- This group of materials contains hardened and tempered steels with hardnesses >45 – 68 HRC.
- Common steels include carburizing steel (~60 HRc), ball bearing steel (~60 HRc) and tool steel (~68 HRc). Hard types of cast irons include white cast iron (~50 HRc) and ADI/Kymenite (~40 HRc). Construction steel (40–45 HRc),
Mn-steel and different types of hardcoatings, i.e. stellite, P/M steel and cemented carbide also belong to this group.
- Typically hard part turning fall within the range of 55–68 HRC.
- Hardened steel is the smallest group from a machining point of view and finishing is the most common machining operation. Specific cutting force: 2550–4870 N/mm². The operation usually produces fair chip control. Cutting forces and power requirements are quite high.
- The cutting tool material needs to have good resistance to plastic deformation (hot hardness), chemical stability (at high temperatures), mechanical strength and resistance to abrasive wear. CBN has these characteristics and allows turning instead of grinding.
- Mixed or whisker reinforced ceramic are also used in turning, when the workpiece has moderate surface finish demands and the hardness is too high for carbide.
- Cemented carbide dominates in milling and drilling applications and is used up to approx. 60 HRc.
Typical components include: transmission shafts, gear box housings, steering
pinions, stamping dies.