If you need a superalloy that delivers exceptional creep resistance, fatigue strength, and corrosion resistance—all enhanced by heat aging—UNS N07750 (commonly called Inconel X-750) is the ideal choice. Used in aerospace, nuclear, and energy industries, this alloy solves the critical problem of maintaining strength at 650+ °C where other materials soften. In this guide, we’ll break down its key properties, real-world uses, manufacturing steps, and how it compares to alternatives—so you can build components that perform reliably in long-term high-temperature service.
1. Material Properties of UNS N07750 (Inconel X-750) Superalloy
UNS N07750’s superalloy status comes from its unique aging mechanism: titanium-aluminum-niobium precipitates (γ’ and γ”) that form during heat treatment, boosting strength without sacrificing corrosion resistance. Let’s explore its properties in detail:
1.1 Chemical Composition
Every element in UNS N07750 is engineered to enable age hardening while preserving corrosion performance. Below is its standard composition (per ASTM B637):
| Element | Content Range (%) | Key Role |
|---|---|---|
| Nickel (Ni) | 70.0 – 75.0 | The base element—provides high-temperature stability and resistance to chloride stress cracking. |
| Chromium (Cr) | 14.0 – 17.0 | Forms a protective Cr₂O₃ layer—resists oxidation and general corrosion (e.g., jet fuel, seawater). |
| Iron (Fe) | 5.0 – 9.0 | Enhances workability and balances cost without reducing aging performance. |
| Titanium (Ti) | 2.25 – 2.75 | The “aging core”—forms γ’ (Ni₃Ti) precipitates, the primary source of high-temperature strength. |
| Aluminum (Al) | 0.40 – 1.00 | Aids γ’ precipitate formation; improves oxidation resistance at extreme heat. |
| Niobium (Nb) | 0.70 – 1.20 | Forms γ” (Ni₃Nb) precipitates—boosts strength and creep resistance; prevents over-aging. |
| Molybdenum (Mo) | ≤ 0.50 | A minor additive that enhances corrosion resistance to pitting. |
| Carbon (C) | 0.04 – 0.10 | Low enough to avoid carbide brittleness; high enough to aid grain boundary strength. |
| Manganese (Mn) | ≤ 1.00 | Enhances weldability; minimizes hot cracking during manufacturing. |
| Sulfur (S) | ≤ 0.015 | Ultra-low to prevent welding defects and corrosion susceptibility. |
1.2 Physical Properties
These properties reflect UNS N07750’s ability to withstand long-term high temperatures—critical for aerospace and nuclear applications. All values are measured at room temperature unless noted:
- Density: 8.28 g/cm³ (higher than steel, due to nickel, titanium, and niobium content).
- Melting Point: 1390 – 1450 °C (high enough to resist softening in gas turbine engines and nuclear reactors).
- Thermal Conductivity: 11.7 W/(m·K) (at 100 °C); 19.3 W/(m·K) (at 600 °C)—low heat transfer, ideal for components needing structural integrity at high temperatures.
- Coefficient of Thermal Expansion: 12.8 × 10⁻⁶/°C (20–100 °C); 17.0 × 10⁻⁶/°C (20–600 °C)—stable expansion for precision parts like jet engine fasteners.
- Specific Heat Capacity: 440 J/(kg·K) (at 25 °C)—efficient at absorbing heat without rapid temperature changes, reducing thermal stress during aging.
- Electrical Conductivity: 7.5 × 10⁶ S/m (at 20 °C)—lower than copper, but suitable for electrical components in high-heat environments (e.g., nuclear reactor sensors).
1.3 Mechanical Properties
UNS N07750’s mechanical properties shine after age hardening—its strength peaks at 650 °C, making it ideal for long-term high-temperature service. Below are typical values (age-hardened condition, per ASTM B637):
| Property | Typical Value (Age-Hardened) | Test Standard | Why It Matters |
|---|---|---|---|
| Hardness (HRC) | 35 – 40 | ASTM E18 | Balanced hardness—strong enough for high stress, tough enough to avoid brittle failure. |
| Tensile Strength | ≥ 1100 MPa | ASTM E8 | Handles extreme pressure (e.g., jet engine combustion chambers, nuclear reactor vessels). |
| Yield Strength (0.2% offset) | ≥ 790 MPa | ASTM E8 | Resists permanent deformation at 650 °C—critical for long-term creep resistance. |
| Elongation (in 50 mm) | ≥ 15% | ASTM E8 | Moderate ductility—allows forming into complex shapes (e.g., turbine blades) without cracking. |
| Impact Toughness (Charpy V-notch) | ≥ 60 J (at 20 °C) | ASTM E23 | Good toughness—prevents failure from sudden stress (e.g., engine startup/shutdown). |
| Creep Resistance | 172 MPa at 650 °C (10⁵ hours) | ASTM E139 | Maintains strength under long-term high-temperature stress—outperforms many superalloys. |
| Fatigue Strength | ~480 MPa (10⁷ cycles) | ASTM E466 | Resists failure from repeated thermal/mechanical stress (e.g., turbine rotation, reactor cycling). |
1.4 Other Properties
- Corrosion Resistance: Very Good. Resists:
- Oxidation up to 870 °C (thanks to chromium and aluminum).
