In high-temperature and high-stress industrial environments, nickel-based superalloys are the materials of choice. Among the most popular members of this family are INCONEL 617 and INCONEL 718, both known for their remarkable combination of strength, oxidation resistance, and corrosion protection.
While they belong to the same INCONEL group, their performance characteristics and alloying principles are quite different. INCONEL 617 is optimized for high-temperature strength and oxidation resistance, whereas INCONEL 718 is engineered for exceptional strength through precipitation hardening at moderate to high temperatures.
In this article, SASAALLOY provides a comprehensive comparison between INCONEL 617 and 718, highlighting their composition, properties, applications, and selection guidelines.
1. Overview of INCONEL 617 and 718
INCONEL 617 – The High-Temperature Performer
INCONEL 617 is a nickel-chromium-cobalt-molybdenum alloy designed for outstanding strength and oxidation resistance at temperatures up to 1100°C. It is a solid-solution strengthened alloy, meaning its strength comes from the uniform distribution of alloying elements within the nickel matrix rather than precipitation hardening.
This makes INCONEL 617 ideal for gas turbines, heat exchangers, and reformers, where long-term stability and oxidation resistance are critical.
INCONEL 718 – The High-Strength Alloy
INCONEL 718 is a nickel-chromium-iron alloy strengthened by niobium and molybdenum, with additional titanium and aluminum. It is a precipitation-hardened superalloy, developed for outstanding strength up to 700°C and excellent weldability.
INCONEL 718 is widely used in aerospace, power generation, and oil & gas industries, especially in components requiring high fatigue strength and stress rupture resistance.
2. Chemical Composition Comparison
| Element | INCONEL 617 (%) | INCONEL 718 (%) | Function |
|---|---|---|---|
| Nickel (Ni) | Balance (≥ 44.5) | 50.0 – 55.0 | Provides corrosion resistance and high-temperature strength |
| Chromium (Cr) | 20.0 – 24.0 | 17.0 – 21.0 | Improves oxidation resistance |
| Cobalt (Co) | 10.0 – 15.0 | ≤ 1.0 | Enhances hot strength (only in 617) |
| Molybdenum (Mo) | 8.0 – 10.0 | 2.8 – 3.3 | Solid-solution strengthening |
| Niobium (Nb + Ta) | — | 4.75 – 5.50 | Precipitation hardening (718 only) |
| Iron (Fe) | ≤ 3.0 | Balance (~18%) | Provides ductility and workability |
| Aluminum (Al) | 0.8 – 1.5 | 0.2 – 0.8 | Enhances oxidation resistance and hardening |
| Titanium (Ti) | ≤ 0.6 | 0.6 – 1.15 | Contributes to γ″ and γ′ precipitate formation |
| Carbon (C) | 0.05 – 0.15 | ≤ 0.08 | Improves creep rupture strength |
Key Difference
-
INCONEL 617 contains cobalt and higher molybdenum, making it ideal for high-temperature oxidation and creep resistance.
-
INCONEL 718 relies on niobium, titanium, and aluminum, allowing precipitation hardening for extremely high strength at lower temperatures.
3. Mechanical Properties Comparison
| Property | INCONEL 617 | INCONEL 718 |
|---|---|---|
| Density | 8.36 g/cm³ | 8.19 g/cm³ |
| Tensile Strength (Room Temp) | 890 MPa | 1375 MPa |
| Yield Strength (0.2% offset) | 440 MPa | 1100 MPa |
| Elongation | 45% | 30% |
| Hardness (HRC) | ~25 | 35–40 |
| Maximum Service Temperature | 1100°C | 700°C |
| Creep Resistance | Excellent | Moderate |
| Fatigue Resistance | Good | Excellent |
Interpretation:
-
INCONEL 617 is superior at very high temperatures (up to 1100°C).
-
INCONEL 718 provides higher strength and fatigue resistance at medium-high temperatures (up to 700°C).
4. Strengthening Mechanisms
INCONEL 617 – Solid-Solution Strengthening
The strength of INCONEL 617 comes from alloying elements such as molybdenum, cobalt, and chromium, which dissolve uniformly in the nickel matrix, providing resistance to deformation and creep.
Advantages:
-
Stable at prolonged high temperatures.
-
No risk of precipitate coarsening.
-
Excellent metallurgical stability.
INCONEL 718 – Precipitation Hardening
INCONEL 718 derives its strength from fine precipitates (γ′ and γ″ phases) formed during controlled heat treatment involving niobium, titanium, and aluminum.
Advantages:
-
Extremely high yield and tensile strength.
-
Outstanding fatigue and creep rupture properties up to 700°C.
-
Excellent dimensional stability during cyclic loading.
5. Oxidation and Corrosion Resistance
INCONEL 617
-
Exceptional resistance to oxidation and scaling up to 1100°C.
-
Performs well in oxidizing, carburizing, and nitriding atmospheres.
-
Commonly used in turbine exhaust systems and chemical reactors.
INCONEL 718
-
Excellent corrosion resistance in acidic and marine environments.
-
Performs exceptionally well in chloride stress cracking conditions.
-
Not ideal for continuous service above 700°C due to precipitate instability.
Summary:
-
INCONEL 617 is preferred for oxidation resistance at very high temperatures.
-
INCONEL 718 is preferred for corrosion resistance and mechanical strength in moderate temperatures.
6. Heat Treatment Processes
INCONEL 617
-
Solution Annealing: 1175–1205°C, air cooled.
-
Typically used in the solution-annealed condition for maximum ductility and oxidation resistance.
-
No precipitation hardening required.
INCONEL 718
-
Solution Treatment: 955°C for 1 hour, followed by air cooling.
