What are the causes of weld cracks in welded steel pipes

There are the following reasons for weld cracking during welding:

Stress, constraint, rigidity, chemical composition, gap reserved for weld, current, weld bead, cleanliness of parent material, etc. These factors may cause weld cracking.

Although there are many reasons for weld cracking, it is caused by multiple factors on different occasions, and there are also two or three factors. But no matter how many factors there are, there must be a main factor. Various conditions have no effect, and only one factor causes weld cracking.

Therefore, when weld cracking occurs, the main and secondary factors of cracking must be correctly analyzed first, and corresponding measures should be taken according to the main and secondary factors causing cracking to be solved.

The weld formed in the welding process is formed by the welding rod and the parent material being melted by the current at high temperature. The welding rod and the parent material change from solid to liquid. The high-temperature liquid is thermal expansion, and cooling to solid is contraction. Due to thermal expansion and contraction, stress is naturally generated in the welded structure.

Some welded structures themselves have constraints and rigidity.

The welding process is from solid to liquid, that is, from solid to liquid (usually molten iron), and then from liquid to solid, forming a weld. Liquid to solid (that is, molten iron to grains). The process of molten iron turning into grains is the crystallization process.

The position where the temperature of the parent material is low begins to crystallize first, gradually stretches to the middle of the weld, and finally crystallizes in the middle of the weld. Due to the effect of thermal expansion and contraction, the welded structure is affected by stress, restraint, or rigidity, so that the parent material grains cannot be connected. In mild cases, small cracks appear in the middle of the weld, and in severe cases, obvious cracks appear in the middle of the weld.

Even if the chemical composition of the parent material and the welding rod is good, cracks or cracks will appear due to the restraint, rigidity, and stress of the welding structure.

If the chemical composition of the parent material and the welding rod is not good (carbon, sulfur, phosphorus, etc. are too high); or the reserved gap of the weld is too large, the parent material has too many impurities at the edge of the weld, or the current is too large, and the welding speed is too fast, too slow, and the weld is too wide, the cracking of the weld will be more serious.