High Temperature Bolt Selection: Environment, Standards, Preload and Cost

Typical applications by temperature band

The temperature table on the selection guide maps each band to its recommended grades. The applications below are what those bands look like in service.

Environment-based material selection matrix

Temperature gets you in the ballpark; the corrosive environment picks the winner. The same alloy that performs brilliantly in dry air at 700 °C can fail within weeks in a wet chloride or sour gas environment.

High-temperature oxidation (air)

Mechanism: scaling and thickness loss above the oxidation threshold.

• A-286 — Good to 650 °C; protective Cr2O3 scale.
• Inconel 718 — Good to 700 °C; higher Cr content aids scale stability.
• Nimonic 80A — Excellent to 815 °C; Al + Ti forms a stable protective scale.
• Hastelloy C-276 — Not recommended above 425 °C; MoO3 volatilisation.
• Monel K-500 — Not recommended above 300 °C; Cu-rich scale spalls.
• Inconel 625 — Good to 650 °C; high Cr provides an oxidation barrier.

Chloride and seawater

Mechanism: pitting, crevice corrosion, chloride stress-corrosion cracking.

• A-286 — Moderate; 15% Cr gives some resistance but pitting risk remains.
• Inconel 718 — Good for general marine; crevice risk in stagnant conditions.
• Nimonic 80A — Fair; high nickel but limited molybdenum for chloride resistance.
• Hastelloy C-276 — Excellent; 16% Mo, effectively immune to chloride SCC and pitting.
• Monel K-500 — Excellent in flowing seawater; resistant to chloride SCC and cavitation.
• Inconel 625 — Excellent; 9% Mo, PREN above 50, immune to chloride SCC in all practical conditions.

Sour gas and H2S (NACE MR0175)

Mechanism: sulphide stress cracking, hydrogen embrittlement.

• A-286 — Acceptable at 35 HRC or below; sulphide films form but do not penetrate.
• Inconel 718 — Good to 40 HRC in most sour conditions; consult NACE for specific limits.
• Nimonic 80A — Good; high nickel and controlled hardness provide SSC resistance.
• Hastelloy C-276 — Excellent; solid-solution alloy, low hardness, no embrittlement risk.
• Monel K-500 — Conditional; limit to 35 HRC maximum, avoid over-ageing for subsea.
• Inconel 625 — Excellent; solid-solution, low hardness, NACE compliant as supplied.

Sulphuric acid (H2SO4)

Mechanism: active corrosion, particularly in intermediate concentrations.

• A-286 — Poor; iron-based alloys are attacked rapidly in H2SO4.
• Inconel 718 — Fair; good in dilute or concentrated acid, poor in the intermediate 40–80% band.
• Nimonic 80A — Fair; the nickel base helps but molybdenum content is limited.
• Hastelloy C-276 — Excellent; resists all concentrations to boiling point.
• Monel K-500 — Good in dilute de-aerated acid; poor in aerated or hot concentrated acid.
• Inconel 625 — Very good; adequate for most concentrations, though C-276 is better.

Hydrochloric acid (HCl)

Mechanism: rapid uniform corrosion of most metals.

• A-286 — Not recommended; the iron matrix dissolves rapidly.
• Inconel 718 — Poor; insufficient molybdenum for HCl resistance.
• Nimonic 80A — Poor; chromium and aluminium offer no HCl protection.
• Hastelloy C-276 — Excellent; resists all concentrations at ambient, and up to 20% boiling.
• Monel K-500 — Fair in dilute (below 5%), de-aerated acid; rapid attack at higher concentrations.
• Inconel 625 — Moderate; better than 718, but C-276 is preferred for HCl service.

High-temperature hydrogen

Mechanism: hydrogen attack, decarburisation, embrittlement.

• A-286 — Fair; iron-based, susceptible above 400 °C at high partial pressure.
• Inconel 718 — Good; nickel-base alloys resist hydrogen attack better than steel.
• Nimonic 80A — Good; the stable gamma-prime structure resists hydrogen degradation.
• Hastelloy C-276 — Not recommended above 425 °C; hydrogen accelerates mu-phase precipitation.
• Monel K-500 — Moderate; the Cu-Ni matrix resists hydrogen, but the age-hardened condition needs care.
• Inconel 625 — Good; the solid-solution structure is inherently resistant to hydrogen attack.

Where high temperature and aggressive corrosion arrive together — wet chloride at 600 °C, for example — the candidate list shrinks rapidly. Inconel 625 is the pragmatic compromise; Hastelloy C-276 wins on corrosion but loses at temperature; Nimonic 80A wins on temperature but loses in chlorides. When both extremes are present, a duplex solution such as a C-276 bolt in a Nimonic 80A joint with a protective sleeve may be required.

Standard cross-reference

Most industrial purchase orders reference a material standard rather than a trade name. Knowing which ASTM, AMS, DIN or EN standard maps to the application avoids costly rework when the wrong one is cited.

When sourcing from multiple regions, reference both the ASTM/AMS standard and the UNS number on the purchase order. Example: “Inconel 718 hex bolts per ASTM B637, UNS N07718, AMS 5662 aged condition, with EN 10204 3.1 material test certificate.” That eliminates ambiguity across American, European and Asian supply chains.

Load, preload and stress relaxation

Temperature is not the only thing that degrades a bolted joint. Three mechanical phenomena have to be accounted for in high-temperature fastener design.

Preload loss (stress relaxation)

At elevated temperature the initial clamp load relaxes as the bolt material creeps. The rate depends on the ratio of service temperature to the alloy’s melting point (T/Tm). Nickel superalloys with T/Tm below 0.5 — roughly below 650 °C for Inconel 718 — relax slowly. Above T/Tm 0.5, specify Nimonic 80A or Waspaloy, and consider Belleville washers or tensioners for live-load maintenance.

Thermal cycling fatigue

Differential thermal expansion between bolt and flange materials cycles the bolt stress with every heat-up and cool-down. The coefficient of thermal expansion for nickel alloys is roughly 13–15 × 10⁻⁶ /°C, which is close to austenitic stainless steel but significantly different from ferritic steels at 11–12. Match CTE where possible; where it cannot be matched, allow for the stress range in the fatigue calculation.

Galling and seizure prevention

Nickel alloys are notoriously prone to galling under high contact pressure. For assembly temperatures above 300 °C, use silver-plated fasteners (to 650 °C), MoS2 dry-film lubricant, or boron-nitride-based anti-seize. Never use copper-based anti-seize on nickel alloys: copper diffuses into nickel grain boundaries at temperature and causes liquid-metal embrittlement.

Cost versus lifecycle

Initial material cost is the wrong metric for high-temperature fasteners. Total lifecycle cost, including replacement labour, downtime and the cost of joint failure, almost always favours the right material over the cheap one.

A single unplanned shutdown in a gas turbine or chemical plant costs orders of magnitude more than the entire bolt order. The price difference between A-286 and Nimonic 80A for a set of M20 stud bolts is typically under $500, while a turbine trip can cost $50,000 to $500,000 in lost production. When the application is critical, specify up.