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

Above approximately 590°C the iron-nitrogen system enters the γ (austenite) field. The α (ferrite) lattice that nitrogen was diffusing through transforms, the compound-layer chemistry changes, and on cooling the part has coarse iron nitrides through a mixed microstructure rather than the intended case. Every standard nitriding process runs below 590°C. The practical window for tool steels is 480-560°C. FNC pushes the upper end to about 580°C. Plasma typically sits at 480-520°C.

The second reason 590°C is a hard ceiling: the substrate has already been tempered, and the tempering temperature defines what it can sit at without softening the core. H13 tempered at 595°C tolerates a 560°C gas cycle. P20 tempered at 595°C tolerates the same. Pre-hard A2 tempered at 175°C cannot be nitrided at gas temperatures at all. The 590°C metallurgical ceiling and the substrate temper bound usually move together. Verify both.