Inconel X-750
Introduction to Inconel X-750
Inconel X-750 is a precipitation-hardenable nickel-chromium alloy renowned for its exceptional high-temperature strength, oxidation resistance, and stress-corrosion cracking resistance. Strengthened through gamma prime (γ′) precipitation via aluminum and titanium additions, this alloy offers stable mechanical properties at temperatures up to 700°C and intermittent exposure up to 980°C.
With its origins in jet engine and nuclear applications, Inconel X-750 is widely used for springs, fasteners, gas turbine blades, and pressure vessel components. It is available in wrought and cast forms and is typically CNC machined in the solution-treated or age-hardened condition, depending on the performance requirements of the end use.
Chemical, Physical, and Mechanical Properties of Inconel X-750
Inconel X-750 (UNS N07750 / AMS 5667 / ASTM B637) is supplied in multiple heat-treated conditions, including solution-annealed, age-hardened, or stress-equalized, for structural and fatigue-critical components.
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Nickel (Ni) | ≥70.0 | Base element; ensures high-temperature strength and corrosion resistance |
Chromium (Cr) | 14.0–17.0 | Provides oxidation resistance and passivation stability |
Iron (Fe) | 5.0–9.0 | Contributes to cost efficiency and structural toughness |
Titanium (Ti) | 2.25–2.75 | Forms γ′ strengthening precipitates |
Aluminum (Al) | 0.40–1.0 | Combines with Ti to enhance high-temp strength |
Manganese (Mn) | ≤1.0 | Improves hot workability |
Silicon (Si) | ≤0.5 | Enhances oxidation resistance |
Copper (Cu) | ≤0.5 | Kept low to avoid corrosion risk |
Carbon (C) | ≤0.08 | Controlled for ductility and weldability |
Sulfur (S) | ≤0.01 | Minimized to prevent hot cracking |
Niobium (Nb+Ta) | 0.5–1.0 | Promotes structural stability under stress |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 8.28 g/cm³ | ASTM B311 |
Melting Range | 1390–1430°C | ASTM E1268 |
Thermal Conductivity | 11.2 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.25 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 13.3 µm/m·°C (20–1000°C) | ASTM E228 |
Specific Heat Capacity | 460 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 214 GPa at 20°C | ASTM E111 |
Mechanical Properties (Age-Hardened, AMS 5667)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 1000–1200 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 750–900 MPa | ASTM E8/E8M |
Elongation | ≥15% (25mm gauge) | ASTM E8/E8M |
Hardness | 320–370 HB | ASTM E10 |
Stress Rupture Strength | ≥120 MPa @ 704°C, 1000h | ASTM E139 |
Key Characteristics of Inconel X-750
High Creep and Stress Rupture Resistance: Retains mechanical properties during extended exposure at 600–700°C, ideal for jet turbine and spring applications.
Excellent Oxidation and Corrosion Resistance: Resists chloride and sulfide attack, with proven performance in marine and nuclear environments.
Age-Hardenable Versatility: Mechanical properties can be tailored using solution annealing and aging treatments per application.
CNC Machinability: Requires careful tool control, but provides precision and stability for critical components with tolerances up to ±0.01 mm and Ra ≤ 1.0 µm.
CNC Machining Challenges and Solutions for Inconel X-750
Machining Challenges
High Work Hardening Rate
The alloy rapidly increases surface hardness during machining, leading to tool wear and dimensional inaccuracy if feeds and speeds are not optimized.
Abrasive Strengthening Phases
Precipitated γ′ and carbides (especially in aged condition) wear down tool edges and coatings, particularly in interrupted cuts.
Heat Retention
Low thermal conductivity concentrates heat at the cutting zone, necessitating high-pressure coolant and advanced tool materials.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | PVD-coated carbide or CBN tools | Withstand thermal fatigue and abrasive phases |
Coating | AlTiN or TiSiN (2–5 µm) | Reduces friction and prolongs tool life |
Geometry | 10–12° rake, edge honed or chamfered | Improves chip evacuation and reduces cutting force |
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 | 0.5–1.0 | 100–150 |
Surface Treatment for Machined Inconel X-750 Parts
Hot Isostatic Pressing (HIP)
HIP enhances creep rupture and fatigue resistance in cast or additively manufactured Inconel X-750 parts by eliminating porosity.
Heat Treatment
Heat Treatment includes solution annealing at 1095°C followed by aging at 705°C for 16–20 hours to optimize γ′ precipitation and tensile strength.
Superalloy Welding
Superalloy Welding employs GTAW with controlled heat input and Inconel X welding filler to reduce susceptibility to microcracking.
Thermal Barrier Coating (TBC)
TBC Coating applies 125–250 µm of YSZ to protect turbine rings and heat shields operating above 900°C.
Electrical Discharge Machining (EDM)
EDM enables precision slotting and profiling in hardened X-750 with tolerances up to ±0.01 mm.
Deep Hole Drilling
Deep Hole Drilling supports internal cooling passages in aerospace actuators and reactor spring systems with L/D ≥ 40:1.
Material Testing and Analysis
Material Testing includes stress rupture testing (ASTM E139), grain structure analysis (ASTM E112), and corrosion qualification (NACE, ASTM G28).
Industry Applications of Inconel X-750 Components
Aerospace
Turbine wheels, exhaust components, and jet engine springs.
Excellent thermal fatigue resistance under cyclic loads at 600–700°C.
Nuclear Reactors
Core springs, bolting, and structural supports.
Withstands neutron exposure and high-pressure steam corrosion.
Gas Turbines
Combustor hardware, transition ducts, and support brackets.
Maintains structural integrity and scale resistance under extreme heat.
Oil & Gas
Valve seats, downhole springs, and completion equipment.
Performs under hydrogen sulfide, chloride, and high-pressure cycling.
FAQs
What distinguishes Inconel X-750 from Inconel 718 or 625 in turbine applications?
What aging treatments optimize the performance of CNC-machined Inconel X-750 components?
How is tool life affected when machining aged Inconel X-750 parts?
Can Inconel X-750 be HIPed or EDM-processed without altering its mechanical integrity?
What industries use Inconel X-750 for nuclear or aerospace-critical spring applications?
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