Carbon steel that has been treated through quenching and tempering, whether it be high or medium, is referred to as hardened steel. When choosing the correct drill bit, hardened steel requires drill bits that can endure the strain of both the heat produced and the intensity of attempting to cut. There are many drill bits that will withstand the rigors of penetrating hard steel, all in different styles, sizes and materials.
The most common drill bits, ones found in home environments, are comprised of High Speed Steel. These bits are inexpensive, but thay have a tendency to break quickly or wear out from extended use. These are not good bits to use on hardened steel, as they are most effective on soft material, such as wood or plastic.
Coated drill bits are more useful when attempting to drill thin metals or woods that are tougher to penetrate. Often these bits are coated in titanium (either carbide or nitride), as this will greatly increase its lifespan, often doubling it.
Longevity and strength
Drill bits that are coated in carbide are very good for home use and are among the strongest available. However, this comes at a price, as carbide drill bits are more expensive. You get what you pay for though, as carbide drill bits stay sharper for longer. They are also the drill bits that are capable of drilling through stainless steel. Other drill bits are not able to handle the rigors of such a dense metal.
In fact, carbide drill bits will usually last a great deal longer, in home use, than any other drill bit. While they are more expensive at the outset, carbide drill bits are a better investment for any sort of home use (woodworking, etc) as a carbide bit will last an average of 25 times as long as a regular drill bit.
Another way to ensure the longevity of your drill bit is to use cutting lubricant. Not only will this help to avoid the heat generated from unlubricated friction, but it will ensure that your drill bit lasts longer. A drill be that is exposed to overheating is more likely to dull more quickly.
How to use
Not overexerting pressure on the drill in use is another way to extend the lifespan of your drill bit. The harder you push, the more friction is created and the more quickly your drill bit will dull. Let the drill do its job.
Once you have decided the coating of your bit, you must chose between the style of bit. Twist bits are the most common and are come in a variety of shapes, depending on the type of material you intend to drill. While the most common angle is 150 degrees, there are other varieties to suit your needs. Newer designs allow for the bit to obtain a better grip to avoid having to start a hole without the bit. This is referred to as a negative edge.
Centre bits are often used with hardened steel to start the hole, but this is often a mistake. The true use of this bit is to drill holes for a cylindrical grinder. While many people use a centre bit to start a hole, the spotting drill is truly the bit that should be used for such a purpose.