Summary of key welding points for composite plates of pressure vessels

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Composite steel plates are bimetallic sheets made by using stainless steel, nickel-based alloys, copper-based alloys, or titanium sheets as the cladding layers, with pearlitic steel as the base layer, through methods such as explosive welding, composite rolling, or surfacing. The base layer of the composite picture steel plate should meet the requirements regarding joint strength and stiffness, while the overlay layer should fulfill requirements such as corrosion resistance. To ensure that the clad steel plate retains its original comprehensive properties, the base layer and the cladding 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 joint design and the composition of the filler metal. Currently, the most commonly used type is clad steel plate with austenitic stainless steel as the cladding layer and pearlitic steel as the base layer; followed by clad steel plate with ferritic steel as the cladding layer and pearlitic steel as the base layer. Common welding methods include shielded metal arc welding, submerged arc welding, tungsten inert gas welding, CO2 gas shielded welding, and plasma arc welding, etc. Currently, tungsten inert gas welding or shielded metal arc welding is commonly used to weld the clad layer, while submerged arc welding or shielded metal arc welding is used to weld the base layer. Groove types: For butt joints, V-shaped, X-shaped, or combined V- and U-shaped grooves can be used. 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. Taking into account the welding characteristics of the transition layer, efforts should be made to minimize the welding workload on the clad layer side. Regardless of whether the laminated layer is on the inner or outer side, the base layer is welded first. When the clad layer is on the inner side, the root of the base layer must be cleaned from the inside before welding the clad layer. 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, a suitable intermediate filler metal is chosen to serve as a transition layer between the steels. This helps control the iron content in the final weld bead of the cladding metal, thereby preventing embrittlement and cracking in both the cladding and base metal welds. It also ensures that the cladding weld exhibits desirable properties such as corrosion resistance and wear resistance. Welding sequence and selection of welding materials: ① Generally, the base metal is welded first. The first weld bead on the base metal (carbon steel or low-alloy steel) must not penetrate into the cladding metal, so as to prevent embrittlement or cracking of the weld metal. ② When welding (surfacing) on one side of the cladding, the effect of dilution must be considered. Therefore, when welding the cladding layer of clad plates, an appropriate filler metal should be selected to deposit one or more transition layers first, and then the cladding layer can be welded. The filler metal for 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 For post-weld heat treatment to eliminate residual welding stresses, when selecting the heat treatment temperature, one must consider the differences between the heat treatment specifications for the base layer and the cladding layer ； Effect on the corrosion resistance of multilayers ； Will the diffusion of elements at the interface between the base layer and the cladding layer result in the formation of 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, etc., 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 of the heat treatment temperature and prolong the holding time.