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Requirements for welding dissimilar steels: (1) The weld joining the two steels should be well-formed, free from defects such as cracks and lack of fusion. (2) The properties of welded joints made from different steel grades generally need not be, nor is it required that they be, the average of the two grades; however, the mechanical properties of such welded joints should not be lower than those of the lower-grade steel, and other properties (such as corrosion resistance) should also not be lower than those of the lower-grade steel. (3) The weld of dissimilar steels is formed by the fusion of the wire metal together with the two base steel types. Generally speaking, when welding dissimilar steels, it is necessary to strive for a uniform chemical composition and a uniform microstructure in the weld joint. (4) The chemical compositions of dissimilar steel welds and the two base metals differ; their thermal conductivity and expansion coefficients are also different. Significant residual stresses exist in the welded joint after welding, and temperature stresses arise when temperatures change. These stresses should not cause damage to the components. Process principles for welding dissimilar steels: To obtain a high-quality welded joint when welding dissimilar steels, special process measures must be taken. Due to the wide variety of dissimilar steels and the fact that the process measures vary, the following common principles must be followed. 1. Select the appropriate welding wire: The welding quality of dissimilar steels depends to a large extent on the welding wire chosen. Due to the unevenness in the chemical composition and microstructure of the weld and fusion zone, poor bonding properties and low performance may arise. When selecting welding wire, bonding performance is considered first, followed by performance in use. Also, when it is not possible to balance the strength and plasticity of the weld metal, a welding wire with good plasticity should be chosen. When welding dissimilar steels of different strength grades, the principle for selecting welding wire is to choose one based on the base material with the lower strength grade; this ensures that the plasticity of the weld is not lower than that of the base material with the higher strength grade. When welding dissimilar heat-resistant steels, the selection of welding wire follows the \"low-matching\" principle – that is, when welding heat-resistant steel with low-carbon steel or low-alloy steel, the welding wire is chosen according to the characteristics of the low-carbon steel or low-alloy steel base material. When welding dissimilar heat-resistant steels with different Cr and Mo contents, the welding wire should be selected based on the heat-resistant steel with the lower Cr and Mo content. 2. Fusion ratio and groove angle The fusion ratio is the percentage of base metal that is melted in the weld metal. A low fusion ratio means that less base material is present in the weld, while more welding wire melts into the weld. When welding dissimilar steels with two different steel microstructures, it is desirable for a larger amount of wire to be incorporated into the weld metal; in this way, the properties of the weld depend mainly on the wire, making it easier to obtain high-quality weld joints. In other words, welding of dissimilar steels requires a low fusion ratio. Increasing the groove width means increasing the amount of welding wire that melts into the weld, which in turn reduces the fusion ratio. When welding dissimilar steels with structures that are relatively similar, large groove angles and low fusion ratios should not be used. 3. Welding process parameters Welding process parameters have a direct impact on the fusion ratio; high wire energy welding means that more heat from the arc is absorbed per unit length of weld, resulting in more melting of the base material, and thus a higher fusion ratio. For welding dissimilar steels, a low fusion ratio is required; therefore, welding with low heat input is necessary, typically using a low current, high welding speed, and multi-layer multi-pass welding. 4. Preheating Preheating is a troublesome task. Steel with poor weldability requires preheating to prevent the formation of cold cracks. One type of steel can be welded without preheating, while another requires preheating; when these two types of steel are welded together, preheating measures should be taken. When pearlitic heat-resistant steel (which requires preheating) is welded to low-carbon steel (which does not require preheating), the preheating temperature should be selected based on the pearlitic heat-resistant steel. One type of steel requires a high preheating temperature, while the other requires a low one; when these two types of steel are welded together, the preheating temperature specified for the steel with the higher requirement should be used as the standard. In conclusion, the preheating temperature for welding dissimilar steels should be determined based on poor weldability. 5. Post-weld heat treatment: Post-weld heat treatment serves two purposes: one is to eliminate welding residual stresses ； Second is to improve the microstructure and properties of the welded joint. For welding of dissimilar steels with similar thermophysical properties (such as Q235 and 16Mn), annealing can fully be used to eliminate welding residual stresses. For welding of dissimilar steels with significant differences in thermophysical properties, annealing is ineffective in eliminating welding residual stresses.