Why welds crack

Why welds crack
We are often amazed at the beauty of welds, but then we have to accept the reality that such beautiful welds can fail the test. This is a headache for engineers. How do they occur? How can they be avoided?

What are weld cracks?
Weld cracks are the most common type of serious defect in welded parts. It is a gap in a welded joint where the metal atomic bond is broken and a new interface is formed as a result of the combined effect of welding stress and other embrittlement-causing factors.

It is characterised by a sharp notch and a large aspect ratio. Cracking affects the safe use of welded parts and is a very dangerous process defect.

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


What are the causes of welding cracks?

Weld cracks in the weld can be caused by the following:

Stress, constraint, rigidity, chemical composition, gap reserved for the weld, current, welding channel, cleanliness of the base material, etc. All of these factors can be responsible for weld cracking.

Although there are many causes of weld cracking, on different occasions it is caused by several factors, or by two or three factors. However, regardless of several factors, there must be a major factor in them. There are also various conditions that have little effect and are only affected by one factor causing the weld to crack.

Therefore, weld cracking must first be correctly analysed out of the main and secondary factors of cracking, according to the main and secondary factors causing cracking to take appropriate measures to solve.

The welding process is the formation of welding seam is the welding rod and the base material both after the current high temperature melting to form a weld, is the welding rod and the base material from a solid into a liquid, high temperature liquid is thermal expansion, cooling into a solid is shrinkage. As a result of thermal expansion and contraction, the welded structure naturally generates stress.

Some welded structures are inherently constrained and rigid.

The welding process is the transformation from solid to liquid, i.e. from solid to liquid (usually referred to as ferrous water) and then from liquid to solid, which forms the weld. The transformation from liquid to solid (i.e. the transformation of the iron into grain). The process of turning the iron into grains is the crystallisation process.

The crystallisation starts at the low temperature of the base material and gradually extends towards the middle of the weld, with the middle of the weld being the last to crystallise. Due to the role of thermal expansion and contraction, the welded structure is affected by stress or constraint force or rigidity, so that the base material grains are not connected together, the lighter in the middle of the weld appears small cracks, the heavier in the middle of the weld appears obvious cracks.

Even if the chemical composition of the base material and electrode are good, by the constraint force of the welded structure, rigidity and the influence of the stresses generated by the welding process, cracks or cracks will appear.

If the chemical composition of the base material and electrode is not good (carbon, sulphur, phosphorus, etc. is high); or the weld gap is too large, the base material is too much impurities at the edge of the weld, or the current is too high, and the welding speed is too fast, too slow, the welding channel is too wide and other factors will make the cracking of the weld more aggravated.

Types of welding cracks and preventive measures.
Welding cracks can be classified in different ways according to their location, size, formation causes and mechanisms. According to the conditions of crack formation, it can be divided into four categories such as hot crack, cold crack, reheat crack and laminar tear.

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

The solution to stress, constraint force, rigidity caused by weld cracking is more effective way is: to take fixed welding, dispersion welding.

The so-called fixed welding: first all the welds of the weld, or important parts of the weld, first take a small current, narrow welding channel, short distance welding, all fixed. This makes the weld less likely to produce large stresses.

Even if the weld is fixed at all points, it must not be in the same position in sequence forward, let alone take a high current and use a large size welding rod. Welding should be done in a different position so that no excessive heat is generated in its local position. Constrained and rigid structures can be tackled in the same way.

The so-called dispersion welding, which for large structures must not be in the same position in the sequence of welding, should be switched position welding.

For large structures, not only must the weld be fixed before the dispersal weld, but the first pass must not be made with high current and large size welding rods. For large structures as a whole, all welds must be dispersed from start to finish, otherwise, although the weld does not crack, the residual stress is too large.
Source：Welding House
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