When a steel specimen is heated, its temperature rises unless there is change of state or a change in structure. Fig. 1 shows heating and cooling curve of steel bearing different structures. Similarly, if heat is extracted, the temperature falls unless there is change in state or a change in structure. This change of structure does not occur at a constant temperature.
It takes a sufficient time a range of temperature is required for the transformation. This range is known as transformation range. For example, the portion between the lower critical temperature line and the upper critical temperature line with hypo and hyper eutectoid steels, in iron carbon equilibrium diagram. This range is also known as critical range.
Over heating for too long at a high temperature may lead to excessive oxidation or decarburization of the surface. Oxidation may manifest itself in the form of piece of scale which may be driven into the surface at the work piece if it is going to be forged. If steel is heated, well above the upper critical temperature, large austenite grains form. In other words steel develops undesirable coarse grains structure if cooled slowly to room temperature and it lacks both in ductility and resistance to shock.