What Is a Super Austenitic Steel?
Stainless steels are grouped by microstructure — ferritic, martensitic, duplex, and austenitic — but within the austenitic family there is an important sub-classification: super austenitic. The term is not a formal standard designation but a widely accepted engineering shorthand for austenitic grades whose alloy content — particularly chromium, molybdenum, and nitrogen — is high enough to deliver corrosion resistance that standard grades like 304 and 316L cannot approach.
The threshold most metallurgists use is a PREN (Pitting Resistance Equivalent Number) ≥ 40. Grades that clear this bar include 1.4547 (254 SMO), 1.4529 (Alloy 926), and the subject of this article: 1.4565, designated X2CrNiMnMoN25-18-6-5 under EN 10088, UNS S34565 under the ASTM system, and commercially marketed as Alloy 24. Under ASTM A182 for forged products it is specified as Grade F65.
EN (Europe): 1.4565 / X2CrNiMnMoN25-18-6-5 (EN 10088-2, EN 10088-3)
ASTM/UNS (North America): UNS S34565 / Alloy 24 / F65 (ASTM A182)
Other standards: NORSOK M-630, VdTÜV Werkstoffblatt 555
What makes 1.4565 distinctive within this class is a compositional philosophy built on three synergistic alloying elements: molybdenum (Mo), nitrogen (N), and manganese (Mn). Each plays a distinct electrochemical and microstructural role, and together they achieve a PREN value comfortably above 40 — making this one of the most corrosion-resistant commercially available stainless steels produced through conventional steelmaking.
The PREN Framework Explained
The Pitting Resistance Equivalent Number (PREN) is the standard index engineers use to rank stainless steels by their resistance to chloride-induced pitting in a single comparable figure.
PREN = %Cr + 3.3 × %Mo + 16 × %N
For 1.4565 at mid-range composition:
PREN = 25 + 3.3 × 4.5 + 16 × 0.45
= 25.00 + 14.85 + 7.20
= 47.05 ← Above the 40-point threshold
Three observations critical for material selection:
- Chromium provides the largest absolute contribution, elevated at 24–26% in 1.4565.
- Molybdenum carries a 3.3× weighting factor — each 1% Mo contributes 3.3 chromium-equivalent PREN points.
- Nitrogen carries a 16× weighting factor — the most powerful PREN contributor per unit weight. Just 0.45% N adds 7.2 PREN points.
PREN is a predictive index, not an absolute guarantee. Real-world performance also depends on surface finish, chloride concentration, temperature, pH, and crevice geometry. A PREN difference of ≥ 5 points between two grades under equivalent conditions is generally considered metallurgically significant and reliably predictive.
Molybdenum: The Passive-Film Fortifier
Stainless steel's corrosion resistance depends on a nanometer-thin chromium oxide passive film. Chloride ions (Cl⁻) can penetrate it at defect sites and initiate pitting. Molybdenum stabilizes this passive film against chloride attack through two mechanisms:
1. Molybdate Re-Passivation (MoO₄²⁻)
When the passive film is breached, dissolved molybdenum forms molybdate anions (MoO₄²⁻) that adsorb at pit initiation sites, blocking active dissolution and triggering re-passivation before a stable pit can establish itself.
2. Raising the Critical Pitting Temperature (CPT)
In ASTM G48 Method C testing (6% FeCl₃ solution):
- 316L: CPT ≈ +15°C to +25°C
- 904L: CPT ≈ +35°C to +45°C
- 1.4565 (4–5% Mo): CPT > +60°C
EN 10088 mandates 4.0–5.0% Mo in 1.4565, versus 2.0–2.5% Mo in 316L — roughly 2× the molybdenum. This difference alone contributes an additional 5–9 PREN points.
Nitrogen: The Dual-Action Alloying Element
Nitrogen simultaneously improves two properties typically in tension: corrosion resistance and mechanical strength — with no meaningful loss of ductility or toughness.
Nitrogen and Corrosion Resistance
Inside an initiating pit, hydrolysis reactions produce H⁺ ions, acidifying the local electrolyte. Nitrogen reacts to form ammonium ions (NH₄⁺) inside the pit, buffering the local pH and interrupting this acidification cycle. This explains the ×16 PREN weighting. Nitrogen also enriches the passive film with oxynitride species that resist chloride displacement.
