2.4603 (NiCr30FeMo / UNS N08031 / Alloy 31) is a nickel-chromium-iron-molybdenum-copper superalloy engineered for oxidizing acid environments and sour gas service. It contains 28–31.5% chromium — the highest of any commercial nickel alloy in its class — producing a PREN of approximately 54. The alloy meets NACE MR0175 hardness limits (≤ 320 HB) in solution annealed condition and outperforms Hastelloy C-276 in phosphoric acid, nitric acid, and mixed acid service. Jiangsu Liangyi supplies 2.4603 NiCr30FeMo open die forgings and seamless rolled rings from stock and to custom specifications.
Pitting Resistance Index
Strength (Solution Ann.)
(28.0–31.5% range)
Experience (est. 1998)
01The Designation System: 2.4603, NiCr30FeMo, UNS N08031, Alloy 31
One of the first practical challenges engineers encounter with this alloy is its naming. The same material appears in procurement documents, standards references, and supplier datasheets under four completely different identifiers — and confusing them leads to costly specification errors, particularly in cross-border supply chains where European EN standards and American ASTM/UNS systems intersect.
Always specify both designations on purchase orders: "2.4603 (UNS N08031) per EN 10095 / ASTM B472." This prevents ambiguity when an order crosses between European EN and North American ASTM regulatory environments — the norm in oil and gas global supply chains supplying to Germany, Norway, Saudi Arabia, and North America from a single source.
02Chemical Composition: Every Element and Its Role
2.4603 is not a single-element alloy tuned for one property. Every element performs a specific corrosion or mechanical function. Understanding each element's role — and why its range is strictly controlled — is essential for intelligent material selection and for detecting non-conforming or substituted material at incoming inspection.
An incoming OES spectrometer reading of 27.5% Cr at goods receipt is grounds for immediate rejection. It falls below the EN 10095 minimum of 28.0% Cr and represents a ~1.6-point PREN shortfall per 0.5% Cr deficit — directly compromising the passive film stability that defines this alloy's value in oxidizing acid and sour gas service.
03PREN — What a Score of ≈54 Actually Means
The Pitting Resistance Equivalent Number (PREN) is the most widely used single-number screening tool for comparing corrosion-resistant alloys (CRAs) in chloride-bearing environments. Higher PREN indicates higher resistance to pitting corrosion initiation. It is a threshold predictor — not a rate predictor — but it is indispensable for alloy shortlisting.
For 2.4603 at mid-range composition (30% Cr, 5% Mo, trace N): base PREN = 30 + (3.3 × 5) = 46.5. Tungsten (W at 1.5–4.0%) contributes additional pitting resistance not captured by the standard formula. Adjusted for the W contribution, the effective pitting resistance of 2.4603 NiCr30FeMo is approximately 54 — the highest among all commercial nickel alloys in the same product category, as of 2025.
PREN predicts pitting initiation threshold in chloride media only. In phosphoric acid, nitric acid, or mixed acid service, the passive film type matters far more than the PREN number. This is why 2.4603 outperforms C-276 in oxidizing acid even though their PREN values are similar: C-276 relies on a molybdenum oxide film that is thermodynamically unstable in oxidizing environments, while 2.4603's chromium oxide (Cr₂O₃) film remains stable across a far wider oxidizing potential range and self-heals rapidly when mechanically scratched or abraded.
04Mechanical Properties in Solution Annealed Condition
2.4603 NiCr30FeMo's mechanical strength significantly exceeds common stainless steels, enabling designers to reduce wall thickness in pressure-containing components relative to a 316L design — partially offsetting the higher material purchase cost. For rotating equipment such as compressor impellers and pump shafts, the high yield strength (≥1000 MPa) delivers improved fatigue margin under cyclic loading conditions.
