2.3mm Spring Wire - Core and Surface Strain Balance
2.3mm Spring Wire: Why the Core Lags the Surface in Cold Drawing
Two coils of 2.3 mm spring wire can pass the same tensile test and still behave differently on the coiler. One bends cleanly and holds a stable pitch; the other splits along its axis during torsion testing, or cracks at a bend radius the first coil handled without trouble. The difference is usually not the steel grade. It is how evenly the drawing strain was distributed between surface and core.
This article explains the strain gradient in medium and coarse spring wire, how to detect it, and which drawing practices keep the section uniform.
Where the Strain Goes
Cold drawing reduces area by forcing the wire through a die. The surface layer passes through the die bearing and takes the full reduction in a short distance; the core deforms under a different stress state, constrained by the surrounding metal. Friction at the die interface increases the surface-to-centre difference further.
On fine wire the section is small enough that the difference is minor. From about 2 mm upward it becomes measurable, and by 4 mm it is a design consideration. The practical result: a wire with correct average tensile strength can have a hard, heavily worked shell and a softer, under-worked core.
How It Shows Up in Service
- Torsion test breaks with a delamination or a star-shaped fracture instead of a clean 45 degree shear
- Springs crack at the inside of a bend where surface strain is highest, not at the outside where it is usually assumed
- Coiling pitch varies along a spring because the wire resists bending unevenly
- Sectioned and polished, the wire shows a hardness difference between edge and centre
The Check That Matters Most
Hardness across the section is the simplest production control. Mount a cross section, polish it, and measure microhardness at the edge, at quarter radius and at the centre. A well drawn 2.3 mm spring wire should show a modest gradient; a large spread indicates the core has not been worked and the wire will be unpredictable in torsion.
Torsion testing alone catches severe cases but not moderate ones, because a delamination only appears when the core is markedly weaker than the shell. Combining torsion with a section hardness check catches both.
Drawing Practices That Keep the Section Even
Limit reduction per pass
Smaller reductions per pass, with more passes, distribute strain more evenly through the section. The cost is more passes and more handling; the benefit is a wire that behaves consistently in forming.
Use an intermediate anneal where the schedule demands it
For large total reductions, an intermediate anneal resets the work hardening and prevents the shell from becoming far harder than the core. The anneal also relieves the residual stress pattern that causes wire to curve off the spool.
Control the die interface
Effective lubrication and the right die geometry reduce friction, which is the main driver of the surface-to-core gradient. Roller dies are an alternative on larger sizes, converting sliding friction into rolling contact.
Do not skip the final stress relief
After the last pass, a controlled low temperature treatment equalises the residual stress field. Wire straightened only by mechanical means keeps a locked-in bending moment that reappears as cast and helix when the coil is opened.
Specification Window for 2.3 mm
| Parameter | Value / Standard |
|---|---|
| Nominal diameter | 2.3 mm |
| Diameter tolerance | h9, tighter on request |
| Grade | AISI 302 hard temper, also 304 and 316 |
| Tensile strength | 1550 - 1800 N/mm2 |
| Section hardness uniformity | Declared on request |
| Torsion test | Per EN 10270-3 class |
| Surface | Bright drawn or soap coated |
| Delivery | Coil, spool, cut length |
Applications
- Heavy duty compression and torsion springs
- Large diameter extension springs
- Industrial return and counterbalance springs
- Wire forms requiring thick sections with tight bends
- Springs coiled on CNC machines at high feed rates
What to Ask a Supplier
Ask for the drawing schedule summary, the number of passes, whether an intermediate anneal is used, and whether section hardness can be declared. A supplier who can answer those four questions is controlling the core, not only the average.
TOPONE has drawn stainless steel wire since 1993 in diameters from 0.15 mm to 12 mm, and controls pass schedule and intermediate annealing as part of the process rather than as a reaction to complaints.
