What is UNS S32550 Seamless Pipe?
Seamless Super Duplex Steel UNS S32550 Pipe (also known as Ferralium 255 seamless pipe) is a high-strength, corrosion-resistant stainless steel pipe manufactured from a solid billet without welding seams.
The seamless production process eliminates longitudinal weld defects, ensuring superior pressure resistance, structural integrity, and reliability in critical industrial applications.
UNS S32550 is classified as a Super Duplex Stainless Steel, featuring a balanced austenitic-ferritic (50:50) microstructure, combining:
- Themechanical strength of ferritic steel
- Thecorrosion resistance of austenitic stainless steel
Key Advantages of UNS S32550 Super Duplex Steel Pipe
1. Superior Corrosion Resistance
- PREN ≥ 41 (Pitting Resistance Equivalent Number)
- Excellent resistance to:
- Pitting corrosion
- Crevice corrosion
- Stress Corrosion Cracking (SCC)
- Ideal for chloride-rich and seawater environments
2.High Mechanical Strength
- Yield strength ≈ 2× higher than 316L stainless steel
- Enables:
- Reduced wall thickness
- Lower system weight
- Cost-efficient designs
3. High Pressure Capability
- Seamless construction supports higher allowable stress values
- Complies with ASME B31.3 piping design code
4. Excellent Erosion Resistance
- High hardness and microstructural stability
Product Specifications and Data
To provide a comprehensive overview of this high-performance material, here are the detailed technical specifications and performance data for Super Duplex UNS S32550.
The composition of UNS S32550 is carefully balanced to achieve its superior corrosion resistance and high strength.
UNS S32550 is specifically designed to maintain its strength and shape in high-stress, corrosive environments.
These physical properties highlight the material’s ability to perform reliably under a wide range of conditions.
| NPS Designator (Inches) | Permissible Variations in Outside Diameter (Inches) |
| 1⁄8 to 1 1⁄2, inclusive | Over: 1⁄64 in. (0.015 in.) |
| Under: 1⁄32 in. (0.031 in.) | |
| Over 1 1⁄2 to 4, inclusive | Over: 1⁄32 in. (0.031 in.) |
| Under: 1⁄32 in. (0.031 in.) | |
| Over 4 to 8, inclusive | Over: 1⁄16 in. (0.062 in.) |
| Under: 1⁄32 in. (0.031 in.) | |
| Over 8 to 18, inclusive | Over: 3⁄32 in. (0.093 in.) |
| Under: 1⁄32 in. (0.031 in.) | |
| Over 18 to 26, inclusive | Over: 1⁄8 in. (0.125 in.) |
| Under: 1⁄32 in. (0.031 in.) | |
| Over 26 to 34, inclusive | Over: 5⁄32 in. (0.156 in.) |
| Under: 1⁄32 in. (0.031 in.) | |
| Over 34 to 48, inclusive | Over: 3⁄16 in. (0.187 in.) |
| Under: 1⁄32 in. (0.031 in.) |
Wall Thickness (WT) Tolerance
The wall thickness tolerances are defined by the nominal wall thickness (t) and the outside diameter (D).
1. Minimum Wall Thickness
The minimum wall thickness at any point on the pipe shall not be more than {12.5%} under the nominal (specified) wall thickness.
2. Maximum Wall Thickness (Percentage Over Nominal)
The maximum allowable wall thickness variation depends on the size and the t/D ratio (ratio of nominal wall thickness to ordered outside diameter):
| NPS Designator (Inches) | t/D Ratio | Permissible Variation (Over Nominal WT) |
| 1/8 to 2, inclusive | All ratios | +20.0% |
| 3 to 18, inclusive | t/D ≤ 5% | +22.5% |
| 3 to 18, inclusive | t/D >5% | +15.0% |
| 20 and larger (Seamless) | t/D ≤ 5% | +22.5% |
| 20 and larger (Seamless) | t/D >5% | +15.0% |
| 20 and larger (Welded) | All ratios | +17.5% |
Length Tolerance
For pipe ordered to a Specific Cut Length, the standard tolerance is +1⁄8 in. [3.2 mm], -0 in. [0 mm].
Pipe is typically supplied in Single Random Lengths (17 to 24 ft) or Double Random Lengths (38 to 40 ft), unless otherwise specified.
Disclaimer: These tolerances are based on the common requirements found in ASTM A790, primarily through its reference to ASTM A999. The full and most current specification document must always be consulted for official design and procurement purposes.
Manufacturing Process of UNS S32550 Seamless Pipe
The production process follows strict metallurgical control:
- Hot Piercing – converting solid billet into hollow shell
- Cold Drawing / Pilgering – improving dimensional accuracy and grain refinement
- Solution Annealing (1050°C – 1120°C)
- Followed by rapid quenching
- Prevents harmful phases such as sigma phase
- Inspection & Testing
- Hydrostatic Testing
- Ultrasonic Testing (UT)
- Eddy Current Testing
Industrial Applications of S32550 Super Duplex Pipe
UNS S32550 is widely used in demanding environments requiring high strength and corrosion resistance:
Oil & Gas Industry
- Subsea pipelines
- Flowlines & manifolds
- Sour gas (H₂S) service
Chemical Processing
- Acid production (nitric, sulfuric, phosphoric)
- Agrochemical systems
Marine & Desalination
- Seawater piping
- Reverse Osmosis (RO) systems
- Offshore platforms
Environmental Engineering
- Flue Gas Desulfurization (FGD) systems
- Pollution control equipment
UNS S32550 vs UNS S32750: Key Differences
Although both are Super Duplex Stainless Steels, UNS S32550 offers:
- Highercopper (Cu) content
- Improved resistance to:
- Sulfuric acid
- Phosphoric acid
- Better performance in non-oxidizing acidic environments
This makes S32550 a preferred choice for chemical processing industries
Why Choose UNS S32550 Seamless Pipe?
- Superior corrosion resistance (PREN ≥ 40)
- High strength-to-weight ratio
- Excellent performance in aggressive environments
- Cost-effective lifecycle compared to standard stainless steels
Standards & Compliance
ASTM A790 / ASME SA790
ASTM A999
NACE MR0175 (for sour service, if applicable)
