In industrial production, pressure vessels are indispensable key equipment in many process flows. They are widely used in various fields such as chemical, petrochemical, pharmaceutical, food processing, and energy industries for the storage, reaction, and transportation of various media, including gases, liquids, and mixtures. The design and manufacture of these vessels must meet strict safety standards, as they often operate under high pressure and high-temperature conditions. A failure could lead to severe safety accidents and economic losses. The safety and economy of pressure vessels largely depend on the structural materials selected. Material selection not only affects the strength, durability, and reliability of the vessel but also directly relates to the manufacturing cost and service life of the equipment. Therefore, the proper selection of steel is one of the key factors in ensuring the successful design of pressure vessels.
The design pressure, design temperature, and characteristics of the medium are key factors in selecting steel for pressure vessels. Suitable materials must possess good mechanical properties, corrosion resistance, weldability, and hot and cold workability under design conditions, while also being as economical as possible to reduce equipment costs. The selection of steel directly affects the safety and service life of pressure vessels, so it must be approached with caution.
The correct selection of steel is crucial for ensuring the safety and economy of pressure vessels. The classification and properties of steels form the basis for material selection. Here is an introduction to the classification and properties of commonly used steels.
Carbon steel is an iron-carbon alloy with a manganese content of less than or equal to 1.2% and a carbon content of less than or equal to 2.0%, without the addition of other alloy elements. Low carbon steel refers to carbon steel with a carbon content of less than or equal to 0.25%, which is commonly used for welded structural pressure elements. Considering weldability, all carbon steels used for welding pressure vessels are low carbon steels.
• Advantages: Low cost, good workability, and mechanical properties that meet general requirements.
• Disadvantages: Poor corrosion resistance and heat resistance, relatively low strength.
• Applications: Suitable for non-corrosive media environments at ambient or lower temperatures, such as water and air.
Low alloy steel is a general term for low alloy high strength steel and pearlitic heat-resistant steel. Low alloy high strength steel improves the strength and comprehensive properties of the steel by adding a small amount of alloy elements (alloy content less than 3.0%), such as 16MnR and 15MnV.
• Advantages: High strength, good comprehensive properties, and moderate cost.
• Disadvantages: Welding and workability are slightly inferior to carbon steel.
• Applications: Suitable for corrosive or non-corrosive media environments under medium pressure and temperature conditions.
Pearlitic heat-resistant steel improves the heat resistance and hydrogen resistance of the steel by adding chromium (Cr≤10%) and molybdenum alloy elements, such as 18MnMoNb and 15CrMo.
• Advantages: Good heat resistance and strong hydrogen resistance.
• Disadvantages: Higher cost, welding performance requires special attention.
• Applications: Suitable for high-temperature environments, especially for heat-resistant steel or hydrogen-resistant steel with a design temperature above 350℃.
The main feature of austenitic stainless steel is that most of the microstructure at room temperature is austenite, such as Cr18Ni9 and Cr17Ni12Mo2.
• Advantages: Excellent corrosion resistance, good toughness, and excellent workability.
• Disadvantages: High cost, large thermal expansion coefficient, and limited high-temperature performance (generally not used for design temperatures above 500℃).
• Applications: Suitable for corrosive media environments, such as in the chemical, food, and pharmaceutical industries.
The microstructure of ferritic stainless steel at room temperature is mostly ferrite, such as Cr13Al.
• Advantages: Lower cost, good corrosion resistance.
• Disadvantages: Poor toughness, average workability.
• Applications: Suitable for mildly corrosive media environments, such as in decoration and construction.
The microstructure of martensitic stainless steel at room temperature is mostly martensite, such as Cr13.
• Advantages: High strength, high hardness.
• Disadvantages: Poor toughness, poor workability.
• Applications: Suitable for environments requiring high strength and hardness, such as tools and bearings.
In the design of pressure vessels, the selection of steel must follow a series of specific principles to ensure that the material properties match the design requirements while balancing economy and safety. Here are detailed explanations of these principles.
Materials for manufacturing pressure vessels must comply with the provisions of GBT 150 "Steel Pressure Vessels." The upper limit of the use temperature for specific steel grades is the highest temperature provided in the allowable stress table. Information on the chemical composition, room temperature mechanical properties, and supply conditions of domestic steel grades and ASME-II similar steel grades can be referenced from relevant standards.
