Saturday, January 07, 2012

Steel Iron Metal

By definition, steel is a combination of iron and less than 2 percent carbon. For centuries, carbon was the only alloying element. The problem in the early days of steel making was getting rid of unwanted elements, not adding new ones. However, there are a variety of alloying elements that are added to modern steels to impart various characteristics.
Iron alone is relatively soft. It does not hold an edge well, wears quickly and has little resistance to bending. Add a little bit of carbon and the story changes dramatically. The carbon combines with the iron to form hard carbide platelets cemented together in a matrix of iron. The combination is resistant to wear and bending and will take a keen edge.
Smaller carbides and a tighter grain structure allow for a stronger, sharper edge. Other carbide formers, like vanadium, can refine the grain of the steel further. Knives with a high vanadium content can take a very keen edge, but are harder to sharpen.
Carbon - Present in all steels, it is the most vital hardening element. Greater than 0.5 percent carbon content qualifies a steel as a “high carbon” steel.
Chromium - Added for wear resistance and corrosion resistance. A steel with at least 13 percent chromium is considered “stainless.” Chromium is a carbide former, so it also increases wear resistance.
Manganese - A carbide former. Manganese aids grain structure, increases hardenability, and wear resistance. Manganese is present in most cutlery steels.
Molybdenum - Another carbide former. Increases hardness, prevents brittleness, makes the steel easier to machine.
Nickel - Adds toughness and possibly aids in corrosion resistance.
Phosphorus - Essentially a contaminant.
Silicon - Increases hardness and strength.
Sulfur - Increases machinability but decreases toughness.
Tungsten - Increases heat, wear and shock resistance. Tungsten is the strongest carbide former behind vanadium.
Vanadium - Another carbide former. Contributes to wear resistance and hardenability. Vanadium refines the grain of the steel, which contributes to toughness and allows the blade to take a very sharp edge.
Most kitchen knives fall into the category of “high carbon stainless.” These knives generally contain between 0.5 and 0.8 percent carbon, 13 to 18 percent chromium and a little manganese, molybdenum, silicon, phosphorus and sulphur. This makes for a steel that is easy to produce, is very stain resistant and reasonably wear resistant. Knives from Global and Mac’s Superior line have some vanadium added for improved wear resistance and a finer grain, which allows the knife to be sharpened to an incredible edge.

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