At present there are in extensive use three processes of steel making. These are, in order of the annual quantity of steel produced by each:

(1) The Bessemer process.

(2) The open-hearth process (Siemens and Siemens-Martin).

(3) The crucible process.

Each process is particularly adapted to removing the impurities from certain grades of iron, and also to producing certain grades of commercial steel. The Bessemer and open-hearth processes have made possible a low-carbon steel (mild steel) which finds extensive use as a structural material in buildings, bridges, hulls of vessels, steel rails, etc. This material is stronger and more uniform in quality than is wrought iron, which was formerly the only suitable material for these uses. The Bessemer process is the least expensive in operation, and the quality of its product depends upon the grade of pig iron used. It is, however, being gradually displaced by the open-hearth process because of the diminishing quantity of ores suitable for supplying pig of the required composition.

High-carbon steel, limited in use to tool making, springs, and for special needs in which extreme hardness is an important feature, is made by the crucible process. Of the three processes, the crucible process produces the highest grade and most expensive steel.