Inconel 718 is a nickel-based superalloy (also referred to as a high-performance nickel-chromium alloy) classified as a non-ferrous metal. It is specifically an age-hardenable (precipitation-hardenable) alloy, meaning its mechanical properties-such as strength, hardness, and creep resistance-can be significantly enhanced through a controlled heat treatment process called "aging." This process promotes the formation of fine, uniformly distributed intermetallic precipitates (primarily γ' and γ'' phases) within the alloy's microstructure, which strengthen the material.
Notably, Inconel 718 exhibits exceptional stability and performance under extreme conditions, including high temperatures (up to approximately 650°C / 1200°F), corrosive environments, and high mechanical stress. These characteristics make it a critical material in aerospace, power generation, and chemical processing industries, where durability under harsh conditions is paramount.
Inconel 718 has a complex chemical composition, with the following key elements (typical ranges per industry standards like ASTM B637):
Nickel (Ni): 50–55% – Serves as the primary base element, providing corrosion resistance and high-temperature stability.
Chromium (Cr): 17–21% – Enhances oxidation and corrosion resistance, particularly in high-temperature and aggressive chemical environments.
Iron (Fe): Balance – Acts as a secondary base element, aiding in workability and reducing material cost.
Niobium (Nb) + Tantalum (Ta): 4.75–5.50% – Critical for age hardening; forms γ'' (niobium-rich) precipitates, the primary strengthening phase.
Molybdenum (Mo): 2.8–3.3% – Improves strength, creep resistance, and resistance to pitting corrosion.
Titanium (Ti): 0.65–1.15% – Assists in forming γ' (nickel-titanium-aluminum) precipitates, contributing to additional strength.
Aluminum (Al): 0.2–0.8% – Works with titanium to form γ' precipitates and enhances age hardening.
Trace elements (in smaller amounts, typically <0.5%) include:
Carbon (C), Manganese (Mn), Silicon (Si), Phosphorus (P), Sulfur (S), Copper (Cu), and Cobalt (Co). These are tightly controlled to avoid detrimental effects on mechanical properties or corrosion resistance.
The primary ASTM International standard governing Inconel 718 is ASTM B637. This standard specifies requirements for precipitation-hardenable nickel-alloy bars, forgings, and forging stock designed for moderate to high-temperature service. It covers chemical composition, mechanical properties (tensile strength, yield strength, elongation), heat treatment procedures, and dimensional tolerances for these forms of Inconel 718.
Other related ASTM standards may apply to specific product forms of Inconel 718, such as:
ASTM B670 – For seamless and welded nickel-alloy tubing.
ASTM B906 – For nickel-alloy wire.
ASTM B861 – For nickel-alloy plate, sheet, and strip.
However, ASTM B637 remains the most widely referenced standard for general Inconel 718 bar and forging applications.
Inconel 718 is known by several alternative names, including:
Alloy 718 (the most common generic name).
UNS N07718 (designation by the Unified Numbering System for metals and alloys).
W.Nr. 2.4668 (German material numbering system).
Trade names such as Haynes 718, Alvac 718, Nicrofer 5219 (by ThyssenKrupp), and Altemp 718.
These names are interchangeable in industrial contexts, referring to the same nickel-based superalloy with the characteristic composition and properties of Inconel 718.
