What are the causes of welding cracks?

Author: admin / 2022-12-02

Weld cracks are the most common serious defects in weldments. Under the joint action of welding stress and other brittle factors, the bonding force of metal atoms in the local area of the welded joint is destroyed and the gap formed by the new interface is formed.

It is characterized by a sharp notch and a large aspect ratio. Cracks affect the safe use of weldments and are a very dangerous process defect.

Welding cracks not only occur during the welding process, some have a certain incubation period, and some occur during the reheating process after welding.

Weld cracking during welding has the following reasons: stress, binding force, rigidity, chemical composition, gap reserved by weld, current, weld bead, cleanliness of base metal, etc. All these factors may cause weld cracking.

Although there are many reasons for weld cracking, they are caused by many factors in different occasions, and there are also two or three factors. But regardless of several factors, there must be a major factor. There are also various conditions that have no effect, and only one factor causes weld cracking.

Therefore, in the occurrence of weld cracking, the main and secondary factors of cracking must first be correctly analyzed, and corresponding measures should be taken to solve them according to the main and secondary factors causing cracking.

The weld seam formed in the welding process is that the welding rod and the base metal are melted by electric current at high temperature to form a weld seam. The welding rod and the base metal change from solid to liquid, high temperature liquid is thermal expansion, and cooling becomes solid is contraction. Due to thermal expansion and contraction, the welded structure is naturally stressed.

Some welded structures are inherently binding and rigid.

The welding process changes 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 into grains). The process of turning molten iron into grains is the crystallization process.

The position where the temperature of the base metal is low starts to crystallize first, gradually extends 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 or restraint or rigidity, so that the grains of the base metal are not connected together. In the lighter case, small cracks appear in the middle of the weld, and in the severe case, obvious cracks appear in the middle of the weld. .

Even if the chemical composition of the base metal and the welding consumables are good, cracks or cracks will appear due to the binding force and rigidity of the welding structure and the stress generated by the welding process.

If the chemical composition of the base metal and welding material is not good (high carbon, sulfur, phosphorus, etc.); Factors such as too fast, too slow, and too wide weld bead will aggravate the cracking of the weld.

Types of welding cracks and preventive measures:

Welding cracks can be classified in different ways according to their location, size, cause of formation and mechanism. According to the conditions of crack formation, it can be divided into four categories: hot crack, cold crack, reheat crack and lamellar tear.

According to the welding seam cracking situation at the welding site, most of them are caused by stress, binding force and rigidity. It can be said that stress, binding force, and rigidity are the main factors for weld cracking.

The more effective way to solve the cracking of weld seam caused by stress, restraint force and rigidity is to adopt fixed welding and scattered welding.

The so-called fixed welding: firstly fix all the welds of the weldment, or the welds of important parts, with small current, narrow bead, and short distance welding, and fix them all. In this way, the weldment is not easy to generate large stress.

Even if the weldment is fixed everywhere, it is not allowed to move forward sequentially at the same position, and it is not allowed to use high current and large-size welding rods. It should be welded in different positions so as not to generate too much heat in its local position. Binding and rigid structures can be addressed in the same way.

The so-called dispersed welding means that for large-scale structures, it is absolutely impossible to weld sequentially at the same position, and the position should be exchanged for welding.

For large structures, it is not only necessary to fix the welding first, and then adopt scattered welding, and the first welding pass cannot use high current and large-size Aluminum Welding Wire. For the overall large structure, all welds have to be welded separately from beginning to end, otherwise, although the welds do not crack, the residual stress is too large.

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