Research On Non-preheating Welding Repair Technology Of U75V Rails

Rails will have defects such as peeling off due to train operation and natural conditions during using,In order to save costs and reduce the blank of operation,surfacing welding repairs are usually used.Because the rail material is mostly high-carbon alloy steel,the carbon equivalent is large,and the hardenability tends to be large,cold cracking is likely to occur after welding,conventional repair methods usually use pre-heating before welding and slow cooling after welding,but they increase costs and processes.This topic proposes the preparation of a preheated and post-heated surfacing electrode,intended to solve the shortcomings of conventional methods.Based on the analysis of welding characteristics and weld-ability of U75V rail material,the influence mechanism of controlling cooling rate on the solidification process of the weld pool is analyzed.It is concluded that the control of the cooling rate after welding can obtain more pearlite and bainite structure,metallographic microscopy,scanning electron microscopy and energy dispersive spectroscopy were used to investigate the causes of post-weld cracking.The results show that the sharp cooling rate and the high carbon sheet martensite which has microcracks in the heat affected zone is the cause of cracking.The feasibility analysis of non-preheating was studied.The CaO-CaF2-SiO2 based slag system was developed and the Cr-Mo-W based alloy system was supplemented which contained nickel,vanadium,titanium and other alloying elements to strengthen the welding rod.Optimized formulas through orthogonal test;it can be found that under no-preheat and post-heat conditions when welding current was over 120 A and welding speed was 150mm/min which has a surfacing layer with lath martensite and retained austenite,a fusion zone with lower bainite structure and a lath horse were prepared,the heat affected zone structure was the bainite and granular bainite;hardness test shows that the surfacing layer was 330HV,fusion line and heat affected zone was around 420HV.The welding repaired temperature and stress fields of U75V rail was calculated by SYSWELD software and the actual thermal cycle curve was measured using K-type thermocouple and paperless recorder.the results show that residual stress in theσ_X direction of the rail surface can reach 350MPa to 520MPa and the tensile stress gradually decreases away from the weld seam;the residual stress in theσY direction can be neglected compared with other two directions;the residual stress in theσ_Z direction in the weld zone which contains weld zone,fusion line and heat affected zone were all tensile stress,in general,the center of the weld can reach 800MPa,it gradually decreases with increasing weld distance,and compressive stress away from the weld zone up to 150MPa;it can be found that the optimal welding current parameter of long rail was 120A to140A through the simulation.The HT-1000 high-temperature friction and wear tester was used to test the friction and wear behavior of the surfacing layer at different currents,the SEM and confocal laser scanning microscopy were used to analyze the morphology of the wear scar.The results show that the wear resistance of the surfacing layer is higher than that of the U75V rail base material when the welding current was 140A and similar to that of the base metal at 100A and 160A;It was verified that the surfacing layer prepared from the preheated electrode had a higher impact toughness than the U75V rail base material at the current of 100A to160 A through the impact test and the impact toughness of the surfacing layer decreased as the current increased,it also can be found that the shock absorbing power is 31.5 J when welding current was 100A.