Walk into any machining plant, and you’ll see high-speed steel tools such as drills, end mills, taps, and reamers.
If you ask a machinist which type of high-speed steel tool to use for machining, you might get different answers. Some insist on using M2 high-speed steel for all machining operations, some insist on using cobalt-containing high-speed steel, and some will tell you that powder metallurgy is the only option.
They’re not entirely wrong.They just have different priorities. So how do you choose the right material for your needs? This guide will detail the important factors, the grades you’ll actually encounter, and provide practical selection advice.
What is High Speed Steel?
High-speed steel got its name in the early 20th century when it was known for its ability to cut metal at speeds far exceeding those of ordinary carbon steel tools.The secret lies in alloying tungsten, molybdenum, chromium, and vanadium, sometimes with the addition of cobalt, combined with precise heat treatment processes to form a hard, tempered martensitic matrix rich in hard carbide particles.
The result is a tool material that stays hard even when the cutting edge gets hot. That property, called red hardness, is what distinguishes high-speed steel from ordinary tool steel.
However, not all high-speed steels are the same.Even minor changes in chemical composition can have a significant impact on the performance and cost of cutting tools.
Six Key Factors in Selecting High Speed Steel
1. Chemical Composition
Each high-speed steel (HSS) grade is defined by its formulation. The following are the roles of the main elements:
- Carbon (0.7–1.1%): Forms hard carbides, achieving martensitic hardening. Higher carbon content results in higher hardness, but also greater brittleness.
- Tungsten (W): Improves red hardness and wear resistance by forming tungsten carbide. T-series high-speed steels primarily rely on tungsten.
- Molybdenum (Mo): Similar in function to tungsten, but lighter and cheaper. M-series high-speed steels use molybdenum as the primary alloying element. Most modern high-speed steels belong to the M-series.
- Chromium (Cr, approx. 3.75–4.5%): Improves hardenability and corrosion resistance. Almost all high-speed steels contain approximately 4% chromium.
- Vanadium (V, 1–5%): Forms very hard vanadium carbide, which is the main factor in improving wear resistance. Higher vanadium content results in longer edge life, but also greater grinding difficulty.
- Cobalt (Co, 0-10%): Does not form carbides. Instead, it dissolves into the matrix, increasing tempering temperature and significantly improving red hardness. Cobalt-containing steels can achieve higher tempering temperatures and faster tempering rates, but are more expensive and slightly more brittle.
The ratio of these elements is what distinguishes M2 from M42 from T15.
2. Hardness vs. Toughness
This is the core trade-off in high-speed steel (HSS) selection.
Hardness (measured in HRC) determines the cutting edge’s resistance to deformation and wear. Harder tools retain their sharpness for longer. However, hardness comes at the cost of toughness, which refers to the tool’s ability to absorb impact without chipping or breaking.
M2 steel typically has a hardness between 62-66 HRC and offers good toughness. M42 steel contains 8% cobalt and can reach a hardness of 65-70 HRC, but it is more prone to chipping under interrupted cutting or heavy loads.
Toughness is more important if your machining involves impact loads, such as milling, broaching, or tapping on hard materials. Hardness is more critical if you are performing high-speed continuous cutting.
3. Red Hardness
Red hardness refers to the ability of steel to maintain its hardness at high temperatures. It is measured by the temperature at which the steel’s hardness drops to a specific value (typically 60 HRC).
- M2 steel can maintain its hardness at approximately 600°C.
- M35 steel (5% cobalt) can maintain its hardness at approximately 620°C.
- M42 steel (8% cobalt) can maintain its hardness above 640°C.
Why is this important? Because cutting generates heat. At high speeds or with insufficient coolant, the cutting edge temperature can easily exceed 500°C. If the steel cannot maintain its hardness at this temperature, the cutting edge will deform, the tool will wear rapidly, and ultimately, you get poor surface finish.
Cobalt is the most important factor affecting the hardness of steel. If you are cutting stainless steel, titanium, or other heat-resistant alloys, it is generally worthwhile to choose cobalt-containing high-speed steel.
4. Wear Resistance
Wear resistance determines how long a cutting tool remains sharp. It primarily depends on the content and type of hard carbide particles in its microstructure, especially vanadium carbides, which are harder than tungsten or molybdenum carbides.
High-vanadium steels (such as T15 with approximately 5% vanadium) offer excellent wear resistance, but these hard carbides also make the steel difficult to grind. High-vanadium steel can be inconvenient if you need to frequently resharpening your tools or machining complex geometries.
