Welding Process and Joint Quality of National Standard Rails
- In which scenarios is flash butt welding applicable for national standard rails?
Flash butt welding is suitable for welding long rails in factories, which can achieve high-quality continuous welding with high strength and good smoothness of welded joints. In railway rail-laying bases, short rails are connected into 500-meter long rails by flash butt welding and then transported to the site for laying. This process has high welding efficiency and is suitable for mass production. The welded rail joints are polished to meet the smoothness requirements of high-speed train operation, and are often used in the laying of long rails for high-speed railways and trunk railways.
- What are the key points in the operation steps of thermit welding?
Thermit welding first requires preprocessing the end of the rail, grinding to remove rust and adjusting the rail gap to 15-20mm. Then fix the sand mold to ensure the gap between the sand mold and the rail is uniform to prevent molten steel leakage. Next, ignite the thermit welding flux in the crucible, and after the reaction is completed, pour the molten steel into the sand mold. After cooling, remove the sand mold and clean the welding bead. Finally, grind and heat treat the welded joint to ensure that the geometric dimensions and mechanical properties of the joint meet the standards. The key is to control the pouring temperature and cooling rate of molten steel to avoid defects such as pores and slag inclusion.
- What is the strength testing method for welded joints?
The strength testing of welded joints usually adopts a tensile test. A rail sample containing the joint is taken, and a tensile force is applied on a tensile testing machine until the sample breaks. The maximum force at break is recorded and compared with the strength of the base metal. It is required that the joint strength is not less than 90% of the base metal. At the same time, a bending test is carried out. The sample is bent to a specified angle, and whether cracks appear at the joint is checked to evaluate its plasticity and toughness. In addition, hardness testing is also performed to detect the hardness distribution at the joint to ensure there are no areas with excessively high or low hardness.
- How to deal with residual stress in joints after welding?
After welding, natural cooling can be used to release part of the residual stress, but for important lines, artificial aging treatment is required. The joint is heated to 250-350°C, kept warm for 2-3 hours, and then slowly cooled to eliminate internal stress. Mechanical stretching can also be used to apply a certain tensile force to the welded rail, causing plastic deformation of the joint, thereby reducing residual stress. Improper handling of residual stress will cause cracks or deformation in the joint during use, affecting the service life of the track.
- What are the differences in service life of joints with different welding processes?
Flash butt welded joints have a longer service life due to stable welding quality and low residual stress, which can reach more than 20 years under normal maintenance. Thermit welded joints have a relatively shorter service life, generally 10-15 years, due to possible micro-defects during welding. In scenarios with large loads such as heavy-haul railways, flash butt welded joints are more durable, while thermit welding is often used for on-site emergency repairs due to its simple operation, but its joints require more frequent inspection and maintenance.