Material Identity: What Is 2.4606?
2.4606 is the European EN/DIN Werkstoffnummer for Nickel Alloy 686 — the same material also identified as UNS N06686 and DIN NiCr21Mo16W. It is a single-phase, solid-solution-strengthened austenitic Ni-Cr-Mo-W superalloy.
Rather than gaining strength from precipitated secondary phases like age-hardening grades, 2.4606 draws its properties from a face-centred cubic (FCC) austenitic matrix saturated with chromium, molybdenum, and tungsten in solid solution. That single-phase architecture is what lets the alloy combine two properties usually in tension: extreme corrosion resistance and excellent weldability without post-weld heat treatment.
The labels 2.4606, N06686, Alloy 686, and NiCr21Mo16W all describe one identical alloy chemistry. Always confirm the governing standard (ASTM, DIN, or ASME) on procurement documents to fix the product form and heat-treatment condition.
The alloy was engineered to outperform C-276 (2.4819) in the harshest mixed-acid and halide environments. Its distinguishing feature is a combined Cr + Mo + W content of about 40–41 wt% — the richest of any commercially established Ni-Cr-Mo wrought alloy — high enough to arrest pitting and crevice corrosion even in chloride-saturated acids at elevated temperature.
Chemical Composition of 2.4606
Per ASTM B574 / DIN 17750: Ni ≥57% (balance), Cr 19–23%, Mo 15–17%, W 3.0–4.4%, Fe ≤5.0%, C ≤0.010%. Combined Cr+Mo+W reaches ~40–41 wt%.
| Element | Symbol | Min % | Max % | Typical % | Function |
|---|---|---|---|---|---|
| Nickel | Ni | 57.0 | Bal. | ~59 | FCC matrix, SCC resistance |
| Chromium | Cr | 19.0 | 23.0 | ~21 | Passive film, oxidizing media |
| Molybdenum | Mo | 15.0 | 17.0 | ~16 | Reducing acid 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 | Prevents weld sensitization |
| Silicon | Si | — | 0.08 | ~0.03 | Deoxidation |
| Manganese | Mn | — | 0.75 | ~0.30 | Minor deoxidizer |
| Phosphorus | P | — | 0.04 | — | Controlled impurity |
| Sulfur | S | — | 0.02 | — | Controlled impurity |
| Cobalt | Co | — | 2.50 | ~0.5 | Residual |
The combined Cr + Mo + W content of ~40–41 wt% is the highest among commercial Ni-Cr-Mo wrought alloys and is the direct reason for the alloy's PREN of 51 and its ASTM G48 pitting temperature above 85°C.
Why Each Alloying Element Matters
Nickel (Ni, ≥57%)
Provides the austenitic FCC matrix, delivering toughness from cryogenic temperatures up to ~500°C and, critically, immunity to chloride stress-corrosion cracking — the failure mode that rules austenitic stainless steels out of hot chloride service.
Chromium (Cr, 19–23%)
Builds the self-repairing Cr₂O₃ passive film that resists oxidizing acids and lets 2.4606 survive mixed-redox environments where oxidizing and reducing species coexist.
Molybdenum (Mo, 15–17%)
Among the highest Mo levels of any commercial nickel alloy. Molybdenum stabilizes the passive film in reducing acids (HCl, H₂SO₄) and raises the pitting-initiation threshold dramatically.
Tungsten (W, 3.0–4.4%)
The defining difference between 2.4606 and C-276. Working with molybdenum, tungsten lifts the critical pitting and crevice temperatures by roughly 15–20°C in identical chloride tests.
Iron (Fe, ≤5%) & Carbon (C, ≤0.010%)
Iron is deliberately capped rather than used as a filler, preserving corrosion performance. The ultra-low carbon ceiling stops chromium-carbide precipitation during welding, keeping the heat-affected zone corrosion-resistant without mandatory PWHT.
Mechanical Properties (Annealed)
Per ASTM B564: tensile 690–860 MPa, yield ≥283 MPa, elongation ≥30%, hardness ≤200 HV. Forged parts reach 310–380 MPa yield from grain refinement.
| Property | Value | Unit | Condition |
|---|---|---|---|
| Tensile Strength | 690–860 | MPa | Annealed |
| 0.2% Proof Strength | ≥283 | MPa | Annealed |
| Elongation (A50) | ≥30 | % | Annealed |
| Hardness | ≤200 | HV | Annealed |
| Elastic Modulus | ~210 | GPa | Room temp |
| Charpy Impact | >130 | J | Room temp |
| Density | 8.90 | g/cm³ | — |
| Yield (forged) | 310–380 | MPa | Grain-refined |
Corrosion Resistance
2.4606 excels in mixed-acid media, chloride pitting, crevice corrosion, and stress-corrosion cracking, tolerating chloride above 100,000 ppm and outperforming C-276 and Alloy 22 in ASTM G48 testing.
The headline benchmark: 2.4606 withstands chloride concentrations over 100,000 ppm without pitting at moderate temperatures — roughly 4–5× the chloride tolerance of super-duplex 2507.
