Duplex stainless steel casting refers to the process of forming complex components from duplex stainless steel alloys, which combine both austenitic and ferritic structures.
The unique properties of duplex stainless steel make it an invaluable material in modern manufacturing, especially in industries that require high strength, Korrosjonsmotstand, og holdbarhet.
Its dual-phase microstructure offers a balance of properties that are difficult to achieve with other materials, making it a preferred choice for a wide range of applications.
I denne bloggen, we will explore the intricacies of duplex stainless steel casting, its properties, the casting process, and how it’s applied across various industries.
1. What is Duplex Stainless Steel?
Dual-Phase Structure
Duplex stainless steel is named for its unique dual-phase structure, which combines austenitic and ferritic grains.
The austenite phase is known for its excellent corrosion resistance, while the ferrite phase provides enhanced strength and resistance to stress corrosion cracking.
This structure makes duplex stainless steel particularly suitable for harsh environments, where both strength and corrosion resistance are critical.
Typical Chemical Compositions of Duplex Stainless Steels
| Karakter | UNS NUMMER | Karbon (C) | Mangan (Mn) | Silisium (Og) | Fosfor (P) | Svovel (S) | Krom (Cr) | Nikkel (I) | Molybden (Mo) | Nitrogen (N) | Andre |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 2205 | S31803/S32205 | ≤ 0.030 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 21.5 - 23.5 | 4.5 - 6.5 | 2.5 - 3.5 | 0.14 - 0.22 | - |
| 2507 | S32750 | ≤ 0.030 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 25 - 27 | 3.5 - 4.5 | 3.5 - 4.5 | 0.25 - 0.35 | - |
| 2304 | S32304 | ≤ 0.030 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 22 - 23 | 1.5 - 2.5 | 1.5 - 2.5 | 0.10 - 0.20 | - |
| 2101 | S32101 | ≤ 0.030 | ≤ 1.50 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 19 - 21 | 0.8 - 1.2 | 0.3 - 0.7 | 0.08 - 0.12 | - |
| 2707H | S32707 | ≤ 0.030 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 26 - 28 | 4.0 - 5.0 | 3.5 - 4.5 | 0.25 - 0.35 | - |
| 2825 | S32825 | ≤ 0.030 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 24 - 26 | 4.0 - 5.0 | 3.0 - 4.0 | 0.20 - 0.30 | - |
| 32760 | S32760 | ≤ 0.030 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 24 - 26 | 6.0 - 7.0 | 3.5 - 4.5 | 0.20 - 0.30 | Cu: 0.5 - 1.5% |
| 329J4L | S32948 | ≤ 0.020 | ≤ 2.00 | ≤ 1.00 | ≤ 0.030 | ≤ 0.020 | 22 - 24 | 3.0 - 4.5 | 3.0 - 4.0 | 0.20 - 0.30 | W: 0.5 - 1.5% |
Equivalent Grade
Standard Duplex Stainless Steel (F.eks., 2205)
- OSS: S31803 / S32205
- ASTM/AISI: 2205
- I: 1.4462
- Han er: SUS329J3L
- Afnor: Z3 CND 22-05 Az
Super duplex rustfritt stål (F.eks., 2507)
- OSS: S32750 / S32760
- ASTM/AISI: 2507
- I: 1.4410
- Han er: SUS329J4L
- Afnor: Z3 CN 25-06 Az
2. Duplex Stainless Steel Casting Process
What is Casting?
Casting is a manufacturing process where molten metal is poured into a mold and allowed to solidify. The basic steps include:
- Smelting: The duplex stainless steel is melted in a furnace.
- Helling: The molten metal is poured into a pre-prepared mold.
- Størkning: The metal cools and solidifies in the mold, taking the shape of the mold.
- Etterbehandling: The cast part is removed from the mold and undergoes finishing processes such as grinding, maskinering, og varmebehandling.
Special Requirements for Duplex Stainless Steel Casting
Casting duplex stainless steel presents unique challenges:
- Maintaining Phase Balance: Controlling the cooling rate and temperature is crucial to maintaining the correct balance between the austenitic and ferritic phases.
- Avoiding Phase Separation: Rapid cooling can lead to the formation of undesirable phases, such as the sigma phase, which can reduce ductility and toughness.
- Controlling Microstructure: Precise control over casting parameters is necessary to achieve the desired microstructure and mechanical properties.
