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Basic requirements for steel used in pressure vessels: High strength, good plasticity and toughness, as well as excellent fabricability and compatibility. Ways to improve the properties of steel: design of chemical composition, modification of microstructure, and surface treatment of parts. This section provides a further analysis of the basic requirements for steel used in pressure vessels. I. Chemical Composition: The chemical composition of steel has a significant impact on its properties and heat treatment. 1. Carbon: As the carbon content increases, the strength of steel rises, while its weldability decreases; cracks are likely to appear in the heat-affected zone during welding. Therefore, the carbon content in steel for pressure vessels should generally not exceed 0.25%. 2. Vanadium, titanium, niobium, etc.: Adding elements such as vanadium, titanium, and niobium to steel can improve its strength and toughness. 3. S and P are the most important harmful elements in steel: Sulfur – it promotes the formation of non-metallic inclusions, thereby reducing plasticity and toughness. Phosphorus – it can increase the strength of steel, but it also increases its brittleness, especially at low temperatures. By keeping the levels of harmful elements such as sulfur and phosphorus at very low levels, that is, **by increasing the purity of the steel, its toughness, resistance to strain aging, resistance to temper embrittlement, resistance to neutron irradiation-induced embrittlement, and corrosion resistance can all be improved. Therefore, compared to ordinary structural steel, steel for pressure vessels requires stricter control over the content of harmful impurity elements such as sulfur, phosphorus, and hydrogen. For example, the sulfur and phosphorus contents in steel used for pressure vessels in China should be below 0.020% and 0.030%, respectively. With the improvement of smelting technology, it is now possible to keep the sulfur content below 0.002%. The chemical composition also has a decisive impact on heat treatment; if the composition is not carefully controlled, the desired heat treatment results cannot be achieved. II. Mechanical Properties: The mechanical behavior of materials – Due to differences in loads (such as the type of load and the way it acts) and stress conditions, as well as the working environment in which the steel is under stress, the various behaviors exhibited by steel when subjected to forces are referred to as the mechanical properties of the material. The mechanical behavior of steel is not only related to its chemical composition and microstructure, but also closely associated with the stress state and environment in which the material finds itself. Mechanical properties determine mechanical behavior. The mechanical properties of steel are primarily criteria used to characterize strength, toughness, and the ability to undergo plastic deformation; they serve as the main basis for material selection and strength calculations in mechanical design. a. In the design of pressure vessels, common strength criteria include tensile strength бb, yield strength бs, endurance limit, creep limit, and fatigue limit б-1. b. In the design of pressure vessels, common plasticity criteria include elongation after fracture δ5 and reduction of area. c. Common criteria for toughness in pressure vessel design: impact absorption energy Akv, ductile-brittle transition temperature, and fracture toughness. Toughness: The size of the critical crack size depends mainly on the toughness of the steel and the tensile stress. If the steel has high toughness, the allowable critical crack size for pressure vessels is larger, and the safety level is also higher. To prevent brittle fracture and rapid crack propagation, pressure vessels often use steel with good toughness. Akv: The Charpy V-notch impact absorption energy Akv provides a good indication of a material’s toughness, and it is closely related to fracture toughness; pressure vessel standards around the world require specification of Akv values. For steel plates such as 16MnR, it is required that the Akv value in the transverse direction (i.e., the direction in which the impact test specimen is taken) at 0°C be not less than 31 J. When the operating temperature is below or equal to -200°C, low-temperature impact toughness must be taken into account, and the temperature for the Charpy V-notch impact test as well as the Akv value must be determined based on the stress level, design temperature, and thickness. III. Properties of Manufacturing Processes ◇ Requirements for Cold Working: For components that undergo cold rolling or cold stamping during manufacturing, the steel must possess good formability and plasticity under cold working conditions; its elongation after fracture, δ5, should be above 15–20%. To test the bending deformation capacity of steel plates, it is generally necessary to select an appropriate bend radius based on the thickness of the plate, and to conduct a bending test at room temperature with a bending angle of 1800 degrees. Steel with no cracks on its outer surface can be used in the manufacture of pressure vessels. ◇The requirement for welding is weldability, which refers to the ease with which high-quality welded joints can be obtained under certain welding process conditions. The weldability of steel depends mainly on its chemical composition. Carbon – the most significant factor among them is the carbon content. The lower the carbon content, the less likely cracks will form, and the better the weldability. Alloying elements – the effect is usually expressed using the carbon equivalent Ceq. The formula recommended by the International Welding Institute is: http://www.cntcw.com/Doc/data.WebNoteBooks/20060601212952/slide0028_image036.gif. The formula specified in China’s \"Requirements for Licensing the Manufacturing of Boilers and Pressure Vessels\" is as follows: The element symbols in this formula represent the percentage content of that element in the steel. It is generally believed that when Ceq is less than 0.4%, the weldability is excellent ； When Ceq is greater than 0.6%, weldability is poor. China has not yet established requirements for carbon equivalent in steels used for pressure vessels, but the aforementioned formula for calculating carbon equivalent can serve as a reference for analyzing the sensitivity to welding cracks. The carbon equivalent calculated according to the above formula shall not exceed 0.45%.