-
Aging Treatment: 720°C for 8 hours + furnace cooling to 620°C for 8 hours.
-
This dual-step aging process precipitates γ′ and γ″ particles, creating extraordinary strength.
7. Fabrication and Weldability
Both alloys are highly weldable, but their behaviors differ.
| Aspect | INCONEL 617 | INCONEL 718 |
|---|---|---|
| Machinability | Moderate | Moderate |
| Weldability | Excellent (GTAW, GMAW) | Excellent (SMAW, GTAW) |
| Hot Forming Temperature | 900–1200°C | 980–1175°C |
| Post-Weld Heat Treatment | Not required | Required (aging) |
| Cold Workability | Good | Good |
Observation:
INCONEL 718 is easier to weld without cracking due to its precipitation-hardenable nature, while INCONEL 617 is preferred for high-temperature assemblies where post-weld heat treatment is not practical.
8. Applications of INCONEL 617
INCONEL 617’s ability to withstand extreme heat and oxidation makes it ideal for energy, chemical, and aerospace sectors.
Typical Applications
-
Gas Turbine Components – Combustor liners, transition ducts, and turbine exhaust casings.
-
Heat Exchangers – In high-temperature gas or steam environments.
-
Petrochemical Plants – Steam reformer tubes and radiant furnace parts.
-
Aerospace – Jet engine afterburners and exhaust systems.
-
Nuclear Power – Reactor internals and heat exchanger tubing.
INCONEL 617’s stable structure allows long-term use at temperatures beyond the limit of most alloys.
9. Applications of INCONEL 718
INCONEL 718’s combination of strength and corrosion resistance has made it one of the most widely used superalloys in aerospace and oil & gas industries.
Typical Applications
-
Aerospace
-
Jet engine disks, shafts, and fasteners.
-
Turbine blades and compressor parts.
-
-
Oil and Gas
-
Wellhead components, valves, and downhole tools.
-
-
Power Generation
-
Steam turbine casings and bolts.
-
-
Marine Engineering
-
Subsea fasteners and springs.
-
-
Industrial Equipment
-
Cryogenic storage vessels and springs for corrosive environments.
-
10. Standards and Specifications
| Alloy | ASTM / ASME Specifications |
|---|---|
| INCONEL 617 | ASTM B166, B167, B168, B564 / ASME SB167 |
| INCONEL 718 | ASTM B443, B444, B446, B637 / ASME SB637 |
Both alloys are certified under ASME Boiler & Pressure Vessel Code and are widely accepted for high-performance engineering applications.
11. Comparative Summary Table
| Feature | INCONEL 617 | INCONEL 718 |
|---|---|---|
| Strengthening Mechanism | Solid-solution | Precipitation hardening |
| Maximum Service Temperature | 1100°C | 700°C |
| Tensile Strength | Moderate | Very High |
| Oxidation Resistance | Excellent | Good |
| Corrosion Resistance | Good | Excellent |
| Creep Resistance | Superior | Good |
| Fatigue Resistance | Good | Excellent |
| Weldability | Excellent | Excellent |
| Primary Alloying Elements | Ni, Cr, Co, Mo | Ni, Cr, Nb, Mo, Ti, Al |
| Common Applications | Turbines, heat exchangers | Jet engines, fasteners, valves |
Summary:
Choose INCONEL 617 for extreme heat and oxidation; choose INCONEL 718 for high strength and corrosion resistance.
12. Cost Considerations
INCONEL 617 is generally more expensive than INCONEL 718 due to its cobalt content and high-temperature capability. However, its long service life in extreme heat can offset higher material costs in critical equipment.
INCONEL 718 offers a cost-effective solution for structural components requiring strength and corrosion resistance without exposure to extremely high temperatures.
13. Why Choose SASAALLOY
As a leading supplier of nickel-based superalloys, SASAALLOY provides both INCONEL 617 and INCONEL 718 in various forms — round bars, plates, pipes, and forged blocks — meeting ASTM, ASME, and EN standards.
Why SASAALLOY Is the Trusted Choice
-
Certified Quality – All materials supplied with EN 10204 3.1 certificates.
-
Wide Product Range – Ready stock of 617 and 718 in multiple forms.
-
Precision Processing – Cutting, forging, heat treatment, and machining available.
-
Global Service – Delivering to over 40 countries with professional export support.
-
Technical Expertise – Metallurgical guidance for material selection and fabrication.
With its focus on quality and reliability, SASAALLOY ensures that each alloy performs optimally under the most demanding industrial conditions.
14. Choosing Between INCONEL 617 and 718
| Condition | Recommended Alloy |
|---|---|
| Continuous service above 1000°C | INCONEL 617 |
| Oxidation and carburization resistance | INCONEL 617 |
| High tensile and yield strength below 700°C | INCONEL 718 |
| Aerospace fasteners and turbine disks | INCONEL 718 |
| Long-term stability in furnaces and reactors | INCONEL 617 |
| Corrosive and marine environments | INCONEL 718 |
15. Conclusion
Both INCONEL 617 and INCONEL 718 are high-performance nickel-based alloys, but they serve distinct engineering purposes:
-
INCONEL 617 is the alloy of choice for high-temperature strength, oxidation resistance, and long-term structural stability.
-
INCONEL 718 is ideal for high strength, fatigue resistance, and corrosion protection at medium to high temperatures.
Understanding their differences allows engineers to optimize material selection for performance, reliability, and cost-effectiveness.
With strict quality control and global distribution expertise, SASAALLOY continues to supply premium INCONEL alloys to industries that demand excellence — from aerospace turbines to power plant heat exchangers.
Post time: Oct-28-2025