Inconel 718 vs A-286 Bolts: Which Superalloy Fastener for Your Application?
RAS Engineering Team
Applications
If your bolts run below 650degC and budget matters, buy A-286. Above 650degC or where tensile strength exceeding 1100 MPa is non-negotiable, buy Inconel 718. That is the 80/20 decision rule. The remaining 20% is environment, geometry, and procurement cost — which this article covers in detail.
TL;DR — A-286 (UNS S66286, AMS 5731): iron-based superalloy, 1000 MPa UTS, service to 650degC, gamma-prime strengthened with 24-27% Ni, 40-60% lower cost than 718. Inconel 718 (UNS N07718, AMS 5662): nickel-based superalloy, 1275 MPa UTS, service to 700degC, gamma-double-prime strengthened with 50-55% Ni. Both are precipitation-hardenable. Selection pivots on temperature ceiling, required tensile strength, and environment.
1. Temperature Limits: 650degC vs 700degC
The first gate in fastener selection is service temperature — and the gap between these two alloys is narrower than most datasheets suggest.
A-286 is rated for continuous service at 650degC with intermittent excursions to 700degC. However, above 650degC the gamma-prime precipitates (Ni3(Al,Ti)) coarsen rapidly. Creep rupture data tells the real story: at 650degC and 400 MPa stress, A-286 delivers approximately 100 hours to rupture. Push to 700degC at the same stress level, and rupture life collapses to under 50 hours. The alloy’s yield strength also drops from 586 MPa at room temperature to approximately 400 MPa at 650degC — a 32% reduction.
Inconel 718 is rated for -253degC to 650degC with shorter-duration service to 700degC. Its gamma-double-prime (Ni3Nb) strengthening phase provides superior thermal stability: at 650degC and 400 MPa, 718 delivers over 1,000 hours to rupture. At 700degC and the same stress, 718 still holds over 200 hours — roughly 4x the life of A-286 at the higher temperature. Room-temperature yield strength of 1035 MPa derates to approximately 780 MPa at 650degC, a 25% drop — less severe than A-286 and from a higher baseline.
The practical takeaway: If your bolted joint sees continuous operation at 650-700degC and you cannot tolerate frequent re-torqueing or replacement, 718 justifies its price premium. Below 600degC, the thermal stability advantage of 718 narrows to the point where A-286 is the economically rational choice for all but the most strength-demanding applications.
| Property | A-286 | Inconel 718 | Delta |
|---|---|---|---|
| Max continuous service | 650degC | 650degC | Same rating |
| Max intermittent service | 700degC | 700degC+ | 718 holds strength longer |
| 100-hr rupture @ 650degC, 400 MPa | ~100 hr | ~1,000+ hr | 10x advantage 718 |
| 100-hr rupture @ 700degC, 400 MPa | <50 hr | ~200+ hr | 4x advantage 718 |
| Yield strength @ 650degC | ~400 MPa | ~780 MPa | 95% higher for 718 |
2. Strength and Mechanical Properties
Room-temperature tensile properties set the baseline, but elevated-temperature performance is where the selection decision lives.
Inconel 718 — solution treated and aged per AMS 5662:
- Ultimate tensile strength (UTS): 1275 MPa minimum
- Yield strength (0.2% offset): 1035 MPa minimum
- Elongation: 12% minimum in 4D
- Hardness: 36-44 HRC (typical)
- Density: 8.19 g/cm3
- Modulus of elasticity: 205 GPa at 20degC
A-286 — solution treated and aged per AMS 5731:
- Ultimate tensile strength (UTS): 896-1000 MPa (minimum 896 per spec; premium heat treatment can reach 1000+)
- Yield strength (0.2% offset): 586 MPa minimum
- Elongation: 15% minimum in 4D
- Hardness: 24-35 HRC (typical)
- Density: 7.94 g/cm3
- Modulus of elasticity: 201 GPa at 20degC
The temperature-derating curve matters. At room temperature, 718 gives you roughly 29% more UTS and 77% more yield strength than A-286. But at 650degC, the gap widens: 718 retains approximately 780 MPa yield strength versus A-286’s 400 MPa — a 95% advantage. If your bolted joint design is strength-limited at elevated temperature, 718 is not just an upgrade; it is a fundamentally different performance class.
Fatigue performance also distinguishes the two. Inconel 718 demonstrates high-cycle fatigue endurance limits in the range of 450-550 MPa (R = -1, 10^7 cycles) at room temperature, compared to A-286’s 300-350 MPa range. For rotating machinery fasteners subjected to vibration — turbine flange bolts, compressor casing studs — the 718 fatigue advantage is a reliability margin worth paying for.
