Why Do Welds Crack? Where Do the Cracks Appear? What Reason?

We are often amazed by the beauty of welds, but we have to accept the reality that such beautiful welds can fail to pass inspection? This is a headache for engineers. Why are there cracks in the welds? How are they produced? How to avoid it?

What are welding cracks?

Welding cracks are the most common serious defects in welded parts. Under the combined action of welding stress and other brittle factors, the bonding force of metal atoms in local areas of the welded joint is destroyed, resulting in the formation of a new interface gap.

It features sharp notches and a large aspect ratio. Cracks affect the safe use of welded parts 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.

What are the causes of welding cracks?

Welds crack during welding for the following reasons:

Stress, restraint force, rigidity, chemical composition, gap reserved for welding seam, current, weld bead, base metal cleanliness, 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 it is also caused by two or three factors. But regardless of several factors, there must be one main factor. There are also various conditions that have no effect, and only one factor causes weld cracking.

Therefore, when weld cracking occurs, we must first correctly analyze the main and secondary factors of cracking, and take corresponding measures to solve the problem based on the main and secondary factors causing cracking.

The weld formed during the welding process is the welding seam formed when the welding rod and the base metal are melted by current at high temperature. The welding rod and the base metal change from solid to liquid. The high-temperature liquid expands due to thermal expansion, and the cooling to solid causes shrinkage. Due to thermal expansion and contraction, stress will naturally occur in the welded structure.

Some welded structures inherently have restraint 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. The liquid state transforms into a solid state (that is, the molten iron transforms into crystal grains). The process of turning molten iron into crystal grains is the crystallization process.

The low-temperature portion of the base metal begins to crystallize first, gradually extending toward 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 force or rigidity, so that the base material grains are not connected together. In mild cases, small cracks will appear in the middle of the weld, and in severe cases, obvious cracks will appear in the middle of the weld. .

Even if the chemical composition of the base metal and the welding rod is good, cracks or cracks may occur due to the restraint force, rigidity of the welded structure and the stress generated by the welding process.

If the chemical composition of the base metal and welding rod is not good (carbon, sulfur, phosphorus, etc. are too high); or the reserved gap for the weld is too large, the base metal has too many impurities at the edge of the weld, or the current is too high, and the welding speed Factors such as too fast, too slow, and too wide a 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 and mechanism of formation. According to the conditions of crack formation, it can be divided into four categories: hot cracks, cold cracks, reheat cracks and lamellar tears.

According to the weld cracking situation at the welding engineering site, most of it is caused by stress, restraint force, and rigidity. It can be said that stress, restraint force and rigidity are often the main factors for weld cracking.

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

The so-called fixed welding: first fix all the welds of the weldment, or the welds of important parts, using small current, narrow weld bead, and short distance welding. This makes the weldment less likely to produce greater stress.

Even if the weldment is fixed everywhere, it cannot be moved forward sequentially at the same position, let alone high current and large-sized welding rods. The welding position should be changed to prevent excessive heat from being generated locally. Constrained and rigid structures can be solved in the same way.

The so-called dispersed welding, for large structures, must not be welded sequentially at the same position, and the positions should be changed for welding.

For large structures, not only fixed welding must be performed first, but then dispersed welding must be performed. High current and large-sized electrodes cannot be used in the first welding pass. For the overall large structure, all welds must be dispersed from beginning to end. Otherwise, although the welds will not crack, the residual stress will be too great.