What Is Waspaloy Alloy?

Waspaloy (UNS N07001 / W.Nr. 2.4654)​ is a precipitation-hardenable, γ′-strengthened nickel-chromium-cobalt-molybdenum superalloy​ developed in the United States (originally by Pratt & Whitney / Special Metals lineage) to provide high creep and stress-rupture strength in the 650–870°C (1200–1600°F)​ range. Its strength derives from coherent γ′ precipitates [Ni₃(Al,Ti)]​ (volume fraction ~20–25%), supplemented by solid-solution strengthening (Co ~13.5%, Mo ~4.2%)​ and grain-boundary purification (B 0.003–0.010%, Zr 0.02–0.12%).

Unlike Inconel 718, which relies on metastable γ″(Ni₃Nb) that dissolves above ~650°C, Waspaloy’s γ′ remains stable to ~1000–1020°C (γ′ solvus), making it suitable for high-pressure turbine disks, compressor disks (rear stages), turbine shafts, seals, and high-temperature fasteners​ where long-term creep resistance above 650°C is required. It is typically produced by VIM + VAR (or ESR)​ double- or triple-melting to ensure low inclusion content and compositional homogeneity for rotating components.

1. Nominal Chemistry & Grade Cross-Reference

Typical Composition (wt%, AMS 5707 / ASTM B637):

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

3. Heat Treatment: The Critical Three-Stage Process

Waspaloy requires three-stage heat treatment​ to achieve design properties and prevent η-phase (Ni₃Ti) embrittlement:

1. Solution Treatment:​ 1050–1080°C​ (commonly 1080°C) × 2–4 h (per section) → oil or water quench.

   • Must exceed γ′ solvus (~1000–1020°C) to dissolve γ′ and carbides.

   • Air cooling is insufficient for sections > ~25–50 mm; oil/water quench ensures retention of supersaturation.

2. Stabilization:​ 843–855°C​ (commonly 845°C) × 4–24 h​ / AC.

   • Critical step:​ Precipitates primary γ′ and M₂₃C₆ at grain boundaries to pin boundaries and prevent η-phase (Ni₃Ti) formation​ during service or final aging. Omitting this step risks long-term embrittlement.

3. Aging:​ 760°C​ × 16 h (sometimes 24 h) / AC.

   • Precipitates fine secondary γ′ (20–50 nm) for peak strength.

Never​ substitute with a simple solution + single-age cycle; the stabilization step is metallurgically essential for Waspaloy.

4. Typical Mechanical Properties (Three-Stage Heat Treated)

Room Temperature (AMS 5707 typical minima / typical ranges):

• Rm: 1200–1450 MPa​ (AMS min ≥ 1275 MPa / 185 ksi)

• Rp0.2: 800–1100 MPa​ (AMS min ≥ 795–827 MPa)

• A₅₀: 15–25 %

• Z: 20–30 %

• Hardness: 34–44 HRC​ (341–401 HB)

Elevated-Temperature Tensile (Aged, Typical):

Stress-Rupture (Representative, NOT Design Allowables):

• 650°C / 415 MPa ≈ 1000 h+

• 760°C / 365 MPa ≈ 100 h; 760°C / 240 MPa ≈ 1000 h

• 815°C / 221 MPa ≈ 100 h; 815°C / 131 MPa ≈ 1000 h

• 870°C / 110 MPa ≈ 1000 h (marginal for sustained design)

→ Waspaloy outperforms Inconel 718 in creep at > 650–700°C​ due to γ′ stability; Inconel 718 retains higher peak strength at ≤ 650°C and better hardenability in thick sections.

5. Microstructural Stability & η-Phase (Ni₃Ti)

• γ′ (Ni₃(Al,Ti)): Cuboidal, coherent, ~20–25 vol.%, stable to ~1000–1020°C.

• η (Ni₃Ti): Hexagonal, platelets/needles, forms at ≥ 850–900°C​ or prolonged 800–850°C exposure​ if stabilization was inadequate.

  • η consumes γ′, reduces creep strength, and creates stress concentrators at grain boundaries → ductility dip and premature rupture.

  • The 845°C stabilization​ pre-precipitates controlled γ′/M₂₃C₆ to occupy boundary sites, suppressing η nucleation during service.

6. Fabricability, Welding & Machining

• Weldability:​ Poor to moderate.​ High Al+Ti (~4.5%) → HAZ γ′ re-precipitates on cooling + residual stress → Strain-Age Cracking (SAC)​ risk, higher than Nimonic 90.

  • Usually welded in solution-annealed condition using matching wire or Inconel 82 (lower strength); post-weld full re-solution + stabilization + aging is mandatory​ for critical parts → expensive.

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

• Hot Working:​ 1040–1170°C; finish ≥ 1000°C. Below ~900°C, γ′ precipitation raises flow stress → cracking.

• Cold Working:​ Difficult; work-hardens rapidly. Done in solution-annealed soft state with intermediate anneals.

• Machining:​ Age-hardened (34–44 HRC) with γ′ hard particles → similar to Inconel 718 but slightly harder. Low SFM (8–15 m/min), rigid setup, TiAlN carbide, flood coolant.

7. Oxidation & Environmental Resistance

• Oxidation:​ Cr ≈ 19.5% + Al ≈ 1.4% → Cr₂O₃ + trace Al₂O₃ sub-layer → continuous useful to ≈ 1038°C (1900°F)​ in static air; cyclic may spall slightly above 980°C but adequate for metal T ≤ 870°C.

• Hot Corrosion (Type I/II):​ Similar to other ~20%Cr Ni-base; in marine/salt environments > 650°C, MCrAlY coating may be considered.

• SCC:​ Better than austenitic SS in Cl⁻; still susceptible in some hot caustic, but not primary concern in gas-path service.

8. Typical Applications

9. Comparison With Selected Alloys

10. Summary

Waspaloy (UNS N07001 / GH738) is a γ′-hardened Ni–Cr–Co–Mo–Ti–Al superalloy​ (Al+Ti≈4.5%, Co≈13.5%, Mo≈4.2%) designed for 650–870°C creep-resistant rotating and stressed components​ (turbine disks, shafts, bolts). Its performance depends critically on:

• VIM+VAR/ESR melting​ for cleanliness,

• Three-stage heat treatment​ (1080°C → 845°C stabilization → 760°C aging) to prevent η-phase embrittlement,

• Strict control of Al/Ti/Co/Mo​ for γ′ volume and solvus (~1000–1020°C).

It is not​ a substitute for welded sheet alloys (Nimonic 263) nor for >950°C disc alloys (Udimet 720 / René 41). When correctly processed, it provides a robust balance of creep strength, fatigue resistance, and microstructural stability​ in the upper intermediate temperature regime of gas turbines.