Material Identity: What Is 2.4606?
2.4606 is the European EN/DIN material number for Nickel Alloy 686 (UNS N06686, Alloy 686, NiCr21Mo16W). It is a single-phase, solid-solution-strengthened, austenitic Ni-Cr-Mo-W superalloy with the highest combined Cr+Mo+W content of any commercially established nickel alloy.
Unlike precipitation-hardened nickel alloys that achieve strength through dispersed secondary phases, 2.4606 derives its properties entirely from a face-centered cubic (FCC) austenitic matrix loaded with chromium, molybdenum, and tungsten in solid solution. This single-phase structure simultaneously delivers extraordinary corrosion resistance and excellent weldability without post-weld heat treatment in most applications.
2.4606, N06686, Alloy 686, Alloy 686, and NiCr21Mo16W are all designations for the same alloy chemistry. Always cross-reference the governing standard (ASTM, DIN, or ASME) to confirm the applicable product form and heat treatment condition required.
Developed by Special Metals Corporation, this alloy was purpose-engineered to outperform C-276 (2.4819) in aggressive mixed-acid and halide environments. The key innovation: a combined Cr+Mo+W of approximately 40–41 wt% — the highest among all commercially established Ni-Cr-Mo wrought alloys — sufficient to suppress pitting and crevice corrosion in chloride-saturated acidic media at elevated temperatures where competing alloys fail.
Chemical Composition of 2.4606 (N06686 / Alloy 686)
Per ASTM B574 / DIN 17750, 2.4606 composition is: Ni ≥57% (balance), Cr 19–23%, Mo 15–17%, W 3.0–4.4%, Fe ≤5.0%, C ≤0.010%. Combined Cr+Mo+W ≈ 40–41 wt%.
| Element | Symbol | Min % | Max % | Typical % | Primary Function |
|---|---|---|---|---|---|
| Nickel | Ni | 57.0 | Balance | ~59 | FCC matrix, SCC resistance |
| Chromium | Cr | 19.0 | 23.0 | ~21 | Passive film, oxidizing media |
| Molybdenum | Mo | 15.0 | 17.0 | ~16 | Reducing media resistance ★ |
| Tungsten | W | 3.0 | 4.4 | ~3.7 | Pitting/crevice suppression ★ |
| Iron | Fe | — | 5.0 | <2 | Controlled — not a diluent |
| Carbon | C | — | 0.010 | ~0.005 | Ultra-low — prevents sensitization |
| Silicon | Si | — | 0.08 | ~0.03 | Deoxidation |
| Manganese | Mn | — | 0.75 | ~0.30 | Minor deoxidizer |
| Phosphorus | P | — | 0.04 | — | Impurity (controlled) |
| Sulfur | S | — | 0.02 | — | Impurity (controlled) |
| Cobalt | Co | — | 2.50 | ~0.5 | Residual from raw materials |
The combined Cr + Mo + W content of 2.4606 is approximately 40–41 wt% — the highest among all commercially established Ni-Cr-Mo wrought alloys. This is the metallurgical basis for its industry-leading PREN of 51 and its Critical Pitting Temperature exceeding 85°C per ASTM G48.
Why Each Alloying Element Matters
Nickel (Ni, ≥57%) — SCC Immunity and Toughness
Nickel at ≥57% provides the austenitic FCC matrix enabling cryogenic toughness through to ~500°C and fundamental immunity to chloride stress-corrosion cracking (SCC) — a failure mode that eliminates austenitic stainless steels from consideration in hot chloride service above approximately 60°C.
Chromium (Cr, 19–23%) — Passive Film and Oxidizing Media
At ~21%, chromium forms a self-repairing Cr₂O₃ passive film resisting oxidizing acids (HNO₃, hot H₂SO₄) and atmospheric oxidation. It enables 2.4606 to perform in mixed-redox environments where conditions shift between oxidizing and reducing — a scenario defeating single-function alloys.
Molybdenum (Mo, 15–17%) — Reducing Acid Defense
At 15–17%, Mo content in 2.4606 is among the highest of any commercial nickel alloy. Molybdenum stabilizes the passive film under reducing acidic conditions (HCl, H₂SO₄, H₃PO₄) and dramatically raises the pitting initiation threshold. It is the primary element responsible for performance in strongly reducing process streams.
Tungsten (W, 3.0–4.4%) — The Key Differentiator
The 3–4.4% tungsten addition is the defining structural difference between 2.4606 and most competing alloys including C-276. Tungsten synergizes with molybdenum: together they suppress pitting in concentrated chloride solutions, elevating CPT and CCT (per ASTM G48) by approximately 15–20°C versus C-276 in the same chloride environment.
Iron (Fe, ≤5%) — Deliberately Limited
In 2.4606, iron is controlled to a maximum of 5% rather than used as a cost diluent. This is a specification intent: higher iron would dilute the effective Mo and W content and reduce corrosion performance in reducing acid service.