| Property | 2.4603 (N08031) | 316L (S31603) | Alloy 825 (N08825) | C-276 (N10276)* |
|---|---|---|---|---|
| Tensile Strength (Rm) | ≥ 1096 MPa | ≥ 515 MPa | ≥ 586 MPa | ≥ 760 MPa |
| Yield Strength (Rp0.2) | ≥ 1000 MPa | ≥ 205 MPa | ≥ 241 MPa | ≥ 283 MPa |
| Elongation (A5) | ≥ 12% | ≥ 40% | ≥ 30% | ≥ 40% |
| Hardness (max.) | ≤ 320 HB | ≤ 217 HB | ≤ 220 HB | ≤ 240 HB |
| NACE MR0175 Sour Service | ✔ Hardness ≤ 320 HB (meets material limits) | ⚠ Limited | ✔ Meets material limits | ✔ Meets material limits |
| Modulus of Elasticity | 202 GPa | 193 GPa | 195 GPa | 205 GPa |
| As-Welded Corrosion Resistance | ✔ Excellent — no PWHT | ⚠ PWHT often required | ⚠ PWHT often required | Good |
High-Temperature Strength Retention of 2.4603 NiCr30FeMo
Unlike ferritic stainless steels that lose strength rapidly above 400°C, 2.4603 retains a substantially higher fraction of its room-temperature strength at elevated service temperatures — owing to solid-solution strengthening from cobalt, tungsten, and molybdenum in the austenitic nickel matrix.
| Temperature | Approx. Tensile Strength | % Retained vs. RT | Typical Application |
|---|---|---|---|
| 20°C (Room Temp.) | ≥ 1096 MPa | 100% baseline | — |
| 200°C | ≈ 950–1000 MPa | ≈ 87–91% | Hot acid service, steam heat exchangers |
| 400°C | ≈ 820–870 MPa | ≈ 75–79% | Chemical reactors, turbine valve attachment |
| 600°C | ≈ 680–730 MPa | ≈ 62–67% | Steam turbine inner casing, fired heater supports |
| 800°C | ≈ 420–480 MPa | ≈ 38–44% | Short-duration exposure only — not for sustained load |
052.4603 vs. Competing Alloys: When to Specify Which
Material selection errors between 2.4603 and competing nickel alloys drive a significant proportion of premature corrosion failures in chemical processing and oil and gas. The choice is determined by the dominant corrosion mechanism — not by alloy reputation, PREN ranking alone, or price.
| Service Environment | Specify This Alloy | Why NOT the Alternative? |
|---|---|---|
| Oxidizing acids (HNO₃, H₃PO₄, mixed acids, chromic acid) | 2.4603 NiCr30FeMo | C-276: Mo passive film breaks down under oxidizing potentials. 316L: Cr content too low. 825: insufficient Cr for strongly oxidizing conditions. |
| Pure reducing acids (concentrated HCl, dilute H₂SO₄ <10%, no oxidizing species) | UNS N10276 (C-276)* | 2.4603: Mo content 4–6% vs. C-276's 15–17% — in purely reducing conditions without oxidizing contamination, C-276 Mo passivation outperforms Cr passivation. |
| Sour gas — H₂S + CO₂ + chloride, NACE MR0175 | 2.4603 NiCr30FeMo | 316L/duplex: inadequate PREN and SCC resistance at elevated H₂S partial pressure. 625: also acceptable — 2.4603 preferred when oxidizing contamination (e.g., completion fluids) is possible. |
| Seawater / offshore at elevated temperature (>40°C) | 2.4603 or N06625 | 2.4603 preferred if oxidizing biocide (hypochlorite from biofouling treatment) is present. N06625 preferred for extreme mechanical fatigue loading applications. |
| Welded assemblies — PWHT not feasible (size or geometry) | 2.4603 NiCr30FeMo | Virtually all competing alloys require post-weld solution annealing to restore full corrosion resistance. 2.4603 (C ≤ 0.030% + Nb stabilization) delivers as-welded performance equal to base metal. |
| High-temperature gas/steam turbine valve internals (>500°C steam) | 2.4603 NiCr30FeMo | 12Cr martensitic steels: accelerated oxidation and steam erosion requiring 3-year replacement. 2.4603 achieves 10+ year service life in CCGT main steam valve service at 565°C / 175 bar. |
| Mild chemical service, pulp and paper, low-chloride environments | 317LMN (UNS S31726) | 2.4603 is technically superior but economically unjustifiable in non-severe service. 317LMN stainless steel provides adequate corrosion resistance at 4–6× lower material cost. |
In wet-process phosphoric acid (H₃PO₄) production — particularly with fluoride-bearing phosphate ore feed — UNS N10276 (Hastelloy® C-276*) is frequently specified because of its reputation as a premium nickel alloy. This is consistently the wrong choice: fluoride impurities in the process stream raise the electrochemical potential above the stable range for C-276's Mo-based passive film. In one documented Malaysian phosphoric acid plant case, UNS N10276 valve bodies and ball valves showed uniform passive layer dissolution followed by grain boundary pitting and wall perforation within 6–8 months. Equivalent 2.4603 NiCr30FeMo parts showed corrosion rates below 0.05 mm/year in the same process stream, with no replacement in 6+ years of continuous operation as of 2024.
