Abstract
In addition to the super piercer, “the new super piercer”, which allows expansion piercing, has been developed by the authors. The development of the skewing technology for disc roll axes and its application to the cone-type piercing technology results in a remarkable increase in the expansion ratio. In this paper, studied in detail were the influences of the expansion ratio, feed and cross angles on the rotary forging effects, those on the redundant shear deformations and those on the inside bore defects in materials with poor hot workability. Furthermore, discussed in detail were the influences of the expansion ratio, rotary forging effects and redundant shear deformations on the inside bore defects in materials with poor hot workability. As a result, it has been quantitatively clarified that the rotary forging effects are the major causes of the initiation of inside bore defects, and that the redundant shear deformations are the major causes of the propagation of inside bore defects. Noteworthy is the influence of the expansion ratio. It should be noted that the larger the expansion ratio is, the more remarkably the inside bore defects decrease.
