Inconel X-750 (UNS N07750) – High-Strength Spring & Turbine Alloy Supplier (ASTM B637)

Section 1: Grade & Introduction

Inconel X-750 (UNS N07750 / W.Nr. 2.4669) is a precipitation-hardenable nickel-chromium alloy used for its corrosion and oxidation resistance and high strength at temperatures up to 704°C (1300°F). Although much of the effect of precipitation hardening is lost with increasing temperature above 704°C, heat-treated material has useful strength up to 982°C (1800°F).

Section 2: Chemical Composition

Inconel X-750 relies on titanium and aluminum for its precipitation strengthening. Below is the wt.% composition per ASTM B637 and SAE AMS 5598 standards.

Section 3: Mechanical & Physical Properties

Physical Properties

• Density: 8.28 g/cm³
• Melting Range: 1393 - 1427°C (2540 - 2600°F)
• Thermal Expansion Coeff. (20-100°C): 12.6 μm/m·°C

Mechanical Properties (Min. values per SAE AMS 5598)

Section 4: Material Characteristics

• High Strength: Excellent mechanical properties after precipitation hardening up to 704°C.
• Relaxation Resistance: Excellent resistance to stress relaxation at high temperatures, making it ideal for springs.
• Corrosion Resistance: Good resistance to various industrial corrosives and seawater.
• Oxidation Resistance: Maintains stability in oxidizing atmospheres up to 982°C.

Section 5: Others

Manufacturing & Processing

Manufactured through hot and cold working processes. Achieving peak properties requires specific heat treatments, typically including solution annealing followed by single or double aging cycles.

Testing

Quality control involves Ultrasonic Testing (UT), PMI analysis, and hardness testing (min 30 HRC for aged material).

Applications

Widely used for gas turbine rotor blades, rocket engine components, high-temperature springs, and fasteners in nuclear reactors.

Global Cases

Used in the Space Shuttle Main Engine and by major gas turbine manufacturers for hot-section seals and springs.

FAQ

1. What is the primary use of X-750? It is most commonly used for high-temperature springs and fasteners.
2. Does it require heat treatment? Yes, its high strength is only achieved through a precipitation-hardening (aging) process.
3. Is it resistant to seawater? Yes, it has good resistance to marine environments.
4. What standards cover X-750 plates? SAE AMS 5542 and SAE AMS 5598 are the most common.
5. Can it be used above 704°C? It retains useful strength up to 982°C, but properties significantly drop beyond 704°C.

Section 6: ASTM B637 Compliance & Spring Temper Heat Treatment

ASTM B637 Composition Limits (Bar, Forgings & Rings)

For round bar, forgings and rings supplied to ASTM B637 (and SAE AMS 5667), Inconel X-750 is certified to the following principal composition limits:

Spring Temper Heat Treatment (Springs & Wire)

For high-temperature spring applications, Inconel X-750 is supplied in the Spring Temper condition — cold drawn wire or cold rolled strip — and is precipitation hardened after forming at 650 °C (1200 °F) for 4 hours, air cooled. This specialized schedule develops the optimum combination of stress relaxation resistance, elastic limit and hardness (typically 35 – 40 HRC) while retaining the ductility required for spring coiling and forming, and is the standard treatment for AMS 5698 / AMS 5699 spring wire.

Standard Heat Treatments for Bar & Forgings (ASTM B637)

For bar, forgings and rings, material is typically solution annealed at 1149 °C (2100 °F) for 2 hours, air cooled, followed by double aging: 843 °C (1550 °F) for 24 hours, air cooled, then 732 °C (1350 °F) for 20 hours, air cooled, achieving the minimum mechanical properties listed in Section 3.