Above roughly 45 HRC, conventional carbide stops working: the edge deforms plastically within seconds. Two options remain — hard turning with a dedicated H-group carbide, or CBN (cubic boron nitride). Hard turning is cheaper and more flexible; CBN is faster and holds tighter tolerances in volume.
Real cutting data for ISO H
Aggregated from 156 insert records published for hardened steel in our catalogue:
| Parameter | Full published range | Typical (median) |
|---|---|---|
| Cutting speed vc | 90 – 180 m/min | 120 – 180 m/min |
| Feed fn | 0.03 – 0.30 mm/rev | shallow, low feed |
| Depth of cut ap | — | shallow (usually 0.05 – 0.5 mm) |
Note how narrow the window is. Hard turning is not “turning with a harder insert” — it is a different process with a much smaller parameter box.
Grades that work
Our hardened-steel grades: PB30, PB60, AP153H, AP151H.
These are fine-grain substrates with thin PVD coatings and a honed (T-land) edge. The hone is not optional: a sharp edge in hardened steel chips instantly. The edge preparation does the work.
Setup requirements — this is what decides success
Most hard-turning failures are machine problems, not tool problems. Before you start:
- Rigidity. Anything that deflects will chatter and destroy the edge. Short overhang, solid workholding, no slop in the tool post or turret.
- Spindle condition. Runout directly translates to uneven edge load.
- Negative geometry. Always. The insert needs the included angle behind the edge.
- Large nose radius where possible — it supports the edge and improves finish, at the cost of higher radial force.
- Dry cutting. Coolant thermally shocks the edge in this process. Some use a light air blast.
If your setup cannot meet these, hard turning will not work no matter which grade you buy. Grind instead.
Hard turning versus grinding
| Hard turning | Grinding | |
|---|---|---|
| Setup / changeover | Fast, one setup for multiple diameters | Slower |
| Material removal rate | Higher in roughing | Lower |
| Dimensional accuracy | IT6–IT7 typical | IT5 and better |
| Surface finish | Ra 0.2–0.8 achievable | Better, and more consistent |
| Geometry flexibility | High — contours, threads, grooves in one setup | Limited by wheel |
| Cost per part | Lower at low–medium volume | Lower at very high volume |
The common practical answer: rough and semi-finish by hard turning, then either finish by turning (if the tolerance allows) or grind only the critical features.
Failure modes
- Plastic deformation of the edge — vc or fn too high. Lower both, use a harder grade.
- Chipping at the edge — setup not rigid, or hone too light. Increase nose radius, check runout.
- Rapid flank wear — normal in hard turning; expect short tool life and plan for it.
- Thermal cracking — you used coolant. Go dry.
Related
- ISO 513 material groups explained
- Carbide grades explained
- Turning insert selection guide
- Cutting parameters calculator
Standards and further reading
The material group definitions used throughout this site follow ISO 513. The manufacturer links below are independent technical references.