Introduction to High-Speed Steel
High-speed steel (HSS) represents a class of tool materials engineered to maintain hardness and cutting performance at elevated temperatures generated during high-speed machining operations. Since its development in the early 20th century, HSS has remained essential for cutting tools, drills, taps, and gear-cutting equipment.
History and Development
Developed by F.W. Taylor and M. White in 1898, the original HSS contained 18% tungsten and 5% chromium. Modern HSS incorporates molybdenum, vanadium, cobalt, and other alloying elements to optimize specific performance characteristics.
HSS Classification and Standards
ASTM Standards
- ASTM A600: Standard specification for tool steel alloy, hot-worked and cold-worked
- T-series (T1-T15): Tungsten-base high-speed tool steels
- M-series (M1-M70): Molybdenum-base high-speed tool steels
AISI/SAE Classification
AISI designates HSS grades with letter-number codes indicating the primary alloying system and composition level. Common designations include M2, M35, M42, and T15.
HSS Grades and Properties
M1 – Molybdenum High-Speed
The standard molybdenum HSS, offering excellent toughness and good wear resistance. Most economical option for general-purpose cutting applications.
Composition: 8% Mo, 4% Cr, 1.5% W, 1% V
Hardness: 62-65 HRC
Red Hardness: 560°C
Applications: Drills, reamers, taps, general-purpose cutting tools
M2 – Molybdenum High-Speed (Standard Grade)
Industry standard HSS with excellent balance of toughness, wear resistance, and cost. Widely available in various forms including bars, sheets, and pre-formed tools.
Composition: 5% Mo, 4% Cr, 6% W, 2% V
Hardness: 63-66 HRC
Red Hardness: 600°C
Applications: Drills, end mills, broaches, lathe tools
M35 – Cobalt High-Speed
Cobalt addition (5%) increases hot hardness and cutting speed capability by approximately 20% over standard M2. Excellent for difficult-to-machine materials.
Composition: 5% Mo, 4% Cr, 6% W, 5% Co, 2% V
Hardness: 64-67 HRC
Red Hardness: 610°C
Applications: High-speed machining, hard materials, stainless steel
M42 – Super High-Speed Cobalt
Premium cobalt HSS with 8% cobalt, providing exceptional hot hardness for the most demanding applications. The choice for high-production cutting operations.
Composition: 9.5% Mo, 4% Cr, 1.5% W, 8% Co, 1.2% V
Hardness: 65-70 HRC
Red Hardness: 640°C
Applications: High-speed finishing, aerospace alloys, exotic materials
T15 – Tungsten High-Speed
Maximum wear resistance among conventional HSS grades, with 15% tungsten and 5% cobalt. Best suited for long production runs with consistent conditions.
Composition: 12% W, 5% Co, 4.5% V, 5% Cr
Hardness: 65-70 HRC
Red Hardness: 650°C
Applications: Finish milling, precision boring, long-run production
Alloying Elements and Their Effects
| Element | Effect on Properties | Typical Content |
|---|---|---|
| Tungsten (W) | Hardness, wear resistance, red hardness | 1.5-18% |
| Molybdenum (Mo) | Hardness, toughness, wear resistance | 1-10% |
| Chromium (Cr) | Hardenability, corrosion resistance | 3.5-4.5% |
| Vanadium (V) | Wear resistance, carbide formation | 1-5% |
| Cobalt (Co) | Hot hardness, cutting speed capability | 0-12% |
| Carbon (C) | Hardness, wear resistance (combined) | 0.7-1.5% |
HSS Tool Types and Applications
Drill Bits
HSS drills remain the most common type for general-purpose drilling in steel, cast iron, aluminum, and wood. Options include:
- Bright finish: General-purpose drilling
- Black oxide: Improved chip evacuation, corrosion resistance
- TiN coating: Extended tool life, higher speeds
- TiAlN coating: Maximum performance in difficult materials
End Mills
HSS end mills serve milling operations in various configurations:
- Two-flute: Aluminum and non-ferrous metals
- Three/four-flute: General-purpose steel machining
- Roughing end mills: Rapid stock removal with chip breakers
- Ball nose: Contour milling and 3D profiling
Taps and Threading Tools
HSS taps create internal threads in most materials:
- Tap types: Spiral point (gun), spiral flute, hand taps
- Coatings: Bright, TiN, TiCN, VAP
- Application: Through holes, blind holes, tough materials
Broaches
HSS broaches produce complex internal profiles including splines, keyways, and specialized shapes. Require specialized equipment but offer excellent accuracy and surface finish.
Surface Treatments and Coatings
Steam Treatment
Oxide layer formation improves chip clearance and provides mild corrosion resistance. Standard treatment for production drills and taps.
TiN (Titanium Nitride)
Gold-colored coating providing excellent wear resistance and reduced friction. Suitable for most steel and cast iron applications with speeds up to 30% higher than uncoated.
TiCN (Titanium Carbonitride)
Harder than TiN with better wear resistance on abrasive materials. Blue-gray color; ideal for aluminum, copper, and highly abrasive materials.
TiAlN (Titanium Aluminum Nitride)
Maximum thermal stability for high-speed machining and difficult materials. Maintains hardness at temperatures exceeding 700°C. Ideal for nickel-based superalloys and titanium.
AlCrN (Aluminum Chromium Nitride)
Superior oxidation resistance for dry machining and high-temperature applications. Excellent choice for drilling and milling in die and mold making.
Heat Treatment
Austenitizing Temperature
HSS requires precise austenitizing (hardening) temperatures to develop optimal properties. Typical ranges:
- M1, M2: 1175-1225°C
- M35: 1190-1230°C
- M42: 1175-1205°C
- T15: 1200-1280°C
Quenching
Oil quench or accelerated gas quench following austenitizing. Proper quench rate ensures maximum hardness without cracking.
Tempering
Multiple tempering cycles (typically 2-3 cycles at 500-600°C) relieve quench stresses and develop secondary hardness. Never temper below 200°C or above the secondary hardening peak.
Selection Guidelines
Material to Machine
| Work Material | Recommended HSS | Preferred Coating |
|---|---|---|
| Carbon Steel | M1, M2 | Bright, TiN |
| Alloy Steel | M2, M35 | TiN, TiCN |
| Stainless Steel | M35, M42 | TiAlN |
| Cast Iron | M1, M2 | TiCN, AlCrN |
| Aluminum | M2 | Bright, ZrN |
| Titanium | M42 | TiAlN |
| Inconel | M42 | TiAlN, AlCrN |
Conclusion
High-speed steel remains a versatile and cost-effective choice for countless cutting tool applications. Matching HSS grade, geometry, and coating to specific machining requirements maximizes tool life and productivity while minimizing per-part cost.
Need high-speed steel or other tool materials for your manufacturing operation?
Contact Tracy at tracy@coremetalsteel.com or call +86 18291910632. Xi’an Coremetal Steel Co., Ltd. supplies quality tool steels and machined components to manufacturers worldwide with ISO 9001 certification.