The basic ingredient is from 11%-30% chromium, however in many stainless steels nickel or manganese are important secondary ingredients.
There are two basic types of stainless steels:
1)”austenitic” stainless steels, and
2)”ferritic” stainless steels.
In austenitic SS the important ingredients are Chromium and Nickel. (Chromium and manganese are occasionally use instead of nickel.) It’s important to note that many austenitic SS’s contain almost no carbon, so by some definitions they are not “steel” at all but iron/chromium/nickel alloys. the additions of nickel (or manganese) actually changes the crystal structure of the iron, so the properties of these types are quite different from normal carbon steels. Austenitic SS have very good to excellent corrosion resistance, and fairly good heat resistance.
In ferritic stainless steels, the important ingredients are chromium and carbon. Ferritic stainless steels have markedly greater strength than austentitic types. However since they contain less chromium than austenitic types they are only moderately corrosion resistant and are much less heat resistant. Without the addition of nickel and/or manganeese adding more than about 20% chromium tends to make the steel brittle.
Most stainless steel is melted under an inert argon atmosphere in an airtight furnace. Melting stainless in open air would preferentially oxidize the chromium, forming slag which would float to the surface of the steel, thus reducing the chromium content. Chromium also tends to react with nitrogen in the air at high temperatures, exposure to nitrogen tends to cause brittleness in stainless steels.
As long as the manufacturer keeps careful control of the composition of the metal and the level of impurities during melting, stainless is endlessly recyclable.