Preparations before manufacturing composite plates
Composite plates are double-layer (or triple-layer) metal plates made by the combination of base materials and laminated materials (or including transition layers) through methods such as explosion or explosion-rolling. Composite plates have special properties such as high strength, corrosion resistance, wear resistance, and low cost. They are mainly used in the manufacturing of equipment like reactors, heat exchangers, and storage tanks.
Two types of composite plates
Currently, there are mainly two types of composite plates: one type has good weldability between the base and laminated materials, such as stainless steel composite plates, nickel-based alloy composite plates, etc.; the other type has poor or no weldability between the base and laminated materials, such as titanium/steel composite plates, zirconium/titanium/steel composite plates, etc. These two types of composite materials have significant differences in the design, manufacturing, and inspection of pressure vessel products and should be treated differently.
Based on GB150.2-2011 “Pressure Vessels – Materials” and NB/T47002-2009 “Explosion Welded Clad Plates for Pressure Vessels,” the fitting rate of the composite plate should be inspected before manufacturing composite plate vessels. If the fitting rate between the base and laminated layers does not meet the requirements, it not only fails to meet special requirements like corrosion resistance and wear resistance but also may lead to bulging of the vessel shell or extensive delamination, significantly reducing the equipment’s service life and safety performance. Poor fitting also makes assembly and welding of the vessel shell difficult. Therefore, the inspection of the fitting rate is crucial for the manufacturing of composite plate vessels.
Control of the stability of composite plate materials is addressed by strengthening technical means for regulation. Before vessel manufacturing, ultrasonic flaw detection should be used to re-examine the fitting rate of the composite plate. During the synthesis of the composite plate, timely inspection of material thickness (considering the possible thinning caused by explosion cladding, sometimes requiring appropriate thickening of the laminated materials) is necessary to ensure that the synthesized composite plate meets standard requirements and drawing specifications.
Fabrication and welding of composite plate cylindrical bodies
When rolling the composite plate cylindrical body, the neutral layer circumference is taken as the reference. The outer circumference of the cylindrical body should be measured promptly to ensure consistency between the cylindrical body’s circumference and that of the other jointed cylindrical body (or head). Before joining cylindrical bodies (or heads), the diameters of the cylindrical bodies or heads should be measured to check for consistency. From a technical perspective, solutions to any errors should be discussed to avoid affecting the final functionality of the pressure vessel. Considering the uniqueness of composite plate materials, the welding process needs to be designed to meet the welding requirements of both the base and laminated materials. It’s crucial to control the welding sequence and process parameters (especially controlling interlayer temperature), reduce welding stress generation, eliminate welding defects, and achieve the desired design effects to ensure stable performance of the pressure vessel.
Structural types of welded joints for composite plate welding
- Structural types of weldable joints for composite plates
- Structural types of unweldable (dissimilar metal) joints for composite plates
- Structural types of unweldable (dissimilar metal) joints for three-layer composite plates
- Arrangement of cover plates and leak detection holes
When welding the base layer, mechanical methods should first remove the laminated layer on both sides of the joint. This manufacturing process requires a width of the laminated layer to prevent the base metal from melting onto the laminated layer and to prevent oxidation of the laminated metal due to high base metal welding temperatures. For unweldable composite plates (dissimilar metals) like titanium, zirconium, etc., the “T” type weld seams at the longitudinal and circumferential weld joint connections are covered with a cover plate and equipped with leak detection holes. This arrangement allows for timely leak inspection in each section of the cylinder. The leak detection channels between each cylinder section cannot be interconnected. Therefore, at the position where the longitudinal and circumferential weld joint connections meet under the cover plate, silver brazing is used to seal.
Layout of cylindrical bodies
The layout of cylindrical bodies is a crucial part of pressure vessel fabrication. To avoid errors during the layout process, the coordinate arrangement method can be used for plate arrangement. Generally, the cylindrical body is unfolded based on its middle diameter size, forming a planar layout drawing. The positions of various fittings, supports, and other welded components on the cylindrical body are corresponded to respective coordinates. This method ensures that the equipment, during both the manufacturing process and upon completion, complies with the design standards specified in the pressure vessel manufacturing acceptance criteria and drawings, achieving the desired design control effects.
Non-destructive testing requirements for composite plate equipment
For dissimilar steel composite plate pressure vessel equipment (such as titanium/steel composite plate equipment), regardless of the pressure vessel category, both A and B class welds require 100% radiographic inspection. All laminated welds undergo 100% penetrant testing. After the final hydrostatic test of the equipment, 100% penetrant testing of the equipment’s laminated welds is also required.
Precautions during the manufacturing process
The forming and assembly of composite plate pressure vessels should take place in specialized processing and assembly workshops, especially for non-ferrous metal composite plates like titanium/steel, nickel/steel, etc. Iron ion contamination will have a fatal impact on the service life and safety performance of the vessel. Therefore, when the composite plate enters the factory, the surface of the plate should be covered or protected by a film. During the equipment processing phase, the surface of the forming equipment contacting the composite plate should be covered with rubber or stainless steel plates to isolate the composite plate from metal forming equipment. During assembly, strict control of the overlap amount should adhere to process specifications and pressure vessel manufacturing inspection standards. The control of the overlap amount should be 50% of the laminated thickness and should not exceed 2mm. Additionally, temporary brackets are not allowed to be welded on the laminated layer during assembly.
Pickling passivation or anodizing treatment
After the completion of the composite plate equipment, chemical pickling or anodizing treatment should be performed on the surface of the laminated layer. The primary purpose is to remove surface contaminants, especially iron ion pollution, and form an oxide film to provide corrosion resistance.
Composite plate pressure vessels, compared to pressure vessels made using pure laminated materials of the same specifications, are cheaper. Users often choose the former when meeting their requirements. However, for manufacturers, the situation is different. On one hand, the specifications and models of composite plates are limited, often requiring outsourced composite processing, which extends the manufacturing cycle and increases manufacturing difficulty.
The composite plate pressure vessels manufactured according to the above requirements have stable quality and have not experienced any issues during practical use, providing a reference for manufacturers of composite plate pressure vessels.