Introduction: Two Members of the Incoloy Family
Incoloy 800 (UNS N08800, W.Nr. 1.4876) and Incoloy 825 (UNS N08825, W.Nr. 2.4858) are among the most widely specified nickel-iron-chromium alloys. They share the same base metallurgy, yet they are engineered for different jobs: Incoloy 800 is optimized for high-temperature strength and oxidation resistance, while Incoloy 825 is optimized for corrosion resistance in acidic and chloride-rich media. Neither alloy is inherently better; the correct choice depends on the service environment, temperature and budget.
Chemical Composition: What Drives the Difference
Incoloy 800 contains 30-35% nickel, 19-23% chromium, with the balance iron and small additions of aluminum and titanium. Incoloy 825 raises nickel to 38-46% and adds molybdenum (2.5-3.5%), copper (1.5-3.0%) and titanium (0.6-1.2%). The molybdenum and copper additions are what give Incoloy 825 its strong resistance to reducing acids such as sulfuric and phosphoric acid, seawater and chloride environments, while the higher nickel content improves resistance to stress corrosion cracking. Incoloy 800, with its lower alloy content, is the more economical choice where high-temperature oxidation and carburization resistance are the primary requirements.
Mechanical and Physical Properties
Typical minimum mechanical properties of annealed material: Incoloy 800 has tensile strength of at least 450 MPa, yield strength of 170 MPa and elongation of 30%; Incoloy 825 shows tensile strength of at least 550 MPa, yield strength of 220 MPa and elongation of 30%. Both alloys are solid-solution strengthened and remain ductile and formable. Physically, Incoloy 800 has a density of about 7.94 g/cm³ and a melting range of approximately 1357-1385°C; Incoloy 825 has a density of about 8.14 g/cm³ and a melting range of approximately 1370-1400°C. Neither alloy is hardened by heat treatment in the conventional sense, so strength comes from composition, cold work and controlled processing.
Corrosion Resistance: Matching the Alloy to the Media
Incoloy 825 excels in sulfuric acid, phosphoric acid, seawater, and chloride-containing environments, showing significantly better resistance to pitting, crevice corrosion and stress corrosion cracking than Incoloy 800. It is a standard choice for chemical processing, marine pollution control, desalination and offshore equipment. Incoloy 800 provides good general corrosion resistance and excellent behavior in high-temperature oxidizing, carburizing and nitriding atmospheres, which suits furnace components, heat exchanger tubing in nitric acid media, and petrochemical cracking units. For strongly reducing acids or chloride-dominated service, Incoloy 825 is the better option; for hot oxidizing service without strong acid attack, Incoloy 800 is both adequate and less expensive.
Applications by Industry
Incoloy 800: heat treatment equipment such as baskets, pallets and clamps; petrochemical heaters and reformers; industrial furnaces, burners and heat exchangers; power generation superheaters; boiler components and pressure vessels. Incoloy 825: sulfuric and phosphoric acid production units; seawater desalination and cooling systems; marine pollution control and offshore oil platforms; chemical storage tanks, heat exchangers and scrubbers; waste treatment and pollution control equipment.
Selection Guidance
Choose Incoloy 800 when the operating environment is dominated by high-temperature oxidation, carburization or nitriding, and when cost is a priority. Choose Incoloy 825 when the media contains sulfuric or phosphoric acid, chlorides, seawater or other aggressive corrosives, or when pitting and stress corrosion cracking are a known risk. When in doubt, evaluate the actual chemical composition of the process stream, the maximum and minimum temperatures, and the applicable design code, then confirm the material certificate against the relevant ASTM specification.
FAQ
Q: Is Incoloy 825 better than Incoloy 800?
A: Neither is universally better. Incoloy 825 has superior corrosion resistance in acids and chlorides; Incoloy 800 offers better value in high-temperature oxidizing service. The right choice depends on the application.
Q: What are the UNS numbers for these alloys?
A: Incoloy 800 is UNS N08800 (W.Nr. 1.4876) and Incoloy 825 is UNS N08825 (W.Nr. 2.4858).
Q: Can Incoloy 825 be used at high temperatures like Incoloy 800?
A: Incoloy 825 can serve at elevated temperatures, but it is not designed for the same long-term creep and oxidation service as Incoloy 800 or the 800H/800HT variants. Select by the governing design temperature.
Q: Which alloy resists seawater better?
A: Incoloy 825, thanks to its molybdenum and copper additions, shows much better resistance to pitting and crevice corrosion in seawater and chloride environments.
Q: Are Incoloy 800 and 825 weldable?
A: Yes, both alloys are readily welded using conventional techniques with matching nickel-based filler metals, and both retain good ductility after welding.
Q: How do I verify the quality of Incoloy 800 or 825 material?
A: Request a material test certificate (MTC) showing the chemical composition and mechanical properties, and confirm that the product is manufactured to the applicable ASTM/ASME specification such as B163, B407 or B423.
