| 摘要 |
Self-tapping screws are made from 18/8 stainless steel by cold forming the head and thread rolling the shank. After forming the heads, the blanks (34) are fed along a rail (7) through a tunnel (40) with thermally insulated walls. As the blanks (34) pass through the tunnel they are sprayed with liq. nitrogen supplied from a bottle (46). The liq. nitrogen cools the blanks to a temp. of -129 deg. C and increases the work-hardening which occurs during the subsequent thread-rolling. A hardness of 48 Rockwell C can be achieved in the threads with the core hardness of 29 Rockwell C. |
