Different proportions of alloying elements give different grades of stainless steel their unique properties.
Carbon is present in both mild and stainless steels. The carbon percentage of stainless steel can range from 0.03% to 1.2%. A steel is considered to be a high carbon steel if it contains between 0.60 to 1.00% carbon, with manganese content ranging from 0.30 to 0.90%. This makes the steel very hard, but also quite brittle and much less ductile than mild steel.
Chromium is the key alloying element in stainless steel. A minimum of 10.5% is necessary for any stainless steel, but more chromium is common for an increased corrosion resistance. The passive surface layer of chromium oxide prevents oxygen diffusing into the metal, protecting the internal structure from corrosion.
Nickel improves corrosion resistance, 75% of stainless steel has some nickel content. 8-9% nickel content obtains a fully austenitic structure providing excellent welding properties. Further increasing the nickel percentage improves workability.
Copper is normally present in stainless steel as a residual element. However, it is added to some alloys to enhance heat hardening properties and/or corrosion resistance particularly for use in sea water environments.
Silicon is used as a deoxidising agent in the melting of steel and consequently most steels contain a small amounts of silicon. Silicon improves resistance to highly concentrated nitric and sulphuric acids. It also prompts the formation of ferrite.
Nitrogen is a strong austenite stabilizer, improves strength and pitting corrosion resistance.
Three other additions to stainless in lesser amounts are Molybdenum, Tungsten and Manganese.