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Key points for welding pressure vessel composite plates: Composite steel plates are bimetallic sheets made by using stainless steel, nickel-based alloys, copper-based alloys, or titanium plates as the overlay layers, with pearlitic steel as the base layer, and are produced through methods such as explosive welding, composite rolling, or cladding. The base layer of the composite steel plate shall meet the requirements for joint strength and stiffness, while the overlay shall meet requirements such as corrosion resistance. To ensure that the composite steel plate retains its original comprehensive properties, the base layer and the overlay layer must be welded separately. The welding at the interface between the base layer and the cladding layer is considered welding of dissimilar steels, and its weldability depends primarily on the physical properties and chemical composition of the base layer and the cladding layer, as well as the type of joint and the composition of the filler metal. Currently, composite steel plates with austenitic stainless steel as the cladding layer and pearlite as the base layer are widely used, followed by those with ferritic steel as the cladding layer and pearlite as the base layer. Common welding methods include shielded metal arc welding, submerged arc welding, tungsten inert gas welding, carbon dioxide gas shielded welding, and plasma arc welding. Currently, tungsten inert gas welding or shielded metal arc welding is commonly used to weld the cladding layer, while submerged arc welding or shielded metal arc welding is used to weld the base layer. Groove shape: For butt joints, the groove shapes that can be used include V-shaped, X-shaped, and a combined V and U-shaped groove. Mechanical machining can also be performed over a short distance on the back side of the joint, removing the cladding metal, to ensure that when welding the base layer, the weld metal from the base layer does not adhere to the cladding. Generally, double-sided welding with an X-shaped groove is preferred: the base layer is welded first, followed by the transition layer, and finally the overlay layer. To ensure that the welded joint has good corrosion resistance. While taking into account the welding characteristics of the transition layer, efforts should be made to minimize the welding workload on the side of the clad layer. Regardless of whether the lamination is on the inner or outer side, the base layer is welded first. When the cladding is on the inner side, the root of the base metal must be cleaned from the inside before welding the cladding. When the double layer is on the outside, the last weld pass of the base layer should be ground smooth. When welding the cladding layer, weld the transition layer first, then the cladding layer. Selection of filler metal: In most cases, an appropriate intermediate filler metal is chosen as a transition layer for the steel, thereby controlling the iron content in the final weld bead of the clad metal. This prevents brittleness and cracks in the weld beads at the clad and base layers, ensuring the special properties such as corrosion resistance and wear resistance of the clad weld bead. Welding sequence and selection of welding materials: ① Generally, the base layer is welded first; the first weld in the base layer (for carbon steel or low-alloy steel) should not penetrate the cladding metal, in order to prevent the weld metal from becoming brittle or developing cracks. ② When welding (cladding) on one side, the effect of dilution must be taken into account. Therefore, when welding composite plates, an appropriate filler metal should be selected to deposit one or more layers of transition layer first, and then the composite layer should be welded. The filler metal of the transition layer must be capable of accommodating the dilution of the base steel. ③ The root can be cleaned using carbon arc gouging, chiseling, or grinding. Before welding the transition layer, any residues in the root groove must be removed. ④ Post-weld heat treatment is required to eliminate welding residual stresses; when selecting the heat treatment temperature, the differences in heat treatment specifications for the base layer and the overlay layer should be taken into consideration ； Impact on the corrosion resistance of multi-layer structures ； Will element diffusion at the base and interlayer interfaces generate brittle phases, leading to a deterioration in the properties of the steel plate? ； Due to the differences in physical properties between the base layer and the cladding layer, stress corrosion cracking occurs in the cladding layer. Post-weld heat treatment: Stress-relief heat treatment can be carried out after welding the base layer, followed by welding the transition layer and then the overlay layer. Use the lower limit for the heat treatment temperature and extend the holding time.