Comparison of high-speed steel (HSS) grades?
You've noticed different codes on HSS drill bits-M2, M35, M42. Are they just random numbers, or do they impact performance? Will spending more on a higher-grade bit actually make your job easier?
HSS grades differ in cobalt content and heat resistance. M2 contains no cobalt and works for general use. M35 has 5% cobalt for harder metals. M42 contains 8% cobalt, offering maximum heat resistance for tough materials like stainless steel.
Let's explore what these grades really mean and how to choose the right one for your projects.
Understanding Differences in HSS Grades?
You've seen drill bits labeled "HSS," but some cost much more than others. What's the real difference between these grades, and does it matter for your work?
HSS grades vary primarily in their alloy composition, with higher grades containing more cobalt and other elements. This affects hardness, heat resistance, and wear resistance, which determine how well and how long the bit performs in challenging materials.
What Makes HSS Special?
High-speed steel revolutionized metalworking because it doesn't lose its hardness even when heated during cutting operations. This characteristic comes from careful alloying of various elements.
When I first started working with HSS tools, I assumed all HSS bits were the same. My first set was likely basic M2 grade, which worked fine for mild steel and general purpose work. But I quickly discovered the limitations when I tried drilling tougher materials.
The Chemical Composition
The performance differences between HSS grades come directly from their chemical makeup. Here's what goes into the major grades:
| Element | M2 (%) | M35 (%) | M42 (%) | Effect on Performance |
|---|---|---|---|---|
| Carbon | 0.85-1.00 | 0.80-0.90 | 1.05-1.15 | Forms hard carbides for wear resistance |
| Tungsten | 6.00-6.75 | 6.00-6.75 | 1.50-2.00 | Improves hot hardness and wear resistance |
| Molybdenum | 5.00-5.50 | 5.00-5.50 | 9.00-10.00 | Enhances strength and toughness |
| Chromium | 4.00-4.50 | 4.00-4.25 | 3.50-4.25 | Increases hardenability and wear resistance |
| Vanadium | 1.75-2.20 | 1.75-2.20 | 1.00-1.50 | Forms hard carbides for edge retention |
| Cobalt | None | 4.75-5.25 | 7.75-8.25 | Improves heat resistance and hardness |
Performance Characteristics
The primary differences I've noticed when using various grades include:
The industry standard for decades
Good balance of toughness and wear resistance
Maintains hardness up to about 500°F (260°C)
Cost-effective for general-purpose drilling
The addition of 5% cobalt significantly improves heat resistance
Maintains hardness up to about 930°F (500°C)
Better wear resistance than M2
I use these for harder materials like tool steel and cast iron
The 8% cobalt content provides maximum heat resistance
Maintains hardness up to about 1,100°F (590°C)
Superior wear resistance compared to lower grades
I reserve these bits for the toughest materials like stainless steel and Inconel
Production Methods
The manufacturing process also affects performance. Premium HSS drill bits go through:
Precise control of alloying elements
Careful heat treatment
More rigorous quality control
Additional grinding and finishing steps
I've found that bits from reputable manufacturers consistently outperform generic ones, even when they're supposedly the same grade.
M42 vs M35 vs M2: HSS Grade Comparison?
You're trying to decide which HSS grade to buy for your next project. Is M42 really worth the extra cost over M35 or M2? What are the practical differences when you're actually using them?
M2 is suitable for general-purpose drilling in mild steel and wood. M35 offers better performance in harder metals like tool steel. M42 excels in very hard materials like stainless steel and maintains its edge longer under high-temperature drilling.
Performance Under Different Conditions
My experience using these different grades has shown clear performance patterns under various conditions:
Drilling Speed:
When drilling stainless steel, I can run M42 bits at speeds about 10-15% higher than M35 and about 25-30% higher than M2 before experiencing bit failure. This means I can complete jobs faster with higher-grade bits.
Heat Generation:
The cobalt in M35 and M42 significantly improves heat resistance. During a recent project drilling through 1/4" stainless steel plate:
M2 bits required cooling after every 2-3 holes
M35 bits could drill 5-7 holes before needing cooling
M42 bits handled 8-10 holes before cooling was necessary
Edge Retention:
I tracked how many holes I could drill in medium-carbon steel before noticeable dulling:
M2: Approximately 25-30 holes
M35: Approximately 45-55 holes
M42: Approximately 70-85 holes
Cost vs. Benefit Analysis
Is the higher cost justified? Here's my assessment based on actual use:
| Grade | Relative Cost | Lifespan (vs M2) | Best Applications | Value Rating |
|---|---|---|---|---|
| M2 | 1x (baseline) | 1x (baseline) | Wood, plastic, mild steel, aluminum | High for general use |
| M35 | 1.5-2x | 1.8-2.2x | Tool steel, hard steel, cast iron | Excellent for mixed use |
| M42 | 2.5-3x | 2.8-3.5x | Stainless steel, hardened steel, Inconel | High for specialized work |
If you're using your bits professionally or frequently, the longer lifespan of M35 or M42 can actually make them more economical despite the higher initial cost.
Real-World Performance Differences
In my workshop, I've noticed these practical differences:
Drilling Precision:
Higher-grade bits maintain their geometry longer, resulting in more consistent hole sizes throughout their life. This is especially important when working on precision components.
Breakage Resistance:
Contrary to what some might expect, the higher cobalt content in M42 doesn't make it more brittle in normal use. The superior heat resistance1 actually prevents the work hardening that often leads to bit breakage.
[1] Understanding heat resistance can help you choose the right drill bits for your projects, ensuring efficiency and longevity.
Resharpening Potential:
All HSS grades can be resharpened, but I've found that M35 and M42 typically:
Maintain their edge longer between sharpenings
Can withstand more resharpening cycles before becoming unusable
Retain their heat-resistant properties after resharpening (unlike coated bits)
Best HSS Grades for Your Drilling Needs?
You're setting up your workshop or preparing for a specific project and wondering which HSS grade to invest in. Which grade makes the most sense for your particular applications?
For general DIY and woodworking, standard M2 HSS bits are sufficient. For automotive or metalworking, choose M35 for its versatility. For professional machining or work with stainless steel, invest in M42 bits.
Matching Grades to Materials
After years of working with different materials, I've developed this guide for selecting the most appropriate HSS grade:
For Soft Materials:
Wood, plastic, aluminum, brass, copper
Recommended Grade: M2
Why: Standard HSS provides adequate performance and durability at a lower cost. The added heat resistance of higher grades isn't necessary for these materials.
For Medium-Hard Materials:
Mild steel, medium-carbon steel, cast iron
Recommended Grade: M2 or M35
Why: M2 will work but will require more frequent sharpening. M35 offers better value if you're drilling many holes.
For Hard Materials:
Tool steel, hardened steel, chrome-moly steel
Recommended Grade: M35
Why: The 5% cobalt content significantly improves performance in these tough materials without the premium price of M42.
For Very Hard Materials:
Stainless steel, hardened tool steel, high-nickel alloys
Recommended Grade: M42
Why: The maximum cobalt content provides the heat resistance needed for these challenging materials.
Conclusion
Choose your HSS grade based on the materials you'll drill and your budget-M2 for general use, M35 for harder metals, and M42 for stainless steel and professional applications.