Manufacturing process and welding process for composite steel plate pressure vessels

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I. Preparations before composite plate fabrication: A composite plate is a two-layer (or three-layer) metal plate formed by combining a base material with a covering material (or including a transition layer) through methods such as explosion or explosion-rolling. Composite panels possess special properties such as high strength, corrosion resistance, and wear resistance, as well as the advantage of low cost; they are mainly used in the manufacture of equipment such as reactors, heat exchangers, and storage tanks. There are currently two main types of composite plates: one type features good weldability between the base material and the overlay material, such as stainless steel composite plates and nickel-based alloy composite plates ； Another category consists of composite materials whose weldability with base materials is poor or non-existent, such as titanium/steel composite sheets and zirconium/titanium/steel composite sheets. For these two types of composite materials, there are significant differences in the design, manufacturing, and inspection of pressure vessel products, and they should be treated separately. Based on GB150.2-2011 \"Pressure Vessels – Materials\" and NB/T47002-2009 \"Explosively Welded Composite Plates for Pressure Vessels\", the adhesion rate of the composite plates must be inspected before the manufacture of composite plate vessels. If the adhesion rate between the base layer and the overlay layer does not meet the required standards, it will not only fail to satisfy requirements such as corrosion resistance and wear resistance, but it may also cause bulging or large-scale delamination of the container shell, significantly reducing the equipment’s service life and safety performance. In addition, poor adhesion will make the assembly and welding of the container shell more difficult. Therefore, the adhesion rate inspection is crucial for the manufacturing of composite panel containers. Control of the stability of composite panel materials, with enhanced technical methods to address regulatory issues. Before container manufacturing, ultrasonic testing is used to re-check the bonding rate of the composite panel. During the synthesis of composite panels, it is necessary to monitor the material thickness in a timely manner (taking into account the thinning that may occur during explosive bonding, sometimes it is required to thicken the composite material appropriately) to ensure that the resulting composite panels meet the standard requirements and specifications. II. Fabrication and Welding of Composite Plate Cylinders: When rolling out composite plate cylinders, the perimeter of the neutral layer is used as a reference; the outer perimeter of the cylinder is measured regularly to ensure that it matches the perimeter of the other cylinder (or end cap) with which it is joined. Before joining the cylinders together (or with the end caps), it is necessary to measure the diameter of each cylinder or end cap to check whether they are consistent, and to explore technical solutions for any discrepancies, in order to prevent the functional performance of the pressure vessel from being affected. Given the special characteristics of clad plate materials, during the assembly welding process, the established welding procedure should meet the welding requirements of both the base layer and the cladding layer. It is also necessary to properly control the welding sequence and process parameters (especially the interlayer temperature) to minimize the occurrence of welding stresses and prevent any welding defects. This will ultimately achieve the desired design outcomes and ensure the stable performance of pressure vessels. III. Structural types of welded joints in composite panels 1. Structural types of weldable joints in composite panels 2. Structural types of non-weldable (heterogeneous metal) joints in composite panels 3. Structural types of non-weldable (heterogeneous metal) joints in three-layer composite panels 4. Layout of cover plates and leak detection holes. When welding the base layer, mechanical methods are used to remove a portion from both sides of the joint; this is required by the manufacturing process to ensure an adequate width for the overlay layer, thereby preventing the base metal from fusing to the overlay layer. It also helps to prevent the overlay metal from oxidizing due to excessive welding temperatures. For weldable composite plates (heterogeneous metals) such as titanium and zirconium, the \"T\"-shaped welds at the joints of the longitudinal and circumferential welds are constructed using cover plates along with leak detection holes. In order to be able to identify promptly at which section of the cylinder leakage occurs, the leak detection channels between each cylinder section are not interconnected; therefore, at the locations where the longitudinal and circumferential welds meet beneath the cover plates, these areas are sealed using silver brazing. IV. Cylinder Arrangement The arrangement of cylinders is an important step in the manufacturing of pressure vessels; to avoid errors during this process, coordinate-based methods can be used for organizing the cylinders. Generally, the cylinder is unfolded according to its middle diameter to form a planar expansion diagram. The locations of various connections, supports, and other welded components on the cylinder are assigned to corresponding coordinates. By using this method, it is ensured that the equipment meets the manufacturing acceptance standards for pressure vessels as well as the design requirements specified in the drawings, thereby achieving the desired level of design control throughout the manufacturing process and after the equipment is completed. V. Requirements for non-destructive testing of composite plate equipment: For pressure vessel equipment made of dissimilar steel composite plates (such as titanium/steel composite plate equipment), regardless of the category of the pressure vessel, 100% radiographic testing is required for welds in categories A and B. 100% penetrant testing must be carried out on all composite welds, and after the final hydrostatic test, 100% penetrant testing is again required on these composite welds. VI. Precautions during processing: The forming and assembly of composite plate pressure vessels should take place in dedicated processing and assembly workshops. This is especially true for composite plate vessels made of non-ferrous metals such as titanium/steel or nickel/steel, as iron ion contamination can have a fatal impact on the vessel’s service life and safety performance. Therefore, when the composite panels arrive at the factory, their surfaces must be covered or fitted with protective films ； During the equipment manufacturing stage, the surface of the forming equipment that comes into contact with the composite panel should be equipped with rubber sheets, stainless steel sheets, etc., to separate the composite panel from the metal forming equipment ； When assembling composite plate containers, it is necessary to strictly control the misalignment amount in accordance with the process specifications and the manufacturing and inspection standards for pressure vessels. The control of the misalignment amount is set at 50% of the double-layer thickness, and shall not exceed 2 mm. Additionally, during assembly, it is not allowed to weld temporary clamps on the laminate. VII. Acid washing, passivation, or anodization: After the fabrication of composite panel equipment, the surface of the composite layer must be subjected to chemical acid washing or anodization. Its main purpose is to remove contaminants from the metal surface, especially iron ion contamination, by forming an oxide film that provides corrosion resistance. Conclusion: Composite plate pressure vessels are cheaper than pressure vessels of the same specifications made from pure cladding materials; therefore, users often opt for the former when the required performance standards are met. However, the situation is different for manufacturers. On the one hand, there are few specifications and models of composite panels, which often requires external composite processing, thereby prolonging the manufacturing cycle and increasing the difficulty of production. The composite plate pressure vessels manufactured in accordance with the above requirements exhibit stable quality, and no problems have occurred during actual use, which can serve as a reference for manufacturers of composite plate pressure vessels.