Materials Substitution in Pressure Vessel Design

In the design and manufacturing of pressure vessels, the choice of metal materials plays a crucial role in the performance and safety of the vessel. The selected materials must not only have good mechanical properties, corrosion resistance, and high-temperature tolerance but also meet the requirements for processing and the working environment. Although the properties of materials are theoretically fixed, the performance demands of pressure vessel materials can vary depending on the specific use case. Therefore, material substitution should not be based solely on the idea of "better" versus "worse", but rather on a thorough and detailed analysis of the actual situation.

Material Performance Requirements

The materials chosen for pressure vessels must meet several key performance requirements, including:

Mechanical Properties: These include strength, toughness, and plasticity, ensuring that the vessel can withstand external pressures, temperature changes, and other loads without failure.
Corrosion Resistance: For pressure vessels used in special environments, the material must have strong resistance to corrosion to prevent structural failure caused by corrosion.
High-Temperature Resistance: Some pressure vessels must operate in high-temperature environments, so the material should have sufficient high-temperature strength and thermal stability.
Processability: This includes factors like weldability and machinability, which directly affect manufacturing costs and timelines.

"Better" and "Worse" in Material Substitution

In pressure vessel design, material substitution is a common practice where a superior material replaces one with inferior performance. However, material substitution is not a simple matter of replacing "worse" material with "better" material, as it involves considering various factors.

1. Balancing Strength and Processability

In terms of strength and mechanical properties, low-alloy steel generally outperforms carbon steel, offering higher tensile and compressive strength. However, its cold-working performance and weldability are inferior to that of carbon steel. Generally, there is an inverse relationship between strength and cold-working performance and weldability—higher strength typically means poorer cold-working and weldability. Therefore, when substituting low-alloy steel for carbon steel, the welding process needs to be adjusted, and necessary changes must be made during heat treatment to ensure the material performs well in actual use.

2. Risk of Stress Corrosion Cracking

In some special environments, such as wet hydrogen sulfide environments, pressure vessels face the risk of stress corrosion cracking. As the material strength increases, the likelihood of crack propagation in high-stress areas also rises. For example, substituting high-strength 20R-series steel for low-alloy steel like 16MnR may increase the risk of stress corrosion cracking. Therefore, in such environments, it's important not to simply substitute "better" materials but to choose the most suitable material based on the specific environmental conditions.

3. Special Applications of Different Steels

Different types of steel show significant performance differences in specific applications. For instance, killed steel generally outperforms boiling steel in many properties, offering lower gas content and better mechanical performance. However, in the manufacture of enamel-lined vessels, killed steel is less effective than boiling steel. In terms of corrosion resistance, although stainless steel performs well in most environments, in chloride-ion environments, low-alloy steel or carbon steel may actually offer better corrosion resistance. Therefore, the selection of materials for pressure vessels must be based on a comprehensive analysis of the vessel's actual working environment.

The Impact of Thickness on Vessel Performance

The choice of material thickness is another key factor in the design of pressure vessels. While increasing the thickness can enhance the strength of the vessel, it may also lead to a range of adverse effects.

1. Geometrical Discontinuities and Stress Concentration

Increasing the material thickness often results in changes to the geometric shape of the pressure vessel, especially at the junctions between the head and shell. These geometrical discontinuities can lead to localized stress concentration, increasing the risk of crack formation. For example, using thick-walled material at the junction between the head and shell, as opposed to thinner materials, may lead to increased local stress, potentially triggering fatigue cracks or fractures, particularly in environments prone to stress corrosion.

2. Changes in Welding Processes

Increasing material thickness often requires adjustments to the welding process. For instance, when increasing the thickness of the shell, edge preparation or even internal edge treatment may be necessary at the junction with the head. Additionally, thicker materials may require special welding processes, increasing manufacturing difficulty and potentially leading to welding quality issues. Therefore, when replacing thin-walled materials with thick-walled materials, careful planning of the welding process is essential.

3. Vessel Weight and Heat Transfer Efficiency

The use of thick-walled materials not only increases the weight of the pressure vessel but also impacts its heat transfer efficiency. As the wall thickness increases, the thermal conductivity typically decreases, reducing the overall heat transfer performance of the vessel. For pressure vessels designed to handle heat transfer, increasing the thickness may result in poor thermal exchange, thus affecting the vessel's operational performance.

Welding Processes and Material Substitution

During material substitution, adjusting the welding process is a critical factor. The welding performance of different materials varies greatly, especially when substituting higher-grade materials for lower-grade ones.
Testing and Acceptance Standards: When replacing a lower-grade material with a higher-grade one, testing and acceptance may follow the lower-grade material's standards without needing to increase the standard. However, if there are significant differences in properties such as high-temperature resistance or toughness, the test temperature during hydrostatic testing should be adjusted accordingly.
Heat Treatment Requirements: If the material thickness exceeds the specified cold-rolled thickness, stress-relieving heat treatment is necessary for the pressure vessel shell. Additionally, when the steel plate reaches a certain thickness, ultrasonic testing may be required, and the water pressure test may need to be conducted at higher pressures.

Conclusion

In conclusion, the selection and substitution of materials for pressure vessels is a complex and detailed process that requires a comprehensive consideration of factors such as mechanical properties, corrosion resistance, processability, and weldability. Material substitution should not be based simply on replacing "worse" materials with "better" ones. Instead, a thorough analysis and judgment based on the specific application, environmental conditions, and usage requirements are essential. Additionally, factors such as material thickness and adjustments to the welding process must be carefully considered during the design phase to ensure the safety, reliability, and efficiency of the pressure vessel.