Study On Damage Of Welded Joints Of Off-line Hypereutectoid Steel Rails

Due to the influences of flux,high temperature and post-welding heat treatment of the welding process,the microstructure and mechanical properties of the rail welded joints are different from those of the base metal.Under the combined loads of impact and rolling of the wheel,the welded joints will become a weak link in the rail damage.Therefore,the damage analysis of the welded joints can directly reflect their service performance.In this paper,the damage of the off-line hypereutectic rail welded joints,which were produced by flash welding and aluminothermic welding respectively,was analyzed by using ATOS three-dimensional scanner,metallographic microscope,scanning electron microscope,MVK-1002 micro-vickers hardness tester and other equipment.By comparing the differences of the tread surface morphology,hardness and microstructure of the rail weld,the heat affected zone and the base material position,the damage evolution rules of the joints welded by the two welding processes in the service process were revealed.The main conclusions are as follows:1.A parametric model of the wear profile of the off-line hypereutectoid rail welded joint was established by using 3D scanning and reverse modeling technology.By comparing with the standard rail profile,the surface wear profile of the off-line joints in the two welding processes was characterized.The contour of the joints on the top of the rail in the straight section showed uniform wear.The contours of the contactless end of the curve segment were almost unchanged,while the contours of the contact end were stepped down significantly,showing severe side grinding.The wear of the joint was inversely proportional to its hardness.The base material with the highest hardness has the smallest wear amount,followed by the position of the welding seam,and the wear at the boundary of the heat-affected zone was the most serious.2.The surface damage of the off-line welding joints is mainly caused by peeling in the straight section,while the crushing fat edge and fatigue crack in the curve section..The damage increased with the decreasing curvature radius.The subsurface of the area where the surface cracks appear will also have the extension of fatigue cracks.The order of the severity of the damage of various parts of the welded joint from the high to the low was: heat-affected zone,weld and base metal.3.The damage characteristics of of each part of the welding joint are directly related to the hardness,while the hardness is related to the pearlite shape,lamellar spacing,grain size and other factors.Due to the influences of welding process and post-welding heat treatment,the grain size and lamellar distance of pearlite at the heat-affected zone and weld were all larger than that of the parent metal.The boundary of the heat affected zone formed granular pearlite due to the uneven heating,which resulted in a gradual decrease in hardness from base metal,weld to heat-affected zone.Thus,the wear degree increased in turn.4.Due to the difference in welding process,the final pearlite grain size and lamellar spacing of the welded joints welded by aluminum heat welding were all larger than that of flash welding,resulting in its poor mechanical properties in weld and heat-affected zone,and greater wear and more serious damage.5.Due to the work hardening phenomenon caused by wheel-rail rolling contact and the reduction of the wear amount after the tread surface enters the stable wear period,the hardened layer of the tread was more stable.The tread hardness of the off-line joint was more than 120 HV hardening compared with that of the unused joint,while the hardness of the base metal was less affected by wheel-rail load,and the curves were more similar.6.The matrix structure of the off-line joint was similar to that of the unused joint.However,at the boundary of the heat affected zone,the spheroidized and hemispheric pearlite spacing arrangement was closer than that of the unused joint because the off-line joint was subjected to impact and extrusion of the wheel rolling contact for a long time,which result in the increase of the hardness in this region.