Why do welds crack? Where do cracks appear? What’s the reason?

We are often amazed at the beauty of the weld, but have to accept a reality, such a beautiful weld would appear unqualified detection situation? This is a headache for engineers. Why are there cracks in the welds? How do they come about? How to avoid it? What is welding crack? Welding crack is one of the most common serious defects in welding parts. Under the joint action of welding stress and other brittleness factors, the new interface caused by the destruction of metal atom bonding force in the local area of the welded joint. It is characterized by sharp notches and a large aspect ratio. Crack is a very dangerous technological defect which affects the safe use of welding parts. Welding cracks not only occur in the welding process, some have a certain incubation period, and some occur in the reheating process after welding. What are the causes of welding cracks? Weld cracking during welding has the following reasons: Stress, binding force, rigidity, chemical composition, reserved seam clearance, current, weld pass, base metal cleanliness, etc. These factors are likely to cause weld cracking. Although there are many reasons for weld cracking, it is caused by a variety of factors on different occasions, and there are also two or three factors. But whatever the factors, there must be one major one. There are also a variety of conditions are not affected by only one factor caused by weld cracking. Therefore, the weld cracking must first correctly analyze the main and secondary factors of cracking, according to the main and secondary factors causing cracking to take corresponding measures to solve. The weld formed in the welding process is the welding seam of the electrode and the base metal after the current is melted at high temperature to form the weld, the electrode and the base metal from solid to liquid, high temperature liquid is thermal expansion, cooling into solid is contraction. Due to thermal expansion and cold contraction, the welding structure naturally produces stress. Some welded structures are inherently constrained and rigid. The welding process is from solid to liquid, that is, from solid to liquid (usually molten iron), and then from liquid to solid, also form a weld. The liquid changes into a solid, ie the molten iron changes into grains. The process of molten iron becoming grains is the process of crystallization. The base metal starts to crystallize at the low temperature position, gradually extends to the middle position of the weld, and finally crystallizes in the middle of the weld. Due to the effect of thermal expansion and cold contraction, the welding structure is affected by stress or binding force or rigidity, so that the base metal grains are not connected together, and the light ones appear small cracks in the middle of the weld, and the heavy ones appear obvious cracks in the middle of the weld. Even if the chemical composition of the base metal and electrode are good, cracks or cracks may occur due to the binding force of the welding structure, rigidity and stress caused by the welding process. If the base metal and electrode chemical composition is not good (carbon, sulfur, phosphorus and other high); Or the reserved gap of the weld is too large, the base metal is too much impurities in the weld edge, or the current is too large, and the welding speed is too fast, too slow, the weld pass is too wide and other factors will make the weld cracking more to aggravate. Types of welding cracks and preventive measures: According to its position, size, forming reason and mechanism, welding crack can be classified by different methods. According to the conditions of crack formation, it can be divided into four types: hot crack, cold crack, reheat crack and lamellar tear. According to the weld cracking situation in the welding project 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 of weld cracking. The effective way to solve the weld cracking caused by stress, binding force and rigidity is to adopt fixed welding and disperse welding. So-called fixed welding: first all welds, or important parts of the weld, the first small current, narrow weld pass, short-distance welding, all fixed. This makes the weldment less prone to great stress. Even if the weldment is fixed everywhere, it should not be in the same position and in the same order, let alone the use of large current and large size electrode. Should change the position of welding, do not make its local position to produce too much heat. Constrained and rigid structures can be solved in the same way. The so-called dispersed welding, which for large structures can not be in the same position in the order of welding, should be replaced by the position of welding. For large structures, not only fixed welding should be taken first, then dispersed welding, the first welding pass is not allowed to use large current and large size electrode. For the overall large structure, all welds must be dispersed from beginning to end, otherwise, although the welds do not crack, the residual stress is too large.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods