Cold heading is a different lubrication problem from wire drawing: surface area multiplies in a single blow and fresh, unlubricated metal appears underneath. The carrier layer has to keep up with that expansion.
The head of a bolt is formed in a single blow of the press. In that blow the material flows, the surface area in the head region multiplies several times over, and underneath that new surface fresh metal appears — metal that has never met a lubricant. In wire drawing the lubrication problem concerns a continuous surface passing through a die. In cold heading the problem concerns a surface that comes into existence and has to be lubricated in the same instant. That difference explains why surface preparation on a fastener line has to be managed more tightly than on a drawing line.
This article covers the preparation of wire used to make bolts, nuts and similar fasteners: where cold heading departs from wire drawing, what is expected of the zinc phosphate coating in this application, how lubricant selection changes, and the mistakes seen most often on the shop floor. For the general mechanism of friction, see friction in cold forming; for a comparison of the lubricant families, see wire drawing lubricant selection.
Both are cold forming and both use the same chemical chain, but what is asked of the lubricant is not the same. The differences come down to this:
The practical consequence is that in cold heading the lubricant cannot be judged on its own. Its performance is inseparable from how the phosphate layer beneath it behaves under deformation.
A zinc phosphate layer is crystalline and porous, and it is chemically bonded to the metal — it is not a paint applied on top. That bond lets the layer deform together with the material. As the surface expands during head formation the crystal structure opens with it, the pores spread apart, and the lubricant held inside them is distributed across the new area. That is how the system works: the layer does not merely carry the lubricant, it distributes it at the moment of deformation.
This mechanism breaks down at both extremes. When coating weight is too low the layer cannot keep up with the expansion, bare metal is left in the opened region, and it contacts the die directly — galling begins. When the coating is heavier than needed and the crystal structure is coarse, the layer fractures rather than holding under deformation and produces debris; the fragments build up on the die face and mark the product. The right point lies between these extremes and shifts with the degree of deformation.
In fastener production the preparation from wire rod to press typically follows this sequence:
The most frequently skipped link in the chain is the first one. When scale is not fully removed from the wire rod, the result is defective even if every following step works correctly: the phosphate layer sits on the oxide, lifts away with it at the first deformation, and bare metal is exposed underneath. This is usually reported as "the coating isn't holding" — but the problem is not in the coating, it is in the surface itself. For how the activation step governs crystal size, see the activation bath and crystal size.
The same bath serves both applications, but the weighting of what is expected from the layer shifts. The table below compares the two.
| Criterion | Wire drawing | Cold heading (fasteners) |
|---|---|---|
| Dominant load | Continuous friction along the die | High contact pressure at a single station |
| Change in surface area | Gradual and limited | Sudden and large; multiplies in the head region |
| Crystal structure requirement | Porosity to hold the lubricant | Porosity plus the ability to stretch with deformation |
| Coating weight decision | Driven by the reduction schedule | Driven by degree of deformation; thickness alone is not enough |
| Dominant failure mode | Film thinning, die wear, surface scoring | Galling, material pick-up on the die, cracking at the head |
| What the lubricant must do | Hold film continuity from die to die | Spread without gaps during sudden expansion |
| Effect of the next step | Residue management if welding, galvanising or heat treatment follows | Residue is usually critical before plating or heat treatment |
The most important row is the second from last. In drawing, film continuity decides the outcome; in heading, spreading behaviour does. If two lines running the same coating weight produce cleanly on one and gall on the other, the first place to look is not thickness but the fineness and uniformity of the crystal structure.
Dry lubricants divide into reactive families, which react with the phosphate layer, and neutral families, which leave only a physical film. The detailed comparison is in wire drawing lubricant selection; here we take only the part specific to cold heading.
Three inputs decide the choice in heading. The first is the degree of deformation: as head volume grows and the number of stations rises, so does the durability asked of the film. The second is material grade; as alloy content rises, both the formation of the phosphate layer and the behaviour of the lubricant change. The third, and usually the last to be noticed, is what the part will see after heading. Fasteners are largely plated or heat treated, and lubricant and phosphate residue left on the surface can spoil plating adhesion or contaminate a furnace atmosphere at those steps. For removal methods, the approaches in removing drawing soap and phosphate residue apply.
Clarifying these points before requesting a sample or a quote noticeably reduces the number of trials:
Once these are filled in, whether the problem comes from the carrier layer or from the lubricant usually separates out at the first assessment. For technical data sheets see the document center, and to determine the right KIMSOL and Kimkal combination for your press layout, get in touch.
In wire drawing the surface area grows gradually and by a limited ratio along the die. In cold heading the surface area multiplies at a single station as the head forms, and fresh metal carrying no lubricant at all appears under the expanding surface. The phosphate carrier layer has to stretch along with that expansion and distribute the lubricant over the new surface.
There is no single correct value. KIMSOL baths are run in the 3–15 g/m² range; where within that range a line should sit is set by the part's degree of deformation, the material grade and the press layout. Thickness on its own is not a sufficient target — the fineness and uniformity of the crystal structure matter at least as much as coating weight.
Before changing the lubricant, verify in order whether scale was fully removed from the wire rod surface, what the coating weight is, and whether the balance of total and free acid in the bath is under control. Galling usually comes not from an inadequate lubricant but from a carrier layer that tears or fractures under deformation.
Fasteners are largely plated or heat treated. Residue left on the surface before those steps can spoil plating adhesion or contaminate the atmosphere in a controlled-atmosphere furnace. For that reason lubricant selection should account not only for heading performance but also for how and at which step the residue will be removed.
In fastener production the surface expands several-fold; a liquid oil cannot carry that contact pressure. We explain how the porous phosphate layer and the soap film work together, and how the pair is selected by severity of deformation.
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 →Phosphate coating is not a single tank but a connected chain: degreasing, rinsing, activation, phosphating, soap and drawing. This guide summarises each stage end to end — what it does, its typical parameters and how it affects the next one.
Read →