The Properties Of Steel
Why They Should Matter To You
Why Are Knife Steel Properties Important to YOU?
Custom knife makers use literally dozens of different alloys of steel to make knives, and their choice will affect your custom knife for its entire life. So you either trust your knifemaker, or question WHY they only use "Steel A" for the knives they make. It's important enough to know that you should ask!!
Do you know there are literally DOZENS of steels suitable for making knives, Every one of those knife steel formulations has a set of properties inherent to the alloys in that formulation. Like the question in the title, what are steel properties? Which ones should be enhanced, and why? What will the enhancement cost in the overall balance of the steel? How does all this affect my knife?
Generally speaking, each steel is relatively good at certain properties, and really good at others. Desirable properties such as tensile strength, impact resistance(toughness), hardness(strength), wear resistance(edge retention), and corrosion resistance are often at odds with each other. For example, higher hardness increases edge retention, but reduces toughness. Therefore it's always a balancing act to enhance some properties without adversely affecting others. This is achieved by the addition of certain elements into the original composition of the steel at the foundry. For example, increased carbon raises hardness, chromium increases corrosion and wear 0resistance, and the addition of elements such as tungsten, vanadium, niobium, and cobalt, generally improve wear resistance. But there is that balancing act to be played out with the addition of alloying elements, as improving one property often comes at the cost of another. Here are some properties of knife steels that matter most. Some matter more to you, the knife owner, and some to me, the maker!
Steel in its simplest form is nothing more than iron with the addition of carbon. This simplest composition has what is called "low hardenability". What that means is the steel will not fully harden through in very thick sections. This is because it must be quenched VERY rapidly from its hardening temperature to achieve full hardness. These simplest steels usually need a water or brine quench, and the result of this violently rapid cooling is a lot of cracked and broken blades. Every knifemaker comes to recognize the sickening "tink" of a blade failing in the quench. Additions of chromium and/or manganese are added to simple steels in small amounts to improve hardenability to the point where the knife can be quenched in oil, a far gentler and safer way to quench a steel.
The most common strength test for knife steels is the Rockwell C hardness test. When a maker says a knife "this knife is 61 Rockwell" it is this scale they are referring to. Hardness positively affects edge stability, wear resistance, and edge retention by having the strength to support fine edge geometries and thin, keen edges. But as always in knifemaking, there is a tradeoff. Hardness(strength) and toughness(resistance to impact, chipping, and breakage) are sometimes like magnets turned so the poles repel each other. If a steel is too hard it becomes brittle, causing microfractures, chips in the edge, or at worst a catastrophic failure of the entire blade. If the blade is softer(less strong), then toughness(impact resistance) increases dramatically at the cost of wear resistance and edge retention. So, as in many aspects of knifemaking, a balance has to be found. Tempering the steel to its optimum hardness for knife use helps to alleviate some of these issues.
Toughness is a measure of a steel's ability to resist breaking, chipping, or cracking under stress. So a tougher steel is better able to resist impact, bending, and twisting forces. A tougher steel can also take a thinner edge without danger of microfractures within reason. One of the standard scientific tests for steel toughness is the Charpy Impact test, which drops a heavy pendulum on a sample of standardized size. The impact breaks the steel and measures the force required to do so in Ft/Lbs of force.
The primary controlling factor of wear resistance and edge retention is hardness of the constituents in the steel. The hardened matrix of a knife, the iron/carbon alloy called martensite, has a maximum theoretical hardness based on carbon content. More importantly, martensite has a limited USEABLE hardness before it becomes too brittle for practical use. This is where carbides come into play. Carbides are formed when elements, including iron and carbon are alloyed into the original steel. The majority of alloying elements can join with carbon and even the iron to form CARBIDES. Carbides contribute greatly to wear resistance and therefore edge retention because they are significantly HARDER than the surrounding steel. As far as edge retention goes, the more carbides the better. Even the softest of them, the chromium carbides, are significantly harder than the hardened steel matrix holding them in place, with Vanadium and Niobium carbides being the hardest. But yet again, there is another tradeoff. Carbides in great volume, and especially in large sizes, make any steel LESS TOUGH. Large carbides create crack initiation points. Steels heated at too high a temperature for too long a time experience grain and carbide growth. Both of these conditions reduce the toughness of steels. It has been found that small additions of Vanadium help keep grain growth to a minimum. Proper heat treating protocols also minimize the risks of grain and carbide growth.
"Sharpenability" is used to describe how easy or hard it is to put a proper edge on a knife. As you might imagine, wear resistance reduces sharpenability, because sharpening is really nothing more than abrading away any material that doesn't form a more or less perfect edge. Extremely wear resistant steels with a large percentage of carbides reduce sharpenability to the point where diamond stones are necessary to abrade the material. Simple carbon steels are easily sharpened, but have minimal edge retention.
Simply put, this is the ability of a steel to resist rust and other damaging forms of oxidation. Chromium is the principal element that bestows corrosion resistance on a knife steel. Nitrogen, nickel, molybdenum also contribute. Simple carbon steels have little to no corrosion resistance, while generally a steel with around 13% chromium can be called "stainless". This happens because free chromium in the steel reacts with oxygen to form a layer of chromium oxide on the surface of the steel, thus preventing further oxidation.
Steel makers, metallurgists, and designers take all these properties into account, or at least the ones applicable to a given steel's intended purpose. Knife makers have considerations too. That's why we should ask a potential buyer "how do you intend to use this knife?" If your knife is going to be wet for long periods, stain resistance will be important. Primarily for breaking down cardboard? Wear resistance will be crucial. Field dressing animals? Wear resistance and sharpenability are important. Kitchen cutlery? Enough toughness and hardness to support the thin edge geometry that will make a truly memorable slicer! Selecting the proper steel is as important a decision in knife design as blade length, handle style, or any other design element in consideration. chosen wisely, the steel for your knife will help the design do what you envisioned it to do!
Visit the "The Steels" page to learn more.
Sign Up For The Keith Nix Knives Newsletter! Click HERE!
Order Your Custom Knife from Keith Nix Knives Shop Now!
Thanks for reading,