Inconel 738
Introduction to Inconel 738
Inconel 738 is a high-strength, cast nickel-based superalloy developed for applications requiring superior creep resistance, oxidation resistance, and thermal fatigue performance at elevated temperatures up to 980°C (1796°F). Designed for turbine components operating under extreme thermal and mechanical stress, Inconel 738 is widely used in aerospace, power generation, and high-efficiency gas turbine systems.
Composed primarily of nickel (~62%) and strengthened by chromium (16%), tungsten (2.6–3.3%), molybdenum (1.5–2.1%), cobalt (8.0–9.0%), titanium (3.3–3.7%), and aluminum (3.2–3.7%), the alloy forms a stable gamma prime (γ′) phase that maintains strength during prolonged exposure to high temperatures. Inconel 738 offers excellent castability and dimensional stability, making it ideal for near-net-shape investment castings followed by CNC machining.
Chemical, Physical, and Mechanical Properties of Inconel 738
Inconel 738 (UNS R30738 / AMS 5389) is typically supplied in cast and precipitation-hardened condition for turbine engines and industrial hot-section components.
Chemical Composition (Typical Cast Analysis)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | ~61.0 | Base matrix for high-temperature strength |
Chromium (Cr) | 16.0 | Provides oxidation resistance |
Cobalt (Co) | 8.5 | Enhances hot strength and fatigue life |
Tungsten (W) | 2.6–3.3 | Solid-solution strengthening |
Molybdenum (Mo) | 1.5–2.1 | Improves creep resistance |
Titanium (Ti) | 3.3–3.7 | γ′ phase formation and strengthening |
Aluminum (Al) | 3.2–3.7 | Contributes to γ′ precipitation |
Tantalum (Ta) | ≤0.05 | Strengthens grain boundaries (optional) |
Carbon (C) | 0.11–0.17 | Carbide formation for grain boundary stability |
Boron (B) | 0.005–0.01 | Enhances ductility and hot strength |
Zirconium (Zr) | ≤0.05 | Grain boundary cohesion |
Silicon (Si) | ≤0.5 | Improves oxidation scale adherence |
Manganese (Mn) | ≤0.5 | Promotes castability |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.15 g/cm³ | ASTM B311 |
Melting Range | 1260–1330°C | ASTM E1268 |
Thermal Conductivity | 11.2 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.28 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.2 µm/m·°C (20–1000°C) | ASTM E228 |
Specific Heat Capacity | 450 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 190 GPa at 20°C | ASTM E111 |
Mechanical Properties (Cast + Aged Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 980–1120 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 700–820 MPa | ASTM E8/E8M |
Elongation | ≥4–8% (25mm gauge) | ASTM E8/E8M |
Hardness | 330–400 HB | ASTM E10 |
Creep Rupture Strength | ≥140 MPa @ 870°C, 1000h | ASTM E139 |
Key Characteristics of Inconel 738
Gamma Prime Strengthening: High γ′ phase content (~60%) provides excellent creep and fatigue resistance at temperatures up to 980°C.
Superior Oxidation Resistance: Forms stable Al₂O₃ and Cr₂O₃ oxide layers for long-term surface protection in turbine exhaust and hot-gas environments.
High Structural Stability: Maintains dimensional accuracy and mechanical strength under cyclic thermal loads and prolonged exposure.
Castability and CNC Compatibility: Excellent flow and feeding behavior during casting allows for complex geometries, followed by CNC machining to tight tolerances (±0.02 mm).
CNC Machining Challenges and Solutions for Inconel 738
Machining Challenges
High Hardness and Brittleness
Aged castings exhibit up to 400 HB hardness, causing accelerated flank wear and limiting tool life during finishing.
Thermal Accumulation
Poor heat dissipation (low thermal conductivity) leads to thermal damage and rapid crater wear without adequate coolant strategies.
Tool Notching and Chipping
Abrasive intermetallic phases and hard carbides result in edge notching and require reinforced cutting-edge geometries.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | SiAlON ceramic or carbide with CBN for finishing | High hot hardness and wear resistance |
Coating | TiAlN, AlCrN (PVD, 3–6 µm) | Prevents heat diffusion and galling |
Geometry | Positive rake (10–12°), strong edge prep | Reduces tool deflection 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 738 Parts
Hot Isostatic Pressing (HIP)
HIP densifies cast structures and eliminates shrinkage porosity, improving fatigue resistance and dimensional stability under cyclic loading.
Heat Treatment
Heat Treatment typically includes solution annealing at 1120–1170°C and aging at 845°C to develop the γ′ phase for optimal high-temperature strength fully.
Superalloy Welding
Superalloy Welding employs advanced techniques like TIG or EB welding with preheat control to reduce hot cracking risks in γ′-strengthened alloys.
Thermal Barrier Coating (TBC)
TBC Coating applies 125–250 µm of yttria-stabilized zirconia (YSZ), reducing surface temperature by 150–200°C in turbine airfoils and exhaust parts.
Electrical Discharge Machining (EDM)
EDM enables the formation of cooling passages, blade root features, and intricate slots with ±0.01 mm accuracy.
Deep Hole Drilling
Deep Hole Drilling achieves L/D ≥ 40:1 for efficient coolant flow paths in turbine blades and vane castings.
Material Testing and Analysis
Material Testing includes non-destructive and destructive tests per ASTM E112, E139, and AMS 5389 to certify mechanical, microstructural, and dimensional compliance.
Industry Applications of Inconel 738 Components
Aerospace Turbines
Turbine blades, vanes, combustor segments, and shrouds.
Operates at sustained temperatures above 950°C under cyclic load and oxidation.
Power Generation
Industrial gas turbine hot section parts, stator rings, and seals.
Combines oxidation resistance and fatigue strength under high-pressure combustion environments.
Defense & Propulsion
Jet engine nozzles, exhaust diffusers, and thermal shields.
Withstands extreme thermal shocks and vibration loads during flight and launch sequences.
Energy Sector
High-efficiency turbine components in combined-cycle power plants.
Maintains mechanical stability and corrosion resistance in aggressive heat transfer systems.
FAQs
How does Inconel 738 differ from Inconel 713 and 718 in high-temperature performance?
What CNC tooling and strategies work best for machining Inconel 738 cast parts?
Is HIP mandatory for aerospace-grade Inconel 738 turbine components?
Can Neway provide EDM and TBC services for complex Inconel 738 geometries?
What quality control standards are followed when manufacturing Inconel 738 parts?
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