Steel Elements

In its most basic form, steel consists of iron and carbon. Modern steels, however, are typically much more sophisticated than that. By adding other elements, or alloys, steel makers have learned to enhance specific aspects of their performance for different applications.

Most modern knife steels are the result of a sophisticated, carefully balanced recipe of alloys. Each of these alloying elements influences the key properties of the steel, such as wear resistance, toughness, corrosion resistance, strength, and ease of sharpening. Understanding the performance potential of various steels, therefore, must begin with sound knowledge of the individual elements and the effects they create.

Spyderco’s Edge-U-Cation® program has been an integral part of our catalog, website, and other marketing materials for decades. It provides our customers with concise, easy-to-understand information that greatly simplifies complex topics, such as the metallurgy of blade steels. One of the cornerstones of this program is our steel brochure—a four-page document that presents the topic of knife steels in layman’s terms that everyone can understand. It also provides brief explanations of the most commonly used alloy elements and their effects when added to steel.

The interactive Steel Chart on the Spyderco website also includes this same information. When viewing the alloy composition of a steel or using the tool to compare two or more steels, simply hover over the name of the element to see a pop-up window with a brief explanation of that alloy’s effects on a steel’s properties. Remembering the complex details of steel elements is challenging. Spyderco’s Edge-U-Cation tools help you tackle that challenge by keeping key information at your fingertips and presenting it in a concise and easy-to-understand format.

Carbon (C)

• Increases edge retention and raises tensile strength.
• Increases hardness and improves resistance to wear and abrasion.

Chromium (Cr)

• Increases hardness, tensile strength, and toughness.
• Provides resistance to wear and corrosion.

Cobalt (Co)

• Increases strength and hardness, and permits quenching in higher temperatures.
• Intensifies the individual effects of other elements in more complex steels.

Copper (Cu)

• Increases corrosion resistance.

Manganese (Mn)

• Increases hardenability, wear resistance, and tensile strength.
• Deoxidizes and degasifies to remove oxygen from molten metal.
• In larger quantities, it increases hardness and brittleness.

Molybdenum (Mo)

• Increases strength, hardness, hardenability, and toughness.
• Improves machinability and resistance to corrosion.

Nickel (Ni)

• Adds strength and toughness.

Niobium (Nb)

• aka columbium. Improves strength and toughness.
• Provides corrosion resistance.
• Improves grain refinement and precipitation hardening

Nitrogen (N)

• Used in place of carbon for the steel matrix. The Nitrogen atom will function similarly to the carbon atom, but offers unusual advantages in corrosion resistance.

Phosphorus (P)

• Improves strength, machinability, and hardness.
• Creates brittleness in high concentrations.

Silicon (Si)

• Increases strength.
• Deoxidizes and degasifies to remove oxygen from molten metal.

Sulfur (S)

• Improves machinability when added in minute quantities.

Tungsten (W)

• Adds strength, toughness, and improves hardenability.

Vanadium (V)

• Increases strength, wear resistance, and toughness.

Steel Production

The world of steel is as fluid as molten metal. It is ever-evolving. Steel, as a matter of opinion, is very subjective as it relates to knives and knife knuts. There is no clear-cut answer as to which steel is the best. We have different requirements and preferences.

Our hope is that this guide will help you understand the world of steel a bit better and perhaps assist you in better defining what your own preferences are and why. A word of caution, this information is not intended to be all-inclusive, nor could it ever be.

We at Spyderco, just like other people, gravitate towards superior products. We are committed to using the best materials available at the time. As the world of steel evolves, so do our products. There are over 3,000 different types of steel, each with its own positive and negative attributes. In order to determine your own preferences, it is perhaps best to first understand the history of steel and how it is made.

Although an exact date of discovery is not known, humans have been forging steel for as long as they've been working iron. Ironworkers learned to make steel by heating wrought iron and charcoal (a source of carbon) in clay boxes for a period of several days. By this process, the iron absorbed enough carbon to become a true steel.

Iron by itself is a relatively soft metal; it does not hold a good edge. However, if you add Carbon, it hardens the iron, making steel. Steel has proven to be ideal for making edged weapons.

At a very simplified level, making steel is like baking a cake. You follow a precise recipe to achieve the type of cake (steel) that you desire. You begin with flour (iron) and then add various ingredients (elements). These additional ingredients will determine what type of cake (steel) you end up with. Once you have added all of the additional ingredients (elements) you are left with a batter that is ready to bake (heat treat). Baking (heat treating) is just as much a part of the "recipe" as the ingredients (elements). If not done properly, several properties can suffer. Once baked, you have a new – completely different – finished product. Your cake will forever be a cake; it can never go back to being batter. Of course, steel can be remelted to a molten state, but that simply is the beginning of becoming a new type of steel.

Steel is an alloy of iron and carbon, just as bronze is an alloy of copper and tin. Historically, steels have been prepared by mixing the molten materials. Alloying elements are melted and dissolved into molten iron to make steel. The molten steel is cast into an ingot, which is then rolled out (while it is still hot) and shaped much like you would roll out cookie dough. As the steel begins to slowly cool below the critical temperature, things start to happen inside the steel. At these elevated temperatures, alloying elements can move around in the steel, or diffuse. Different elements diffuse at different rates; typically, the larger the atom, the slower it diffuses. If the alloying contents are too high for some elements to assimilate with, the excess will separate or segregate out of the steel and form inclusions or possibly combine with another element to form large, undesirable carbides. These diffusional processes are also controlled by the austenite grain size of the steel – grains are little packets of specifically oriented crystals. Grain boundaries act as barriers to diffusion; the smaller the grains, the more boundaries, and the slower the steel. This limits the performance capabilities of the steel in both corrosion resistance and wear-resistant carbide formation.

