Wire EDM of Tool Steel: Recast Layer, Stress Relief and Skim Cuts

What EDM leaves behind

Wire EDM erodes metal with sparks, and each spark melts a microscopic crater. The resolidified surface — the recast layer, often called the white layer because it etches featureless white — is typically on the order of a few micrometres to a few tens of micrometres thick depending on the energy of the cut. Its properties differ sharply from the parent steel:

• It is untempered, brittle martensite, often harder than the bulk, frequently containing microcracks.
• Beneath it lies a shallow heat-affected zone whose temper has been altered.

On a punch or die edge, the recast layer is where chipping and premature fatigue cracks start. On low-duty parts it may be acceptable as-cut; on working tool edges it is managed deliberately.

Skim cuts: the standard remedy

Production die work rarely uses a single cut. The usual sequence is:

1. One roughing cut at high energy for speed — thick recast, rough surface (often Ra 1.6 µm or coarser).
2. Two to four skim (trim) cuts at progressively lower discharge energy, each removing a few hundredths of a millimetre or less. Each skim removes the previous layer's damage and leaves a thinner, less cracked one of its own.

With multiple skims the final recast can be reduced to roughly 1–2 µm or less, surface finish improves toward Ra 0.2–0.4 µm, and dimensional accuracy tightens because low-energy cuts deflect the wire less. The cost is machine time, which is why the number of skims is specified per part class — cutting dies and fine blanking tools at the generous end, rough clearance work at the lean end.

After cutting, stoning or light polishing of critical edges removes the last recast mechanically; on fatigue-critical punches this step is not cosmetic.

Stress relief and distortion

Hardened tool steel carries residual stress from quenching, and wire EDM removes material without adding mechanical stress — but splitting a block lets the existing stress redistribute, and the part moves. Classic symptoms: a slit plate closing onto or springing away from the wire, and long parts bowing after a centre section is removed.

Standard countermeasures:

• Stress-relieve before EDM where the steel's temper allows — a temper roughly 25–30 °C below the final tempering temperature, after rough machining, stabilizes blanks for finish work.
• Cut strategy: leave tabs/bridges so the slug and frame stay connected until the end; rough out large openings first, then finish; avoid cutting a hardened blank into long, thin, unsupported sections in one pass.
• Rough-machine oversize, stress-relieve, then finish-EDM to size — the sequence used for precision die plates.

Wire and process notes

Brass wire is the general standard; coated and fine wires serve tight radii and taper work. Because EDM is indifferent to hardness, it is equally the method for repairing and modifying finished dies — with the same recast discipline applied to the repaired edge as to a new one.