A brief discussion on thermal cracking, reheat cracking, cold cracking, and layered tearing

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By their nature, welding cracks can be classified into thermal cracks, reheat cracks, cold cracks, layered tearing, etc. Below, a detailed explanation is provided solely on the causes, characteristics, and prevention methods of various cracks. 1. Thermal cracks occur at high temperatures during welding, which is why they are called thermal cracks; their characteristic is cracking along the original austenite grain boundaries. Depending on the material of the metal being welded (such as low-alloy high-strength steel, stainless steel, cast iron, aluminum alloys, and certain special metals), the pattern, temperature range, and main causes of heat cracks also vary. Currently, thermal cracks are classified into three main categories: crystalline cracks, liquefaction cracks, and polyhedral cracks. 1) Crystalline cracks mainly occur in welds of carbon steel and low-alloy steel containing high levels of impurities (such as S, P, C, Si), as well as in welds of single-phase austenitic steel, nickel-based alloys, and certain aluminum alloys. This type of crack occurs during the crystallization of the weld seam, near the solidus line; due to the contraction of the solidifying metal, there is insufficient residual liquid metal to fill in the gaps, and cracking along the grain boundaries takes place under stress. The control measures are as follows: in terms of metallurgical factors, appropriately adjust the composition of the weld metal, narrow the range of the brittle temperature zone, and control the levels of harmful impurities such as sulfur, phosphorus, and carbon in the weld ； Refine the primary grains of the weld metal by appropriately adding elements such as Mo, V, Ti, and Nb ； In terms of processing, it can be prevented and controlled through preheating before welding, controlling wire energy, and reducing joint restraint. 2) The near-seam zone liquefaction crack is a microcrack that propagates along the austenite grain boundaries; it is very small in size and occurs in the near-seam area or between layers of the HAZ. Its formation is generally due to the fact that, during welding, the metal in the area near the weld seam or the metal between the weld layers causes the low-melting eutectic compositions at the austenite grain boundaries in these areas to melt again; under tensile stress, cracks form along the austenite grains, resulting in liquefied cracks. The preventive measures for this type of crack are essentially the same as those for crystalline cracks. Especially in metallurgy, it is highly effective to minimize the content of low-melting eutectic elements such as sulfur, phosphorus, silicon, and boron ； In terms of the processing technique, it is possible to reduce the line energy and decrease the concavity of the weld pool fusion line. 3) Multilateral cracks occur during the process of polygonization due to the low plasticity at high temperatures. Such cracks are not common, and preventive measures include adding elements to the weld such as Mo, W, Ti, etc., which increase the polyfacetation activation energy. 2. Reheat cracks usually occur in certain steel grades and superalloys that contain precipitation-hardening elements (including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-hardening superalloys, and certain austenitic stainless steels). No cracks are detected after welding in these materials, but cracks appear during the heat treatment process. Reheat cracks occur in the overheated, coarse-grained areas of the weld heat-affected zone, and they propagate along the grain boundaries of the austenite coarse grains in the fusion line. To prevent and control reheat cracking, fine-grained steel can be selected from the perspective of material choice. In terms of the welding process, a lower wire energy should be used, along with a higher preheating temperature and subsequent post-weld heat treatment measures; furthermore, welding materials with low mismatch should be selected to avoid stress concentration. 3. Cold cracks mainly occur in the weld heat-affected zones of high and medium carbon steels, as well as low and medium alloy steels; however, in some metals such as certain ultra-high strength steels, titanium, and titanium alloys, cold cracks can also appear within the weld itself. Generally, the hardening tendency of the steel grade, the hydrogen content and distribution in the welded joint, as well as the degree of restraint stress on the joint, are the three main factors that cause cold cracks during the welding of high-strength steels. The martensitic structure formed after welding, under the influence of hydrogen combined with tensile stress, leads to the formation of cold cracks. Its formation is generally through the grain or along the grain. Cold cracks are generally classified into weld toe cracks, cracks beneath the weld bead, and root cracks. Preventing and controlling cold cracking can be approached from three aspects: the chemical composition of the workpiece, the selection of welding materials, and process measures. Materials with a lower carbon equivalent should be preferred as much as possible ； Low-hydrogen electrodes should be used for welding materials, and the weld strength should be matched accordingly; for materials with a high tendency to cold cracking, austenitic welding materials can also be employed ； Reasonably controlling wire energy, preheating, and post-heating treatment are process measures to prevent cold cracking. In welding production, due to variations in the types of steel used and welding materials, as well as differences in structure type, stiffness, and specific construction conditions, various forms of cold cracks can occur. However, the main issue encountered in production is delayed cracking. Delayed cracks come in the following three forms: 1) Root crack – This type of crack originates at the junction between the base material and the weld, where there is significant stress concentration. The direction of the crack is often parallel to the weld bead, and it generally starts from the surface of the weld toe and extends deeper into the base metal. 