Liu Baohai

(Qinhuangdao West Works Section,DAQIN Railway Co．,LTD,Henan 066000)

Abstract:Rail welding is an important link in the process of seamless laying of the line,and the quality of its welded joints is directly related to the safe operation of the line.Good welding joint is an important precondition for safe operation of rail transit.This paper analyzesseveral common methods of rail welding and deeply explorethequality control of welded joints.

Keywords:rail welding;joint;quality control

I.Rail Traffic and Rail Welding

In 1876,the completion of Wusong Railwaythe first railway in China,marked the official start of China"s railway transportation. With the development of history, China"s railway transportation industry is making continuous progress.By the end of 2016,the total length of China"s railway track has exceeded 120,000 km,and the current railway transportation is developing rapidly towards the direction of passenger highspeed and freight heavy load[1].The progress of the railway industry is closely related to the support of science and technology.Including seamless track laying technology is the cornerstone of railway industry development,the technology is to weld the rail as a whole,eliminating the joint problem in the traditional railway.It greatly reduces the collision and impact between wheels and tracks,effectively extends the life of parts,and at the same time,enables passengers to have a more comfortable riding environment.The key to seamless line laying is the quality of rail welding technology.Generally speaking,the object of rail welding is PG4,U71Mn,U75V,U77MnCr,Bainite alloy steel and other types of steel.These steels can basically meet the requirements of seamless line laying,but there is no lack of fatigue fracture in actual rail welding,and the solution can only rely on technological breakthroughs[2].At present,the domestic and foreign rail welding mainly includes flash welding,pneumatic welding,aluminothermic welding and arc welding,etc.,while in the market of our country,what basically put intouse is 3 kinds.

II.Classification and Working Principle of Rail Welding

Railway transportation is a highly valued industry in many countries,and academic exchanges between different countries are frequent.Therefore,the welding methods adopted in rail welding process at home and abroad are largely the same,among which flash welding, pneumatic welding,aluminothermic welding and arc welding are the four most widely used welding methods,while the first three welding methods are frequently used in China[3].

(i)Flash Welding lding

Flash welding,also known as contact welding,is a resistance welding,the main process including welding end preheating and hydraulic upset.According to different flash characteristics,it can be further divided into continuous flash welding and pulse flash welding.The process history of continuous flash welding is longer and its technical system is more mature.Pulsed flash welding is a new technology based on the defect of continuous flash technology.Although there are differences in the process between the two,their working principles are basically similar.First,contact the two steel rails that need to be welded in a reasonable way and then pass a strong current.As the rail itself has a certain resistance,resistance heat will be generated when a strong current passes through to form a high temperature,and the contact surface of the two rail will form a melting state under the high temperature.At this time,the surface fusion is completed by the hydraulic jacking force applied externally.After cooling for a certain time,the metal crystallizes,and the two rail will be welded into a whole.

(ii)Pneumatic Welding lding

Pneumatic welding,as the name suggests,is to complete the welding process through the use of pneumatic equipment,the principle is similar to flash welding,the rail to be welded is heated after affixed,but the heating method is generally completed by oxyacetylene flame.The heat energy of the flame changes the area between the two contact surfaces into a plastic state.After the surrounding metal atoms have a high enough activation energy,the metal bond between the atoms is accelerated through the hydraulic jacking forging on the bonding surface,and then the welding is completed after cooling and crystallization.Compared with flash welding,pneumatic welding equipment is more portable,simple,less investment cost,high process flexibility,However,the rail needs longitudinal movement during welding,and the butt processing of the two joints is very demanding,and it is difficult to weld the rail with a large length.Therefore,air pressure welding is often used in thelater maintenancework[4].

(iii)Thermic Welding lding

Thermite welding is a thermite welding using iron oxide as oxidant and aluminum powder as reducing agent.Aluminum powder,iron oxide and other additives are proportionally prepared into thermite welding flux in the reaction crucible.Through high temperature matches the activation reaction,hot melt alloy additives and reaction generated under high temperature iron formation of liquid steel,watering to the sand mold cavity according to the size to be welded rail joint out of the shape,the molten rail crystallizes with it under the constant influence of high temperature,cooling after finalize the two rails also become a pieces.It is worth noting that the highest temperature in the whole process may reach 3000C.Compared with flash welding and pneumatic welding,this welding method omits the steps of top forging.It has high process flexibility.However,the overall performance of its welding seams is slightly worse.Therefore,thermit welding is often used in the line stress release and cross-zone line junction welding and fracture maintenance.

