Adaptor subs are made from top-grade alloy steel. They possess great hardness and excellent thread smoothness. In addition, we have developed a kind of special hardening technique which is used for the secondary hardening of the threads of adaptor subs. This technique can overcome the thread jam and lapped seam problem. It allows us to make durable and long-lasting adaptor subs. In comparison with ordinary subs, the China Drilling adaptor sub increases lifespan by 60% to 70%.