Common Casting Methods
| Metode | Beskrivelse | Fordeler |
|---|---|---|
| Investeringsstøping | Ideal for producing complex and precise parts with smooth surfaces. | Høy presisjon, glatt overflatebehandling, suitable for intricate designs. |
| Sandstøping | Suitable for larger parts and more complex geometries, often used for prototyping and small production runs. | Cost-effective for small to medium production runs, flexibility in design. |
| Die casting | Less common for duplex stainless steel but can be used for high-volume production of smaller, simpler parts. | High production speed, consistent quality, cost-effective for large volumes. |
Advantages of Casting Duplex Stainless Steel
- Komplekse geometrier: The casting process enables the creation of intricate and detailed shapes, which are often difficult or impossible to machine.
- Kostnadseffektiv: For large production runs, casting reduces manufacturing costs while maintaining consistent quality.
- Høy presisjon: Casting duplex stainless steel allows for components with precise dimensions, minimizing the need for extensive post-processing.
3. Key Properties of Duplex Stainless Steel
Duplex stainless steels are a family of rustfrie stål that combine the microstructural features of ferritic and austenitic stainless steels.
This unique combination provides a balance of properties that make duplex stainless steel highly desirable for a wide range of applications,
particularly in environments where high strength, Korrosjonsmotstand, and toughness are required.
Chemical Properties
Korrosjonsmotstand
- Pitting og sprekker korrosjonsmotstand: Duplex stainless steels exhibit excellent resistance to pitting and crevice corrosion, which are common issues in chloride-containing environments.
This is due to their higher chromium and molybdenum content compared to standard austenitic stainless steels. - Stresskorrosjonssprekker (SCC) Resistance: Duplex stainless steels have superior resistance to stress corrosion cracking, especially in chloride environments.
This property is crucial in applications involving hot, saline solutions, such as those found in offshore oil and gas platforms. - General Corrosion Resistance: The balanced microstructure of duplex stainless steels provides good general corrosion resistance,
making them suitable for a variety of corrosive media, including acids and alkaline solutions.
Intergranular Corrosion
- Low Carbon Content: Duplex stainless steels typically have a low carbon content, which minimizes the risk of intergranular corrosion.
This is achieved by controlling the carbon content to levels below 0.03%, which helps prevent the formation of chromium carbides at grain boundaries.
Sveisbarhet
- Welding Characteristics: Despite their high strength, duplex stainless steels can be welded using conventional techniques.
Imidlertid, care must be taken to control the heat input and cooling rate to avoid the formation of undesirable phases,
such as the sigma phase, which can reduce ductility and toughness.
Environmental Stability
- Chloride Environments: Duplex stainless steels are particularly well-suited for chloride-rich environments,
such as seawater and brines, where they provide excellent resistance to chloride-induced corrosion.
Fysiske egenskaper
Tetthet
- Verdi: Omtrent 7.8 g/cm³
- Betydning: The density of duplex stainless steels is similar to that of other stainless steels, making them suitable for applications where weight is not a critical factor.
Imidlertid, their high strength-to-weight ratio still offers advantages in certain applications.
Mekaniske egenskaper
- Avkastningsstyrke: Duplex stainless steels have a yield strength that is typically twice that of austenitic stainless steels.
For eksempel, the yield strength of 2205 duplex stainless steel can range from 450 til 750 MPA. - Strekkfasthet: The tensile strength of duplex stainless steels is also higher than that of austenitic stainless steels, often ranging from 550 til 850 MPA.
- Forlengelse: Despite their high strength, duplex stainless steels maintain reasonable elongation, Vanligvis rundt 25-30%, which provides good ductility and formability.
- Påvirke seighet: Duplex stainless steels exhibit excellent impact toughness, Selv ved lave temperaturer, making them suitable for cryogenic applications.
Termiske egenskaper
- Termisk konduktivitet: Duplex stainless steels have a higher thermal conductivity than austenitic stainless steels, som kan variere fra 15 til 30 W/m · k.
This property is beneficial in applications where efficient heat transfer is required. - Termisk ekspansjon: The coefficient of thermal expansion for duplex stainless steels is lower than that of austenitic stainless steels, Vanligvis rundt 10.5 til 12.5 µm/m·°C.
This property reduces thermal stresses and deformation in high-temperature applications.
Electrical Properties
- Elektrisk resistivitet: The electrical resistivity of duplex stainless steel is higher than that of carbon steel but lower than that of austenitic stainless steel.
It typically ranges from 70 til 80 µω · cm, which affects their suitability for electrical applications.
Magnetiske egenskaper
- Ferromagnetic Behavior: Unlike austenitic stainless steels, duplex stainless steels are ferromagnetic due to their ferritic phase.
