1. Chemical Composition
2. Mechanical Properties
Inconel 718: Exhibits high tensile strength (≈1,300–1,600 MPa in the aged condition) due to Nb-induced γ″ phase strengthening. It retains strength well up to ~650°C (1,200°F).
Inconel 925: Has moderate tensile strength (≈1,100–1,300 MPa in the aged condition), primarily strengthened by Ti/Al-induced γ′ phases. It performs reliably up to ~550°C (1,020°F).
Inconel 718: Superior creep resistance at elevated temperatures (up to 650°C) due to the stable γ″ phase, making it suitable for long-term high-stress applications like turbine components.
Inconel 925: Lower creep resistance compared to 718, limiting its use in extremely high-temperature, high-stress environments.
Inconel 718: Excellent low-temperature toughness, maintaining ductility even at cryogenic temperatures (e.g., -270°C), which is critical for applications like cryogenic storage.
Inconel 925: Good room-temperature toughness but less effective at extreme low temperatures compared to 718.
Inconel 718: Highly weldable using standard techniques (e.g., TIG, MIG) without significant cracking issues, thanks to its Nb content that suppresses hot cracking.
Inconel 925: Weldable but requires more careful heat treatment post-welding to avoid embrittlement, as its γ′ strengthening is sensitive to thermal cycles.
3. Corrosion Resistance
Resistant to general corrosion, oxidation, and pitting in mild to moderate environments (e.g., air, water, and some acids).
Less effective in aggressive chloride or sulfide environments, as it is prone to chloride stress corrosion cracking (SCC) compared to 925.
Superior resistance to chloride-induced SCC, sulfide corrosion, and acids (e.g., hydrochloric acid) due to higher Mo content and optimized Cr/Mo balance.
Excels in marine, oilfield, and chemical processing environments where exposure to harsh electrolytes is common.
4. Applications
Aerospace and Aviation: Turbine discs, blades, shafts, and casings in jet engines (due to high-temperature strength and creep resistance).
Cryogenics: Low-temperature storage tanks, rocket propulsion systems, and LNG equipment (thanks to excellent low-temperature toughness).
Nuclear Engineering: Components in nuclear reactors, where radiation resistance and high strength are required.
Oil and Gas Industry: Downhole tools, wellhead equipment, and pipelines (resistant to sulfide stress cracking and brine corrosion).
Marine Engineering: Seawater handling systems, propeller shafts, and offshore platforms (resists seawater corrosion and biofouling).
Chemical Processing: Reactors, pumps, and valves handling acids, chlorides, and other corrosive chemicals.
5. Cost
Inconel 718 is generally more expensive than Inconel 925. This is due to its higher nickel content and the inclusion of niobium, a relatively costly alloying element.
Inconel 925's lower nickel content and absence of niobium make it a more cost-effective choice for applications where extreme high-temperature strength is not critical.
