Stainless Screw Wire 0.15-12mm - Tensile Spread in a Coil
Stainless Screw Wire 0.15-12mm
,Tensile Spread Wire Coil
,EPQ Stainless Wire Coil
One Coil, One Value? Tensile Spread Along Stainless Screw Wire
Product Introduction
Stainless screw wire is bought by coil, but it is measured by specimen. A certificate that states one tensile figure describes the piece that was pulled, not the 300kg around it. On a cold heading line the difference appears as scattered head heights, occasional die overload, and a last fifty kilos that behaves unlike the first.
This page explains where within-coil variation comes from, how to sample a coil so that the tail is actually covered, and how to write a spread limit into the purchase specification. It applies to 201, 302, 304, 316 and 304Cu wire from 0.15mm to 12mm.
A Coil Is a Process Record
Every coil carries the history of its own drawing run. The first metres pass through fresh dies; the last metres pass through the same dies after several tonnes of wear. The same strand sits at a different position in the annealing furnace depending on where it was on the pay-off, and take-up tension changes as the coil builds.
None of these effects is a defect. They are the normal gradient of a continuous process, and the spread they produce is the reason a single specimen cannot represent a coil.
Where the Spread Comes From
Die wear across the run
A die that has drawn several tonnes has a slightly larger bearing and a warmer surface. Wire drawn late in the run comes out marginally larger and marginally softer than wire drawn at the start. The change per metre is small, but it accumulates across a full coil.
Position in the annealing load
For annealed or softened conditions, the outer and inner parts of a furnace load see different heating and cooling rates. A coil at the edge of the load and a coil at the centre can leave the furnace with different structures even when the recorder shows one set point. This produces coil-to-coil spread rather than within-coil spread, and it is why two coils of the same nominal grade can head differently.
Take-up tension
Wire wound under rising tension stores more residual stress in the outer layers. That stress does not change tensile directly, but it changes how the wire behaves at pay-off and how the first forming station sees it.
How Wide Is Normal
A practical specification states two limits, agreed per grade and diameter:
- Within a coil: the highest and lowest tensile from head, middle and tail should fall inside one band, for example 60 MPa.
- Between coils of one lot: a wider band, for example 100 MPa, because it also carries the annealing position effect.
These figures are a specification choice, not a property of the grade. What matters is that the limit is written down and that the sampling plan can actually detect a violation. One mid-coil specimen cannot.
Sampling That Covers the Tail
Take three specimens: the first few metres after pay-off, one from the middle of the coil, and one from the last few metres. Test them at the same gauge length and the same rate, and report all three rather than the average.
The tail specimen matters most for heading. It is the wire that runs last on a long shift, often at the end of a die campaign, when the operator is least likely to stop the machine.
What the Heading Line Sees
Cold heading is a volume process: slug volume is fixed by wire diameter and cut length, and head height follows from that volume. Wire that is slightly softer at the tail shears with a different cut face and upsets at a lower load. The head is not necessarily out of tolerance, but the scatter grows, and the adjustment that fixed the start of the coil is no longer correct at the end.
Thread rolling adds a second sensitivity. A softer tail fills the thread form more readily and can produce a lap at the crest on a die setting that was correct for the head of the coil.
Specifications
| Parameter | Specification |
|---|---|
| Grade options | 201, 302, 304, 316, 304Cu (UNS S20100, S30200, S30400, S31600, S30430) |
| Diameter range | 0.15mm - 12mm |
| Condition | Cold drawn; bright, matte or soap-coated surface |
| Tensile strength | Quoted per grade and diameter class, confirmed per order |
| Within-coil spread | Highest minus lowest of three specimens, typically 60 MPa, tighter on request |
| Coil-to-coil spread | Typically 100 MPa within one lot |
| Sampling | Head, middle and tail; same gauge length and same test rate |
| Test method | ISO 6892-1 or ASTM A370 |
| Coil form | 100-500kg coils, spools for fine sizes; one continuous length, no butt welds |
| Documentation | Mill certificate EN 10204 3.1, three tensile results per coil on request |
Values are typical for the grades and are confirmed per order against the agreed purchase specification.
Where This Wire Is Used
- Screws, bolts and self-tapping screws formed on high-speed headers
- Rivets and cold-formed fasteners with short, repeatable head heights
- Thread-rolled parts where crest fill must stay constant across a coil
- Wire forms and small upset parts produced from a single coil per shift
Product Advantages
- One tensile target per diameter, not per order. Each size is drawn against its own target, so a fine coil and a heavy coil do not share a single band.
- Three-point sampling reported. Head, middle and tail values are recorded against the coil number, so a drift is visible before the wire reaches the header.
- Die sequence monitored by running tonnage. Dies are changed on a tonnage schedule rather than at failure, which keeps the late-run size drift inside the tolerance band.
- Annealing loads mapped by position. Coils from the edge and the centre of a load are identified, so a structural outlier can be kept away from a critical part.
- No butt welds in the coil. Every coil is one continuous length, so the tail specimen and the wire actually used are from the same unit.
- Grade selection kept open. 201 for dry, low-load screws; 302 and 304 for general fasteners; 316 for chloride exposure; 304Cu for severe upsetting.
How to Write the Clause
Five lines cover the requirement:
- Grade and standard: 201, 302, 304, 316 or 304Cu to the agreed ASTM or EN grade.
- Diameter and tolerance band for each size ordered.
- Tensile range for that diameter, not a single band across all sizes.
- Within-coil and coil-to-coil spread limits, with the sampling plan stated.
- Reporting: three individual results per coil, not one averaged figure.
FAQ
Can one coil really differ enough to affect heading?
Yes, once the part is small and the tolerance is tight. A screw with a short head has little room for volume scatter, so a tensile difference at the soft end of a coil shows up as a head-height spread rather than as a rejected coil.
Should the specification use the average or the extremes?
The extremes. An average can hide one soft tail. Limits on the highest and lowest values are what the heading line actually experiences.
Does tighter spread cost more?
It changes the process: slower finishing speeds, more frequent die changes, and more sorting of annealing loads. That is why the limit should be set from the part, not from a default number copied between grades.
