Unlocking the Potential of UNS N08800 High Temperature Corrosion Resistance Hex Head Bolts

High-temperature service asks a lot from a bolted joint: the fastener must hold its load while the surrounding parts resist oxidation, carburization and nitridation. UNS N08800 hex head bolts — the fastener form of the alloy commonly supplied as Incoloy 800 — meet that requirement in engines, reactors and heat-recovery equipment, where ordinary steel bolts scale and lose strength. Bolt selection therefore starts from the service conditions rather than from the dimensions alone, and the mating parts — flange material, facing finish and flange gaskets — must survive the same medium and the same thermal cycling.

What UNS N08800 Brings to High-Temperature Bolting

UNS N08800 is a nickel-iron-chromium alloy built for service where heat and an aggressive atmosphere act together. It resists oxidation, carburization and nitridation at elevated temperatures, and it keeps useful mechanical properties where a carbon steel bolt would soften, scale and lose clamp load. That combination is why hex head bolts machined from this grade stay tight through repeated heating and cooling.

• Nickel-iron-chromium base — the same alloy family supplied under the trade name Incoloy 800.
• Oxidation resistance at elevated temperature — the bolt surface does not scale away as fast as unprotected steel.
• Resistance to carburization and nitridation — pick-up of carbon and nitrogen from process gases stays limited.
• Retained strength and ductility — clamp load remains stable through heating and cooling cycles.

Designers usually compare this grade with standard carbon or low-alloy bolting before specifying it. The deciding factors are the peak temperature and the chemistry of the surrounding atmosphere, not the bolt size alone.

Where UNS N08800 Hex Head Bolts Are Applied

Because the alloy tolerates heat and aggressive gases at the same time, it is specified wherever a bolt failure would stop production or endanger people. Engineers also review the complete joint — pipe flanges, facing finish, gasket grade and bolting — as one system, because a mismatch in any element shortens service life.

Table 1. Service conditions where UNS N08800 hex head bolts are commonly specified.

Aircraft engines and gas turbines push exhaust parts through extreme temperatures, while petrochemical reactors add corrosive chemicals and pressure. Power equipment runs continuously, automotive exhaust parts face hot corrosive gases, and renewable energy assemblies must survive long outdoor exposure. In every case the alloy keeps its properties where ordinary bolting would not.

Selection, Installation and Inspection

The alloy grade sets the limit; assembly practice decides whether the joint reaches it. Four details usually separate a reliable high-temperature bolted joint from a troubled one.

• Match the grade to the peak process temperature and to the gas chemistry, not to the average operating value.
• Tighten the bolts in a cross pattern and raise tension gradually so the load spreads evenly over the flange face.
• Use a calibrated torque tool and consistent lubrication, because friction changes make torque readings unreliable.
• Inspect for scaling, corrosion staining and loose nuts at every shutdown, and replace bolts that show cracks or heavy oxidation.

Keep the bolting set consistent as well: mixing grades inside one joint invites uneven corrosion and uneven loading. Hex head bolts fit standard wrenches and give maintenance crews a familiar grip, which is one reason this bolt form stays the common choice in the sectors listed above.

Frequently Asked Questions

What is UNS N08800, and how is it related to Incoloy 800?

UNS N08800 is a nickel-iron-chromium alloy supplied under the trade name Incoloy 800. It is known for resistance to oxidation, carburization and nitridation at elevated temperatures.

Why are UNS N08800 hex head bolts used in high-temperature service?

Ordinary steel bolts oxidize, scale and lose strength as temperature climbs. The alloy keeps its mechanical properties and resists corrosion, so clamp load is maintained through long heating and cooling cycles.

Where are UNS N08800 hex head bolts typically installed?

Aircraft engines, gas turbines and exhaust systems; heat exchangers, reactors and piping in petrochemical plants; boilers, turbines and heat recovery systems in power generation; hot engine and exhaust parts; and renewable energy assemblies.

How should UNS N08800 hex head bolts be tightened?

Tighten in a cross pattern with a calibrated tool, raising tension in steps so the load spreads evenly across the flange face. Even loading protects the gasket and keeps the joint tight.

What should be checked on UNS N08800 hex head bolts during shutdown?

Look for surface scaling, corrosion staining, cracked heads and loose nuts. Replace damaged bolts rather than reusing them, since a bolt that has lost material will not hold the same load.