rail welding methods

Classification of Rail Welding Methods and Welding Defects

Rail welding refers to a rail connection method in which standard-length rails are welded into long rails of the required length using welding methods at the factory or on site, and then laid on seamless lines. The development of seamless line technology, which eliminates rail joints, can not only improve driving stability, reduce traction resistance, reduce maintenance and repair workload, but also greatly reduce rail joint damage, which is one of the effective measures for rational use of rails. At present, there are three main types of rail welding: contact welding, air pressure welding and thermite welding. Contact welding is divided into two types: factory welding and on-site welding. These rail welding methods account for different proportions of rail welds in seamless lines, with contact welding being the most common, followed by thermite welding and moving air pressure welding.

1. Contact welding

The first method is flash contact welding. This welding method is mainly used in various rail welding factories, because in-factory welding can ensure the welding quality of the rails. When welding in the factory, a 100m-long fixed-length rail is usually placed in a specific machine, and then heated according to the thermal effect principle of electric current. When the rail is heated to a plastic state, it is then extruded at an extremely fast speed. The advantage is that the strength of the rail welded by this method is equivalent to that of the base material, and the surface is smooth and easy to grind and shape. The disadvantage is that this method has certain limitations and the rails can only be welded in the factory.

According to the characteristics of the flash process, rail flash welding is divided into: continuous flash welding, preheated flash welding, and pulsating flash welding. The most widely used ones are continuous flash welding and pulsating flash welding.

2. Gas pressure welding

Pneumatic welding is divided into two types: fusion pneumatic welding and plastic pneumatic welding. Most of them use plastic gas pressure welding. During plastic pressure welding, the two clean end faces of the rail are closely bonded, the veneer is heated with a gas flame, and when the veneer and its vicinity are heated to a plastic state, the metal atoms have enough “activation energy” and sharply spread across the interface, that is, the bonding surface is overwrought to achieve the metal chain connection between the atoms, complete the re-recrystallization, and the two rails are welded together. When laying seamless lines, small moving air pressure welding is commonly used in the field to weld the long rail to the longer rail.

3. Thermite welding

The third welding method is thermic welding, with air pressure welding this welding method is also suitable for outdoor sites, the welding principle is the use of iron and aluminum REDOX reaction, in the oxidation process will release a lot of heat, the generated hot metal will weld the two steel rails together.

First, heat the joint width of the two rails (16±2) mm, and then ignite the mechanical mixture of aluminum and steel scraps (aluminum powder) in the colander with a special match, so that the instantaneous temperature reaches 1200~1300 ℃, and the mixture In a violent reaction, the molten iron sinks, and the alumina floats on top of the molten metal in the form of slag. After the end of the reaction, the cyanotic plug is opened, and the molten material is poured into the prefabricated sand mold through the funnel, and the entire section of the rail end with the preheating temperature of more than 900 ℃ is fused, and the internal welding between the two rails is completed.

Types of weld defects

1. The defects of contact welding seams are gray spots, cracks, burns, etc., of which gray spots can exist in any part of the rail, especially the bottom edge of the rail, cracks exist in the waist of the rail and the heat affected zone, and burns exist in the area of 130~133 mm from the weld.

2. The weld defects of thermite welding include slag inclusion, porosity, sand inclusion, shrinkage hole, porosity, incomplete welding and cracks, etc., among which slag inclusion and porosity can be produced in any part of the weld, sand inclusion mostly exists on both sides of the bottom of the rail, porosity mostly exists in the triangle area of the bottom of the rail, and cracks mostly occur between the weld and the base material.

3. The common defects of air pressure welding are light spot, over-burning, and incomplete welding, among which the light spot exists in the rail head or the bottom of the rail, the over-burning exists in both sides of the rail bottom and the protruding part of the joint of the rail head and the rail waist, and the incomplete welding exists in any part of the weld.

Among the above defects, most of the contact welding defects are gray spots. The main reason is that the silicon oxide formed by the silicon in the steel during the high-temperature flash is not ejected; overburning and incomplete penetration are more common in air pressure welding. In addition, there will also be a small amount of defects caused by slag inclusion and segregation in the material itself, and a decrease in strength and plasticity caused by the coarse grains of the rail itself, which directly affects the quality of the weld.