Is Incoloy 800 the Perfect High-Temperature Solution?

┃ Overview

UNS N08800, also known as Alloy 800, W.Nr. 1.4876, or Incoloy® 800, is a nickel-based alloy specifically designed to withstand corrosive and high-temperature environments. Its composition, primarily consisting of nickel, enables it to resist stress corrosion cracking and maintain a ductile, austenitic structure. The presence of chromium in the alloy provides excellent resistance to both aqueous corrosion and high-temperature oxidation. Additionally, the iron content contributes to its ability to resist internal oxidation. Moreover, Incoloy 800 exhibits favorable formability, weldability, and machinability. These exceptional properties make it a widely utilized material for constructing equipment that requires resistance to corrosion, heat, strength, and stability in service conditions of up to 1500°F (816°C).

┃ Application

In High Temperatures Applications: Heat-treating equipment such as baskets, trays, and fixtures

In Chemical Processing: Heat exchangers and other piping systems in nitric acid media

InPetroleum: Heat exchangers that air cool the process stream

InDomesticAppliances: Sheathing of electric heating elements

InPaperPulp Production: Digester-liquor heaters

InNuclearPowerPlants: Steam-generator tubing

┃ Chemical Composition (%)

┃ Mechanical Property

┃Corrosion Resistance

Incoloy 800, 800H and 800HT alloys share identical compositions of nickel, chromium, and iron, resulting in similar corrosion resistance characteristics. Similar to many austenitic stainless steels, Incoloy 800 can become sensitized, making it susceptible to intergranular attack in certain aggressive environments when exposed to temperatures ranging from 1000 to 1400°F (540–760°C). Therefore, when subjecting Incoloy 800 to heat during welding or other processes, it is crucial to exercise caution to prevent sensitization, particularly if the material is intended to be pickled or exposed to aggressive surroundings. However, sensitization is generally not a concern in most high-temperature applications.

┃Heat Treatment

Incoloy 800 is a nickel-iron-chromium alloy renowned for its outstanding resistance to oxidation, carburization, and high-temperature corrosion. Heat treatment techniques can be employed to improve specific properties of Incoloy 800, such as controlling grain size and relieving stress. The commonly employed heat treatment methods for Incoloy 800 include annealing, solution treatment, and stress relieving. It is essential to acknowledge that the precise parameters for heat treatment may vary based on factors such as the form and condition of the Incoloy 800 material, as well as the desired properties required for a particular application.

┃ Heating and Pickling

Prior to heating, it is imperative to thoroughly clean Incoloy 800 to remove any oil, paint, grease, shop soil, or other foreign substances. During the heating process, all operations should be conducted within a low-sulfur atmosphere. If heating is carried out in an open-air environment, it is necessary to utilize low-sulfur fuel and maintain a reducing condition in the furnace atmosphere to prevent excessive oxidation.

Due to the susceptibility of chromium in Incoloy 800 to oxidation by air, carbon dioxide, or water vapor, the alloy cannot undergo bright annealing in standard industrial annealing furnaces. Instead, the alloy is typically annealed in box or muffle furnaces using specially prepared reducing atmospheres. By burning low-sulfur natural gas with a controlled air deficiency, a thin, adherent, green-black oxide film forms on the material. These oxide films and scales can be removed through a process called pickling. However, it is important to note that due to Incoloy 800's inherent resistance to chemical attack, specialized pickling procedures are required.

┃ Hot and Cold Forming

For hot forming, it is recommended to work within a temperature range of 1600°F to 2250°F (870°C to 1230°C). When conducting heavy forging, the temperature range should be between 1850°F and 2200°F (1010°C to 1200°C). Forming at temperatures ranging from 1200°F to 1600°F (650°C to 870°C) can lead to cracking of the workpiece. The cooling rate after hot forming does not significantly impact thermal cracking. However, it's important to note that Incoloy is susceptible to carbide precipitation within the temperature range of 1000°F to 1400°F (540°C to 760°C), and therefore it should be rapidly cooled through this range to prevent sensitization.

The cold forming process for Incoloy 800 follows similar procedures to those used for Inconel 600 and stainless steel. The work-hardening rate of Incoloy 800 is similar to that of Inconel 600, higher than that of mild steel, but lower than that of 304 stainless steel.

┃ Machining

Incoloy 800 can be machined using standard methods with ease. Turning operations, in particular, can achieve high metal removal rates, extended tool life, and excellent surface finish when employing coated carbide tools. Additionally, satisfactory outcomes have been achieved using high-speed-steel tools, which are particularly suitable for interrupted cutting processes.

┃Welding

Inconel 800 exhibits excellent weldability across various welding processes. When using shielded metal-arc welding, the recommended welding product for Alloy 800 is the INCO-WELD A Electrode. For gas-tungsten-arc welding, gas-metal arc welding, and submerged-arc welding, it is advisable to utilize INCONEL Filler Metal 82. In the case of submerged-arc welding, INCOFLUX 4 Submerged Arc Flux is commonly employed as a complementary flux.

┃ Product Forms

Ø Bar & Rod

Ø Plate & Sheet

Ø Coil & Strip

Ø Pipe & Tube

Ø Fitting: Flange, Tee, Elbow, Reducer etc.

Ø Forging: Ring, Shaft, Circle, Block etc.

┃ Note

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