Inconel 718 Superalloy | UNS N07718 | W.Nr. 2.4668

1. Introduction

Inconel 718 is a precipitation-hardened nickel-based superalloy known for its high strength at elevated temperatures (up to 650°C), excellent corrosion resistance, and good manufacturability.

Its strength comes from a unique combination of alloying elements—especially niobium, which forms hardening phases that enhance mechanical performance without compromising weldability.

Developed in the 1960s by Special Metals Corporation, Inconel 718 addressed key shortcomings in earlier alloys, such as poor weldability and limited high-temperature strength.

Today, it plays a vital role in aerospace, power generation, and oil & gas industries, where structural integrity under extreme conditions is critical.

2. What Is Inconel 718？

Inconel 718 (UNS N07718; W.Nr. 2.4668) is a nickel-chromium-niobium superalloy widely regarded as a “workhorse” material due to its exceptional balance of performance and processability.

Unlike many high-strength superalloys, it offers excellent high-temperature strength alongside superior weldability and machinability, making it highly versatile across complex manufacturing environments.

Its global adoption is reflected in numerous international standards, including ASTM B637 (bars and rods), AMS 5662 (aerospace-grade forgings), and ISO 9723 (European rod and wire specifications), ensuring consistent quality and reliability across industries.

Chemical Composition

Inconel 718’s properties come from its carefully balanced chemical composition:

3. Mechanical Properties of Inconel 718 at Different Temperatures

4. Corrosion and Oxidation Resistance

Inconel 718 offers outstanding resistance to a wide range of corrosive environments, making it highly reliable in applications exposed to aggressive chemicals, moisture, and high-temperature oxidizing conditions.

Its corrosion resistance is primarily due to its high nickel (Ni) and chromium (Cr) content, along with molybdenum (Mo) and niobium (Nb) for enhanced pitting and crevice corrosion resistance.

Corrosion Resistance

Inconel 718 is often used in downhole oilfield tools, chemical reactors, fasteners, and heat exchangers where corrosion is a critical factor.

Oxidation Resistance

Inconel 718 performs well in oxidizing atmospheres up to ~980°C, although its mechanical strength limits practical service to ~650°C.

• Protective oxide film: The alloy forms a dense Cr₂O₃ (chromia) layer, which resists spalling and protects against further oxidation.
• Thermal cycling: Maintains surface integrity during repeated heating and cooling, suitable for jet engine components and turbine hardware.
• Sulfidation: Better resistance compared to stainless steels in high-sulfur combustion environments, though less than some higher Mo-containing alloys.

Wear and Galling Resistance

Though not a primary wear alloy, Inconel 718 shows good resistance to adhesive wear and galling, especially in bolted joints and valve components operating under high loads and temperatures.

5. Manufacturing and Processing Techniques

While Inconel 718 exhibits excellent mechanical properties and corrosion resistance, its high strength and work-hardening tendency introduce processing challenges.

Nevertheless, its fabricability surpasses many other nickel-based superalloys, owing to its balanced chemistry and microstructural stability.

Welding

• Preferred Process: Gas Tungsten Arc Welding (GTAW or TIG) is the most common, utilizing matching filler metals such as ERNiFeCr-2 to ensure compositional compatibility.
• Key Considerations:

• • Precise control of heat input (typically 100–150 A current) is vital to prevent liquation cracking and hot cracking in the heat-affected zone (HAZ).
  • Pre-weld cleaning and shielding gas purity reduce contamination and porosity.
  • Post-weld heat treatment (PWHT) involves solution annealing around 980°C, followed by double aging to restore mechanical strength and precipitate distribution.

• Performance: Welded joints typically retain approximately 85% of the base metal’s tensile strength at room temperature and up to 90% at elevated temperatures (~650°C), making welded Inconel 718 components highly reliable in critical environments.

Machining

• Challenges:

• • Inconel 718’s rapid work hardening—where the machined surface hardness can increase by up to 50%—leads to accelerated tool wear and thermal stresses.
  • Low thermal conductivity causes heat to concentrate at the cutting zone.

• Solutions:

• • Use carbide tools with advanced coatings such as TiAlN to improve tool life and reduce adhesion.
  • Apply low cutting speeds (5–10 m/min) combined with higher feed rates to minimize heat buildup.
  • Employ high-performance coolants with extreme pressure (EP) additives for efficient heat dissipation.

• Cost Impact: Due to tooling wear and slower speeds, machining Inconel 718 can be 3 to 4 times more expensive than common stainless steels like 316L, influencing part design and production economics.

Forging and Forming

• Hot Forging:

• • Conducted between 980°C and 1,040°C to reduce yield strength and increase ductility during deformation.
  • Allows effective grain refinement and uniform distribution of strengthening precipitates.
  • Subsequent heat treatments restore full mechanical properties.

