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With the development of industries such as steel, petrochemicals, shipbuilding, and power, welded structures are trending towards larger sizes, greater capacities, and higher parameters; some even operate in environments involving low temperatures, cryogenic conditions, and corrosive media. Therefore, the use of various low-alloy high-strength steels, medium and high-alloy steels, ultra-high-strength steels, as well as various alloy materials is becoming increasingly widespread. However, with the use of these steel grades and alloys, many new problems have arisen in welding production, among which welding cracks are a common and particularly serious issue. Cracks sometimes occur during the welding process, and at other times during placement or operation, as well as in the case of so-called delayed cracks. Since such cracks cannot be detected during manufacturing, their severity is even greater. There are various types of cracks that occur during the welding process. Based on current research, and classified by the nature of their formation, these cracks can be roughly divided into the following five categories: 1. Thermal cracks. Thermal cracks arise at high temperatures during welding, which is why they are called thermal cracks. Depending on the material of the metal being welded, the form, temperature range, and main causes of the resulting hot cracks also vary; therefore, hot cracks are classified into three categories: solidification cracks, liquefaction cracks, and polygonal cracks. 1. Crystal cracks: During the later stages of crystallization, the liquid film formed by the eutectic with low volume weakens the bonds between the grains, leading to cracking under tensile stress. It mainly occurs in the welds of carbon steels and low-alloy steels with relatively high levels of impurities (high contents of sulfur, phosphorus, iron, carbon, and silicon), as well as in the welds of single-phase austenitic steels, nickel-based alloys, and certain aluminum alloys. In some cases, crystalline cracks can also occur in the heat-affected zone. 2. High-temperature liquefaction cracks: Under the effect of the peak temperature in the welding thermal cycle, remelting occurs in the heat-affected zone and between layers in multi-pass welding; these cracks are generated under stress. It mainly occurs in the near-weld zone or between multiple weld layers of high-strength steels containing chromium and nickel, austenitic steels, and certain nickel-based alloys. When the levels of sulfur, phosphorus, silicon, and carbon in the base metal and welding wire are high, the tendency for liquefaction cracking increases significantly. 3. Multilateral cracks: At the solidified crystallization front, under the influence of high temperature and stress, lattice defects move and accumulate, forming secondary boundaries. These boundaries exhibit low plasticity at high temperatures, and cracks form under stress. Multilateral cracks mostly 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. II. Reheat cracks: Cracks that occur in the coarse-grained areas of the weld heat-affected zone in thick plate welded structures made of steels containing certain precipitation-hardening alloy elements, during stress-relief heat treatment or while operating at certain temperatures, are known as reheat cracks. 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. III. Cold cracks: Cold cracks are a relatively common type of crack that occurs during welding; they form when the metal is cooled to lower temperatures after welding. Cold cracks mainly occur in the weld heat-affected zones of low-alloy steel, medium-alloy steel, 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 involved, cold cracks can be classified into different categories. Generally, they can be divided into the following three types: 1. Delayed cracks. This is a common form of cold crack. Its main characteristic is that it does not appear immediately after welding; instead, there is a certain incubation period. These cracks arise under the combined effects of hardened microstructure, hydrogen, and restrained stresses, exhibiting delayed characteristics. 2. Quenching cracks: These cracks generally show no delay in appearance; they are detected immediately after welding. They sometimes occur on the weld itself, and at other times in the heat-affected zone. Mainly, there are cracks caused by hardened structures under the action of welding stress. 3. Low-plasticity embrittlement cracks: Cracks that occur in certain materials with low plasticity when cooled to low temperatures; these are caused by strains resulting from contraction forces exceeding the material’s inherent plastic reserve, or due to the material becoming brittle. Since it occurs at lower temperatures, it is also another form of cold crack, but without any delay phenomenon. IV. Layered tearing: During the manufacturing of large oil production platforms and thick-walled pressure vessels, stepped cracks parallel to the rolling direction sometimes occur, known as layered tearing. This is mainly due to the presence of layered inclusions within the steel plate (along the rolling direction); the stresses generated during welding, which are perpendicular to the rolling direction, cause stepped, layered tearing to occur at a distance from the heat-affected zone. V. Stress corrosion cracking: Delayed cracks that occur in certain welded structures (such as containers and pipes) under the combined action of corrosive media and stress. Factors affecting stress corrosion cracking include the material of the structure, the type of corrosive environment, the shape of the structure, the manufacturing and welding processes, the welding materials, as well as the degree of stress relief. Stress corrosion occurs during service.