Nitrogen and Mechanical Strength
The 0.3–0.6% N content elevates 0.2% proof strength (Rp0.2) to ≥ 420 MPa in solution-annealed condition, versus approximately 220 MPa for 316L — enabling thinner wall sections and higher design stresses.
At 0.45% N, nitrogen contributes 7.2 PREN points while adding approximately 150–200 MPa yield strength over a comparable N-free grade — with no adverse effects on toughness.
Manganese: Enabling High Nitrogen Without Gas Porosity
Nitrogen solubility in liquid steel is limited at atmospheric pressure. Exceeding this limit causes N₂ gas porosity during solidification, making forgings unusable. Manganese significantly increases nitrogen solubility in the austenitic melt — approximately 0.04–0.05 wt% per 1% Mn increase. At 5–7% Mn, this enables stable dissolution of 0.4–0.6% N during conventional EAF + LF + VD atmospheric melting, without expensive pressurized remelting (PESR) equipment.
This is why 1.4565 achieves super austenitic performance at a cost point accessible to industrial applications.
Manganese as an Austenite Stabilizer
Manganese also partially substitutes for nickel as an austenite stabilizer. The combination of 16–19% Ni and 5–7% Mn provides a thermodynamically stable, fully austenitic microstructure resistant to strain-induced martensitic transformation — critical for non-magnetic requirements in valve and pump components.
Passive film fortifier via MoO₄²⁻ re-passivation. Raises CPT to >+60°C. PREN: ×3.3 per % Mo.
Buffers pit acidification via NH₄⁺. Oxynitride film enrichment. Adds ~150–200 MPa yield strength. PREN: ×16 per % N.
Enables 0.4–0.6% N via increased melt solubility at atmospheric pressure. Austenite stabilizer, partially substituting Ni.
Grade Comparison: 1.4565 vs. 316L, 904L, 2205, and 2507
| Grade | EN No. | Cr % | Mo % | N % | Ni % | PREN* | Structure |
|---|---|---|---|---|---|---|---|
| 316L | 1.4404 | 17 | 2.2 | 0.05 | 12 | 25 | Austenitic |
| 904L | 1.4539 | 21 | 4.5 | 0.05 | 25 | 36 | Austenitic |
| 2205 Duplex | 1.4462 | 22 | 3.1 | 0.17 | 5.5 | 35 | Duplex |
| 2507 S.Duplex | 1.4410 | 25 | 3.9 | 0.27 | 7 | 42 | Super Duplex |
| 1.4565 / Alloy 24 † | 1.4565 | 25 | 4.5 | 0.45 | 17.5 | 47 | Super Austenitic |
1.4565 and 2507 super duplex overlap in PREN range but serve different needs. The fully austenitic microstructure of 1.4565 is preferable where non-magnetic properties, SCC resistance in hot chloride, or cryogenic toughness is required. Super duplex grades suit applications prioritising high strength combined with pitting resistance at temperatures below ~260°C.
Corrosion Mechanisms That 1.4565 Is Engineered to Resist
Pitting Corrosion
The primary threat in most 1.4565 applications. PREN > 40 prevents stable pit formation across the temperature and chloride concentration ranges where 316L routinely fails.
Crevice Corrosion
In crevice geometries — gasket faces, threaded connections, flange overlaps — oxygen depletion creates an aggressive acidified electrolyte. 1.4565's high Mo raises the Critical Crevice Temperature (CCT) significantly above 316L and 904L in ASTM G48 Method B testing.
Stress Corrosion Cracking (SCC)
Fully austenitic grades are generally more SCC-resistant than duplex grades above 100°C in hot chloride. 1.4565's austenitic microstructure provides reliable SCC resistance in moderately aggressive conditions.
Intergranular Corrosion (IGC)
The "X2" designation specifies maximum 0.03% carbon, preventing chromium carbide precipitation at grain boundaries during welding heat cycles. 1.4565 is inherently resistant to IGC without requiring post-weld solution annealing in most fabrication scenarios.
Industries and Applications for 1.4565 Forged Parts
In all these environments, the common requirement is resistance to chloride-driven localized corrosion at moderate-to-elevated temperatures — exactly the performance niche where 1.4565 excels. If you are sourcing components for any of these industries, view our 1.4565 forged parts for oil & gas, FGD, desalination, and chemical processing to see available product forms, weight range, and certification options.
