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A crack is a gap that forms when the atomic bonds in a weld are disrupted, creating new interfaces. Cracks are the most dangerous defects in welded joints and are not allowed to exist. A crack is a type of area-based defect; its presence significantly reduces the load-bearing area. What’s more, sharp notches form at the ends of the cracks, leading to high stress concentration, which makes it easy for the cracks to propagate and cause failure. Cracks are extremely hazardous, especially cold cracks; due to their delayed onset and rapid brittle fracture behavior, the damage they cause is often catastrophic. Practical experience shows that the failure of welds in most electrical equipment is caused by cracks. Depending on the conditions and timing of their formation, cracks can be classified into thermal cracks, cold cracks, reheat cracks, layered tears, etc. Since welding joints contain significant residual stresses, stress corrosion cracks caused by these welding residues are sometimes also categorized as welding cracks. Based on the location of formation, they can be classified as longitudinal cracks, transverse cracks, weld line cracks, root cracks, crater cracks, heat-affected zone cracks, etc. Macroscopic morphology and distribution of welding cracks. 1) Thermal crack. Thermal cracks occur at high temperatures during welding, which is why they are called thermal cracks. Thermal cracking occurs at the end of the solidification of the weld metal (during the primary crystallization of the liquid metal), with the sensitive temperature range being in the high-temperature area near the solidus line. Its characteristic is cracking along the original austenite grain boundaries. The patterns, temperature ranges, and main causes of thermal cracking in different metal materials also vary. Based on their causes, thermal cracks are mainly classified into the following three types: ① Crystal cracks: Crystal cracks occur during the crystallization of the weld, near the solidus line, as the contraction of the solidifying metal results in a shortage of residual liquid metal that cannot fill in the gaps in time; under stress, cracking along the grain boundaries takes place, which is why they are called crystal cracks. In most cases, an oxidized color can be seen on the fracture surface of the weld, indicating that such cracks were formed at high temperatures. Crystalline cracks mainly occur in welds of carbon steel and low-alloy steel containing high levels of impurities (high sulfur, phosphorus, carbon, and silicon content), as well as in welds of single-phase austenitic steel, nickel-based alloys, and certain aluminum alloys. In some cases, crystalline cracks can also occur in the heat-affected zone. ② High-temperature liquefaction cracking: In the area near weld seams or between layers in multi-layer welding, under the influence of the peak temperature during the welding heat cycle, the metal to be welded, which contains a high amount of low-melting eutectics, melts again; cracks then occur along the austenite grain boundaries due to tensile stress. Liquefaction cracks mainly occur in the zone near weld seams or between multiple weld layers in high-strength steels containing chromium and nickel, austenitic steels, and certain nickel-based alloys. When the levels of sulfur, phosphorus, carbon, and silicon in the base metal and welding wire are high, the tendency for liquefaction cracking increases significantly. ③ Multilateralized cracks: During welding, in the high-temperature region just below the solidus line in the weld area or the area near the weld, due to the presence of numerous lattice defects (mainly dislocations and vacancies) in the newly solidified metal, as well as severe physical and chemical irregularities, under certain temperature and stress conditions, the migration and aggregation of these lattice defects result in the formation of secondary boundaries, namely the so-called multilateralized boundaries. Due to the accumulation of a large number of lattice defects at the boundaries, its structural properties are fragile; its strength and ductility at high temperatures are poor. Even under slight tensile stress, it will crack along the polygonalized boundaries, resulting in what are known as polygonal cracks. Multilateral cracks commonly occur in the welds or near-weld areas of pure metals or single-phase austenitic alloys, and they belong to the type of thermal cracks. (2) Cold cracks. Cold cracks refer to cracks that occur when the welded joint cools below the martensite transformation temperature Mₛ (200–300°C). They generally appear after a period of time following welding (several hours, days, or even longer), which is why they are also known as delayed cracks. Cold cracking is a common type of crack in welding processes; it occurs when the welded material is cooled to lower temperatures. In the case of low-alloy high-strength steels, it occurs around the martensite