A lot can change in 2.5 million years. Take knives, for example. Our early ancestors crafted knives by chipping and flaking off bits of rock to make an edge. On occasion, they attached those tools to a handle.
Naturally, man eventually discovered new and better materials for knives. As technology improved, knives improved. Stone became copper. Then bronze, an alloy of copper and tin. Iron moved into the line-up. Then steel, an alloy of iron and carbon. In some parts of the world, exotic materials, like meteorite, even got pounded into blades. When purchasing a knife, you want to make an informed decision, whether you’re planning to use the tool for survival, utility or protection. In the following story, we’ll provide the details you need to know.
KNIFE STEEL TERMS
When looking at different characteristics for knife blades, you may have to balance several different attributes. For example, improving one attribute usually deteriorates another. It is also important to keep in mind that no single knife steel does everything. Typically, steels made to hold an edge become more brittle. Steels made to take impact often have less corrosion resistance. Sometimes it makes sense to carry more than one knife (a beater and a biter). With that in mind, following are the properties of the steels and their descriptions.
This may be the most common one you hear. Hardness in knife steels correlates to strength. It is the ability to resist deforming and is measured using the Rockwell C scale (HRC). Most knife blades fall in the 56 to 60 range. The harder it is, the harder it is to sharpen.
This is used to describe the ability to resist damage like chips and cracks. A chipped blade is not easy to fix. Hardness and toughness are directly opposite of each other. In general, the harder the steel, the less tough (or more brittle) it is likely to be. High-carbon steels offset this balance by sacrificing some corrosion resistance.
This is abrasion (and also adhesion) resistance. This generally correlates with hardness. The harder the steel, the better it wears, although alloying materials do contribute greatly to this property.
This is the ability to resist rust caused by the environment, such as humidity, moisture and salt. This property has the highest effect on overall edge performance as the alloying material (chromium) can reduce toughness and the ability to hold a sharp edge. A true stainless blade has a chromium level of at least 14%. D2 is an example of a semi-stainless blade at 12% chromium, while something like M4 falls far from stainless at 4% (often referred to as tool steel or high-carbon steel).
Finally, the property that’s inherent in the tool itself, edge retention, is the ability of the knife edge to retain its sharpness when used. This is a balance between the other properties and determining how you will use the knife. That will determine which properties are important in your choice of blade materials.
Interestingly, the edge is composed of tiny, very hard glass-like structures called carbides that are exposed during sharpening to create a microscopic “forest” of teeth. A 3D cross-section image of this structure looks like pick-up sticks. For the best cutting edge, modern steels attempt to create a balanced product with the most non-intersecting carbine. This leads us to the next topic.
GENERAL STEEL TYPES
Steel used for knife blades falls into three general categories: Tool steel, carbon steel and stainless steel.
This steel is the hardened alloy steel that’s used in your cutting tools, your wrenches and yes, the blades in your pocketknives. Molybdenum, tungsten and other alloying elements give this steel strength, hardness and resistance to wear.
This type of steel is named for the amounts of carbon in the alloy (low, medium or high). Carbon hardens, so blades for hard use, such as axes and machetes, are made of this steel. High-carbon steel is also often used for knives, as it holds an edge very well and is easy to sharpen. The drawback is that knives made of this material require more care, as they don’t resist corrosion as well as stainless steel.
This steel is one of the most popular choices for knife blades. The addition of at least 14% chromium helps this steel resist corrosion. However, they do stain in certain environments, do not hold as sharp an edge as high-carbon steel, and can be difficult to sharpen. Note that some will argue that most modern blade steels do hold a superior edge to high-carbon steels and hold it longer.
STEEL ALLOYS: The Manufacturing Process
Early ironworkers created steel by heating iron and charcoal (a source of carbon) in clay boxes for several days. Historically, mixing the molten materials creates steel. The molten steel is cast into an ingot and then rolled out and shaped (while hot). These steels are then heat-treated to harden, and tempered (further heating the steel at lower temperatures) to decrease brittleness and improve ductility and toughness.
“Hardness in knife steels correlates to strength. … Most knife blades fall in the 56 to 60 range.”
