Material Selection for Disc Springs Under High-Temperature Conditions: An Engineering Selection Guide to Nickel-Based Alloys Inconel 718, Inconel X-750, and Nimonic 90

Disc springs, also known as Belleville springs, can generate high spring forces within a compact axial space and are widely used in preload systems, sealing assemblies, valve assemblies, and other mechanisms. Conventional disc springs are typically manufactured from spring steels such as 50CrV4 and 51CrV4. However, under high-temperature, corrosive, nuclear-radiation, or long-term static-load conditions, conventional spring steels may experience load loss and fail to meet long-term service-life requirements. In such applications, nickel-based precipitation-hardened high-temperature alloys such as Inconel 718, Inconel X-750, and Nimonic 90 become important material options.

Material selection for nickel-based alloys should not be based solely on the maximum operating temperature. High-temperature disc spring design requires a comprehensive assessment of temperature, stress, deformation, heat treatment, creep, stress relaxation, fatigue, and corrosion. Although all three alloys are precipitation-strengthened nickel-based alloys, their performance characteristics differ significantly and they should not be considered directly interchangeable.

Inconel 718, DIN designation 2.4668, has a room-temperature elastic modulus of approximately 208,000 N/mm². It offers a balanced combination of high strength, corrosion resistance, and fatigue resistance. It is relatively stable in supply and is suitable for oil and gas equipment, conventional high-pressure valves, chemical processing equipment, and moderately high-temperature aerospace components.

Inconel X-750 is one of the most established nickel-chromium precipitation-hardened alloys for high-temperature spring applications. It has been widely used in nuclear reactor hold-down springs and high-temperature valves. Its γ′ precipitates are central to its mechanical performance, and extensive literature has demonstrated its excellent resistance to stress relaxation, making it a preferred material for high-temperature preload applications.

Nimonic 90, DIN designation 2.4969, has a room-temperature elastic modulus of approximately 220,000 N/mm². It is a nickel-chromium-cobalt alloy whose key advantages are high-temperature strength and creep resistance. Its maximum applicable temperature is approximately 600°C, and it is commonly used for high-temperature springs associated with turbines. However, an important point must be emphasized: creep data obtained from raw-material tensile specimens cannot be directly equated with the stress-relaxation behavior of finished disc springs.

There are two additional key considerations in high-temperature disc spring design:
1. The elastic modulus decreases as temperature increases. Therefore, the load calibrated at room temperature is not necessarily the actual operating load at elevated temperature.
2. Disc spring geometry—including outer diameter, inner diameter, thickness, free height, and stacking configuration—is fundamental. Simply changing the alloy cannot compensate for deficiencies in geometric design.

The complete engineering evaluation should follow the logic of:
Temperature → Stress → Deformation → Environment → Creep → Stress Relaxation → Fatigue → Design Life

For critical operating conditions, the product should be subject to appropriate testing, including room-temperature load-deflection testing, high-temperature load testing, stress-relaxation testing, fatigue testing, and, where necessary, metallographic examination.

High-temperature disc springs are not simply conventional disc springs made from more expensive alloys. They are a system-engineering solution involving material, geometry, heat treatment, and stress distribution. Only through the coordinated optimization of these four factors can long-term load-retention capability be ensured.