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Welding of dissimilar steels: low-carbon steel and low-alloy steel. Both low-carbon steel and low-alloy steel belong to ordinary ferritic steels. The welding between them, as well as the welding between low-alloy steels of different compositions, falls under the category of welding of dissimilar steels. Welding between such steels is carried out using welding materials corresponding to the lower-grade material, that is, materials with a lower strength grade or lower content of alloying elements, to ensure that the mechanical properties of the weld meet those required for the lower-grade material. Using lower-grade materials also results in better weldability compared to higher-grade materials, and they are cheaper, which helps to reduce manufacturing costs. Welding of low-alloy and medium-alloy heat-resistant steels: Due to the discontinuity in the chemical composition of the welds formed between steels of different types, there is also discontinuity in their properties. If this discontinuity has a significant impact on performance, welding materials cannot be selected based on the low-grade principle. For example, when welding SA213-T91 and SA213-T22 materials, if welding material of 2.25Cr-1MO is chosen following conventional guidelines, the T91 base metal near the weld line on the T91 side will experience severe carbon enrichment, resulting in a decarburized layer; simultaneously, the weld metal near the weld line on the T91 side will also suffer from severe decarburization, giving rise to a decarburized layer. This is because T91 contains approximately 9% chromium, while the 2.25Cr-1Mo welding wire has a carbon content of around 2.25%. After post-weld annealing, the chromium content in the heat-affected zone on the T91 side is significantly higher than that on the weld side. A large amount of carbon migrates toward the base metal, forming a carburized layer that increases hardness and makes the microstructure more hardenable. Meanwhile, the weld side experiences severe decarburization, resulting in lower hardness and a softer microstructure, thereby deteriorating the overall properties of the joint. If 9Cr-1Mo welding material is used, weld carbonation and base metal decarburization will occur on the side of the T22 fusion line. Selection of welding materials for welding dissimilar steels: When welding carbon steel, low-alloy steel, and austenitic stainless steel together, the welding materials should be chosen based on the operating temperature of the joint as well as the mechanical stresses to which the joint is subjected. When the operating temperature of such dissimilar steel joints subjected to compressive loads is below 315°C, austenitic stainless steel welding materials with high Cr and Ni alloy contents can be used. Based on the chemical composition of carbon steel (alloy steel) and austenitic steel, as well as the welding fusion ratio, appropriate austenitic stainless steel welding materials with specific Cr and Ni contents are selected using a nickel equivalent and chromium equivalent diagram, in order to prevent the formation of large amounts of martensite in the weld. Of course, narrow martensite bands are formed near the fusion zone in carbon steel or low-alloy steel; by reducing the carbon content of the welding material, the martensite structure becomes low-carbon martensite with better plasticity, which also ensures that the joint possesses good properties. When dissimilar steel joints subjected to compressive loads are in operation at temperatures above 315°C, nickel-based welding materials should be used. For example, ECrNiFe-2, ERCrNiFe-3, etc., and the main reasons are as follows. If ordinary austenitic stainless steel welding materials are used, the following problems will occur: a) Due to the significant difference in thermal expansion coefficients between ferrite and austenite, thermal stress and thermal fatigue damage will arise during operation at high temperatures. b) Due to the significant variations in alloy element contents, welding joints operating at high temperatures develop severe decarburized and carburized zones, which deteriorate their high-temperature performance. c) The local microstructure of the weld becomes hardened due to the martensite band structure near the fusion line. The use of nickel-based welding materials can prevent the aforementioned phenomena from occurring for the following reasons: a) The coefficient of thermal expansion of nickel-based materials lies between that of ferritic and austenitic steels. b) Nickel-based materials do not cause decarburization or carburization in welded joints. c) Welding of nickel-based materials does not produce martensitic structure. However, for non-load-bearing welded joints operating at high temperatures, although the use of nickel-based electrodes can also meet the performance requirements, it involves high manufacturing costs and is not necessary. Other cheaper welding materials can also achieve the same goal. Numerous experimental studies abroad have shown that for the non-load-bearing fillet welds of tubes and fittings in boiler manufacturing, when the tubes are made of carbon steel or low-alloy steel and the fittings are made of austenitic stainless steel, the welding material should be selected based on the material of lower grade.