Steel

Though steel had been produced by various inefficient methods long before the Renaissance, its use became more common after more efficient production methods were devised in the 17th century. With the invention of the Bessemer process in the mid-19th century, steel became a relatively inexpensive mass-produced material. Further refinements in the process, such as basic oxygen steelmaking, further lowered the cost of production while increasing the quality of the metal. Today, steel is one of the most common materials in the world and is a major component in buildings, infrastructure, tools, ships, automobiles, machines, and appliances. Modern steel is generally identified by various grades of steel defined by various standards organizations.

Iron, like most metals, is not usually found in the Earth's crust in an elemental state. Iron can be found in the crust only in combination with oxygen or sulfur. Since the oxidation rate itself increases rapidly beyond 800 °C, it is important that smelting take place in a low-oxygen environment. Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel.

The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in water or oil, cooling it so rapidly that the transformation to ferrite or pearlite does not have time to take place. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy. Martensite has a lower density than austenite, so that transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, these internal stresses can cause a part to shatter as it cools; at the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when water quenched, although they may not always be visible.

Blister steel, produced by the cementation process, was first made in Italy in the early 16th century and soon after introduced to England. It was produced by Sir Basil Brooke at Coalbrookdale during the 1610s. The raw material for this were bars of wrought iron. During the 17th century it was realised that the best steel came from oregrounds iron from a region of Sweden, north of Stockholm. This was still the usual raw material in the 19th century, almost as long as the process was used. Crucible steel is steel that has been melted in a crucible rather than being forged, with the result that it is more homogeneous. Most previous furnaces could not reach high enough temperatures to melt the steel.