318 Stainless Steel (Duplex): Properties and Key Applications
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Table Of Content
318 Stainless Steel, also known as Duplex Stainless Steel, is a unique alloy that combines the beneficial properties of both austenitic and ferritic stainless steels. Classified as a duplex stainless steel, it typically contains a balanced microstructure of approximately 50% austenite and 50% ferrite. The primary alloying elements include chromium (Cr), nickel (Ni), and molybdenum (Mo), which significantly influence its corrosion resistance, mechanical strength, and overall performance.
Comprehensive Overview
318 Stainless Steel is particularly known for its high strength and excellent resistance to corrosion, making it suitable for various demanding applications. The alloy typically contains around 24% chromium, 6% nickel, and 3% molybdenum, which contribute to its robust performance in harsh environments. The dual-phase microstructure provides enhanced toughness and ductility compared to traditional austenitic stainless steels.
Advantages:
- Corrosion Resistance: Exceptional resistance to pitting and crevice corrosion, especially in chloride environments.
- Mechanical Strength: Higher yield strength compared to austenitic grades, allowing for thinner sections in structural applications.
- Weldability: Good weldability with appropriate filler materials, making it suitable for fabrication.
Limitations:
- Cost: Generally more expensive than standard austenitic stainless steels due to alloying elements.
- Brittleness at Low Temperatures: Potential for reduced toughness in cryogenic applications.
- Sensitivity to Sigma Phase Formation: At elevated temperatures, prolonged exposure can lead to the formation of sigma phase, which can embrittle the alloy.
Historically, duplex stainless steels like 318 were developed to overcome the limitations of austenitic and ferritic grades, particularly in terms of strength and corrosion resistance. Today, they occupy a significant position in the market, especially in industries such as oil and gas, chemical processing, and marine applications.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | S31803 | USA | Closest equivalent to 318L |
| AISI/SAE | 318 | USA | Minor compositional differences to be aware of |
| ASTM | A240/A240M | USA | Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and general applications |
| EN | 1.4462 | Europe | European equivalent with similar properties |
| JIS | SUS318 | Japan | Japanese standard designation |
The differences between equivalent grades can be subtle but significant. For instance, while S31803 and 1.4462 have similar compositions, their mechanical properties and corrosion resistance can vary slightly due to differences in manufacturing processes and heat treatment.
Key Properties
Chemical Composition
| Element (Symbol and Name) | Percentage Range (%) |
|---|---|
| Chromium (Cr) | 24.0 - 26.0 |
| Nickel (Ni) | 4.5 - 6.5 |
| Molybdenum (Mo) | 2.5 - 3.5 |
| Manganese (Mn) | 0.5 - 1.5 |
| Nitrogen (N) | 0.08 - 0.20 |
| Carbon (C) | ≤ 0.03 |
| Phosphorus (P) | ≤ 0.03 |
| Sulfur (S) | ≤ 0.02 |
The primary role of chromium is to enhance corrosion resistance, while nickel contributes to toughness and ductility. Molybdenum further improves resistance to pitting and crevice corrosion, particularly in chloride environments. Nitrogen is added to increase strength and improve resistance to stress corrosion cracking.
Mechanical Properties
| Property | Condition/Temper | Typical Value/Range (Metric - SI Units) | Typical Value/Range (Imperial Units) | Reference Standard for Test Method |
|---|---|---|---|---|
| Tensile Strength | Annealed | 620 - 850 MPa | 90 - 123 ksi | ASTM E8 |
| Yield Strength (0.2% offset) | Annealed | 450 - 650 MPa | 65 - 94 ksi | ASTM E8 |
| Elongation | Annealed | 25 - 40% | 25 - 40% | ASTM E8 |
| Hardness (Rockwell B) | Annealed | 85 - 95 HRB | 85 - 95 HRB | ASTM E18 |
| Impact Strength (Charpy) | -20°C | 40 J | 29.5 ft-lbf | ASTM E23 |
The combination of high tensile and yield strength makes 318 Stainless Steel suitable for applications requiring structural integrity under mechanical loading. Its elongation and impact strength indicate good ductility and toughness, essential for dynamic loading conditions.
Physical Properties
| Property | Condition/Temperature | Value (Metric - SI Units) | Value (Imperial Units) |
|---|---|---|---|
| Density | - | 7.8 g/cm³ | 0.283 lb/in³ |
| Melting Point | - | 1400 - 1450 °C | 2552 - 2642 °F |
| Thermal Conductivity | 20 °C | 15 W/m·K | 86 BTU·in/(hr·ft²·°F) |
| Specific Heat Capacity | 20 °C | 500 J/kg·K | 0.12 BTU/lb·°F |
| Electrical Resistivity | 20 °C | 0.72 µΩ·m | 0.00000072 Ω·m |
| Coefficient of Thermal Expansion | 20 - 100 °C | 16.5 x 10⁻⁶ /K | 9.2 x 10⁻⁶ /°F |
The density of 318 Stainless Steel indicates a robust material, while its thermal conductivity and specific heat capacity suggest it can handle thermal stresses effectively. The coefficient of thermal expansion is crucial for applications involving temperature fluctuations, ensuring dimensional stability.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C/°F) | Resistance Rating | Notes |
|---|---|---|---|---|
| Chlorides | 3-10 | 20-60 °C / 68-140 °F | Excellent | Risk of pitting in high concentrations |
| Sulfuric Acid | 10-30 | 20-40 °C / 68-104 °F | Good | Limited resistance at high temperatures |
| Hydrochloric Acid | 1-5 | 20-25 °C / 68-77 °F | Fair | Not recommended for high concentrations |
| Sea Water | - | Ambient | Excellent | Highly resistant to seawater corrosion |
318 Stainless Steel exhibits excellent resistance to a variety of corrosive agents, particularly in marine environments and chemical processing applications. Its performance against chlorides is noteworthy, making it suitable for offshore and coastal applications. However, caution is advised in environments with high concentrations of sulfuric and hydrochloric acids, where alternative materials may be more appropriate.