- Seawater corrosion and pitting (due to molybdenum and nickel).
- Mild acids and alkalis (suitable for chemical processing and marine applications).
- Oxidation Resistance: Excellent. Forms a dense oxide layer that prevents further oxidation at 800–870 °C—ideal for gas turbine components.
- Weldability: Good (with post-weld aging). Requires preheating (200–300 °C) and post-weld solution annealing + age hardening to restore strength; use ERNiFeCr-3 filler metal.
- Machinability: Fair. Work hardens rapidly—requires sharp carbide tools, slow cutting speeds (6–10 m/min for turning), and high-pressure cutting fluids to reduce friction.
- Formability: Moderate. Can be hot-formed (at 980–1150 °C) into complex shapes; cold forming is possible but requires intermediate annealing to reduce work hardening.
2. Applications of UNS N07750 (Inconel X-750) Superalloy
UNS N07750 is used in applications where long-term high-temperature strength is non-negotiable—industries where component failure risks safety or costly downtime. Here are its most common uses, with real examples:
2.1 Aerospace and Jet Engines
- Examples: Jet engine turbine blades, combustion chamber liners, aircraft fasteners (for high-temperature zones), and rocket motor casings.
- Why it works: Age-hardened strength resists creep at 650+ °C, while corrosion resistance handles jet fuel and atmospheric pollutants. A U.S. aerospace manufacturer used UNS N07750 for turbine blades—blade life increased by 450% vs. Inconel 625.
2.2 Nuclear Reactors
- Examples: Reactor pressure vessel components, control rod guide tubes, and fuel assembly brackets.
- Why it works: Resists radiation-induced embrittlement and corrosion from reactor coolants (e.g., water, liquid sodium). A French nuclear operator used UNS N07750 for control rod tubes—no maintenance issues in 20 years.
2.3 Gas Turbines (Energy Industry)
- Examples: Gas turbine stator vanes, rotor components, and exhaust liners for power generation (natural gas plants).
- Why it works: Creep resistance handles long-term operation at 1000+ °C, while oxidation resistance resists exhaust gases. A German energy firm used UNS N07750 for stator vanes—vane life extended to 18 years (vs. 10 years for Inconel 718).
2.4 Oil and Gas Industry
- Examples: Downhole tools (for high-temperature, high-pressure reservoirs) and subsea wellhead components (exposed to seawater).
- Why it works: Resists sulfide stress cracking and creep at 200+ °C. A Saudi Arabian oil company used UNS N07750 downhole tools—tools operated for 11 years without failure (vs. 3 years for stainless steel).
2.5 High-Temperature Fasteners
- Examples: Bolts, nuts, and washers for furnace components, aerospace structures, and nuclear reactors.
- Why it works: Age-hardened strength maintains clamp load at high temperatures, preventing fastener loosening. A Japanese heat treatment shop used UNS N07750 bolts for furnace doors—bolt replacement frequency dropped by 80%.
3. Manufacturing Techniques for UNS N07750 (Inconel X-750) Superalloy
UNS N07750’s manufacturing is centered on precise age hardening—this step is critical to unlock its full strength. Its work-hardening nature also demands careful machining. Here’s a step-by-step breakdown:
- Melting:
- Raw materials (high-purity nickel, chromium, titanium, aluminum) are melted in a vacuum induction furnace (VIF) followed by vacuum arc remelting (VAR). This dual melting ensures ultra-low impurities and uniform composition (critical for consistent precipitate formation).
- Casting/Forging:
- Molten alloy is cast into ingots (up to 4 tons for turbine components) or investment-cast into near-net-shape parts (e.g., turbine blades).
- Ingots are hot-forged at 980–1150 °C—forging aligns grain structure to maximize creep resistance; complex shapes use precision forging.
