Product Description
ASTM A790 UNS S31803/S32205 Welded Stainless Steel Pipe
1. Introduction
ASTM A790 UNS S31803 and S32205 welded stainless steel pipes are duplex stainless steels, characterized by a microstructure composed of approximately equal proportions of austenite and ferrite phases. These pipes are highly sought - after in multiple industries due to their exceptional combination of mechanical and corrosion - resistant properties. The ASTM A790 standard, set by the American Society for Testing and Materials, specifically defines the requirements for seamless and welded ferritic/austenitic stainless steel pipes, with S31803 and S32205 being two prominent grades within this standard. Although they are closely related, S32205 has a relatively higher alloy content in certain elements, which leads to some differences in performance compared to S31803.
2. Chemical Composition
The chemical composition of these alloys is a key determinant of their properties. Table 1 shows the typical chemical composition ranges for ASTM A790 UNS S31803 and S32205:
|
Element
|
Content in S31803 (%)
|
Content in S32205 (%)
|
|
Chromium (Cr)
|
21.0 - 23.0
|
22.0 - 23.0
|
|
Nickel (Ni)
|
4.5 - 6.5
|
4.5 - 6.5
|
|
Molybdenum (Mo)
|
2.5 - 3.5
|
3.0 - 3.5
|
|
Nitrogen (N)
|
0.08 - 0.20
|
0.14 - 0.20
|
|
Carbon (C)
|
≤0.030
|
≤0.030
|
|
Manganese (Mn)
|
≤2.00
|
≤2.00
|
|
Silicon (Si)
|
≤1.00
|
≤1.00
|
|
Phosphorus (P)
|
≤0.030
|
≤0.030
|
|
Sulfur (S)
|
≤0.020
|
≤0.020
|
Chromium forms a passive oxide film on the surface, providing basic corrosion resistance. Nickel improves the toughness and ductility of the steel. Molybdenum further enhances corrosion resistance, especially in chloride - rich environments. Nitrogen contributes to strength and pitting resistance. The slightly higher molybdenum and nitrogen content in S32205 endows it with potentially better corrosion - related performance.
3. Mechanical Properties
The mechanical properties of ASTM A790 UNS S31803 and S32205 welded pipes are well - suited for a wide range of applications. Table 2 presents the typical mechanical properties:
|
Property
|
Value for S31803
|
Value for S32205
|
|
Yield Strength (MPa)
|
≥450
|
≥450
|
|
Tensile Strength (MPa)
|
620 - 820
|
650 - 850
|
|
Elongation (%)
|
≥25
|
≥25
|
|
Hardness (HB)
|
≤290
|
≤290
|
Both grades offer high yield and tensile strength, making them suitable for applications where structural integrity under stress is crucial. The good elongation ensures that the pipes can withstand a certain degree of deformation without fracture. The controlled hardness provides a balance between machinability and wear resistance. S32205, with its slightly higher tensile strength, may be more suitable for applications with more severe mechanical stress requirements.
4. Corrosion Resistance
4.1 Pitting and Crevice Corrosion Resistance
ASTM A790 UNS S31803 and S32205 exhibit excellent resistance to pitting and crevice corrosion. The high chromium, molybdenum, and nitrogen content in the alloy significantly contribute to this property. In chloride - rich environments, such as seawater or chemical processing plants, these alloys' ability to resist the initiation and growth of pits is much better than many other stainless - steel grades. The pitting resistance equivalent number (PREN) for S31803 is typically calculated as PREN = %Cr + 3.3 x %Mo+ 16 x %N, usually around 32 - 35. For S32205, with its higher alloy content, the PREN is typically around 34 - 37, indicating a higher level of pitting resistance.
4.2 Stress Corrosion Cracking Resistance
The duplex microstructure of S31803 and S32205 gives them outstanding resistance to stress corrosion cracking (SCC). Unlike some austenitic stainless steels that are prone to SCC in certain environments, these duplex stainless steels can withstand high tensile stresses in corrosive media without cracking. This makes them ideal for applications in the oil and gas industry, where pipelines are often exposed to harsh environments with high - pressure fluids. S32205 may offer marginally better SCC resistance in extremely harsh conditions due to its higher alloy content.
