EPQ Wire Annealing - Softening Control for Cold Heading
EPQ Wire: Why Annealing Decides Whether Cold Heading Works
EPQ is usually sold on surface quality: no seams, no rolled-in scale, tight diameter tolerance, clean bright finish. All of that matters, but surface alone does not make a wire cold headable. A coil can look perfect and still split at the first blow of a heading die, because the property that governs cold forming is the structure inside the wire, and that is set by annealing.
This article explains what each stainless family needs from annealing, which three structural figures should be on the inspection sheet, and how to recognise under-annealed and over-annealed wire before it reaches the press.
What EPQ Actually Covers
Extra press quality means the wire is intended for deformation rather than for bending or welding. In practice it combines three things: a surface free of longitudinal seams and handling damage, a diameter and ovality held tight enough for automatic feeding, and a metallurgical condition soft enough to flow under a heading punch without cracking.
Most incoming inspection tests the first two. The third is where field failures come from.
Annealing Route Depends on Structure
Austenitic grades: 201, 302, 304, 304Cu, 316
These are soft annealed, not spheroidised in the classical sense, because their carbon content is low and their carbides are few. The aim is to remove the work hardening from drawing, keep the structure fully austenitic, and avoid sensitisation. Cooling practice matters as much as holding temperature: slow cooling through the carbide precipitation range can leave grain boundary chromium depletion, which shows up later as corrosion at the head of a cold formed part.
Ferritic grades: 430
430 responds to classical spheroidise annealing. Lamellar carbides are converted to spheroids, hardness falls and ductility rises. Without it, cold heading of 430 at high reduction produces edge cracking that starts at the head fillet.
Martensitic grades: 410, 420
Hardness after annealing is the controlling figure. These grades must be delivered soft, and the softening must be uniform through the section, otherwise the head deforms unevenly and the part needs straightening that reintroduces stress.
Three Structural Figures That Predict Formability
1. Hardness and its uniformity
The single most useful number. Too hard and the head cracks; too soft and the head deforms without filling the die. Measure at the surface, at quarter radius and at the centre: a large spread means the anneal did not penetrate, which is common on larger diameters.
2. Grain size
Fine grains resist cracking but raise forming load; coarse grains lower load but produce orange peel and rough surfaces at high strain. A middle range, agreed between supplier and user, gives the best compromise. A sudden change of grain size between lots is a warning sign even if the hardness number looks correct.
3. Carbide form and distribution
For ferritic and martensitic grades, spheroidised carbides deform with the matrix instead of cracking. Banded or network carbides act as crack paths and should be rejected regardless of hardness.
Under-Annealed vs Over-Annealed
| Condition | Symptom in Production |
|---|---|
| Under-annealed | Head splits at first or second blow, die wear rises, crack surfaces show little deformation |
| Correctly annealed | Head fills the die, crack-free, stable load on the press |
| Over-annealed | Orange peel and rough finish at high strain, head underfills, part bends after ejection |
Coating: The Other Half of Cold Heading
EPQ wire is normally supplied with a carrier coating, such as a borax or oxalate film, which holds lubricant at the die interface. Without it the wire galls the die, and galling is often misdiagnosed as a material defect because the crack appears on the part rather than on the tool.
Specification Window
| Parameter | Range / Standard |
|---|---|
| Grades | 201, 302, 304, 304Cu, 316, 430, 410 |
| Wire diameter | 0.15 - 12 mm |
| Condition | Soft annealed, spheroidised for ferritic grades |
| Hardness | Declared per grade and diameter |
| Grain size | Agreed range, declared per lot |
| Surface | EPQ bright, seam free |
| Coating | Borax, oxalate, or bare on request |
| Standards | ASTM A580, JIS G4309, EN 10088-3 |
Applications
- Bolts, screws and nuts formed by cold heading
- Rivets, pins and fasteners
- Precision components for instruments and appliances
- Cold forged fittings and connectors
- Parts with high heading reduction ratio
What to Put on the Enquiry
State the grade, the heading reduction ratio, the number of blows, whether the part is subsequently thread rolled, the hardness range you can tolerate, and whether a carrier coating is required. With the reduction ratio stated, the annealing condition can be matched to the operation instead of being guessed from a standard table.
TOPONE has produced stainless steel wire since 1993 in diameters from 0.15 mm to 12 mm, with annealing, coating and hardness declared per lot for cold heading applications.
