AISI M42 Cobalt High-Speed Steel: Premium Super-Hard Cutting Tool Material
ASTM A600 · Published: 2026-06-24 · Updated: 2026-06-25
AISI M42 is a cobalt-bearing high-speed steel that represents the pinnacle of cutting tool metallurgy. With 8% cobalt, it maintains hardness up to 1,100°F (593°C) — the temperature at which standard high-speed steels like M2 begin to soften...
AISI M42 is a cobalt-bearing high-speed steel that represents the pinnacle of cutting tool metallurgy. With 8% cobalt, it maintains hardness up to 1,100°F (593°C) — the temperature at which standard high-speed steels like M2 begin to soften significantly. M42 achieves a maximum working hardness of 68-70 HRC, among the highest of any tool steel. It is used for the most demanding machining operations: cutting hardened steels above 50 HRC, high-temperature aerospace alloys (Inconel, titanium), and high-speed production tooling where tool life directly determines manufacturing economics.
M42's cobalt content serves a critical metallurgical function: cobalt raises the temperature at which martensite softens by inhibiting carbide coalescence. This is why M42 costs 5-8× more than M2 — the cobalt content alone (at ~$35/lb for cobalt metal) accounts for a significant portion of the material cost. For applications that do not require red-hardness above 1,000°F, M2 or M7 provide better value.
Quick Facts
| Category | |
| Standard | ASTM A600 |
| Density | 8.05 g/cm³ |
| Yield Strength | N/A (HRC 66-70, not typically measured as yield) |
| Tensile Strength | N/A (hardened tool steel — measured by hardness, not tensile) |
Global Equivalents & Cross-Reference
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Frequently Asked Questions
How does M42 compare to carbide tooling?
M42 bridges the gap between conventional HSS and solid carbide. Hardness: M42 at 68-70 HRC vs. carbide at 75-80 HRA (different scales — carbide is harder). Toughness: M42 is significantly tougher than carbide — it can withstand interrupted cuts and vibration that would chip carbide. Cost: M42 tools cost 3-5× standard HSS vs. carbide tools at 10-20×. Best application: M42 for difficult materials with interrupted cuts or where tool breakage risk makes carbide uneconomical. Carbide for continuous high-speed production of abrasive materials.
What is the correct heat treatment for M42?
Preheat: 850°C (1,560°F), hold for thorough soaking. Austenitize: 1,190-1,210°C (2,175-2,210°F) — precise control within ±5°C is critical. Quench: oil or salt bath to below 150°C. Temper: triple temper at 540-560°C (1,000-1,040°F) for 2 hours each, cooling to room temperature between tempers. Triple tempering is essential for M42 due to retained austenite transformation during cooling between tempers. Sub-zero treatment (-85°C) after the first temper maximizes transformation and final hardness.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu
Tool & High-Performance Steel — Engineering Reference
Tool steels are high-carbon, high-alloy grades engineered for cutting, forming, and shaping other materials. They are classified by application: high-speed (M-series), hot-work (H-series), cold-work (D/A/O-series), and shock-resisting (S-series).
ASTM A681, ISO 4957, JIS G4403/G4404, GB/T 1299
Cutting tools, dies, molds, punches, shear blades, bearings, drill bits, injection molding tooling
Tool steel selection starts with the operating temperature. Cold-work steels (D2, A2, O1) for room-temperature forming. Hot-work steels (H13) for die casting and extrusion above 600°F. High-speed steels (M2, M42) retain hardness at cutting temperatures up to 1,000°F.