2) Cracks under the weld bead – These cracks often occur in the welding heat-affected zone, which has a high tendency to harden and contains high levels of hydrogen. Under normal circumstances, the crack direction is parallel to the weld line. 3) Root cracks — This type of crack is a common form among delayed cracks, and it primarily occurs in situations with high hydrogen content and insufficient preheating temperature. This type of crack is similar to weld toe cracks, originating from the area of the weld root where stress concentration is greatest. Root cracks may occur in the coarse-grained region of the heat-affected zone, or they may appear in the weld metal. The hardening tendency of the steel grade, the hydrogen content in the welded joint and its distribution, as well as the constraint stress condition acting on the joint, are the three main factors that cause cold cracks during the welding of high-strength steels. Under certain conditions, these three factors are interrelated and mutually reinforcing. The hardening tendency of a steel grade is primarily determined by factors such as chemical composition, plate thickness, welding processes, and cooling conditions. During welding, the greater the hardening tendency of the steel grade, the more likely cracks will occur. Why does steel crack after being quenched? It can be summarized into the following two aspects. a: It forms a hard and brittle martensitic structure – martensite is a supersaturated solid solution of carbon in α-iron; carbon atoms exist as interstitial atoms within the lattice, causing the iron atoms to deviate from their equilibrium positions and leading to significant distortion of the lattice, which results in a hardened structure. Especially under welding conditions, the heating temperature in the near-weld zone is very high, causing severe growth of austenite grains; when cooled rapidly, these large austenite grains transform into large martensite grains. According to the strength theory of metals, martensite is a brittle and hard structure that requires less energy to fracture; therefore, when martensite is present in a welded joint, cracks tend to form and propagate easily. b: Hardening leads to the formation of more lattice defects – metals develop a large number of lattice defects under conditions of thermodynamic imbalance. These lattice defects are mainly vacancies and dislocations. As the thermal strain in the welding heat-affected zone increases, under conditions of stress and thermomechanical imbalance, vacancies and dislocations move and aggregate. When their concentration reaches a certain critical value, crack sources are formed. Under continued stress, expansion continues to occur, resulting in the formation of macroscopic cracks. Hydrogen is one of the key factors causing cold cracks in high-strength steel welding, and it exhibits a delayed characteristic; therefore, in many studies, the cracks induced by hydrogen are referred to as \"hydrogen-induced cracks\". Experimental studies have shown that the higher the hydrogen content in high-strength steel welded joints, the greater their susceptibility to cracking. Cracking begins to occur when the hydrogen content in a particular area reaches a certain critical value; this value is known as the critical hydrogen content for crack formation, denoted as cr. The CR value at which various steels develop cold cracking varies, and it is related to factors such as the steel’s chemical composition, hardness, preheating temperature, and cooling conditions. 1: During welding, moisture in the welding materials, rust and oil on the joint edges, as well as environmental humidity, are all factors that contribute to hydrogen accumulation in the weld. Under normal circumstances, the hydrogen content in the base material and welding wire is very low, but the moisture in the electrode coating and the humidity in the air cannot be ignored, as they constitute the main sources of hydrogen addition. 2: The ability of hydrogen to dissolve and diffuse in different metal structures varies; the solubility of hydrogen in austenite is much higher than that in ferrite. Therefore, during welding, when austenite transforms into ferrite, the solubility of hydrogen drops suddenly. Meanwhile, the diffusion rate of hydrogen behaves in the opposite way, increasing suddenly during the transformation from austenite to ferrite. During welding, under high temperatures, a large amount of hydrogen dissolves in the molten pool. During the subsequent cooling and solidification process, as the solubility of hydrogen drops sharply, it tries to escape. However, due to the rapid cooling, hydrogen does not have time to escape and remains in the weld metal as diffused hydrogen. 