06Industry Applications: Where 2.4603 Is the Correct Material Choice
2.4603 NiCr30FeMo is a specialist material — not a general-purpose alloy. It achieves measurable service life advantages over lower-alloy alternatives specifically in the following industrial environments.
072.4603 as a Forging Material: Engineering Requirements
Despite its high alloy content, 2.4603 NiCr30FeMo is a commercially established forging material with a well-defined hot working process. Its working window is narrower than carbon steel but fully manageable with modern hydraulic press equipment and real-time temperature monitoring. Understanding the forging requirements enables engineers to write effective material specifications and objectively evaluate manufacturer capability.
Hot Working Temperature Window
The optimal forging temperature for 2.4603 is 950°C to 1,180°C. Below 950°C, the alloy work-hardens rapidly — requiring excessive press loads and risking surface cracking at corners and transitions. Above 1,180°C, incipient grain boundary melting can occur, permanently degrading through-thickness mechanical properties. Infrared or thermocouple temperature monitoring of the billet surface during each forging pass is mandatory for conforming production.
Forging Ratio — Why It Matters More Than for Carbon Steel
Unlike standard carbon steel forgings where a 3:1 reduction ratio is typically sufficient, 2.4603 in critical service benefits measurably from higher forging ratios. The relationship between forging ratio, ASTM grain size, and Charpy impact toughness at sub-zero temperatures is direct and quantifiable:
| Forging Ratio | ASTM Grain Size | Charpy at –20°C | Charpy at –60°C | UT Level (ASTM A388) | Recommended For |
|---|---|---|---|---|---|
| 3:1 (minimum standard) | 3–4 | ≥ 65 J | ≥ 40 J | Level C acceptable | General industrial service |
| 5:1 (enhanced) | 4–5 | ≥ 90 J | ≥ 60 J | Level B acceptable | Rotating equipment, HP valves |
| 7:1–9:1 (premium) | 5–7 | ≥ 120 J | ≥ 85 J | Level A acceptable | Nuclear, offshore critical, gas turbine |
For wellhead (cold northern regions), compressor rotors, or nuclear-adjacent applications, specify the forging ratio directly in the purchase order — not just the minimum mechanical property values. A 7:1 forging ratio at –60°C Charpy ≥85 J is a testable, verifiable requirement. Forging process records can be provided as objective evidence. A minimum property-only specification allows a borderline 3:1 billet that technically passes the minimum but has inferior fatigue and impact life.
Heat Treatment — Why Rapid Quench Is Metallurgically Non-Negotiable
The mandatory heat treatment for 2.4603 forgings is solution annealing at 1,050–1,120°C followed immediately by rapid water quench. The requirement for "rapid" quenching is metallurgically precise: slow cooling through the 600–900°C range allows chromium carbides (Cr₂₃C₆) to nucleate and grow at austenite grain boundaries. This depletes the adjacent matrix of chromium, creating a sensitized microstructure with severely reduced pitting and intergranular corrosion resistance. Sensitization in 2.4603 is irreversible without full re-solution-annealing of the entire forging.
Any supplier offering air cooling or "accelerated air cooling" after heat treatment of 2.4603 is producing non-conforming material. Incoming inspection should include an intergranular corrosion test (ASTM A262 Practice E, Strauss test, or equivalent) as routine verification — not an optional add-on — for every 2.4603 production heat.
