28Cr, more commonly known in the industry by its generic name Super Duplex Stainless Steel or its standard designations like UNS S32707, is a premium-grade corrosion-resistant alloy (CRA). It is characterized by a mixed microstructure of approximately 50% austenite and 50% ferrite.
This balanced structure provides a combination of extremely high strength and outstanding resistance to corrosion, making it suitable for the most aggressive environments, particularly in the oil & gas, chemical, and offshore industries.
Key Features & Advantages:
Exceptional Corrosion Resistance: Superior to standard duplex (2205) and 316L stainless steels. It offers excellent resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC).
Very High Strength: Approximately twice the yield strength of standard austenitic stainless steels (like 304/316), allowing for thinner wall designs and weight savings.
Excellent Erosion-Corrosion & Abrasion Resistance: Ideal for piping systems handling slurries or high-velocity fluids.
Good Weldability: Can be welded using suitable filler metals and procedures, though strict heat input control is required to maintain the beneficial duplex structure.
Product Parameters & Technical Data
Table 1: Typical Chemical Composition (Weight %)
| Element | Minimum | Maximum | Typical / Note |
|---|---|---|---|
| Chromium (Cr) | 26.0 | 28.0 | Primary contributor to corrosion resistance. |
| Nickel (Ni) | 4.5 | 6.0 | Stabilizes the austenite phase. |
| Molybdenum (Mo) | 3.5 | 4.5 | Enhances pitting and crevice corrosion resistance. |
| Manganese (Mn) | - | 1.50 | - |
| Nitrogen (N) | 0.30 | 0.40 | Crucial for strengthening and phase balance. |
| Copper (Cu) | 0.50 | 1.00 | Improves corrosion resistance in sulfuric acid. |
| Tungsten (W) | 0.50 | 1.00 | Further enhances pitting resistance (PREN). |
| Silicon (Si) | - | 0.70 | - |
| Phosphorus (P) | - | 0.030 | - |
| Sulfur (S) | - | 0.020 | - |
| Carbon (C) | - | 0.030 | Low carbon to prevent sensitization. |
| Iron (Fe) | Balance |
Key Parameter: PREN (Pitting Resistance Equivalent Number)
PREN = %Cr + 3.3x(%Mo + 0.5x%W) + 16x%N
For 28Cr, the PREN typically exceeds ** >41**, classifying it as a "Super Duplex" grade.
Table 2: Typical Mechanical Properties (at Room Temperature)
| Property | Value (Typical) | Standard / Condition |
|---|---|---|
| Tensile Strength (Rm) | ≥ 900 MPa (130 ksi) | ASTM A790 / A928 |
| Yield Strength (Rp 0.2) | ≥ 650 MPa (94 ksi) | ASTM A790 / A928 |
| Elongation (A5) | ≥ 15 % | ASTM A790 / A928 |
| Hardness (HBW) | ≤ 310 | Solution Annealed Condition |
Table 3: Manufacturing Standards & Specifications
Pipes and tubes made from 28Cr are manufactured according to international standards that define chemical, mechanical, and dimensional requirements.
| Product Form | Standard Number | Standard Title |
|---|---|---|
| Seamless Pipe | ASTM A790 | Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe |
| Welded Pipe | ASTM A928 | Standard Specification for Ferritic/Austenitic (Duplex) Stainless Steel Pipe Electric Fusion Welded with Addition of Filler Metal |
| Bar & Billet | ASTM A182 | Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service |
| Universal Material | UNS S32707 | Unified Numbering System designation for this specific alloy. |
| European Designation | 1.4501 | Common European (EN) material number. |
Physical Properties
Density: 7.8 g/cm³
Melting Range: ~ 1420 - 1460 °C
Thermal Expansion: ~ 13.0 x 10⁻⁶ /K (20 - 100°C)
Thermal Conductivity: ~ 15 W/m·K (at 100°C)
Modulus of Elasticity: ~ 200 GPa
Common Applications
28Cr pipes are specified for critical and severe service applications:
Oil & Gas:
Offshore subsea flow lines & risers
Downhole tubing & casing in corrosive wells
Process piping in topside facilities handling production fluids with high chloride, H₂S, and CO₂ content.
Chemical Process Industry (CPI):
Piping for aggressive acids, chlorides, and solvents.
Desalination & Seawater Systems:
High-pressure reverse osmosis (RO) pipes, seawater cooling systems.
Pollution Control & FGD Systems:
Important Considerations for Use
Fabrication & Welding: Requires strict procedures to avoid the formation of detrimental intermetallic phases (like sigma phase) that can embrittle the steel. Post-weld heat treatment may be necessary.
Cost: 28Cr is a premium material with a significantly higher cost than standard stainless steels. It is selected based on a life-cycle cost analysis where its longevity and reliability justify the initial investment.
Availability: It is a specialty grade and may not be as readily available as 316L or 2205 duplex. Lead times can be longer.