2.4606 is not the first choice for strongly oxidizing acid service (e.g. concentrated nitric acid), where Alloy 22 performs better, and it offers lower strength than precipitation-hardened grades above 300°C.
PREN Index & Localized Corrosion
PREN = 51, from PREN = %Cr + 3.3(%Mo + 0.5×%W). Critical Pitting Temperature exceeds 85°C per ASTM G48 — among the highest of any wrought nickel alloy.
PREN — 2.4606 / Alloy 686
For reference: 316L stainless ~24; duplex 2205 ~35; super-duplex 2507 ~42; Alloy 625 ~51. The tungsten addition gives 2.4606 the edge in real ASTM G48 localized-corrosion testing.
Critical Pitting / Crevice Temperature in acidified 6% FeCl₃ (ASTM G48 C & D): > 85°C — ahead of C-276 (~70°C) and Alloy 22 (~75°C).
Thermal & Physical Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Melting Range | 1325–1370 | °C | — |
| Density | 8.90 | g/cm³ | 20°C |
| Thermal Conductivity | 10.1 | W/(m·K) | 100°C |
| Thermal Expansion | 12.6 | µm/(m·K) | 20–100°C |
| Specific Heat | 377 | J/(kg·K) | 20°C |
| Electrical Resistivity | 1.37 | µΩ·m | 20°C |
| Max Service (VdTÜV) | 400 | °C | Werkstoffblatt 515 |
| Max Service (ASME) | 425 | °C | Code Case 2198 |
| Solution Anneal | 1120–1175 | °C | Rapid quench |
Forging Process & Manufacturing
Hot forging range 1000–1200°C (optimal 1050–1150°C), followed by mandatory solution anneal at 1120–1175°C with rapid quench. All forgings supplied annealed per ASTM B564.
Hot Working Window
Working below 1000°C risks surface cracking and excessive press tonnage; above 1200°C risks incipient grain-boundary melting in thick sections. The 1050–1150°C band gives the best grain refinement for pressure-boundary parts.
Manufacturing Realities
The Mo + W + Cr solid-solution strengthening makes 2.4606 far harder to deform than 316L, demanding high-tonnage presses and heated tooling. Its low thermal conductivity produces steep temperature gradients in large billets, so multi-step heating with intermediate reheats is standard. Every forging then requires a solution anneal to dissolve secondary phases (P, μ, σ) and restore full corrosion resistance — skipping it can cut corrosion performance in half.
Jiangsu Liangyi manufactures custom 2.4606 (NiCr21Mo16W) open die forgings and seamless rolled rings from 30 kg to 30,000 kg per piece, solution-annealed and supplied with full EN 10204 3.1/3.2 material test reports traceable to melt heat.
Standards & Certifications
2.4606 / N06686 is covered by the following standards. Specify both the product standard and the alloy designation for unambiguous procurement:
Industrial Applications
2.4606 is the material of choice wherever mixed-acid exposure, chloride contamination, and elevated temperature combine to defeat stainless steels. Common product forms include 2.4606 forged flanges, valve bodies, rings, and pump casings produced to ASTM B564 for these service environments:
Reactors & Heat Exchangers
Mixed acids and halide streams where redox conditions shift within a single operating cycle.
Sour Service Components
NACE MR0175 approved downhole connectors, wellhead bodies, and subsea fittings for H₂S service.
FGD Scrubber Systems
Absorber internals, slurry pump casings, and ductwork in hot chloride-rich sulfuric acid mist.
Bleach Plant Equipment
Digester and bleach-line parts against ClO₂, Cl₂, and kraft liquors.
Incineration Systems
High-chloride leachate and HCl flue-gas streams in waste-to-energy plants.
Seawater Systems
Heat exchangers and pump casings where chloride defeats duplex stainless in crevice geometries.
2.4606 vs Competing Alloys
A technical starting point for material selection — always validate against site-specific corrosion data:
| Alloy | UNS | Ni% | Cr% | Mo% | W% | PREN | Edge vs 2.4606 |
|---|---|---|---|---|---|---|---|
| 2.4606 / Alloy 686 | N06686 | ≥57 | 21 | 16 | 3.7 | 51 | Best pitting + crevice in Cl⁻ |
| C-276 | N10276 | 57 | 15.5 | 15.5 | 3.7 | ~69* | Reducing acids; lower cost |
| Alloy 22 | N06022 | 56 | 22 | 13 | 3 | ~66 | Oxidizing media; stability |
| Alloy 625 | N06625 | 61 | 21.5 | 9 | — | ~51 | Higher strength; high-temp |
| Super Duplex 2507 | S32750 | 7 | 25 | 4 | — | ~42 | Lower cost; high strength |
| 316L Stainless | S31603 | 11 | 16.5 | 2 | — | ~24 | Much lower cost |
Choose 2.4606 for pitting/crevice corrosion in chloride-rich mixed-acid media; choose C-276 for strongly reducing low-chloride acids; choose Alloy 22 when oxidizing acid resistance at temperature dominates.