ISO H — Hardened Steel · H20
H20 — Hardened steel (high)
Hardness range: 60–68 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 — H20
- ISO designation
- H20 (ISO H — Hardened Steel)
- Material
- Hardened steel (high)
- Hardness
- 60–68 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 (60–68 HRC) — starting point
- ISO application class
- H (hardened steel). At 60–68 HRC, CBN / PCBN is mandatory; carbide and most ceramics will not survive. Match the ISO H class across brands rather than the brand name (Sandvik, Kennametal, Iscar, Walter, Seco and Mitsubishi each publish one), then confirm the exact grade for your condition.
- Grade & coating
- High-content CBN / PCBN with a honed or chamfered edge. This is a finishing operation, not roughing.
- Geometry
- Negative geometry with a strong chamfer plus hone, a large nose radius, and an extremely rigid setup. Run dry.
- Starting parameters
- CBN Vc ≈ 100–180 m/min, fn ≈ 0.05–0.12 mm/rev, ap ≈ 0.05–0.15 mm. Finishing only — confirm in the advisor or on a sample part.
Selection table: hardened steel (60–68 HRC)
| Operation | Geometry | ISO class | Coating family | Coolant | First check |
|---|---|---|---|---|---|
| Roughing | Not a roughing material: this is a finishing operation | H | High-content CBN / PCBN | Dry | Confirm the process is finishing, not stock removal |
| Finishing | Negative with strong chamfer plus hone, large nose radius | H | High-content CBN / PCBN | Dry: avoid water-based coolant on CBN | Edge wear shifts size on tight tolerances: index on a schedule |
| Interrupted cut | Strong chamfer plus hone; avoid interruptions where possible | H | Robust CBN grade | Dry | CBN is hard but brittle: a too-keen edge chips |
| Low rigidity / thin wall | Shortest, most rigid tool and workholding | H | CBN / PCBN | Dry | Rigidity is everything at this hardness |
hardened steel (60–68 HRC): what goes wrong, and what to check first
- Edge chipping
- CBN is hard but brittle; any interruption or too keen an edge chips it. Use a strong chamfer plus hone, a robust grade, and zero-interruption passes where possible.
- Thermal cracking
- Avoid water-based coolant on CBN; thermal shock cracks the edge. Run dry.
- Dimensional drift
- Edge wear shifts size on tight-tolerance finishing. Monitor and index on a schedule.
- Chatter
- Rigidity is everything at this hardness. Use the shortest, most rigid tool and workholding.
Reference data
Frequently asked questions
What insert do I need for 60–68 HRC steel?
High-content CBN / PCBN with a honed or chamfered edge; carbide and most ceramics will not survive. Match the ISO H class across brands and confirm the CBN grade in the advisor.
Can you machine 65 HRC steel?
Yes, with CBN in a rigid finishing setup at light depth and feed; it is how hardened dies and bearing races are finish-turned instead of ground.
Should I use coolant on CBN at high hardness?
No, run dry; coolant causes thermal-shock cracking on CBN.
Why does my CBN edge chip on very hard steel?
CBN is brittle; interruptions or too keen an edge chip it. Use a strong chamfer plus hone and an extremely rigid setup.
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