Why do welds crack?

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We are often amazed by the beauty of welds, but we must accept the reality that such beautiful welds can still fail inspection This is a headache for engineers: why are there cracks in the welds? How were they created? How can it be avoided? What are welding cracks? Welding cracks are one of the most common serious defects in welded parts. A gap that forms at a new interface, resulting from the breakdown of the bonding forces between metal atoms in local areas of the welded joint, under the combined effect of welding stress and other embrittlement factors. It is characterized by sharp notches and a high aspect ratio. Cracks affect the safe use of welded parts and represent a very dangerous process defect. Welding cracks occur not only during the welding process; some have a certain incubation period, while others arise during reheating after welding. What are the causes of welding cracks? Welds crack during welding due to factors such as stress, restraint forces, rigidity, chemical composition, gaps left in the weld, current, weld beads, and the cleanliness of the base material. All of these factors can cause cracks in the weld. Although there are many reasons for weld cracking, in different situations it is caused by multiple factors, or by two or three factors. But no matter how many factors there are, there must be one main factor. There are also cases where various conditions have no impact, and weld cracking is caused by only one factor. Therefore, when weld cracks occur, it is necessary to first accurately analyze the primary and secondary factors causing the cracking, and then take appropriate measures to address them based on those factors. The weld formed during the welding process is created when the electrode and the base material are melted by electric current at high temperatures; it results from the transformation of the electrode and base material from solid to liquid. The hot liquid expands, and as it cools, it contracts back to a solid state. Thermal expansion and contraction naturally cause stress in welded structures. The welding process involves changing from a solid state to a liquid state, that is, transforming from a solid form (usually molten iron) to a liquid state, and then from a liquid state back to a solid state, thus forming the weld. The liquid transforms into a solid state (that is, molten iron turns into crystals). The process by which molten iron turns into grains is the crystallization process. Crystallization begins at the areas where the base metal temperature is low, and gradually spreads toward the center of the weld, with the center of the weld being the last to crystallize. Due to the effect of thermal expansion and contraction, and under the influence of stress, restraint forces, or rigidity, the grain boundaries of the base material fail to connect with each other; in mild cases, small cracks appear in the weld area, while in severe cases, obvious cracks appear there. Even if the chemical compositions of the base material and the welding electrode are good, cracks or fissures can still occur due to the constraints imposed by the welded structure, its stiffness, and the stresses generated during the welding process. If the chemical composition of the base metal and the welding electrode is poor (with high levels of carbon, sulfur, phosphorus, etc.) ； Factors such as an excessive gap left in the weld, too many impurities at the edges of the base material, excessive current, as well as a welding speed that is too fast or too slow and a weld bead that is too wide, can all exacerbate weld cracking. Types of welding cracks and preventive measures: Welding cracks can be classified in various ways depending on their location, size, causes, and formation mechanisms. Based on the conditions under which cracks form, they can be classified into four categories: thermal cracks, cold cracks, reheat cracks, and delamination tears. Based on the weld cracking occurrences in welding projects, most cases are caused by stress, restraint forces, and rigidity. It can be said that stress, restraining forces, and rigidity are often the main factors causing weld cracking. An effective way to address weld cracking caused by stress, restraint forces, and rigidity is to employ fixed welding and distributed welding. The so-called fixed welding involves first fixing all the welds on the workpiece, or those in critical areas, by using a low current, narrow weld bead, and short welding distances. This prevents the welded joint from experiencing high stress. Even if the welded parts are secured in all areas, it is not allowed to move forward sequentially at the same position, nor is it permissible to use high currents or large-sized electrodes. Welding should be performed in a different position to prevent excessive heat from being generated in a specific area. Constrained and rigid structures can be solved using the same method. The so-called dispersed welding means that for large structures, welding must not be carried out in sequence at the same location; instead, the positions should be changed for welding. For large structures, welding must first be carried out in a fixed manner before moving on to scattered welding; moreover, high current levels and large-sized electrodes cannot be used for the first weld pass. For the overall large structure, all welds must be welded in a dispersed manner from start to finish; otherwise, although the welds will not crack, the residual stress will be too high.