Application and development of nickel-based alloy Incoloy 800HT fasteners

Incoloy 800HT is a nickel-iron-chromium alloy used for studs, nuts and bolting that must hold their load at high temperature. Its high-temperature strength, oxidation resistance and resistance to chloride stress corrosion cracking make it a common choice for heaters, reformers, boilers and high-temperature piping, where carbon steel softens and many stainless grades crack in chloride-bearing service. For the seal, such joints use high-temperature flange gaskets together with a controlled bolt load.

Composition and Structure of Incoloy 800HT

Incoloy 800HT is a modified Incoloy 800 with tighter control of carbon, aluminium and titanium. The composition limits are listed below.

Chemical composition of Incoloy 800HT in percent by mass; iron is the balance.

The alloy has a face-centred cubic lattice. Its very low carbon content and raised Ti:C ratio increase structural stability, oxidation resistance and resistance to intergranular corrosion, while annealing at about 950 °C keeps the grain size fine. For bolting, that stability means a stud keeps its preload through repeated heating and cooling cycles.

Mechanical, Physical and Corrosion Properties

At room temperature the alloy shows a tensile strength of 450 N/mm2, a 0.2% proof strength of 180 N/mm2 and 35% elongation. Density is 8.0 g/cm3 and the melting range is 1350–1400 °C.

• High-temperature strength — the alloy keeps its mechanical properties in aqueous media up to about 500 °C.
• Oxidation resistance — the chromium content builds a stable oxide scale under cycling between air and hot process gas.
• Stress corrosion resistance — the nickel base limits chloride stress corrosion cracking in wet, chloride-bearing service.
• Machinability — good machinability allows studs, nuts and non-standard bolting shapes to be produced to drawing.

Corrosion resistance depends on the medium:

• Nitric acid and organic acids — good resistance.
• Sulfuric acid and hydrochloric acid — limited resistance, so confirm the corrosion rate for the exact concentration before use.
• Halides — pitting is possible, but oxidising and non-oxidising salts are handled well.
• Water, steam, and mixtures of steam, air and carbon dioxide — good resistance.

In refinery and chemical service this resistance also protects bolting exposed to chlorides, so the joint keeps its sealing load instead of failing early.

Where Incoloy 800HT Fasteners Are Used

The alloy is specified wherever a joint must stay tight at high temperature and in corrosive media. In plant piping it bolts pipe flanges, heat exchanger heads and vessel covers, and it often replaces stainless bolting that would fail by chloride stress corrosion cracking.

• Petrochemical and refining — bolting for heat exchangers, piping systems and process equipment exposed to high temperature and chlorides.
• Power generation — studs and nuts for boilers, superheaters and reheaters that run through repeated start-up and shutdown cycles.
• Heat treatment equipment — baskets, trays and fixtures that see repeated high-temperature cycles and corrosive atmospheres.
• Aerospace and defence — components that need thermal stability and strength under extreme conditions.
• Nuclear power — bolting for key components in high-temperature, high-radiation environments.
• Pharmaceutical and food processing — equipment that must stay corrosion resistant during high-temperature processing.

Frequently Asked Questions

At what temperature can Incoloy 800HT fasteners be used?

The alloy keeps its mechanical properties in aqueous media up to about 500 °C and resists oxidation well above that range. Confirm the design temperature of the joint against the applicable piping or pressure vessel code before final selection.

What does the raised Ti:C ratio do for the alloy?

The very low carbon content and the higher titanium-to-carbon ratio raise structural stability, oxidation resistance and resistance to intergranular corrosion. Annealing at about 950 °C keeps the grain size fine, so bolting stays dimensionally stable through thermal cycles.

Which media limit the use of Incoloy 800HT fasteners?

Resistance is limited in sulfuric acid and hydrochloric acid, and pitting is possible in halide-bearing media. Nitric and organic acids, oxidising and non-oxidising salts, water, steam and steam-air-carbon dioxide mixtures are handled well.

How should Incoloy 800HT studs be tightened on a flange joint?

Tighten studs in a cross pattern with calibrated tooling to the bolt load required by the gasket and the flange class, and re-check the load after the first thermal cycle. Avoid repeated re-torquing of the same stud, which work-hardens the material.