Inconel 718C
Introduction to Inconel 718C
Inconel 718C is a variant of the standard Inconel 718 nickel-based superalloy, optimized specifically for casting processes while preserving the high-temperature mechanical properties, corrosion resistance, and precipitation-hardening capability that make the base alloy a staple in aerospace, energy, and high-performance engineering applications. Designed for near-net-shape components via investment casting, Inconel 718C is well suited for large or geometrically complex parts requiring final CNC machining.
Containing nickel (50–55%), chromium (17–21%), niobium (4.75–5.50%), molybdenum (2.80–3.30%), and iron (balance), Inconel 718C gains strength from γ′ and γ″ phase precipitation after aging. It delivers stable mechanical performance up to 704°C (1300°F), and its castability makes it ideal for turbine vanes, combustor rings, and other structural parts subject to both mechanical and thermal stress.
Chemical, Physical, and Mechanical Properties of Inconel 718C
Inconel 718C (UNS N07718C / ASTM B670 cast grade) is generally delivered in cast, solution-treated, and aged condition, meeting rigorous demands for aerospace and industrial gas turbine applications.
Chemical Composition (Typical Cast Analysis)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | 50.0–55.0 | Base element; high-temperature mechanical strength |
Chromium (Cr) | 17.0–21.0 | Provides oxidation and corrosion resistance |
Iron (Fe) | Balance | Structural support and stability |
Niobium (Nb) + Tantalum (Ta) | 4.75–5.50 | Key for γ″ strengthening and stability |
Molybdenum (Mo) | 2.80–3.30 | Creep resistance and solid solution strengthening |
Titanium (Ti) | 0.65–1.15 | Forms γ′ phase for additional precipitation hardening |
Aluminum (Al) | 0.20–0.80 | Forms γ′ phase with Ti for high-temperature strength |
Cobalt (Co) | ≤1.00 | Optional for enhanced hot strength |
Carbon (C) | ≤0.08 | Controlled to reduce hot cracking risk |
Manganese (Mn) | ≤0.35 | Improves castability |
Silicon (Si) | ≤0.35 | Enhances oxidation resistance |
Sulfur (S) | ≤0.015 | Minimized to avoid cracking and weld defects |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.19 g/cm³ | ASTM B311 |
Melting Range | 1260–1336°C | ASTM E1268 |
Thermal Conductivity | 11.0 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 | 198 GPa at 20°C | ASTM E111 |
Mechanical Properties (Cast + Aged Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 1120–1260 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 960–1100 MPa | ASTM E8/E8M |
Elongation | ≥6–10% (25mm gauge) | ASTM E8/E8M |
Hardness | 320–360 HB | ASTM E10 |
Creep Rupture Strength | ≥160 MPa @ 650°C, 1000h | ASTM E139 |
Key Characteristics of Inconel 718C
High-Temperature Mechanical Strength: Retains over 1100 MPa tensile strength and stable γ′/γ″ precipitate structure up to 704°C, suitable for demanding thermal and structural environments.
Castability and Integrity: Excellent for precision investment casting; reduced hot cracking and improved feeding behavior versus wrought variants.
Corrosion Resistance: Excellent resistance to chloride pitting, sulfide stress corrosion, and high-pressure oxidation environments.
Post-Casting Machinability: CNC machining is typically employed to achieve final dimensional tolerances (±0.02 mm) and surface finishes (Ra ≤ 0.8 µm).
CNC Machining Challenges and Solutions for Inconel 718C
Machining Challenges
High Strength in Aged Condition
Material hardness of up to 360 HB causes high tool wear rates and limits cutting speeds, especially in finishing operations.
Thermal Conductivity Limitations
Generates intense heat zones during cutting, requiring effective coolant delivery and thermal shock-resistant tooling.
Notch Sensitivity and Built-Up Edge
Ductile yet abrasive behavior leads to edge buildup and notching, especially at depth-of-cut transitions.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | High-performance carbide or ceramic inserts | Withstands heat and maintains edge sharpness |
Coating | TiAlN, AlCrN PVD coatings (3–6 µm) | Improves thermal resistance and wear life |
Geometry | Positive rake (8–12°), edge-honed profiles | Controls cutting forces and prevents edge failures |
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 | 35–50 | 0.05–0.10 | 0.3–0.8 | 100–150 |
Surface Treatment for Machined Inconel 718C Parts
Hot Isostatic Pressing (HIP)
HIP eliminates casting porosity and increases fatigue strength by up to 25%, making it critical for aerospace-grade integrity.
Heat Treatment
Heat Treatment includes solution annealing at 980–1065°C and aging at ~718°C to optimize γ′/γ″ hardening and dimensional stability.
Superalloy Welding
Superalloy Welding with Nb-stabilized filler alloys and precise arc control allows strong joints without micro fissuring or HAZ cracking.
Thermal Barrier Coating (TBC)
TBC Coating applies YSZ ceramic layers (125–300 µm) to extend thermal fatigue life in high-velocity gas environments.
Electrical Discharge Machining (EDM)
EDM is ideal for final shaping of turbine segments, cooling holes, and fine contours at ±0.01 mm precision.
Deep Hole Drilling
Deep Hole Drilling supports cooling channel formation and tube features with L/D ratios ≥ 40:1 in cast sections.
Material Testing and Analysis
Material Testing ensures full compliance with ASTM E139, AMS 5663, and E112 for mechanical, microstructural, and defect inspection.
Industry Applications of Inconel 718C Components
Aerospace Gas Turbines
Turbine vanes, nozzle rings, and combustion supports.
Delivers high fatigue strength and thermal resistance in rotating and static parts.
Power Generation
Steam turbine hot zone castings and transition pieces.
Operates continuously at high temperatures and cyclic loads.
Oil & Gas Extraction
Valves, impellers, and sealing components in downhole tools.
Resists H₂S-rich corrosion and chloride pitting in deep well operations.
Defense and Rocketry
Thrust chambers, nozzle skirts, and control vanes.
Maintains strength and geometry during launch and reentry thermal cycling.
FAQs
What differentiates Inconel 718C from standard Inconel 718 in casting and machining?
Which tools and parameters are most effective for CNC machining Inconel 718C?
Is HIP mandatory for Inconel 718C aerospace turbine components?
Can Neway handle complex post-processing like EDM and TBC for Inconel 718C?
What quality certifications and test methods are used for Inconel 718C castings?
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