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Heat treatments are used to endow the steel with specific properties according to how it is to be used 

Heat treatment is the operation (or series of operations in the case of complex treatments) during which one or more thermal cycles are carried out on the steel, i.e. modifying the temperature within specific limits according to time. During a thermal cycle, the steel is usually heated to a given temperature, held at this temperature for a certain time and then left to cool to room temperature in various ways according to the effects required.

The various cycles are selected according to the hardness, toughness, microstructure and workability properties required. The treatment cycle must be defined considering not only the grade of steel but also the dimensions of the parts, the characteristics of the heating and cooling medium.

Before describing the individual heat treatments, consideration must be given to the critical points of steel as these must be known if the various cycles are to be carried out precisely.
The transformation point or critical point indicates the temperature at which a change of phase is generated during heating and cooling.

The main critical points are:

AC1 = temperature at which, during heating, austenite starts to form:
AC3 = temperature at which, during heating, transformation of the ferrite into austenite ends;
AR3 = temperature at which, during cooling, transformation of the austenite into ferrite starts;
AR1 = temperature at which, during cooling, transformation of the austenite into ferrite + cementite ends;
MS = temperature at which, during cooling, transformation of the austenite into martensite starts;
MF = temperature at which, during cooling, transformation of the austenite into martensite ends.

In order to chart the behavior of a specific steel during the various treatment cycles, in particular as regards the effect of modifying the cooling rate, we will refer to the austenite transformation diagrams, generally known as:

a) Bain “S” curves or TTT (temperature, time, transformation) curves, particularly suitable for isothermal treatments

b) CCT (continuous, cooling, transformation) curves.