What High-Speed Tool Steel Is
High-speed tool steel is an alloy tool steel engineered to hold a cutting edge at temperatures that would soften plain carbon tool steel. The name reflects the fact that tools made from it cut metal at speeds several times higher than carbon steel tools of the same geometry. The decisive property is red hardness: a tempered martensitic matrix reinforced by a dense dispersion of hard alloy carbides resists softening even when the cutting edge reaches roughly 540 to 600 C.
Commercial supply divides into two alloy families. Tungsten grades depend on tungsten-rich carbides and are still selected where maximum hot hardness is required. Molybdenum grades use molybdenum with a smaller tungsten addition and account for the largest share of world consumption, because they deliver comparable performance at moderate cutting speeds and at lower cost.
Chemical Composition of Widely Used Grades
The ranges below follow ASTM A600, the standard specification for high-speed tool steel, together with the ISO 4957, DIN and GB/T 9943 designations used for the same materials. Values are percentages by mass.
| Grade | ISO 4957 / DIN | GB/T 9943 | C | Cr | Mo | W | V | Co |
|---|---|---|---|---|---|---|---|---|
| M2 | HS6-5-2 / 1.3343 | W6Mo5Cr4V2 | 0.78-0.88 | 3.75-4.50 | 4.50-5.50 | 5.50-6.75 | 1.75-2.25 | - |
| T1 | HS18-0-1 / 1.3355 | W18Cr4V | 0.65-0.80 | 3.75-4.50 | - | 17.0-18.5 | 0.90-1.30 | - |
| M35 | HS6-5-2-5 / 1.3243 | W6Mo5Cr4V2Co5 | 0.78-0.88 | 3.75-4.50 | 4.75-5.75 | 5.50-6.50 | 1.75-2.25 | 4.75-5.75 |
| M42 | HS2-9-8 / 1.3247 | W2Mo9Cr4VCo8 | 1.05-1.15 | 3.50-4.25 | 9.00-10.00 | 1.15-1.85 | 1.00-1.40 | 7.75-8.75 |
A frequently repeated summary states that high-speed steel contains 0.6% carbon, 3% chromium and 14-18% tungsten. Those figures describe the tungsten grades, such as ASTM A600 grade T1 with 17.0-18.5% W, and not the molybdenum grades that dominate modern supply. Grade M2, for example, carries 4.50-5.50% Mo and 5.50-6.75% W at a carbon level of 0.78-0.88%, which is why the two families must be quoted separately.
Heat Treatment and Working Hardness
High-speed steel is normally supplied in the annealed condition so that it can be machined to shape before hardening. A typical sequence for the molybdenum grades runs as follows:
Anneal to a soft, machinable condition, then stress relieve after heavy stock removal to limit distortion.
Austenitise at approximately 1180-1220 C for molybdenum grades, with higher temperatures for tungsten and cobalt grades.
Quench in oil, molten salt or vacuum gas at a controlled cooling rate.
Temper two or three times at about 540-570 C to develop secondary hardening, which brings cutting tools to roughly 63-66 HRC.
Cobalt-bearing grades such as M35 and M42 follow the same cycle with adjusted austenitising and tempering temperatures to exploit their higher hot hardness.
Performance Characteristics
Red hardness keeps the cutting edge hard during dry or high-speed machining where the tip temperature climbs steeply.
Wear resistance comes from vanadium and tungsten carbides that limit abrasive and adhesive wear in interrupted cuts.
Toughness is generally higher in the molybdenum grades, while cobalt grades trade shock resistance for additional hot hardness.
Grindability favours the molybdenum family, which reduces tool room cost and reconditioning time.
Dimensional stability achieved by correct austenitising and multiple tempering keeps distortion low in long tools such as reamers and broaches.
Typical Applications
Twist drills, centre drills, taps, reamers and counterbores for general engineering.
End mills, slot drills, face milling cutters and side and face cutters.
Gear hobs, shaper cutters and broaches for gear and spline production.
Band saw and circular saw blades for metal cutting, including bimetal constructions.
Punches, forming tools, rolls and shear blades that need wear resistance with moderate toughness.
Grade M2 covers the large majority of these duties. Grade T1 is chosen for heavy-duty cutting where the highest hot hardness is wanted, and M35 or M42 are used for high-strength alloys, stainless steels and titanium where heat concentrates at the cutting edge.
Before a grade and a supply form are fixed for a project, the following points should be agreed with the mill:
Confirm the required hardness and cutting duty before fixing the grade, because higher hot hardness always reduces shock resistance.
State the tool design, stock removal and expected cutting speed so that the heat treatment schedule matches the application.
Common supply forms are annealed hot rolled or forged bars in round, square and flat sections, peeled and ground bars for tool rooms, and cut lengths or near-net shapes.
Bars should be ordered to a stated tolerance, straightness and surface condition, with a test report showing the heat analysis.
Frequently Asked Questions
Q: What is high-speed tool steel used for?
It is used for cutting tools such as twist drills, taps, reamers, end mills, milling cutters, gear hobs, broaches and saw blades, and for punches and dies that need wear resistance.
Q: Which grade is the most widely used?
M2 is the highest-volume grade because it combines good wear resistance with reasonable toughness and cost. T1 remains in use where maximum hot hardness is required.
Q: Does high-speed tool steel always contain cobalt?
No. Only the cobalt grades do. M35 and M42 add 4.75-5.75% and 7.75-8.75% cobalt respectively, which raises hot hardness for high-speed machining of difficult materials.
Q: What hardness can high-speed steel reach?
After correct austenitising, quenching and multiple tempering, cutting tools commonly reach about 63-66 HRC in M2, with slightly higher values possible in the cobalt grades.
Q: Can high-speed tool steel be joined?
Brazing, friction welding and electron beam welding are used in production, but fusion welding of a cutting edge is normally avoided because it destroys the hardened microstructure.
Q: How is high-speed tool steel supplied?
As annealed hot rolled or forged bars in round, square and flat sections, as peeled and ground bars for tool rooms, and in cut lengths or near-net shapes for specific tools.