Powder metallurgy (PM) high-speed steel has changed this. By atomizing the steel into a fine powder before compaction, powder metallurgy steel can achieve a uniform, fine-grained microstructure with evenly distributed carbides. As a result, its wear resistance is comparable to or even better than that of conventional high-vanadium high-speed steel, while toughness and grinding performance are also significantly improved. Powder metallurgy steels (such as ASP 2030 or S390) have a service life 2-3 times longer than M2 steel, but they are also much more expensive.
5. Machinability and Grindability
This factor is often overlooked, but it directly impacts total cost.
High-speed steel tools almost always require grinding to their final shape. The easier the steel is to grind, the faster the tool can be manufactured or regrinded, and the lower the cost. High-vanadium and high-cobalt steels are more difficult to grind, requiring more expensive grinding wheels and slower feed rates.If you plan to grind your own tools or perform frequent regrinding, grindability must be taken into account when making your selection.
6. Cost
Cost is the real key factor. The following lists approximate cost ranges based on M2 cost.
- M35: Approximately 30-40% higher than M2
- M42: Approximately 60-100% higher than M2
- Powder metallurgy high-speed steel: 2-4 times or more higher than M2
The issue isn’t which grade is best, but which grade offers the lowest cost per unit. A more expensive tool that lasts three times longer and runs 30% faster is often the cheaper choice in high-production environments. But for a mill running small batches, M2 might be perfectly adequate and much easier on the budget.
International General HSS Grades & Characteristics
| Grade | Key Chemistry | Hardness (HRC) | Red Hardness | Best For |
| M2 | 6% W, 5% Mo, 2% V, no Co | 62–66 | ~600°C | General-purpose drilling, milling, tapping |
| M35 | M2 + 5% Co | 64–68 | ~620°C | Stainless steel, cast iron, harder alloys |
| M42 | 9.5% Mo, 8% Co, 1.2% V | 65–70 | ~640°C | High-speed production, hard materials, heat-resistant alloys |
| T1 | ~18% W, 1% V | 62–66 | ~600°C | Traditional heavy-duty tools, dimensional stability |
| T15 | ~12% W, 5% V, 5% Co | 65–68 | ~620°C | High-wear applications, fine blanking, forming tools |
| PM HSS (ASP/S390) | Varies, fine-grained | 64–70 | Excellent | Demanding precision machining, high-volume production |
How to Choose High-Speed Steel?
When choosing a grade, please answer the following five questions in order, and you will eventually find the answer.
1. What materials will you be machining?
Mild steel, aluminum, brass, cast iron → M2 may be sufficient.
Stainless steel, titanium, high-temperature alloys → Consider M35 or M42.
Hardened steel, abrasive materials, high-volume production → M42 or powder metallurgy high-speed steel (PM HSS).
2. What are your machining operations?
Intermittent cutting (milling, broaching, sawing) → Prioritize toughness. Avoid excessively high hardness.
Continuous cutting (turning, reaming) → Prioritize hardness and red hardness.
Constrained cutting (tapping, deep hole drilling) → Balance toughness and heat resistance.
3. What is your production volume?
Small batch, single-piece machining → M2 or M35. Tool cost is more important than tool life.
High-volume production → Choose M42 or powder metallurgy high-speed steel (PM HSS). Calculate part cost, not tool cost.
4. Can it be resharpened?
If you are grinding the tool yourself, consider its grinding performance. High-vanadium steel (such as T15) is more difficult to grind. Powder metallurgy high-speed steel has better grinding performance than traditional high-alloy high-speed steel.
5. What is your budget?
Be sure to honestly assess whether high-quality steel is truly worth the price. If you produce 10 parts per month, M2 steel is sufficient. If you produce 10,000 parts per week, the situation is different.
Choosing the right high-speed steel isn’t difficult, but it’s certainly not easy either.The right grade depends on your material, your operation, your volume, and your budget.
Start with M2. It is the default for a reason. When you run into a limitation,tools wearing too fast, edges deforming under heat, chipping under load ,then choose a steel grade that addresses that limitation.This isn’t about using the best steel, but rather the steel that allows for machining the part with the lowest cost and least hassle. Sometimes it’s M2, sometimes M42. Sometimes, when machining requirements are very high, powder metallurgy is also worthwhile.
Understand the various trade-offs, perform cost calculations, and make a decision based on the actual application.
