M42
1.3247Cobalt super high speed steel — 8% Co for maximum hot hardness (HRC 69 at 550°C). THE HSS for machining superalloys, titanium, and pre-hardened steels where M2 burns out. Mo-series (not W-series like T15). Reaches HRC 68-70 after hardening — among the hardest HSS grades. Also known as HS2-9-1-8 (EN) or SKH59 (JIS). Used for drills, end mills, taps, broaches, and milling cutters for difficult-to-machine materials.
International equivalents
| Flag | Standard | Country | Grade | Number | Match |
|---|---|---|---|---|---|
| 🇺🇸 | AISI | USA | AISI M42 / UNS T11342 | T11342 | REF |
| 🇪🇺 | EN | Europe | HS2-9-1-8 / 1.3247 | 1.3247 | 100% |
| 🇯🇵 | JIS | Japan | SKH59 | — | 100% |
Sources: ASTM A600, EN ISO 4957, JIS G4403
Mechanical properties
Compatibility verdict
M42 and HS2-9-1-8 / 1.3247: 100% composition match
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