The modern method of making steel alloys is called powder metallurgy. The molten steel is atomized into liquid nitrogen in which the steel is instantly frozen. The powder is then cleaned, sorted by size (the larger particles and contamination removed) and the ideal powder sintered (heated to just below the melting point) in a hot, high-pressure press to solidify the steel. Alloy blades made this way have excellent edge-holding capabilities. Most recently, many modern steels improve on this process by creating powered steels in cryogenic chambers.
Finally, we have exotic materials. Some non-steels, such as ceramic and titanium, are also used for knife blades, as are artisanal Damascus and san mai, which are “folder” or laminated steels.
Ceramic blades are made by the firing and pressing of powdered zirconium dioxide. Their edge retention is amazing, and they do not corrode, but brittleness is their drawback. Plus, they can more easily chip and break.
Titanium is a metal generally regarded as an exotic material for knife blades. Stronger, lighter and corrosion-resistant, knives made of titanium can be also pricey, but they are a popular choice in dive knives. They hold a great edge, but they generally have to have the edge retooled rather than sharpened by traditional means.
“WHEN PURCHASING A KNIFE, YOU WANT TO MAKE AN INFORMED DECISION, WHETHER YOU’RE PLANNING TO USE THE TOOL FOR SURVIVAL, UTILITY OR PROTECTION.”
Makers create Damascus steel, a patterned steel, by hammer-welding strips of steel and iron followed by repeated heating and forging. Today, most makers choose two to three alloys that work well together to make their patterned steel. The welding of different alloys mixes the qualities of those alloys together, and these different alloys can aid in the abrasive, sawing action of the blade, acting like a micro-serration. Often though, these blades are collector’s items.
Most true Damascus is high-carbon steel and is more prone to rust. In recent years, companies such as Damasteel have created stainless powdered Damascus to better fit the needs of the daily carry user.
San mai is a Japanese term for three layers. To create san mai, you create a “steel sandwich,” welded together through hammer-welding, much like Damascus steel, but without the repeated folding. Often, san mail consists of a high-carbon (harder) core sandwiched between two lower carbon (softer) outer layers, creating a strong yet flexible weapon for the samurai or high-end chef’s knife.
There are now steels that fall into the “super steel” category. Super steels don’t consist of just powdered steels but next-gen steels like M390, S90V and 20CV.
WHICH STEEL IS BEST?
There isn’t a “best” steel for a knife blade. Instead, select a knife for its application. Consider the intended use of your knife. Material choice in a blade depends on the usage.
While high-carbon steel is extremely sharp, you have to be meticulous in keeping it clean and dry. Plus, you will want to oil it often. If it’s going to be your diving knife, corrosion-resistance is going to be paramount. Do you do a lot of trailblazing each summer, hacking away undergrowth to find the ultimate camping spot? In this case, toughness is what you need. If this is your EDC (everyday carry) knife and you use it for tasks like opening packages, cutting the errant thread of a shirt, or whittling a branch, you might want a blade that holds an edge well but you can easily sharpen.
“Educate yourself on the types of blade materials available and then handle the knife.”
Don’t get caught up researching the best steel type for the application, either. Modern steel makers create proprietary alloy blends, and knife companies manipulate the steel they use, hardening steel by heat treatment and tempering, and sometimes using specialty finishes to impart the qualities they want in the blades. Thus, a blade made of the same steel by two different knife companies can have quite different properties, especially in the ease of sharpening. Fortunately, most knifemakers state the qualities of each model on their website or in their catalog.
In reality, most modern steels perform well enough that a novice knife buyer should pay more attention to how the knife handles and its intended use. The shape of the blade (and handle) also play large roles in determining a knife’s performance. Educate yourself on the types of blade materials available and then handle the knife. It should fit and feel good in your hand as you pantomime the movements for which you’d use it. And then sit back and enjoy the knife … and think about how far technology has come.
Blade Coatings and Finishes
The earliest forms of coating were “bluing.” This is a process that helps guard against corrosion. If done well, it can also produce a great color pattern.
The reasons for coating blades these days are many: better performance, looks or for the properties of the coating itself. Black-and-earth colored Teflon coatings exist for military applications, and some powder coats create better textures for specialty work or better corrosion resistance.
Special thanks to Nolan Kidwell of Cascadia Cutlery LLC for his technical editing and contribution. Find his company on the web at CascaidaCutlery.com or call (503) 505-2567. He is also very responsive by email at email@example.com.