In the knife industry, different types of steel are created by varying the types of additive elements as well as how the blade is rolled and heated (i.e. the finishing process).
Ultimately, the different types of steel used in knife blades each exhibit varying degrees of these five key properties:
Hardness is the ability to resist deforming when subject to stress and applied forces. Hardness in steel knife is directly correlated to strength and is generally measured using the Rockwell C scale (aka “HRC”).
Testing for hardened steel.
It is not easy to determine if the steel has undergone the hardening and tempering process by simply looking at it, but there is a reliable and simple test. To examine a piece of steel, obtain a hand file and file an edge of the selected metal. If the piece of steel has not undergone the hardening process, the metal file should easily 'bite' into the sample. If the metal has been hardened, the file fails to cut into the sample and glances off with little visible effect.
Toughness is the ability to resist damage like cracks or chips when subject to impact or “sudden loads”. Chipping is a knife’s worst enemy and never easy to fix. There are a number of different ways to measure toughness (i.e. Charpy, Izod) thus it’s less standardized than hardness when it comes to knives. In general, the harder the steel the less tough it’s likely to be.
Wear resistance is the steel’s ability to withstand damage from both abrasive and adhesive wear. Abrasive wear occurs when harder particles pass over a softer surface. Adhesive wear occurs when the debris is dislodged from one surface and attaches to the other. Wear resistance generally correlates with the steel’s hardness but is also heavily influenced by the specific chemistry of the steel. In steel of equal hardness, the steel with larger carbides (think microscopic, hard, wear-resistant particles) will typically resist wearing better. However, carbides can become brittle and crack, thus decreasing toughness.
Corrosion resistance is the ability to resist corrosion such as rust caused by external elements like humidity, moisture, and salt. Note that high resistance to corrosion does involve a sacrifice in the overall edge performance.
Edge Retention represents how long the blade will retain its sharpness when subject to periods of use. It’s what everyone talks about these days but unfortunately, the measurement of edge retention lacks any defined set of standards and so much of the data is subjective.
Unfortunately the “best knife steel” is not simply a case of maximizing each of the properties above….it’s a trade-off. The biggest trade-off is balancing strength or hardness with toughness. Some blades can be made to be exceptionally hard but will chip or crack if you drop them onto a hard surface. Conversely, a blade can be extremely tough and able to bend but will struggle to hold it’s the edge. Basically, the stuff that makes steel strong (high amount carbon/carbides) generally lowers the toughness. Also, note that the term ‘stainless steel‘ is generally misleading as almost all types of steel will show some kind of discoloration if left exposed to the elements for long enough. By knowing how you plan to use the knife you will generally be able to determine the best steel for your situation.
Common Knife Steel Types
The most common blade steel types generally fall into the following categories:
- Tool Steel – primarily hard steel alloys used in cutting tools. Some popular steels in this group include D2, O1 and Crucible’s CPM series (i.e. CPM 3V) plus more advanced high-speed steels like M4.
- Carbon Steel – generally made for rough use where toughness and durability are important. Common in survival knives and machetes. They take a sharp edge and are relatively easy to re-sharpen. The trade-off is being more prone to corrosion given the low chromium content. The most popular carbon knife steel is 1095.
- Stainless Steel – basically carbon steel with added chromium to resist corrosion and other elements that increase performance levels but usually at the expense of inferior toughness. Easily the most popular category today for EDC knives and includes the 400, 154CM, AUS, VG, CTS, MoV, Sandvik and Crucible SxxV series of steels. Note that to qualify as true stainless steel there must be at least 13% chromium.