Intro
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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.
A press shop finish-EDMs a closed-cavity H13 die at 48-50 HRC, sends it straight to the nitride house, and the certificate comes back clean. Surface microhardness HV 1050. Case depth 0.32 mm by DIN 50190. Compound layer 6 µm, gamma-prime dominant. The cavity goes into production and spalls at 4,200 hits, well under the 80,000-cycle program target. The lab cross-section finds a 22 µm recast layer under the nitrided case, full of microcracks, with the diffusion zone formed cleanly through and around the cracks. The case did exactly what the certificate said. The substrate underneath it was already fractured before the nitride job started.
This is the most common forge-die failure on the shop floor, and the build sequence prevents it.