A Time-Temperature-Transformation (TTT) diagram describes the decomposition of the austenite (after prior heating) and the structure generated by this.
In order to set up a TTT diagram for continuous cooling rod-shaped test pieces are heated to austenitising temperature in a dilatometer. They are kept at this temperature for a certain time and are then cooled down at a defined cooling tem-perature. From the cooling curve of the dilatometer test piece a temperature-time diagram can be set up where the time axis is drawn as a logarithm.
Cooling curve of a dilatometer test piece of low alloyed steel with 0.25 % C, 1.4 % Cr, 0.5 % Mo und 0.25 % V. Heating and cooling rate 4 K/min.
If the transformation temperatures are introduced into this diagram, a characteristic cooling curve will be obtained, having the additional information on the structural composition and the hardness at RT. During the continous cooling of the austenite and passing of the Ar3-line ferrite will be formed. Further cooling reaches the Ar1-line where at this point (Ar1a) the formation of ferrite is completed (55 %). At this point the formation of perlite starts untile the point Ar1e is reached and no further formation takes places. The structure of the material contents of 55 % ferrite and 45 % perlite.
Such a cooling curve is to be taken for one cooling speed only. A higher cooling speed will shift the curve to the left.
In this cases other structures beneath ferrite and perlite like bainite and/or martensite may be found. Regardless the cooling speed all coolings are continious.