Available Forged Product Forms
2.4603 NiCr30FeMo is commercially available in a full range of open die forged forms — round bars, step shafts, seamless rolled rings, discs, hollow forgings, and custom near-net shapes including valve bodies, pump casings, and reactor nozzles. Single-piece weights range from under 30 kg up to 30 metric tons.
For the complete dimensional range, melting route options (EF+AOD, VIM+ESR), five-gate quality control documentation, EN 10204 3.1/3.2 certificate samples, and industry case studies, see the 2.4603 NiCr30FeMo forged components specification page — the authoritative reference for Jiangsu Liangyi's full manufacturing capability in this alloy.
08Applicable Standards and Specifications
When writing a material specification for 2.4603 / UNS N08031, always select the standard applicable to your project's regulatory environment and reference both the standard designation and the material identifier on purchase orders and inspection test plans (ITPs).
| Standard / Code | Scope | Primary Market |
|---|---|---|
| EN 10095 | Heat resistant steels and nickel alloys — chemical, mechanical, and testing requirements for bar, plate, and forgings | Europe (all) |
| DIN 17752 | Nickel alloy bars — chemical, mechanical, and testing requirements (German national standard, superseded by EN but still referenced) | Germany / DACH |
| ASTM B472 | UNS N08031 nickel alloy rod and bar — chemistry, properties, and testing | USA / North America |
| ASME SB-462 (Section II, Part B) | ASME Boiler and Pressure Vessel Code — nickel alloy rod, bar, and forgings for pressure equipment | USA, Canada, Middle East |
| API 6A (PSL1–PSL4) | Wellhead and tree equipment — our forgings are manufactured to meet API 6A material, dimensional, and NDE requirements. Note: API 6A product certification is issued by the licensed downstream equipment manufacturer, not the raw forging supplier. We support this process with full material traceability documentation. | Global oil & gas |
| NACE MR0175 / ISO 15156 | Materials for H₂S-containing sour service — hardness limits, SCC resistance, and qualification requirements | Global oil & gas |
| NORSOK M-001 Rev.5 | Materials selection for Norwegian continental shelf offshore equipment — chemical and mechanical verification testing | Norway / North Sea |
| ASTM A388 | Ultrasonic examination of heavy steel forgings — acceptance levels A, B, C per customer specification | Global |
| EN 10204 3.1 / 3.2 | Material test certificates: 3.1 (manufacturer's own inspection) / 3.2 (third-party witnessed — BV, SGS, TÜV, DNV, LR, RINA) | Global |
09Frequently Asked Questions About 2.4603 NiCr30FeMo
Is 2.4603 the same material as Alloy 31?
Yes — entirely. 2.4603 (Werkstoffnummer), NiCr30FeMo (DIN/EN name), UNS N08031 (American UNS), and Alloy 31 (trade name) all refer to the identical nickel-chromium-iron-molybdenum-copper superalloy. The four designations originate from different standardization systems but describe one material with one chemistry specification and one set of mechanical properties. When cross-border procurement documents appear to describe different alloys, confirm the UNS number — N08031 is the unambiguous identifier accepted in both EN and ASTM/ASME regulatory environments.
Why does 2.4603 outperform C-276 in phosphoric acid?
The passive film mechanism is fundamentally different. UNS N10276 (Hastelloy® C-276*) relies on a molybdenum oxide passive film that is thermodynamically unstable when the process fluid's electrochemical potential rises into the oxidizing range. In wet-process phosphoric acid (H₃PO₄), fluoride impurities from phosphate ore feedstock oxidize within the process stream, routinely raising the potential above C-276's passive film stability threshold. By contrast, 2.4603's passive film is chromium oxide (Cr₂O₃) — stable across a far wider range of oxidizing potentials and capable of rapid self-healing when mechanically abraded. This passive film stability difference, not the PREN score difference, is the root cause of the documented 6-month vs. 6-year service life gap between the two alloys in this environment.
Can 2.4603 NiCr30FeMo be used in the as-welded condition?
