Reseach On Normalizing Technology About Strengthening-Toughening Of Rail Welding Joints

Railway is the basis of rail transport, nowadays, the world’s rail is developing rapidly in the direction of high-speed and heavy-load, it needs to build a lot of continuous welded rail (CWR) and across section-CWR. Rail welded joints become the weak link in the railway line, because of coarse grain, poor toughness and plasticity after welding. In Chinese standard of TB/T1632<Welding of rails>, it requires that, flash welded joints and gas pressure welded joints must be normalizing heat treatment after welding. To ensure the joints quality, research a method that is applicable to rail welded joints strengthening-toughening technology is important.In this paper, I used orthogonal test method, to carry out the research of normalizing process simulation, designed orthogonal test program about three factors and three levels. The three factors are normalizing gas flow, normalizing process time and cooling rate after normalizing, that influence the effect of normalizing. I studied the properties of hardness, impact, microstructure, grain size and Pearlite lamellar distance under the 9 sets of normalizing process parameters. Then it obtained the best normalizing process parameters.The results show that, the degree of three factors in effects on normalizing, the cooling rate is maximum, followed by normalizing gas flow, time is minimized of normalizing. The best normalizing process parameters is that, acetylene gas flow is 80% of welding acetylene gas flow (oxygen gas flow is calculated by C2H2/O2=1.09), normalizing process time is 230s, cooling rate after normalizing is 2.5℃/s. After the heat treatment of the best normalizing process parameters, the joints’ HJ/Hp=1.05, softened zone is the narrowest, the impact energy is 18J～20J, the degree of grain size is 8～9.5, the Pearlite lamellar distance is minimum, the effect of joints’strengthening-toughening is the most significant.Flash welded joints of U75VG, after the heat treatment of the best normalizing process parameters, the joints’HJ/Hp=1.05, the widest area of softened zone is 1.9mm, the degree of grain size is 9～10, that is higher than welding joints of 2 degree, the average impact energy is 24J, that is higher than the standard value (6.5J) of 3.69 times. With fine-tuning the best normalizing Process of gas pressure welded joints of U75 V, the joints’ HJ/Hp at the top and profile of rail joints is 1.05 and 0.9. Softened zone width less than 10mm, the degree of grain size is 7～9, that is higher than base metal of 1.3 degree, than welding joints of 1.5 degree, the average impact energy is 15J, that is higher than the standard value (6.5 J) of 2.45 times in the rail head.The best normalizing process parameters is applied in the rail type inspection of gas pressure welding rail vehicle, we carried out drop-weight test of 27 welded joints and 67 normalized joints. As a result, statistical analysis showed that:the hardness of Joints is equivalent to the base metal, the width of softened zone and the size of grain is reduced obviously, the impact toughness is increased Substantial, the strengthening-toughening and the quality of joints is improved obviously.