Why do cracks occur when welded pipes are welded?

What is welded pipe welding crack?

Welding cracks are a common serious defect in welded pipes. Under the embrittlement factors such as welding stress, the bonding force of metal atoms in the local area of ​​the welded joint is damaged, resulting in gaps caused by new interfaces, which are characterized by sharp notches and large gaps. The length-to-width ratio of the crack affects the safe use of the welded pipe. It is a very dangerous process defect. Welding cracks have a certain incubation period, and some occur during the reheating process after welding.

Cause of cracks

Stress, restraint force, rigidity, chemical composition, current, weld bead, cleanliness of base metal, etc., these factors may lead to weld cracking of welded pipe;

The welding seam in the welding process is formed by the welding rod and the base metal being melted by the current at high temperature. The welding rod and base metal change from solid to liquid. Due to thermal expansion and contraction, the welding structure will naturally generate stress, and some welding structures are inherently binding. And rigidity, the welding process is from solid to liquid, and then from liquid to solid, thereby forming a weld, the liquid is transformed into a solid (that is, molten iron is transformed into grains), and 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. Small cracks, even if the chemical composition of the base metal and electrode is good, cracks or cracks can occur due to the restraint and rigidity of the welded structure.

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

Precaution

Most of the cracking of welded pipe welds is caused by stress, restraint force and rigidity, which are the main factors of weld cracking. The effective method is to adopt fixed welding and dispersion welding. Fixed welding is to fix the weld of the welded pipe or the weld of the important part with a small current, narrow weld bead, and short distance welding, so that the welded pipe is not easy to generate large stress.

Even if the welded pipe is fixed in various places, it is not allowed to move forward in sequence at the same position, and it is not allowed to use large current and large size welding rod. The welding position should be changed to avoid excessive heat generation in its local position, with restraint force and rigid structure can be solved in the same way

Dispersion welding cannot be sequentially welded at the same position for large structures. To change the welding position, large structures must be fixed and welded first, and then distributed welding. Large current and large-sized electrodes cannot be used. For large structures, the welded pipe weld needs to be remade Disperse welding to the end, otherwise the weld will not crack, but the residual stress will be too large.

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