| Rail welding is the basic work of railway seamless line laying.Rail welding quality affects the safety of railway transportation,the comfort of passengers and the economy of line maintenance.This paper selects the engineering problems of microcracks and high detection alarm rate on the undersurface of the welded rail joint that have plagued the railway welded rail factory for many years.From the perspective of material metallurgy and welding process,macro-micro analysis and ultrasonic testing are adopted to conduct a comprehensive analysis and research on the characteristics of microcracks on the undersurface of the welded rail joint and ultrasonic testing of different batches of rail base material.The following results were obtained:1.The characteristics of microcracks on the bottom surface of the rail flash butt welding head are mainly distributed in the overheated area of the weld on the bottom surface of the rail.Usually,the longitudinal direction of the rail is about 1mm away from the overheated area of the weld.In the direction of rail height,microcracks exist on the bottom surface of the rail and near the surface.From the bottom surface of the rail to the rail matrix,the microcracks disappear after 2mm.When the microcrack is more serious,it will affect the drop hammer performance of the welded joint.Sometimes the microcrack can be seen by the naked eye on the metallographic grinding surface.When the ultrasonic testing is carried out according to Q/CR707-2019,the wave height of the echo will generally be higher than 20%.2.The relationship between the welding process and the microcracks on the bottom of the rail was studied.Through microscopic analysis of the bottom surface and near surface of the weld rail under different push-convex phase processes,it was found that the low push-convex phase temperature was one of the main reasons for microcracks in the weld under the rail surface.By shortening the time from the beginning of the upset forging to the beginning of the pushing(the pushing convex knife contact the overlap),the joint is ensured to complete the protruding at a higher temperature,which can avoid or significantly reduce the weld microcracks on the bottom surface of the rail.Increasing welding heat input and reducing upset forging amount can help to increase the pushing temperature and reduce the probability of microcracks to some extent.3.It is found in the study that there are A class,B class or C class inclusions in the area where abnormal echo occurs in the rail base material to be welded,and the severity of inclusions is different.In most cases,the inclusions reflected by a single field of view are not serious.Through testing and drawing the inclusion map of the abnormal echo area of the rail base metal,it is found that the inclusion has regional distribution characteristics.It is proposed for the first time that the method of evaluating rail inclusions using ultrasonic testing spectrum(including sampling and processing of rail base material,sample heat treatment,fixed point ultrasonic testing,calculation and analysis of average echo height,etc.)is superior to the metallographic analysis method of single field of view.Therefore,the influence degree of different batches of rail base material to be welded on the weld inspection results can be predicted,and the solution and means to realize the pre-evaluation of rail weldability in the pre-welding preparation stage can be creatively provided. |
