Steel is one of the most diverse materials, the different methods of alloying and heat treating the steel can give many different properties all from the same basic material.
In its most basic form, steel is an alloy of iron and 0-2% carbon, though often there are other alloying ingredients that can change its properties, either for ease of processing, or to change its properties such as toughness or wear resistance. There are many different kinds of steel and different grading systems used to describe each alloy.
The American Iron and Steel Institute and Sea International worked together to come up with a system that designates 4-5 digit codes to describes a steel alloy. The first two digits display the two main alloying materials, while the last 2-3 show the carbon content in hundredth of a percent. For example, 1095 steel can be broken down as follows, 1 is carbon steel, 0 is plain carbon steel (no major alloying ingredients), and 95 denotes .95% carbon content. 52100 steel can be broken down as follows, 5 is chromium steel, 2 is 1.45 chromium content and minimum of 1% carbon, and 100 denotes 1% carbon. In both of these cases, the first two digits each mean a different alloy, and the last two to three digits mean the carbon content.
There are other systems in place that each have their own methods of describing steel, O1, S6 and A2 are all examples of the SAE designation of tool steels, and the first letter denotes a property of that steel, for example O1 and A2 can each be quenched in Oil and Air respectively (for more info of quenching and similar concepts, check out Heat Treat Basics) and lastly, S6 steel is a steel that is designed for Shock resistance.
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