Application of Ni-base Alloy in Dissimilar Steel Welding

1. Main problems in the welding process of dissimilar steels

    Dissimilar steel welding, in layman's terms, is the process in which steel materials of different materials are melted together through welding methods to become dissimilar steel welding. There are essential differences between the welded joints of dissimilar steel and the welded joints of the same steel, mainly due to the inhomogeneity between the deposited metal, the heat affected zone and the base metal on both sides of the welding. When welding dissimilar steel, due to the difference between the selected welding materials and the base metal, it is necessary to calculate the composition, structure and performance of the weld metal. Heat conduction temperature field of welding arc; Microstructure and properties of welded joint; Weldability test and process approval test; Equipment for high-efficiency welding of ships and corresponding high-efficiency welding technology; Welding process of various metal materials; The metal structure welding process of petrochemical industry, large beam and column, pressure vessel manufacturing, offshore engineering pipe structure and ship hull structure are all within the scope of dissimilar steel welding. The following is an analysis of the problems existing in the welding process of dissimilar steel.

1.1 Weld metal dilution

    There are differences between the alloy composition of the weld seam and the metal around the weld seam in the welding of dissimilar steels. The melting amount of the base metal is related to the thickness of the base metal, the size and shape of the weld seam, the arc voltage, the welding position and the welding speed. When welding different steels, the mutual dilution between the melting area of the base metal and the deposited metal will change, resulting in the change of the chemical composition of the welded joint. The chemical composition of the joint is not uniform, which causes the change of the metal structure at the welded seam, thereby affecting the welding joint. If the nickel-based alloy is used as an isolation layer in the welding process, it can prevent the change of metal chemical composition, regulate the temperature around the weld, and avoid the dilution of the weld metal.

  1.2 Residual stress of joint

    The difference of metal structure between dissimilar steels leads to the difference of linear coefficients between the two, and at the same time, there are differences in plasticity and thermal conductivity in the joint position, which will cause abnormal thermal cycle temperature field during welding, resulting in residual stress in the welded joint, affecting the stability of the joint position, and affecting the service life of welded parts.

  1.3 Carbon migration

    Filled metal and molten base metal mix at the inside and edge of the molten pool, but there are some differences in the mixing situation. The incomplete mixing area appears at the weld boundary, and a decarburized layer appears on one side of the dissimilar steel, and a carburized layer appears on the other side, which affects the plasticity and durability of the weld structure. The content of nickel in the weld can effectively inhibit the formation of carbide through graphitization, so as to avoid carbon migration and form a diffusion layer.

2. Solutions to problems in the welding process of dissimilar steels

    The problems that often occur in the welding process of dissimilar steels are introduced above. In the actual construction process, if there is a problem with dissimilar steel in the welding process, it will affect the entire welding and the entire construction, so it is very necessary to study and solve the existing problems. The first problem that should be paid attention to is the dilution of weld metal. Usually, when welding dissimilar steels, the composition of weld alloy will be different from the composition of surrounding metals, and it will be affected by some factors. When the weld metal is diluted during welding, it is prone to deformation. During the dilution process, the composition of the welded part will also change, and cause the distribution of these components to be very uneven, which will cause the microstructure of the metal at the weld seam to change, affecting the overall welding. Therefore, the application of Ni-based alloys in the welding process, as the isolation layer between the weld seam and the weld joint, can effectively solve the problem of changes in metal chemical composition. And the temperature around the weld will be controlled, so as to ensure that the metal is not diluted. Therefore, the use of nickel-based alloys in the welding process of dissimilar steels will effectively avoid the impact of dilution of weld metal and changes in chemical composition.

    The problem of residual stress in welded joints should also be effectively solved. Because there are differences between metals in the process of dissimilar steel welding, the coefficients of the two coefficients change and there are differences during welding. In this way, the problem of abnormal thermal cycle will appear in the actual working process, and there will be a certain difference between the thermal conductivity of the joint position and the metal, which will cause residual stress in the welded joint, which will affect the welding process. The stability of the head and the service life of the welded parts will also have a certain impact. Therefore, removing the residual stress of the joint will effectively improve the service life of the welded part and use nickel-based alloys as welding materials can effectively improve the performance difference between the metal and the joint. The thermal conductivity of the joint part, so that there will be no problems in the thermal cycle link during the welding process. While ensuring the residual stress of the welded joint, it also brings better improvement to the stability of the entire joint part and the efficiency during the welding process. Therefore, in the process of dissimilar steel welding, the application of nickel-based alloys will effectively solve the problem of residual stress in the joint.

