Requirements for milling hardened steels· In general hard milling involves cutting primarily tool steel or precipitation hardening stainless steel such as 15 5 or 17 4 that has been hardened to at least 50 HRC After a workpiece is roughed in the soft state it is sent to the furnace for hardening and then finish machined with coated carbide ceramic or PCBN tools The amount of metal removal in the hardened state is minimal