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Welding cracks are one of the most common serious defects in welded joints. The weldability of metals involves two main types of issues: one is the deterioration in the material properties caused by welding, which results in the welded joint losing its original characteristics; for example, stainless steel loses its corrosion resistance after welding ； Another type involves defects such as cracks and pores arising in the base metal of the welded joint or in its vicinity. Cracks affect the safe use of welded components 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. Welding cracks can be classified in different 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 layered tearing. Thermal cracks generally occur at high temperatures near the solidus line, and they are characterized by their distribution along grain boundaries (see interfaces) ； But sometimes it can also form along a “polymerized boundary” at temperatures below the solidus line. Thermal cracks usually occur within the weld metal, but they can also form in the base metal adjacent to the weld joint. Based on the characteristics of their formation process, they can be further divided into the following three cases. Crystalline cracks occur in the \"brittle temperature\" range toward the end of the crystallization process of the weld metal; at this stage, there is a thin layer of liquid phase between the grains, resulting in extremely low plasticity of the metal. When the tensile deformation caused by uneven cooling contraction exceeds the allowable limit, cracking takes place along the liquid phase layer at the grain boundaries. The main metallurgical measures to eliminate crystalline cracks involve adjusting the composition, refining the grain structure, and strictly controlling impurity elements that can lead to the formation of low-melting eutectics, in order to improve the plasticity of the material in the brittle temperature range ； Furthermore, efforts should be made from the design and processing perspectives to minimize internal tensile deformation in this temperature range. Liquefaction cracks primarily occur in the base metal near the weld seam fusion line, and sometimes in the previously laid weld layers of multi-layer welding. The cause is local melting along the grain boundaries in the metal outside the weld fusion line due to the effect of welding heat, followed by cracking of the liquefied layer along those grain boundaries during subsequent cooling and contraction. There are two reasons for such cracks: one is the presence of a large amount of low-melting substances at the grain boundaries of the material ； Another reason is rapid heating, which causes certain metal compounds to decompose before they can diffuse, resulting in an enrichment of some alloying elements at local grain boundaries, sometimes even reaching eutectic compositions. The principle to prevent such cracks is to strictly control the impurity content, select welding materials appropriately, and minimize the effect of welding heat. Multilateral cracks are formed at temperatures below the solidus temperature. It is characterized by being distributed along a \"polyhedral boundary\" and having no clear relationship with the grain boundaries of primary crystallization ； It tends to occur in single-phase austenitic metals. This phenomenon can be explained by the high temperatures and uneven crystallization conditions during welding, which cause a large number of vacancies and dislocations to form within the crystal. Under certain temperatures and stresses, these defects arrange themselves into subgrain boundaries (polyhedralized grain boundaries). When such boundaries coincide with areas enriched in harmful impurities, microcracks often form. A way to eliminate this defect is to add alloying elements that can increase the polymerization activation energy, such as W, Mo, Ta, etc., to Ni-Cr alloys; on the other hand, it is to reduce overheating and welding stress during welding. Cold cracks can be classified into quenching cracks, hydrogen-induced delayed cracks, and deformation cracks based on their primary causes. Quenching cracks are cracks that occur near the martensitic transformation point of steel (as shown in the diagram of rapid austenite transformation) or at temperatures below 200°C. They mainly appear in medium and high-carbon steels, low-alloy high-strength steels, and titanium alloys, with the most common occurrence areas being in the heat-affected zone and within the weld metal. The crack direction is grain-boundary or transgranular. The main factors contributing to the formation of cold cracks are: ① a high hydrogen content in the metal; ② brittle microstructures or microstructures sensitive to hydrogen embrittlement ； ③Welding restraint stress (or strain). Hydrogen-induced delayed cracking: During welding, hydrogen dissolved in the weld metal diffuses and accumulates in the heat-affected zone, particularly concentrating in areas prone to crack initiation due to triaxial tensile stress, thereby causing hydrogen embrittlement. This reduces the critical stress of the metal at the crack initiation site (or the front end of the crack); when the local stress in these areas exceeds this critical value, cracking occurs. The formation of such cracks is characterized by a significant time delay, as hydrogen diffusion and enrichment require time (a incubation period). The conditions for the formation of such cracks are the presence of hydrogen and hydrogen-sensitive tissues, along with high constraint stresses. Therefore, it often occurs at the root and edges of welds with severe stress concentration, as well as in overheated areas. Preventive measures include: ① Reducing the hydrogen content in the weld, such as using low-hydrogen electrodes and thoroughly drying the welding materials ； ②Appropriate preheating and post-heating ； ③Choose raw materials with a lower carbon equivalent ； ④Reduce constraint stress and avoid stress concentration (see hydrogen in metals). Deformation cracks: The formation of such cracks is not necessarily due to high hydrogen content; they occur in multi-layer welds or fillet welds where strain concentration exists, as the tensile strain exceeds the metal’s capacity for plastic deformation. Reheat cracking occurs during the subsequent high-temperature heating after welding certain low-alloy high-strength steels, pearlitic heat-resistant steels, austenitic stainless steels, and nickel-based alloys. The main reason is generally believed to be that when reheated to 500–700 degrees after welding, within the overheated zone of the heat-affected area, secondary strengthening of the grains occurs due to the precipitation of specific carbides; trace elements that weaken the grain boundaries precipitate as well; and additional deformation that occurs as welding stresses are relieved concentrates at the grain boundaries, leading to intergranular cracking. Therefore, these cracks exhibit the characteristics of intergranular cracking, and all occur in the coarse-grained regions of the heat-affected zone where severe stress concentration exists. To prevent the occurrence of such cracks, first, materials with low sensitivity to reheat cracking should be selected during design; second, from a process perspective, internal stresses and stress concentrations in the area near the weld seam should be minimized as much as possible. Layered tearing occurs mainly during the corner welding of thick plates, and is characterized by a stepped progression parallel to the surface of the steel plate in the rolling direction. Such cracks are often not limited to the heat-affected zone; they can also appear in the base material far from the surface. The main reason for this is the layered distribution of non-metallic inclusions in the metal, which results in lower plasticity of the steel plate in the thickness direction compared to the rolling direction. Additionally, significant welding stresses are generated in the thickness direction during the corner welding of thick plates, leading to layered tearing. Flake-shaped sulfide inclusions are generally considered to be the most harmful, while layered silicates and excessively dense alumina inclusions also have an impact. To prevent such defects, it is essential to strictly control the quantity and distribution of inclusions during the metallurgical process. Additionally, improving joint design and welding processes also plays a certain role.