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1. Principles for selecting welding materials: When selecting welding materials for steel used in low-temperature pressure vessels, it is necessary to ensure that the welded joints contain as few harmful impurities as possible, such as sulfur, phosphorus, oxygen, and nitrogen. In particular, for steels containing nickel, the level of impurities must be strictly controlled, as an increase in impurity content significantly reduces the toughness of the welded joints. Welded joints of unpressurized attachments welded to low-temperature pressurized components shall meet the same requirements as those of pressurized component weld joints when bearing large loads at low temperatures ； When the load is low, appropriate welding materials can be selected based on the welding requirements. For manual arc welding electrodes used for welding the pressure-bearing components of low-temperature pressure vessels, or for welding pressure-bearing components to non-pressure-bearing components, low-hydrogen basic electrodes specified in GH/T 5117 \"Carbon Steel Welding Electrodes\" and GB/T 5118 \"Low-Alloy Welding Electrodes\" should be selected. For submerged arc welding, alkaline or neutral fluxes should be used. 2. Product welding test plates: The requirements for low-temperature impact testing of welding test plates for ferritic steel products are as follows: Welding between ferritic steels should generally use ferritic-type welding materials (except for 9%Ni steel). The low-temperature impact test temperature for welded joints, as well as the requirements for the low-temperature impact energy of the weld metal, fusion zone, and heat-affected zone, shall comply with the provisions in the drawings or relevant technical documents, and shall not be less than 27 J. Welding materials for welding dissimilar ferritic steels are generally selected based on the base material with higher toughness requirements. The impact test temperature for the welded joint metal shall not be higher than the lower of the temperatures specified for the base materials on either side. The welding procedure qualification for dissimilar steel welds and the heat treatment condition of the product welding test plates shall be the same as those in the final service condition of the vessel. The following requirements shall be met during performance testing: ① The tensile and bending test requirements for welded joints shall be in accordance with the lower requirement among the base materials on both sides ； ②The low-temperature impact energy requirement shall comply with the specifications in the drawings or relevant technical documents, and shall not be less than 27 J. Under strict regulations, the impact energy requirements for the weld seam and heat-affected zone on the side with lower metal strength in the welded joint shall be in accordance with the requirements for the base material on that lower-strength side, while the impact energy requirements for the weld seam and heat-affected zone on the side with higher strength shall be in accordance with the requirements for the base material on that higher-strength side. 3. Precautions for selecting welding materials for austenitic steel: The selection of welding materials for austenitic steels shall meet the following requirements: ① The carbon content in the welded joint metal shall be less than or equal to 0.01% ； ②The chemical composition of the metal in the welded joint shall meet the requirements specified for E0-19-10, E00-19-10, and E00-23-10 in GB/T 983 \"Stainless steel electrodes\", as well as those for H0Cr21Ni10, H00Cr21Ni10, and H0Cr26Ni21 in GB/T 4233 \"Stainless steel bars and wires for welding under inert gas protection\" and GB/T 4242 \"Stainless steel wires for welding\" ； ③When the design temperature is below -100°C, low-temperature Charpy V-notch impact tests on the welded joints shall be conducted in accordance with JB 4708 \"Welding Procedure Qualification for Steel Pressure Vessels\", and the requirements specified in Table 4-5 of this standard shall be met. 4. Precautions for welding between ferritic steel and austenitic steel. For welding dissimilar steels such as ferritic steel and austenitic steel, high-chromium-nickel or nickel-based welding materials of the Cr23Ni13 or Cr26Ni21 type are generally recommended; in principle, no stress-relief heat treatment is required after welding. The welding procedure qualification and product welding test plates for such dissimilar steel combinations shall meet the following requirements: ① The tensile strength of the welded joint shall be not less than the lower value of the minimum tensile strengths of the base metals on both sides ； ②The impact energy of the fusion line and heat-affected zone on the ferritic steel side shall meet the requirements specified in Table 4-5, based on the tensile strength of the ferritic steel; the impact energy of the welded joint metal shall also comply with the requirements in Table 4-5 ； ③The joint shall be subjected to a lateral bending test; the testing method shall comply with GB150. With d=4a, after a 180° bending test, no cracking defects exceeding 1.5 mm shall be present in any direction on the tensile surface, and no cracking defects exceeding 3 mm shall be present at the weld line either. 5. Key points for quality inspection and control of on-site welding defects: The on-site welding of low-temperature pressure vessels must be carried out strictly in accordance with the welding procedures that have been approved as suitable for use. First is the control of welding wire energy; an increase in welding wire energy leads to a decrease in the toughness of the weld and the heat-affected zone. Therefore, to minimize overheating, a low welding wire energy should be used. The supervision of welding wire energy can be carried out by controlling current, voltage, welding speed, and specifying the welding length per electrode. During multi-pass welding, the temperature between weld passes (interpass temperature) should be reduced as much as possible, that is, welding should not be carried out continuously. Fast multi-pass welding facilitates grain refinement and improves the toughness of the weld, but this effect is not achieved if the interpass temperature is too high.