- Rolling/Forming:
- Hot rolling (at 950–1100 °C) produces plates, bars, or tubes; cold rolling is limited to thin sheets and requires intermediate annealing (at 900–1000 °C).
- Heat Treatment (Critical for Aging Strength):
- Solution Annealing: Heat to 980–1065 °C, hold 1–2 hours, water quench. Dissolves excess precipitates and carbides, preparing the alloy for aging.
- Intermediate Aging (Optional): Heat to 700–760 °C, hold 2–4 hours, air cool. Forms small γ’ precipitates to boost preliminary strength.
- Final Aging: Heat to 620–650 °C, hold 16–24 hours, air cool. Forms large γ’ and γ” precipitates—the main source of UNS N07750’s ultra-high high-temperature strength.
- Machining:
- Use carbide tools with negative rake angles and sharp cutting edges to minimize work hardening.
- Cutting speeds: 6–9 m/min (turning), 4–6 m/min (milling); feed rates: 0.06–0.10 mm/rev.
- Use high-pressure (100–140 bar) cutting fluids (water-soluble with EP additives) to cool the tool and flush chips—prevents re-cutting work-hardened material.
- Welding:
- Preheat to 200–300 °C to reduce thermal stress.
- Use TIG welding with ERNiFeCr-3 filler metal (matches composition).
- Post-weld heat treatment: Solution anneal (1020 °C) + full age hardening to restore strength (critical for load-bearing joints).
- Surface Treatment (Optional):
- Aluminizing (applying an aluminum coating) enhances oxidation resistance for components operating above 870 °C (e.g., turbine blades).
- Shot peening (cold working the surface) improves fatigue strength by creating compressive stress—used for fasteners and turbine components.
4. Case Study: UNS N07750 in Nuclear Reactor Control Rod Tubes
A U.S. nuclear power plant faced a problem: their Inconel 600 control rod tubes failed after 12 years due to creep deformation and radiation embrittlement. They switched to UNS N07750, and here’s what happened:
- Process: UNS N07750 tubes (25 mm diameter, 2 mm wall) were vacuum-melted, hot-rolled, solution annealed (1040 °C), age-hardened (630 °C for 20 hours), and shot-peened to improve fatigue strength.
- Results:
- Tube life extended to 20 years (67% improvement)—no creep deformation or embrittlement even after 85,000 hours of reactor operation.
- Reactor safety margins increased—tube dimensional stability ensured control rods operated smoothly.
- Maintenance costs fell by $600,000/year (fewer tube replacements, no unplanned shutdowns).
- Why it works: γ’ and γ” precipitates in UNS N07750 prevented creep at high temperatures, while nickel’s radiation resistance avoided embrittlement—solving the plant’s core reliability issue.
5. UNS N07750 (Inconel X-750) vs. Other Superalloys
How does UNS N07750 compare to alternatives for long-term high-temperature applications? Let’s evaluate key properties:
| Material | Creep Resistance (MPa at 650 °C, 10⁵h) | High-Temp Stability (Max °C) | Tensile Strength (MPa) | Cost (vs. UNS N07750) | Best For |
|---|---|---|---|---|---|
| UNS N07750 (Inconel X-750) | 172 | 870 | ≥ 1100 | 100% | Long-term high temp (nuclear, aerospace fasteners) |
| UNS N07718 (Inconel 718) | 207 | 700 | ≥ 1240 | 90% | High stress (turbine blades) |
| UNS N06625 (Inconel 625) | 138 | 1095 | ≥ 827 | 80% | High heat (less creep resistance) |
| Hastelloy C276 | 90 | 1010 | ≥ 690 | 150% | Extreme corrosion (low creep) |
| 316 Stainless Steel | 10 | 870 | ≥ 515 | 20% | Mild heat (no long-term creep) |
Key takeaway: UNS N07750 is the top choice for long-term high-temperature applications (10+ years) like nuclear reactors and aerospace fasteners. It balances creep resistance, corrosion performance, and cost better than Inconel 718 (which has higher stress strength but lower temperature stability) and Hastelloy C276 (which excels at corrosion but not creep).
Yigu Technology’s View on UNS N07750 (Inconel X-750) Superalloy
At Yigu Technology, UNS N07750 is our go-to for clients needing long-term high-temperature reliability—like nuclear operators and aerospace manufacturers. Its unique aging mechanism solves the biggest challenge: maintaining strength over decades of high-heat service. We specialize in precise age hardening (controlling time/temperature to optimize precipitates) and machining, often shot-peening critical components to boost fatigue life.
