Pre-nitride heat treatment — you can't nitride a soft die

Heavy roughing, finish milling, grinding, and especially wire and ram EDM leave residual tensile stress in the surface, 100-200 µm deep on EDM-cut surfaces. EDM also leaves a recast white layer 5-30 µm thick: remelted and rapidly resolidified steel with cracks, retained austenite, and stress on the order of the material's yield strength. Nitriding does not heal any of that. Nitrogen diffusing into a stressed, cracked recast layer amplifies the stress and propagates the cracks. That is the network failure pattern in the opening example.

The fix is a stress-relief cycle before nitriding: 30°C below the temper temperature, 1-2 hours per inch of section, slow cool. For an H13 or P20 die tempered at 600°C, that is a 570°C stress relief. The cycle removes residual stress without re-tempering the bulk. For EDM-finished surfaces, light bench stoning or polishing to knock down the worst of the recast layer before the stress relief is worthwhile; the stress relief alone does not eliminate recast.

Stress relief is not a temper. A temper sets bulk hardness. A stress relief at sub-temper temperature removes residual machining and EDM stress without changing the temper. Vendors and schedulers conflate the two routinely. Specify both on the traveler if both are required. AMS 2759/11 covers the stress-relief operation for aerospace-grade work.