Current Status and Analysis of Steel Rails after Heat Treatment
With the continuous development of high-speed and heavy-duty railways, the construction of seamless lines has become an important part of railway development, and the welding joints of the steel rail flat bar are related to the key aspects of seamless line quality. Therefore, at present, domestic and foreign railway construction units and related management departments have strictly controlled the quality of the rail welding process, and have also adopted corresponding post-weld treatment measures to ensure the quality of the on-line steel rail flat bar welded joints. Among them, an important means for improving the internal structure of the welded joint during post-weld heat treatment can eliminate the superheating zone near the fusion line, refine the grain, improve the weld strength and toughness. In the construction of railways in China, post-weld heat treatment of steel rail flat bar is an indispensable part. Flash welding and gas welding heads must be heat treated after welding, including normalizing and any process measures to increase or decrease the cooling rate of the welded joint. Generally, the fixed flash joints in the welding steel rail flat bar factory are mostly heated by medium frequency electric induction, and then subjected to normalizing treatment by means of forced air cooling. The on-site moving flash welding head and the air pressure welding head are limited by the construction environment and equipment conditions, and are often heated by a gas pressure welding heater flame swing method, and then normalized in a natural cooling manner during the construction period. With the advancement of technology, mobile medium frequency electric induction and heating equipment for on-site flash welding and air-welding head heat treatment have also been successfully developed, and satisfactory test results have been obtained.
The electric induction heating equipment used for post-weld heat treatment of steel rail flat bar is mostly medium frequency inverter induction heating equipment, including power supply, intermediate frequency power supply, water cooling system, compensation capacitor, intermediate frequency transformer, heater, air spray (spray) cooling device and data acquisition and Management system and other parts. The production process of steel rail flat bar heat treatment after welding with medium frequency electric induction heating method should be controlled from heating start temperature, maximum heating temperature, heating time, heating power, frequency, DC voltage, DC current, air injection starting temperature, air blowing end temperature, wind pressure, joint heat uniformity, and other parameters by. The steel rail flat bar material is mostly high carbon steel in the hypoeutectoid steel, and the normalizing heat treatment temperature is usually from 820 °C to 840 °C, and after a period of heat preservation, it is cooled to room temperature in air or forced flow of air. However, there is no heating and post-heating device for the base and on-site rail welding. Only the method of instantaneous overheating can promote the full transformation of the internal structure of the rail joint. Therefore, the maximum heating temperature of the rail head is required to be controlled between 850 °C and 950 °C. In addition, since the normalizing temperature is too high, the grain will continue to grow and the quality requirements of the relevant standards cannot be met. Therefore, the normalizing temperature should be lower than 500 °C.
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