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Both F53 (UNS S32750) and F55 (UNS S32760) are super duplex stainless steels with identical yield strength, overlapping PREN values above 40, and full qualification under NACE MR0175 and NORSOK M-650. The decisive difference is alloy chemistry: F55 adds tungsten and copper, which provide measurably superior resistance to reducing acids such as dilute H₂SO₄ and HCl.
For standard offshore, seawater, and chloride service, F53 is the industry default because it is more widely stocked and typically 5–15% lower in material cost. Choose F55 only when process media includes reducing acids, acidified produced water, or when the engineering specification explicitly calls out UNS S32760.
Metric | Value |
F53 PREN (standard formula, typical) | > 41 |
F55 PREN-W (with W factor, min guaranteed) | ≥ 40 |
Yield Strength (both grades, min) | 550 MPa (80 ksi) |
Tensile Strength – F53 / F55 (min) | 795 MPa / 750 MPa |
Max Continuous Service Temperature | 300°C (572°F) |
Critical Pitting Temperature (F53 / F55) | > 50°C / > 40°C (ASTM G48-C) |
Ferrite Phase Content (target) | 40–60% (both) |
Sour-Service Qualification | NACE MR0175 / ISO 15156 (both) |
Source: ASTM A182 / A182M, EN 10088-3, NACE MR0175, Special Metals Corporation, Rolled Alloys Zeron 100 data sheet.
Super duplex stainless steel is defined by a pitting resistance equivalent number greater than 40. This microstructure is roughly 50% austenite and 50% ferrite (called a “duplex” structure), giving it a strength level approximately twice that of 316L while retaining excellent toughness.
F53 and F55 are both super duplex, meaning they clear the PREN ≥ 40 threshold comfortably. The grade designations come from ASTM A182 — the standard for forged flanges and fittings.
F53 maps to UNS S32750, widely known commercially as SAF 2507®. F55 maps to UNS S32760, the proprietary grade Zeron® 100. Both share the same European EN 1.4xxx classification family and are interchangeable in most offshore and chemical applications.
Grade Designation
Source: ASTM A182/A182M, EN 10088-3, ISO 15510, Sandvik SAF 2507 data sheet, Rolled Alloys Zeron 100 data sheet.
Standard System | F53 (Grade/Code) | F55 (Grade/Code) |
ASTM A182 (Forgings) | Grade F53 | Grade F55 |
UNS Number | S32750 | S32760 |
EN / DIN | 1.4410 | 1.4501 |
EN Steel Name | X2CrNiMoN25-7-4 | X2CrNiMoCuWN25-7-4 |
Proprietary / Trade Name | SAF 2507® (Sandvik) | Zeron® 100 (Rolled Alloys) |
Chinese GB Standard | 022Cr25Ni7Mo4N | 022Cr25Ni7Mo4WCuN |
The two grades share an almost identical elemental backbone: 24–26% chromium, 6–8% nickel, and 0.20–0.32% nitrogen. The fundamental difference is that F55 deliberately adds tungsten (W, 0.5–1.0 wt%) and copper (Cu, 0.5–1.0 wt%).
Composition | F53 (S32750) wt% | F55 (S32760) wt% | Significance |
Carbon (C) | ≤0.030 | ≤0.030 | Weld sensitization control |
Chromium (Cr) | 24.0–26.0 | 24.0–26.0 | Primary pitting/oxidation barrier |
Nickel (Ni) | 6.0–8.0 | 6.0–8.0 | Austenite stabilizer, toughness |
Molybdenum (Mo) | 3.0–5.0 | 3.0–4.0 | PREN contributor; active-passive stability |
Nitrogen (N) | 0.24–0.32 | 0.20–0.30 | Pitting resistance; austenite strengthener |
Copper (Cu) | ≤0.50 | 0.50–1.00 ★ | F55 key: reducing acid resistance |
Tungsten (W) | — | 0.50–1.00 ★ | F55 key: PREN-W enhancement |
Silicon (Si) | ≤0.80 | ≤1.00 | Deoxidizer |
Manganese (Mn) | ≤1.20 | ≤1.00 | Minor austenite stabilizer |
Source: ASTM A182/A182M Table 1; EN 10088-3; Special Metals UNS S32750 data sheet; Rolled Alloys Zeron 100 (UNS S32760) technical data sheet. ★ = key differentiating elements.
Both deliver at least 550 MPa yield strength (0.2% proof stress), which is roughly double that of 316L stainless steel. Both have a minimum tensile strength above 750 MPa, elongation at or above 25%, and the same hardness ceiling of 32 HRC.
F53 carries a slightly higher minimum tensile strength (795 MPa vs 750 MPa for F55) under ASTM A182 in the forged product form. In practice, typical values for both grades usually exceed 800 MPa in tensile strength and 600 MPa in yield. Both grades cannot be strengthened through heat treatment; only cold-working raises strength above the annealed condition minimum.
