Isothermal transformation diagram
In the time-temperature -transformation diagram ( TTT diagram in the English TTT, " Time Temperature Transformation" ), the structure is also shown at different time - temperature curves. One of the most common applications is carried out in the hardening of steel. TTT diagrams are often created by a Abschreckdilatometers. In addition, other methods such as differential scanning calorimetry and electrical resistance measurements are applied. A basic distinction isothermal and continuous TTT diagram. For TTT diagram usually include additional information such as material designation (including chemical composition ), initial microstructure, previous time-temperature paths (eg Austenitisierungsbedingung ), portion of structure or hardness.
Determination of TTT diagrams
About the change of a physical, measurable size as the length of a sample during the temperature control of the beginning and possibly the end of a conversion is determined. Often, it is also more transformations clearly identified. Since most of the physical parameters are themselves dependent on temperature, with the length on the thermal expansion coefficient, this is taken into account. For several temperature profiles of the conversion start and end are entered in a time - temperature diagram. The time is usually plotted logarithmically. Often the time-temperature paths used are for information included in the drawing.
The isothermal TTT diagram
After austenitization (heat treatment in the hardening of steel), the workpiece is brought very quickly to the desired temperature and held until all conversions are completed, which can be determined from the change in length ( dilatometer ).
The continuous TTT diagram
After austenization the workpiece with different cooling rates is cooled to room temperature. The conversion points are recorded. In addition, the cooling curve is usually still listed the achievable hardness at the end.
The two types of diagrams must be considered strictly separated.
More or related types of charts
Next there is the welding CCT diagram for a description of material behavior during welding and the ZTA diagram ( where the A for austenitization is ) that the structural transformations during heating of steel describes a defined chemical composition and present initial microstructure including grain growth. Time-temperature austenitizing graphs were created by the major steel to represent achievable Austenitisierungszustände with minimum grain growth.
The temperatures at which there is a conversion are determined by means of a dilatometric study. The structure at this temperature in each phases present and its volume fractions are metallographic observations, eg with an optical microscope, is determined.