Carbon (C, ≤0.010%) — Weld HAZ Integrity
The 0.010% maximum is among the most stringent for any commercially available nickel alloy. Ultra-low carbon prevents chromium carbide precipitation (sensitization) at grain boundaries during welding, maintaining corrosion resistance in the heat-affected zone and eliminating mandatory PWHT in most applications.
Mechanical Properties of 2.4606 (Annealed Condition)
Per ASTM B564: tensile strength 690–860 MPa, 0.2% yield strength ≥283 MPa, elongation ≥30%, hardness ≤200 HV. Forged parts typically achieve 310–380 MPa yield strength due to grain refinement.
| Property | Value | Unit | Standard / Condition |
|---|---|---|---|
| Tensile Strength (UTS) | 690 – 860 | MPa | ASTM B564 / Annealed |
| 0.2% Proof Strength (YS) | ≥ 283 | MPa | ASTM B564 / Annealed |
| Elongation (A50mm) | ≥ 30 | % | ASTM B564 / Annealed |
| Reduction of Area | ∼ 50 | % | Typical |
| Hardness (Vickers) | ≤ 200 | HV | Annealed |
| Hardness (Rockwell B) | ≤ 95 | HRB | Annealed |
| Elastic Modulus | ∼ 210 | GPa | Room temperature |
| Charpy Impact | > 130 | J | Room temperature |
| Charpy at −196°C | > 100 | J | Cryogenic service |
| Density | 8.90 | g/cm³ | — |
| Yield (forged + annealed) | 310 – 380 | MPa | Grain-refined forging — typical |
Closed-die and open-die forgings of 2.4606 typically achieve 10–20% higher yield strength than annealed plate (310–380 MPa vs 283 MPa minimum), with improved fatigue resistance and better dimensional stability at temperature. This is the primary engineering justification for specifying forged 2.4606 over machined plate in pressure-boundary applications.
Corrosion Resistance of 2.4606 (Alloy 686)
2.4606 provides superior resistance to mixed-acid environments, chloride pitting, crevice corrosion, and stress-corrosion cracking. Chloride tolerance exceeds 100,000 ppm. Critical Pitting Temperature >85°C per ASTM G48, outperforming C-276 and Alloy 22.
Key benchmark: 2.4606 withstands chloride concentrations exceeding 100,000 ppm without pitting initiation at moderate temperatures — approximately 4–5 times the chloride tolerance of super-duplex 2507, and materially higher than most competing nickel alloys in standardized ASTM G48 testing.
2.4606 is not the optimal choice for strongly oxidizing acid service (e.g., concentrated nitric acid >65%) where Alloy 22 or high-Cr stainless grades perform better. It also has lower strength than precipitation-hardened alloys (Alloy 725, Alloy 718) for structural applications above 300°C.
PREN Index and Localized Corrosion Performance
PREN of 2.4606 = 51. Formula: PREN = %Cr + 3.3(%Mo + 0.5×%W). Critical Pitting Temperature >85°C per ASTM G48 Method C. Highest PREN among commercially established wrought Ni-Cr-Mo alloys.
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N
For 2.4606: PREN = 21 + 3.3 × (16 + 0.5 × 3.7) + 0 ≈ 51
PREN Index — 2.4606 / Alloy 686 / N06686
A PREN of 51 is among the highest for any commercially available wrought nickel alloy. Reference scores: 316L stainless ~24; duplex 2205 ~35; super-duplex 2507 ~42; Alloy 625 ~51; C-276 ~69 (calculated) though 2.4606 outperforms in standardized ASTM G48 localized corrosion testing due to its tungsten content.
Critical Crevice Temperature (CCT) and Critical Pitting Temperature (CPT) in acidified 6% FeCl₃ per ASTM G48 Method C & D: > 85°C — exceeding most competing alloys under the same standardized protocol, including C-276 (~70°C) and Alloy 22 (~75°C).
Thermal and Physical Properties of 2.4606
| Property | Value | Unit | Temperature / Standard |
|---|---|---|---|
| Melting Range | 1325 – 1370 | °C | — |
| Density | 8.90 | g/cm³ | 20°C |
| Thermal Conductivity | 10.1 | W/(m·K) | 100°C |
| Thermal Expansion (CTE) | 12.6 | µm/(m·K) | 20–100°C |
| Specific Heat Capacity | 377 | J/(kg·K) | 20°C |
| Electrical Resistivity | 1.37 | µΩ·m | 20°C |
| Magnetic Permeability | ∼ 1.001 | — | Essentially non-magnetic |
| Max Service Temp (VdTÜV) | 400 (752°F) | °C | Werkstoffblatt 515/12.97 |
| Max Service Temp (ASME) | 425 (800°F) | °C | ASME Code Case 2198 / Sec. VIII Div.1 |
| Solution Anneal Temperature | 1120 – 1175 | °C | Rapid water quench required |
Forging Process and Manufacturing of 2.4606
Hot forging range: 1000–1200°C (optimal 1050–1150°C). Mandatory post-forge solution anneal at 1120–1175°C followed by rapid quench. All forgings must be supplied in the annealed condition per ASTM B564.
