302 Spring Wire - Reading a Wire Specification Sheet

A wire specification sheet looks complete and often is not. It lists a grade, a diameter and a tensile range, and leaves out the three properties that decide whether the wire will run on a particular machine: surface condition, test method and coil form. The result is a purchase that meets the sheet and still fails in production. This page walks through the lines that matter, shows which ones are usually missing, and works through a short example of turning a part drawing into an order that can actually be checked on arrival.

Product Specifications

The Six Lines That Decide the Fit

Grade and Temper

Grade alone says little. 302 in the soft annealed condition at 600 MPa and 302 in spring temper at 1700 MPa are different materials for practical purposes, with different forming behaviour and different fatigue limits. The specification must name the temper or the tensile range, and the two must be consistent: a sheet quoting spring temper and a 700 MPa band describes nothing that can be supplied.

Diameter and Tolerance

Diameter is the line everyone reads and the tolerance is the line everyone skips. For a spring, the diameter enters the rate calculation to the fourth power, so the tolerance is not a rounding detail. A plus or minus 0.02 mm band on 1 mm wire allows a rate variation of about eight percent before any tooling variation is added.

Tensile Strength and Elongation

Both are needed. Tensile strength sets the load capacity and the fatigue limit, while elongation indicates how much deformation the wire will take before cracking. A sheet that quotes only tensile strength cannot distinguish a ductile wire from a brittle one at the same strength level. Check also whether the elongation gauge length is stated, because values from 50 mm and 100 mm gauge lengths are not comparable.

The Lines That Are Usually Missing

Surface Condition

Bright, soap coated, matte and coated with a specific carrier are four different products. The surface decides friction in the feed rolls, whether the wire can be welded without cleaning, and whether a plating step will adhere. A sheet that says only stainless steel 302 has not specified the surface at all.

Test Method and Sampling

A tensile value without a test standard is a number without a definition. State the standard, the specimen type and how many tests per lot. The same applies to grain size and torsion, where the comparison method and the intercept method can give different results on the same sample.

Coil Form and Packing

Cast, helix, coil weight and whether the wire arrives as a free coil or a level wound spool all affect the coiler. These are cheap to specify and expensive to correct after the container has sailed.

Turning a Drawing into an Order

A Worked Example

Take a compression spring: 1.2 mm wire, mean coil diameter 12 mm, eight active coils, required rate 1.3 N/mm. Using the standard rate formula with a shear modulus of 70 GPa, the calculated rate is about 1.31 N/mm, so the geometry works. Now check the tolerance: at 1.22 mm, still inside a plus or minus 0.02 mm band, the rate becomes about 1.40 N/mm, which is nearly seven percent high. If the spring has to hold a load window of plus or minus five percent, the wire tolerance has to be tightened to plus or minus 0.01 mm, or the spring has to be sorted.

What to Ask the Supplier

Three questions close most of the gap. What is the temper and the tensile band for this diameter? What is the diameter tolerance and how is it measured? And what surface condition will the wire arrive with? A supplier who can answer all three in numbers is describing a product; one who answers in adjectives is describing a catalogue page.

Applications

First Article and Prototype Work

Prototypes are where a specification error is cheapest to find. Ordering a small quantity against a fully written specification, including surface and coil form, exposes the mismatch before tooling is committed.

Second Sourcing an Existing Part

When a part is moved to a second supplier, the original drawing rarely contains enough information. Reconstruct the missing lines from the part itself: measure the coil, calculate the index, and derive the required rate, then specify the wire against those numbers rather than against the previous supplier's invoice description.

Switching Grade

Moving from 302 to 304 or 316 changes the shear modulus only slightly but changes the corrosion behaviour and the work hardening rate significantly. The wire specification has to be rewritten rather than edited, because the acceptable temper range shifts with the grade.

Product Advantages

• Full specification available in numbers: grade, temper, tensile band, diameter tolerance, surface and coil form.
• Test results reported per lot with the method named, so the values can be compared with an existing specification.
• Diameter tolerance of ±0.01 mm on fine sizes, supporting spring designs with a tight load window.
• Surface options stated explicitly rather than left to default, so the wire matches the next process.
• Technical support to check a spring design against the available wire range before the order is placed.

Quality Control

Each lot is tested for diameter, tensile strength, elongation and, where specified, torsion and grain size. Results are reported individually against the heat number. Surface is inspected over the full coil length, and cast and helix are checked on samples from each packing lot. Where a customer supplies a specification sheet, the order acknowledgement repeats the agreed values so that both sides are working from the same document.

Selection and Ordering Notes

1. Write the temper and the tensile band together, and check that the two are physically compatible for the diameter.
2. State the diameter tolerance and the measurement method, not only the nominal diameter.
3. Name the surface condition required by the next process: coiling, welding, plating or weaving.
4. Specify the test standards and the number of tests per lot for every property that matters to the part.
5. Add cast, helix, coil weight and packing form for any wire going onto automatic equipment.

A specification that can be checked is worth more than one that looks thorough. Send the part drawing and the process, and the missing lines can be filled in before the order is confirmed rather than after the first coil is rejected.