Best Drill Bits for Hardened Steel: Cobalt and Carbide

Hardened steel is one of the most difficult materials to machine, with a hardness of HRC 45-65, compared to only HRC 10-25 for ordinary steel. 

Hardened steel is resistant to wear and high impact pressure, so ordinary HSS drill bits will soften and become unusable within seconds when drilling hardened steel. Read the following article to learn about high-quality drill bits for hardened steel.

Drill Bits for Hardened Steel 

There are two types of materials for drilling into hardened steel, cobalt drill bits and solid carbide drill bits, this selection introduces the characteristics of those two types of drill bits. 

Cobalt Drill Bits (M35 vs M42) 

Cobalt drill bits are the best bit for drilling into hardened steel. The key lies in the red hardness of cobalt drill bits. When drilling through hardened steel, the cutting edge temperature can reach 600-1000℃. Ordinary HSS drill bits for stainless steel will soften and become dull within seconds, but cobalt drill bits can still maintain a hardness of HRC 63-65 at 625℃ and a hardness of HRC 60+ at 650℃.

Cobalt drill bits come in two hardness levels: M 35 and M 42. M 35 drill bits contain approximately 5% cobalt, while M 42 drill bits contain approximately 8% cobalt. The higher the cobalt content in the drill bit, the higher its hardness, but the more brittle it is.

Comparison Item	M35 (5% Cobalt)	M42 (8% Cobalt)
Hardness	HRC 64-67	HRC 66-70
Hot Hardness (Red Hardness)	Maintains HRC 63-65 at 625°C	Maintains HRC 65+ at 650°C, can withstand higher cutting temperatures
Lifespan	Baseline (average 20 holes)	50% longer than M35 (average 30 holes)
Toughness	Better	Slightly more brittle, more prone to breakage than M 35, requires more secure clamping
Recommended Applications	Stainless steel, medium-hardness steel, DIY	Hardened steel, titanium alloys, high-temperature alloys, industrial-grade applications

Solid Carbide Drill Bits

Solid carbide drill bits are also the strongest drill bit material for drilling hardened steel. Tungsten carbide is a compound composed of carbon and metal. Compared to cobalt drill bits, solid carbide drill bits are much harder, reaching HRC 70-75, and can easily cut hardened steel with an HRC of less than 60. Solid carbide drill bits have extremely high red hardness, maintaining their hardness even at 1000℃.

Solid carbide drill bits have lower toughness than HSS. They are prone to breakage, more expensive, and are designed to be used on drill presses or CNC machines machining, making them suitable for industrial-grade precision machining.

How to Choose Between Cobalt and Solid Carbide Drill Bits for Hardened Steel

From the article above, you already know that cobalt drill bits and solid carbide drill bits are suitable for drilling through hardened steel. However, the two drill bits materials differ in hardness, toughness, cost, and applicable scenarios. This section will tell you which type of drill bits to choose:

1.Based on Equipment Type

For common equipment such as handheld drills and bench drills, we recommend that you choose cobalt drill bits. Cobalt drill bits have good toughness and are not easy to chip or break under slight vibration or unstable conditions, making them suitable for daily maintenance and DIY projects.

If you use CNC drilling machines daily, consider using solid carbide drill bits. The machine speed is stable, which can fully utilize the high hardness and wear resistance of solid carbide drill bits.

2.Material Hardness of Drill Bits for Hardened Steel

For steels with a hardness below HRC50, such as stainless steel, alloy steel, and pre-hardened steel, M35 or M42 cobalt drill bits are generally sufficient for machining requirements. When the material hardness reaches HRC 55 or higher, drilling generates higher cutting forces and heat. In this case, solid carbide drill bits can maintain a sharp cutting edge and reduce wear.

3.Select Based on Your Processing Volume

Your usage frequency determines the cost and lifespan of your drill bits. If you only need occasional repairs, cobalt drill bits are the most economical choice for home DIY projects.

For jobs requiring long-term machining or mass production, solid carbide drill bits are more durable.

4.Consider the Budget of Drill Bits

Budget is also a crucial consideration when choosing drill bits. M35 drill bits are relatively inexpensive and offer the best value for money for hardened steel; they are suitable if your machining requirements are not demanding. 

M42 drill bits are more expensive than M35 but offer better wear resistance and heat resistance, which makes them the most balanced choice between performance and cost. 

Solid carbide drill bits are the most expensive and are specifically designed for industrial-grade machining environments.

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FAQ

1.What RPM speed should I use when drilling hardened steel?

Generally, drilling hardened steel should be done at lower speeds. For small diameter drill bits (3-6mm), a speed of 100-300 RPM is recommended; for larger drill bits, the speed should be further reduced. Low speed and stable pressure help the drill continue cutting. Drill speed affects drilling results and drill bits’ life. 

Unlike wood and soft metals, drilling hardened steel generates heat, causing the workpiece surface to harden. If you are searching for the best way to drill stainless steel or hardened steel, remember one principle: low speed, high pressure, and continuous cutting.

2.What are common mistakes when drilling hardened steel? 

• Excessive drilling speed causes brick overheating when drilling hardened steel.

• Continuous force causes material surface hardening.

• Failure to use cutting fluid or lubricant.

• Using a worn drill bit.

3.Should I use cutting fluid when drilling hardened steel?

Yes, generally speaking, you should use cutting fluid when drilling hardened steel. If you are a professional user, we recommend using a professional metal cutting oil. For general maintenance work, light engine oil is enough. The main functions of cutting fluid include:

• Lowering drilling temperature.

• Reducing friction between the drill bits and the workpiece.

• Preventing premature drill bits wear.

• Improving drilling accuracy and surface quality.

• Reducing the risk of work hardening.

4.Can I use HSS drill bits on hardened steel?

When you ask what type of drill bit is for hardened steel, we generally recommend using cobalt drill bits or solid carbide drill bits. Ordinary HSS drill bits can also drill hardened steel in some cases, but they are inefficient and the drill bits are easily damaged.

5.Are titanium drill bits good for hardened steel? 

In fact, titanium drills are not made of titanium, but rather have a layer of titanium nitride coated on the surface of HSS drill bits. This titanium nitride coating helps dissipate heat when drilling hardened steel, reducing friction and extending drill bit life. However, the titanium coating does not increase the hardness of the drill bit itself, so cobalt and carbide drill bits remain more advantageous.

Final Thoughts

The article above provides comprehensive guidance for choosing drill bits. If you are looking for tools of cutting hardened steel, check out Diversitech's drill bits kits. We have extensive OEM/ODM experience and can customize tool kits to suit your target customers. Feel free to contact us!