With the rapid development of high-speed and heavy-duty railways, the track structure is gradually replaced by seamless lines from ordinary lines. Compared with ordinary lines, the seamless line eliminates a large number of rail joints in the factory, so it has the advantages of smooth running, low track maintenance costs, and long service life. It has become the main method of high-speed railway line construction at present. The seamless line is an important new technology of railway track. The line formed by welding ordinary steel rails into long rails of a certain length, welding and laying long rails with a certain length is called a seamless line. Rail welding is an important part of laying seamless lines.
At present, the welding methods of seamless line rail joints mainly include rail contact welding, gas pressure welding and aluminothermic welding:
01 Contact welding method and process
Rail contact welding (flash welding) is generally used in factory welding. 95% of the seamless line is completed by this process, that is, the standard rail with a length of 25 meters and no holes is welded into a long rail of 200-500 meters.
The principle is to use electric current to generate heat through the contact surface of the rail to melt the partial end face of the rail, and then complete the welding by upsetting. Since the welding heat source of contact welding comes from the internal heat source of the workpiece, the heat is concentrated, the heating time is short, the welding process does not require filler metal, the metallurgical process is relatively simple, the heat affected zone is small, and it is easy to obtain a better quality welded joint.
The welding process adopted by the rail welding factory is basically the same, including: rail matching, flaw detection, repairing the end face of the rail, entering the station to be welded, welding, rough grinding, fine grinding, straightening, normalizing, flaw detection, entering the rail platform, installing The welding process is the most critical among all the processes. The quality of the welding is directly related to the workload of line maintenance. If there is a problem, it will endanger the driving safety in serious cases. Compared with other rail welding methods, Flash welding has a high degree of automation and is less affected by human factors. The welding equipment is equipped with computer control, with small fluctuations in welding quality and high welding productivity. Under normal circumstances, compared with gas pressure welding and thermite welding, the strength of the contact welding seam of the rail is higher, and the breakage rate on the line is about 0.5/10000 or less. However, compared with the base material, its strength is still lower than that of the base material for the following reasons:
(1) The rail is a large-section bar material, and its core material is poor, with low melting point inclusions, loose, and coarse grains. During the welding and upsetting process, the edge material is extruded, and the core material is It is replaced by outward expansion, and the fibrous tissue is interrupted and bent, and the greater the amount of upsetting, the more obvious this situation is.
(2) Due to the thermal influence of welding high temperature, the grains are coarse in the 1-2mm area around the weld, and the grains are reduced to 1-2 grades
(3) The cross section of the rail is uneven, the top and bottom of the rail are compact sections, and the two corners of the bottom of the rail are expanded sections. The temperature of the two corners of the bottom of the rail is low during welding. temperature stress
(4) There are defects that are difficult to eliminate on the weld — gray spots.
02 Gas pressure welding welding method and process
At present, the widely used gas pressure welding of rails is a small mobile gas pressure welding machine, which is mainly used for welding joint joints of long rails on site, and can also use the closed skylight for welding of damaged rails.
The principle is to heat the welded end face of the rail to a plastic state, and generate an upsetting amount under the action of a fixed upsetting force. When the upsetting amount reaches a certain amount, the rail is welded into a whole.
The current small air pressure welding machines are basically domestic welding, and the welding process is generally divided into stages such as oxy-acetylene flame preheating, pre-pressurization, low-pressure upsetting, high-pressure upsetting, and pressure-holding and pushing. It is necessary to manually align the rails and observe the heating conditions with the naked eye, so it is greatly affected by human factors, and it is prone to weld joint errors and joint defects.
But because it has the characteristics of simple equipment, small size and light weight, it is easy to move online, offline and on the construction site, and the operation is relatively simple, so it is widely used for welding long rails on the construction site.
03 Thermite welding method and process
Thermite welding is generally used in the on-site welding of railway rails, and is an indispensable method for line laying, especially for seamless line locking and repair of broken rails. The aluminothermic welding of rails is based on the strong chemical affinity between aluminum in the flux and oxygen under high temperature conditions. It reduces heavy metals and releases heat at the same time, melting the metals into molten iron for casting and welding.
The important process is to put the prepared thermite flux into a special crucible, ignite the flux with a high-temperature match, produce a strong chemical reaction, and obtain high-temperature molten steel and slag. After the reaction calms down, pour the high-temperature molten steel into the Fasten the rails in the preheated sand mold, melt the ends of the butted rails in the sand mold, remove the sand mold after cooling, and reshape the welded joints in time, and the two sections of rails are welded into one. Although the aluminothermic welding equipment has the characteristics of low investment, simple welding operation, and good smoothness of the joint, the weld seam is a relatively thick cast structure with poor toughness and plasticity. It is best to perform post-weld heat treatment to improve the performance of the welded joint. .
In short, the welding quality of long rails should be better with contact welding and gas pressure welding. The ultimate strength, yield strength and fatigue strength of contact welding and gas pressure welding can reach more than 90% of the base metal. The quality of aluminothermic welding is slightly worse, its ultimate strength is only about 70% of the base metal, the fatigue strength is even worse, only reaching 45% to 70% of the base metal, and the yield strength is slightly better, which is close to that of contact welding.
Post time: Mar-01-2023