This property can be advantageous in certain applications, such as magnetic separation processes, but maybe a disadvantage in others where non-magnetic materials are required.
| Eiendom | Typisk verdi | Description and Application Benefits |
|---|---|---|
| Avkastningsstyrke | 450-550 MPA | Approximately twice the yield strength of austenitic stainless steels like 304 og 316, making duplex steels ideal for structural and load-bearing applications. |
| Tetthet | ~7.8 g/cm³ | Similar to other stainless steels, suitable for components requiring a high strength-to-weight ratio. |
| Elastisk modul | 190-210 GPA | Offers rigidity, which is beneficial in applications requiring structural integrity under load. |
| Termisk konduktivitet | ~25 W/m·K | Higher than that of austenitic stainless steels, advantageous for heat transfer applications in chemical processing and energy industries. |
| Termisk ekspansjon | 13.5 x 10⁻⁶ /° C. | Lower thermal expansion rate than austenitic grades, making it well-suited for applications with temperature fluctuations to reduce risks of thermal stress and deformation. |
4. Applications of Duplex Stainless Steel Castings
Olje- og gassindustri
- Offshore -plattformer: Ventiler, rørledninger, and pressure vessels that require excellent corrosion resistance in seawater and harsh chemicals.
- Onshore Facilities: Components for refining and processing plants, such as heat exchangers and storage tanks.
Marine Applications
- Skipsbygging: Hull components, propeller, and other parts exposed to seawater.
- Avsaltningsplanter: Equipment for water treatment and desalination processes, where corrosion resistance is critical.
Chemical Processing and Pulp & Papirindustri
- Reactors and Heat Exchangers: Components that handle aggressive chemicals and high pressures.
- Storage Tanks: Vessels for storing and transporting corrosive substances.
Kraftproduksjon
- High-Pressure Systems: Components for steam turbines, kjeler, og varmevekslere.
- Nuclear Power Plants: Parts that require high strength and corrosion resistance in radioactive environments.
Food and Beverage Industry
- Processing Equipment: Pumper, ventiler, and machinery parts that need to resist corrosion from cleaning chemicals and food-related substances.
- Storage Tanks: Containers for storing and transporting food and beverages.
5. Advantages of Duplex Stainless Steel Castings
Overlegen korrosjonsmotstand
- Atmospheric and Underwater Environments: The combination of austenitic and ferritic phases enhances the material’s resistance to corrosion in both atmospheric and underwater environments.
- Chloride-Rich Environments: Excellent resistance to pitting and crevice corrosion in chloride-rich environments, such as seawater and brines.
Higher Strength-to-Weight Ratio
- High Mechanical Strength: Duplex stainless steel castings offer high tensile and yield strength, making them ideal for applications where weight reduction is critical.
- Lightweight Design: The high strength-to-weight ratio allows for the design of lighter and more efficient components.
Cost-Effective for Large Production Runs
- Efficient Production: Casting processes enable the efficient production of large volumes of parts with complex shapes at a lower cost compared to other manufacturing methods.
- Reduced Machining: The ability to produce near-net shapes reduces the need for extensive machining, saving time and material.
Enhanced Durability
- Long-Term Performance: Due to its high strength and resistance to stress corrosion cracking, duplex stainless steel is ideal for critical, long-lasting applications in harsh environments.
6. Challenges in Duplex Stainless Steel Casting
Casting Defekter
- Porosity and Shrinkage: These defects can affect the quality and integrity of the castings.
- Inneslutninger: Foreign particles or impurities can weaken the material and reduce its performance.
Welding and Fabrication Issues
- Special Procedures: Welding duplex stainless steel may require special procedures and filler materials to avoid compromising its corrosion resistance and mechanical properties.
- Varmebehandling: Post-weld heat treatment may be necessary to optimize the microstructure and properties of the welded joints.
Complexity in Production
- Precise Control: Managing the balance between the ferritic and austenitic phases during casting requires precise control over casting parameters such as temperature and cooling rate.
- Kvalitetssikring: Rigorous quality control measures are essential to ensure that the final product meets the required specifications and performance standards.
7. Duplex stainless steel vs Super Duplex stainless steel
Duplex rustfritt stål og Super duplex rustfritt stål are different alloys, although they share some similarities.
Both are designed with a dual-phase microstructure, consisting of a mix of Austenittisk og ferritisk faser, which gives them excellent mechanical properties and high resistance to corrosion.
