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In recent years, the use of low-alloy high-strength materials in special equipment has become increasingly common, which is related to the high temperature and pressure conditions encountered in boilers and pressure vessels. However, during the manufacturing process of such equipment, cracks are often detected in the welds after heat treatment, especially in materials such as 2.25Cr-1Mo and 13MoNiMoR, which has drawn the attention of manufacturers. 1. There are many types of cracks in welded joints. Crystal cracks: These occur when the weld pool solidifies; within the temperature range where both the liquid and solid phases coexist, due to crystallization segregation and contraction stress and strain, cracks form in the weld metal along the boundaries of the primary crystals. Such cracks occur only in the welds (including the crater). Liquefaction crack: During welding, under the influence of the peak temperature of the welding heat cycle, in the interlayer metal of multi-layer welds as well as in the metal in the area near the weld in the base material, cracking occurs along the austenite grain boundaries due to the remelting of the intergranular metal upon heating, under the effect of certain contraction stresses. This phenomenon is referred to as \"thermal tearing\" in some literature. High-temperature low-plasticity cracking: After liquid-phase crystallization is complete, the metal of the welded joint begins to cool down starting from the temperature at which the material regains its plasticity. In certain special materials, when cooled to a specific temperature range, the interaction between the strain rate and various metallurgical factors leads to a decrease in plasticity, resulting in cracking of the metal in the welded joint along the grain boundaries. It generally occurs in the heat-affected zone, which is located further from the weld line than the site of the liquefaction crack. Reheat crack: A crack that develops along the austenite grain boundaries under certain conditions, during service at a specific temperature, in weldments that have undergone heat treatment to remove residual stresses or that have not received any heat treatment at all after welding. In fact, reheat cracking is one of the main problems that need to be addressed regarding the weldability of low-alloy high-strength steels. Especially in the welds of thick plates made from such steels, which contain large amounts of carbide-forming elements such as Cr, Mo, V and can produce precipitated carbides, reheat cracking often occurs during the post-weld stress-relief heat treatment process. Addressing these defects is both time-consuming and labor-intensive, having a significant impact on production. Next, a brief analysis is provided on the formation mechanism of reheat cracks, as well as the preventive measures and inspection methods during the manufacturing process. 2. Mechanism of reheat cracks: Simply put, reheat cracks arise because the strength within the grains is very high while the strength of the grain boundaries is lower. During post-weld heat treatment, the deformation that occurs as a result of stress relaxation concentrates on the grain boundaries. Once the strain at these boundaries exceeds their strength limit, cracking along those boundaries takes place. (1) Internal causes of reheat cracking: During welding, the heat-affected zone near the weld seam is heated to around 1200°C. Especially in thick plates that are heated multiple times, the grain size becomes larger. During cooling, the precipitation of strong carbides occurs more slowly. In submerged arc welding as well, due to the high line energy, the grains in the weld seam are also large. During the subsequent SR treatment at 480–680°C, carbides such as V4C3, NbC, and MoC precipitate dispersedly within the grains, thereby strengthening those grains (improving their thermal strength). This results in strain concentration at the grain boundaries during stress relaxation during heat treatment. The larger grain size reduces the number of grain boundaries capable of bearing stress, which in turn increases the strain per unit grain boundary. Additionally, during post-weld SR treatment, low-melting-point impurities and trace elements such as B, Sb, Sn, and As accumulate at the grain boundaries, reducing their plasticity. When the strain exceeds the plastic limit of these grain boundaries, cracking occurs. (2) External factors contributing to the formation of reheat cracks. The internal factors behind reheat cracks were briefly described above, but for such cracks to occur, external factors are also necessary. These external factors should be considered in terms of welding residual stress and expansion stress. During post-weld stress-relief heat treatment, the welding residual stresses are reduced through relaxation and creep deformation; cracks occur when the deformation of the material is unable to meet such requirements. In the welded area, the presence of low-melting-point compounds, segregation, and coarse-grain embrittlement zones, coupled with insufficient grain boundary strength and toughness, prevents it from resisting creep expansion deformation, leading to crack failure. Creep deformation is essentially a process of thermal expansion; during this process, expansion tensile stresses are generated to counteract some of the compressive stresses produced during welding. When cooling and contracting, contraction forces are created to offset some of the tensile stresses arising from welding, thereby reducing the peak stress levels. Therefore, in stress concentration areas such as microdefects, pores, and slag in the welding zone, when the expansion force combines with the stresses in these areas, it results in peak tensile stresses. When these peak values exceed the strength of the material, the equilibrium that previously maintained the material’s integrity is disrupted, leading to the formation of cracks. The stress distribution in these stress-concentration areas is very complex; it varies depending on the thickness and location, as well as whether there are any restraints such as nozzles in the surrounding area. For example, when such defects are located in the lower part during V-groove welding, they are subject to tensile stress; when they are in the upper part, they are subject to compressive stress. This is also why many reheat cracks occur at the root of the weld area. Due to the welding of dissimilar steels, the microstructure of the composite surfacing transition layer is very complex, and as it is in a region under tensile