Table of Contents
Introduction
Engineered for extreme environments, Inconel 718 is a nickel-based alloy offering exceptional strength, corrosion resistance, and temperature stability. Developed for demanding industries like aerospace and energy, Alloy 718’s properties make it ideal for high-stress applications. This article covers Inconel 718’s key specifications, advantages, applications, and comparisons with other alloys.
What is Inconel 718?
Inconel 718 (UNS N07718) is a high-strength nickel-chromium alloy known for its corrosion resistance and stability in extreme temperatures, from cryogenic up to 1300°F (704°C). Developed in the 1960s, it is widely used in aerospace, oil & gas, and power generation industries. With elements like niobium and molybdenum, Alloy 718 offers exceptional tensile, yield, and creep-rupture strength, making it ideal for high-stress environments. Its unique composition also ensures excellent weldability and resistance to oxidation, reinforcing its role as a leading superalloy for critical applications.
What Are the Specifications of Inconel Alloy 718?
718 Inconel Equivalent
| Standard | Equivalent Alloy |
| UNS | N07718 |
| ASTM | ASTM B637, ASTM B670 |
| DIN | 2.4668 |
| AMS | AMS 5662, AMS 5663 |
| ISO | ISO 15156-3, ISO 6208 |
| AFNOR | NC19FeNb |
| BS | NA51 |
| SAE | SAE J467 |
Inconel 718 ASTM Standard
| Form | ASTM Standard |
| Bar | ASTM B637 |
| Sheet, Strip, Plate | ASTM B670 |
Inconel 718 Chemical Composition
| Element | Percentage (%) |
| Nickel (Ni) | 50.0 – 55.0 |
| Chromium (Cr) | 17.0 – 21.0 |
| Iron (Fe) | Balance |
| Niobium (Nb) + Tantalum (Ta) | 4.75 – 5.50 |
| Molybdenum (Mo) | 2.8 – 3.3 |
| Titanium (Ti) | 0.65 – 1.15 |
| Aluminum (Al) | 0.20 – 0.80 |
| Cobalt (Co) | 1.0 max |
| Manganese (Mn) | 0.35 max |
| Silicon (Si) | 0.35 max |
| Carbon (C) | 0.08 max |
| Phosphorus (P) | 0.015 max |
| Sulfur (S) | 0.015 max |
Inconel Metal 718 Mechanical Properties
| Property | Value |
| Tensile Strength | 965 MPa (140,000 psi) minimum |
| Yield Strength | 550 MPa (80,000 psi) minimum |
| Elongation | 12% minimum |
| Hardness (Rockwell) | 35 HRC maximum |
| Creep-Rupture Strength | Excellent at temperatures up to 1300°F (704°C) |
Inconel Metal 718 Physical Properties
| Property | Value |
| Density | 8.19 g/cm³ (0.296 lb/in³) |
| Melting Point | 1260 – 1340°C (2300 – 2440°F) |
| Thermal Conductivity | 11.4 W/m·K at 200°C |
| Specific Heat Capacity | 435 J/kg·K at 20°C |
| Electrical Resistivity | 1.29 μΩ·m at 20°C |
| Coefficient of Thermal Expansion | 13.0 µm/m·K (20 – 1000°C) |
Nickel Alloy 718 Corrosion Resistance
| Environment | Corrosion Resistance |
| Oxidizing Environment | Excellent resistance; can withstand temperatures up to 1300°F (704°C) |
| Reducing Environment | High resistance in reducing conditions due to its chromium content |
| Chloride-Ion Stress | Resistant to stress-corrosion cracking in chloride environments up to 500°F (260°C) |
| Seawater | Good resistance to pitting and crevice corrosion in seawater up to 600°F (316°C) |
| High-Temperature Oxidation | Excellent oxidation resistance at temperatures up to 1300°F (704°C) |
Inconel 718 Heat Treatment
| Heat Treatment Process | Details |
| Solution Annealing | Heat to 1750 – 1800°F (954 – 982°C) for 1 hour, then air cool |
| Precipitation Hardening Treatment | Heat to 1325°F (718°C) for 8 hours, then furnace cool to 1150°F (621°C) and hold for 10 hours; air cool |
| Aging | Aging treatment improves tensile strength and hardness, resulting in optimized mechanical properties for high-stress applications |
What Are the Advantages of Inconel Alloy 718?
