Causes and solutions of weld cracks and cold welding defects

Weld seam cracks mostly appear in the weld seam, showing a small hair shape, most of which occur in the superficial part of the weld seam, some can be seen with the naked eye, and some need to be carefully identified or even enlarged to find.

Reasons for these tiny cracks:

1. Reflow and slag inclusion caused by high soldering temperature.

2. The tube blank is thin and narrow, and the extrusion force is low. Some oxides are not extruded from the weld and form non-metallic inclusions after cooling.

3. There are non-penetrating lack of meat and edge micro-cracks on the edge of the formed tube.

4. The V-shaped butt joint of the weld, the superficial layer of the fusion structure is loose, and the cold shrinkage stress will tear the loose fusion structure.

5. The chemical composition of the edge of the tube is segregated, and the oxide layer is thick, resulting in high melting point non-metallic inclusions.

Solution: Remove it by appropriately increasing the opening angle, increasing the extrusion force, increasing the welding temperature, and improving the edge joint state.

Cold welding

The salient feature of cold welding is that there are obvious (serious) or inconspicuous (slight) grooves or dark lines on the weld. Most cold welding occurs on the outer wall of thick-walled pipes, and sometimes also on the inner wall. The principle of cold welding is The metal crystallization at the weld is only completed in a part of the thickness direction of the tube blank. Although the other part of the gap is also heated by the high-frequency current, due to some reasons, the metal crystallization is not completed or does not meet the conditions of the metal crystallization.

Cold welding is a serious quality defect. The reasons for cold welding of the outer wall of welded pipe are roughly as follows:

Low temperature welding

(2) Insufficient squeezing force

(3) The welding speed is too fast

(4) The magnetic bar was demagnetized, and the welding speed slowly decreased, and the operator did not notice in time.

The edge of the formed tube is deformed, and the upper edge of the squeeze roll hole is severely worn, resulting in sharp V-shaped butt joints on the edge of the tube.

If the coolant is improperly sprayed, it will be sprayed directly on the V-shaped loop of the heating tube billet

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

Solution: reduce the welding speed, increase the welding heat, detect the state of the magnetic rod (cooling and arrangement), avoid the direct spray of the coolant on the V-shaped loop of the heating tube, the solution is to reach the tube metal melting condition, (2) Increase the squeezing force, observe the burr shape, (5) strengthen the shape of the edge of the tube blank, and ensure that the I-shaped butt joint of the tube blank welding edge is realized.

Tip: The dark line at the welding seam position is often misjudged as the scratching trace left by the removal of the burr. One way of identification is to wipe it with sandpaper. The black line can still be seen after wiping off the surface. This is cold welding, and the second is to do side pressure. Flat experiment.


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