stress, it also has a high tendency to develop reheat cracks. Preventive measures: Based on an analysis of the mechanisms underlying the formation of reheat cracks, the preventive measures include the following aspects: Strict control of raw materials – When purchasing raw materials, elements in steel such as Cr, Mo, V, Nb, Ti, and B, which are strong carbide-forming elements, have a significant impact on the formation of reheat cracks; therefore, their levels need to be strictly controlled. Similarly, the contents of S and P, which can form sulfur-phosphorus eutectics, also need to be under control. The same requirements apply when purchasing welding materials. Such measures are among the most effective ways to address the internal factors that lead to reheat cracks. Select welding materials with low thermal crack sensitivity (strictly control the contents of elements such as S, P, V, Nb, etc.), and use the lower limit for the strength of the weld metal. Develop reasonable welding specifications: ① Minimize the welding heat input as much as possible, and control the temperature between preheating layers. These two factors determine the cooling conditions of the weld metal, having a significant impact on the microstructure of the weld area. Generally speaking, using a low wire energy with multiple passes and layers, along with appropriately increasing the cooling rate in the weld area, is beneficial for improving the microstructure, enhancing impact toughness, and preventing the formation of heat cracks. However, an excessively low interlayer temperature is unfavorable for the escape of hydrogen and poses a risk of cold cracking; therefore, to control the cooling rate and achieve refined grains, emphasis should be placed on regulating the amount of heat input. ②Take appropriate preheating measures. By taking appropriate preheating measures, the hardness of the hardened layer can be reduced, toughness can be increased, and crack resistance can be enhanced. Control the welding process to reduce the number of minor defects: Strictly follow the welding specifications to minimize minor defects and reduce the amount of weld metal used; employing narrow-gap welding is also an effective measure to control reheat cracks. As discussed above, these minor defects, those that do not exceed the specified limits, act as stress concentration points; therefore, during the stress-relief process in heat treatment, stress accumulation occurs, leading to reheat cracks. Therefore, it is also necessary to control these defects. Controlling welding residual stresses: Under the action of thermal treatment creep expansion forces, and especially when the combined stresses are tensile, the stress concentration points in the weld, as well as the weak areas at the grain boundaries resulting from precipitation hardening caused by carbides, are unable to resist strain and thus crack. Therefore, before heat treatment, methods to reduce residual stress can also decrease the occurrence of reheat cracks. ①Semi-intermediate heat treatment is employed. ②The high-frequency ultrasonic shock method is employed. Both of these methods can effectively reduce welding residual stress. Post-weld heat treatment: During post-weld heat treatment, the rate of heating and cooling is controlled to allow for a slow and uniform expansion and contraction, thereby reducing the occurrence of reheat cracks. 3. Methods for inspecting, testing, and identifying defects: Surface inspection methods commonly used can only determine whether defects are present; to identify the actual cause of these defects, the following methods must be employed: Replicated metallography: This method is often used for non-destructive inspection on-site. When the workpiece is vibrating or has a narrow shape, the replica metallography method can be used. The resulting replicas can be stored for a long time, and can be observed, analyzed, and photographed under a microscope in the laboratory. Using a large-workpiece metallographic inspector in combination with the replica metallography method yields better results. 4. Preparation of surface specimens from the area to be inspected: For composite materials, acrylic sheets 1–2 mm thick can be used, or acetate or nitrate fiber films (AC paper) can also be employed. Organic solvents that can be used include chloroform, propane, ethyl acetate, etc. First, cut the film into small pieces of the desired size. During the operation, an appropriate amount of organic solvent is dropped onto the surface of the prepared sample, and then a plexiglass sheet or film is quickly placed over it. It is pressed gently with fingers or rubber to allow any bubbles between them to escape. Once it has dried completely, it can be removed for observation and photographing. To increase tissue contrast, the surface to be examined can be etched a bit more deeply, or an appropriate amount of dye can be added to the organic solvent. 5. Microscopic inspection using a macroscopic metallographic inspector. Microscopic inspection includes examination before etching and after etching: before etching, it is mainly necessary to check for cracks, non-metallic inclusions, and defects resulting from the sample preparation process; after etching, the microstructure of the sample is examined. During observation, the overall structure of the tissue is generally first viewed at 75–100x magnification using a microscope. When observing fine tissues, select an appropriate high magnification. 6. Microscopic inspection of pipes and components: a. Identify the types of non-metallic inclusions and microscopic cracks in the material; observe their morphology and distribution; measure their quantity and size. b Identify the composition of the microstructure of the specimen, as well as the morphology, distribution, and quantity of various structures. Evaluations are made on grain size, banded structure, non-metallic inclusions, Widmanstatten structure, spheroidized structure, decarburized layer, etc. c Identify tissue characteristics, determine the state of the heat treatment process, and provide a basis for formulating a new heat treatment process if necessary. d Determine whether there is a connection between the aforementioned defects and the cracks examined, etc. Since reheat cracks do not occur during the welding process but rather during heat treatment or operation, they are somewhat hidden, which makes accidents unpredictable and can lead to greater losses. Therefore, the possibility of reheat cracking must be taken into account in advance at all stages of special equipment design, manufacturing, inspection, etc.