Inconel X750 vs N07718 alloy - Knowledge

1. Chemical Composition

The primary distinction lies in their alloying elements, which drive their unique properties:

Element	Inconel X750 (UNS N07750)	Inconel 718 (UNS N07718)
Nickel (Ni)	70% min (balance)	50–55% (balance)
Chromium (Cr)	14.0–17.0%	17.0–21.0%
Iron (Fe)	5.0–9.0%	17.0–21.0%
Molybdenum (Mo)	0.05% max (trace)	2.8–3.3%
Niobium (Nb)	0.7–1.2% (often with Ta)	4.75–5.50% (key alloying element)
Aluminum (Al)	0.4–1.0%	0.2–0.8%
Titanium (Ti)	2.25–2.75%	0.65–1.15%
Copper (Cu)	0.5% max	0.3% max

Key notes: Inconel 718 relies heavily on niobium (and tantalum, often grouped with Nb) for strengthening, while X750 uses higher titanium and aluminum. Inconel 718 also contains significant molybdenum, absent in X750.

2. Strengthening Mechanisms

Inconel X750: Strengthened primarily by precipitation of the γ' phase (Ni₃(Al, Ti)), a coherent intermetallic compound that forms during heat treatment. This phase provides high-temperature strength and stability.

Inconel 718: Strengthened by the γ" phase (Ni₃Nb), a metastable, disc-shaped precipitate that forms at lower aging temperatures (593–816°C). It also contains γ' phase, but γ" is the dominant strengthening phase. This mechanism gives 718 exceptional strength at intermediate temperatures.

3. Mechanical Properties

Their properties diverge, especially at elevated temperatures:

Property	Inconel X750 (Annealed + Aged)	Inconel 718 (Solution Annealed + Aged)
Tensile Strength (RT)	~1,240 MPa (180,000 psi)	~1,240 MPa (180,000 psi)
Yield Strength (RT)	~860 MPa (125,000 psi)	~1,100 MPa (160,000 psi)
Elongation (%)	15–20%	12–16%
High-Temp Strength (650°C)	~820 MPa (119,000 psi)	~965 MPa (140,000 psi)
Creep Resistance	Excellent at 650–815°C	Superior at 427–760°C; drops above 760°C (γ" phase dissolves)
Fatigue Resistance	Good, especially under thermal cycling	Excellent, even with cold working

4. Heat Treatment

Inconel X750:

Standard process: Solution annealing at 1149°C (2100°F) for 1 hour, air cooling; stabilization at 843°C (1550°F) for 24 hours; and precipitation aging at 691°C (1300°F) for 20 hours. This optimizes γ' formation for high-temperature strength.

Inconel 718:

Typical treatment: Solution annealing at 982–1066°C (1800–1950°F) for 1 hour, water quenching; followed by double aging: 720°C (1328°F) for 8 hours, furnace cooling to 621°C (1150°F), holding 8 hours, then air cooling. This promotes γ" precipitation for maximum strength.

5. Temperature Resistance

Inconel X750: Performs well up to ~760°C (1400°F) due to stable γ' phases. Beyond this, it retains better creep resistance than 718.

Inconel 718: Excels at intermediate temperatures (up to ~650°C) but loses strength above 760°C because the γ" phase becomes unstable and dissolves.

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6. Fabrication & Weldability

Inconel X750: Weldability is limited due to sensitivity to cracking during welding (hot cracking risk from titanium/aluminum). Pre- and post-weld heat treatments are often required.

Inconel 718: Highly weldable-one of its key advantages. It can be welded using conventional methods (TIG, MIG) without severe cracking, thanks to its niobium content, which reduces grain boundary sensitization. Post-weld aging can restore full strength.

7. Applications

Inconel X750: Used in high-temperature structural components, e.g., gas turbine rotor blades, jet engine fasteners, rocket motor casings, and pressure vessels. Its stability at 650–815°C makes it ideal for long-term service under thermal stress.

Inconel 718: Dominates in applications requiring weldability and strength at intermediate temperatures: aircraft engine components (discs, shafts), rocket nozzles, cryogenic tanks, oil and gas downhole tools, and industrial turbine parts. Its versatility makes it one of the most widely used superalloys.

Inconel X750 is preferred for high-temperature, non-welded applications, while Inconel 718 shines in weldable, intermediate-temperature scenarios due to its γ" strengthening and superior fabricability.