Sustainable progressive-stamping of brass American snaps by using the low-temperature plasma nitrided dies

Abstract

The semi-dry progressive stamping was strongly required to make mass production of clothing parts and beverage cans with minimum use of mineral-oil base lubricants. Without the surface treatment and protective coating, the bare punches had to suffer from severe wear and high friction to shorten their life time. Among several candidate treatments, the low temperature plasma nitriding at 673 K was employed to harden the seven kinds of punches for semi- dry progressive stamping after previous works in our research. The microstructure and nitrogen mapping in these nitrided punches were investigated by SEM (Scanning Electron Microscopy) - EDX (Energy Dispersive X-ray Spectrometry) to demonstrate that nitrogen super-saturation into punch materials could prolong their life time. In the present paper, the engineering impact of this approach on the semi-dry stamping was discussed with use the sustainability index (Stotal). This Stotal was defined by (Senergy + Swear + Swaste) / Saccuracy, where Senergy measured the energy consumption for stamping, Swear, the punch wear, Swaste, the material emission, and, Saccuracy, the quality ratio of products. The endurance experiments were performed to describe the dry progressive stamping behavior by 300 strokes per minute in fabrication of American snaps from the copper-base alloy sheets. Senergy was calculated by (number of products) x (energy cost per one shot). The micro-hardness was employed to prove that the seven punches and dies have uniform hardness even at the vicinity of punch head edges. In second, the wear of punches was investigated by using the optical microscopy in every million shots to estimate the time history of tool wear. Without use of the low temperature plasma nitriding, severe scratches were observed on the SKD11 (13% Cr alloy tool steel for cold mold) with case hardened punches enough to exchange to new ones after continuously stamping for 6 x 105 shots. When using the plasma nitrided SKD11 punches, this tool life limit was found to prolong up to 1.1 x 107 shots. This impact of low temperature nitriding was explained by using the calculated sustainability indices.