Prestressing strand and spring steel wire see the heaviest total reductions in wire drawing. What is asked of the carrier layer here is not survival through one die but through dozens.
Among wire drawing applications, prestressing strand and spring steel wire sit apart. The difficulty here is not contact pressure at a single die but the magnitude of the total reduction: high carbon rod passes through a long series of dies down to a small fraction of its starting diameter, and the carrier layer together with the lubricant is expected to keep working throughout. A system that performs well at the first die may be exhausted by the twentieth.
This article covers surface preparation on lines producing high carbon wire: what patenting does to the surface, what is expected of the zinc phosphate coating in this segment, how lubricant selection shifts, and the control points that bring the break count down. For the general process sequence see the phosphate coating process; for fault tracking on the line see wire drawing troubleshooting.
The third point turns surface preparation from a question of process convenience into a question of product quality. A scratch formed during drawing may be a cosmetic defect on an ordinary wire line, while on prestressing strand it can be a notch that initiates a stress concentration.
Before heavy drawing, high carbon wire goes through a heat treatment called patenting. The purpose is to give the material a fine lamellar structure able to take high reduction. From the surface preparation standpoint what matters is this: patenting leaves an oxide layer on the wire coming out of the furnace, and everything that follows depends on how that layer is removed.
The oxide is removed mechanically or by acid pickling. Whichever route is taken, the criterion is the same: the surface entering the phosphate bath must be metallic and clean. A line that leaves oxide residue will not get a result even if every subsequent step runs correctly — the coating sits on the oxide and lifts away with it at the first die.
The job of the phosphate layer is the same in every application: hold the lubricant and prevent metal-to-die contact. But on PC strand and spring steel lines a further requirement is added — endurance. The layer has to still be working at the last die, not only at the first.
That expectation has three consequences. First, coating weight is set by the needs of the total reduction rather than the first die; in heavy multi-die drawing, moving toward the upper part of the range is often necessary. KIMSOL baths are run in the 3–15 g/m² range, and where within it a line should sit is established against the drawing schedule; for the measurement method see coating weight measurement.
Second, the uniformity of the crystal structure becomes more decisive than thickness. A coarse, irregular crystal structure runs out in places partway through the total reduction; where it runs out the film thins and the risk of a break appears. When breaks on a line cluster after a particular die number, that is a sign the layer ends there. For how crystal size is controlled see the activation bath and crystal size.
Third, bath stability is not a luxury in this segment but a requirement. When the balance of total and free acid drifts the crystal structure drifts with it; a fluctuating bath produces variable coating weight within a batch and makes the break count unpredictable. For the control method see phosphate bath maintenance.
| Criterion | PC strand / prestressing wire | Spring steel wire |
|---|---|---|
| End use | Under sustained tension; loaded for life inside concrete | Cyclic loading; millions of cycles |
| Critical surface defect | Notches and scratches that initiate stress concentration | Surface irregularity that initiates a fatigue crack |
| Post-drawing operation | Stranding and stress relieving | Coiling, heat treatment and often surface coating |
| Residue sensitivity | Matters for bond with concrete and corrosion behaviour | Cleanliness critical before heat treatment and plating |
| Carrier layer priority | Endurance across the whole total reduction | Endurance plus final surface quality |
| Lubricant tendency | Film strength comes first in heavy drawing | A balance of surface quality and cleanability is sought |
In practice the distinction comes to this: both products involve heavy drawing, but on spring steel wire the appearance and cleanliness of the final surface becomes a binding constraint earlier. On prestressing wire the priority is a drawing run that is free of breaks and free of notches across a long reduction chain.
Dry lubricants divide into reactive families, which react with the phosphate layer, and neutral families, which leave a physical film; the detailed comparison is in wire drawing lubricant selection. In high carbon heavy drawing the general tendency is toward the reactive side, with its higher film carrying capacity — but this is not an automatic rule.
Two constraints shift the decision. The first is final diameter: as the line comes down to fine sizes, a thick, coarse-grained film can cause dusting and build-up at the die entry; finer-grained formulations run cleaner in that region. The second is what follows drawing: if spring wire will be plated or heat treated in a controlled atmosphere, residue left on the surface can turn into a process fault. For how to remove it, removing drawing soap and phosphate residue sets out the methods.
A common misjudgement on multi-die lines is to try to rescue film continuity by raising lubricant consumption. Where the carrier layer is the thing falling short, this hides the symptom for a while without removing its cause, and it opens a new source of defects by building up at the die entry. For the variables that actually drive consumption, see reducing wire drawing lubricant consumption.
The fifth item is the most informative diagnostic in this segment. If breaks are distributed randomly along the line the problem is usually on the material or die side; if they cluster after a particular die, the carrier layer is being consumed at that point and the answer lies on the coating side.
To determine the right KIMSOL and Kimkal combination for your own drawing schedule, get in touch; for technical data sheets see the document center.
For two reasons. First, total reduction is very high; the carrier layer has to keep working not through one die but through dozens without being consumed. Second, the finished product works under sustained tension; a scratch or notch formed during drawing can initiate a stress concentration and affect service life directly.
Patenting is the heat treatment that gives the material a structure able to take high reduction, and it leaves an oxide layer on the wire coming out of the furnace. If that layer is not fully removed mechanically or by acid pickling, the phosphate coating sits on the oxide and lifts away with it at the first deformation. A metallic, clean surface entering the phosphate bath is the precondition for the whole chain.
That distribution points to the carrier layer being used up at that point. If breaks are spread randomly along the line the problem is usually on the material or die side; if they cluster after a particular die number, coating weight is not sufficient for the total reduction or the crystal structure is not uniform. In that case the answer should be sought on the coating side, not on the lubricant side.
The general tendency is toward the reactive side with its higher film carrying capacity, but this is not an automatic rule. As the line comes down to fine diameters a coarse-grained film can cause dusting and build-up at the die entry. In addition, if the wire will be plated or heat treated after drawing, formulations whose residue is easier to remove may come to the fore.
Problems on a wire drawing line become visible at the die, but they usually originate in degreasing, activation, phosphating or the soap stage. This guide takes the seven symptoms most often reported and sets out their likely causes and what to check, in what order.
Read →Coating weight is the criterion that reduces phosphate coating quality to a single number. The steps of gravimetric measurement, sampling discipline, the likely causes of deviations, and how to monitor the trend.
Read →Reactive soap reacts with the phosphate layer to form zinc soap; neutral soap leaves a physical film. Which suits which line is decided by drawing speed, reduction ratio, wire diameter and what the wire has to go through after drawing.
Read →