Super Duplex Stainless Steel: Properties and Key Applications
Bagikan
Table Of Content
Table Of Content
Super Duplex Stainless Steel is a high-performance material 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, which contributes to its exceptional strength and corrosion resistance. The primary alloying elements in super duplex stainless steel include chromium (Cr), nickel (Ni), molybdenum (Mo), and nitrogen (N), each playing a crucial role in enhancing the steel's properties.
Comprehensive Overview
Super duplex stainless steel is characterized by its high chromium content (usually around 25%), which provides excellent resistance to corrosion, particularly in chloride environments. The addition of molybdenum (up to 7%) further enhances pitting resistance, while nitrogen improves strength and resistance to stress corrosion cracking (SCC). This unique combination of elements results in a material that exhibits superior mechanical properties, including high tensile strength and yield strength, alongside good ductility.
Advantages:
- Corrosion Resistance: Excellent resistance to pitting and crevice corrosion, making it ideal for harsh environments.
- Strength: High strength-to-weight ratio, allowing for thinner sections in applications without compromising structural integrity.
- Versatility: Suitable for a wide range of applications across various industries, including oil and gas, chemical processing, and marine environments.
Limitations:
- Cost: Generally more expensive than standard stainless steels due to alloying elements.
- Weldability: While weldable, it requires careful control of heat input and filler materials to avoid issues like hot cracking.
- Availability: May not be as readily available as more common stainless steel grades.
Historically, super duplex stainless steels have gained prominence in industries where corrosion resistance and strength are paramount, particularly in offshore oil and gas applications.
Alternative Names, Standards, and Equivalents
| Standard Organization | Designation/Grade | Country/Region of Origin | Notes/Remarks |
|---|---|---|---|
| UNS | S32750 | USA | Closest equivalent to EN 1.4410 |
| ASTM | A890/A890M | USA | Covers various duplex grades |
| EN | 1.4410 | Europe | Commonly used in Europe |
| JIS | G 4305 | Japan | Similar properties to UNS S32760 |
| ISO | 1.4462 | International | General specification for duplex steels |
The subtle differences between these grades, such as variations in nitrogen content or specific alloying elements, can significantly influence their performance in specific applications. For instance, while S32750 and S32760 are often considered equivalents, S32760 typically has higher molybdenum content, enhancing its resistance to localized corrosion.
Key Properties
Chemical Composition
| Element (Symbol) | Percentage Range (%) |
|---|---|
| Chromium (Cr) | 24.0 - 26.0 |
| Nickel (Ni) | 6.0 - 8.0 |
| Molybdenum (Mo) | 3.0 - 5.0 |
| Nitrogen (N) | 0.1 - 0.3 |
| Iron (Fe) | Balance |
The primary role of chromium in super duplex stainless steel is to enhance corrosion resistance, particularly against pitting and crevice corrosion. Nickel contributes to the austenitic phase stability, improving ductility and toughness. Molybdenum further enhances resistance to localized corrosion, while nitrogen increases strength and resistance to stress corrosion cracking.
Mechanical Properties
| Property | Condition/Temper | Typical Value/Range (Metric) | Typical Value/Range (Imperial) | 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) | Annealed | 28 - 32 HRC | 28 - 32 HRC | ASTM E18 |
| Impact Strength | -20°C | 50 - 100 J | 37 - 74 ft-lbf | ASTM E23 |
The combination of high tensile and yield strength makes super duplex stainless steel suitable for applications requiring high mechanical loading and structural integrity. Its notable elongation percentage indicates good ductility, allowing for deformation without fracture.
Physical Properties
| Property | Condition/Temperature | Value (Metric) | Value (Imperial) |
|---|---|---|---|
| Density | - | 7.8 g/cm³ | 0.28 lb/in³ |
| Melting Point | - | 1350 - 1400 °C | 2462 - 2552 °F |
| Thermal Conductivity | 20°C | 14 W/m·K | 81.0 BTU·in/(hr·ft²·°F) |
| Specific Heat Capacity | 20°C | 500 J/kg·K | 0.12 BTU/lb·°F |
| Electrical Resistivity | 20°C | 0.7 µΩ·m | 0.7 µΩ·in |
| Coefficient of Thermal Expansion | 20-100°C | 16.5 x 10⁻⁶/K | 9.2 x 10⁻⁶/°F |
The density of super duplex stainless steel contributes to its strength, while its relatively high melting point allows for use in high-temperature applications. The thermal conductivity and specific heat capacity are critical for applications involving heat exchange, ensuring efficient thermal management.
Corrosion Resistance
| Corrosive Agent | Concentration (%) | Temperature (°C) | Resistance Rating | Notes |
|---|---|---|---|---|
| Chlorides | 3-10 | 20-60 | Excellent | Risk of pitting at higher concentrations |
| Sulfuric Acid | 10-30 | 20-40 | Good | Limited resistance at high temperatures |
| Hydrochloric Acid | 5-20 | 20-40 | Fair | Not recommended for prolonged exposure |
| Sea Water | - | Ambient | Excellent | Suitable for marine applications |
Super duplex stainless steel exhibits excellent resistance to various corrosive environments, particularly in chloride-rich conditions, making it ideal for offshore and marine applications. However, it is susceptible to localized corrosion in highly acidic environments, particularly with sulfuric and hydrochloric acids.
