Inconel 718 vs Waspaloy: Nickel Superalloy Selection for Turbine Disk Applications

What Are Inconel 718 and Waspaloy, and How Do Their Strengthening Mechanisms Differ?

Both alloys are precipitation-hardened nickel superalloys, but they develop their strength from different intermetallic phases: Inconel 718's niobium content forms gamma-double-prime (Ni3Nb), a metastable body-centered-tetragonal phase, while Waspaloy's aluminum and titanium content forms classical gamma-prime (Ni3(Al,Ti)), a thermally stable face-centered-cubic phase - and this single chemistry difference is the root cause of nearly every other distinction covered in this article.

Source: Special Metals Corporation nominal composition data for INCONEL alloy 718 and Waspaloy technical bulletins (SMC-045 and SMC-011).

The most visible chemistry difference is Waspaloy's substantial cobalt content (12-15%) against 718's essential absence of cobalt (1% maximum) - cobalt raises the temperature at which gamma-prime remains stable and slows its coarsening, which is part of why Waspaloy holds up better at sustained elevated temperature.

The absence of niobium in Waspaloy, and the corresponding absence of significant aluminum and titanium in 718, is the other defining split: 718 was deliberately engineered around gamma-double-prime specifically because that phase forms more slowly during aging, which (as later sections explain) is the direct source of 718's excellent weldability.

Which Alloy Has the Higher Maximum Service Temperature for Turbine Disk Applications?

The gap is smaller than commonly assumed for the specific case of turbine disks: Special Metals' own Inconel 718 literature states the alloy remains useful continuously to about 704°C (1,300°F), while its own Waspaloy literature caps critical rotating applications - the exact category turbine disks fall into - at 650°C (1,200°F), reserving Waspaloy's higher, often-quoted 870°C (1,600°F) ceiling specifically for other, less demanding, non-rotating applications.

Source: Special Metals Corporation, INCONEL alloy 718 and Waspaloy technical bulletins (SMC-045 and SMC-011).

This is a genuinely useful correction to a common oversimplification. Much of the secondary literature comparing these two alloys states flatly that "Waspaloy handles higher temperatures than 718," citing the 870°C figure without the qualifier that this ceiling applies to non-rotating hardware, not the turbine and compressor disks this article is specifically about.

For the disk application itself, both alloys' own manufacturer literature places the continuous-service ceiling within about 50°C of each other. The more accurate and more useful framing is not "Waspaloy runs hotter" but "Waspaloy retains more strength within the temperature range both alloys can tolerate" - which is the subject of the next two sections.

Why Does Gamma-Prime Retain Strength Better Than Gamma-Double-Prime at Sustained High Temperature?

Gamma-double-prime is a metastable phase that gradually transforms into orthorhombic delta phase (also Ni3Nb, but non-strengthening) during prolonged high-temperature exposure, progressively weakening 718; gamma-prime does not undergo an equivalent transformation and instead simply coarsens more slowly, which is why gamma-prime-strengthened alloys like Waspaloy generally offer better microstructural stability during extended elevated-temperature service.

This distinction explains why 718's maximum service temperature is described in terms of a phase transformation threshold, while Waspaloy's is described more in terms of oxidation resistance and short-term strength retention - the two alloys are approaching their limits through genuinely different failure mechanisms. It also explains why 718 was never intended to compete with gamma-prime alloys at the very hottest points in a gas turbine's hot section; 718's real competitive advantage lies in the temperature range below its delta-transformation threshold, where its combination of strength, weldability, and cost outperforms alternatives, not in matching Waspaloy or newer powder-metallurgy disk alloys degree-for-degree at the highest disk rim temperatures.

How Do the Two Alloys Compare in Creep and Stress-Rupture Strength?

Waspaloy's published 1,000-hour rupture strength falls from 89 ksi (615 MPa) at 1,200°F down to 16 ksi (110 MPa) at 1,600°F - and because Special Metals' own patent and product literature separately confirms Inconel 718's yield and tensile strength both fall below 50 ksi at 1,600°F, the general direction is clear even without a perfectly matched side-by-side test: within the shared operating range, Waspaloy's gamma-prime retains load-bearing capability more effectively than 718's gamma-double-prime.

Source: Special Metals Corporation, typical creep-rupture properties, Waspaloy technical bulletin SMC-011 (optimum high-temperature creep and stress-rupture heat treatment).

Readers should note that rupture strength and tensile/yield strength are related but distinct properties, so this comparison should be read directionally rather than as a precise, matched benchmark - the point is that both figures, drawn independently from each alloy's own literature, point the same way. For a disk design where the governing failure mode is creep or stress-rupture at sustained temperature (rather than low-cycle fatigue or tensile overload), this strength retention is the single strongest technical argument for choosing Waspaloy over 718 within their shared operating window.