Forging Considerations for 1.4565
Elevated Flow Stress
High interstitial nitrogen increases lattice friction stress, requiring higher press tonnage per unit cross-section than 316L or 904L. Forge shops rated only for standard austenitic grades may be unable to fully consolidate large 1.4565 components.
Narrow Forging Temperature Window
Recommended range: 1,100°C to 1,200°C. Above 1,220°C, incipient melting can initiate. Below 1,050°C, work-hardening and surface cracking risk increase. Large components require continuous pyrometer monitoring and multiple reheating cycles.
Mandatory Solution Annealing
Post-forging solution annealing at 1,100–1,150°C with rapid water quench is non-negotiable. 1.4565's high Cr and Mo content makes it more susceptible to sigma phase formation than 316L. Sigma phase drastically reduces toughness and corrosion resistance.
Verify press capacity, furnace dimensions relative to component size, and quench tank depth. Ensure the manufacturer can supply heat-specific EN 10204 3.1 (or 3.2 if required) MTC with full traceability from melt to finished part. NDT reports (UT per EN 10228-3, PT/MT per EN 10228-1/2) should be provided per the agreed inspection scope.
Jiangsu Liangyi Co., Limited has manufactured 1.4565 open die forgings and seamless rolled rings since 1997, with press capacity up to 6,300T and furnaces up to 18 m length for full solution annealing. For technical specifications, available product forms, and weight range, visit our 1.4565 (X2CrNiMnMoN25-18-6-5) open die forgings and seamless rolled rings page.
Summary: The Three-Element Formula Behind Super Austenitic Performance
Manganese (5–7%): Enables high nitrogen content through melt solubility enhancement at atmospheric pressure — making super austenitic performance achievable through standard production routes.
Together, these three elements produce a PREN consistently in the 43–50 range — well above the 40-point super austenitic threshold and approximately double 316L's ceiling of PREN ≈ 25. For engineers specifying forged components in aggressive chloride environments, 1.4565 / Alloy 24 represents the most corrosion-resistant fully austenitic stainless steel achievable through conventional steelmaking.
Custom open die forgings, seamless rolled rings, bars and precision-machined components. EN 10204 3.1/3.2 MTC supplied per order. Founded 1997 · ISO 9001:2015 certified · Free technical review and quotation within 24 hours.
Frequently Asked Questions About 1.4565 Steel
What is 1.4565 steel and what are its equivalent designations?
1.4565 is a super austenitic stainless steel with EN chemical designation X2CrNiMnMoN25-18-6-5 (EN 10088). The North American equivalent is UNS S34565, marketed as Alloy 24. Under ASTM A182 for forged products it is Grade F65.
What PREN value does 1.4565 achieve?
At mid-range composition, 1.4565 achieves PREN ≈ 47 (= 25 + 3.3×4.5 + 16×0.45). This exceeds the 40-point super austenitic threshold and is approximately double the PREN of 316L (≈ 25).
Why does 1.4565 contain 5–7% manganese?
Manganese increases nitrogen solubility in the liquid steel melt during atmospheric melting. Without sufficient Mn, dissolving 0.4–0.6% nitrogen would cause N₂ gas porosity during solidification. High Mn enables stable high-nitrogen content without expensive pressurized remelting equipment.
How does 1.4565 compare to 316L stainless steel?
Key differences: PREN 47 vs 25; Mo 4.5% vs 2.2%; N 0.45% vs 0.05%; CPT above +60°C vs +15–25°C; yield strength ≥420 MPa vs ~220 MPa. In high-chloride environments 316L typically fails by pitting while 1.4565 provides multi-year service life.
Is 1.4565 the same as Alloy 24 and UNS S34565?
Yes. 1.4565 (EN), Alloy 24 (trade name), and UNS S34565 (ASTM/UNS) all refer to the same super austenitic stainless steel X2CrNiMnMoN25-18-6-5.
What certifications does Jiangsu Liangyi hold?
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 certification. EN 10204 3.1 MTC is supplied per order; 3.2 with third-party countersignature available on request. NDT reports per agreed inspection scope.
What are the key forging challenges for 1.4565?
Three main challenges: (1) elevated flow stress from high nitrogen requires higher press tonnage; (2) narrow forging window of 1,100–1,200°C requires continuous monitoring; (3) mandatory solution annealing at 1,100–1,150°C with rapid water quench to dissolve sigma phase.