304 Bright EPQ Wire: Defining Bright So It Can Be Inspected

Bright is the most used and least defined word in stainless wire purchasing. Two suppliers can both quote bright 304, both ship wire that looks shiny in the sample bag, and both be correct against their own internal standard. When the coil arrives at a forming line and the parts do not polish, plate or weld as expected, the argument has no technical basis because no measurable requirement was ever written down.

This article describes what produces a bright surface, what bright does not guarantee, and how to write a bright requirement that a third party can inspect.

Three Routes to a Bright Surface

Bright drawing

The wire is drawn with clean lubricant through polished carbide dies and leaves the die with a reflective surface. This is the normal route for bright EPQ wire, and the finish reflects the die condition as much as the process: worn dies produce a dull, longitudinally streaked surface that no later operation will remove.

Bright annealing

After drawing, the wire may be annealed in a protective atmosphere, typically hydrogen or a nitrogen-hydrogen mixture, with the dew point controlled. If the atmosphere is right, the surface stays metallic and oxide free. If the dew point drifts, the wire comes out grey, straw coloured or blue, and the oxide is thin enough to pass a visual check on a coil while ruining adhesion in a later coating or welding step.

Mechanical or chemical finishing

Used where a specific roughness is required, for example before electro-polishing. It adds cost and is normally only specified for visible parts.

What Bright Does Not Guarantee

• Freedom from drawing lubricant residue, which interferes with welding, brazing and PVD adhesion
• Absence of subsurface seams, which are closed by the drawing action and reopen when the wire is bent
• Uniform appearance from lot to lot, since die condition and annealing atmosphere both change over time
• Corrosion resistance at the level of a passivated part, because a bright surface is not necessarily a passive one

Four Figures Worth Putting on the Drawing

1. Surface roughness

Average roughness gives a number to what the eye calls bright. A typical fine bright range sits well under one micrometre Ra; the exact figure should be agreed between supplier and user, because too smooth can reduce lubricant retention in forming.

2. Defect limits

State a maximum depth for longitudinal scratches and a complete prohibition of seams, slivers, rolled-in scale and die marks. Expressing defects in depth rather than in appearance is what makes them rejectable.

3. Residue

If the part will be welded, plated or coated, require a degreased surface and verify it with a water break test: a clean surface holds a continuous water film, a contaminated one breaks into droplets. It takes seconds and settles disputes that visual inspection cannot.

4. Colour and oxide

For bright annealed wire, state that the surface must be free of visible oxide tint. A simple comparison against an agreed sample under daylight is enough, provided the sample is kept and dated.

Packing and Transport Matter More Than Expected

Bright wire can leave the factory perfect and arrive with white rust, caused by condensation inside a container or a coil that was packed damp. VCI paper, a dry desiccant and a sealed wrap cost very little against a rejected shipment, and the requirement is worth writing into the order rather than assuming.

Specification Window

Applications

• Precision wire forms with visible surfaces
• Kitchen baskets, racks and appliance trim
• Decorative and architectural wire
• Components to be welded or brazed after forming
• Parts requiring uniform appearance after polishing

How to Write the Requirement

Name the route (bright drawn or bright annealed), give a roughness figure or a reference sample, state defect limits in depth, say whether the surface must be degreased, and state the packing method for sea freight. Five lines on a purchase order remove most of the disagreement that the single word bright creates.

TOPONE has produced stainless steel wire since 1993 in diameters from 0.15 mm to 12 mm, with finish, roughness and packing agreed per order rather than described in general terms.