Step 1 of 4
29%
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.
Step 1 of 4What the recast layer actually is
Every EDM spark melts a small volume of substrate, the dielectric flushes most of it away, and a thin film of molten steel resolidifies on the cavity surface. That resolidified film is the recast layer, also called the white layer because it etches white under nital. Three things are true about it on tool steel.
It is hard. Microhardness on H13 recast runs 800-1100 HV, well above the tempered bulk at 550-600 HV. The hardness comes from quench-rate solidification, retained austenite, and trapped carbides.
It is cracked. The volume contraction during resolidification, the cooling rate, and the thermal mismatch with the substrate all drive tensile residual stress through the layer, and the layer relieves that stress by microcracking. Cracks run perpendicular to the surface and bottom out at or just past the recast-to-substrate interface.
It is tensile-stressed. The layer is in tension, and the heat-affected zone immediately under it is in tension as well, to a depth of 50-200 µm depending on cut energy. Nitriding does not relieve any of this. Nitrogen diffuses into the cracks, the diffusion zone bridges over them, and the case grows on a fractured foundation.
Quick check
A microsection of a nitrided cavity surface under nital etch shows a 14 µm bright white band on the surface, a darker band immediately underneath that runs 80 µm deep, and a clean tempered martensite structure below 100 µm. The nitride case depth measurement reads 0.30 mm by DIN 50190. What are the two upper bands, and what does the case-depth reading miss about the substrate condition?