Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt.

The process of alloying is used to change the chemical composition of steel and improve its properties over carbon steel or adjust them to meet the requirements of a particular application.

1. Chromium:

Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.

1. Nickel:

Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.

1. Cobalt:

Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually approximately 0.2% maximum. This problem is emphasized because there is residual cobalt content in the nickel used in producing these steels.

1. Tungsten:

Used in small amounts, tungsten combines with the free carbides in steel during heat treatment, to produce high wear resistance with little or no loss of toughness. High amounts combined with chromium gives steel a property known as red hardness. This means that the steel will not lose its working hardness at high temperatures.

An example of this would be tools designed to cut hard materials at high speeds, where the friction between the tool and the material would generate high temperatures.