Material preparation for pressure vessels

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Material preparation: During transportation and storage, raw materials can develop rust and oxide scales, as well as become contaminated with soil and oil. If these contaminants are not removed, they will significantly affect the quality of pressure vessel manufacturing after processes such as marking, cutting, shaping, and welding are carried out. Therefore, the blank must be purified before being put into production, in order to remove oil and rust from the edges of the welds, maintain the corrosion resistance of the container, and prepare it for the subsequent processing steps. The main methods of purification include manual purification, mechanical purification, chemical purification, flame purification, etc. Manual purification refers to the method in which workers use wire brushes, sandpaper, manual grinders for grinding, or files and scrapers for scraping. This method has advantages such as flexibility, convenience, and freedom from constraints, but it also has disadvantages including high labor intensity for workers, low efficiency, and a harsh working environment; it is generally used for the local purification of welds. Mechanical purification includes methods such as electric grinding wheels, sandblasting machines, and shot blasting. A portable electric grinder (also known as an electric fine grinder) uses the high-speed rotation of its grinding wheel to remove rust; due to its grinding capability, it is also commonly used for polishing grooves, smoothing welds, and removing burrs from edges. When a sanding wire wheel is used instead of a regular grinding wheel for rust removal, it achieves a better rust-removal effect and higher efficiency, making it suitable for rust removal on large areas. Sandblasting is a cleaning method that uses a high-speed stream of compressed air carrying sand particles to impact the work surface at high speed, thereby removing rust and oxide layers. The sand particles used are uniform quartz sand, and the pressure of the compressed air is generally between 0.5 and 0.7 MPa ； The sandblasting nozzles suffer significant erosion wear, and are commonly made of wear-resistant materials such as cemented carbide or ceramics. It is mainly used for large-area surface treatment of carbon steel and low-alloy steel. Although this method is efficient, it generates a lot of dust that is harmful to human health, and it must be carried out in a closed sandblasting chamber. Since sandblasting poses a serious threat to human health and pollutes the environment, shot blasting is now widely used abroad. Its main features are improved working conditions, easy automation, and convenient quality control of the surface of the materials being processed. The impeller of the shot blasting machine’s blasting head is generally Φ380～Φ500mm ； The shot blasting rate is 200–600 kg/min, and the size of the steel shots is Φ0.8–1.2 mm. In addition, there is also a steel shot recycling and dust removal system. Chemical cleaning is an efficient method that uses acids, bases, or other solvents to dissolve rust, oil, and oxide films. For large-scale cleaning, the steel plate to be cleaned is immersed in an acid bath or an alkali bath; for local cleaning, a special rust remover or cleaning agent is applied to the area that needs to be cleaned. After chemical purification, it must be washed with water to remove solvents such as acids and bases that have adhered to it; the chloride content in the water used for rinsing stainless steel equipment shall not exceed 25 mg/kg. Chemical cleaning methods are commonly used for large-scale rust removal from aluminum, stainless steel, and similar materials. The effectiveness of chemical rust removal is influenced by the type of material to be cleaned, the degree of rusting, as well as the type, concentration, temperature, and time of the cleaning agent. Flame purification involves using high-temperature combustion to remove grease adhering to the surface, but it often leaves behind unburned \"char ash\" on the surface. The basic principle of flame cleaning is that, during the heating and cooling process, the material to be cleaned undergoes sliding due to the difference in linear expansion coefficients between it and the rust, thereby separating the rust from the metal. After the metal has cooled, a wire brush can be used to remove any remaining rust layer. Flame cleaning is primarily used for surface derusting and degreasing of carbon steel and low-alloy steel.