What Is Steel? 

At its simplest, steel is a whole lot of iron and a little bit of carbon. If you want to understand steel, the first step is understanding what steel is made of, and how these ingredients contribute to the structure of the steel. Knife steels typically have from 0.5% carbon to 1.0% carbon. Some of today’s “super steels” have carbon contents greater than 1.5% to 2%. Chromium, molybdenum, manganese, nickel, tungsten, vanadium and silicon are other common ingredients. Sulfur, and phosphorus are common impurities that can have a detrimental effect on the knife steel but are very, very difficult—and thus costly—to remove.

If you look at a prepared wafer of steel, regardless of which ingredients it has in it, under a metallurgical microscope you will see what looks like a drawing of grains of sand. These little grains aren’t actually 2D like they appear, they have 3D shapes and have been polished down into a nice flat plain in preparation for viewing. The little grains you are viewing show the crystal state of the steel. Unhardened steel will have one crystal state, hardened steel will have another. There are several different crystal states of steel, but we will only focus on two: austenite and martensite. Austenite is responsible for the softness of unhardened steel, while martensite is responsible for the hardness of hardened steel.

The whole point of heat treating is to change the crystal structure of the steel from one that isn’t particularly useful (austenite) to one that is strong and wear resistant (martensite). In this process, a particular alloy is heated to a specific temperature and then cooled rapidly. The act of achieving this temperature and then cooling transforms austenite to martensite. After the steel has cooled to room temperature, it is necessary to re-heat it slightly to alter the martensite in such a way as to impart toughness. This process is called tempering. The proper combination of heat treating and tempering takes soft useless steel and transform it into something that can stand up to hard use.

For more information on what steel is, read the Heat Treating.