Research On The Improvement Of Rail Thermit Welding Joint Performance

Alumino-thermit welding technology has the characters of filling metal welding, simple equipment, easy operation and smooth joints. So it is widely adopted in switches welding, positioned welding, stress relief, broken rail repairing and routine repair work. With the development of speed up and heavy haul transportation, the impact on the joints increased and the repairing time shortened accordingly. So alumino-thermit welding technology should have high joint performance and short operating time.Researches on joint performance show that the tensile strength of joint metal is the lowest in the rail web, higher in the rail foot, and the highest in the rail head. The distribution of tensile strength contributes to the micro porosity in the rail web and inclusions in the rail foot. In addition, the carrying capacities of joints mainly are affected by welding defects, welding collar shapes, as well as the welding residual stress. So the improvement of joint performance should firstly aimed on reducing or eliminating of welding defects that may exist in joints, optimizing solidification sequence of weld metal, optimizing the structure of weld collar.Researches on welding powder formula show that the temperature of liquid steel produced by thermit reaction is affected by the initial temperature of reactants, additives proportion, and FeO proportion in the iron oxide. If the initial reactants temperature increases 1℃, the liquid steel temperature increases 0.6℃. And If the additives proportion increases 1%, the liquid steel temperature decreases 25.7℃. In addition, the increase of FeO proportion in the iron oxide will lead to the decrease of liquid steel temperature. The reaction rate thermit reaction is affected by molten steel temperature, reactants particle size and particle size of additives. The higher the reaction temperature is, the faster the reaction rate is. If reactants particle size is appropriate, the reaction rate is the fastest. Larger or smaller reactants particle size can make the produced liquid substance penetrate the gap formed by welding powder fully, and results in the decrease of reaction rate. If the particle size of additives for 60 ~ 80 mesh, the additives can fill the reactants particle due to reduce the reaction rate and prolong the reaction time.Researches on sand mould show that the physical characteristics of sand mould are affected by raw materials and producing processes. Higher SiO2 mass fraction and lower mud mass fraction can improve the strength and refractoriness of sand mould. And higher amount of sodium silicate and compaction densities can improve the strength and refractoriness of sand mould as well. Drying temperature and holding time affect the strength of sand mould. The optimum drying process can be used for lower drying temperature with long time holding or higher drying temperature with short time holding. The concrete drying process should comprehensively consider the energy efficiency and time efficiency in the actual production situation. ZTK pouring system has good welding capabilities and joints performance, so it is recommended in thermit welding for large scale rails.Alumino-thermit welding powder with Ni and ZTK casting system are suitable for 75kg/mPG4 rail welding. The performance of welded joints as follows: the average tensile strength is 853MPa, static bending breaking load is 1759kN for rail head up and 1662kN for rail head down, physical fatigue is over two million cycles, and the hardness of weld metal is 334HB. So the joints are matched with 75kg/mPG4 hot-rolling rail steel.