| 摘要 |
<p>A drill string steel (13) has an outer layer (13a), at least 6mm deep, hardened to a hardness of at least 600 Brinell, using an induction heating head (31) and a quenching head (34), moved along the drill steel (13) as it rotates in a supporting lathe (20). Portions or recesses (13d) of the drill steel (13), to be engaged by tools, have an outer layer hardened to a hardness in the range of 280-500 Brinell.</p> |
