Selection of Tool Steel for High-Speed Cutting (HSC)
High-speed cutting (HSC) operates at very high cutting speeds and material removal rates, generating extreme strain rates and temperatures at the tool–workpiece interface. Plastic deformation produces shear strains of 2–5 and strain rates up to 105 s−1 (locally reaching 107 s−1), with interface temperatures often exceeding 1000℃.
At these temperatures, tool performance is no longer controlled by room-temperature hardness. Tool failure is dominated by diffusion wear (crater formation), abrasive flank wear, edge chipping, and rapid thermal softening. Once hardness decreases at elevated temperatures, plastic deformation and wear accelerate, leading to a loss of cutting-edge stability.
Key Selection Factors
In HSC, material selection is primarily limited by high-temperature performance rather than conventional hardness or strength. The key is maintaining cutting-edge integrity under combined thermal and mechanical loading.
Hot Hardness (Red Hardness)
This is the primary constraint. The steel must retain hardness and resist tempering softening above 600–1000°C. If the hardness drops under the cutting temperature, the edge deforms plastically, and the wear rate increases sharply.
Wear Resistance
Wear resistance depends on matrix hardness and carbide characteristics. Fine and uniformly distributed carbides (MC, M6C) improve resistance to abrasion and diffusion wear. However, excessive carbide volume or coarse carbides increase brittleness and promote edge chipping under thermal cycling.
Toughness
Thermal cycling and intermittent cutting loads introduce repeated mechanical shock. Insufficient toughness leads to micro-chipping and edge failure, especially in milling or interrupted cutting. Increasing wear resistance by raising alloy content typically reduces toughness, so the balance must match the cutting condition.
Recommended Tool Steels
AISI M42 Tool Steel | 1.3247 | SKH59
M42 is used where cutting temperature is the dominant limiting factor. Its ~8% cobalt content improves secondary hardening and delays thermal softening, allowing working hardness of 68–70 HRC.
It is suitable for machining titanium alloys and nickel-based superalloys where cutting temperatures remain high. Compared to M4, it has lower toughness, so stable cutting conditions and rigid setups are required to avoid chipping.
AISI T15 (Tungsten-Vanadium-Cobalt HSS)
T15 is selected when abrasive wear is the primary failure mode. High vanadium (~5%) forms a large volume of hard MC carbides, significantly improving abrasion resistance compared to conventional HSS grades such as M2.
It is suitable for high-strength, high-tensile materials where edge wear is dominant. However, reduced toughness limits its use in interrupted cutting.
AISI M4 (High-Vanadium HSS)
M4 is used in conditions where abrasive wear is important, but some toughness is still required. With ~4% vanadium, it provides good wear resistance while maintaining better toughness than T15.
Typical hardness is 62–64 HRC. It is suitable for machining cast iron, brass, and hardened steels where edge stability and resistance to chipping are required.
Powder Metallurgy (P/M) Steels (e.g., ASP 60 / CPM Rex 76)
P/M steels address carbide segregation in conventional HSS by producing a uniform microstructure. This allows higher alloy content while maintaining usable toughness.
These steels typically reach 67–69 HRC and provide more stable performance under combined high temperature, wear, and shock conditions. They are suitable for demanding HSC applications that require both hot hardness and edge stability.
Summary Table
| Tool Steel Grade | Hardness | Key Feature | Primary Advantage |
| AISI M42 | 68–70 HRC | High Co (~8%) | Maintains hardness at high cutting temperatures |
| AISI T15 | Up to 67 HRC | High V + Co | High resistance to abrasive wear |
| AISI M4 | 62–64 HRC | Balanced V content | Better toughness with good wear resistance |
| P/M Steels | 67–69 HRC | Uniform microstructure | Stable performance under combined conditions |