Structural materialization of stainless steel molds and dies by the low temperature high density plasma nitriding

Various kinds of stainless steels have been widely utilized as a mold substrate material for injection molding and as a die for mold-stamping and direct stamping processesSince they suffered from high temperature transients and thermal cycles in practice, they must be surface-treated by dry and wet coatings, or, by plasma nitridingMartensitic stainless steel mold was first wet plated by the nickel phosphate (NiP), which was unstable at the high temperature stamping condition; and, was easy to crystalize or to fracture by itselfThis issue of nuisance significantly lowered the productivity in fabrication of optical elements at presentIn the present paper, the stainless steel mold was surface-treated by the low-temperature plasma nitridingThe nitrided layer by this surface modification had higher nitrogen solute content than 4 mass%; the maximum solid-solubility of nitrogen is usually 0.1 mass% in the equilibrium phase diagramOwing to this solid-solution with high nitrogen concentration, the nitrided layer had high hardness of 1400 Hv within its thickness of 40 μm without any formation of nitrides after 14.4 ks plasma nitriding at 693 KThis nitrogen solid-solution treated stainless steel had thermal resistivity even at the mold-stamping conditions up to 900 K.