ISO H — Hardened Steel · H10
H10 — Hardened steel (medium)
Hardness range: 55–60 HRC. Part of the ISO H — Hardened Steel group. Hard turning replaces grinding for many finishing operations on hardened steel above 45 HRC. CBN (cubic boron nitride) and PCBN inserts are required above 55 HRC. Dry machining is preferred — coolant causes thermal shock cracking. Tight radius inserts (rε 0.4–0.8 mm) and low feed rates preserve surface finish.
Material profile — H10
- ISO designation
- H10 (ISO H — Hardened Steel)
- Material
- Hardened steel (medium)
- Hardness
- 55–60 HRC
- ISO group colour
- Hardened Steel
Tooling guidance — H-group
- Group summary
- CBN/PCBN required above 55 HRC. Dry machining preferred — coolant causes thermal shock.
Insert selection for hardened steel (55–60 HRC) — starting point
- ISO application class
- H (hardened steel). Above ~45 HRC hard turning replaces grinding; above ~55 HRC CBN / PCBN is required. Match the ISO class across brands (Sandvik, Kennametal, Iscar, Walter, Seco and Mitsubishi) and confirm the CBN grade.
- Grade & coating
- CBN / PCBN for 55–60 HRC. A honed or chamfered edge prep is essential; carbide will not survive above ~55 HRC.
- Geometry
- Negative geometry with a T-land or chamfer plus a hone for edge security, and a large nose radius for finish. Very rigid setup. Run dry — coolant causes thermal-shock cracking on CBN.
- Starting parameters
- CBN Vc ≈ 100–200 m/min, fn ≈ 0.05–0.15 mm/rev, small ap ≈ 0.05–0.25 mm — a finishing regime, dry. Typical starting band — confirm in the advisor or on a sample part.
Selection table: hardened steel (55–60 HRC)
| Operation | Geometry | ISO class | Coating family | Coolant | First check |
|---|---|---|---|---|---|
| Roughing | Not a roughing material at this hardness: small ap, finishing regime only | H | CBN / PCBN; carbide will not survive above about 55 HRC | Dry | Carbide in the holder by mistake: use CBN |
| Finishing | Negative with T-land or chamfer plus hone, large nose radius | H | CBN / PCBN | Dry: coolant causes thermal-shock cracking on CBN | Edge replaced on schedule, speed in band |
| Interrupted cut | Honed or chamfered edge prep for edge security | H | Robust CBN grade | Dry | CBN is brittle: minimise interruptions, avoid a too-keen edge |
| Low rigidity / thin wall | Negative with chamfer plus hone; rigid tool and workholding | H | CBN / PCBN | Dry | Setup rigidity and overhang before the grade |
hardened steel (55–60 HRC): what goes wrong, and what to check first
- Edge chipping (CBN is brittle)
- Interruptions or too keen an edge chip CBN. Use a honed / chamfered edge, a robust CBN grade, a rigid setup, and minimise interruptions. See the tool-wear guide.
- White layer / thermal damage
- A worn edge or too-high speed overheats the surface and forms a white layer. Replace edges on schedule and keep speed in band, running dry.
- Chatter (rigidity is everything)
- In hard turning, chatter is setup, not grade. Shorten overhang and use a rigid tool and workholding.
- Rapid wear with the wrong grade
- Carbide will not survive above ~55 HRC — use CBN.
Reference data
Frequently asked questions
What insert should I use for hard turning 55–60 HRC steel?
CBN / PCBN with a honed or chamfered edge; carbide will not survive above ~55 HRC. Match the ISO H class across brands and confirm the CBN grade in the advisor.
Can hard turning replace grinding?
For many finishing operations above ~45 HRC, yes — CBN hard turning reaches tight tolerance and good finish with a far more flexible setup, given a rigid machine.
Should I use coolant when hard turning with CBN?
Usually dry — coolant can cause thermal-shock cracking on CBN. Keep the setup rigid and the edge fresh instead.
Why does my CBN edge chip on hardened steel?
CBN is hard but brittle; interruptions or too keen an edge chip it. Use a honed or chamfered edge, a robust grade, and a very rigid setup.
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