High forging speed (150-200mm/s) is required for metal welding of large flanges with good thermal conductivity.  For the same metal, the temperature gradient of the interface area is large, because the cooling speed of the joint is fast, the forging speed also needs to be improved.  Forging pressure Fu is usually expressed as pressure per unit area, i.e. forging pressure.  The size of the top forging pressure should ensure that the liquid metal in the interface can be extruded, and produce a certain plastic deformation at the joint.  

 
Because large flange has good comprehensive performance, so it is widely used in chemical industry, construction, water supply, drainage, petroleum, light and heavy industry, refrigeration, sanitation, water heating, fire, electric power, aerospace, shipbuilding and other basic engineering.  
 
When large flange is welded, the top forging pressure is too small, then the deformation is insufficient and the joint strength decreases.  If the top forging pressure is too high, the deformation is too large and the grain bending is serious, which will reduce the impact toughness of the joint.  Due to the high flash speed will lead to an increase in temperature gradient, so the welding of good thermal conductivity of metal (copper, aluminum alloy), special steel flange needs a large top forging strength (150-400mpa).  

 
In addition to the above technological parameters, preheating temperature and preheating time should also be considered.  Preheating temperature according to the workpiece section and material properties, welding low carbon steel, generally not more than 700-900 degrees.