Which Alloy Is Easier to Weld and Fabricate?

Inconel 718 is one of the most weldable age-hardenable nickel superalloys precisely because gamma-double-prime forms slowly during aging, allowing welds to be made in the solution-annealed condition without pre-cracking; Waspaloy, by contrast, is explicitly described by Special Metals as a material "not readily weldable outside very carefully controlled circumstances," prone to hot-short behavior and strain-age cracking, and must be welded only in the solution-treated condition with the entire component re-solution treated afterward before service.

This is arguably the single largest practical difference between the two alloys for fabricators and repair shops. Strain-age cracking occurs when a rapidly forming strengthening precipitate develops stress in and around a weld heat-affected zone faster than the material can relax it, cracking the joint during the very heat treatment meant to strengthen it.

Gamma-prime's comparatively fast precipitation kinetics make Waspaloy genuinely susceptible to this failure mode, which is why Special Metals recommends the fastest practical heating rate to the solution-annealing temperature specifically to minimize time spent in the strain-age-cracking-prone range, and why Waspaloy weld strength after heat treatment is explicitly lower than that of heat-treated wrought material - Special Metals directly advises against placing Waspaloy welds at high-stress locations. Gamma-double-prime's slower formation gives 718 a much wider, more forgiving processing window for exactly the same reason 718 shows a wide, forgiving hot-working finishing-temperature range in forging, as covered in a companion article in this series.

Which Alloy Is Easier to Machine?

Waspaloy is explicitly identified by Special Metals as "among the more difficult of the superalloys to machine," while Inconel 718, though still substantially harder to machine than stainless steel, is generally regarded as more machinable than gamma-prime-strengthened alloys like Waspaloy - a real, cumulative cost driver across the multiple machining operations a finished turbine disk typically requires.

Machinability differences compound across a disk's full manufacturing sequence - rough machining after forging, finish machining after heat treatment, and any subsequent rework - so even a moderate per-operation difference in tool life, cutting speed, or achievable surface finish becomes a meaningful total cost and lead-time factor over a complete disk. This machining difficulty is directly connected to the same cobalt and gamma-prime volume fraction that gives Waspaloy its elevated-temperature strength advantage: the properties that make the alloy resist deformation in service are largely the same properties that make it resist material removal in the machine shop.

How Do the Two Alloys Compare in Hot-Working Range and Forgeability?

Waspaloy's hot-working range (1,800-2,140°F) sits at a similarly high temperature band to Inconel 718's (1,650-2,050°F), but Waspaloy carries sharper failure modes at both ends - possible cracking below 1,800°F and hot-shortness above 2,150°F - and work-hardens very rapidly, which is why intermediate annealing is normally required during cold forming operations that 718 can often tolerate with fewer interruptions.

Source: Special Metals Corporation, INCONEL alloy 718 and Waspaloy technical bulletins (SMC-045 and SMC-011).

How Do the Standard Heat-Treatment Cycles Compare?

Waspaloy's standard heat treatment involves an additional step 718 does not require - a stabilization anneal between solution treatment and aging - making Waspaloy's process both longer and more sequence-sensitive, consistent with its greater susceptibility to strain-related cracking if the schedule is not followed precisely.

Source: Special Metals Corporation, INCONEL alloy 718 and Waspaloy technical bulletins (SMC-045 and SMC-011).

Waspaloy's intermediate stabilization step exists to precipitate grain-boundary carbides in a controlled way before the aging step develops the bulk gamma-prime strengthening - a sequencing choice that reduces sensitivity to subsequent grain-boundary attack in service but adds real processing time and a further point at which the schedule can be gotten wrong. This is one more expression of the same underlying theme running through this article: Waspaloy's superior high-temperature strength retention is purchased at the cost of a narrower, less forgiving, more heavily sequenced manufacturing process.

Does Cobalt Content Create a Cost and Supply Consideration for Waspaloy?

Yes - Waspaloy's 12-15% cobalt content is a genuine, structural cost and supply-chain difference from Inconel 718, which contains essentially none (1% maximum); cobalt is a smaller, more geographically concentrated, and historically more price-volatile market than nickel, chromium, or molybdenum, and its presence at double-digit weight percent in Waspaloy is not a minor alloying footnote.

This is not a claim about a specific current price premium, which fluctuates with cobalt market conditions and should be confirmed against current mill quotes - it is a structural observation about the two alloys' underlying material cost exposure. A material specification decision between 718 and Waspaloy is, in part, a decision about how much of the finished part's cost base should be exposed to cobalt market volatility rather than the comparatively more stable nickel, chromium, and molybdenum markets both alloys otherwise share.

