Salt-bath nitriding (Tufftride / Melonite / QPQ) — fast, dirty, effective

Precision tooling with tight dimensional or distortion budgets is the wrong fit. The 570°C bath thermally shocks the parts on entry and on quench, and dimensional growth from compound-layer formation runs 3-7 µm per side. On a long, thin precision punch or a close-tolerance mold core, that is enough to move the part outside tolerance with no finish-grind option.

Complex internal geometry is the wrong fit. Blind holes, deep recesses, internal threads, and porous powder-metal parts trap molten salt. Cyanate residue left in a part becomes a corrosion site in service and a regulatory issue in shipment. Shops will reject parts they cannot rinse cleanly.

Environmental and worker-safety constraints disqualify some plants outright. The salts contain cyanate; under upset conditions or in spent-salt regeneration the chemistry can include cyanide. Spent salts are a regulated hazardous waste stream (RCRA-listed in the US). A salt-bath FNC operation requires permitted waste disposal, fume capture, and worker exposure controls that do not exist in a typical job shop. Salt-bath FNC is concentrated in dedicated process houses, not added to a general heat-treat line.