Cryogenic performance is a niche where 718 stands alone. At -253degC (liquid hydrogen), 718 retains approximately 1800 MPa UTS and 1400 MPa yield with 20% elongation. A-286 also performs well at cryogenic temperatures but with roughly 30% lower strength values. For LNG and liquid hydrogen service, 718 is the aerospace-standard choice.
3. Corrosion and Environment
Corrosion is rarely the sole selection criterion for superalloy fasteners, but it eliminates candidates fast when overlooked.
Oxidation resistance: Inconel 718, with 50-55% nickel and 17-21% chromium, forms a protective chromia (Cr2O3) scale effective to approximately 980degC. A-286, with 24-27% nickel and 13.5-16% chromium, oxidizes more aggressively above 700degC. In exhaust gas environments with thermal cycling, the lower chromium content of A-286 means oxide spallation begins earlier and progresses faster. If your fasteners live in a hot oxidizing gas stream — turbine exhaust flanges, afterburner casings, turbocharger housings — 718’s oxidation margin is real and measurable.
Chloride stress corrosion cracking (SCC): Neither alloy is immune, but 718 is measurably better. In boiling 45% MgCl2 tests (ASTM G36), 718 in the aged condition shows time-to-failure typically 10x that of solution-treated A-286. For offshore topside bolting exposed to salt spray, 718 is preferred — not because it is immune, but because the mean time between SCC failures is substantially longer. Both alloys require proper heat treatment to minimize SCC susceptibility: over-aged A-286 is more resistant than peak-aged.
Hydrogen embrittlement: This is one environment where A-286 can hold an advantage. A-286 in the solution-treated and aged condition, with hardness controlled below 35 HRC, demonstrates good resistance to hydrogen embrittlement per ASTM F519 (Type 1a.1) testing. Inconel 718 at higher hardness levels (above 40 HRC) is known to be susceptible to hydrogen-assisted cracking in sour service environments with H2S partial pressure exceeding 0.05 psi. NACE MR0175/ISO 15156 lists both alloys for sour service, but A-286’s lower hardness and ferrous matrix make it the safer default choice for hydrogen-charging environments.
Galvanic corrosion: When paired with carbon steel or low-alloy steel structures, both 718 and A-286 are cathodic and will accelerate corrosion of the less-noble partner. The potential difference for 718 versus carbon steel is larger than for A-286 due to higher nickel content. In multi-material joints, isolate with coatings or specify the correct washer material.
| Environment | Preferred Alloy | Reason |
|---|---|---|
| Oxidizing gas, 700degC+ | 718 | Higher Cr, oxidation resistance |
| Chloride SCC (salt spray, offshore) | 718 | Longer time-to-failure in Cl- |
| Hydrogen / sour gas (H2S > 0.05 psi) | A-286 | Better H2 embrittlement resistance at controlled hardness |
| Nitric acid (HNO3) | A-286 | Adequate; 718 acceptable but not necessary |
| Cryogenic (LNG, LH2) | 718 | Superior strength and toughness at -253degC |
| Galvanic couple with carbon steel | A-286 | Smaller potential difference |
4. Cost Analysis: Where the 40-60% Comes From
The price gap between A-286 and Inconel 718 is driven almost entirely by nickel content.
- A-286 contains 24-27% nickel. At current (August 2026) LME nickel prices of approximately $16,200/MT, the raw nickel cost per kilogram of A-286 is roughly $4.00-4.40.
- Inconel 718 contains 50-55% nickel. Raw nickel cost per kilogram: approximately $8.10-8.90.
On raw material alone, 718 costs 2x more than A-286. Add niobium (4.75-5.5% in 718 vs essentially none in A-286) at approximately $45/kg for ferro-niobium, plus the higher processing cost of vacuum induction melting (VIM) followed by vacuum arc remelting (VAR) — the standard triple-melt route for aerospace-grade 718 — and the finished product price gap settles at roughly 40-60% per kilogram.
Machining cost also differs. Inconel 718 in the aged condition (36-44 HRC) is harder on tooling than A-286 (24-35 HRC). Thread rolling and tapping operations on 718 produce higher tool wear rates, adding 10-20% to the manufacturing cost per fastener. For large-diameter studs (M20 and above) with rolled threads, the machining cost delta can be larger than the raw material delta.