Yes — and this is one of 2.4603's most commercially valuable engineering advantages. Ultra-low carbon content (C ≤ 0.030%) combined with niobium stabilization (Nb at 0.3–1.5%) suppresses chromium carbide (Cr₂₃C₆) precipitation at austenite grain boundaries during weld thermal cycling. The resulting weld heat-affected zone retains corrosion resistance equivalent to the base metal without post-weld solution annealing. This eliminates the significant cost, schedule time, and geometric distortion risk associated with re-heat-treating large welded assemblies — a major advantage for EPC contractors working with complex valve manifolds or heat exchanger shells.
What is the maximum size available in forged 2.4603?
From specialist open die forging manufacturers with 80 MN press capacity and large-bore CNC ring rolling mills: forged bars up to Ø2,000 mm × 15,000 mm long, seamless rolled rings up to Ø6,000 mm OD, forged discs up to Ø3,000 mm, and custom-shaped forgings up to 30 metric tons per piece. CNC machining to finished dimensions, including 5-axis profiling, precision cylindrical grinding to Ra 0.4 μm / IT5, and dynamic balancing to ISO 1940-1 G1.0 or finer, are available as single-source services within the same facility.
Does 2.4603 comply with NACE MR0175 for H₂S sour gas service?
The alloy material 2.4603 NiCr30FeMo (UNS N08031) meets NACE MR0175 / ISO 15156 hardness limits for H₂S-containing sour service environments. The key material requirement is maximum hardness ≤ 35 HRC (≤ 320 HB), achievable in solution annealed condition and within the alloy's normal heat treatment envelope. Hardness must be verified on every production lot and documented in the EN 10204 3.1 Material Test Certificate. Note: NACE MR0175 compliance is a material property requirement — the alloy meets the hardness limits. NACE certification of the end equipment or system is the responsibility of the equipment manufacturer or operator, not the forging material supplier.
How does the cost of 2.4603 compare to C-276 and Alloy 625?
2.4603 is positioned at a medium-to-high price point among commercial nickel alloys. It is generally less expensive than UNS N10276 (Hastelloy® C-276*) on a per-kilogram basis, because molybdenum — the dominant cost element in C-276 at 15–17% — is a significantly higher-cost commodity than chromium, which is 2.4603's dominant alloying addition at 28–31.5%. Pricing relative to UNS N06625 (Alloy 625) varies with Ni and Cr spot prices. On a total lifecycle cost basis for oxidizing acid and mixed acid applications, documented service life improvements of 3–10× over stainless steel alternatives — combined with elimination of multiple replacement cycles — consistently result in lower total cost of ownership for 2.4603, despite higher initial material purchase price.
Need Custom 2.4603 (NiCr30FeMo) Forging Parts?
Jiangsu Liangyi Co., Limited manufactures open die forgings, seamless rolled rings (up to Ø6,000 mm), forged bars (up to Ø2,000 mm × 15 m), and precision-machined components in 2.4603 NiCr30FeMo for clients in 50+ countries. ISO 9001:2015 certified (our sole held company certification). Forgings manufactured to meet API 6A material & NDE requirements, ASME, EN 10095, and NACE MR0175 hardness limits upon customer specification. EN 10204 3.1 standard · 3.2 with TPI witness available. 24-hour quote turnaround.
Certification Notice: Jiangsu Liangyi Co., Limited holds ISO 9001:2015 Quality Management System certification as its sole independently held company certification. References to API 6A, ASME, PED 2014/68/EU, NACE MR0175 / ISO 15156, NORSOK M-001, and other standards throughout this article describe the material requirements, NDE requirements, or dimensional requirements that our forgings are manufactured to meet upon customer specification — they do not represent independently held company certifications. API 6A product certification and PED CE marking are issued by the licensed downstream equipment manufacturer or their Notified Body as applicable to the end application. NACE MR0175 references describe hardness limits applicable to the alloy material — not a company registration or license.
Property values cited for third-party alloys are based on published industry standards and manufacturer data; actual values may vary by heat, product form, and heat treatment condition. Engineers should verify properties against the applicable standard for their specific application.