- High Strength in Extreme Temperatures: Maintains excellent tensile, yield, and creep-rupture strength from cryogenic to 1300°F (704°C), ideal for aerospace and oil & gas.
- Corrosion and Oxidation Resistance: Effective in oxidizing and reducing environments, resists pitting, crevice corrosion, and chloride stress-corrosion cracking.
- Weldability and Fabrication: Easier to weld than many superalloys, with resistance to postweld cracking; retains strength after welding.
- Machinability for Fabrication: Age-hardenable, allowing easier machining, beneficial for aerospace and automotive uses.
- Long-Term Durability: Structural stability and thermal fatigue resistance make it ideal for components exposed to heat, like turbine blades.
What is Inconel 718 Used For?
- Aerospace: Jet engines, turbine blades, high-stress components
- Oil & Gas: Downhole tools, valves, fasteners
- Power Generation: Gas turbines, nuclear power components
- Marine Applications: Seawater-resistant hardware, subsea equipment
- Automotive and Motorsports: Turbocharger rotors, exhaust valves
- Chemical Processing: Reactors, heat exchangers, piping systems
Comparison with Other Inconel Alloys
Inconel Alloy 718 stands out among other Inconel alloys for its unique properties, especially its high strength, corrosion resistance, and weldability. Below is a comparison of Inconel Alloy 718 with other popular Inconel alloys, such as Inconel 625 and Inconel X750.
Inconel 718 vs 625
Inconel 625 withstands higher temperatures (up to 2000°F/1093°C), while Inconel 718 is optimized for up to 1300°F (704°C). Alloy 718 has superior tensile and yield strength due to precipitation hardening, ideal for high-stress applications. Both resist corrosion, but Inconel 625 excels in seawater and harsh environments. 718 Inconel offers better weldability, with resistance to postweld cracking.
Inconel 718 vs x750
Inconel X750 handles temperatures up to 1500°F (816°C), higher than Inconel 718’s 1300°F (704°C). 718 Inconel provides greater strength, suitable for high-stress uses, while X750 is often used in gas turbines and high-temperature fasteners. Both alloys resist corrosion, though Inconel Alloy 718 generally performs better in aggressive environments.
Conclusion
Inconel 718’s unmatched strength, corrosion resistance, and thermal stability make it indispensable in high-stress industries. From aerospace to energy, its reliability in extreme conditions secures its position as a top material choice. For more on how Inconel 718 can meet your project needs, reach out to us or explore our catalog for full specifications.
FAQs
Is Inconel 718 hard to machine?
Yes, Inconel 718 is challenging to machine due to its high strength, work-hardening properties, and heat resistance. It requires specialized tools and careful machining techniques, such as lower cutting speeds and increased cooling, to prevent excessive tool wear and ensure precision.
What is the machinability rating of Inconel 718?
Inconel 718 has a machinability rating of approximately 12% relative to standard steel. This low rating indicates that it is much more difficult to machine than common metals, and additional time and resources may be needed for effective processing.
What are the disadvantages of Inconel 718?
The main disadvantages of Inconel 718 include its high cost and difficulty in machining, which can increase production time and expenses. Additionally, it requires careful heat treatment to avoid brittleness and is limited to maximum operating temperatures of around 1300°F (704°C), which is lower than some other superalloys.
Can Inconel 718 be hardened?
Yes, Inconel 718 can be hardened through precipitation hardening, also known as age hardening. This heat treatment process increases the alloy’s strength and durability, making it suitable for high-stress applications.
Can you stress relieve Inconel 718?
Yes, Inconel 718 can undergo stress relief to reduce residual stresses from fabrication processes. This is typically done by heating the alloy to around 1100°F (593°C) for a specified period, allowing it to achieve improved stability and performance without significantly altering its mechanical properties.
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