Total cost of ownership (TCO): A bolted joint that requires replacement every 5,000 hours in service costs far more in downtime, labor, and production loss than any material saving. If A-286 meets your strength and corrosion requirements with acceptable service life, buy it. If your operating conditions demand 718, the price premium is amortized over fewer failures and longer maintenance intervals. There is no generic formula — TCO must be calculated per application, not per kilogram.
| Cost Factor | A-286 | Inconel 718 |
|---|---|---|
| Nickel content | 24-27% | 50-55% |
| Raw material cost index | 1.0 (baseline) | 1.8-2.0 |
| Finished fastener cost index | 1.0 (baseline) | 1.4-1.6 |
| Machinability (aged condition) | Good (24-35 HRC) | Moderate (36-44 HRC) |
| Typical lead time (mill production) | 4-6 weeks | 6-8 weeks |
5. Application Decision Matrix
Match the alloy to the application, not the application to the alloy. Here is how that breaks down across the four industries that consume the most superalloy fasteners:
Aerospace
Winner: Inconel 718. Gas turbine engine fasteners — compressor discs, turbine discs, combustion casings, exhaust nozzle bolts — operate in temperature ranges where 718’s strength retention and fatigue resistance are required by design. AMS 5662/5663 are the default specifications for engine rotating-component bolting. A-286 sees use in secondary structures, access panel fasteners, and lower-temperature airframe applications, but the engine hot section belongs to 718.
Industrial Gas Turbines (IGT)
Winner: 718, with A-286 in cooler sections. Land-based gas turbines for power generation share the same metallurgical logic as aero but with longer service intervals (24,000+ hours between overhauls). Inconel 718 turbine flange studs and casing bolts are standard in the compressor discharge and turbine inlet sections. A-286 is frequently specified for inlet casing and exhaust diffuser bolting where temperatures remain below 550degC. For IGT operators, A-286 can reduce spares inventory cost by 30-40% in the cooler flange positions.
Turbochargers (Automotive and Marine)
Winner: A-286, unless EGT exceeds 700degC. Turbocharger turbine housing bolts see rapid thermal cycling from 200degC to 700degC with every throttle transient. A-286 handles this thermal fatigue well and is the standard alloy for production turbocharger fasteners from Garrett, BorgWarner, and Mitsubishi. Inconel 718 is specified only in high-performance and motorsport applications where exhaust gas temperature (EGT) consistently exceeds 700degC or where the bolted joint is preloaded to 75%+ of A-286’s yield — a design choice that 718 accommodates more comfortably.
Oil and Gas (Sour Service)
Winner: Application-dependent. NACE MR0175/ISO 15156 lists both alloys. For wellhead components and downhole tool fasteners where yield strength above 827 MPa (120 ksi) is required, 718 is the primary choice — A-286 cannot reliably reach those strength levels. For subsea bolting in hydrogen-charging environments with controlled hardness limits, A-286’s better hydrogen embrittlement resistance makes it the safer specification. In practice, many operators stock both: 718 for high-strength wellhead studs, A-286 for flowline flanges and manifold bolting.
| Application | Recommended Alloy | Key Specification | Deciding Factor |
|---|---|---|---|
| Aero engine hot section | 718 | AMS 5662/5663 | 650degC+ strength retention |
| Aero secondary structure | A-286 | AMS 5731 | Cost-effective at lower temps |
| IGT turbine inlet flange | 718 | AMS 5662 | Creep resistance, fatigue |
| IGT exhaust diffuser | A-286 | AMS 5731 | Adequate strength below 550degC |
| Turbocharger (OEM production) | A-286 | AMS 5731/5737 | Thermal fatigue, cost |
| Turbocharger (motorsport) | 718 | AMS 5662 | EGT above 700degC, higher preload |
| Wellhead studs (>827 MPa YS) | 718 | NACE MR0175 | Strength requirement |
| Subsea flange bolting | A-286 | NACE MR0175 | H2 embrittlement resistance |
| Cryogenic (LNG/LH2) | 718 | AMS 5662 | Strength and toughness at -253degC |
Bottom Line
Select A-286 when your bolt temperature stays below 600degC, your required tensile strength is under 1000 MPa, and cost per fastener matters — specify Inconel 718 when any of those three constraints is violated, because the failure mode on the wrong side of the decision is not a higher invoice, it is a joint that loosens, leaks, or fractures in service.
Standards referenced: AMS 5662 (718 bar and forgings), AMS 5663 (718), AMS 5731 (A-286 bar), AMS 5732 (A-286), AMS 5737 (A-286), ASTM B637 (718), ASTM A453 (A-286 bolting), NACE MR0175/ISO 15156, ASTM G36, ASTM F519.
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