content and the O content.The Mn content in the flat spot directly affects the O content change.(2)The flat spots at the fracture of the flash butt welding head are mainly distributed at the bottom of the rail,and the macroscopic appearance is mostly elongated or oblong.The shapes of the ends of the rails are various.According to the characteristics of the flash blasting at the end of the rail,during the flash blasting,the resistance at both sides of the waist or bottom is small.The flash beam blasting direction is along the cross section of the rail,and the perforation direction is along the flash blast direction.Therefore,after the formation of the flat spot,the distribution is along the direction of flash explosion,and the flat spot found at the rail waist or rail bottom is macroscopic.The distribution of the topography is generally parallel to or inclined at a small angle to the cross section of the rail waist.In the end of the rail,because of its three free surface directions,the flash beam blasting can be performed in any one or more directions,so the flat spots at the end of the rail bottom are more varied in appearance.(3)A model for the formation of flat spot was established.In the process of flash butt welding,silicate-type non-metallic inclusions scattered at the weld seam are brittle and have a poor ability.When the joint receives an external force,it is most likely to crack at this point and form non-metallic inclusions and flat spots in local dimple tissue.Because the normal weld toughness is good and the whole weld width is only 0.2-0.4mm,and the heat affected zone and the rail base metal near the weld are high in carbon content,the toughness is relatively poor,and the cracks along the weld heat affected zone are toward the base metal.Expansion,the formation of cleavage fractures.Because the fracture mechanism is different between the flat spot and the fracture matrix,different fracture structures are formed.Therefore,the flat spot and the matrix form a clear boundary,and the flat spot is different from the matrix color.(4)Formation mechanism of flat spot: The surface to be welded of the steel rail is gradually formed into a layer of liquid metal film under the early welding process,and large lintel blasting occurs in the final continuous flashing stage to spray the liquid metal film along the free end surface of the rail.The end face is welded to form perforations that are exposed to air,so that elements such as Mn and Si in the weld react with oxygen in the air at high temperatures to form silicate inclusions,which are generated later in the rail welding continuous flash stage.The new effective lintel blasting is again produced or the forging phase has been entered at this time.Therefore,the silicate inclusions formed at the perforation cannot be discharged from the welding end face and remain in the weld seam to form a flat spot.(5)The microcracks occur mainly in the underheated zone near the bottom of the rail where the flash welding head is close to the weld,and the depth is within 1 mm to 2.5 mm from the bottom surface of the rail.The existence of microcracks was not found near the bottom weld of the flash welding head rails of the untuned rails,indicating that the micro-cracks in the superheated zone of the secondary surface of the track were less caused during the upset forging.The micro-cracks in the hot zone near the weld are mainly generated during the push-convex phase of flash welding.The reason for the analysis of microcracks is that the upset causes the strip-like structure to be deflected,and the push-convex causes the strips in the overheated zone near the weld to bend further and cause large-angle deflections,which causes flaking of the MnS inclusions in the banded structure.The metal matrix is subjected to lateral stress,and microcracks occur when the critical strength is exceeded.(6)For the U71 Mn and U75 V rails,the base material inclusions were classified and statistically analyzed.The main steel rails were mainly manganese sulfide inclusions.The content of Type A(manganese sulfide)inclusions in the U71 Mn and U75 V rail base materials was much higher.For the sum of the contents of B(silicate)and C(alumina)inclusions,the contents of Class A,B,and C inclusions in the U71 Mn rail are more than twice that of the U75 V rail.The difference in drop weight performance and static bending test of the overall performance of the two types of rail flash welding joints is related to the Mn content of the parent steel rail and the content of MnS inclusions.Ther higher Mn content and MnS inclusion content of U71 Mn affect the weldability,resulting in the incidence of flat spot defects increased,resulting in higher drop weight and static bending breakage rates.