Property | F53 (S32750) — Min | F55 (S32760) — Min | Standard |
0.2% Proof Strength (Yield) | 550 MPa (80 ksi) | 550 MPa (80 ksi) | ASTM A182 |
Ultimate Tensile Strength | 795 MPa (115 ksi) | 750 MPa (109 ksi) | ASTM A182 |
Elongation (A50 or A4) | ≥25% | ≥25% | ASTM A182 |
Reduction of Area | ≥45% | ≥45% | ASTM A182 / EN 10088 |
Hardness (max) | 32 HRC / 310 HBW | 32 HRC / 310 HBW | ASTM A182 |
Charpy Impact (0°C, transverse) | ≥27 J (typical 60–80 J) | ≥27 J (typical 60–80 J) | EN ISO 148-1 |
Elastic (Young’s) Modulus | ~196 GPa | ~196 GPa | Ref. data |
Source: ASTM A182/A182M; EN 10088-3; Special Metals UNS S32750 technical bulletin; Rolled Alloys Zeron 100 data sheet. Values represent specified minimums; typical production values are higher.
Physical properties — density, thermal conductivity, elastic modulus, and thermal expansion — are determined primarily by the iron-chromium-nickel matrix. Because both grades share this matrix, their physical properties are essentially identical within measurement uncertainty. Engineers designing heat exchangers, calculating thermal stresses, or sizing support structures can use the same physical data for both grades.
Physical Property | F53 (S32750) | F55 (S32760) | Unit |
Density | 7.80 | 7.80 | g/cm³ |
Elastic Modulus (20°C) | ~196 | ~196 | GPa |
Thermal Conductivity (20°C) | ~14 | ~14 | W/m·K |
Mean Thermal Expansion (20–100°C) | 13.0 | 13.0 | ×10⁻⁶ /°C |
Electrical Resistivity (20°C) | ~0.80 | ~0.80 | μΩ·m |
Specific Heat Capacity | ~480 | ~480 | J/kg·K |
Max Service Temperature | 300 | 300 | °C |
Source: Outokumpu Stainless Corrosion Handbook (10th ed.); Special Metals Corporation S32750 technical bulletin; Rolled Alloys Zeron 100 data sheet. All values are nominal/reference; actual production may vary slightly.
Both F53 and F55 clear the PREN ≥ 40 threshold that defines the super duplex category.
Corrosion Parameter | F53 (S32750) | F55 (S32760) | Remarks |
PREN (Cr + 3.3Mo + 16N) | > 41 | ≥40 (guaranteed) | Standard formula; F55 guaranteed minimum |
PREN-W (Cr + 3.3(Mo+0.5W) + 16N) | ~41–43 | ~41–42 | W-inclusive formula; both grade similar |
Critical Pitting Temp (ASTM G48-C) | > 50°C | > 40°C | In 6% FeCl₃ solution |
Critical Crevice Temp (ASTM G48-D) | > 25°C | > 25°C | Both excellent vs 316L (~10°C) |
Stress Corrosion Cracking (SCC) | Excellent | Excellent | Both far exceed 316L & duplex 2205 |
Reducing Acid (H₂SO₄, HCl) | Good | Superior | Cu addition in F55 is decisive |
Seawater / Brine (chloride) | Excellent | Excellent | Interchangeable performance |
H₂S Sour Service | Qualified | Qualified | Both per NACE MR0175 / ISO 15156 |
Oxidizing Acid / Nitric Acid | Good | Good | Not primary choice for strong oxidizers |
Source: ASTM G48 (pitting and crevice corrosion testing); NACE MR0175 / ISO 15156; Outokumpu corrosion data; Rolled Alloys Zeron 100 corrosion guide. ★ in F55 column indicates superior performance.
For procurement and compliance purposes, F53 and F55 fall under essentially the same set of codes and standards. Both carry the ASTM A182 designation (as different grades, F53 and F55), appear in EN 10088-3 under their respective EN numbers, and are fully qualified under NACE MR0175/ISO 15156 for sour service. Both are covered by NORSOK M-650, which governs material qualification for Norwegian offshore installations and is widely adopted globally.
Key procurement note: always verify that the specific form (bar, pipe, plate, forging) and corresponding product standard are cited. A flange order references ASTM A182 F53 or F55; a pipe order references ASTM A790 S32750 or S32760. These are different standards covering different product forms even for the same alloy.
