The Properties Of Steel
What Are They And Why They Should Matter To You
Why Are Knife Steel Properties Important to YOU?
Some Custom knife makers use a dozen different steel alloys to make knives, others only one. 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.
Did you know that there are numerous types of steel that can be used to make knives? Every steel formulation comes with its own set of properties, which are determined by the alloys used in the formulation. It's important to understand why steel properties matter, which ones should be improved, and what the costs of those improvements will be in relation to the overall quality of the steel. Ultimately, all of these factors can have an impact on the quality of your knife.
In general, each type of steel has its own unique set of desirable properties, with some being better at certain traits than others. These properties can include tensile strength, impact resistance (or toughness), hardness, wear resistance (edge retention), and corrosion resistance. However, these properties often conflict with each other, making it a challenge to improve one without negatively impacting another.
To enhance a specific property, certain elements can be added to the steel's original composition during production. For instance, adding carbon can increase hardness, while chromium can improve corrosion and wear resistance. Additionally, elements like tungsten, vanadium, niobium, and cobalt can enhance hardness and wear resistance. However, it's important to carefully balance the addition of alloying elements, as improving one property can often come at the expense of another. As a knife maker, some properties of knife steels matter more to me than they do to you, the knife owner.
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HARDENABILITY--
Steel, at its most basic level, is composed of iron and carbon. However, this simple composition has low hardenability, meaning that the steel will not fully harden if the cross section is too thick. To achieve full hardness, the steel must be rapidly quenched from its hardening temperature down to room temperature. For this reason, water or brine quench, or a "fast oil" engineered quenchant is needed, but this can result in cracked and broken blades due to the violent cooling process. Sometimes, the center of a thick piece of steel cannot be quenched fast enough to harden it. To solve this problem, steel makers add small amounts of chromium and/or manganese to improve hardenability, allowing the knife to be quenched in slower oil, which is a gentler and safer method of cooling steel quickly. As a result, knife makers can avoid the sickening "tink" of a blade failing in the quench.
HARDNESS--
The most commonly used strength test for knife steels is the Rockwell C hardness test. When a knife maker states that their knife is "61 Rockwell," they are referring to this particular scale. Hardness plays a crucial role in enhancing edge stability, abrasive wear resistance, and edge retention. This is possible because it provides the necessary strength to support fine edge geometries and thin, sharp edges. However, in the world of knifemaking, there is always a tradeoff to consider. The relationship between hardness (strength) and toughness (resistance to impact, chipping, and breakage) can be likened to the repelling poles of a magnet. If steel is too hard, it tends to become brittle, leading to microfractures, edge chips, or even catastrophic failure of the entire blade. On the other hand, if the blade is softer (less strong), it becomes more resistant to impact, but at the cost of wear resistance and edge retention. Thus, finding a balance between the two is crucial in knifemaking. One way to achieve this balance is by tempering the steel to its optimum hardness for knife use, which can help alleviate some of these issues.
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Toughness--
Steel's toughness is determined by its capacity to withstand stress and resist breaking, chipping, or cracking at a particular hardness level. Tougher steel displays greater resistance against impact, bending, and twisting forces. It can also take a finer edge without the risk of microfractures, to a reasonable extent. The Charpy Impact test is a widely-accepted scientific method for determining steel toughness. This test involves dropping a heavy pendulum on a standardized steel sample, measuring the force required to break the steel in Ft/Lbs of force.
EDGE RETENTION--
The quantity and hardness of carbides in the hardened matrix of a steel knife are crucial in determining its wear resistance and edge retention. The iron/carbon phase known as martensite has a maximum theoretical hardness based on carbon content, but it has a limited useable hardness before it becomes too brittle for practical use.
Carbides, which are formed when alloying elements are added to the original steel, play a significant role in increasing edge retention and wear resistance. Most alloying elements can join with carbon to create carbides that are considerably harder than the surrounding steel matrix. Chromium carbides, even though they are the softest, are significantly harder than the matrix holding them in place. The hardest carbides are Vanadium and Niobium carbides.
However, there is a tradeoff with carbides, as with all things knife and steel related. Carbides in significant volume, especially in large sizes, can make any steel less tough by creating crack initiation points that weaken the steel. Machinists often use carbide cutting tools that are very hard but also very brittle. Therefore, some carbide in knives is good, but not too much.
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SHARPENABILITY--
The term "sharpenability" refers to the ease or difficulty of achieving a proper edge on a knife made from a particular type of steel. This aspect is affected by wear resistance, as sharpening involves removing any material that doesn't contribute to a precise edge. If a steel is highly wear-resistant and contains a large amount of hard carbides, it may be difficult to sharpen without the use of diamond stones. On the other hand, simple carbon steels can be sharpened easily but may not have a long edge retention.
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CORROSION RESISTANCE--
Corrosion resistance is the ability of a steel alloy to withstand rust and other harmful forms of oxidation. Chromium is the primary element that provides stain resistance to knife steel, while nitrogen, nickel, and molybdenum also contribute to the cause. Generally, simple carbon steels have minimal to no corrosion resistance, whereas steel with approximately 13% chromium is referred to as "stainless." This is because free chromium in the steel reacts with oxygen, creating a chromium oxide layer on the surface of the steel that prevents further oxidation of the iron underneath. Although other elements also offer corrosion resistance, chromium is undoubtedly the most effective.
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SUMMARY--
When creating steel products, such as knives, it is essential to take into account the necessary properties for their intended use. Steel makers, metallurgists, and designers consider these factors carefully. As a knife maker, it is important to ask potential buyers about their intended use to ensure that the knife meets their needs. For instance, if the knife will be exposed to water for extended periods of time, stain resistance will be necessary. If the knife is primarily for breaking down cardboard, wear resistance is crucial. Field dressing animals requires both wear resistance and sharpenability. For kitchen cutlery, the steel must possess enough toughness and hardness to support a thin edge for optimal slicing. The selection of steel is a crucial element in knife design, as important as blade length, heat treating/tempering, and handle style. Choosing the right steel will help the design achieve its intended purpose.
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Visit the "The Steels" page to learn more.
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Thanks for reading,
Keith