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Lesson 07·Process sequencing

EDM after heat treat — recast, stress, and the nitriding handoff

Why the EDM recast layer ruins a downstream nitriding job, what to specify for Ra and recast depth, the required stress relief between EDM and nitriding, and how the most common shop-floor screwup produces a cavity that spalls in under 5,000 hits.

8 min readLesson 7 of 13

Tying it together

What this means on the shop floor

For hot-forge dies in H13 and premium H13, run finish-EDM, hand stone, stress relieve at 570°C for 2 hours, then nitride. The stress relief sits in the routing as its own step with its own paperwork.

For aluminum forging dies that will see PVD on top of the nitride, the cavity finish target tightens to Ra 0.4-0.8 µm because PVD adhesion drops on rougher surfaces. The recast removal has to be cleaner.

For hot-trim dies, the cutting edge is the critical zone. A 3-5 µm recast at the edge under a nitrided case will chip in the first thousand hits. Stone the edge geometry by hand before stress relief.

For closed-cavity sinker work where parts of the impression cannot be reached with a stone, the finishing-skim EDM pass has to do all the work. Specify the recast depth on the witness coupon, and inspect by cross-section before releasing to nitride.

Pushback questions for the EDM shop and the nitride house

  1. What is the recast depth on a witness coupon from my finishing-skim pass, measured by metallographic cross-section?
  2. Has the part been stress-relieved between EDM and shipment to the nitride vendor, at what temperature, and for how long?
  3. Where in the cavity geometry is mechanical finishing impossible, and how is recast being controlled in those zones?
  4. What is your nitride cycle temperature, and does it sit at least 20-30°C below the post-EDM stress-relief temperature so the stress relief is not undone in the retort?

Common confusions

Stress relief is not a temper. The temper sets bulk hardness. The stress relief sits below the temper and removes residual stress without changing hardness. Many shop schedulers conflate the two, and a temper specified in the stress-relief slot of the traveler will drop the bulk hardness on the next job.

A clean surface finish does not mean the recast is gone. A 5 µm Ra cavity can still carry 15 µm of recast. Ra measures topography, not microstructure. Always specify recast depth as its own number.

The nitride certificate cannot detect what is under the case. A clean surface microhardness traverse will look identical whether the substrate is sound or fractured. The witness coupon from the EDM operation, sectioned before nitride, is the document that catches a bad handoff before production does.

Up next: surface treatment sequencing.

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