Application of nickel-based alloys in welding dissimilar steels

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1. Main problems in the welding of dissimilar steels. In simple terms, welding of dissimilar steels refers to the process of fusing together steels made of different materials through welding. There are fundamental differences between welded joints of dissimilar steels and those of similar steels, mainly due to the unevenness in the deposited metal as well as in the welding heat-affected zones and base material on both sides. When welding dissimilar steels, since the welding material used is different from the base material, it is necessary to determine the composition, microstructure, and properties of the weld metal. Temperature field of heat conduction in the welding arc ； Microstructure and properties of welded joints ； Weldability tests and process qualification tests ； Equipment for efficient ship welding and the corresponding efficient welding techniques ； Welding processes for various metal materials ； The welding of metal structures in the petrochemical industry, large beams and columns, pressure vessel manufacturing, offshore engineering pipe structures, and ship hulls all falls under the category of welding dissimilar steels. The following discusses the problems that arise during the welding of dissimilar steels. 1.1 Weld metal dilution: When welding dissimilar steels, there is a difference in the composition of the weld alloy compared to the metal surrounding the weld. The amount of base material that melts depends on factors such as the thickness of the base material, the size and shape of the weld, arc voltage, welding position, and welding speed. When welding different types of steel, the degree of dilution between the melted base material and the deposited weld metal changes, resulting in variations in the chemical composition of the welded joint. This uneven chemical composition leads to changes in the metal structure at the weld site, thereby affecting the quality of the welded joint. By using nickel-based alloys as an isolation layer in welding processes, changes in the metal’s chemical composition can be prevented, and the temperature around the weld can be controlled to avoid dilution of the weld metal. 1.2 Residual stresses in joints: The differences in metal structure between different steel grades result in varying linear expansion coefficients. Additionally, there are differences in plasticity and thermal conductivity at the joint site, which lead to abnormal temperature patterns during welding. This results in residual stresses in the welded joint, affecting the stability of that area and thereby impacting the service life of the welded components. 1.3 Carbon migration: Mixing occurs between the filler metal and the molten base metal inside and at the edges of the weld pool, but this mixing varies to some extent. Areas of incomplete mixing appear at the weld boundaries; a decarburized layer forms on one side of the dissimilar steels, while an carburized layer forms on the other side, affecting the plasticity and durability of the welded structure. Increasing the nickel content in the weld area can effectively suppress the formation of carbides through graphitization, thereby preventing carbon migration and the formation of a diffusion layer. 2. Countermeasures for problems in the welding of dissimilar steels. Above, the common problems that occur during the welding of dissimilar steels were discussed. In the actual construction process, if problems arise with dissimilar steels during welding, it will affect the quality of the welding as a whole, and it will also have a significant impact on the overall quality of the construction. Therefore, it is very necessary to study and resolve these issues. The first issue to consider is the problem of weld metal dilution. Generally, when welding dissimilar steels, the composition of the weld alloy differs from that of the surrounding metal. Affected by various factors, the weld metal gets diluted during welding, which can lead to deformation. During the dilution process, the composition of the welded area also changes, resulting in a very uneven distribution of these components. This leads to changes in the microstructure of the metal at the weld site, affecting the overall quality of the weld. Therefore, by applying nickel-based alloys to welding processes as an isolation layer between the weld seam and the weld joint, the issue of changes in the metal’s chemical composition can be effectively resolved. Moreover, the temperature around the weld is controlled, thereby ensuring that the metal is not diluted. Therefore, the use of nickel-based alloys in the welding of dissimilar steels effectively prevents the effects resulting from the dilution of the weld metal and changes in its chemical composition. The issue of residual stress in welded joints also needs to be effectively addressed. Because during the welding of dissimilar steels, there are differences between the metals, which causes changes in their expansion coefficients during welding, resulting in variations. This leads to problems of thermal cycling anomalies during actual operation; there are differences in the thermal conductivity at the joint location compared to that of the metal itself, which results in residual stresses in the welded joint. This affects the stability of the weld during the welding process, and it also has an impact on the service life of the welded components. Therefore, removing residual stresses in the joints will effectively improve the service life and quality of the welded areas. Using nickel-based alloys as welding materials can help reduce the performance differences between the metal and the joints. Reduce the thermal conductivity in the joint area, thereby preventing issues during the heat cycles that occur during welding. It ensures the residual stress in the welded joint, while simultaneously improving the stability of the entire joint area as well as the quality and efficiency of the welding process. Therefore, in the welding of dissimilar steels, the use of nickel-based alloys can effectively address the issue of residual stress in the joint. During the welding of dissimilar steels, carbon migration is also a common issue. The main cause of carbon migration is melting under the influence of the metal inside and at the edges of the molten pool, resulting in mixing between the metal material and the liquid. This results in certain differences in carbon content in the dissimilar steel joint areas, leading