UNS N07718 (Inconel 718) Superalloy: Properties, Uses & Manufacturing

If you need a superalloy that delivers unmatched strength, creep resistance, and high-temperature stability for the most demanding applications—UNS N07718 (commonly called Inconel 718) is the industry standard. Used in aerospace jet engines, gas turbines, and nuclear reactors, this alloy solves the critical problem of material failure under extreme heat and pressure. In this guide, we’ll break down its key properties, real-world uses, manufacturing steps, and how it compares to alternatives—so you can build components that perform reliably in life-or-death scenarios.

1. Material Properties of UNS N07718 (Inconel 718) Superalloy

UNS N07718’s superalloy status comes from its unique composition: niobium-titanium-aluminum precipitates for strength, chromium for corrosion resistance, and nickel for a tough, heat-resistant base. Let’s explore its properties in detail:

1.1 Chemical Composition

Every element in UNS N07718 is engineered to work in harmony—maximizing strength at high temperatures without sacrificing corrosion resistance. Below is its standard composition (per ASTM B637):

1.2 Physical Properties

These properties reflect UNS N07718’s ability to withstand extreme heat and pressure—critical for aerospace and energy applications. All values are measured at room temperature unless noted:

• Density: 8.19 g/cm³ (higher than steel, due to nickel, molybdenum, and niobium content).
• Melting Point: 1260 – 1320 °C (high enough to resist softening in gas turbine engines, which operate at 1000+ °C).
• Thermal Conductivity: 11.4 W/(m·K) (at 100 °C); 19.0 W/(m·K) (at 600 °C)—low heat transfer, ideal for components that need to retain structural integrity at high temperatures.
• Coefficient of Thermal Expansion: 12.6 × 10⁻⁶/°C (20–100 °C); 16.8 × 10⁻⁶/°C (20–600 °C)—stable expansion for precision parts like jet engine blades.
• Specific Heat Capacity: 435 J/(kg·K) (at 25 °C)—efficient at absorbing heat without rapid temperature changes, reducing thermal stress.
• Electrical Conductivity: 7.3 × 10⁶ S/m (at 20 °C)—lower than copper, but suitable for electrical components in high-heat environments.

1.3 Mechanical Properties

UNS N07718’s mechanical properties are unmatched for high-stress, high-temperature applications—its strength actually increases with heat (up to 650 °C) due to precipitate formation. Below are typical values (age-hardened condition, per ASTM B637):

1.4 Other Properties

• Corrosion Resistance: Very Good. Resists:
  • Oxidation up to 870 °C (thanks to chromium and aluminum).
  • Seawater corrosion and pitting (due to molybdenum).
  • Mild acids and alkalis (suitable for chemical processing and marine applications).
• Oxidation Resistance: Excellent. Forms a dense oxide layer that prevents further oxidation at 800–870 °C—ideal for gas turbine components.
• Weldability: Good (with care). Requires preheating (200–300 °C) and post-weld heat treatment (solution annealing + age hardening) to restore strength; use ERNiFeCr-2 filler metal.
• Machinability: Fair. Work hardens rapidly—requires sharp carbide tools, slow cutting speeds (5–10 m/min for turning), and high-pressure cutting fluids to reduce friction.
• Formability: Moderate. Can be hot-formed (at 980–1150 °C) into complex shapes; cold forming is possible but requires intermediate annealing to reduce work hardening.

2. Applications of UNS N07718 (Inconel 718) Superalloy

UNS N07718 is used in applications where failure is catastrophic—industries where component strength and reliability directly impact safety and efficiency. Here are its most common uses, with real examples:

2.1 Aerospace and Jet Engines

• Examples: Jet engine turbine blades, combustion chambers, afterburner components, and aircraft structural parts (e.g., landing gear for high-temperature environments).
• Why it works: High-temperature strength (up to 650 °C) resists engine heat, while creep resistance ensures long blade life. A U.S. aerospace manufacturer used UNS N07718 for turbine blades—blade life increased by 500% vs. Inconel 625.

2.2 Gas Turbines (Energy Industry)

• Examples: Gas turbine rotors, stator vanes, and combustion liners for power generation (natural gas or coal-fired plants).
• Why it works: Creep resistance handles long-term operation at 1000+ °C, while corrosion resistance resists turbine exhaust gases. A German energy firm used UNS N07718 for turbine rotors—rotor life extended to 20 years (vs. 12 years for other superalloys).

2.3 Oil and Gas Industry

• Examples: Downhole tools (for high-temperature, high-pressure reservoirs), subsea wellheads, and pipeline components (for sour gas with high sulfur content).
• Why it works: Resists sulfide stress cracking and creep at 200+ °C. A Saudi Arabian oil company used UNS N07718 downhole tools—tools operated for 10 years without failure (vs. 3 years for stainless steel).