Standard / Specification | F53 (S32750) | F55 (S32760) | Scope |
ASTM A182 / A182M | ✓ Grade F53 | ✓ Grade F55 | Forged fittings & flanges |
ASTM A276 / A276M | ✓ | ✓ | Stainless steel bars |
ASTM A479 / A479M | ✓ | ✓ | Stainless steel wire, bars for pressure vessels |
ASTM A240 / A240M | ✓ S32750 | ✓ S32760 | Plate, sheet, strip |
ASTM A789 / A790 | ✓ | ✓ | Seamless & welded duplex tubes |
EN 10088-3 | ✓ 1.4410 | ✓ 1.4501 | Bars, rods, wire (stainless) |
EN 10028-7 | ✓ | ✓ | Flat products for pressure vessels |
NACE MR0175 / ISO 15156 | ✓ Qualified | ✓ Qualified | Sour service (H₂S environments) |
NORSOK M-650 | ✓ Rev.5+ | ✓ Rev.5+ | Qualification of manufacturers |
ISO 15510 | ✓ | ✓ | Chemical composition of stainless steels |
Source: ASTM International (A182, A276, A479, A240, A789/A790); EN 10088-3; NACE International MR0175/ISO 15156; NORSOK M-650 Rev. 5.
Both grades must be supplied in the solution-annealed condition to achieve their specified corrosion resistance. The anneal dissolves any intermetallic phases formed during hot working and re-establishes the balanced ferrite-austenite microstructure.
F53 is typically annealed at 1020–1100°C followed by rapid water quenching. Duplex F55 requires a slightly higher anneal window of 1050–1150°C, also water quenched. The difference reflects the higher tungsten and copper content of F55, which shifts phase boundaries. Both grades must avoid the 600–1000°C danger zone where the chi phase (χ) and sigma phase (σ) precipitate rapidly, particularly at the ferrite-austenite grain boundaries.
•F53 solution anneal: 1020–1100°C, water quench (per ASTM A182 / Sandvik data)
•F55 solution anneal: 1050–1150°C, water quench (per ASTM A182 / Rolled Alloys Zeron 100)
•Sigma phase risk zone (both): 600–1000°C — never allow slow cooling through this range
•Ferrite target (both): 40–60% ferrite by volume (verify per ASTM E562 or feritscope measurement)
•Neither grade can be strengthened by heat treatment — only cold work raises strength above anneal minimum
Q1: Is F55 stronger than F53?
No. Both grades have the same minimum yield strength of 550 MPa (80 ksi) under ASTM A182. F53 has a slightly higher specified minimum tensile strength (795 MPa vs 750 MPa for F55), but in practice both grades typically produce similar tensile results. Neither is stronger in the structural design sense.
Q2: Can F53 and F55 be substituted for each other?
For most seawater and chloride service applications, yes — both grades perform equivalently. However, F55 is preferred (and sometimes required) in reducing acid environments due to its copper content. Always verify with the project engineer before substituting, particularly when a specification explicitly calls out one UNS number.
Q3: What is the PREN of F53 and F55, and how do they compare?
Using the standard PREN formula (Cr + 3.3×Mo + 16×N), F53 typically achieves PREN > 41 and F55 has a guaranteed minimum of PREN ≥ 40. When the PREN-W formula (which credits tungsten at 0.5× the Mo coefficient) is used for F55, both grades reach similar PREN-W values of 41–43, confirming equivalent pitting resistance in chloride service.
Q4: Are both grades qualified for sour service (H₂S)?
Yes. Both F53 (UNS S32750) and F55 (UNS S32760) are qualified under NACE MR0175 / ISO 15156 for use in sour service environments containing H₂S. Both are also covered by NORSOK M-650, which governs material qualification for the Norwegian Continental Shelf and many other offshore jurisdictions globally.
Q5: Why does F55 have less molybdenum than F53?
F55 deliberately limits molybdenum to a maximum of 4% (versus 5% for F53) because tungsten — added at 0.5–1.0% — provides an equivalent PREN contribution when assessed using the PREN-W formula. High combined Mo+W content can also increase sigma phase precipitation risk; capping Mo while adding W optimizes the balance between corrosion resistance and microstructural stability during welding and heat treatment.
Q6: What filler metal should I use when welding F55?
The preferred filler for welding F55 (UNS S32760) is Zeron 100X® (or equivalent) which contains W and Cu to match the parent metal’s alloy chemistry. Using standard ER2594 filler (designed for F53/2507) on F55 will deposit slightly lower corrosion resistance in the weld metal due to absent W and Cu. For critical applications, match the filler to the base metal grade; for general service, confirm acceptability with the corrosion engineer.
Q7: What is the difference between 2507 and Zeron 100?
2507 is the common commercial name for UNS S32750 (ASTM A182 F53), originally developed by Sandvik as SAF 2507®. Zeron 100® is the proprietary name for UNS S32760 (ASTM A182 F55), developed by Rolled Alloys. Both are super duplex stainless steels. The principal difference is the W and Cu additions in Zeron 100 (F55) that provide superior reducing acid resistance, as detailed throughout this article.
Q8: Which grade is more readily available?
F53 (UNS S32750 / 2507) is produced by more mills globally and is more widely held in stock by distributors. It is the industry default for offshore and chemical processing applications. F55 (UNS S32760 / Zeron 100) is available but the supply base is smaller, and delivery lead times and material premiums are typically higher. Specify F55 only when the process chemistry clearly demands it.
