Steel, in the most basic terms, is refined elemental iron mixed with carbon to form a stronger material. However, there are thousands of different forms of steel, as it is a chemical process with dozens of possible chemicals added for specific properties. Despite the wide range of different compositions, steel can be categorized into one of three basic families: stainless steels, tool steels, and mild or low-carbon steel.
The most defining difference between tool, mild and stainless steel is the chemical makeup of the steel. Generally, mild steels are simple steels with very few elemental additives. Tool steels are tailored for specific properties using multiple additives, and stainless steels have very large concentrations of one or two elements. Carbon is perhaps the most critical chemical additive, since it directly determines the hardenability of steel. Mild steels typically contain less than 1 percent carbon by mass; tool steels can contain up to 15 percent or more carbon, as do stainless steels. Stainless steels contain up to 20 percent chromium and relatively high concentrations of nickel, chemicals not typically added to mild or tool steels. Tool steels can contain multiple additives in trace amounts or in higher concentrations. These elements determine specific physical and mechanical properties.
A steel's physical properties are determined by its chemistry. Generally, mild steels have moderate toughness and are quite easy to weld, due to their simple composition, but are prone to corrosion. Stainless steels and moderate to difficult to weld, due to their complex composition, but are very resistant to corrosion. Tool steels have excellent machinability, hardenability and wear resistance, but can be prone to corrosion.
The hardenability, or ability to turn steel from malleable to hard, is determined by the carbon content, and to a lesser extent, trace elements. As a rule, mild steels are generally not able to be hardened, since they contain so little carbon. Some stainless steels can be hardened, but not every form of stainless steel has this ability. Nearly all tool steels can be hardened, but with the use of different mediums. Some need to be quenched in oil or water to be properly hardened, while others are able to harden in the air after proper heat treatment.
Mild steels are generally the cheapest steels, and the ones made in the greatest abundance. They are used as structural materials, and as sheeting in building trades. Stainless steels are used in finished products, like food service materials or as a cladding, where corrosion resistance is necessary. Tool steel is used almost exclusively in machining and tool making, though some cutlery is also made from certain grades.