Do you know the annealing heat treatment process of rails?

The material and mechanical properties of the rail mainly depend on the chemical composition, physical and mechanical properties, metallographic structure and heat treatment process of the rail. In order to meet the working requirements of the rail and ensure uniform wear along the entire length of the rail, the heat treatment processes used include: continuous quenching and tempering of the rail ends to give them a tempered troostite structure; underspeed quenching of the rails , to obtain a wear-resistant lamellar pearlite structure, with the purpose of improving hardness, wear resistance and corrosion resistance.

In order to obtain the required hardness and structure to meet the technical needs of service, according to the heat treatment characteristics of the rail, there are usually two types of heat treatment for the rail: offline and online heat treatment. The heat treatment for the rail includes QT heat treatment process, SQ heat treatment process and deformation heat treatment process etc.. Zhengzhou Gou's only introduces you to the first two processes.

(1) QT heat treatment process (quenching and ten tempering). It heats the rail to the austenitizing temperature of the steel, then sprays the cooling medium to rapidly cool the rail surface layer below the martensite transformation temperature, and then performs tempering treatment to obtain tempered martensite or soxhlet. The heat treatment process of the bulk structure, this is a traditional heat treatment process, which can increase the hardness and strength of the rail and improve the fatigue and wear resistance of the rail. The disadvantages are that the rail has a large curvature after quenching and needs to be straightened, and bainite collapse occurs on the rail surface. The process method is divided into medium frequency induction heating rail head quenching and ten tempering processes, and overall heating and overall quenching and tempering processes.

①Medium frequency induction hardening. Through medium frequency induction heating, the rail is heated to AC3+(50~70)℃, and then air-cooled to about 750℃. Compressed air is sprayed to cool the rail to about 500℃, and a self-tempering heat treatment process is performed. Its structure is tempered. Sorbite. This process has stable production, energy saving, no pollution to the environment, and flexible production methods. There are two types of intermediate frequency quenching inductors: pointed rail and continuous. The working principles of the two are the same. The intermediate frequency continuous quenching inductor includes the functions of the pointed rail inductor, so it is more widely used.

a. The medium-frequency continuous quenching inductor is used, which can not only quench the entire length of the rail, but also be suitable for quenching the point rail and rail end. The quenching layer is hat-shaped. The moving speed of medium-frequency induction quenching of rails should be determined according to specific dimensions, requirements, etc. The cooling medium after medium-frequency induction heating can be selected according to the material composition and technical requirements, including spray, air spray, air first and then water processes, etc. They have different cooling properties and therefore should be applied differently.

The heat treatment process of the rail is: medium frequency preheating - medium frequency heating - cooling (when the surface temperature is below 750°C, compressed air cooling is carried out) - 500°C natural aging or tempering. If air is used first and then water is used, the quenching speed is fast, the production efficiency is high, the process is stable, the distortion is small and the cost is low, so it is widely used. Several important process parameters are described below.

The preheating temperature of the rail is 550~650℃, the temperature of the rail waist and rail bottom is in the range of 400~500℃; the heating temperature on the rail tread should be 900~920℃, and the temperature on both sides and fillets is 1000~1020 ℃; wind pressure is 0.125~0.130MPa, water volume is 280~450L/h, cooling water temperature is less than 40℃; the distance between the sensor and the tread is 5~8mm.

b. The pointed rail medium frequency inductor is a continuous saddle-shaped inductor with low temperature difference, which uses 1000-2500Hz medium frequency current. Its characteristic is that the temperature difference between the rail angle and the tread is very small. By increasing the gap at the rounded corner, the ring effect and sharp corner effect are reduced. The gap is 10~12mm, and the gap between the rail angle and the tread is 2mm, so that the gap between the two places is 10~12mm. The temperature difference is reduced to 20~30℃, and the quenching method is continuous spray (mixed gas of water and oil) or air jet cooling. The intermediate frequency quenching uses a 10OkW, 1000Hz thyristor silicon intermediate frequency power supply. The moving speed of the rail for continuous quenching is 292~350mm/ min, the hardness at 9mm after quenching and the depth of the quenching layer meet the requirements.

②Integral heating and overall quenching process. Generally, gas is used to heat the rail as a whole. After reaching the austenitizing temperature, the rail is quenched in oil or warm water, and then tempered at 450 to 500°C to obtain a tempered troostite structure. . The heat treatment furnace used is a special gas heating equipment and is equipped with professional heating and cooling fixtures. This process has high output and uniform quenching hardness, which can improve the strength and toughness of the entire rail section.

(2) SQ heat treatment process (quenching to obtain pearlite structure). This process is to heat the rail to the austenitizing temperature, use a quenching medium to slowly cool it for quenching, and directly quench it into sorbite, that is, to obtain a fine pearlite structure. The characteristic of this process is underspeed quenching and no tempering treatment is required, so the manufacturing cost is low. There are currently three types of underspeed quenching processes.

①Use the waste heat of rolling to carry out under-speed quenching process. This process makes full use of the 800-900°C temperature of the rail after rolling, and sprays or compresses air quenching directly on a special cooling bed. It is a relatively advanced heat treatment process in the world.

②Medium frequency induction heating under-speed quenching process. It uses power frequency or medium frequency current to preheat the entire section of the rail, then uses medium frequency current to heat the rail head to the austenitizing temperature, and uses blowing compressed air to quench. The quenching speed is generally 1.O-1.3m/min. . This process directly obtains quenched sorbite, which is a fine lamellar pearlite structure. It can be seen that the method of medium frequency induction heating and slow cooling is used to obtain fine flake pearlite structure.

③ Gas heating overall underspeed quenching process. Using fuel such as coal gas or natural gas, the rail is preheated to 450-500°C, then rapidly heated to the austenitization temperature of the steel, and compressed air is blown to directly quench the rail into a troostite structure, that is, fine pearlite.

Rails must have sufficient strength, hardness and wear resistance, high fatigue strength and good impact toughness, as well as good atmospheric corrosion characteristics. Therefore, the heat treatment process of rails is very standardized. The heat treatment specifications of some rails were described earlier. If you encounter any problems during the operation, you can ask Zhengzhou Gou's for information. Zhengzhou Gou's is a manufacturer specializing in the manufacturing of induction heating equipment and has rich experience.