Hot Forging Temperature Window
The recommended hot working range for 2.4606 is 1000–1200°C (1832–2192°F). Below 1000°C risks surface cracking and excessive deformation resistance. Above 1200°C risks incipient melting at grain boundaries in thick sections. Optimal range for grain refinement in pressure-boundary forgings: 1050–1150°C.
Key Manufacturing Challenges
High flow stress: Combined solid-solution strengthening from Mo + W + Cr makes 2.4606 significantly harder to deform than 316L stainless at equivalent temperatures. Closed-die forging requires high-tonnage hydraulic presses (≥3,000-ton for medium sections) and heated tooling to prevent die chilling.
Thermal gradient management: Low thermal conductivity (~10 W/m·K) causes significant core-to-surface temperature gradients in thick billets. Standard practice: minimum 4-hour soak per 100mm section thickness, plus intermediate reheats during multi-pass forging sequences.
Mandatory solution anneal: All 2.4606 forgings require 1120–1175°C anneal + rapid water quench to dissolve secondary phases (P-phase, μ-phase, σ-phase) that may precipitate during cooling. Inadequate annealing can reduce corrosion performance by up to 50% in aggressive media.
Jiangsu Liangyi Co., Limited manufactures custom 2.4606 (NiCr21Mo16W) open die forgings and closed-die forgings (flanges, valve bodies, pump casings, rings, discs, custom near-net shapes) per ASTM B564 and DIN 17752. ISO 9001 certified. All forgings solution-annealed, supplied with full MTRs traceable to melt heat. NDT (UT, PT, MT), hydrostatic testing, and third-party inspection available.
Standards and Certifications for 2.4606 / N06686
2.4606 (N06686) is covered by the following international standards. Specify both the product standard and alloy designation on procurement documents for unambiguous specification:
Industrial Applications of 2.4606 Forged Parts
2.4606 is the material of choice in a well-defined set of industrial sectors where the combination of mixed-acid exposure, chloride contamination, and elevated temperature creates conditions that defeat stainless steels and many competing nickel alloys. Common product forms include 2.4606 forged flanges, valve bodies, rings, and pump casings produced to ASTM B564 for direct use in these service environments:
Reactors, Heat Exchangers & Piping
Primary selection for vessels and piping handling mixed acids, halide-bearing streams, and organic process fluids where conditions shift between oxidizing and reducing environments within the same operating cycle.
Sour Service Downhole Components
NACE MR0175 / ISO 15156-3 approved for H₂S-containing production. Downhole tubing connectors, wellhead valve bodies, subsea fittings, and completion hardware where H₂S partial pressure exceeds stainless limits.
Flue Gas Desulfurization Systems
One of the primary designed-for applications. Absorber tower internals, slurry pump casings, and ductwork exposed to hot SO₂/HCl-bearing wet gas. Standard where stainless and C-276 have failed prematurely.
Bleach Plant Equipment
Digester components, bleach plant piping in contact with ClO₂, Cl₂, hypochlorite, and kraft pulping liquors. Outperforms C-276 in crevice-forming bleach plant geometries.
Incineration & Leachate Treatment
Waste-to-energy scrubbing systems and high-chloride leachate plants where HCl concentrations in gas streams render other alloys unsuitable within 12–18 months of service.
Seawater-Cooled Systems
Shell-and-tube heat exchangers, sea chests, and pump casings in seawater or produced water service where chloride and particulate loads cause crevice attack on duplex stainless steels.
2.4606 vs Competing Alloys: Side-by-Side Technical Comparison
Selection should always be validated against site-specific corrosion testing data. The table below provides a technical starting point for material selection decisions:
| Alloy | UNS / DIN | Ni % | Cr % | Mo % | W % | PREN | Best Advantage vs 2.4606 |
|---|---|---|---|---|---|---|---|
| 2.4606 (Alloy 686) ★ | N06686 | ≥57 | 21 | 16 | 3.7 | 51 | Best pitting+crevice in mixed Cl⁻ media |
| C-276 (2.4819) | N10276 | 57 | 15.5 | 15.5 | 3.7 | ~69* | Strongly reducing acids; lower cost |
| Alloy 22 (2.4602) | N06022 | 56 | 22 | 13 | 3 | ~66 | Better in oxidizing media; thermal stability |
| Alloy 625 (2.4856) | N06625 | 61 | 21.5 | 9 | — | ~51 | Higher strength; high-temp structural use |
| Super Duplex 2507 | S32750 | 7 | 25 | 4 | — | ~42 | Lower cost; high strength; moderate Cl⁻ |
| 316L Stainless | S31603 | 11 | 16.5 | 2 | — | ~24 | Much lower cost; adequate mild service |
Choose 2.4606 when the dominant failure mode is pitting or crevice corrosion in chloride-rich or mixed-acid media. Choose C-276 when the environment is predominantly strongly reducing with low chloride content. Choose Alloy 22 when oxidizing acid resistance at elevated temperature is the primary requirement.
Frequently Asked Questions About 2.4606 Alloy
Jiangyin City
Jiangsu Province, China