302 Spring Wire - Stress Relaxation at Temperature
302 spring wire temperature resistance
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302 Spring Wire - Stress Relaxation at Temperature
A spring that holds its working load on a bench can lose part of that load in service without any visible change in shape. The mechanism is stress relaxation: under a fixed deflection, the stress inside the wire gradually falls as the material rearranges itself, and the spring that held 12 N now holds 10 N. At room temperature the effect is slow and usually small. Above about 100 °C it becomes the dominant design limit, and it decides whether a stainless spring is acceptable in a hot location at all. This page explains the mechanism, the temperature ranges where it matters, and the four measures that control it.
Product Specifications
| Item | Specification |
|---|---|
| Product Name | 302 Soap Coated Stainless Steel Spring Wire |
| Material | AISI 302 / SUS302 / UNS S30200 |
| Diameter Range | 0.15-12 mm |
| Diameter Tolerance | ±0.01 mm (fine wire), ±0.02 mm (0.5-3 mm), ±0.03 mm (above 3 mm) |
| Tensile Strength | 1400-1800 MPa, spring temper (size dependent) |
| Recommended Service Temperature | Up to about 250 °C continuous for general spring duty; above this a stabilised grade should be evaluated |
| Relaxation Resistance | Improved by cold work and by a stress relieving treatment after coiling |
| Shear Modulus | Approximately 70 GPa at room temperature, falling slowly with temperature |
| Surface | Soap coated, bright drawn base |
| Coil Weight | 1-100 kg |
| Applicable Standards | ASTM A313, EN 10270-3, GB/T 24588 |
How Stress Relaxation Works
Constant Strain, Falling Stress
Relaxation is often confused with creep, and the two are different tests of the same underlying process. Creep holds the stress constant and watches the strain grow; relaxation holds the strain constant and watches the stress fall. A compressed spring in a housing is a relaxation case: the length does not change, but the force it exerts decreases. The mechanism is thermally activated movement of dislocations, which allows the elastic strain to convert slowly into plastic strain, and the rate rises steeply with temperature.
Why the Temperature Threshold Is Sharp
At room temperature the thermal energy available is small compared with the barriers that pin dislocations in a cold drawn structure, so relaxation proceeds very slowly and is usually lost in the scatter of the spring rate itself. As the temperature climbs, the fraction of dislocations that can move increases, and above roughly 100 °C the rate becomes measurable within the life of a part. Above 250-300 °C the cold worked structure also starts to recover, the strength gained from drawing begins to fall, and relaxation stops being a slow drift and becomes a design failure.
Why the Initial Stress Level Matters Most
Relaxation is driven by the stress already in the wire. A spring working at 30% of its tensile strength relaxes far less at the same temperature than one working at 55%, because the driving force for dislocation motion is lower from the start. This is the single most effective lever a designer has, and it costs nothing except a slightly larger spring.
The Four Measures That Control It
Lower the Working Stress
Reducing the design stress from the upper part of the allowable range to the middle of it removes most of the relaxation risk before any process change is considered. Where the envelope allows a larger wire or a larger coil diameter, the gain is immediate and permanent.
Stress Relieve After Coiling
A low temperature heat treatment after coiling, typically in the 250-400 °C range depending on grade and temper, relieves the residual stresses locked in by winding and lets the spring settle before it is put into service. The part then relaxes less in the first hours of use, which is where the largest part of the total loss usually occurs. This treatment is not the same as the bake used after electroplating, which is a hydrogen removal step at a lower temperature and a different purpose.
Preset or Scragging
Deliberately over-compressing a spring to a height beyond its service position, once or a few times, produces the same settlement in the factory instead of in the application. The spring becomes shorter by a controlled amount and its subsequent load is more stable. Scragging does not stop relaxation, but it moves the fast early stage to a point where it can be measured and compensated.
Choose the Grade Against the Temperature
302 is a general purpose grade and suits most applications below about 250 °C. Where the service temperature is higher, or where the load must be held for years at temperature, a stabilised grade with niobium or titanium addition holds its structure better and should be evaluated rather than relying on a larger 302 spring.
Applications
Springs in Warm Enclosures
Motors, power supplies, cooking appliances and lighting fixtures all create local temperatures well above ambient, and a spring compressed inside the housing sits in that heat continuously. Load loss appears as a weakened contact pressure or a valve that opens early, and it is usually diagnosed long after the spring was made.
Outdoor and Solar Exposed Parts
A dark metal spring in direct summer sun can reach surface temperatures far above air temperature. Springs clamped in tension in outdoor equipment are the classic relaxation case, because the length is fixed and any loss shows directly as a load loss.
Valve and Clamp Springs
Any spring that must hold a sealing force for years is a relaxation application whether or not temperature is involved, because the requirement is a minimum load at end of life. These parts are normally specified with a stress level, a stress relieving treatment and an end of life load, rather than with a single initial load figure.
Product Advantages
- Spring temper 302 with a stated tensile range per diameter, so the working stress ratio can be calculated before the design is fixed.
- Consistent diameter and roundness, keeping the initial load predictable so that relaxation appears as a trend rather than as scatter.
- Coil set controlled, so a spring does not start service with an unintended pre-stress that accelerates relaxation.
- Grade and temper advice against the service temperature, since a higher grade is often cheaper than a redesigned housing.
- Traceability by heat number, supporting an investigation when a load loss appears after a period in service.
Quality Control
Diameter, tensile strength and elongation are tested on every lot, and coil set is checked against the agreed value. Surface is inspected over the full coil length. Where a customer specifies a service temperature, the recommended stress relieving treatment and the working stress limit are stated with the delivery so that the spring maker and the end user are working from the same assumptions.
Selection and Ordering Notes
- State the service temperature, not only the room temperature load, because relaxation is what changes the design in warm locations.
- Give the working stress as a fraction of tensile strength, or the design load and the spring geometry, so the ratio can be checked.
- Ask for the stress relieving treatment after coiling to be defined, since it is the cheapest single measure against early load loss.
- Specify whether scragging is permitted, as it changes the free length and must be agreed before the parts are made.
- For long life parts, specify an end of life load rather than an initial load, and let the spring design carry the relaxation allowance.
Stress relaxation is a slow loss that is invisible until the part stops doing its job, and it is controlled by the stress level, the heat treatment and the grade rather than by the wire diameter alone. Send the service temperature, the load history and the required end of life force, and the wire specification can be matched to the life the spring has to deliver.