• Cold Forming:

• • Generally limited to light bending and shaping due to the alloy’s high strength and work hardening.
  • Requires intermediate annealing (around 900°C) to relieve stresses and restore ductility for more complex shapes.

• Careful control of forming parameters prevents cracking and surface defects.

Casting

• Investment Casting is often used for complex geometries like turbine blades and rocket motor components.
• However, wrought (forged or rolled) forms of Inconel 718 are more common for critical load-bearing applications due to:

• • Better mechanical properties—typically, cast Inconel 718 exhibits ~10% lower tensile strength and reduced fatigue life compared to wrought material.
  • More uniform microstructure and fewer casting defects.

• Casting requires strict control of solidification rates and post-cast heat treatments to reduce segregation and porosity.

Additive Manufacturing

• Recent advances have enabled Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM) of Inconel 718.
• AM offers:

• • Complex geometries without tooling.
  • Reduced material waste.
  • Fine microstructures with potentially improved mechanical properties.

• Post-processing (hot isostatic pressing, heat treatment) remains essential to reduce porosity and optimize precipitation phases.

6. Heat Treatment of Inconel 718

Heat treatment is fundamental to achieving the exceptional mechanical properties of Inconel 718.

The process primarily involves solution annealing followed by precipitation (age) hardening, which controls the size, distribution, and volume fraction of key strengthening phases—mainly γ″ and γ′—within the nickel-based matrix.

• Solution Annealing: Heating to 980–1,065°C for 1–2 hours, followed by water quenching. This dissolves excess precipitates and homogenizes the microstructure.
• Intermediate Aging: Heating to 720°C for 8 hours, then furnace-cooling to 620°C at 50°C/hour.
• Final Aging: Holding at 620°C for 8 hours, then air-cooling. This promotes the formation of γ″ and γ′ precipitates, maximizing strength.

δ-Phase Control

• The δ-phase (Ni₃Nb) forms primarily along grain boundaries between 750°C and 900°C.
• While small amounts of δ-phase help control grain growth and improve creep resistance, excessive δ-phase reduces the niobium available for γ″ precipitation, leading to diminished strength.
• Heat treatment schedules are carefully controlled to balance δ-phase formation, ensuring optimal mechanical properties and toughness.

7. Applications of Inconel 718

Inconel 718’s unique combination of high strength, excellent corrosion resistance, and thermal stability makes it a material of choice across a wide range of demanding industries.

Aerospace Industry

• Turbine Engines:
  Inconel 718 is extensively used for turbine discs, blades, and seals in jet engines, where it withstands high centrifugal forces and elevated temperatures (up to 650°C) without compromising mechanical integrity.
• Rocketry and Spacecraft:
  Components such as rocket motor cases, thrust chambers, and fasteners benefit from its strength-to-weight ratio and excellent weldability, critical for launch vehicle reliability.
• Airframe Components:
  Its corrosion resistance and fatigue strength are ideal for landing gear parts, bushings, and structural components exposed to harsh environments.

Power Generation

• Gas Turbines:
  Inconel 718 is commonly used in compressor blades, discs, and turbine components due to its high-temperature strength and creep resistance.
• Steam Turbines:
  Utilized in parts exposed to high temperature and steam environments, where oxidation resistance is essential.

Oil & Gas Sector

• Downhole Tools:
  Inconel 718’s corrosion resistance and strength under high pressure and temperature make it ideal for drill collars, stabilizers, and packers in harsh subterranean environments.
• Valves and Fittings:
  Resistant to sulfide stress cracking and chloride-induced corrosion, it’s widely used in valves, pumps, and pressure vessels handling sour gas and corrosive fluids.
• Offshore Platforms:
  Exposure to marine environments necessitates alloys like Inconel 718 to combat saltwater corrosion and mechanical stress.

Emerging and Specialized Applications

• Additive Manufacturing:
  The alloy’s compatibility with laser powder bed fusion (LPBF) and electron beam melting (EBM) enables the production of complex, lightweight parts previously impossible with traditional manufacturing.
• High-Speed Machining Tools:
  Cutting tools made from Inconel 718 withstand high thermal and mechanical loads, extending tool life in demanding operations.
• Cryogenic Applications:
  Its retained toughness at extremely low temperatures (down to -270°C) makes it suitable for liquefied gas storage and transport equipment.

8. Performance Limitations and Failure Modes

• Thermomechanical Fatigue (TMF): Fails after 500–1,000 cycles (25°C to 650°C) under combined thermal and mechanical stress, a risk in turbine engines.
• Thermal Embrittlement: Prolonged exposure above 700°C causes δ-phase coarsening, reducing toughness by 30% and increasing fracture risk.
• Stress-Corrosion Cracking (SCC): Occurs in hot (>100°C) chloride solutions (e.g., offshore cooling systems) at stresses >70% of yield strength, though rare in well-designed systems.

