INCONEL®718 screws for high-temperature fastening systems

Production of INCONEL® 718 screws

The production of INCONEL® 718 screws follows a strict sequence to ensure optimal material properties and compliance with required specifications.

1. Selection of raw material:
   • INCONEL® 718 bars must meet the standards of international regulations to ensure the quality of the material. Common specifications include:
     • ASTM B637: Standard for bars, billets and forgings in INCONEL® 718.
     • AMS 5662/5663: Forging and bar specifications for INCONEL® 718.
     • ASME SB-637: Specification used in the context of pressure applications.
     • JIS NCF 718: Japanese standard for high-strength nickel alloys.
2. Hot stamping:
   • Hot stamping is a key step in the production of INCONEL® 718 screws. This process results in a rough screw shape with optimal mechanical properties and a uniform microstructure.
     • Warm-up phase:
       • The raw material is heated to temperatures between 980 °C and 1200 °C, depending on the size and complexity of the component to be made. This allows for reduced deformation resistance and improved processability.
     • Molding:
       • The heated bar is inserted into a mold and subjected to high pressure by mechanical or hydraulic presses, shaping it into the desired shape.
       • During this stage, it is essential to avoid defects such as cracks or inclusions by strictly monitoring temperature and processing conditions.
     • Controlled cooling:
       • After molding, the component is cooled in a controlled manner (in air or by a forced cooling process) to prevent the formation of unwanted residual stresses.

   This process not only ensures a shape close to the finished product, but also improves the quality of the microstructure and ensures homogeneous distribution of alloying elements.

3. Mechanical processing:
   • Machining INCONEL® 718 requires CNC machine tools and cemented carbide tools or heat-resistant materials. Machining speeds must be reduced because of the material’s tendency to harden during machining (work hardening).
   • Turning, milling and drilling processes must be carried out with adequate cooling to avoid overheating and deterioration of material properties.
4. Heat treatment: Heat treatment is a crucial step in optimizing the mechanical and thermal properties of INCONEL® 718. Typical sequence includes:
   • Solubilization:
     • Heat the material to a temperature of 980-1010 °C, maintaining the heat long enough to dissolve unwanted secondary phases and achieve a homogeneous microstructure.
     • Rapid cooling (usually in air or water) follows.
   • Aging:
     • After solubilization, aging is performed to increase hardness and strength by precipitation of the γ” (Ni3Nb) and γ” (Ni3(Al,Ti)) phases:
       1. Heat to about 720-760 °C for 8-10 hours.
       2. Cool slowly to 620 °C (cooling rate of about 50 °C/h).
       3. Keep at 620 °C for 8 hours and then cool in air.

Basic requirements

To ensure that INCONEL® 718 screws meet the needs of critical applications, the following requirements must be met:

1. Mechanical strength:
   • INCONEL® 718 can withstand stresses up to about 1,200 MPa under aged conditions.
   • Creep resistance is optimal up to temperatures of 700-750 °C, making it ideal for turbines, aerospace engines, and energy sector applications.
2. Resistance to high temperatures:
   • The material properties remain excellent up to 700 °C with minimal degradation due to the microstructural stability offered by the precipitation of the γ” and γ” phases.
3. Corrosion resistance:
   • INCONEL® 718 offers high resistance to oxidation and stress corrosion in aggressive chemical environments, such as chlorides and high-temperature gases.
4. Dimensional accuracy:
   • Screws must be machined to strict tolerances (ISO, ASME or specific standards), considering the dimensional stability of the material after heat treatments.

Main applications

Due to its properties, INCONEL® 718 screws find application in:

• Gas and steam turbines.
• Aerospace engines and exhaust systems.
• Petrochemical sector (offshore and onshore plants).
• Fastening systems for critical high-temperature applications.

INCONEL® 718’s ability to maintain excellent properties in extreme environments makes these screws indispensable for long-term safety and reliability.