Welding Technology for National Standard Rails and Seamless Railway Construction
- What are the common welding methods for national standard rails?
Flash butt welding uses the resistance heat generated by the current passing through the end face of the rail to melt the end face metal, and then applies upsetting force to complete the welding. The joint quality is high and it is often used for welding long rails in the factory. The welding speed can reach 1 - 2 minutes per joint. Gas pressure welding heats the end face of the rail through a gas flame until it reaches a plastic state, and then applies upsetting force for welding. It is convenient for on - site operation and suitable for rail connection at the construction site, but it has high requirements for the welding environment. Thermite welding uses the high - temperature molten steel generated by the thermite reaction to fill the rail gap and solidify to achieve welding. The equipment is simple and can be used in complex field environments. However, the strength of the welded joint is slightly lower than the previous two.
- How does the welding process affect the performance of rails?
The heat - affected zone of welding will change the local structure and performance of the rail, such as grain growth and hardness change. The heat - affected zone of flash butt welding is narrow (about 1 - 3mm), which has little impact on the overall performance; the heat - affected zone of gas pressure welding is slightly wider (3 - 5mm). If not properly controlled, the toughness of the joint may decrease; the heat - affected zone of thermite welding is the widest (5 - 8mm), and due to the fast cooling speed, residual stress is likely to be generated, reducing the fatigue strength of the joint. Therefore, heat treatment is often required after welding to improve performance.
- How to detect the quality of national standard rail welding joints?
Visual inspection requires that the weld surface is smooth, without pores, cracks, slag inclusions and other defects, and the weld reinforcement meets the standard (0 - 0.5mm). Ultrasonic flaw detection can detect internal defects such as cracks and lack of penetration, and the flaw detection sensitivity should be able to detect defects with a length of ≥3mm. Tensile test is used to test the joint strength, and the tensile strength of the welded joint shall not be less than 95% of that of the base metal. In addition, hardness testing can also be carried out to check whether the hardness of the heat - affected zone is within a reasonable range (the deviation from the base metal hardness is ≤20HB).
- What are the precautions for rail welding in the laying of seamless tracks?
Before welding, the end face of the rail needs to be ground to remove impurities such as oxide scale and oil stains to ensure that the welding surface is clean with a roughness Ra≤12.5μm. The welding environment temperature should be strictly controlled, generally welding is carried out between - 5℃ and 40℃. Too low a temperature is likely to produce cold cracks, and too high a temperature will affect the joint structure performance. During the welding process, the alignment accuracy of the rails must be ensured, and the deviation of the rail top, the side of the rail head and the rail bottom shall not exceed ±0.5mm to ensure the welding quality and track smoothness.
- What suggestions are there for the selection of national standard rail welding processes for different railway lines?
High - speed railways have extremely high requirements for track smoothness. Flash butt welding is preferred for welding long rails in the factory, and a small amount of gas pressure welding is combined for on - site unit rail section welding to ensure stable welding quality. For ordinary trunk railways, flash butt welding can be used in the factory according to the actual situation, and gas pressure welding or thermite welding can be used at the construction site. Heavy - haul railways have high requirements for the strength of welding joints due to the large loads they bear. Flash butt welding can be used for both factory and construction site welding. If conditions are limited, improved thermite welding can be selected at the construction site, and quality inspection should be strengthened.