Why do welds crack? Where do the cracks appear?

What is welding crack?

Welding crack is a common serious defect in welding parts. Under the brittle factors such as welding stress, the bonding force of metal atoms in the local area of the welding joint is damaged, resulting in the gap caused by the new interface. Its characteristics are sharp gap and large aspect ratio. The crack affects the safe use of the welding parts. It is a very dangerous process defect. The welding crack has a certain incubation period, Some are produced during reheating after welding.

There are the following reasons for weld cracking during welding:

Stress, restraint force, rigidity, chemical composition, current, weld bead, base metal cleanliness and other factors may lead to weld cracking. In case of weld cracking, the main and secondary factors should be analyzed, and corresponding measures should be taken to solve it.

In the welding process, the welding rod and base metal are melted by electric current at high temperature. The welding rod and base metal change from solid to liquid. Due to thermal expansion and cold contraction, the welding structure will naturally produce stress. Some welding structures are naturally binding and rigid. The welding process is from solid to liquid, and then from liquid to solid, thus forming the weld, Liquid into solid (that is, molten iron into grains), the process of molten iron into grains is the crystallization process.

The base metal begins to crystallize at low temperature, then gradually extends to the middle of the weld, and finally crystallizes in the middle of the weld. Due to thermal expansion and cold contraction, the weldment is affected by stress, restraint force and rigidity, and the base metal grains cannot be connected together, resulting in small cracks. Even if the chemical composition of the base metal and the electrode is good, cracks or cracks will appear due to the restraint force and rigidity of the welding structure.

If the chemical composition of base metal and electrode is not good, or the reserved gap of weld is too large, the impurity of base metal is too much, the current is too large, and the welding speed and other factors will aggravate the weld crack.

Preventive measures:

Most of the weld cracking is caused by stress, restraint force and rigidity, which is the main factor of weld cracking. The effective way is to adopt fixed welding and scattered welding.

Fixed welding is to fix the welding seam of the weldment or important parts with small current, narrow welding pass and short distance welding, so that the weldment is not easy to produce large stress.

Even if the weldment is fixed everywhere, it is not allowed to move forward in the same position in sequence. It is not allowed to use large current and large size welding rod. The welding position should be changed to avoid excessive heat in its local position. The same method can be adopted for binding force and rigid structure.

For large-scale structures, distributed welding can not be carried out in the same position in sequence. To change the welding position, large-scale structures should be fixed welding first, and then distributed welding. Large current and large-scale welding rod can not be used. For large-scale structures, the weld should be welded from beginning to end, otherwise the weld will not crack, but the residual stress will be too large.

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