Determination of Rail Material and Heat Treatment Process

Determination of Rail Material and Heat Treatment Process

The steel rail square bar material usually has metal carbon content of 0.68% and 0.80%, which is a kind high-carbon steel in the sub-eutectoid steel. After heating the metal of this component, the crystal grows rapidly. When it’s heated to the welding temperature, it is basically in the first-order crystal particle size, belonging to the austenitic superheated structure. It is preserved under the condition of rapid cooling in the air after welding and extinguishing. The coarse-grained austenite structure is characterized by low plasticity, toughness and strength, hard and brittle, and cannot be used in the actual application. The coarse-grained austenite structure must be eliminated by heat treatment. For the hypoeutectoid steel, the most suitable treatment method is complete annealing. It not only refines the grains, uniformizes the structure, but also improves the strength, plasticity, toughness and hardness. It also eliminates the internal stress, but it takes long time and the necessary equipment. It is difficult to implement in the appearance field. With normalization, the pearlite structure can be also obtained, but the grain is finer and the dispersion is larger. In addition, the strength, toughness, impact resistance by normalization are higher than the microstructure obtained by annealing, and the process is simple, the time is short, the equipment is simple, and it is convenient for on-site construction.

The specific normalizing process is as follows. After the tumor is removed, the weld line is immediately normalized when it is cooled between 400℃ and 500 ° C. If it is prematurely normalized, the core temperature of the steel rail square bar is still high, and there will be tendency for the crystal grains to continue to grow. However, if the normalizing fire is too late and there will be possibility that the normalizing fire does not completely leave the coarse crystal nucleus, it will affect the performance of use and will not achieve the purpose of normalizing. Normalizing after the steel rail square bar welding with welding heater, the flame uses weak carbonized flame, which can reduce the combustion ratio or reduce the gas supply. This kind of flame is soft, the heating time is longer, and the organization is better. The gas-welded steel rail square bar joint has a normalizing temperature between 870℃ and 920°C or between 850℃ and 900°C. When the steel rail square bar temperature rises to this temperature, the flame is extinguished and allowed to cool naturally in the air. For steel rail square bar with high carbon content, we should use a lower heating temperature and use a higher temperature for the steel rail square bar with lower carbon content. It has been proved that the post-weld heat treatment of the steel rail square bar adopts the above two-stage normalizing process, the grain size is stable from grade 7 to grade 8, and various mechanical properties are also ideal.

The steel rail square bar post-weld heat treatment equipment is mainly composed of a heater, a gas flow control box and an automatic swaying machine. The components can be coordinated and organically connected to jointly realize the post-weld heat treatment process requirements of the steel rail square bar. The heater is a heating device for heat-treating the steel rail square bar. The flame is an oxy-acetylene flame. The structure is a split-type single-mixing chamber heater. The heater contains a pneumatic system and a waterway system. The cycle takes forced water cooling.