What Standards Govern Each Alloy for Turbine Disk Procurement?

Both alloys share ASTM B637 as a common bar and forging-stock specification, but each carries its own distinct family of AMS specifications reflecting their different heat-treatment sequences - Waspaloy's specification set explicitly separates solution-treated-only material (AMS 5706, AMS 5708) from fully solution-stabilized-and-aged material (AMS 5704, AMS 5707, AMS 5709), the same logical split covered for Inconel 718's AMS 5662/5663 pair in a companion article.

Source: Special Metals Corporation, "Available Products and Specifications," INCONEL alloy 718 and Waspaloy technical bulletins.

Which Alloy Should You Specify for a Turbine Disk Application?

Specify Inconel 718 for compressor disks, fan disks, and lower-temperature turbine disks where operating temperatures stay comfortably below roughly 600-650°C, where fabrication cost, weldability, and machinability matter, and where cobalt-market cost exposure is a concern; specify Waspaloy where the disk sees sustained operation in the 540-650°C range and creep/stress-rupture strength retention over long service intervals is the governing design driver.

Choose Inconel 718 for: Compressor, fan, and lower-temperature turbine disks operating well within 718's continuous-service range.

Choose Inconel 718 for: Disks or hardware that will require welding, weld repair, or extensive machining as part of fabrication.

Choose Inconel 718 for: Programs where lower material cost and reduced exposure to cobalt-market price volatility are priorities.

Choose Waspaloy for: Turbine disks and rotating hardware operating in the roughly 540-650°C range where sustained creep and stress-rupture strength governs disk life.

Choose Waspaloy for: Applications where the fabricator has qualified, tightly controlled welding procedures able to manage Waspaloy's strain-age cracking sensitivity, or where the design avoids welds at high-stress locations entirely.

Choose Waspaloy for: Legacy engine programs and repair/overhaul work where Waspaloy is the qualified, drawing-called-out material and substitution is not an option.

For the very hottest turbine disk positions beyond either alloy's practical range, note that both 718 and Waspaloy have been substantially supplemented in the newest engine designs by powder-metallurgy gamma-prime/gamma-double-prime superalloys developed specifically to push disk temperature capability further - a reminder that this two-alloy comparison describes a well-established, still-relevant choice, not the entire modern turbine disk materials landscape.

Frequently Asked Questions

Q: Is Waspaloy stronger than Inconel 718?

A: At sustained elevated temperature within their shared operating range, yes - Waspaloy's gamma-prime strengthening phase retains load-bearing capability more reliably than 718's gamma-double-prime, which is prone to transforming into non-strengthening delta phase over time. At room temperature, both alloys can be heat treated to broadly comparable high-strength conditions.

Q: Does Waspaloy really operate at higher temperatures than Inconel 718?

A: Not by as much as commonly assumed for turbine disks specifically. Special Metals' own literature caps Waspaloy at 650°C (1,200°F) for critical rotating applications like disks, versus 718's 704°C (1,300°F) continuous-service ceiling. Waspaloy's often-cited 870°C (1,600°F) capability applies to less demanding, non-rotating applications, not turbine disks.

Q: Why is Inconel 718 easier to weld than Waspaloy?

A: 718's gamma-double-prime strengthening phase forms slowly during aging, so welds can be made in the solution-annealed condition without cracking during subsequent heat treatment. Waspaloy's gamma-prime phase forms much faster, making the alloy prone to strain-age cracking; Special Metals states Waspaloy is not readily weldable outside carefully controlled circumstances and must be welded only in the solution-treated condition, with the whole component re-solution treated afterward.

Q: Is Waspaloy more expensive than Inconel 718?

A: Waspaloy's 12-15% cobalt content (versus 718's essentially cobalt-free chemistry) is a structural cost and supply-chain difference, since cobalt is a smaller, more price-volatile market than nickel or chromium. Combined with Waspaloy's more difficult machining and more demanding fabrication requirements, it generally carries a higher total delivered cost, though exact premiums should be confirmed against current market pricing.

Q: Can Inconel 718 be used as a direct substitute for Waspaloy in a turbine disk?

A: Not automatically. While 718 is easier and cheaper to fabricate, its continuous-service temperature ceiling and elevated-temperature strength retention differ from Waspaloy's, and turbine disk substitutions require full engineering requalification against the specific engine's temperature, stress, and service-life requirements - not just a like-for-like material swap.

Q: Which alloy is easier to machine, Inconel 718 or Waspaloy?

A: Inconel 718 is generally considered more machinable. Special Metals explicitly describes Waspaloy as among the more difficult of the superalloys to machine, a direct consequence of the same cobalt content and gamma-prime volume fraction that give it superior elevated-temperature strength.