Fine piercing of electromagnetic steel sheets by the plasma-nitrided tools

The fine piercing process under the nearly zero-clearance die-punch set-up was proposed as a non-traditional process with use of the plasma-nitrided punch - die pair as well the diamond-coated WC (Co) - plasma nitrided die pair. With use of the high density plasma nitriding system, the SKD11 punch was nitrided to have the surface hardness up to 1000 HV. The annealed SKD11 die-substrate was shaved into a core die by accurately piercing the nitrided punch. After shaping and plasma-nitriding the shaved core die, both the punch and core-die was placed into the cassette die set for piercing experiments under the nearly zero-clearance. The electromagnetic steel sheets with the thickness of 0.5 mm were prepared to describe the shearing behavior in piercing. The piercing load and stroke histories were traced with increasing the number of shots; the engineering durability was also discussed with comments on the wear of punch and die. The brittle electromagnetic sheets were accurately blanked with the burr height ratio of 10 % and the burnished surface area ratio of 70 %. With increasing the clearance from nearly zero to 4 % or 8 % in thickness, this piercing process turned to be normal shearing one with much higher burr height ratio and fractured surface ratio.