It is possible to produce steel plates that have more than twice the strength of those produced from mild steel through a heat treatment process known as quenching and tempering. Steel is heated to a point above its upper critical temperature and is then very quickly cooled in water. This method of cooling, known as quenching, induces a transformation in the crystalline structure of the steel which leaves it much harder. Unfortunately the steel in this condition is also very brittle and it is not possible to use it in many applications. A method of heat treatment for alleviating this problem is called tempering and requires the quenched steel to be re-heated although this time to below the lower critical temperature. The result is to put toughness and ductility back in to the steel and, although some of the yield strength of the quenched steel is lost, the end result of the quenching and tempering process is to produce a strong steel which is also easy to weld, bend, and fabricate generally.
