Welding problems of dissimilar steels and solutions

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I. Definition of welding of dissimilar steels Generally, welding of dissimilar steels refers to the process of joining steel materials of different types through welding, causing them to melt together. Common examples of such welding include welding of martensitic steel with pearlitic steel, welding of non-austenitic steel with austenitic steel, and welding of bainitic steel with pearlitic steel. II. Characteristics of joints formed by welding dissimilar steels In terms of the essence of welding, there are differences between joints formed by welding dissimilar steels and those formed by welding similar steels. The reason for this phenomenon lies in the uneven distribution of the welding heat-affected zones, as well as the heat-affected zones of the deposited metal and the base material on both sides. First, the chemical composition is not uniform enough. During the welding of steel, the amount of base metal that melts on both sides during heating, as well as the various components of the melted base metal and the deposited metal, are affected by dilution, resulting in varying degrees of variation and leading to severe unevenness in the chemical composition. Second, the organizational composition is not uniform enough. All the tissues inside the joint suffer from unevenness due to the thermal cycles during welding. Furthermore, in extremely rare regional areas, complex organizational structures will also emerge. Third, the stress field distribution is not uniform enough. The thermal conductivity and coefficient of expansion at the welded joints of dissimilar steels vary due to factors such as compositional and microstructural changes. The plastic region also varies due to differences in the thermal expansion coefficients. Thermal stress also varies due to the influence of the thermal conductivity. When thermal stress and mechanical stress act together, stress peaks are formed at the joints of dissimilar steel welds, leading to fracture of the joints. III. Control of chemical composition and microstructure in welds of dissimilar steels: Since the base metal and the welding material are different when welding dissimilar steels, it is necessary to estimate the properties, microstructure, and chemical composition of the welded metal. There are 4 factors that affect the dilution ratio: First, the impact of preheating. If the preheating temperature is increased, the dilution rate will increase due to the increased penetration depth. If the preheating temperature drops, the dilution rate will decrease due to a reduced penetration depth. Therefore, when carrying out the treatment, it is necessary to control the preheating temperature properly and carry out the treatment at an appropriate level. Second, the influence of welding parameters. The greater the current flow, the higher the dilution rate; whereas when the welding speed decreases, the dilution rate becomes lower. The welding parameter values are affected by the amount of base material melted per unit area. Third, the type of weld joint. As the groove size increases, the dilution rate will decrease. As the groove size decreases, the variation in the dilution rate remains essentially stable. Fourth, welding method. IV. Issues related to carbon handling during welding of dissimilar steels: During the welding of dissimilar steels in power plant pipelines, welders face the problem of carbon migration. If appropriate measures are not taken, this will result in the formation of diffusion zones around the weld seam, as well as decarburized layers on the side corresponding to the pearlite structure. In addition, it also causes a carburized layer to form on the side of the adjacent austenitic weld. If the decarburized layer transforms from pearlite to ferrite, its hardness will decrease, resulting in softening; as a consequence, it becomes very easy for large-sized particles to form. Chromium carbides precipitate, hardening the carburized layer and significantly reducing its high-temperature strength. The brittleness of the weld seams in pipes made of dissimilar steels in thermal power plants increases accordingly. If carbon migration occurs around the weld seam during the welding of dissimilar steels, it indicates a high likelihood of low-stress creep fracture at the weld joint, resulting in the loss of its integrity. When the welding temperature for dissimilar steels does not exceed 650°C, it takes a longer time for carbides to precipitate, and as time passes, the amount of carbides that precipitate continues to increase. When the welding temperature of dissimilar steels exceeds 650°C, the precipitated carbides increase at first and then decrease over time. If temperature control is not accurate enough when welding dissimilar steels, it will inevitably affect the carbides, resulting in the loss of effectiveness of the welded joint. V. Optimizing welding techniques for dissimilar steels: The welding technique chosen for welding dissimilar steels is of critical importance; in addition to having a direct impact on the welding quality, it is also necessary to consider the effects on various post-welding treatments. Therefore, when welding dissimilar steels, the selection of welding techniques should be considered from the following perspectives. First, it is necessary to ensure that the methods used when welding dissimilar steels meet the quality requirements specified for welding, thereby minimizing the fusion ratio and effectively reducing the likelihood of cracks occurring. In addition, when optimizing the selection of welding methods, their economic viability and advancement also need to be taken into consideration. Second, when welding dissimilar steels, electrodes can be used. Welding dissimilar steels by shielded metal arc welding is mainly due to the relatively large variety of electrodes, which enable good adaptability during welding. When welding high-chromium martensitic steel and pearlitic steel, carbon dioxide can be used as the shielding gas. When welding high-alloy dissimilar steels, TIG welding can be used for the welding process. If welding dissimilar steel components is relatively simple, then diffusion welding or brazing can be used to accomplish this. VI. Optimizing the welding of dissimilar steels: First, it is necessary to select an appropriate welding method for welding dissimilar steels, improving the rationality and scientific basis of such methods in order to ensure welding quality and efficiency. Therefore, when choosing the welding method, it is necessary first to take into account the actual conditions of the project, appropriately reduce the fusion ratio, and prevent cracks from occurring. Secondly, it is necessary to fully consider the economic efficiency and advancement of welding technology. When determining the weld joint configuration, the dilution rate must be taken into account. For dissimilar steels with greater thicknesses, a U-shaped or X-shaped weld groove should be used during welding. When selecting the welding current, welding speed, and number of weld layers for dissimilar steel welding, the melting of the base material can be appropriately reduced. If there are two types of base materials being used during welding, and one of them is hardening steel, then preheating is necessary before welding; the heating temperature should be selected appropriately based on the welding material. The pre-treatment should rely on the type of steel to be welded, with the heating temperature selected appropriately. Conclusion: The use of dissimilar steels has become extremely common. During welding, various problems arise due to a number of factors. To ensure welding quality, it is necessary to continuously optimize processes and develop appropriate countermeasures in order to improve the quality of welds.