General requirements for the use of welding electrodes

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This post was last edited by Desert Fish on 2022-4-29 at 11:55. I. Use of stainless steel electrodes 1. Chromium-containing stainless steels possess certain corrosion resistance (against oxidizing acids, organic acids, and cavitation), as well as heat and wear resistance. It is commonly used in equipment materials for power plants, the chemical industry, petroleum, and so on. Chromium stainless steel has poor weldability; attention should be paid to the welding process, heat treatment conditions, and the selection of appropriate welding electrodes. 2. Chromium 13 stainless steel has a high degree of hardening after welding, making it prone to cracking. When welding with chromium stainless steel electrodes of the same type (G202, G207), preheating at over 300°C and slow cooling at around 700°C after welding are necessary. If post-weld heat treatment is not possible for the welded parts, chromium-nickel stainless steel electrodes (A107, A207) should be used. 3. Chromium 17 stainless steel: Appropriate amounts of stabilizing elements such as Ti, Nb, and Mo are added to improve its corrosion resistance and weldability; its weldability is better than that of chromium 13 stainless steel. When using chromium stainless steel electrodes of the same type (G302, G307), preheating at over 200°C and tempering at around 800°C after welding are required. If the welded parts cannot be heat-treated, chromium-nickel stainless steel electrodes (A107, A207) should be used. 4. Chromium-nickel stainless steel welding electrodes possess excellent corrosion resistance and oxidation resistance, and are widely used in the chemical, fertilizer, petroleum, and medical machinery manufacturing industries. 5. When welding chromium-nickel stainless steel, repeated heating leads to the precipitation of carbides, which reduces its corrosion resistance and mechanical properties. 6. The fluxes for chromium-nickel stainless steel come in titanium-calcium type and low-hydrogen type. The titanate-calcium type can be used for both AC and DC welding, but the penetration depth is shallower during AC welding, and red discoloration occurs easily; therefore, a DC power source should be used whenever possible. Diameters of 4.0 and below can be used for all-position welding, while those of 5.0 and above are suitable for flat welding and fillet welding. 7. Welding electrodes should be kept dry when in use. Titanium-calcium type electrodes should be dried at 150°C for 1 hour, while low-hydrogen type electrodes should be dried at 200–250°C for 1 hour (repeated drying should be avoided, as this can cause the coating to crack and peel off). This is to prevent the coating from absorbing oil and other contaminants, which could increase the carbon content in the weld and affect the quality of the welded parts. 8. To prevent intergranular corrosion caused by heating, the welding current should not be too high; it should be about 20% lower than that used with carbon steel electrodes. The arc length should also be kept short, and rapid cooling between layers is necessary, with narrow weld beads being preferred. II. Use of low-temperature steel electrodes 1. Generally, “low temperature” refers to temperatures ranging from -10 to -196°C; in China, it starts from -40°C. Temperatures below -196 to -273°C are referred to as “ultra-low temperatures”. 2. Low-temperature welding electrodes at -40 to -50°C, such as J507RH and J607Ni. III. Use of molybdenum and chromium-molybdenum heat-resistant steel electrodes 1. Heat-resistant steel refers to steel grades that possess resistance to creep failure at high temperatures, as well as resistance to oxidation and hydrogen embrittlement, and are also resistant to corrosion by certain media. Based on chemical composition and metallographic microstructure, they can be classified into pearlitic steel, martensitic steel, and austenitic steel. Note that the alloying elements include chromium, molybdenum, vanadium, niobium, titanium, tungsten, nickel, etc., which ensure the steel’s heat strength and corrosion resistance. 2. Welding electrodes designed for use with heat-resistant steel are primarily used for welding pipes, containers, petroleum refining and cracking units, as well as steel reactors and heat exchangers under conditions of high temperature and pressure and in environments prone to hydrogen embrittlement. To improve weld crack resistance, low-hydrogen electrodes are commonly used. 3. Heat-resistant steel contains many alloying elements, resulting in high hardenability during welding; therefore, preheating is necessary along with control of the interpass temperature. After welding, slow cooling or high-temperature tempering should be carried out before the metal has fully cooled down. 4. When welding dissimilar steels, select welding materials that are suitable for the intermediate composition, and choose appropriate preheating temperatures and post-weld heat treatment conditions depending on the materials with poor weldability. 5. When used alone or for welding thick-walled pipes, low-hydrogen heat-resistant steel electrodes should be selected. 6. Before welding, it is necessary to remove impurities such as rust, oil, and moisture from the weldment, especially for electrodes such as R517A, R717A, and R727A. 7. For electrodes such as R307B and R407B, use reverse polarity with direct current; during welding, employ a short arc and prefer narrow weld beads. 8. When the Cr content is below 5%, a pearlitic structure is formed; when it is above 5%, a martensitic structure is formed. Cr-Ni steel has an austenitic structure. 9. In the welding of thin-walled pipes, TIG welding is commonly used for the root pass, with acidic electrodes used for the cover pass. 10. When welding martensitic heat-resistant steel, pay close attention to the issue of crater cracks, and fill the craters using the multiple \"spot welding\" method. IV. Use of structural steel welding electrodes 1. In situations where the cooling rate of the weld is high, resulting in increased strength and an increased tendency for cracks to form in the weld joint, it is possible to use welding electrodes of a grade with strength one level lower than that of the base material. In cases such as multi-layer welding of thick plates or normalization after welding, it is necessary to prevent excessively low strength. 