Comparatively, super duplex stainless steel outperforms standard austenitic stainless steels (like 316L) in terms of pitting resistance and strength, while also offering better corrosion resistance than ferritic grades (like 430) in chloride environments.
Heat Resistance
| Property/Limit | Temperature (°C) | Temperature (°F) | Remarks |
|---|---|---|---|
| Max Continuous Service Temp | 300 | 572 | Above this, oxidation may occur |
| Max Intermittent Service Temp | 350 | 662 | Short-term exposure only |
| Scaling Temperature | 600 | 1112 | Risk of scaling above this temp |
At elevated temperatures, super duplex stainless steel maintains its strength and corrosion resistance, although prolonged exposure to temperatures above 300°C can lead to oxidation and reduced mechanical properties. Careful consideration of service temperatures is essential in design to avoid degradation.
Fabrication Properties
Weldability
| Welding Process | Recommended Filler Metal (AWS Classification) | Typical Shielding Gas/Flux | Notes |
|---|---|---|---|
| TIG | ER2594 | Argon | Preheat may be required |
| MIG | ER2594 | Argon + 2% Oxygen | Control heat input to avoid cracking |
| SMAW | E2594 | - | Suitable for field applications |
Super duplex stainless steel is weldable, but it requires specific filler materials and careful control of heat input to prevent issues such as hot cracking. Preheating may be necessary to minimize thermal stresses during welding.
Machinability
| Machining Parameter | Super Duplex Stainless Steel | AISI 1212 | Notes/Tips |
|---|---|---|---|
| Relative Machinability Index | 20% | 100% | Requires high-speed tooling |
| Typical Cutting Speed (Turning) | 30-50 m/min | 80-120 m/min | Use sharp tools and coolant |
Machining super duplex stainless steel can be challenging due to its high strength and work hardening characteristics. Optimal conditions include using high-speed tooling and adequate cooling to prevent overheating.
Formability
Super duplex stainless steel exhibits moderate formability. Cold forming is feasible, but care must be taken to avoid excessive work hardening. Hot forming is preferred for complex shapes, with recommended bend radii being larger than those for standard stainless steels to prevent cracking.
Heat Treatment
| Treatment Process | Temperature Range (°C/°F) | Typical Soaking Time | Cooling Method | Primary Purpose / Expected Result |
|---|---|---|---|---|
| Solution Annealing | 1020 - 1100 / 1868 - 2012 | 30 min | Air or Water | Dissolution of precipitates, uniform microstructure |
| Aging | 800 - 900 / 1472 - 1652 | 1 - 2 hours | Air | Improve strength and hardness |
Heat treatment processes such as solution annealing are critical for achieving the desired microstructure and properties in super duplex stainless steel. This treatment dissolves precipitates and promotes a balanced austenitic-ferritic structure, enhancing corrosion resistance and mechanical properties.
Typical Applications and End Uses
| Industry/Sector | Specific Application Example | Key Steel Properties Utilized in this Application | Reason for Selection |
|---|---|---|---|
| Oil & Gas | Offshore platforms | High strength, corrosion resistance | Durability in harsh environments |
| Chemical Processing | Storage tanks | Resistance to aggressive chemicals | Safety and longevity |
| Marine | Shipbuilding | Pitting resistance, strength | Performance in seawater |
| Power Generation | Heat exchangers | Thermal conductivity, corrosion resistance | Efficiency and reliability |
Other applications include:
* - Desalination plants
* - Pharmaceutical equipment
* - Food processing machinery
Super duplex stainless steel is chosen for these applications due to its exceptional combination of strength, corrosion resistance, and durability, ensuring long service life and reduced maintenance costs.
Important Considerations, Selection Criteria, and Further Insights
| Feature/Property | Super Duplex Stainless Steel | AISI 316L | Inconel 625 | Brief Pro/Con or Trade-off Note |
|---|---|---|---|---|
| Key Mechanical Property | High strength | Moderate | High | Super duplex offers a balance of strength and cost |
| Key Corrosion Aspect | Excellent in chlorides | Good | Excellent | Inconel offers superior resistance but at a higher cost |
| Weldability | Moderate | Good | Fair | Super duplex requires careful welding techniques |
| Machinability | Challenging | Good | Moderate | Requires specialized tooling and techniques |
| Formability | Moderate | Good | Fair | Super duplex is less formable than austenitic grades |
| Approx. Relative Cost | Moderate | Low | High | Cost-effective for high-performance applications |
| Typical Availability | Moderate | High | Low | Super duplex is less common than standard grades |
When selecting super duplex stainless steel, considerations include cost-effectiveness, availability, and specific application requirements. While it offers superior performance in corrosive environments, its higher cost compared to standard grades may be a factor in decision-making. Additionally, its unique properties make it suitable for niche applications where strength and corrosion resistance are critical.
