Causes and Solutions of Weld Cracks and Cold Welding Defects

The causes of these micro cracks are as follows

1. Reflow slag inclusion caused by high welding temperature.

2. The billet is thin and narrow, the extrusion force is low, some oxides are not extruded from the weld, and non-metallic inclusions are formed after cooling.

3. There are non penetrating defects and micro cracks on the edge of the tube blank.

4. In the V-joint of the weld, the fusion structure in the shallow layer is loose, and the cold shrinkage stress will crack the loose fusion structure.

5. The chemical composition of the edge of the tube is segregated, the oxide layer is thick, and the nonmetallic inclusions with high melting point are produced.

Solution: by increasing the opening angle, increasing the extrusion force, increasing the welding temperature, improving the edge butt joint state and other measures to remove.

Cold welding

The remarkable feature of cold welding is that there are obvious (serious) or not obvious (slight) grooves or dark lines on the weld. Most of the cold welding occurs on the outer wall of the thick wall pipe, sometimes also on the inner wall. The principle of cold welding is that the metal crystallization at the weld is only completed in the part of the zone in the thickness direction of the pipe billet. Although the other part of the interval is also heated by high frequency current, due to some reasons, the metal crystallization at the weld is only completed in the part of the zone in the thickness direction of the pipe billet, Cause the metal crystallization is not completed or does not meet the conditions of metal crystallization.

Cold welding is a serious quality defect. There are six reasons for cold welding of welded pipe outer wall

Low temperature welding

Insufficient extrusion force

The welding speed is too fast

The magnetic bar was degaussed, the welding speed decreased slowly, and the operator didn't notice it in time.

The sharp V-shaped butt joint appears on the edge of the tube blank due to the deformation of the edge of the tube blank and the serious wear of the upper edge of the extrusion roll hole

If the coolant is not sprayed properly, it will be sprayed directly on the V-shaped circuit of the heating tube blank

The reasons (1), (3), (4) and (6) are interrelated, and the key is insufficient welding heat.

The solution: reduce the welding speed, increase the welding heat, detect the state of the magnetic bar (cooling and arrangement), avoid the direct spraying of coolant to the V-shaped circuit of the heating tube blank, the key points are to reach the metal melting condition of the tube blank, (2) increase the extrusion force, observe the burr shape, (5) strengthen the shape of the tube blank edge, to ensure the realization of the I-type butt joint of the tube blank welding edge.

Tip: the dark line of weld position is often misjudged as the scratch line trace left by deburring. One identification method is to wipe it with sandpaper, and the black line can still be seen after wiping off the surface layer, which is cold welding. The other is to do the side flattening experiment