4. Layered tearing is a type of internal low-temperature cracking. It is limited to the base metal of thick plates or the heat-affected zone of welds, and occurs frequently in “L”, “T”, and “+” type joints. It is defined as a stepped cold crack that occurs in the base material of rolled thick steel plates, resulting from insufficient plasticity in the thickness direction to withstand the welding contraction strain in that direction. Generally, this is because during the rolling process of thick steel plates, some non-metallic inclusions within the steel are rolled into strip-shaped inclusions parallel to the rolling direction; these inclusions cause anisotropy in the mechanical properties of the steel plate. To prevent layered tearing, refined steel can be used as the material of choice, that is, steel plates with high Z-direction properties, or the design of the joints can be improved by avoiding single-sided welds or by not providing grooves on the side subjected to Z-directional stress. Layered tearing is different from cold cracking; its occurrence is not related to the strength grade of the steel, but is primarily associated with the amount and distribution of impurities in the steel. Layered tearing can occur in commonly rolled thick steel plates, such as low-carbon steel, low-alloy high-strength steel, and even aluminum alloy sheets. Based on the location where the layered tearing occurs, it can be broadly divided into three categories: The first category is layered tearing resulting from the initiation of cold cracks at the weld toe or weld root in the welding heat-affected zone. The second type is cracking along inclusions in the weld heat-affected zone, which is the most common type of layered tearing in engineering applications. The third type involves cracking along inclusions in the base material away from the heat-affected zone; this generally occurs in thick plate structures with numerous flaky inclusions of MnS. The morphology of layered tearing is closely related to the type, shape, and distribution of inclusions, as well as their location. When flaky MnS inclusions predominate in the rolling direction, the layered tearing exhibits distinct stepped patterns; when silicate inclusions are predominant, it takes on a linear shape, while when Al inclusions are dominant, it shows irregular stepped patterns. During the welding of thick plate structures, especially T-joints and corner joints, under conditions of rigid restraint, weld contraction generates significant tensile stresses and strains in the thickness direction of the base metal. When these strains exceed the plastic deformation capacity of the base metal, separation occurs between the inclusions and the metal matrix, resulting in microcracks. Under continued stress, the crack tips propagate along the plane where the inclusions are located, thus forming what is known as a \"platform\". There are many factors that affect layered tearing, the main ones of which are as follows: 1. The type, quantity, and distribution of non-metallic inclusions are the fundamental causes of layered tearing; they are the root reasons for the anisotropy and differences in mechanical properties of steel. 2: Z-direction restraint stress. Thick-walled welded structures are subjected to various Z-direction restraint stresses, residual stresses after welding, and loads during the welding process; these are the mechanical factors that cause layered tearing. 3: The effect of hydrogen – It is generally believed that in the vicinity of the heat-affected zone, hydrogen is an important factor contributing to the transformation of cold cracks into layered tears. Given that the effects of lamellar tearing are significant and its hazards are quite serious, it is necessary to assess the susceptibility of steel to lamellar tearing prior to construction. Common evaluation methods include the Z-direction tensile area reduction rate and the pin Z-direction critical stress method. To prevent delamination, the reduction in cross-sectional area should be no less than 15%; generally, a value of 15–20% is desirable. A value of 25% indicates excellent resistance to delamination. To prevent laminar tearing, measures should be taken primarily in the following areas: First, refine the steel by widely adopting pre-desulfurization of molten iron along with vacuum degassing; this allows for the production of ultra-low sulfur steel with a sulfur content of only 0.003–0.005%, and its shrinkage rate in the Z direction can reach 23–25%. Second, controlling the morphology of sulfide inclusions involves converting MnS into sulfides of other elements, making it difficult for them to elongate during hot rolling and thereby reducing anisotropy. The elements widely used as additives at present are calcium and rare earth elements. The steel subjected to the above treatments can be used to produce laminar tear-resistant steel plates with a Z-direction shrinkage rate of 50~70%. Thirdly, from the perspective of preventing layered tearing, design and construction methods should focus on avoiding Z-directional stress and stress concentration; specific measures are as follows: 1) Single-sided welds should be avoided as much as possible, as using double-sided welds can alleviate the stress conditions in the weld root area and help prevent stress concentration. 2) Use symmetric fillet welds with less welding volume in place of fully penetrated welds that require more welding, to avoid excessive stress. 3) A groove should be made on the side subjected to Z-directional stress. 4) For T-joints, a layer of low-strength welding material can be pre-welded on the cross plate to prevent cracks at the weld root and to reduce welding stresses as well. 5) To prevent lamellar tearing caused by cold cracking, measures to prevent cold cracking should be taken as much as possible, such as reducing hydrogen content, appropriately increasing preheating, and controlling the interlayer temperature.