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Tooling protection (1) Back support cover: The back support cover moves in conjunction with the movement of the welding pool during welding. On one hand, it provides gas protection, and on the other hand, it helps to cool the weld seam and the heat-affected zone. When designing the back gas-shielding hood, care should be taken to ensure that the curvature of its enclosure matches that of the components being welded, in order to prevent excessive loss of argon and thus maintain an effective shielding effect. The feature of this back support cover is its flexibility and good protective effect. However, the movement of the cover requires manual operation; thus, it is limited by the dimensions of the product’s structure, and an additional assistant is needed to operate it. Therefore, this shroud is commonly used for protecting the back side during welding of petrochemical vessels with large inner diameters. (2) Back shielding cover: In pressure vessels, many cylinders or nozzles have an inner diameter of only 200–300 mm, or even less. It is difficult to implement back shielding for the welds of such cylinders using shielding support fixtures; therefore, shielding cover fixtures are commonly used in practice. This type of protective cover has a simple structure, good stiffness, and can be reused multiple times. However, if the product structures are diverse and their specifications have limited adaptability, it is often necessary to customize multiple sets of tooling to meet different size requirements, resulting in high costs for such tooling. Protection is provided by a protective agent. This agent is made by mixing adhesives and is applied to the back side of the weld; it can be used easily regardless of the structure of the weld joint. After welding, a slag layer forms on the back side of the weld, providing effective protection. Its application relies entirely on manual application by the operator, which means it is subject to human factors; as a result, the thickness of the coating applied varies from one operator to another. From a corporate management perspective, it is not possible to calculate costs accurately using fixed quotas, and the usage cost is relatively high. Argon-filled paper or water-soluble paper is used to seal the appropriate areas on both sides of the groove; air is then injected into it, thereby creating an air chamber on the back side of the weld, which provides effective protection. Generally, boiler manufacturing standards require hydraulic testing or post-weld heat treatment, and this type of product dissolves easily in water or burns completely, leaving no residues behind. The only drawback is that argon-filled paper or water-soluble paper cannot withstand high temperatures, which limits their use for welding materials that require preheating. Therefore, this argon filling method is mostly used for the butt welding of small-diameter pipes that do not require preheating. Nitrogen block: This is another method that requires no post-weld treatment. Nitrogen is pressurized to take on a solid form; during welding, the heated gas evaporates, creating a chamber filled with inert gas on the back side of the weld, which also serves to protect the weld. Due to the limited volume of the nitrogen block, the space available for gas evaporation is restricted; therefore, it is only suitable for welding small-diameter pipes and not applicable to materials sensitive to nitriding.