Inconel 718LC
Introduction to Inconel 718LC
Inconel 718LC is a low-carbon variant of the widely used Inconel 718 superalloy, engineered for improved weldability, reduced segregation, and enhanced structural integrity in critical aerospace and industrial gas turbine castings. By lowering carbon and certain trace elements, Inconel 718LC minimizes hot cracking and facilitates defect-free welding and casting of large-section components requiring precise CNC finishing.
With a base of nickel (50–55%) and additions of chromium (17–21%), niobium (4.75–5.50%), molybdenum (2.80–3.30%), and iron (balance), Inconel 718LC delivers excellent mechanical strength and corrosion resistance up to 704°C (1300°F). Its compatibility with investment casting and post-machining processes makes it a reliable choice for high-performance, dimensionally stable parts.
Chemical, Physical, and Mechanical Properties of Inconel 718LC
Inconel 718LC (UNS N07718LC / AMS 5383) is typically supplied in cast, solution heat-treated, and age-hardened conditions, meeting the demanding performance requirements of aerospace, nuclear, and industrial gas turbine components.
Chemical Composition (Typical Cast Analysis)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | 50.0–55.0 | Base element for thermal strength and corrosion resistance |
Chromium (Cr) | 17.0–21.0 | Enhances oxidation and corrosion resistance |
Iron (Fe) | Balance | Structural matrix and cost efficiency |
Niobium (Nb) + Tantalum (Ta) | 4.75–5.50 | Strengthening via γ″ precipitate formation |
Molybdenum (Mo) | 2.80–3.30 | Improves high-temperature creep resistance |
Titanium (Ti) | 0.65–1.15 | Forms γ′ phase for strength retention |
Aluminum (Al) | 0.20–0.80 | Contributes to γ′ phase precipitation |
Carbon (C) | ≤0.02 | Lowers hot cracking sensitivity and segregation |
Cobalt (Co) | ≤1.00 | Enhances elevated temperature strength (optional) |
Manganese (Mn) | ≤0.35 | Improves castability |
Silicon (Si) | ≤0.35 | Oxidation resistance at high temperatures |
Sulfur (S) | ≤0.010 | Controlled for weldability and hot ductility |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.19 g/cm³ | ASTM B311 |
Melting Range | 1260–1336°C | ASTM E1268 |
Thermal Conductivity | 11.2 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 | 1100–1250 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 950–1080 MPa | ASTM E8/E8M |
Elongation | ≥6–10% (25mm gauge) | ASTM E8/E8M |
Hardness | 310–360 HB | ASTM E10 |
Creep Rupture Strength | ≥160 MPa @ 650°C, 1000h | ASTM E139 |
Key Characteristics of Inconel 718LC
Low-Carbon Advantage: Reduces the risk of hot cracking during casting and welding while enhancing weldability and microstructural homogeneity in large-section components.
High-Temperature Performance: Maintains tensile and yield strength >1100 MPa and ≥950 MPa, respectively, at elevated temperatures up to 704°C.
Precipitation Strengthening: Double hardening via γ′ (Ni₃(Al, Ti)) and γ″ (Ni₃Nb) phases enables long-term strength retention under cyclic thermal and mechanical loads.
Machinability After Casting: Supports tight CNC tolerances (±0.02 mm) and fine surface finish (Ra ≤ 0.8 µm) when machined with optimized parameters and tooling.
CNC Machining Challenges and Solutions for Inconel 718LC
Machining Challenges
High Strength and Age Hardening
Brinell hardness up to 360 HB in aged condition shortens tool life and necessitates rigid setups with low deflection tolerances.
Heat Retention and Tool Wear
Low thermal conductivity (~11 W/m·K) leads to high tool-tip temperatures, requiring high-pressure coolant systems and wear-resistant coatings.
Surface Finish and Notching
γ′ and γ″ precipitates contribute to edge buildup and notching if improper tool geometry or worn inserts are used.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Coated carbide or SiAlON ceramic for high-temp ops | Maintains hardness and stability under heat |
Coating | TiAlN, AlCrN (PVD 3–6 µm) | Resists wear, oxidation, and diffusion |
Geometry | Positive rake (8–12°), honed/chamfered edge | Reduces cutting forces and prevents edge chipping |
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 718LC Parts
Hot Isostatic Pressing (HIP)
HIP closes internal porosity in cast structures and boosts fatigue strength by 25–30%, crucial for aerospace and turbine parts.
Heat Treatment
Heat Treatment involves solution treatment at 980–1065°C and aging near 718°C to precipitate γ′/γ″ phases and optimize high-temperature properties.
Superalloy Welding
Superalloy Welding employs TIG/EB welding with low-carbon Ni-based fillers for defect-free joining and minimal HAZ cracking.
Thermal Barrier Coating (TBC)
TBC Coating applies 125–300 µm of YSZ via APS or EB-PVD to protect against thermal cycling and oxidation.
Electrical Discharge Machining (EDM)
EDM delivers ±0.01 mm accuracy for cooling holes, fir-tree slots, and intricate cast details.
Deep Hole Drilling
Deep Hole Drilling creates precise bore features with L/D ≥ 40:1 for turbine cooling systems and structural castings.
Material Testing and Analysis
Material Testing includes mechanical, ultrasonic, and X-ray NDT, as well as metallographic analysis per AMS 5383 and ASTM E112.
Industry Applications of Inconel 718LC Components
Aerospace Engines
Turbine vanes, stator rings, and nozzle guide castings.
Operates reliably under high thrust, thermal, and oxidation stress cycles.
Power Generation
Hot-section blades and combustor transition castings.
Delivers prolonged life at 650–700°C under high-cycle fatigue loads.
Oil & Gas and Marine
Pump housings, high-pressure manifolds, and subsea components.
Withstands chloride-induced SCC, brine exposure, and H₂S corrosion.
Defense and Rocket Propulsion
Engine support structures and thermal control elements.
Maintains mechanical strength through extreme temperature changes and vibration.
FAQs
How does Inconel 718LC compare to Inconel 718C and standard 718 in castability and weldability?
What are the best CNC strategies for minimizing tool wear during Inconel 718LC machining?
Is HIP treatment required for aerospace-grade Inconel 718LC parts?
Can Neway provide full-service processing for Inconel 718LC—including casting, machining, and surface treatments?
What certifications and inspections are applied to Inconel 718LC turbine or defense components?
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