ASTM A240 UNS S32550 (Alloy 255) Super Duplex Stainless Steel
ASTM A240 UNS S32550 (Alloy 255) Super Duplex Stainless Steel - Properties & Data
Complete technical data for UNS S32550 super duplex stainless steel to ASTM A240, including chemical composition, mechanical and thermophysical properties, international equivalents, and application guidance.
Hot rolling, cold rolling, forming, welding, machining
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ASTM A240 UNS S32550 Super Duplex Stainless Steel Introduction
UNS S32550, commonly known as Alloy 255, is a super duplex stainless steel covered by ASTM A240/A240M for plate, sheet, and strip. It features a balanced ferrite-austenite microstructure with additions of copper and tungsten, delivering an exceptional combination of high strength, superior resistance to pitting and crevice corrosion in chloride-rich environments, and excellent resistance to stress corrosion cracking. Typical yield strength is nearly double that of standard austenitic grades, allowing for thinner, lighter designs. The alloy is solution annealed and quenched to achieve the desired phase balance and corrosion resistance. It is widely used in aggressive chemical process, offshore oil and gas, desalination, and marine engineering applications.
ASTM A240 UNS S32550 Super Duplex Stainless Steel Chemical Composition per ASTM A240 for UNS S32550
The chemical composition of UNS S32550 as specified in ASTM A240/A240M. Single values are maximum unless a range is given. The alloy relies on chromium, molybdenum, nitrogen, copper, and tungsten for corrosion resistance and strength.
| Element | Min (%) | Max (%) | Notes |
|---|---|---|---|
| Carbon (C) | 0.04 | ||
| Manganese (Mn) | 1.50 | ||
| Silicon (Si) | 1.00 | ||
| Phosphorus (P) | 0.04 | ||
| Sulfur (S) | 0.030 | ||
| Chromium (Cr) | 24.0 | 27.0 | |
| Nickel (Ni) | 4.5 | 6.5 | |
| Molybdenum (Mo) | 2.9 | 3.9 | |
| Nitrogen (N) | 0.10 | 0.25 | |
| Copper (Cu) | 1.50 | 2.50 | |
| Tungsten (W) | 0.50 | 1.50 | |
| Iron (Fe) | Balance |
ASTM A240 UNS S32550 Super Duplex Stainless Steel Thermophysical & Electrical Properties
Typical physical properties for UNS S32550 in the solution-annealed condition. These are not specification minima or maxima but are representative values for engineering calculations.
| Property | Value | Unit | Test Condition / Temperature |
|---|---|---|---|
| Density (ρ) | 7.80 | g/cm³ | at 20 °C |
| Modulus of Elasticity (E) | 200 | GPa | at 20 °C |
| Shear Modulus (G) | 77 | GPa | calculated from E and ν |
| Poisson's Ratio (ν) | 0.30 | - | at 20 °C |
| Thermal Expansion (α), 20-100 °C | 12.5 | 10⁻⁶/°C | mean linear coefficient |
| Thermal Expansion (α), 20-200 °C | 13.0 | 10⁻⁶/°C | mean linear coefficient |
| Thermal Expansion (α), 20-300 °C | 13.5 | 10⁻⁶/°C | mean linear coefficient |
| Thermal Conductivity (λ) | 13.5 | W/m·K | at 20 °C |
| Thermal Conductivity (λ) | 16.0 | W/m·K | at 100 °C |
| Thermal Conductivity (λ) | 18.0 | W/m·K | at 200 °C |
| Specific Heat Capacity | 490 | J/kg·K | at 20 °C |
| Electrical Resistivity (ρ_e) | 0.85 | μΩ·m | at 20 °C |
ASTM A240 UNS S32550 Super Duplex Stainless Steel Mechanical Properties at Room Temperature per ASTM A240
Tensile properties for solution-annealed UNS S32550 plate, sheet, and strip as per ASTM A240. Requirements apply to thicknesses up to 5.00 in. (127 mm). The high strength allows down-gauging in many structural applications.
| Property | Standard Value | Unit | Test Condition |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 795 | MPa | Room temperature |
| Yield Strength (0.2% offset) | ≥ 550 | MPa | Room temperature |
| Elongation (A) in 2 in. (50 mm) | ≥ 15 | % | Per ASTM A240 table for thicknesses |
| Hardness | ≤ 32 HRC / 302 HBW | - | Maximum; Brinell equivalent provided |
ASTM A240 UNS S32550 Super Duplex Stainless Steel Complete Equivalent Material Standards & Substitute Grades
| Country/Region | Standard | Grade | Notes |
|---|---|---|---|
| Europe | EN 10088-2 / EN 10088-3 | 1.4507 (X2CrNiMoCuWN 25-7-4) | Super duplex with matching Cr, Ni, Mo, Cu, W, N |
| International | ISO 15510 | X2CrNiMoCuWN 25-7-4 | Chemical composition equivalent to EN 1.4507 |
| USA | ASTM A789/A789M (Seamless) & A790/A790M (Welded) | UNS S32550 | Pipe and tube standards for the same alloy |
| China | GB/T 20878 | 022Cr25Ni7Mo3WCuN | Approximate grade; verify exact composition and properties |
| Japan | JIS G 4304/G 4305 (approximate) | SUS329J4L | Similar duplex with low carbon; may differ in copper and tungsten content |
ASTM A240 UNS S32550 Super Duplex Stainless Steel Application Introduction
UNS S32550 is tailored for severe environments where high mechanical strength and resistance to localized corrosion are critical. It is employed in both product forms and as a material for fabricating components.
Product Applications: Pressure Vessels and Storage Tanks, Process Piping and Pipeline Systems, Valves, Pumps, and Fittings, Heat Exchanger Tubes and Tube Sheets, Fasteners (Bolting), Wire and Welding Consumables
Processed into products: Seawater Cooling Water Pipes and Fittings, Heat Exchanger Tubing for Sour Service, Shell and Tube Exchangers, Chemical Reactor Internals, Hydraulic and Instrumentation Tubing, Impellers and Pump Shafts, Flanges and Weld Neck Rings
Application industries: Chemical and Petrochemical Processing, Oil and Gas (Offshore and Subsea), Marine Engineering and Shipbuilding, Desalination Plants, Heat Exchanger and Condenser Equipment, Pulp and Paper Industry (Bleach Washers, Black Liquor Heaters), Flue Gas Desulfurization Systems
ASTM A240 UNS S32550 Super Duplex Stainless Steel Similar / Alternative Material Recommendations
| Country/Region | Standard | Grade | Notes |
|---|---|---|---|
| USA | ASTM A240 | UNS S32750 (2507) | Super duplex with higher Cr, Mo, N; higher strength, similar PRE |
| USA | ASTM A240 | UNS S32760 (Zeron 100) | Contains W and Cu, similar corrosion resistance to 255 |
| USA | ASTM A240 | UNS S32520 | Another 25Cr duplex often used as alternative |
| Europe | EN 10088 | 1.4410 (X2CrNiMoN 25-7-4) | Super duplex without deliberate Cu and W additions |
| Europe | EN 10088 | 1.4501 (X2CrNiMoCuWN 25-7-4) | Close composition with Cu and W, similar to 1.4507 |
Notes:
UNS S32550 is magnetic due to its ferritic phase. Standard delivery condition is solution annealed (minimum 1880 °F / 1025 °C) followed by rapid quenching. Post-weld heat treatment is typically not required for many service conditions, but adequate controls are essential to maintain phase balance and avoid intermetallic precipitation. For design data at elevated temperatures, consult the latest edition of the ASME Boiler and Pressure Vessel Code, Section II, Part D.
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