The main manufacturing processes for pressure vessels

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Manufacturing process: The manufacturing process of pressure vessels includes the preparation of raw materials, marking, cutting, bending, shaping, edge processing, assembly, welding, inspection, etc. Preparation of raw materials: Before marking the steel, it is necessary to pre-treat the steel first. The pre-treatment of steel refers to the purification, straightening, and application of a protective primer to materials such as steel plates, pipes, and sections. Cleaning treatment mainly involves removing rust, scale, oil, weld slag, and other contaminants from the surfaces of steel plates, pipes, and profiles prior to marking, cutting, and welding; it also applies after the steel has been cut, beveled, formed, and welded. Straightening is the process of correcting the deformations that occur in steel during transportation, lifting, or storage. Applying a protective coating is primarily aimed at enhancing the corrosion resistance of steel, preventing oxidation, and extending the lifespan of components and equipment by applying a protective layer to their surface. Marking is the first process in the manufacturing of pressure vessels. It directly determines the dimensional accuracy and geometric shape accuracy of the parts after forming, and has a significant impact on subsequent assembly and welding processes. Marking involves drawing cutting lines, machining lines, various position lines, and inspection lines on raw materials or preliminarily processed blanks, and applying (or writing) the necessary marks and symbols. The marking process usually includes steps such as unrolling the part, lofting it, and making markings. The dimensions of the blank should be determined before marking. The blank size consists of the expanded size of the part and various machining tolerances. The main methods for determining the developed dimensions of parts are as follows: 1) Graphical method: This involves using geometric drafting techniques to develop the part into a planar figure. 2) Calculation method: It refers to deriving the calculation formula based on the principle of expansion or the principle that the area remains unchanged before and after compression (tension) deformation. 3) Experimental method: This refers to determining the unrolled dimensions of blanks for parts with complex shapes using experimental formulas; it is a simple and convenient approach. 4) Comprehensive method: For overly complex parts, the drawing method and calculation method can be used separately for different sections to determine the unfolded dimensions of the blank; sometimes, experimental methods can also be employed for verification. The parts used to manufacture containers can be divided into two categories: expandable parts and non-expandable parts; for example, circular cylinders and elliptical heads belong to the expandable and non-expandable parts respectively. Cutting, also known as blanking, is the process of separating the desired blank from the raw material by making cuts along predetermined lines. There are two cutting methods: mechanical cutting and thermal cutting. 1. Mechanical cutting: Mechanical cutting mainly includes shearing, sawing, milling, and punching, etc. Its characteristic is that mechanical force plays a key role in the cutting process. (1) Shearing: Shearing involves pressing scissors into the workpiece so that the shear stress exceeds the material’s shear strength, thereby achieving the purpose of cutting. This method is efficient and provides high precision in cutting; it can be used as long as the material’s hardness and dimensions are appropriate. However, the metal within 2–3 mm of the cut site experiences significant hardening. Based on the shape of the plane being sheared, it can be divided into straight-line shearing and curved-line shearing. 1) Straight shear: There are two types of shearing machines that use straight, long cutting blades for shearing, namely straight-blade shears and mitered-blade shears. In flat-nose shears, the two straight cutting edges are parallel, and the shearing process takes place simultaneously along the length of the cutting edges; as a result, the shearing force is high and the impact is strong, making them suitable for shearing thick but narrow strips. In oblique-edge shears, the two straight cutting edges intersect at a certain angle, and the shearing process takes place gradually along the length of the cutting edges; as a result, the shearing force is lower compared to that used in straight-edge shears when cutting workpieces of the same thickness, the impact is reduced, and such shears are suitable for cutting thin and wide sheets of material. In equipment manufacturing, gantry shear machines are commonly used for cutting straight-shaped workpieces. This shearing machine is easy to use, simple to feed, fast in cutting speed, and high in precision. (2) Sawing Sawing is a type of cutting process; the equipment used includes abrasive wheel saws, circular saws, etc. Sawing is generally used for cutting pipes and profiles. 2. Oxygen cutting: Oxygen cutting is also known as gas cutting or flame cutting. Oxygen cutting is a type of thermal cutting; a preheating flame is required for cutting, but the flame alone is not sufficient for cutting – what’s essential is also a high-velocity stream of pure oxygen. 3. Plasma cutting: Plasma is a state of matter in which all particles are ionized into positive and negative ions. Plasma cutting uses a high-temperature, high-speed plasma jet to melt the material and create a cut; it falls under the category of high-temperature melting cutting in thermal cutting methods. It is not restricted by material properties; it can cut both metals and non-metals, but it is mainly used for cutting stainless steel, aluminum, copper, nickel, and their alloys. Forming: Formation of the cylinder. The cylinder is formed by welding several cylindrical sections together using circumferential welds; these sections, in turn, are created by rolling sheet metal and welding longitudinal seams. The principle of tube section rolling: Tube section rolling, also known as rolling or sheet rolling, is the basic manufacturing method for tube sections. The principle of roll bending involves using a rolling machine to apply continuous and uniform plastic bending to the steel plate in order to create a cylindrical surface. Forming of heads: There are mainly three methods for forming heads: stamping, spin forming, and explosive forming. Currently, the commonly used methods are stamping and spin forming. Welding is a process in which, through heating or pressure, or both, the components being welded are brought together at the atomic level to form a permanent joint. 50% of the world’s annual steel consumption involves welding processes. Welding can be divided into three main categories: fusion welding, pressure welding, and brazing. (1) Fusion welding: A manufacturing method in which the parts to be welded are locally heated to melting point, and a weld is formed upon cooling, thereby joining the components together. Including arc welding, gas welding, electroslag welding, electron beam welding, laser welding, etc. Welding by fusion is a widely used welding method, and most low-carbon steels and alloy steels are welded using this method. Special fusion welding can also be used to weld non-metals such as ceramics and glass. (2) Pressure welding: Pressure must be applied during the welding process; it can be completed with or without heating. The main purpose of heating in this process is to soften the metal, enabling it to be deformed under pressure so that the atoms come close enough to exert a strong attractive force on one another; this is fundamentally different from the heating used in fusion welding. Pressure welding includes resistance welding, friction welding, ultrasonic welding, cold pressure welding, explosive welding, diffusion welding, and magnetic welding. Its features include low welding deformation, few cracks, and ease of automation. (3) Brazing: A welding method in which a filler metal with a lower melting point than the base material is heated to melt, but at a temperature below the melting point of the base material; the melted filler metal is used to fill the weld seam, wet the base material, and diffuse with it to form a single integrated structure. Soldering is divided into two main categories: hard soldering and soft soldering. Hard soldering involves heating temperatures above 450°C and tensile strengths greater than 200 MPa; silver-based and copper-based solders are commonly used, and this method is suitable for applications with high working stresses and high ambient temperatures, such as the welding of cemented carbide cutting tools and geological drill bits. The heating temperature for soft soldering is below 450°C, and the tensile strength is less than 70 MPa. It is suitable for environments with low stress and low operating temperatures, such as tin-based soldering of electrical circuits.