Incoloy grades

Chemical composition

Alloy 800HT also requires Al + Ti 0.85–1.20%. Alloys 800H and 800HT have grain size and heat-treatment requirements for elevated-temperature service, as given in the standards.

Alloys 800, 800H and 800HT share the same base composition; they differ in carbon range, aluminium and titanium control, grain size and heat treatment, which together set the creep and rupture strength at high temperature. Material is often dual certified 800H/800HT.

Incoloy products and specifications

Flanges are made to ASME B16.5 and B16.47, butt-weld fittings to ASME B16.9 and forged fittings to ASME B16.11.

Incoloy 800, 800H and 800HT – high-temperature service

With around 32% nickel and 21% chromium, alloys 800, 800H and 800HT resist oxidation, carburisation and sulphidation at high temperatures and keep a stable austenitic structure through long service. Alloy 800 is used up to about 600 °C, where strength is governed by tensile properties, for heat exchangers, process piping, sheathing for electric heating elements, and chemical and food processing equipment.

Above about 600 °C, design is governed by creep and rupture strength, so the higher-carbon, coarse-grained 800H and 800HT are specified. They are used for steam methane reformer outlet manifolds and pigtails, ethylene cracking furnace components, hydrocarbon cracking, heat-treatment furnace parts and radiant tubes, and superheater and reheater tubing in power plants.

Incoloy 825 – corrosion resistance

Alloy 825 adds molybdenum and copper to a higher nickel content, giving resistance to sulphuric and phosphoric acids, to chloride stress corrosion cracking and to pitting and crevice corrosion in many environments. Titanium stabilisation protects it against intergranular corrosion after welding. It is used for sulphuric acid pickling equipment, phosphoric acid evaporators, pollution control and flue-gas scrubbers, nuclear fuel reprocessing, and oil and gas – where 825 is used for downhole tubing, flowlines, and as a cladding or lining inside carbon steel pipe for sour and wet gas service.

Compared with 316L stainless steel, alloy 825 offers much better resistance to chloride stress corrosion cracking and to reducing acids, at a lower cost than the nickel-chromium-molybdenum alloys such as Inconel 625 and Hastelloy C276. This makes it a common step up when stainless steel is no longer adequate but the most highly alloyed materials are not needed.

Working with Incoloy

Incoloy alloys are formed hot or cold and welded with matching or nickel-based filler metals using the common arc processes. Joints must be clean and free of sulphur, lead and low-melting contaminants. For alloys 800H and 800HT used above about 600 °C, the solution annealed, coarse-grained condition must be maintained, and any cold-forming strain should be considered against the design code's requirements for post-forming heat treatment. Like other nickel alloys, Incoloy work-hardens during machining, so rigid set-ups and sharp tools are needed.

Documents and inspection

Incoloy material is supplied with mill test certificates (MTC) showing heat number, chemical composition, mechanical properties, heat treatment and, for 800H and 800HT, grain size. Third-party inspection is available on request, and alloy 825 can be supplied to NACE MR0175 / ISO 15156 on request for sour service. Other nickel alloys are covered on our materials page.