Inconel 718: Properties and Applications of Precipitation-Hardened Nickel-Based Superalloy
Inconel 718 (UNS N07718), also known as Alloy 718, is a niobium and molybdenum strengthened precipitation-hardened nickel-iron-chromium alloy. First developed in the 1960s, it has become one of the most mature and widely used nickel-based superalloys worldwide. Featuring outstanding comprehensive performance across an extreme temperature range from -253°C to 700°C, it holds an irreplaceable position in aerospace, power energy, petrochemical and other industries with harsh service conditions.
1. Chemical Composition and Microstructure
1.1 Main Chemical Composition (wt%)
| Element | Ni | Cr | Fe | Nb | Mo | Ti | Al | C | B |
|---|
| Content | 50-55 | 17-21 | Balance | 4.75-5.5 | 2.8-3.3 | 0.65-1.15 | 0.2-0.8 | ≤0.08 | ≤0.006 |
Nickel stabilizes the austenite matrix and delivers excellent low-temperature toughness and high-temperature corrosion resistance. Chromium forms dense protective oxide film to improve high-temperature oxidation resistance and acid-alkali corrosion resistance. As the core strengthening element, niobium forms nano-scale γ'' precipitates (Ni₃Nb), which provide the main strength contribution. Molybdenum enhances solid solution strengthening, creep resistance and chloride ion corrosion resistance. Titanium and aluminum assist in forming γ' phase to further optimize fatigue resistance and structural stability.
1.2 Strengthening Mechanism
Inconel 718 possesses a non-magnetic face-centered cubic austenite structure. Its superior mechanical properties mainly come from dual precipitation strengthening phases. The disc-shaped γ'' phase serves as the primary strengthening phase and remains stable below 650°C, greatly boosting tensile and yield strength. Spherical γ' phase plays an auxiliary strengthening role and improves fatigue performance. Uniformly distributed carbides restrain grain growth and enhance grain boundary stability against high-temperature creep cracking.
2. Physical and Mechanical Properties
2.1 Basic Physical Properties (Room Temperature)
- Density: 8.19 g/cm³
- Melting point: 1260-1338°C
- Thermal conductivity: 11.2 W/(m·K) at 100°C
- Resistivity: 1.24 μΩ·m
- Linear expansion coefficient: 13.0×10⁻⁶ /°C (20-100°C)
2.2 Standard Mechanical Properties after Heat Treatment
In accordance with AMS 5662 specification, the alloy reaches excellent mechanical performance after standard aging treatment. It maintains high strength, stable toughness and reliable ductility both at room temperature and elevated temperature. It shows no brittle fracture at ultra-low temperature and retains good creep resistance under long-term high-temperature service. Besides, it owns remarkable fatigue resistance to inhibit crack initiation and propagation.
3. Standard Heat Treatment Process
Heat treatment directly determines the final service performance of Inconel 718. The mainstream industrial process consists of solution treatment and double aging treatment.
Solution treatment is carried out at 950-980°C for 1 to 2 hours followed by air cooling, which dissolves precipitates and homogenizes the internal structure. The first-stage aging is kept at 720°C for 8 hours then furnace cooled to 620°C for secondary aging holding for another 8 hours before air cooling. This complete process can fully precipitate strengthening phases and achieve the optimal balance between high strength and good plasticity.
4. Main Industrial Applications
4.1 Aerospace Industry
This is the largest consumption field of Inconel 718. It is widely used to manufacture aero-engine turbine discs, blades, combustion chamber components and afterburner parts, which need to withstand high temperature, high pressure and thermal fatigue impact. It is also an essential material for rocket engine turbine pumps and cryogenic structural parts adapting to huge temperature difference environment.
4.2 Power and Energy Industry
It is applied in large-scale ground gas turbine key components, nuclear reactor internal structural parts, control rod conduits and low-temperature hydrogen storage equipment, meeting the requirements of radiation resistance, high-temperature water medium corrosion resistance and ultra-low temperature toughness.
4.3 Petroleum, Chemical and Marine Engineering
In deep and ultra-deep oil and gas exploitation fields, Inconel 718 is used for high-temperature high-pressure pipeline pipes, valves and pump shafts, with strong resistance to hydrogen sulfide and chloride corrosion. It is also adopted in high-temperature chemical reactors, heat exchangers and offshore platform load-bearing fasteners for long-term stable operation in harsh marine atmospheric environment.
4.4 Other High-end Fields
The alloy is also used for medical implant devices due to good biocompatibility, high-temperature precision sensor parts and vacuum equipment components. In recent years, it has been widely applied in additive manufacturing industry to produce complex-shaped high-strength structural parts.
5. Machining Characteristics and Application Notes
In practical processing, Inconel 718 has relatively high machining difficulty. Its high strength and poor thermal conductivity lead to high cutting temperature and severe tool wear, so matched cutting tools and reasonable cutting parameters are required. During forging, the forming temperature range must be strictly controlled to avoid structural defects. Although it has good weldability, reasonable welding heat input control is still needed to prevent performance decline.
In actual service, long-term continuous working above 700°C shall be avoided to prevent coarsening of strengthening phases and strength attenuation. Strict control of impurity elements is required to avoid grain boundary embrittlement. Proper surface anti-oxidation coating can further extend its service life under extreme high-temperature conditions.
6. Conclusion
Thanks to reasonable chemical composition design, stable dual-phase precipitation strengthening system and all-round excellent service performance, Inconel 718 has become a versatile core material in the field of high-temperature alloys. It covers almost all high-end manufacturing fields requiring extreme working condition resistance. With the continuous upgrading of forging, casting and additive manufacturing technologies, its processing efficiency and structural stability are further improved, and it will continue to play an irreplaceable core role in high-end equipment manufacturing in the future.