III.Defects in Different Ways of Rail Welding and Countermeasures

The scope of application of different welding methods is different,and their working principles are also different,and different problems have been exposed in the long-term use process.In view of these technical defects,it is necessary to respond with targeted strategiesand methods.

(i)Flash Welding lding

Flash welding is now the most widely used in the laying of seamless lines,and its weld quality is relatively stable.In comparison,the occurrence rate of problems is the lowest,but there are still three possible technical defects:1.The joint may produce martensite structure due to imprecise feed control,and such defects can usually be dealt with by polishing the contact part of the electrode holder before welding.In addition,the normalizing treatment after welding and the composition control of Mn and Celements in the base metal are effective methods to limit the formation of martensite structure.2.Due to the low upset force,incomplete elimination of molten metal layer will result in incomplete fire pits.Generally,when the pit is not closed,it is necessary to polish the two welding surfaces before welding,and make appropriate adjustment to the upset parameters to ensure the normal completion of the upset process.3.Under high temperature,the silicate inclusions formed by rail oxidation form gray spots,and this kind of problem is more simple to deal with,only need to strictly control the specific parameters in the welding process,and timely clean the oxide layer on the welding surface.

(ii)Pneumatic Welding lding

The welding quality of pneumatic welding is mainly affected by human factors,and the working environment and climatic conditions will also have some influence on it.Therefore,it is hard to avoid some defects in the welding seams of pneumatic welding.They are mainly divided into defects in appearance and in the inner of the welding seams.Common types are as follows:1.The dislocation and bending of the welding seam, usually, the dislocation and bending of the welding seam is caused by the difference between the height of the rail low bolt of the bonding machine or the improper placement of the heater.These problems can be avoided by an accurate inspection before welding;2.Too long heating time leads to the collapse of rail Angle of welded joint.Such defects can be reduced by improving the accuracy of heating instrument parameters,but it is difficult to avoid completely;3.Burn the surface of welding seam when removing the weld nodules.For such defects,the flame can be preset before cutting the weld nodules to prevent overheating and burn the welding seam in the process.

(iii)Thermic Welding lding

Thermite welding is similar to pneumatic welding,manual welding is also required in the field,so its main defects are reflected in both internal and external aspects,the main forms are as follows:1.Due to the high camber reserved for the rail end face correction,the welded joints are camber up,and the appearance shape and size exceed the standard.Generally,the three parts of rail head,rail waist and rail bottom are precisely aligned before welding and timely reset after welding can avoid such problems;2.Due to flux quality or tool dampness resulting in porosity,there is no special way to solve this problem,only through the operator before welding a comprehensive and careful inspection of equipment and materials;3.Sand inclusion.Sand inclusion refers to the defects caused by molding sand entering the welding seam during operation.Generally speaking,the selection of suitable mold sand with more stringent requirements can greatly reduce the occurrence of sand entrainment,and strict operation in technology can also avoid the occurrence of such defects to a certain extent[5].

(iv)attention to ensure the quality of welding lding joint joint

The above article analyzes and discusses the possible defects in different welding methods.Although they have different problems in the quality of welded joints,generally speaking,the quality of welded joints should be guaranteed from three aspects:Carry out comprehensive maintenance and commissioning inspection of the equipment before welding to ensure the normal operation of the equipment;Pay attention to the situation of welding seam at all times,and make corresponding adjustment at any time when different situations occur;Check the welding result in time after welding,and strictly control the quality of welding joint.

IV.Conclusion

In a word,no matter in what way to do rail welding,there will be more or less certain defects.What we need to do is not only to explore the causes of these defects,but also to be able to address these problems in rail welding and strictly ensure the performanceand quality of welded joints.