When compared to other stainless steels, such as 316L and 2205, 318 offers superior resistance to pitting and stress corrosion cracking, particularly in chloride-rich environments. However, 316L may perform better in highly acidic conditions, while 2205 provides enhanced strength.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 300 °C | 572 °F | Suitable for continuous use at this temperature |
| Max Intermittent Service Temp | 350 °C | 662 °F | Short-term exposure can be tolerated |
| Scaling Temperature | 600 °C | 1112 °F | Risk of oxidation beyond this temperature |
| Creep Strength considerations | 500 °C | 932 °F | Creep resistance begins to diminish at this temperature |
At elevated temperatures, 318 Stainless Steel maintains good mechanical properties, although prolonged exposure can lead to oxidation and the formation of sigma phase, which can embrittle the alloy. It is essential to consider the service environment and temperature fluctuations when selecting this material for high-temperature applications.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| TIG | ER318L | Argon | Good results with proper technique |
| MIG | ER318L | Argon/CO2 mix | Suitable for thicker sections |
| SMAW | E318-16 | - | Requires preheat for thicker sections |
318 Stainless Steel is generally considered to have good weldability, particularly with the appropriate filler metals. Preheating may be necessary for thicker sections to avoid cracking. Post-weld heat treatment can enhance the mechanical properties and relieve residual stresses.
Machinability
| Machining Parameter | 318 Stainless Steel | AISI 1212 | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 40% | 100% | Requires slower cutting speeds |
| Typical Cutting Speed | 20-30 m/min | 60-80 m/min | Use carbide tools for best results |
Machining 318 Stainless Steel can be challenging due to its strength and toughness. It is advisable to use high-speed steel or carbide tools and to maintain lower cutting speeds to achieve optimal results.
Formability
318 Stainless Steel exhibits good formability, allowing for cold and hot forming processes. However, due to its strength, it may require higher forces compared to austenitic grades. The material can be bent and shaped with appropriate tooling, but care must be taken to avoid work hardening.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Solution Annealing | 1020 - 1100 °C / 1868 - 2012 °F | 30 minutes | Air or water | Dissolve carbides and improve ductility |
| Stress Relieving | 300 - 600 °C / 572 - 1112 °F | 1 hour | Air | Reduce residual stresses |
Heat treatment processes such as solution annealing are crucial for optimizing the microstructure and properties of 318 Stainless Steel. This treatment helps dissolve carbides and enhances ductility, making the material more suitable for fabrication.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection (Brief) |
|---|---|---|---|
| Oil and Gas | Offshore platforms | High strength, corrosion resistance | Essential for harsh marine environments |
| Chemical Processing | Storage tanks | Excellent resistance to aggressive chemicals | Ensures longevity and safety |
| Marine | Shipbuilding | Resistance to seawater corrosion | Critical for structural integrity |
| Power Generation | Heat exchangers | Good thermal conductivity and corrosion resistance | Efficient heat transfer in harsh conditions |
Other applications include:
- Pulp and Paper Industry: Used in bleaching and chemical recovery processes.
- Food Processing: Equipment requiring high hygiene standards and corrosion resistance.
- Pharmaceuticals: Equipment and piping systems where cleanliness and corrosion resistance are paramount.
The selection of 318 Stainless Steel for these applications is primarily due to its superior mechanical properties and corrosion resistance, which are essential for maintaining performance and safety in challenging environments.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | 318 Stainless Steel | 316L Stainless Steel | 2205 Duplex Stainless Steel | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | High strength | Good ductility | Higher strength | 318 offers a balance of strength and ductility |
| Key Corrosion Aspect | Excellent in chlorides | Good in acids | Excellent in chlorides | 318 is superior in chloride environments |
| Weldability | Good | Excellent | Good | 316L is easier to weld |
| Machinability | Moderate | Good | Moderate | 316L is easier to machine |
| Formability | Good | Excellent | Moderate | 316L offers better formability |
| Approx. Relative Cost | Higher | Moderate | Higher | Cost varies with market demand |
| Typical Availability | Moderate | High | Moderate | 316L is more commonly stocked |
When selecting 318 Stainless Steel, considerations such as cost-effectiveness, availability, and specific application requirements are crucial. While it may be more expensive than standard austenitic grades, its superior performance in corrosive environments often justifies the investment. Additionally, its unique properties make it suitable for niche applications where other materials may fail.
In summary, 318 Stainless Steel is a versatile and robust material that excels in various demanding applications, particularly in environments where corrosion resistance and mechanical strength are paramount. Its unique properties and advantages make it a preferred choice in many industries, ensuring safety and longevity in critical applications.