2.4 Nuclear Reactors

• Examples: Reactor pressure vessel components, control rod housings, and fuel handling systems.
• Why it works: Resists radiation-induced embrittlement and corrosion from reactor coolants (e.g., water, liquid sodium). A French nuclear operator used UNS N07718 for control rod housings—no maintenance issues in 18 years.

2.5 Automotive (High-Performance)

• Examples: Turbocharger rotors and exhaust components for high-performance cars or racing vehicles.
• Why it works: Withstands turbocharger heat (up to 900 °C) and resists exhaust gas corrosion. A Japanese automaker used UNS N07718 for turbo rotors—turbo life doubled vs. stainless steel rotors.

3. Manufacturing Techniques for UNS N07718 (Inconel 718) Superalloy

UNS N07718’s manufacturing is complex—its precipitate strengthening requires precise heat treatment, and its work-hardening nature demands careful machining. Here’s a step-by-step breakdown:

1. Melting:
   • Raw materials (high-purity nickel, chromium, niobium, titanium) are melted in a vacuum induction furnace (VIF) followed by vacuum arc remelting (VAR) or electroslag remelting (ESR). This dual melting ensures ultra-low impurities and uniform composition (critical for precipitate formation).
2. Casting/Forging:
   • Molten alloy is cast into ingots (up to 5 tons for turbine rotors) or investment-cast into near-net-shape components (e.g., turbine blades).
   • Ingots are hot-forged at 980–1150 °C—forging aligns grain structure to maximize creep resistance; complex shapes (like blades) use precision forging.
3. Rolling/Forming:
   • Hot rolling (at 950–1100 °C) produces plates, bars, or tubes; cold rolling is limited to thin sheets and requires intermediate annealing (at 900–1000 °C).
4. Heat Treatment (Critical for Strength):
   • Solution Annealing: Heat to 950–1050 °C, hold 1–2 hours, water quench. Dissolves excess carbides and precipitates, preparing the alloy for age hardening.
   • Intermediate Aging: Heat to 700–760 °C, hold 2–4 hours, air cool. Forms small γ’ (gamma prime) precipitates to boost strength.
   • Final Aging: Heat to 620–650 °C, hold 8–12 hours, air cool. Forms large γ” precipitates—the main source of UNS N07718’s ultra-high strength.
5. Machining:
   • Use carbide tools with negative rake angles and sharp cutting edges to minimize work hardening.
   • Cutting speeds: 5–8 m/min (turning), 3–5 m/min (milling); feed rates: 0.05–0.10 mm/rev.
   • Use high-pressure (100–150 bar) cutting fluids (water-soluble with EP additives) to cool the tool and flush chips—prevents re-cutting work-hardened material.
6. Welding:
   • Preheat to 200–300 °C to reduce thermal stress.
   • Use TIG welding with ERNiFeCr-2 filler metal (matches composition).
   • Post-weld heat treatment: Solution anneal (980 °C) + full age hardening to restore strength (critical for load-bearing joints).
7. Surface Treatment (Optional):
   • Aluminizing (applying an aluminum coating) enhances oxidation resistance for gas turbine components operating above 870 °C.
   • Shot peening (cold working the surface) improves fatigue strength by creating compressive stress—used for turbine blades and rotors.

4. Case Study: UNS N07718 in Gas Turbine Rotors

A U.S. power generation company faced a problem: their Inconel 625 gas turbine rotors failed after 12 years due to creep deformation (loss of shape) at 1050 °C. They switched to UNS N07718, and here’s what happened:

• Process: UNS N07718 ingots were vacuum-melted, forged into rotors (2 meters diameter), solution annealed (1000 °C), age-hardened (730 °C + 630 °C), and shot-peened to improve fatigue strength.
• Results:
  • Rotor life extended to 20 years (67% improvement)—no creep deformation even after 80,000 hours of operation.
  • Power output increased by 5%—UNS N07718’s higher strength allowed the turbine to operate at higher temperatures.
  • Maintenance costs fell by $800,000/year (fewer rotor replacements, no unplanned shutdowns).
• Why it works: γ” precipitates in UNS N07718 prevented creep at high temperatures, while shot peening reduced fatigue failure risk—solving the company’s core reliability issue.

5. UNS N07718 (Inconel 718) vs. Other Superalloys

How does UNS N07718 compare to alternatives for high-stress, high-temperature applications? Let’s evaluate key properties:

Key takeaway: UNS N07718 is the strongest superalloy for high-stress, high-temperature applications. It outperforms Inconel 625 in strength and creep resistance, and is more cost-effective than Hastelloy C276—making it the top choice for aerospace, energy, and oil industries.