When steels cool from high temperatures at which they have an austenitic microstructure, they can transform to a number of very different microstructures, such as pearlite, bainite and martensite. The structure formed depends on the composition of the steel and the rate at which it cools. With very slow cooling rates, a mixture of ferrite and pearlite is likely. This is relatively weak but very ductile. At higher cooling rates, bainite will form. It is very strong and very tough. At still higher rates, martensite can be expected. This structure is very strong but tends to be brittle. In most cases, the martensite is considered to be undesirable, so high cooling rates must be avoided. However, heavily alloyed steels are called hardenable because they will produce martensite at low cooling rates, so the critical cooling rates for steels with high hardenability are much lower than those for steels with low hardenability. With knowledge of these critical cooling rates, and using the cooling rate equations, welding personnel can determine what the welding procedures should be to avoid undesirable microstructures, or to obtain those that are specified. See CONTINUOUS COOLING DIAGRAM, METALLURGY, and MARTENSITE.