Process Optimization On Automatic Narrow Gap Arc Rail Welding And Its Engineering Application In Low Atmospheric Pressure Environment

Seamless rail is the foundation of developing high-speed passenger train and heavy load freight, and welding is the key technologies of laying and maintainingseamless rail. Flash welding and gas-pressure welding(mobile welding) are automatic welding, thire technology and equipment are mature. The welding quality of rail joint is excellent and stable.The quality stability andmechanical properties in thermite welding( in-situ welding) joints are relatively low for its specific welding method. A t present, the performance of arc welding(in-situ welding) joints is good, but this type of welding mainly use forced forming manual electric arc welding technology which greatly affected by human factors, and the quality stability is poor. Therefore, the development of automatic narrow gap arc welding has an important significance and application value to improve the in-situ rail welding technology.According to the key technology that need to be addressed of current rail automaticarc welding, the droplet transfer form of self-shielded flux-cored wire for rail arc welding, the stability and parameter optimization of welding process, the effect of post-weld heat treatment, and the arc welding inlow pressure environment were studied intensively in this paper.First, welding electric information(welding current and arc voltage) and high-speed camera(droplet and the arc, etc.) synchronous acquisition and analysis system was built, which provides the necessary tools for in-depth research and analysis of arc characteristics of self-shielded flux cored wire, the wire melting and droplet transfer behaviorwith its corresponding welding electrical information characteristics.Second, the longer duration arc bridge transition and short-circuit transition easily causing an explosion spatter and fluctuation of welding current was found, which provides a basis for optimization and adjustment of process parameters. Therelationship between a typical droplet transfer behavior and the corresponding welding electrical information was gotten, and thewelding process control setting law of avoiding short-circuit transfer was found.Using the welding parameters after optimization could effectively reduce the rail joint welding defects, enhance the welding process repeatability, and improve rail joints static bending properties.Third, the effects of flame heat treatment on the rail arc welding jointmicrostructure and residual stresses were studied. Before heat treatment, welding zone is bainite and martensite, heat affected zone is coarse pearlite, ferrite and cementite structure, and there are two kinds of structre phase infusion zone with clear interface. There is a big residual stress in rail waist and rail jaw.After heat treatment, the heat-affected zone of coarse structure is refined, and the rail surface and weld rail waist part refinement were improved significantly, meanwhile the maximum residual stress decreased by 1/2 to 2/3. The hammer rail joint performance greatly improved after the heat treatment.Fourth, proposed that in low pressure conditions(positive polarity welding) cathode pressure drop increase is the primary cause of wire feeding speedand melting speed could maintain balancewhen welding current declines. In low pressure conditions, cathode temperature decreases, the electron thermal emission diminished, the electric field strength increases, the electron field emission capacity enhancement. Welding current falling and arc expansion result in arc stiffness decreasing and worse weld qaulity.By an appropriate increasing welding current(wire feed speed) to increases arc stiffness, and reducing the arc voltage to enhance the arc stability, at an altitude of 4447 m in Nagqu successfully realized arc welding rail joints and its hammer performance reach to flash welding standards.