transformation temperature M₃, as a result of the combined effects of constraint stress, hardened microstructure, and hydrogen. Cold cracks mainly occur in the weld heat-affected zones of low-alloy steels, medium-alloy steels, medium-carbon and high-carbon steels. In some cases, such as when welding ultra-high-strength steels or certain titanium alloys, cold cracks also appear in the weld metal. Depending on the type of steel being welded and the structure, cold cracks can be divided into the following three categories: ① Delayed cracks: This is a common form of cold crack. Its main characteristic is that it does not appear immediately after welding; rather, there is a certain incubation period during which the crack develops, which is why it is called a delayed crack. The formation of such cracks is primarily determined by the hardening tendency of the steel grade, the stress condition of the welded joint, and the hydrogen content in the deposited metal. ② Hardening embrittlement cracks (also known as quenching cracks): In some steel grades with a high tendency to harden, cracking can occur even in the absence of hydrogen-induced effects, solely due to compressive stresses. Such cracks can occur when welding Ni-Cr-Mo steels with high carbon content, martensitic stainless steels, tool steels, and dissimilar steels. It is entirely caused by the brittleness resulting from the martensitic phase transformation during cooling, and it is generally considered to have little relation to hydrogen. This type of crack shows no delay; it can be detected immediately after welding, and it appears sometimes in the heat-affected zone and sometimes on the weld itself. Generally speaking, using a higher preheating temperature and high-toughness electrodes can basically prevent the occurrence of such cracks. ③ Low-plasticity embrittlement cracks: In some materials with low plasticity, when cooled to low temperatures, the strain induced by contraction forces exceeds the plasticity reserve of the material itself, or the material becomes brittle, resulting in cracks known as low-plasticity embrittlement cracks. Such cracks occur, for example, during the repair welding of cast iron, the surfacing welding of cemented carbide, and the welding of high-chromium alloys. Since it occurs at lower temperatures, it is also a type of cold crack, but without any delay phenomenon. (3) Reheat crack. Reheat cracks refer to cracks that occur when a welded joint is cooled and then reheated to a certain temperature. In steel with thick plate welding structures that contain certain precipitation-hardening alloying elements, cracks that occur in the coarse-grained areas of the weld heat-affected zone during stress-relief heat treatment or while operating at certain temperatures are known as reheat cracks. Since this type of crack is formed during the reheating process, it is called a reheat crack, also known as a stress-relief treatment crack, abbreviated as SR crack. Reheat cracks mostly occur in the coarse-grained areas of the weld heat-affected zones of low-alloy high-strength steels, pearlitic heat-resistant steels, austenitic stainless steels, and certain nickel-based alloys. The sensitive temperature for reheat cracking varies depending on the steel grade, ranging from about 550 to 650°C. This type of crack also exhibits characteristics of intergranular cracking, but it is fundamentally different from crystalline cracks. (4) Layered tearing. Layered tearing is a stepped crack that appears in thick components along the rolling direction of the steel plate. The main cause of layered tearing is the presence of stratified inclusions of varying degrees within the rolled steel (especially sulfide and oxide inclusions). When subjected to stresses perpendicular to the rolling direction during welding, this leads to the formation of stepped-shaped layered tears in the area near or slightly beyond the heat-affected zone, which then propagate along or through the grain boundaries. Layered tearing belongs to low-temperature cracking; in the case of low-alloy steels, the tearing occurs at temperatures not exceeding 400°C. However, its characteristics are quite different from those of cold cracks. Layered tearing tends to occur in T-joints, cross-joints, and corner joints of wall-thick structures; it is a type of failure that is difficult to repair and can even lead to catastrophic accidents. Using Z-direction steel with resistance to delamination is a common practice to prevent delamination in large structures. There are many factors that affect the formation of layered tears, such as the material of the steel plate, the distribution and type of inclusions, the hydrogen content in the weld joint, the type of joint and the stress conditions, as well as the welding procedures used. Furthermore, when other defects are present in the welded joint, such as microcracks, microporosities, undercutting, and lack of penetration, these notch effects can develop into layered tearing under stress.