More recently, Powder Metallurgy has become the chosen method of preparation. The difference in the processing of powdered metal allows for steel chemistries that are not possible with traditional steelmaking practices. The process begins the same as that of wrought steels – alloying elements are added and dissolved into molten iron. Then comes the main difference. The molten steel is atomized (misted into microscopic droplets) into liquid nitrogen, where the steel is instantly frozen, leaving no time for diffusional processes. The chemistry of the resulting powder is identical to that in the molten vat. Additionally, there are no inclusions or large carbides that form. The austenite grain size is the size of the powder at the very largest, which is small. The powder is then cleaned and sorted by size, and then the remaining ideal powder is sintered in a hot isostatic press to solidify the steel. Sintering is heating the steel to a temperature just below its melting point, and then pressing it together at high pressures to solidify or remove the voids between powder spheres. This allows for drastic changes in the steel chemistry, namely in carbon and vanadium. A larger volume of highly wear-resistant vanadium carbides forms upon heat treatment. Since Vanadium has a greater propensity to interact with carbon and form carbides than it does with Chromium, most of the excess carbon is utilized in the formation of vanadium carbides. These leave the Chromium free to help keep the steel corrosion resistant. The result is a premium steel product with exceptional wear resistance and good corrosion resistance properties.

Heat treating the steel to its critical temperature allows the carbon atoms to enter into the crystalline molecules of the iron, which have expanded due to the heating. Quenching the steel at this point causes the molecules to contract, trapping the carbon atoms inside. More specifically, the process of hardening steel through heat treatment involves heating the steel to a temperature at which austenite is formed. Austenite has the property of dissolving all the free carbon present in the steel. Quenching is then used to "freeze" the austenite changing it to martensite. These treatments set up large internal strains in the steel; these are relieved by tempering (further heating the steel at lower temperatures). Tempering the steel decreases the hardness, strength, and brittleness. It, however, increases the ductility and toughness.

Steels are classified according to the elements used in their production. These classifications are Carbon Steels, Alloy Steels, High-Strength Low-Alloy Steels, Stainless Steels, Tool Steels and Exotic Steels (non-steel).

 

Properties of Steel

Alloy
A material that is dissolved in another metal in a solid solution; a material that results when two or more elements combine in a solid solution.

Alloy Steels
Have a specified composition, containing certain percentages of vanadium, molybdenum, or other elements, as well as larger amounts of manganese, silicon, and copper than do regular carbon steels.

Austenetized
The basic steel structure state in which an alloying element is uniformly dissolved into iron.

Carbon Steels
Contain varying amounts of carbon and not more than 1.65% of manganese and .60% of copper. There are 3 types of Carbon Steels, Low (.3% or less), Medium (.4-.7%) and High (.8% and up). High carbon steel is commonly used for knives.

Corrosion Resistance
The ability of a material to resist deterioration as a result of a reaction to its environment. Provided by the elements Chromium (Cr), Copper (Cu), Molybdenum (Mo), and Nitrogen (N).

Critical Temperature
The temperature at which steel changes its structure to austenite in preparation for hardening.

Ductility
The ability of a material to be stretched or drawn plastically and deform appreciably before fracturing. Provided by the element Manganese (Mn).

Edge Retention
The ability of a material to resist abrasion and wear. Provided by the elements Carbon (C), Chromium (Cr), Manganese (Mn), Nitrogen (N), and Vanadium (V).

Exotic Steels
Are generally accepted as steel, but by definition are not steel. Examples of Exotic Steels include H1, ZDP-189, Talonite, and Titanium. There is an old proverb, "There was never a good knife made of bad steel." This statement, just like steel itself, is entirely subjective as it relates to knives and knife knuts. We hope this information provides you with a foundation to make your own determinations where steel is concerned.

Grit
The physical size of the austenite grains during austenizing. The actual size can vary due to thermal, time, and forging considerations.

Hardenability
The ability of steel to be hardened by a heat-treating process. Provided by the elements Manganese (Mn), Molybdenum (Mo), and Tungsten (W).

Hardness
The resistance of a steel to deformation or penetration analogous to strength.

Heat Treating
Heating and cooling metal to a prescribed temperature, and the limits for the purpose of changing the properties and behavior of the metal.

High-Strength Low-Alloy Steels
Known as HSLA steels are relatively new. They cost less than regular Alloy Steels because they contain only small amounts of the expensive alloying elements. They have been specially processed, however, to have much more strength than Carbon Steels of the same weight.

Impact Strength
The ability of a material to resist cracking due to a sudden force.

Martensite
A very hard and brittle steel with a distorted body-centered tetragon crystal structure.

Precipitation
The separation of a substance that was previously dissolved in another substance.

Quenching
Soaking steel that has reached a high temperature (above the recrystallization phase) in a medium of air, liquid, oil, or water to rapidly cool it. Quenching steel creates martensite.

Rockwell Test
A measurement of steel hardness based on the depth of penetration of a small diamond cone pressed into the steel under a constant load.

Stainless Steels
Contain a minimum of 12% Chromium. Chromium provides a much higher degree of rust resistance than Carbon Steels. Various sources cite differing minimum amounts of Chromium required to deem a steel as stainless (10-13%). It is important to note that the amount of Chromium required can depend on the other elements used in the steel.

Tempering
Reheating to a lower temperature after quenching for the purpose of slightly softening the steel, precipitating carbides, and stress relieving.

Tensile Strength
Indicated by the force at which a material breaks due to stretching.

Tool Steels
Contain Tungsten, Molybdenum, and other alloying elements that give them extra strength, hardness, and resistance to wear.

Toughness
The ability of a material to resist shock or impact.

Yield Strength
The point at which a steel becomes permanently deformed; the point at which the linear relationship of stress to strain changes on a Stress/Strain curve.