Inconel 718
Introduction to Inconel 718
Inconel 718 is a precipitation-hardenable nickel-chromium alloy renowned for its outstanding high-temperature strength, corrosion resistance, and weldability. Capable of operating up to 704°C (1300°F) with excellent tensile, fatigue, and creep-rupture performance, Inconel 718 is widely used in aerospace, power generation, and oil and gas industries.
This alloy contains significant amounts of nickel (50–55%), chromium (17–21%), niobium (4.75–5.50%), molybdenum (2.80–3.30%), and iron (bal.). Its unique hardening mechanism—age hardening via Ni₃Nb (γ″ phase) and Ni₃(Al, Ti) (γ′ phase)—delivers exceptional strength and dimensional stability even under prolonged thermal cycling.
Chemical, Physical, and Mechanical Properties of Inconel 718
Inconel 718 (UNS N07718 / AMS 5662, AMS 5663, ASTM B637) is available in wrought, cast, and powder-metallurgy forms and is typically heat-treated to the solution-annealed and aged condition.
Chemical Composition (ASTM B637)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | 50.0–55.0 | Base element; high-temperature strength |
Chromium (Cr) | 17.0–21.0 | Corrosion and oxidation resistance |
Iron (Fe) | Balance | Structural support, cost control |
Niobium (Nb) + Tantalum (Ta) | 4.75–5.50 | Strengthens via γ″ precipitation |
Molybdenum (Mo) | 2.80–3.30 | Enhances creep and corrosion resistance |
Titanium (Ti) | 0.65–1.15 | γ′ phase strengthening |
Aluminum (Al) | 0.20–0.80 | Forms γ′ precipitates for high-temperature strength |
Cobalt (Co) | ≤1.00 | Enhances hot strength (optional) |
Carbon (C) | ≤0.08 | Controlled for weldability and toughness |
Manganese (Mn) | ≤0.35 | Improves hot workability |
Silicon (Si) | ≤0.35 | Oxidation control |
Sulfur (S) | ≤0.015 | Minimizes hot cracking |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.19 g/cm³ | ASTM B311 |
Melting Range | 1260–1336°C | ASTM E1268 |
Thermal Conductivity | 11.4 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.23 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.0 µm/m·°C (20–1000°C) | ASTM E228 |
Specific Heat Capacity | 435 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 200 GPa at 20°C | ASTM E111 |
Mechanical Properties (AMS 5662/5663 – Aged Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 1240–1380 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 1030–1180 MPa | ASTM E8/E8M |
Elongation | ≥12% (25mm gauge) | ASTM E8/E8M |
Hardness | 330–380 HB | ASTM E10 |
Creep Rupture Strength | ≥160 MPa @ 650°C, 1000h | ASTM E139 |
Key Characteristics of Inconel 718
High-Temperature Strength: Maintains mechanical strength above 1000 MPa up to 650°C and creep resistance at 700°C for extended durations, making it ideal for aerospace turbines and energy systems.
Excellent Corrosion Resistance: Resists chloride pitting, sulfide stress corrosion, and acidic/alkaline media—suitable for downhole tools and marine equipment.
Stable Microstructure: Double-phase precipitation (γ′ + γ″) ensures long-term mechanical integrity and phase stability under thermal cycling.
Weldability: Unlike many superalloys, Inconel 718 is easily weldable without cracking due to its low carbon and high Nb/Al/Ti balance.
CNC Machining Challenges and Solutions for Inconel 718
Machining Challenges
High Work Hardening Rate
Strain hardens quickly (n ≈ 0.4), increasing surface hardness by >30% during cutting, accelerating tool wear and deflection.
Thermal Management Issues
Poor thermal conductivity (11.4 W/m·K) causes cutting temperatures to exceed 900°C, leading to crater wear and reduced dimensional accuracy.
Built-Up Edge and Notching
Ductile flow combined with carbide-precipitate abrasiveness results in notching at depth-of-cut transitions and tool edge chipping.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Carbide (PVD-coated), ceramic for high-speed ops | High hot hardness, wear resistance |
Coating | TiAlN, AlCrN, or TiSiN, 3–6 µm | Reduces heat transfer and wear |
Geometry | Positive rake (8–12°), strong edge prep | Reduces work hardening and BUE |
Cutting Parameters (ISO 3685)
Operation | Speed (m/min) | Feed (mm/rev) | DOC (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 20–30 | 0.20–0.30 | 2.0–3.0 | 80–100 |
Finishing | 40–60 | 0.05–0.10 | 0.3–0.8 | 100–150 |
Surface Treatment for Machined Inconel 718 Parts
Hot Isostatic Pressing (HIP)
HIP eliminates porosity and improves fatigue life by up to 30% in high-pressure turbine and aerospace castings.
Heat Treatment
Heat Treatment involves solution annealing at 980–1065°C and aging at 718°C to optimize γ′/γ″ precipitation and mechanical properties.
Superalloy Welding
Superalloy Welding uses GTAW or EB welding with Nb-stabilized fillers to maintain microstructural integrity without post-weld cracking.
Thermal Barrier Coating (TBC)
TBC Coating applies 125–300 µm ceramic coatings via APS or EB-PVD, reducing surface temperatures and enhancing thermal fatigue resistance.
Electrical Discharge Machining (EDM)
EDM ensures ±0.01 mm tolerance and excellent finish in hardened or aged Inconel 718, ideal for cooling slots and mold details.
Deep Hole Drilling
Deep Hole Drilling achieves L/D ≥ 40:1 with high straightness and surface finish required in engine bores and tubing.
Material Testing and Analysis
Material Testing includes tensile, fatigue, ultrasonic, and metallographic analysis (ASTM E112, E139, AMS 5663) to ensure aerospace-grade reliability.
Industry Applications of Inconel 718 Components
Aerospace Engines
Turbine discs, shafts, fasteners, and combustor liners.
Operates at high thrust/load without creep deformation or fatigue failure.
Power Generation
Steam turbine blades, seals, and transition ducts.
Performs reliably under high pressure, oxidation, and vibration.
Oil & Gas
Downhole tools, valves, and completion equipment.
Resists sour gas, high-pressure brine, and chloride-induced SCC.
Industrial Molding and Tooling
Injection molds and hot runner systems.
Retains mechanical integrity under rapid cycling and heat stress.
FAQs
What machining practices minimize tool wear and thermal damage when processing Inconel 718?
How does Inconel 718 compare to Inconel 625 or 925 in mechanical and corrosion properties?
What heat treatment cycle is required to optimize strength in Inconel 718?
Can Neway machine, weld, and surface-treat complex Inconel 718 turbine or oilfield parts?
What quality standards are used to validate Inconel 718 components for aerospace and nuclear use?
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