Chipping Resistant Property of Nitrided Age-hardening Steels for Blanking Punches

Abstract

In a previous paper, the chipping phenomena on blanking punches are described as follows: During the blanking operation, many wear pits are initiated on the side surface of the blanking punch. The side surface of the blanking punch is subjected to tensile stress and compressive stress alternately. The chipping phenomenon is initiated from fatigue cracks caused by the stress concentration at the bottom of the wear pits.
In this study, the chipping resistance of various nitrided steels for blanking punches was tested. The tested steels were age-hardening steels of the composition of Fe-3%Mn-15%Ni-V-Mo-Co, where the quantities of V, Mo and Co were varied to several levels. The steels were ion-nitrided at 723 K (450°C) for 3.6∼187.2 ks (1 to 52 h).
The results were obtained as follows:
(1) The blanking punches made of these ion-nitrided age-hardening steels are free from chipping damage.
(2) If the high-vanadium steels containing vanadium more than 3% are ion-nitrided for an excessively long time, internal cracks are initiated in the nitrided layer. But chipping does not occur during the blanking operation with these internal cracks in the nitrided layer, while slight spallings may be observed in the surface layer whithin 1.7×10⁴ cycles.
(3) The lives of the punches made of the ion-nitrided high-vanadium steels are extremely long, if the internal cracks are not present. For instance, the life of a punch made of 3%Mn-15%Ni-3%Mo-9%Co steel which is ion-nitrided for 18 ks (5 h) at 723 K is three times longer than that of an SKD-1 steel punch, which was quenched from 1253 K (980°C) and tempered at 473 K (200°C).