抄録
The present study was undertaken to clarify the mechanism of chipping phenomenon frequently found on pressing punch, since no definite theory to describe the phenomenon 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 fractography and optical microscopy.
The results of observation were summarized as follows.
(1) 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. As blanking continued, the metal fell off and adhered repeatedly, while a small pit was formed at the side surface of the punch.
(2) When blanking out mild steel sheets, wear was observed at the side surface of the punch after several thousands or more of blankings. And the carbides present at the surface of the tool steel fell off from the matrix and pits were formed.
(3) After pit formation, cracks appeared at the corner of the pits.
(4) When chipping occurred on a punch, the fractured surface was examined. As a result, fatigue fracture took place near the side surface of the punch and brittle fracture inside the punch.
(5) It was surmised that the working stress of blanking was concentrated at the pits formed on the side surfaces. From these observations, it was concluded that fatigue cracks started from the side surfaces of punches and brittle fracture propagated inwards.
The proposed mechanism of chip formation is in contrast to the conventional concept of simple cracks starting from the punches.
