302 Spring Wire - Drawing Die Material and Surface
302 spring wire drawing die material
,stainless steel spring wire surface finish
,302 spring wire corrosion resistance
302 Spring Wire - Drawing Die Material and Surface
Two suppliers can draw the same rod down to 1.4 mm in 302 and deliver wire that looks the same on a certificate but differs on the surface. The difference is usually the die: which material it was made from, how long it has run, and whether the bearing length still matches the pass. Die choice is the quiet variable behind surface roughness, ovality, and the die marks that become fatigue origins in a finished spring. This page explains how carbide and diamond dies differ, what a worn die does to the wire, and what to ask for when the surface has to be consistent from coil to coil.
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) |
| Drawing Dies | Tungsten carbide for rough and intermediate passes, polycrystalline diamond for final passes on fine wire |
| Die Bearing Length | 30-50% of the wire diameter, adjusted per pass |
| Approach Angle | 4-8 degrees half angle, depending on reduction per pass |
| Tensile Strength | 1400-1800 MPa, spring temper (size dependent) |
| Surface | Bright drawn, soap coated, die mark depth controlled |
| Coil Weight | 1-100 kg |
| Applicable Standards | ASTM A313, EN 10270-3, GB/T 24588 |
Carbide and Diamond Dies
Tungsten Carbide
A carbide die is a cobalt bonded tungsten carbide insert pressed into a steel casing. It is the workhorse of the drawing shop: robust, repairable by repolishing, and cheap enough to keep a full range of sizes on the shelf. Its limitation is wear rate. Carbide loses its edge steadily under stainless, which is harder and more abrasive than carbon steel, and the wear appears first as a polished oval in the bearing zone. Carbide is the right choice for the rough and intermediate passes, where reductions are large and the die is changed on a schedule anyway.
Polycrystalline Diamond
A PCD die uses a sintered diamond compact, which is far harder than carbide and much more resistant to the adhesive wear that stainless causes. The practical consequence is a bearing zone that stays circular and keeps its polish for far longer, so the wire leaves the final pass with a lower surface roughness and a rounder section. PCD dies cost many times more than carbide, and the economics only work when the die is used on the final pass at small diameters, where the wire length per die is large and a die change stops a whole line.
Where the Choice Actually Matters
For coarse wire above roughly 4 mm, the surface consequence of die material is small compared with the cost of the die, and carbide dominates. Below 1 mm the situation reverses: the same absolute wear that is negligible on a 5 mm wire is a large fraction of a 0.5 mm diameter, and a PCD final die pays for itself in reduced breaks, fewer die marks and less downtime. The boundary is commercial rather than metallurgical, and most shops place it somewhere between 1 and 2 mm.
What a Worn Die Does to the Wire
Diameter Drift and Ovality
As the bearing zone wears, the hole grows and loses its circularity. The wire that leaves it is slightly oversize and slightly oval, and both effects accelerate as the die continues to run. Because the diameter tolerance has a width, a worn die can stay inside specification on the mean diameter while the ovality has already gone out of specification, which is why the two readings should be recorded together.
Die Lines and Surface Roughness
A polished bearing zone leaves a smooth surface. A scored or worn zone leaves longitudinal lines, and those lines are not cosmetic: on a spring they become stress raisers at the inside of the coil, where the bending stress is highest. A single deep die mark running along the wire is enough to start a fatigue crack at a fraction of the expected life. This is the main reason spring wire specifications sometimes name a maximum die mark depth rather than asking for a bright surface in general.
Heat and Lubrication Breakdown
A worn die raises friction, and friction raises temperature at the die entry. Once the soap or oil film breaks down, metal contacts metal and the surface tears rather than flows. The result is a roughness that looks like a coating defect but originates in the tooling, and it will not be fixed by changing the lubricant.
Applications
Spring Temper Wire for Coiling
Spring temper wire is drawn to final size without a final anneal, so the surface produced by the last die is the surface the coiler sees. Fine sizes for instrument and appliance springs are normally finished through a PCD die, with soap coating applied afterwards as a coiling lubricant.
Fine Wire for Weaving and Knitting
Woven and knitted products expose the full length of the wire, and any die line becomes a visible defect in the mesh. Diameters below 0.5 mm are almost always finished through diamond, and the surface is inspected over the full coil rather than at the ends only.
Cold Heading and Forming Grades
Heading wire carries a coating that must bond to a clean surface. A scratched surface holds the carrier unevenly, so the lubricant film breaks down first exactly where the deformation is greatest. Controlling the final die condition is part of controlling the coating, not a separate concern.
Product Advantages
- Final passes run through PCD dies on fine sizes, giving a lower surface roughness and a rounder section than a carbide finish.
- Die bearing length and approach angle set per pass, so the deformation is controlled rather than simply forced through the die.
- Die marks monitored as a surface acceptance criterion, because a longitudinal line is a fatigue origin in a spring.
- Diameter and ovality recorded together, catching a worn die before the mean diameter drifts out of tolerance.
- Diameter tolerance of ±0.01 mm on fine sizes, held by die condition rather than by sorting afterwards.
Quality Control
Diameter and ovality are measured at the coil ends and at intermediate points, and the surface is inspected over the full length for die lines, scratches and laps. Dies are replaced or repolished on a count basis per size and grade, not only when the wire fails, because a die that has started to wear produces wire that still passes the diameter check. Tensile strength and elongation are tested per lot, and results are recorded against the heat number and the drawing route.
Selection and Ordering Notes
- State the surface requirement in terms the mill can control: maximum die mark depth, or a reference sample, rather than the word bright on its own.
- For diameters below 1 mm, ask whether the final pass is run through PCD, since this is the single largest factor in surface consistency.
- Add an ovality limit next to the diameter tolerance, as both are produced by the same die.
- Name the next process, because a coating that must bond needs a cleaner surface than a coating that is only a handling aid.
- For critical fatigue parts, request the surface inspection result per lot, not only a statement that the wire is bright drawn.
Die material is invisible in the finished wire until the surface is measured, and by then the coil has already been drawn. Send the diameter, the application and the surface requirement, and the finishing route can be specified together with the temper rather than left to the drawing shop's default.