9. Advantages and Limitations

Inconel 718 stands out as one of the most versatile and widely used nickel-based superalloys, offering a unique combination of properties that make it suitable for extreme and demanding environments:

Advantages of Inconel 718

Exceptional High-Temperature Strength

• Maintains high tensile, yield, and creep strength up to approximately 650°C, outperforming many other alloys in similar conditions.

Outstanding Corrosion and Oxidation Resistance

• Forms a stable, protective oxide layer that resists oxidation at elevated temperatures.
• Highly resistant to chloride, sulfur, and acidic environments, making it ideal for harsh chemical and marine applications.

Good Fabricability and Weldability

• Unlike many other nickel-based superalloys, Inconel 718 can be welded reliably with minimal cracking risks.
• Its machinability, while challenging, is better than many other superalloys, enabling efficient manufacturing.

Excellent Creep and Fatigue Resistance

• Exhibits superior resistance to creep deformation and thermomechanical fatigue, essential for aerospace turbine components and power generation equipment.

Wide Temperature Range Performance

• Maintains mechanical properties from cryogenic temperatures (-270°C) to high temperatures (~650°C).

Microstructural Stability

• Controlled precipitation hardening and stable microstructure reduce phase instability and prolong component life.

Compatibility with Advanced Manufacturing

• Adaptable to additive manufacturing techniques such as laser powder bed fusion (LPBF) and electron beam melting (EBM), enabling complex part geometries.

Limitations of Inconel 718

• High Cost: Raw material ($40–60/kg) is 8–10× 316L; machining adds 30–50% to fabrication costs.
• Processing Complexity: Requires specialized melting (VIM-VAR) and tooling, limiting accessibility.
• Temperature Ceiling: Ineffective above 650°C; replaced by Inconel 738 or single-crystal alloys for >700°C.

10. Comparison with Other Materials

Inconel 718 is often compared to other superalloys, stainless steels, and titanium alloys when selecting materials for extreme environment applications.

Understanding these comparisons helps highlight its strengths and limitations.

Inconel 718 vs. Other Nickel-Based Superalloys

• Summary: Inconel 718 offers a balanced combination of high strength and relatively good fabricability, unlike some other superalloys that prioritize high-temperature strength at the expense of manufacturability.

Stainless Steel vs. Inconel 718

• Summary: Inconel 718 outperforms stainless steels at elevated temperatures and under aggressive corrosion conditions, making it the preferred choice when strength and oxidation resistance are critical.

Inconel 718 vs. Titanium Alloys

• Summary: Titanium alloys excel in strength-to-weight ratio and corrosion resistance at moderate temperatures, but Inconel 718 remains superior for higher temperature applications where strength retention is essential.

Key Takeaways

• Strength and Temperature: Inconel 718 offers exceptional strength and stability at temperatures up to about 700°C,
  surpassing most stainless steels and titanium alloys, but slightly less than some specialized superalloys.
• Fabricability: It provides better weldability and machinability compared to many other nickel-based superalloys, reducing manufacturing complexity and cost.
• Corrosion Resistance: Inconel 718 is highly resistant to oxidation, chloride, and acid corrosion, making it suitable for marine, chemical, and high-temperature oxidizing environments.
• Cost: Generally more expensive than stainless steels and titanium alloys but offers superior performance in harsh environments where failure is not an option.

11. Conclusion

Inconel 718 remains one of the most versatile and widely used superalloys in high-performance engineering.

Its unique ability to combine high strength, fatigue life, corrosion resistance, and manufacturing adaptability makes it indispensable in mission-critical applications.

While newer alloys may offer better high-temperature properties, Inconel 718’s processability, cost-effectiveness, and well-documented performance ensure its continued dominance in aerospace, energy, and advanced manufacturing sectors.

FAQs

What is the maximum operating temperature for Inconel 718?

It performs reliably up to 650°C (1,200°F) for continuous service. For short durations (minutes to hours), it can withstand up to 800°C, but strength declines significantly above 650°C.

Is Inconel 718 magnetic?

No. Its austenitic microstructure remains non-magnetic in all conditions, unlike duplex stainless steels or some nickel-iron alloys.

How does Inconel 718 compare to titanium alloys?

Inconel 718 offers higher strength at >400°C (titanium alloys lose strength rapidly above 300°C) but is denser (8.1 g/cm³ vs.

Ti-6Al-4V’s 4.43 g/cm³), making it less suitable for weight-critical, low-temperature applications.

Can Inconel 718 be used in seawater?

Yes. Its corrosion rate in seawater is <0.02 mm/year, making it suitable for subsea components, though it is more costly than 316L or duplex stainless steels for non-high-temperature marine applications.

What causes Inconel 718 to fail?

Common modes include thermomechanical fatigue (TMF) in turbine engines, thermal embrittlement above 700°C, and rare SCC in hot chloride environments.

Proper design (stress <70% yield) and heat treatment mitigate these risks.