Causes of cracks and preventive measures

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Cracks can be classified into cold cracks, hot cracks, and reheat cracks depending on the temperature and time at which they occur. Based on the location where they form, they can be divided into longitudinal cracks, transverse cracks, root cracks, crater cracks, weld line cracks, and cracks in the heat-affected zone, as shown in Figure 1. Cracks are one of the most dangerous defects in welded structures; they not only lead to the rejection of products but may also cause serious production accidents. (1) Thermal cracks are welding cracks that occur during the welding process when the metal in the weld and heat-affected zone cools to a high temperature range near the solidus line. It is a dangerous welding defect that should not be allowed to exist. Based on the mechanism of formation, temperature range, and morphology, thermal cracks can be classified into crystalline cracks, high-temperature liquefaction cracks, and high-temperature low-plasticity cracks. The main cause of hot crack formation is that the low-melting-point eutectics and impurities in the molten metal lead to severe intragranular and intergranular segregation during crystallization; simultaneously, under the effect of welding stresses, these elements are pulled apart along the grain boundaries, resulting in hot cracks. Thermal cracking generally occurs frequently in austenitic stainless steels, nickel alloys, and aluminum alloys. When welding low-carbon steel, hot cracks generally do not occur easily; however, as the carbon content in the steel increases, the tendency for hot cracking also rises. Preventive measures: Strictly control the content of harmful impurities such as S and P in steel and welding materials to reduce susceptibility to hot cracks; adjust the chemical composition of the weld metal, modify the weld structure, refine the grain size, improve plasticity, and reduce or disperse the degree of segregation. Use basic welding electrodes to lower the impurity content in the weld and reduce segregation. Select appropriate welding process parameters, appropriately increase the weld formation factor, and employ multi-layer, multi-pass welding methods. When stopping the arc, use a backing plate identical to the base material, or extinguish the arc gradually while filling the crater, in order to avoid the formation of hot cracks at the crater site. (2) Cold cracks: Cracks that occur when a welded joint is cooled to lower temperatures (below the Ms temperature for steel) are known as cold cracks. Cold cracks can appear immediately after welding, or they may emerge after a period of time (several hours, days, or even longer); such cracks are also known as delayed cracks. It is a relatively common form of cold crack, carrying a greater degree of danger. The causes of cold cracking include the hardened tissue formed at the martensite transformation interface, as well as the welding residual stresses and hydrogen present in the weld seam. The three main measures to prevent cold cracking are: using alkaline, low-hydrogen welding electrodes, baking them strictly according to the instructions before use, removing oil and moisture from the workpieces prior to welding in order to reduce the hydrogen content in the weld seam; selecting appropriate welding process parameters and heat input levels to minimize the tendency for the weld seam to harden; carrying out hydrogen-removal treatments immediately after welding to allow hydrogen to escape from the welded joint. For steel materials with a high tendency to harden, preheating before welding and performing heat treatment promptly after welding is necessary to improve the structure and properties of the joint. Various process measures can also be employed to reduce welding stresses. (3) Reheat cracks are cracks that occur when the welded part is reheated within a certain temperature range after welding (as part of stress-relief heat treatment or other heating processes). The cause of these cracks is that they generally appear in low-alloy high-strength steels, pearlitic heat-resistant steels, and stainless steels containing alloying elements such as V, Cr, Mo, and B; they arise after undergoing one welding heat cycle and then being reheated to sensitive temperature ranges (between 550–650°C). This is due to the re-deposition of supersaturated solid solute carbides (mainly V, Mo, and Cr carbides) during the first heating process, which leads to intragranular strengthening. As a result, slip strain is concentrated at the original austenite grain boundaries; when the plastic strain capacity of these grain boundaries is insufficient to withstand the strain during the stress relaxation process, reheat cracks occur. Cracks mostly originate from the coarse-grained region in the weld heat-affected zone. Reheat cracks mostly occur in thick parts and areas with stress concentration; they can also appear during multi-layer welding. Preventive measures: While meeting the design requirements, select electrodes with low strength so that the strength of the weld is lower than that of the base material; this helps to reduce stress within the weld and prevents cracks from forming in the heat-affected zone. Minimize residual welding stresses and stress concentrations as much as possible. Control the amount of heat input during welding, choose appropriate heat treatment temperatures, and avoid temperatures within the sensitive zones whenever possible.