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
Chemical composition comparison (wt%)
Side-by-side: M42 (AISI) (ASTM A600) vs HS2-9-1-8 / 1.3247 (EN)
| Element | M42 (AISI) | HS2-9-1-8 / 1.3247 (EN) | Overlap |
|---|---|---|---|
| C | 1.05–1.15% | — | — |
| Si | 0–0.65% | — | — |
| Mn | 0–0.4% | — | — |
| P | 0–0.03% | — | — |
| S | 0–0.03% | — | — |
| Cr | 3.5–4.25% | — | — |
| Mo | 9–10% | — | — |
| V | 0.95–1.35% | — | — |
| W | 1.15–1.85% | — | — |
| Co | 7.75–8.75% | — | — |
Mechanical properties
Hardened Tempered
| Property | Value | Unit |
|---|---|---|
| Hardness HRC | 68–70 | HRC |
| Elastic Modulus | ≥ 210 | GPa |
| Density | ≥ 7.98 | g/cm3 |
Annealed
| Property | Value | Unit |
|---|---|---|
| Hardness HB | ≤ 269 | HB |
Heat treatment
| Process | Temperature | Medium | Holding Time | Notes |
|---|---|---|---|---|
| hardening | 1160–1190 °C | salt bath or vacuum | short hold | Salt 1160-1180°C, vacuum 1180-1190°C |
| tempering | 540–595 °C | air | 3x 1h minimum | Triple temper for secondary hardness peak — HRC 69 at 550°C |
Compatibility verdict
M42 and HS2-9-1-8 / 1.3247: 100% composition match
Close equivalents but NOT identical.
Related materials
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M2 / HS6-5-2
1.3343The most widely used high-speed steel worldwide. W-Mo-Cr-V composition gives excellent red hardness (cuts at temperatures up to 600°C). Used for twist drills, end mills, taps, reamers, bandsaw blades, and general-purpose cutting tools. The benchmark HSS grade.
X45NiCrMo4
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