Under what conditions will the creep phenomenon of stainless steel torsion springs be more significant

Creep is the slow, permanent plastic deformation of a solid material under constant stress over time. For stainless steel torsion springs, creep manifests as a gradual decrease in the restoring torque (technically known as stress relaxation under constant deflection) or a continuous increase in the deflection angle under constant load. This phenomenon directly affects the spring's long-term precision and reliability. From a professional perspective, the significant occurrence of creep in stainless steel torsion springs is primarily influenced by the synergistic effects of the following three integrated factors.

1. Critical Temperature Effect

Temperature is the primary factor determining whether creep will occur significantly. While creep theoretically occurs at any temperature, its rate only materially impacts engineering applications once it exceeds a specific threshold.

Melting Point Correlation: Traditional metal material theory suggests that creep typically becomes significant around 0.4Tm​ above the material's absolute melting temperature. Stainless steels (such as the 300 series) have a higher melting point, but because the spring wire is under high stress, the actual temperature at which creeping occurs is much lower.

Stainless steel service temperature: Generally speaking, the recommended maximum service temperature for a torque spring for standard austenitic stainless steels (such as SUS 304 or 302) is approximately 250°C to 300°C.

When the working temperature is below 100°C, the creep rate is extremely low and can be ignored.

When the working temperature exceeds 150°C, especially in the 200°C to 300°C range, dislocation movement and vacancy diffusion within the stainless steel are activated by thermal energy, accelerating plastic deformation and causing creeping to become noticeable.

2. The Catalytic Effect of High Stress Levels

Under the same temperature conditions, applied stress levels are the primary driving force that accelerates creep. For torsion springs, this stress specifically refers to bending stress.

Stress and Yield Strength: Creep is unique in that it occurs at stress levels far below the material's yield strength. However, the closer the stress approaches the elastic limit, the higher the creep rate.

Spring Design: When designing a torsion spring, if the Maximum Working Stress exceeds a critical percentage of the stainless steel material's Proportional Limit (e.g., 60% or 70%), creep may accumulate over an extended period, generating significant dimensional instability, even at room temperature. High stress provides the activation energy required to overcome lattice resistance, accelerating the occurrence of dislocation creep.

Stress Relaxation: In constant deflection applications, high stress directly leads to accelerated stress relaxation. This relaxation ultimately manifests as torque loss, which is the primary reason the spring cannot maintain its intended function.

3. Sustained Loading Duration

Creep is a typical time-dependent deformation. The longer the spring remains under load, the greater the cumulative creep strain.

Three Stages of Creep: The creep process is typically divided into three stages:

Primary Creep: The strain rate gradually decreases. This is the stage dominated by strain hardening when the spring is first loaded.

Secondary Creep: The strain rate remains essentially constant. This is a stage of equilibrium between hardening and softening (i.e., recovery), and accounts for the majority of the spring's service life.

Tertiary Creep: The strain rate increases sharply until fracture. In practical applications of torque springs, this stage is generally not permitted.

Long-Term Static Load: For static load applications that require maintaining a fixed angle for extended periods, such as valve springs or certain clamping mechanisms, time is crucial. Even at relatively low stress and temperature, cumulative loads over years or even decades can cause the spring's permanent set to exceed tolerances.

4. Influence of Material Microstructure

The microstructure and manufacturing process of stainless steel wire have a decisive influence on creep resistance.

Cold Work Hardening: Stainless steel spring wire typically undergoes a high percentage of cold drawing to achieve high strength. The high density of dislocations introduced by cold working improves creep resistance at room temperature. However, as the temperature rises, these dislocations may begin to recover, reducing stress relaxation performance.

Precipitation Hardening: Some high-strength stainless steel grades (such as 17-7 PH Stainless Steel) utilize a precipitation hardening mechanism. Proper heat treatment and aging can form fine precipitates, effectively pinning dislocations and significantly improving elevated temperature creep resistance.