High Machinability of Plasma-Nitrided HPM80 Dies at 673K by PCD-Tools for Hot Mold-Stamping

Abstract

HPM80 had been utilized as one of the martensitic stainless steel mold and die substrates for stamping. In particular, HPM80 mold was often used as hot-stamping molds for oxide glasses and optical polymers to fabricate the optical elements and DOE (Diffractive Optical Elements) with the aid of NiP (Nickel Phosphate) and NiWP (Nickel-Tungsten Phosphate) coatings. These machinable amorphous coatings were indispensable to cut and finish the tailored micro textures for DOE since HPM80 was difficult or nearly impossible to be finished by using the PCD (Poly-Crystal Diamond)-chipped tools. However, those coatings were fragile above 473 K; the original HPM80 must be modified to be fine machinable even by the diamond-chipped and coated tools for fine finishing to mirror-polished die surfaces with far less damage to cutting tools. These HPM80 substrates were plasma nitrided at 673 K under the nitrogen and hydrogen mixed gas flow to form the high content nitrogen super-saturated layer. Precise machinability experiment was performed with the use of the PCD coated WC (Co) tools to investigate the machinability of these nitrogen supersaturated HPM80 substrates. Two-phase fine-grained layer with higher nitrogen solute content than 2 mass% was machined in the ductile mode with adaptively high quality to shaping and finishing the mold die surface for stamping. Through the hardness testing, this nitrogen super-saturated die surface had higher hardness than 1400 HV; it had sufficient durability for long-term stamping operations. Finally, the turning test was employed to demonstrate that this nitrogen supersaturated HPM80 layer with the thickness of 40μm was machined with high quality to form the mirror-polished surface.