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Selection of welding method: While ensuring welding quality, choose a method that is easy to operate, has a simple process, and low costs. First, consider traditional welding methods–fusion welding, among which shielded metal arc welding is given priority. This is because stick welding technology is mature, easy to operate, offers a wide variety of electrodes, has strong adaptability, and provides ample options for selection. The selection of welding method is equally important for welding any material. Since hetero-metal welding involves two or more metals, it is necessary to take these metals into account, giving priority to meeting the requirements of the metal with poorer weldability. Since fusion welding involves heating the metal to high temperatures, causing significant changes in its chemical composition, and also entails a crystallization process, the microstructural properties of the joint undergo substantial changes ； Furthermore, since welding of dissimilar metals involves joining different metals together, there is the issue of their compatibility; as a result, fusion welding has considerable limitations when used for welding such metals. Non-fusion welding methods (i.e., solid-state welding methods such as pressure welding, including resistance welding, friction welding, diffusion welding, and brazing) as well as fusion-brazing (where one of the materials melts while the other does not) are more commonly employed. Since the basic element of different steel grades is iron, fusion welding can generally meet the requirements. Selection of welding materials: From a material perspective, there are three types of welding between different metals: welding of different steel grades, welding of steel with non-ferrous metals, and welding between different non-ferrous metals. Most welding methods can be used for welding dissimilar metals, such as various fusion welds, various pressure welds, brazing, and diffusion welding. In fusion welding, processes such as shielded metal arc welding, submerged arc welding, and gas shielded welding are the most widely used. Choosing the right welding material is key to welding dissimilar metals, and the quality of the joint is closely related to the welding material. Due to the characteristics of welding dissimilar metals, it is necessary to select welding materials based on the chemical composition, properties, joint configuration, and application requirements of the base material. 1. Requirements for welding materials 1) The most important aspect in selecting welding materials is to prevent the formation of undesirable welding defects in the weld joint (such as pores, inclusions, and various types of welding cracks). 2) It is necessary to maintain the stability of the weld metal microstructure, meaning that no significant structural changes occur under operating conditions. 3) Under the aforementioned conditions, the requirements regarding performance in use must be met (including mechanical properties as well as other specific requirements such as corrosion resistance, wear resistance, high-temperature resistance, low-temperature resistance, etc.), with particular attention being paid to ensuring the properties of the weld metal and the transition zone. If the structure is to operate under special conditions (which is often the reason for using welding of dissimilar metals), meeting specific performance requirements takes precedence. Mechanical properties should be considered while meeting specific performance requirements, provided that the mechanical properties of the weld metal are not lower than those of the base material with lower properties. In terms of mechanical properties, if it is not possible to balance strength and ductility, welding materials with better ductility should be selected. It should be noted here that an appropriate combination of weld metal chemical composition and welding parameters is necessary to first meet the requirements regarding the cooling rate (which is very important for the welded heat-affected zone). 4) When welding processes are limited (such as the inability to perform pre-welding heating or post-welding heat treatment), nickel-based or austenitic welding materials can be used to improve the plasticity and toughness of the weld metal. 2. Selection criteria for welding materials 1) The selected welding materials must meet the performance requirements of the structure, especially with regard to special properties (such as mechanical properties and corrosion resistance). 2) The selected welding material must contribute to improving weldability, possess appropriate thermophysical properties such as melting temperature, and still maintain the required performance characteristics of the structure after being diluted (such as mechanical properties, wear resistance, corrosion resistance, etc.). 3) The selected welding materials should have low levels of harmful impurities and gases (carbon, oxygen, hydrogen, nitrogen). 4) When welding dissimilar metals, the choice of welding material is usually to opt for the higher grade rather than the lower one. When welding low-alloy steel with stainless steel, stainless steel welding materials should be selected ； When welding low-alloy steel to nickel-based alloys, or stainless steel to nickel-based alloys, nickel-based welding materials should be selected. 5) When the weldability of two metal materials differs too greatly and no suitable welding material is available, a transition material should be used; it is first welded to one of the metals, and then an appropriate welding material is chosen to weld the transition material to the other metal. Selection of welding parameters: Two factors need to be considered when selecting welding parameters. The first is the fusion ratio, which is particularly important for fusion welding of dissimilar metals (including different types of steel), especially when welding the transition layer (i.e., the first layer in multi-layer welding). The chemical composition of the weld metal is determined by the welding materials used and the fusion ratio, which in turn is adjusted based on the joint groove geometry and welding parameters such as pre- and post-heating temperatures as well as the welding heat input. The second is the microstructure of the welded joint, which is determined by the chemical composition and the welding heat cycle, especially in the case of the welding heat-affected zone. There are several scenarios regarding the welding of two different metal materials. Firstly, there are variations in operating temperature; for example, different parts of a gas turbine have varying temperatures, with the outer circumference of the impeller reaching temperatures above 6000°C, which requires the use of high-alloy heat-resistant steel for fabrication ； The operating temperature at the center of the impeller is **lower than this value, so using alloy structural steel for its manufacture will meet the requirements. By using a composite structure welded together from two metal materials with different properties and compositions, and leveraging the advantages of each, it is possible to save a large amount of high-alloy heat-resistant steel material, thereby achieving significant economic benefits. Secondly, for many containers and pipes that are exposed to high temperatures and corrosive substances, their inner walls need to be able to withstand the erosion caused by these factors. Meanwhile, the supporting components that are in contact with air, as well as those parts that are subject to wear, can be made of carbon steel or low-alloy steel. By welding these different metals together to form composite structures, it is possible to make the most of each type of material. From another perspective, this goal can be achieved by welding a layer of corrosion-resistant and wear-resistant high-alloy steel material onto its working surface; this is also a form of dissimilar metal welded joint. Again, weld joints that use different welding materials despite having the same metal to be welded are also commonly used for welding components made of difficult-to-weld materials. At this time, if welding material identical to that of the metal to be welded is used, crack defects will inevitably occur in the weld joint. To prevent cracks in the welded joint, preheating is required before welding this material; the interlayer temperature must be controlled during the welding process, and slow cooling or heat treatment is necessary after welding. This makes the welding process more complex and makes it difficult to ensure the quality of the welded joint. If a welding material with good weldability is selected, one that does not require the aforementioned process measures during welding, the resulting welded joint can possess both sufficient strength as well as adequate plasticity and toughness, making it an optimal form of welded joint.