Inconel X-750 Disc Springs — A Preferred Spring Alloy for Nuclear Power and High-Temperature Valves

Among the three nickel-based high-temperature disc spring materials, Inconel X-750 has the most extensive research data and engineering service history in spring applications. It is a preferred precipitation-hardened nickel-chromium alloy for high-temperature preload applications, nuclear power equipment, and high-temperature valves.

The alloy achieves precipitation strengthening through γ′ precipitates. The size and volume fraction of the γ′ phase directly influence the alloy's strength, creep resistance, and resistance to stress relaxation.

The evolution of the γ′ phase is affected by aging temperature, holding time, applied stress, and thermal history. This is why heat treatment is an integral part of X-750 disc spring design.

Published literature and U.S. Department of Energy materials indicate that Inconel X-750 has been used extensively for hold-down springs and bolting components inside light-water reactors and can withstand neutron irradiation environments. Its stress-relaxation behavior under irradiation is a key performance criterion.

It is also widely used in:
• High-temperature process valves
• Safety valves
• High-temperature aerospace mechanical assemblies

Many high-temperature valves rely on disc springs to continuously maintain sealing preload. Loss of spring load can directly result in sealing failure, which is an application area where X-750 offers important advantages.

Many purchasing drawings specify only the material grade “Inconel X-750.” For critical disc springs, this specification is incomplete.

A complete technical specification should define:
• Material grade
• Material condition
• Heat-treatment process
• Mechanical properties
• Dimensional tolerances
• Load requirements

Two disc springs manufactured from sheet material of the same alloy grade can exhibit significantly different long-term stress-relaxation behavior if their cold-working levels, aging processes, grain structures, or residual stresses differ.

Engineering recommendation: For X-750 disc springs used in critical applications, stress-relaxation testing should be added to verify long-term load-retention capability. Acceptance should not rely solely on room-temperature load testing.