RESEARCH OF CHIPPING PHENOMENON ON BLANKING-PUNCH MADE OF TOOL STEELS.

This study was undertaken to clarify the mechanism of chipping frequently found on the pressing punch, for which phenomenon no definite theory has been established. The punches made of several different types of tool steel were used to stamp out sheets of mild steel and stainless steel. Blanking punches, after use, were examined by means of electron fractorgraphy and optical microscopy. When stamping out 18Cr-8Ni type stainless steel sheets, some of the metal from the sheets adhered to the side surface of the punch after several thousand blankings. When blanking out mild steel sheets, wear was observed at the side surface of the punch after several thousands or more of blankings. The carbides present at the surface of the tool steel fell off from the matrix and pits were formed. After pit formation, cracks appeared at the corner of the pits. It is concluded that fatigue cracks started from the side surfaces of punches and brittle fracture propagated inward.