2. Selection of acidic or basic welding electrodes of the same strength grade. Alkaline welding electrodes are chosen when good plasticity, high impact toughness, good low-temperature performance, and strong crack resistance are required. If there are difficulties in using a DC power source, AC/DC-compatible alkaline electrodes can be used instead. 3. For weld joints between different types of steel, such as low-carbon steel and low-alloy steel, or between low-alloy steels themselves, welding materials suitable for the steel with the lower strength grade are generally selected. 4. Welding of medium carbon steel. Due to the high carbon content in medium-carbon steel, which increases the tendency for welding cracks, low-hydrogen electrodes or those that yield weld metal with good plasticity and toughness can be used, along with preheating and slow cooling of the welded parts. 5. Welding of cast steel. Cast steel has a high carbon content, large thickness, and complex shapes, making it highly prone to cracking, especially when it contains high levels of alloying elements. Low-hydrogen electrodes can be used, along with preheating and appropriate alloying techniques. 6. If the workpiece is preheated, the welding current can be reduced by 5–15% compared to when no preheating is done ； Using direct current can reduce consumption by 10% compared to alternating current ； It is reduced by 10–15% compared to vertical welding and overhead welding. 7. When welding indoors, attention should be paid to ventilation ； When welding outdoors, the operator should position themselves upwind to reduce inhalation of welding fumes. 8. Welding electrodes with “B”(RH) and “Ni” in their designation are high-toughness ultra-low hydrogen series electrodes, used for welding critical structural components. V. Selection of welding electrodes: The selection of welding electrodes must be made on the basis of ensuring the safety and functional suitability of the welded structure. This involves a comprehensive evaluation of factors such as the chemical composition and mechanical properties of the material to be welded, the plate thickness and type of joint, the characteristics of the welded structure, the stress conditions, the requirements regarding the properties of the welds under the structural usage conditions, as well as the welding procedures and technical-economic considerations. Appropriate welding electrodes should be chosen accordingly, and welding feasibility tests may be necessary when required. 1. Key points for selecting welding electrodes when welding the same type of steel 1.1 Consider 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, welding 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 form in the weld; therefore, alkaline low-hydrogen electrodes with good crack resistance should be used. 1.2 Consider the service performance and operating conditions of welded components. For welded parts subjected to loads and impact forces, in addition to meeting strength requirements, 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 weldments exposed to corrosive media, stainless steel electrodes or other corrosion-resistant electrodes should be selected based on the properties of the medium and its corrosion characteristics. 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. 1.3 Consider the characteristics of the welded structure and its loading conditions. For thick welded components with complex shapes and high rigidity, large 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 high stresses 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. 1.4 Consider construction conditions and economic benefits. Provided that the required performance characteristics of the product are met, acidic welding electrodes with good processability should be selected. 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 whenever possible, such as iron powder electrodes and high-efficiency gravity electrodes, or specialized electrodes like root pass electrodes and vertical-down electrodes, in order to improve welding productivity. 2. Key points for selecting welding electrodes when welding dissimilar steels 2.1 Carbon steel + low-alloy steel (or low-alloy steel + high-strength low-alloy steel) with different strength grades: 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 should ensure that the strength of the weld and joint is at least as high as that of the material with the lower strength. Meanwhile, the plasticity and impact toughness of the weld metal should be no less than those of the material with higher strength but poorer plasticity. Therefore, the welding electrode can be selected from the steel grades with the lower strength level between the two. However, to prevent welding cracks, the welding procedure must be determined based on the steel grade with a higher strength level and poorer weldability, including the welding specifications, preheating temperature, and post-weld heat treatment. 2.2 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, Cr25-Ni13 type austenitic steel electrodes with higher chromium and nickel contents, as well as better plasticity and crack resistance, are chosen to prevent cracks resulting from the formation of brittle hardened structures. However, the welding process and specifications should be determined based on the stainless steel with poor weldability. 2.3 For stainless clad steel plates, three types of electrodes with different properties should be selected based on the welding requirements for the base layer, cladding layer, and transition layer. For welding of base metals (carbon steel or low-alloy steel), use structural steel electrodes of the appropriate strength grade ； Since the cladding is in direct contact with the corrosive medium, welding electrodes made of austenitic stainless steel with appropriate compositions should be used. The key is the welding of the transition layer (i.e., the interface between the clad layer and the base layer); the dilution effect of the base material must be taken into account, and Cr25-Ni13-type austenitic steel electrodes with high chromium and nickel contents, as well as good plasticity and crack resistance, should be used.