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

A finishing-skim EDM pass is the first line of defense. After roughing the cavity to within 0.10-0.20 mm of final, the operator switches to fine-finish settings and removes the rough recast with low-energy cuts. Done right, this lands the cavity at Ra 0.8-1.6 µm with 5-10 µm of recast remaining.

The remaining recast comes off mechanically. Hand stoning with diamond or ceramic stones is standard practice for forging cavities. The objective is to break through the recast at the surface and blend the radii. For cavities the stone cannot reach, abrasive flow machining or vapor honing is the alternative. For external features, grinding removes recast cleanly.

The post-EDM stress relief follows the mechanical finishing. The cycle is 550-580°C for 2 hours, below the last temper temperature by at least 20°C. For H13 tempered at 595°C to 48 HRC, a 570°C stress relief sits 25°C under the temper and removes residual stress without re-tempering the bulk. The cycle is non-negotiable for any cavity going to nitriding. Skipping it means the cavity arrives at the nitride retort with tensile residual stress in the surface and a partially relieved recast layer, and the nitride cycle locks the stress state in.