Key Engineering Risks of High-Temperature Disc Springs: Stress Relaxation, Material Data Sheets, and Differences in Finished Spring Performance

In conventional spring design, engineers mainly focus on the load-deflection curve. For high-temperature disc springs, however, the most critical failure risk is stress relaxation: the spring may remain visually intact while its preload gradually decreases over time.

When disc springs are used for valve sealing preload or equipment clamping assemblies, loss of load can directly result in sealing failure or equipment loosening.

Stress relaxation is influenced by:
• Operating temperature
• Initial stress
• Deformation
• Duration
• Material condition
• Heat treatment
• Environmental medium

There are two common engineering misconceptions:

1. Selecting material solely according to maximum temperature.
For example, selecting Nimonic 90 simply because the operating temperature is 600°C while ignoring stress, static/dynamic loading, service life, corrosion, and other coupled factors.

2. Treating material handbook data as equivalent to finished spring performance.

The tensile strength and creep data listed in nickel-based alloy material data sheets are generally obtained from standardized tensile specimens.

A disc spring, however, operates under a combined bending-stress condition. Forming introduces residual stresses, while cold working and precipitation-hardening heat treatment further influence the final material state.

As a result, the stress-relaxation behavior of the finished spring can differ significantly from that of the raw-material specimen.

Existing literature and testing have shown that Nimonic 90, René 41, and Haynes 282 springs tested at 600–700°C can experience force loss significantly faster than predicted by the creep data of the raw materials.

High temperature also changes the elastic modulus. The elastic modulus of Inconel 718, X-750, and Nimonic 90 decreases as temperature increases.

Therefore, a disc spring that passes load testing at room temperature may produce a lower load under actual high-temperature operating conditions. For critical equipment, load should be measured at the actual operating temperature.

High-Temperature Disc Spring Validation Testing

For conventional industrial disc springs, dimensional inspection plus room-temperature load testing may be sufficient.

For nickel-based high-temperature alloy disc springs, critical applications should include additional specialized validation:
1. Room-temperature load-deflection testing — to confirm basic spring characteristics;
2. High-temperature load testing — to measure actual spring output at the operating temperature;
3. Stress-relaxation testing — under controlled temperature, deformation, and holding time to determine load loss;
4. Fatigue testing — to evaluate service life under cyclic compression;
5. Metallographic examination — to verify microstructure and heat-treatment condition for critical components.

Conclusion

High-temperature disc spring design cannot be based solely on the material grade. Material, geometry, heat treatment, and stress state must be evaluated as an integrated system, and finished-product testing must be used to verify long-term load-retention capability.