Nickel Based alloy Incoloy 800H Characteristics and Applications

Incoloy 800H is a nickel-iron-chromium alloy built for service where mechanical strength and corrosion resistance must both survive at high temperature. It belongs to a family with grades 800 and 800HT, and the differences between the three decide which grade is specified for a reactor tube, a fastener or a bolted joint, including the flange gaskets that keep that joint tight.

Composition and Metallurgical Structure

The nominal composition of the 800 family balances nickel at 30-35% and chromium at 19-23%, with iron as the remainder. Carbon is the controlling element: grade 800 allows up to 0.10%, while 800H starts at 0.05% and 800HT at 0.06%. Aluminium and titanium are added for structural stabilisation.

800H has a face-centred cubic lattice. Its very low carbon content and raised Ti:C ratio improve structural stability and resistance to sensitisation and intergranular corrosion, and annealing at around 950 degC leaves a fine grain structure.

• Nickel 30-35%, chromium 19-23%, iron as the balance.
• Carbon rises to 0.05-0.10% in grade 800H and 0.06-0.10% in grade 800HT, against a 0.10% maximum in grade 800.
• Aluminium and titanium 0.15-0.60% each; on grade 800HT the two total 0.85-1.20%.
• Annealing at around 950 degC produces a fine grain structure.

Chemical composition limits of Incoloy 800, 800H and 800HT, % by weight (Fe = balance). On grade 800HT, Al+Ti totals 0.85-1.20.

Physical and Mechanical Properties

Two physical values drive high-temperature design: density is 8.0 g/cm3 and the melting range is 1350-1400 degC. The specified minimum mechanical properties at room temperature are listed below.

• Alloy 800 - tensile strength Rm 500 N/mm2, yield strength Rp0.2 210 N/mm2, elongation A5 35%.
• Alloy 800H - tensile strength Rm 450 N/mm2, yield strength Rp0.2 180 N/mm2, elongation A5 35%.
• For applications up to 500 degC the alloy is supplied in the annealed condition.

In service the alloy keeps strength and dimensional stability at elevated temperature, and its oxidation and creep resistance are markedly better than those of ordinary stainless steels and carbon steels.

Corrosion Resistance and Service Limits

800H resists most corrosive media. The high nickel content gives good resistance to stress corrosion cracking in aqueous conditions, while the high chromium content improves resistance to pitting and crevice corrosion.

• Good resistance: nitric acid, organic acids, oxidising and non-oxidising salts, water and steam, and steam/air/carbon dioxide mixtures.
• Limited resistance: sulfuric acid and hydrochloric acid.
• Risk to watch: pitting in media that contain halides.
• Mechanism: very low carbon plus a raised Ti:C ratio keeps sensitisation and intergranular corrosion in check.

Selection is therefore driven by the medium. Halide content, temperature and the presence of oxygen decide whether the alloy fits, and welded or heavily cold-worked parts should be assessed separately for intergranular attack.

Applications in Equipment and Fasteners

The alloy is specified where high temperature and corrosion meet: petrochemical reactors, furnace tubes, steam heating lines, heater element tubes and nitric acid condensers. The piping between them is joined through pipe flanges and bolting that has to tolerate the same heat.

• Nitric acid condenser tubes - resistance to nitric acid corrosion.
• Steam heating pipes and heater element tubes - mechanical properties held at temperature.
• Fasteners for critical joints - bolts, stud bolts and nuts exposed to heat and oxidation.
• Aircraft parts and reactor fasteners - high-temperature and oxidation resistance.
• For service up to 500 degC the material is supplied in the annealed condition.

Because the alloy keeps its properties through repeated thermal cycles, it is also chosen for bolts and nuts in joints where ordinary stainless steel would oxidise or lose strength.

Frequently Asked Questions

What is the difference between Incoloy 800, 800H and 800HT?

All three share nickel at 30-35% and chromium at 19-23%. The difference is carbon: 800H carries 0.05-0.10% C and 800HT carries 0.06-0.10% C against a 0.10% maximum in grade 800, and on 800HT aluminium plus titanium total 0.85-1.20%.

At what temperature is Incoloy 800H used?

The data above covers service up to 500 degC, where the alloy is supplied in the annealed condition. Above that range creep behaviour governs, and the design has to be verified case by case.

Why does 800H resist intergranular corrosion?

Very low carbon and a raised Ti:C ratio limit sensitisation, and annealing at around 950 degC produces a fine grain structure.

Which acids can Incoloy 800H handle?

Nitric acid and organic acids are handled well, as are many oxidising and non-oxidising salts. Sulfuric and hydrochloric acid give only limited resistance, and halides may cause pitting.

Is Incoloy 800H suitable for fasteners?

Heat, corrosion and oxidation resistance make it a candidate for critical bolting in petrochemical equipment, aircraft parts and reactor fasteners, provided the joint is designed for the same temperature range.