Research On The Microstructures And Properties Of The Stationary Flashed Welded Joints Of U75V Rail

High-speed and heavy-haul are two major trends in world’s railway. It is necessary to construct the continuously welded rail track in order to achieve the high-speed and heavy-haul in railway. At present, eighty percent of the welds on the seamless line are completed by flash welding that has stable welding quality in China. In the operation of the railway, welded joints are the weakest link of the whole line, which easily occur abrasion, brittle fracture and collapse. So, it is very necessary to further study the failure mechanism, structure and properties of the welded joints. The research results have very important practical significance for optimizing the welding quality, improving train running safety and reducing maintenance cost.Tensile strength, impact toughness, hardness and microstructure of U75 V hot-rolled joints before heat treatment were investigated in this paper, so as to the joints that were normalized in air cooling and forced-air cooling. Through the comparison, there are the following conclusions:(1) The mechanical properties of the welded joints have fallen sharply before the heat treatment. The microstructure of welding joints were coarsened seriously and netted texture ferrite exist in grain boundary.(2) After normalizing treatment, welding microstructure and mechanical properties are improved significantly and the original heat affected zone disappears. Meanwhile, the new heat affected zones formed were got wider and softening band of hardness also appeared about 46mm away from the weld.(3) Instead of improving, it occurred deterioration about the hardness distribution of joint parts due to the slow cooling rate of normalizing air cooling.(4) Comparing with the way of air cooling, the treatment of forced-air cooling can match the hardness distribution of joint parts with parent metal better. The global hardness meet the request of the technique standard provision except a small amount of soft spots. In addition, strength and toughness of joints are more close to or even better than parent metal. The welded joints by the treatment of forced-air cooling showed well comprehensive mechanical properties.