4.3 General Corrosion Resistance
In addition to pitting, crevice, and stress corrosion resistance, ASTM A790 UNS S31803 and S32205 also show good general corrosion resistance. The passive oxide layer formed by chromium protects the surface from uniform corrosion in a wide range of chemical environments, including acids, alkalis, and neutral solutions. However, the corrosion rate may vary depending on the specific composition and concentration of the corrosive medium.
5. Welding Process
For ASTM A790 UNS S31803/S32205 welded stainless steel pipes, the following welding methods are commonly used:
5.1 Electric Resistance Welding (ERW)
- Process: In the ERW process, stainless - steel sheets or coils are first formed into a tubular shape. Then, an electric current is applied to the edges of the tube, generating heat to melt and fuse the edges together. This method is relatively fast and cost - effective for producing straight - seam welded pipes.
- Considerations: When welding S31803 and S32205 using ERW, it is crucial to control the welding parameters precisely to ensure that the weld quality meets the high corrosion - resistance and mechanical - property requirements of these duplex stainless steels. The heat - affected zone (HAZ) needs to be carefully monitored to avoid any adverse effects on the microstructure and properties of the alloy.
5.2 Arc Welding
- Process: Arc welding, such as gas tungsten arc welding (GTAW) or gas metal arc welding (GMAW), uses an electric arc to melt the filler metal and the base metal at the joint. For S31803 and S32205, appropriate filler metals with similar chemical compositions are selected to ensure good weld properties.
- Advantages: Arc welding allows for greater flexibility in welding complex geometries and thicker materials. It can also provide better control over the heat input, which is beneficial for maintaining the integrity of the duplex microstructure in the weld zone. However, it is generally more time - consuming and may require more skilled operators compared to ERW.
After welding, the pipes usually undergo heat treatment to relieve welding stresses and optimize the microstructure and properties of the weld zone. This helps to restore the corrosion resistance and mechanical properties of the pipes to the desired levels.
6. Applications
Due to their excellent properties, ASTM A790 UNS S31803/S32205 welded stainless steel pipes are widely used in various industries:
6.1 Oil and Gas Industry
- Subsea Pipelines: The high strength, corrosion resistance, and SCC resistance make them suitable for transporting oil and gas in subsea environments, where the pipes are exposed to seawater and high - pressure fluids. S32205 may be more preferred in deeper subsea applications due to its better corrosion and mechanical properties.
- Refinery Equipment: Used in refinery units such as heat exchangers, reactors, and distillation columns, which are often in contact with corrosive chemicals and high - temperature process streams.
6.2 Chemical Processing Industry
- Reactors and Tanks: S31803/S32205 pipes are used in the construction of reactors and storage tanks for handling corrosive chemicals, such as acids, alkalis, and organic solvents. The choice between the two grades may depend on the specific corrosive nature of the chemicals and the required mechanical strength of the equipment.
- Piping Systems: In chemical plants, the pipes are used to transport various process fluids, ensuring long - term service life and reliability.
6.3 Desalination and Water Treatment
- Seawater Intake and Discharge Pipes: In desalination plants, these pipes are used to intake seawater and discharge brine. Their corrosion resistance to seawater and ability to withstand the mechanical stress of water flow make them a preferred choice. S32205 may offer a longer service life in this application due to its better corrosion resistance.
- Water Treatment Equipment: In wastewater treatment plants, the pipes are used in systems for treating and transporting effluent, as they can resist the corrosion caused by various chemicals and biological agents in the wastewater.
6.4 Marine and Shipbuilding
- Hull Structures: Used in the construction of ship hulls and other marine structures, providing resistance to seawater corrosion and ensuring the structural integrity of the vessel.
- Marine Equipment: Such as seawater cooling systems, where the pipes need to withstand the corrosive effects of seawater and maintain efficient heat transfer.
7. Conclusion
ASTM A790 UNS S31803/S32205 welded stainless steel pipes are high - performance materials with excellent chemical, mechanical, and corrosion - resistant properties. Their duplex microstructure and specific alloy compositions enable them to meet the demanding requirements of various industries. Although S31803 and S32205 have many similarities, their minor differences in chemical composition result in some variations in performance, allowing users to select the more appropriate grade according to specific application needs. As industries continue to evolve and demand materials with better performance, ASTM A790 UNS S31803/S32205 welded stainless steel pipes are likely to maintain their popularity and find even more applications in the future.
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