The metallurgy you actually need — Fe-N phase diagram in plain English

γ'-Fe₄N is the lower-nitrogen phase, about 5.7-5.9 wt% N, face-centered cubic. It is more ductile than ε and more crack-resistant under impact, with lower hardness as the tradeoff. A γ' compound layer is the right answer for cold heading punches, blanking punches, and percussion tooling. Typical surface microhardness is HV 0.05 around 700-900.

ε-Fe₂₋₃N is the higher-nitrogen phase, 8-11 wt% N (with carbon entering the lattice in nitrocarburizing as ε-Fe₂₋₃(N,C)), hexagonal close-packed. It is harder than γ' and more wear-resistant and corrosion-resistant, but more brittle. Typical surface microhardness is HV 0.05 around 900-1100. The outer portion of an ε layer often develops a network of fine pores called the porous zone (Porensaum), which is useful for holding lubricant in sliding wear but can be a corrosion path on storage-sensitive parts.

The pathological case is a thick ε layer over a γ' sublayer with a sharp phase boundary between them. The two phases have different crystal structures, thermal expansion coefficients, and nitrogen content, and under load the layer cracks along the interface and spalls. A competent vendor grows either a single-phase compound layer or a graded transition.