• Q235 series: The selection of steel grades such as Q235-A, F, Q235-A, Q235-B, and Q235-C must comply with the specific regulations of GB150.
• Wall thickness less than 8mm: When the wall thickness of the pressure element is less than 8mm, carbon steel steel plates are preferred.
• Stiffness requirement: When the wall thickness of the pressure element depends on stiffness, carbon steel is preferred.
• Strength requirement: When the wall thickness of the pressure element depends on strength, low carbon steel and low alloy steel, such as 20R, 16MnR, and 15MnVR steel plates, are selected in sequence within the applicable range.
• Graphitization tendency: Carbon steel and carbon-manganese steel used for a long time at 425℃ will develop a graphitization tendency due to the decomposition of carbides, leading to material embrittlement. Therefore, low carbon pearlitic heat-resistant steel must be used.
• High-temperature environment: Pearlitic heat-resistant steel is generally used for heat-resistant steel or hydrogen-resistant steel with a design temperature above 350℃.
• Corrosion resistance: Austenitic stainless steel is mainly used for corrosion resistance of materials or in conditions where the material requires cleanliness and cannot be contaminated by iron ions.
• High-temperature limitation: Generally not used for heat-resistant steel with a design temperature above 500℃.
• Low-temperature application: When low alloy steel cannot be selected as low-temperature steel, austenitic stainless steel can be used.
• Thickness requirement: When the required thickness is greater than 12mm, austenitic stainless steel composite steel should be considered first.
• Low-temperature environment: When the design temperature is less than or equal to -20℃, low-temperature steel is generally selected (except for low-stress conditions).
• Brittle fracture: If the steel is used below the material's brittle transition temperature and the stress reaches a certain value, the material will undergo brittle fracture. Therefore, low-temperature steel must meet the requirements for impact toughness.
• Hydrogen embrittlement: When pearlitic heat-resistant steel is used as high-temperature hydrogen-resistant steel, hydrogen dissolved in the steel reacts chemically with carbon to form methane over long-term high-temperature use, causing internal cracking and even cracking of the steel (i.e., hydrogen embrittlement).
• Nelson curve: In high-temperature hydrogen-containing conditions, the Nelson curve should be consulted based on the hydrogen partial pressure of the material (design pressure multiplied by the volume percentage of hydrogen) and the design temperature to determine the steel grade suitable for the condition. The Nelson curve can be referenced from HG20581.
In the actual design and manufacturing of pressure vessels, in addition to following the above material selection principles, attention must also be paid to some key details to ensure the fully utilization of material properties and the safe operation of equipment.
Welding is a critical process in the manufacturing of pressure vessels, so the weldability of steel is extremely important. Low carbon steel and low alloy steel have relatively good weldability, but attention must still be paid to the control of welding process parameters during welding to avoid welding defects. Austenitic stainless steel has good weldability, but its large thermal expansion coefficient means that the effect of thermal stress must be considered during welding.
The workability of steel directly affects manufacturing costs and production efficiency. Carbon steel and low alloy steel have good workability and are suitable for mass production. Austenitic stainless steel has excellent workability, but it is more costly. Ferritic and martensitic stainless steel have relatively poor workability and are suitable for specific processing requirements.
In corrosive media environments, corrosion resistance is a key factor in steel selection. Austenitic stainless steel has excellent corrosion resistance, but it is more expensive. Low alloy steel and carbon steel have poorer corrosion resistance and are suitable for mildly corrosive media environments.
Under the premise of meeting design requirements, economical steel should be selected as much as possible to reduce equipment costs. Carbon steel and low alloy steel are less costly and suitable for general conditions. Austenitic stainless steel and pearlitic heat-resistant steel are more expensive and suitable for special conditions.
The design and manufacture of pressure vessels is a complex process, and the selection of steel is a crucial part of it. The correct selection of steel not only ensures the safety and reliability of pressure vessels but also effectively reduces equipment costs. In practical applications, steel should be selected reasonably based on factors such as design pressure, design temperature, and media characteristics, combined with the mechanical properties, corrosion resistance, weldability, and workability of steel, while also considering economy. At the same time, strict compliance with relevant standards and regulations is essential to ensure that the design and manufacture of pressure vessels meet the requirements. Through the introduction of this article, it is hoped that readers will have a better understanding of the principles and methods of steel selection in pressure vessel design, providing reference and guidance for practical work.
