302 Fine Spring Wire - Torsion Test as Quality Gate

At 0.2 mm, a surface defect that would be visible on a 2 mm wire is invisible, and an internal inclusion is impossible to see at all. The torsion test exists for exactly this gap. It twists a short length of wire until it breaks and counts the turns, and a low count tells you something is wrong inside the wire even though the diameter, the tensile strength and the certificate are all in order. This page explains how the test works, which defects it exposes, and how to use the result as an incoming acceptance check rather than as a number on a certificate nobody reads.

Product Specifications

What the Torsion Test Reveals

The Test Itself

A length of wire is clamped at both ends and twisted at a controlled rate until it fractures, and the number of turns is counted. The test is run on a gauge length proportional to the diameter, and the standard requires a defined rate so that results from different labs can be compared. A reverse torsion variant twists in one direction, unwinds, and continues in the other, which is harsher on surface condition and closer to what some forming processes actually do.

Defects It Exposes

Torsion is a shear test, and shear strain concentrates at any discontinuity. Non-metallic inclusions, centre porosity, longitudinal seams, laps from an overlapping pass, and decarburised or otherwise altered surface layers all reduce the turn count before fracture. The fracture face is as informative as the number: a normal ductile break is a clean helical surface, while a wire with a longitudinal defect splits along its axis and shows a ragged, layered break.

Why Fine Wire Needs It Most

The Inspection Problem at Small Diameter

Optical surface inspection at 0.2 mm resolves very little, and eddy current testing needs the correct frequency and coil for each diameter, which is not practical for a mixed product range. Torsion testing a short sample costs a few minutes and needs only a simple machine. For diameters below about 0.5 mm it is often the only practical check on internal quality that a customer can run without a metallurgical lab.

Limits and How They Are Set

Acceptance limits are usually agreed as a minimum turn count over a defined gauge length, sometimes with a requirement that no specimen falls more than a set amount below the average. The limit should be set from the process capability of the wire you actually buy, not copied from a generic table, because the required count changes with diameter, temper and drawing route. A limit that is too high rejects good wire; one that is too low passes the defect that will stop a coiler.

Reading a Torsion Result

Counting and Criteria

A lot is normally represented by several specimens taken from different positions in the coil, since defects are not distributed evenly. Report the mean and the lowest value, not only the mean. A mean of 30 turns with a single specimen at 12 is a different situation from a consistent 25, even though the average looks similar.

What a Low Count Means

A single low result points to a local defect and is usually resolved by taking more specimens from the same area. A whole lot running low points to a process problem: a dirty melt, an inclusion level that has drifted, or a drawing route that has been shortened. Neither is fixed by re-testing, and both are worth raising before the coil is consumed.

Applications

Fine Springs for Instruments

Instrument springs from 0.1 mm to 0.5 mm fail by fatigue at the coil, and a fatigue crack almost always starts at an inclusion or a seam that a torsion test would have flagged. For this application the torsion result is a more useful acceptance criterion than tensile strength.

Knitted Mesh and Filters

Knitting bends the wire repeatedly at a small radius, and a wire with a longitudinal seam will split at the needle. Torsion testing catches the seam before knitting starts, when the cost of a failure is a rejected roll of mesh rather than a rejected part.

Braided and Stranded Constructions

In braided hose and stranded cable, one broken filament ends the life of the assembly under flexing. Fine wire for these constructions is usually specified with both a torsion limit and a bend test, since the two expose different defects.

Product Advantages

• Torsion tested per lot to ASTM A938 or ISO 7800, with turns to fracture reported rather than only stated as compliant.
• Individual specimen results available on request, so the spread within a lot can be judged and not only the average.
• Diameter tolerance of ±0.01 mm on fine sizes, keeping the gauge length and the test conditions consistent.
• Surface free of seams and laps, which is what the torsion count is largely measuring at small diameters.
• Traceable to heat number, so a low count can be traced back to a specific melt and drawing lot.

Quality Control

Torsion specimens are taken from both ends and the middle of each lot, and the mean and minimum are recorded. Diameter, tensile strength and elongation are tested alongside. Surface is inspected over the full coil length. Where a torsion result falls below the agreed limit, the lot is held and re-sampled before any decision on release, and the outcome is recorded against the heat number.

Selection and Ordering Notes

1. State the torsion requirement as a minimum turn count over a defined gauge length, and name the test standard and the twist rate.
2. Ask for the individual specimen results, not only the mean, since the lowest value is what predicts a failure in production.
3. For diameters below 0.5 mm, treat the torsion test as the primary internal quality check and specify it explicitly.
4. Where the wire will be knitted, braided or bent repeatedly, add a bend or wrap test alongside the torsion test.
5. Confirm whether reverse torsion is required, as it is harsher and may need a different acceptance limit from single direction torsion.

The torsion test answers a question that diameter and tensile strength cannot: whether the inside of the wire is sound. Send the application and the diameter, and the test method and acceptance limit can be agreed before the first coil is shipped.