Imidlertid, they differ in their composition, ytelse, and suitable applications.
| Trekk | Duplex rustfritt stål | Super duplex rustfritt stål |
|---|---|---|
| Phase Composition | Roughly 50% austenite and 50% ferritt | Omtrent 40-50% austenite and 50-60% ferritt |
| Legeringselementer | Contains less molybdenum and chromium than super duplex | Higher levels of chromium, Molybden, og nitrogen |
| Korrosjonsmotstand | Good resistance to pitting and crevice corrosion, especially in chloride environments | Superior resistance to pitting, sprekk korrosjon, and stress corrosion cracking in more aggressive environments |
| Strekkfasthet | Generally lower than super duplex | Higher tensile strength due to the addition of more alloying elements |
| Avkastningsstyrke | Omkring 450 MPA | Omkring 550-720 MPA, higher yield strength |
| Applikasjoner | Suitable for marine, kjemisk, and food industries | Used in more aggressive environments like offshore oil and gas platforms, avsaltningsplanter, og kjemisk prosessering |
| Koste | Less expensive compared to super duplex | More expensive due to higher alloy content |
Super duplex rustfritt stål
Super duplex stainless steel, slik som Karakter 2507, contains higher levels of krom, Molybden, og nitrogen compared to duplex stainless steel.
These additional elements improve its resistance to extreme environments, particularly in highly corrosive and high-pressure applications.
Super duplex steels offer Overlegen korrosjonsmotstand, especially in environments containing chloride and acidic substances.
They are used in more demanding industries such as offshore oil and gas platforms, avsaltningsplanter, and chemical reactors, where harsh conditions require a stronger, more corrosion-resistant material.
8. Konklusjon
Dupleks stainless steel casting provides a robust solution for industries that require materials with superior mechanical properties, high corrosion resistance, og holdbarhet.
Its unique combination of austenitic and ferritic phases offers numerous advantages, including enhanced strength and resistance to stress corrosion cracking.
Understanding the casting process, Fordeler, and applications of duplex stainless steel will help ensure the right material is selected for your next project, maximizing both performance and cost-efficiency.
If you have any Duplex Stainless Steel processing needs, Ta gjerne Kontakt oss.
Additional Content
Key Alloying Elements in Duplex Stainless Steel
Krom
To form a stable chromium oxide passive film that protects against atmospheric corrosion, steel must contain at least 10.5% krom. Increasing chromium content enhances the corrosion resistance of stainless steel.
Chromium promotes the formation of body-centered cubic (BCC) ferritt, a ferrite-forming element. Higher chromium levels require more nickel to achieve austenite or duplex (ferrite-austenite) structures.
High chromium content also encourages the formation of intermetallic phases. Austenitic stainless steels typically have at least 16% krom, while duplex stainless steels have at least 20%.
Chromium also improves high-temperature oxidation resistance, crucial for forming and removing oxide scales or temper colors after heat treatment or welding.
Pickling and removing temper colors are more challenging for duplex stainless steel compared to austenitic stainless steel.
Molybden
Molybdenum significantly enhances pitting and crevice corrosion resistance in stainless steel. In chloride environments, molybdenum is three times more effective than chromium when the steel contains at least 18% krom.
Molybden, a ferrite-forming element, also increases the tendency to form intermetallic phases.
Derfor, austenitic stainless steels usually contain less than 7.5% Molybden, while duplex stainless steels contain less than 4%.
Nitrogen
Nitrogen boosts the pitting and crevice corrosion resistance of austenitic and duplex stainless steels and significantly increases their strength.
It is the most effective solid-solution strengthening element and a low-cost alloying element.
The improved toughness of nitrogen-containing duplex stainless steel results from higher austenite content and reduced intermetallic phase formation.
Although nitrogen does not prevent intermetallic phase precipitation, it delays it, allowing sufficient time for processing and fabrication.
Nitrogen is added to high-corrosion-resistant austenitic and duplex stainless steels with high chromium and molybdenum content to counteract the tendency to form σ phase.
Nitrogen, a strong austenite-forming element, can replace some nickel in austenitic stainless steels.
It reduces stacking fault energy and increases the work-hardening rate of austenite.
It also enhances austenite strength through solid-solution strengthening.
Duplex stainless steels typically contain nitrogen and have their nickel content adjusted to achieve the appropriate phase balance.
Balancing ferrite-forming elements (chromium and molybdenum) with austenite-forming elements (nickel and nitrogen) is essential for achieving a duplex structure.
Nikkel
Nickel stabilizes austenite, promoting the transformation of the crystal structure from body-centered cubic (BCC) ferrite to face-centered cubic (FCC) Austenitt.
Ferritic stainless steels contain little to no nickel, while duplex stainless steels have a low to moderate nickel content, vanligvis 1.5% til 7%.
Austenitic stainless steels in the 300 series contain at least 6% nikkel.
Adding nickel delays the formation of harmful intermetallic phases in austenitic stainless steels, though this effect is less significant in duplex stainless steels compared to nitrogen.
The face-centered cubic (FCC) structure gives austenitic stainless steel excellent toughness.
Since nearly half of the structure in duplex stainless steel is austenite, duplex steel is significantly tougher than ferritic stainless steel.