to incomplete fusion at the weld boundaries. As a result, decarburized zones and carburized zones are formed, and these two areas will have an impact on the quality of the welded structure. Effective use of nickel-based alloys can reduce the occurrence of carbon migration, thereby suppressing the formation of carbides resulting from carbon migration. This prevents the emergence of decarburized and carburized layers after carbon migration, leading to an improvement in the overall structure and quality of the weld. 3. Welding process for dissimilar steels: Common austenitic stainless steels and low-alloy heat-resistant steels are used as dissimilar steels for welding purposes. Low-alloy heat-resistant steels contain relatively high levels of chromium and molybdenum; therefore, appropriate heat treatment is required after welding to stabilize the properties and microstructure of the joint and to suppress the tendency toward rapid cooling. However, heat treatment after welding can easily degrade the corrosion resistance of austenitic stainless steel, leading to cracks at the welded joint and affecting the stability of the joint. Therefore, during the welding process, alloy materials with high levels of nickel and chromium can be used to suppress carbon migration, prevent dilution of the weld metal, reduce stress at the joint, and ensure good welding results. First, a nickel-based alloy layer is welded on between the dissimilar steels using shielded metal arc welding or tungsten inert gas welding; after passing quality inspections, specialized heat treatment is carried out to eliminate residual stresses. Thereafter, shielded metal arc welding and gas tungsten arc welding were used to weld austenitic stainless steel and low-alloy heat-resistant steel together. 4. Welding characteristics and application effects of nickel-based alloys: The weld metal of nickel-based alloys has a simple compositional structure, poor liquid flowability, and is of austenitic structure. As a result, the penetration depth during welding is shallow, and small pores and minor thermal cracks tend to form, which affects the quality of the weld. Therefore, it is necessary to carry out proper preparations before welding: purchase high-quality nickel-based alloy materials, select the appropriate alloy based on the properties of the different steel types and the required welding quality, and thoroughly clean the welding wire and workpieces before starting the welding process to prevent impurities from affecting the welding results. During welding, it is necessary to control the amount of heat input in order to avoid prolonged exposure of the joint to high temperatures, thereby improving its corrosion resistance and reducing the number of cracks. When welding, it is necessary to select an appropriate welding speed based on the actual conditions; the speed should not be too high. A welding process with narrow weld beads should be used, and short arcs should be employed for TIG and arc welding. The interpass temperature should be kept below 100°C, to prevent excessive heat from causing the weld to expand and affecting the quality of the weld. First, when welding dissimilar steels using shielded metal arc welding, a barrier layer needs to be placed around the nickel-based alloy material. During the installation of the isolation layer, a special inspection of its quality must be carried out. Once the insulation layer passes the inspection, it requires professional heat treatment. The purpose of the treatment is to eliminate the residual stresses present in the isolation layer. After heat treatment, shielded metal arc welding and gas tungsten arc welding are used to weld austenitic stainless steels and low-alloy heat-resistant steels. During the actual welding process, the operations should be carried out in the sequence described above. When welding the isolation layer, special attention must be paid to the welding materials used for arc welding and manual tungsten inert gas welding. Certain considerations also apply when using these welding materials according to their specific material types. Firstly, the welding materials used must meet **specified standards; when welding the isolation layer, nickel-based welding materials are usually chosen. The metal composition of nickel-based welding materials (wires and electrodes) must meet the corresponding standards. When welding an insulation layer, it is essential to choose and control the welding materials properly. Because if the composition of the welding material is not chosen properly, it will lead to some accuracy errors. During the welding of the isolation layer electrodes, the current level and polarity must meet the specified requirements. Using nickel-based welding materials for welding not only ensures that the quality of the isolation layer meets the required standards, but also improves the quality of the entire welded joint, thereby contributing positively to the development of welding different types of steel together. It also ensures the quality of the welded joints as well as safety during the welding process, and guarantees the service life and performance of these welded joints. After welding was completed, 100% radiographic testing was carried out on the weld joints in accordance with the relevant inspection requirements for such joints. In addition, impact tests, bending tests, and tensile tests were performed on the welded parts in line with the quality assessment requirements for steel containers. The test results show that using a nickel-based alloy as an interlayer in welding processes can effectively increase the tensile strength of the welded joint, raising it significantly above the specified value. After bending the specimen, no defects or cracks appeared, indicating that the specimen has good plasticity and that the joint is dense and continuous. It can be observed from the tests that the proper application of nickel-based alloys during the welding of dissimilar steels can significantly improve welding quality, ensure the performance of the welded joints, and prolong the service life of components. In summary, nickel-based alloys hold great practical value in the welding of dissimilar steels. By using nickel-based alloy interlayers, carbon migration within the weld can be effectively suppressed, metal dilution in the weld can be reduced, and residual stresses can be lowered. This helps to ensure the quality of the welded joints and improves the efficiency of welding processes, making it a valuable reference for welding dissimilar steels.