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The selection of welding electrodes must be carried out in such a way as to ensure the safety of the welded structure and its reliable performance. This involves taking into account factors such as the chemical composition and mechanical properties of the material to be welded, the plate thickness and joint type, the characteristics of the welded structure, the stress conditions, the requirements regarding the properties of the welds based on the conditions under which the structure will be used, the welding conditions, and economic considerations. When necessary, welding feasibility tests are also required. 01 When welding the same type of steel, the selection of welding electrodes takes into account the mechanical properties and chemical composition of the weld metal. For ordinary structural steel, it is generally required that the weld metal have the same strength as the base material; therefore, electrodes whose tensile strength of the deposited metal is equal to or slightly higher than that of the base material should be used. For alloy structural steel, it is sometimes also required that the alloy composition be the same as or similar to that of the base material. In situations where welded structures have high rigidity, high stress at the joints, and a tendency for cracks to form in the welds, it is advisable to consider using electrodes with a strength lower than that of the base material. When the contents of elements such as carbon, sulfur, and phosphorus in the base metal are high, cracks are likely to occur in the weld; therefore, alkaline low-hydrogen electrodes with good crack resistance should be used. 2. Considering the service performance and operating conditions of welded components, in addition to meeting strength requirements for welds subjected to dynamic or impact loads, it is essential to ensure that the weld metal possesses high impact toughness and plasticity; therefore, low-hydrogen electrodes with high plasticity and toughness values can be used. For welded components that operate under high-temperature, low-temperature, wear-resistant, or other special conditions, corresponding heat-resistant steels, low-temperature steels, cladding materials, or welding electrodes for special purposes should be used. 3. Considering the characteristics of welded structures and their stress conditions, for thick welded components with complex shapes and high rigidity, significant internal stresses are generated during welding, which can easily lead to cracks in the welds; therefore, alkaline low-hydrogen electrodes with good crack resistance should be used. For weldments that are not subject to heavy stress and whose welding areas are difficult to clean thoroughly, acidic electrodes that are not sensitive to rust, scale, and oil should be used. For weldments that cannot be flipped due to constraints, electrodes suitable for all-position welding should be used. 4. Taking into account the welding conditions and economic benefits, acid electrodes with good workability should be selected as long as the product’s performance requirements are met. In confined or poorly ventilated areas, acidic electrodes or low-dust electrodes should be used. For structures with large welding volumes, high-efficiency electrodes should be used when possible, such as iron powder electrodes and high-efficiency gravity electrodes, or specialized electrodes like root pass electrodes and vertical downward welding electrodes, in order to improve welding productivity. 02 When welding dissimilar steels, electrodes of grade 01 should be used. For carbon steel + low-alloy steel combinations, it is generally required that the strength of the weld metal or joint be no less than the lowest strength of the two metals being welded. The strength of the weld metal produced by the selected electrode must ensure that the strength of the weld and joint is at least as high as that of the base material with the lower strength; simultaneously, the plasticity and impact toughness of the weld metal should be no less than those of the base material with the higher strength but lower plasticity. Therefore, the electrode can be selected from the steel grades with the lower strength level among the two. However, to prevent welding cracks, the welding process should be determined based on the steel grade with a higher strength level and poorer weldability, including welding specifications, preheating temperature, and post-weld heat treatment. 02 For low-alloy steel + austenitic stainless steel, electrodes should be selected in accordance with the specified limits for the chemical composition of the deposited metal. Generally, OC25Ni13-type austenitic steel electrodes with high chromium and nickel contents, as well as good plasticity and crack resistance, are chosen to prevent cracks resulting from the formation of brittle hardened structures; however, the welding process should be determined based on the poor weldability of the stainless steel. 03 For stainless steel composite plates, three welding electrodes with different properties should be selected to meet the welding requirements of the base layer, coating layer, and transition layer. For welding base metals (carbon steel or low-alloy steel), structural steel electrodes of the appropriate strength grade should be used; when the coating is in direct contact with corrosive media, austenitic stainless steel electrodes with suitable compositions should be selected. The key lies in the welding of the transition layer (i.e., the interface between the layer and the base material); the dilution effect of the base material must be taken into account, and austenitic steel electrodes of the 0Cr25Ni13 type, which have high chromium and nickel contents as well as good plasticity and crack resistance, should be used. 03 Common empirical principles (1) Equal strength principle – Use welding electrodes of the same strength grade as the base material. It is generally used for welding low-carbon steel and low-alloy steel. (2) Principle of identical composition – Use welding electrodes with a chemical composition that is the same as or similar to that of the base material. It is generally used for welding special-purpose materials such as heat-resistant steel and stainless steel. (3) Crack resistance principle – In welded structures with high welding stiffness, complex shapes, and those subjected to dynamic loads during operation, alkaline electrodes with good crack resistance are typically used to prevent cracks from forming in the joints during welding and use. Low-hydrogen welding electrodes shall be used for pressure vessels of Category II and its subcategories that meet the requirements of the \"Regulations on Safety Inspection of Pressure Vessels\". When welding cast steel and medium-carbon steel (for non-compressed components), the high carbon content or large thickness of the steel leads to a higher tendency for cracks during welding; therefore, low-hydrogen electrodes should be used, along with slow cooling procedures. (4) Porosity resistance principle – Due to the limitations imposed by welding process conditions, such as the difficulty in removing oil, rust, and other contaminants from the joint area of the welded parts, acidic electrodes with strong porosity resistance should be used to prevent gases from remaining in the weld during welding, which could lead to the formation of pores. (5) Principle of low cost – Under the premise of meeting usage requirements, welding electrodes with good process performance, low cost, and high efficiency should be selected as much as possible. (6) Equal toughness principle — The toughness of the weld metal deposited by the electrode should be equal to or close to that of the base metal; in practice, most failures of welded structures are not due to insufficient strength, but rather to inadequate toughness. Therefore, when selecting welding rods, their strength should be slightly lower than that of the base material, while their toughness should be the same or similar. This is also the principle of low mismatch toughness during the welding of high-strength steel. (7) Principle for weldment thickness – When the weldment thickness is ≤ 4 mm, use an electrode with a diameter not exceeding that of the weldment; for weldment thicknesses of 4–12 mm, use an electrode with a diameter of 3–4 mm; and for weldment thicknesses > 12 mm, use an electrode with a diameter > 4 mm.