Factors Affecting Welding Thermal Cycles

Welding thermal cycle refers to the process in which the temperature of a certain point on the edge of tube billet changes with time under the action of heat source generated by high frequency welding current. Welding thermal cycle reflects the thermal effect of welding heat source on weld and nearby metal, causing different changes in microstructure and properties. At the same time, it also provides reference for finding good welding process, improving weld microstructure and predicting weld stress through process means This is the way.

Factors affecting welding thermal cycle

1.Welding process and linear energy, under the premise of other conditions stable and input welding heat (power) unchanged, the welding speed is fast, heating time is short, heating width is narrow, cooling is fast; when the welding speed is slow, it is the opposite.

The line energy can comprehensively reflect the influence of welding current, welding voltage, welding speed, extrusion force, welded pipe type and operation quality on the weld. When the line energy increases, the heat affected zone becomes wider, the heating time becomes longer, the oxidation zone at the edge of tube blank becomes wider, and the cooling intensity increases, which is unfavorable to the weld quality. Similarly, when the welding line energy decreases, the heating range and heat affected zone become wider Narrow, shorten the heating time, also affect the weld quality.

2.Welded pipe specification, an important feature of high-frequency welding current is skin effect, which takes the tube blank as the loop. When the tube to be welded is large and thick, the longer loop and longer heat transfer path will consume energy, resulting in the heating of other parts of the tube, which leads to the increase of linear energy and affects the thermal cycle.

3.Induction coil and magnetic rod, induction coil and welded pipe are closely coupled, that is, the gap between induction coil and welded pipe is small, the turns match, the skin effect and proximity effect of tube blank edge induction are strong, the tube blank edge receives more high-frequency current, so as to increase the active power of line energy. The role of magnetic rod is to concentrate as much induction current as possible to the two edges of the billet to be welded, So as to increase the efficiency of welding energy. In this way, as long as the seemingly low but effective line energy is used to complete the welding, on the contrary, if the induction coil is large, the magnetic rod is small, and the permeability is low, more welding current will be lost in the pipe body, and less current will gather at the edge to be welded, which leads to the fact that although the energy of the characterization line is large, the energy actually used for welding is not much, thus affecting the thermal cycle of the pipe body .

4.Forming quality of formed tube blank. High quality formed tube blank can ensure the parallel butt joint of the edge to be welded, so as to achieve high quality welding with low linear energy. If the edge to be welded is V-shaped butt joint, the shape of internal and external opening angle will increase and decrease the welding heat, and the thermal cycle curve of welded pipe will also change.

5.The larger the opening angle, the weaker the proximity effect of high frequency current, the longer the time for the billet to reach the welding temperature, the wider the heating zone, and the higher the line energy; on the contrary, the smaller the opening angle, the stronger the proximity effect of current, the narrower the heating zone at the welding edge, and the less line energy required, which changes the thermal cycle of the welded pipe. In addition, the welding extrusion force, the chemical composition of the billet, and so on, all affect the line energy The actual effect and welding thermal cycle are affected.