What is Nimonic 80 (80A) Alloy?

Nimonic 80A (UNS N07080 / W.Nr. 2.4631 / 2.4952), often generically referred to as Nimonic 80, is a precipitation-hardenable (γ′ strengthened) nickel-chromium superalloy​ originally developed by Henry Wiggin (UK) in the 1940s as an evolution of Nimonic 75 for jet engine turbine blades. Its nominal chemistry is Ni–19.5Cr–2.3Ti–1.4Al–(Co ≤ 2.0%), distinguished from later Nimonic grades (90, 263) by its very low cobalt content. It derives high-temperature strength from coherent γ′ phase [Ni₃(Al,Ti)] precipitation, offering good creep resistance in the 650–800°C​ range and a maximum continuous service temperature of ≈ 815°C (1500°F).

In modern practice, "Nimonic 80" usually implies Nimonic 80A​ (the stabilized, low-Co version), qualified to BS HR 1, BS 3076 NA 20, ASTM B637, and used for gas turbine blades/vaned rings, high-temperature springs, fasteners, exhaust valves, and nuclear boiler tube supports​ where low Co is advantageous (radiation activation concerns).

1. UNS, Trade Name & International Equivalents

⚠️ Pure "Nimonic 80" (without "A") sometimes refers to early, less controlled compositions; Nimonic 80A​ is the modern, specification-controlled grade with tighter Ti/Al and low Co.

2. Nominal Chemical Composition (per BS HR 1 / ASTM B637)

Key metallurgical note:​

(Al+Ti) total ≈ 3.4–4.2 wt%​ → generates ≈ 15–18 vol.% γ′​ after aging, slightly less than Nimonic 90 (≈18–22%). Low Co means γ′ solvus ≈ 950–970°C, lower than Nimonic 90 (~985–1010°C), thus creep strength tapers off above ~800–815°C sooner.

3. Key Physical Properties (20°C, Solution-Annealed)

4. Typical Mechanical Properties

Condition: Solution Annealed (SA, as-delivered for machining/form):​

• Rm ≈ 800–1000 MPa

• Rp0.2 ≈ 350–450 MPa

• A₅₀ ≥ 30–40%

• Hardness ≈ 85–95 HRB

Condition: SA + Aged (typical: 1080°C × soak → oil/water quench + 700°C × 16 h / AC):​

(Note: Nimonic 80A often uses single-stage 700°C aging; double-stage 650°C optional for slight tweak)

Elevated-Temperature Tensile (Aged — Typical):

Stress-Rupture (representative, not design allowables):​

• 650°C / 415 MPa ≈ 100–300 h

• 750°C / 180 MPa ≈ 100–300 h

• 750°C / 240 MPa ≈ 50–150 h (lower than Nimonic 90 at same stress)

• 815°C / 105 MPa ≈ 50–100 h (marginal for sustained load)

→ Creep strength adequate to ~ 730–750°C design metal T; above 800°C, Nimonic 90 or Waspaloy preferred.

5. Heat Treatment Practice

• Solution Anneal (SA):​ 1050–1080°C​ (typ 1080°C) × soak per section (e.g. 8 h for bar) → oil or water quench​ (must retain γ′ solutes; thinner sections air cool acceptable but oil/water safer for full quench).

  Note: Lower than Nimonic 90's 1120–1140°C due to lower γ′ solvus (~950–970°C).

• Aging (Precipitation):​

  • Single-stage (most common for 80A):​ 700°C × 16 h / Air Cool​

  • Double-stage (less common, optional):​ 700°C × 16 h + 650°C × 8–16 h (slight refinement of γ′ size)

• Over-aging:​ > 950°C or long 800°C+ exposure → η (Ni₃Ti) forms → strength drop.

• Spring Temper:​ Cold-drawn wire + age (600–700°C) → UTS 1300–1500 MPa for valve springs.

6. Welding, Forming & Machining

• Weldability:​ Fair — Al+Ti ≈ 4% → HAZ γ′ re-precipitates on cooling → Strain-Age Cracking (SAC) risk​ in restrained joints (similar to Nimonic 90 but slightly less severe due to lower γ′ volume).

  • Filler: matching N07080 wire preferred; Inconel 82 (ERNiCr-3) used for non-critical (lower creep).

  • Usually welded in SA condition; best to re-solution + age post-weld for critical parts.

  • Not for large thin-wall welded fabrications​ (use Nimonic 263 / Hastelloy X).

• Forming:​ Good in SA condition (similar to Inconel 625); springback noticeable. Intermediate anneals (1050–1080°C) if heavy cold work.

• Machining:​ Work-hardens; rigid setup, low SFM (8–15 m/min), TiAlN carbide, flood coolant — typical γ′ Ni-base behavior.

7. Oxidation & Environmental Resistance

• Static Air Oxidation:​ Cr₂O₃ scale (Cr≈19.5%, Al trace → minor Al₂O₃ sub-layer) → continuous useful limit ≈ 1040°C (1900°F)​ for oxidation alone; service metal T usually ≤ 815°C due to creep limit.

• Nuclear / Low-Co Advantage:​ Co ≤ 2% → 59Co(n,γ)⁶⁰Co​ activation product minimized → specified for nuclear boiler tube supports, spring hangers in reactor auxiliaries​ where Co-60 gamma activity is a lifetime concern (Nimonic 90 with 18% Co avoided here).

• Sulfidation / Hot Corrosion:​ Similar to other 20%Cr Ni-base; not as good as Co-base (Haynes 25/188) in low-pO₂ high-S; adequate for normal combusted air.

8. Typical Applications

Summary

Nimonic 80A (UNS N07080 / W.Nr. 2.4631 / GH4080A) is a γ′-hardened Ni–Cr–Ti–Al superalloy with Co ≤ 2%, offering:

• Creep resistance in the 650–750°C​ range (max. continuous 815°C);

• Low cobalt advantage​ for nuclear applications where Co-60 activation is restricted;

• Typical uses: legacy turbine blades, high-temp springs, exhaust valves, nuclear boiler supports;

• Heat treatment: 1050–1080°C solution + 700°C × 16 h aging​ (single-stage common);

• Not​ a substitute for Nimonic 90 in > 750°C high-creep zones nor for Nimonic 263 in welded sheet-metal fabrications.