    In the process of dissimilar steel welding, the problem of carbon migration is also easy to appear. The main cause of carbon migration problem is melting under the influence of metals inside and around the solvent pool, and the metal materials and liquids are mixed with each other. This makes the carbon content in dissimilar steel joints have some differences, resulting in incomplete stitching at the weld boundary. As a result, a decarburized layer and a carburized layer appear, and these two parts will have an impact on the welding structure. The effective use of nickel-based alloys can lead to the emergence of carbon migration, play a certain inhibitory role in the formation of carbides generated by carbon migration, avoid the emergence of decarburized layers and carburized layers formed after carbon migration, and have a negative impact on the entire The welded structure and brings better improvement.

3. Dissimilar steel welding process

    The common austenitic stainless steel and low-alloy heat-resistant steel are used as dissimilar steels for welding operation. In low-alloy heat-resistant steel, the content of chromium and molybdenum is relatively high. It is necessary to do a good job of corresponding heat treatment after welding to stabilize the joint performance and structure, and to restrain the tendency of air quenching. However, the heat treatment after welding is easy to cause the deterioration of the corrosion resistance of austenitic stainless steel, which leads to cracks in the welded joint and affects the stability of the joint. Therefore, alloy materials with high nickel and chromium elements can be used in the welding process to inhibit carbon migration, avoid dilution of weld metal and joint stress, and ensure welding effect.

    First of all, use electrode arc welding or tungsten argon arc welding to surface the nickel-based alloy material isolation layer between dissimilar steels. After passing the inspection, professional heat treatment work is carried out to eliminate residual stress. After that, welding rod arc welding and manual argon tungsten arc welding are used to weld austenitic stainless steel and low-alloy heat-resistant steel.

4. Welding characteristics and application effects of nickel-based alloys

    The weld metal of nickel-based alloy has a single composition, poor liquid flow, and is an austenitic structure. During the welding process, the penetration is shallow, and small pores and small hot cracks are prone to appear, which affects the welding. Therefore, before welding It is necessary to do a good job, purchase high nickel-based alloy materials, and select nickel-based alloys according to the properties of dissimilar steel and welding requirements for operation, and strictly and thoroughly clean the welding wire and workpiece before the process to avoid impurities affecting the welding effect. During the welding period, it is necessary to control the amount of heat input, avoid the high temperature time process of the joint, improve the corrosion resistance of the joint, and reduce and control the number of cracks. When welding, it is necessary to choose the appropriate welding speed according to the actual situation. It is not suitable to be too fast. Choose the narrow bead process to weld, use short arc for TIG welding and arc welding, and control the interlayer temperature below 100 °C to avoid excessively high welding. Seam affects welding effect.

    First of all, when the dissimilar steel is processed by electrode arc welding, the nickel-based alloy material needs to place an isolation layer. In the process of installing the isolation layer, special inspection should be carried out on the isolation layer. When the isolation layer passes the inspection, it needs to be treated by professional heat treatment. The purpose of the treatment is to eliminate the residual stress existing in the isolation layer. After the heat treatment work, the welding rod arc welding and manual tungsten argon arc welding are used to weld austenitic stainless steel and low-alloy heat-resistant steel. In the actual welding operation process, the operation shall be carried out in the order described above. When welding the isolation layer, the welding materials for arc welding and manual argon tungsten arc welding must be used, and some problems should be paid attention to when applying the welding materials according to the material type.

    First off, the use of welding materials must be the standard stipulated by the state. In this paper, when welding the isolation layer, nickel-based welding materials are usually used. The metal composition of nickel-based welding materials (welding wires and electrodes) must meet the corresponding standards. It is very necessary to select and control the welding material when the isolation layer is welded. Because if the composition of the welding material is not selected properly, some accuracy errors will be caused. During the welding process of the isolation layer electrode, the magnitude and polarity of the current must meet the requirements. Selecting nickel-based welding materials for welding will not only make the isolation layer meet the standard, but also improve the entire welded joint, which will bring better help to the welding of dissimilar steel. At the same time, it will also ensure the safety of the welded joint and the construction process, and ensure the service life and use effect of the welded joint.

    After the welding is completed, 100% radiographic inspection of the welded joint is carried out according to the relevant inspection requirements of the welded joint of the specimen, and impact experiments, bending experiments and tensile experiments are carried out on the welded joint according to the relevant steel vessel evaluation requirements. The test results show that the tensile strength of the welded joint can be effectively improved by using nickel-based alloy as the welding method of the isolation layer, making it significantly higher than the specified value. After bending the specimen, there are no defects and cracks, which shows that the specimen has good plasticity and the joint is dense and continuous. It can be found from the test that the reasonable application of nickel-based alloy in the welding process of dissimilar steel can significantly improve the welding, ensure the use effect of welded joints, and prolong the service life of devices.

    To sum up, nickel-based alloys have great application value in the welding process of dissimilar steels. Through the nickel-based alloy isolation layer, the carbon migration behavior in the weld can be effectively suppressed, the weld metal is diluted and the residual stress can be reduced, so as to ensure the welding of welded joints and improve the use efficiency during welding. It has great reference value for the welding of related dissimilar steels.