302 Spring Wire - Choosing Tensile Grade for Forming
302 Spring Wire: Choosing a Tensile Grade That Survives Forming
When a spring wire specification is written without a conversation, one number tends to get pushed upward: tensile strength. The assumption is that stronger wire makes a better spring. In practice, tensile strength and formability move in opposite directions, and a wire that is 100 MPa stronger than necessary is a wire that cracks at the coiling radius or splits at the end hook.
This article explains where 302 spring wire strength comes from, how the EN 10270-3 classes differ, and how to match tensile range to the forming operation instead of defaulting to the maximum.
Strength Comes From Cold Reduction
302 is an austenitic grade that cannot be hardened by heat treatment. Its spring properties are produced mechanically: the wire is drawn through a sequence of dies, and each pass reduces area and elongates the grains, raising tensile strength and hardness. The total reduction from the last annealing step sets the final strength.
The trade-off is ductility. As tensile rises, the number of twists the wire survives falls, and the elongation at break drops. Torsion testing is the most practical indicator: it loads the wire in the same mode as a spring and shows surface defects and residual ductility at the same time.
The Three EN 10270-3 Classes
Static class
For springs that are loaded a limited number of times: setting springs, one-time assembly springs, locating springs. Surface and inclusion requirements are the least severe and the price is the lowest of the three.
Medium class
The general purpose class. Suitable for springs with moderate cycling in appliances and machinery. Tighter control of surface condition and cross section than the static class, and tighter diameter tolerance.
Dynamic class
For springs in continuous or high frequency cycling, such as valve springs and vibration components. This class demands the cleanest steel, the finest surface and the narrowest diameter tolerance, because fatigue life is controlled by exactly those three things. It is also the most expensive, and it is wasted on a static application.
Matching Grade to Forming Severity
Forming severity, not application name, should drive the tensile choice. The following practical rules apply to 302 wire in the 1 mm range:
- Large coiling ratio with generous bend radii: the upper part of the medium range is fine and gives the best resistance to relaxation
- Coil index below about 6, or bend radius below twice the wire diameter: reduce tensile to the lower part of the range, or accept scrap
- Severe end hooks, multi-slide forming with several tight bends: lower tensile again, and specify a soap or lubricant coating
- Parts that will be welded after forming: lower tensile, because the heat affected zone loses the cold work regardless
As a working figure, moving from the top to the bottom of a normal 302 tensile band buys roughly one additional diameter of minimum bend radius before cracking starts. That is often the difference between a stable process and a 5% reject rate.
Surface and Coating
Bright drawn wire is standard for springs that will be plated, polished or used as formed. Soap coated wire carries a dry film that protects the surface during feeding and reduces die wear in automatic coilers; the coating is normally removed after forming. Both are available with a controlled surface roughness, which matters for fatigue in dynamic applications.
Specification Window
| Parameter | Range / Standard |
|---|---|
| Grade | AISI 302, also 304, 304Cu, 316, 201 |
| Wire diameter | 0.15 - 12 mm |
| Class | EN 10270-3 static, medium, dynamic |
| Tensile range | Agreed per diameter and class |
| Diameter tolerance | h9, tighter on request |
| Torsion test | Declared per diameter and class |
| Surface | Bright drawn or soap coated |
| Delivery | Coil, spool, cut length |
Applications
- Compression, tension and torsion springs in appliances
- Automotive clips and retaining springs
- Valve and pump springs where dynamic class applies
- Industrial machinery return springs
- Wire forms with integral spring sections
How to Write the Requirement
State the class, the diameter and tolerance, the tensile range rather than a single target value, the minimum torsion figure, and the surface condition. If the forming operation is severe, say so: it is easier to supply a slightly softer wire than to explain a cracked batch.
TOPONE has produced stainless steel spring wire since 1993 in 0.15 mm to 12 mm, with tensile and torsion declared per lot so that forming performance can be traced back to the wire rather than guessed at.
