Inconel 738LC
Introduction to Inconel 738LC
Inconel 738LC is a low-carbon version of the cast nickel-based superalloy Inconel 738, designed to improve weldability, reduce hot cracking susceptibility, and enhance the structural integrity of cast components. It is engineered for service in high-temperature environments where mechanical strength, oxidation resistance, and creep performance are critical, particularly in aerospace turbines and industrial gas turbines.
Comprising nickel (~62%), chromium (16%), cobalt (8.5–9.5%), titanium (3.4–3.8%), and aluminum (3.2–3.7%), Inconel 738LC is strengthened primarily by the γ′ phase. Its optimized carbon content (0.02–0.06%) lowers the risk of microfissuring during welding and solidification, while preserving the high-temperature performance characteristics of the base alloy.
Chemical, Physical, and Mechanical Properties of Inconel 738LC
Inconel 738LC (UNS R30738 / ASTM A297, AMS 5391) is typically supplied in precision-cast, solution-treated, and age-hardened condition for gas turbine hot-section and aerospace structural applications.
Chemical Composition (Typical Cast Analysis)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | ~62.0 | Base matrix for heat resistance and strength |
Chromium (Cr) | 15.5–16.5 | Enhances oxidation and corrosion resistance |
Cobalt (Co) | 8.5–9.5 | Increases fatigue strength and hot corrosion resistance |
Tungsten (W) | 2.6–3.3 | Solid solution strengthening |
Molybdenum (Mo) | 1.5–2.1 | Improves creep and rupture strength |
Titanium (Ti) | 3.4–3.8 | Forms γ′ phase for age hardening |
Aluminum (Al) | 3.2–3.7 | Contributes to γ′ precipitation |
Carbon (C) | 0.02–0.06 | Reduced content improves weldability and casting reliability |
Boron (B) | 0.005–0.01 | Enhances grain boundary ductility |
Zirconium (Zr) | ≤0.05 | Grain boundary stabilization |
Silicon (Si) | ≤0.5 | Oxidation resistance |
Manganese (Mn) | ≤0.5 | Castability and cleanliness improvement |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.15 g/cm³ | ASTM B311 |
Melting Range | 1260–1330°C | ASTM E1268 |
Thermal Conductivity | 11.1 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.28 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.3 µm/m·°C (20–1000°C) | ASTM E228 |
Specific Heat Capacity | 450 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 188 GPa at 20°C | ASTM E111 |
Mechanical Properties (Cast + Aged Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 980–1100 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 680–800 MPa | ASTM E8/E8M |
Elongation | ≥4–8% (25mm gauge) | ASTM E8/E8M |
Hardness | 320–390 HB | ASTM E10 |
Creep Rupture Strength | ≥135 MPa @ 870°C, 1000h | ASTM E139 |
Key Characteristics of Inconel 738LC
Low Carbon Content: Reduces hot cracking during welding and casting, enhancing reliability in structural turbine parts.
High Gamma Prime Content: Strengthened primarily by γ′ precipitates, it delivers excellent creep and fatigue resistance at elevated temperatures.
Dimensional and Structural Stability: Maintains geometry and load-bearing performance up to 980°C under thermal cycling.
CNC Machinability: Compatible with high-performance cutting tools, Inconel 738LC can be CNC machined to tight tolerances (±0.02 mm) with surface finishes of Ra ≤ 0.8 µm.
CNC Machining Challenges and Solutions for Inconel 738LC
Machining Challenges
High Surface Hardness
Brinell hardness near 390 HB leads to rapid wear on cutting edges, requiring optimized tool materials and geometries.
Poor Heat Dissipation
Low thermal conductivity accumulates heat at the tool–chip interface, causing crater wear and tool failure without adequate cooling.
Abrasive Microstructure
γ′ and carbide phases promote notching and galling during interrupted or high-feed machining operations.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Ceramic (SiAlON) or coated carbide | Maintains cutting edge under thermal load |
Coating | TiAlN, AlCrN (3–6 µm PVD) | Reduces heat diffusion and tool oxidation |
Geometry | Positive rake (10–12°), edge-honed inserts | Minimizes cutting resistance and chipping |
Cutting Parameters (ISO 3685)
Operation | Speed (m/min) | Feed (mm/rev) | DOC (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 15–25 | 0.20–0.30 | 2.0–3.0 | 80–100 |
Finishing | 30–45 | 0.05–0.10 | 0.3–0.8 | 100–150 |
Surface Treatment for Machined Inconel 738LC Parts
Hot Isostatic Pressing (HIP)
HIP eliminates porosity and strengthens grain structure, improving fatigue life and creep resistance by up to 25%.
Heat Treatment
Heat Treatment uses solution annealing at 1120–1170°C and aging at 845°C to fully precipitate γ′, enhancing high-temperature strength.
Superalloy Welding
Superalloy Welding is feasible with reduced cracking risk due to low carbon. Preheat and post-weld heat treatment further stabilize the microstructure.
Thermal Barrier Coating (TBC)
TBC Coating applies 125–250 µm of YSZ ceramics via APS or EB-PVD to reduce thermal fatigue and oxidation in turbine blades.
Electrical Discharge Machining (EDM)
EDM achieves intricate geometries, cooling slots, and sharp features with ±0.01 mm precision post-casting.
Deep Hole Drilling
Deep Hole Drilling enables high L/D ratio cooling bores and oil channels essential in turbine airfoils and rotor structures.
Material Testing and Analysis
Material Testing verifies alloy integrity through tensile, creep, hardness, and microstructure analysis per ASTM E112 and AMS 5391.
Industry Applications of Inconel 738LC Components
Aerospace Turbines
Guide vanes, shroud segments, and nozzle components.
Reliable under high rotational stress and extreme thermal cycling.
Power Generation
Hot section castings in gas turbines including combustion chambers and seals.
Maintains shape and strength during extended base-load operation at 950°C+.
Marine & Energy
High-temperature pump housings, exhaust valves, and turbine discs.
Resistant to corrosion and thermal distortion in harsh offshore environments.
Defense Propulsion Systems
Jet engine hot parts and afterburner elements.
Delivers consistent performance through rapid heating and cooling cycles.
FAQs
How does Inconel 738LC improve casting and welding reliability compared to Inconel 738?
What CNC machining practices are best for maintaining tool life with Inconel 738LC?
Is HIP necessary for high-cycle fatigue applications in Inconel 738LC?
Can Neway provide integrated services from casting to TBC for Inconel 738LC parts?
What quality standards are used to inspect and validate Inconel 738LC turbine components?
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