Time-temperature conversion diagram
In the time-temperature conversion diagram (TTT diagram, in English TTT, "Time Temperature Transformation"), the structure development is shown with different time-temperature curves for a single material or alloy. One of the most common uses is in the hardening of steel. ZTU charts are often created using a quenching dilatometer . In addition, other methods such as dynamic differential calorimetry or electrical resistance measurement are also used. In principle, a distinction is made between the isothermal and the continuous ZTU diagram. The ZTU diagram usually includes additional information such as material designation (including chemical composition), initial structure, previous time-temperature paths (e.g. austenitizing condition), structural proportions or hardness.
Determination of ZTU diagrams
The change in a physical, measurable variable such as the length of a sample during temperature control determines the beginning and, if necessary, the end of a conversion. Often, multiple conversions can also be clearly identified. Since most of the physical parameters themselves are temperature-dependent, with the length over the thermal expansion coefficient , this must be taken into account. The resulting transformation structure is determined metallographically . The start and end of transformation as well as the respective structure are entered in a time-temperature diagram for several temperature controls. The time is usually plotted logarithmically . The time-temperature paths used are often drawn in for information.
The isothermal ZTU diagram
After austenitizing (heat treatment when hardening steel), the workpiece is brought to the desired temperature very quickly and held until all transformations have been completed, which can be determined via the change in length (dilatometer).
The continuous ZTU diagram
After austenitizing, the workpiece is cooled down to room temperature at various cooling speeds. The different transformation points for different types of cooling (e.g. in air, water or oil) are recorded, which then form a cooling curve that depends on the type of cooling. In addition, the achievable hardness is usually noted at the end of the cooling curve . A continuous TTT diagram can only be read along the cooling curves, even if the beginning and end of the transformations, similar to the isothermal TTT diagram, are marked with lines crossing the cooling curves.
The two types of diagrams must be viewed strictly separately.
Other or related types of charts
There is also the welding ZTU diagram to describe the material behavior during welding and the ZTA diagram (where the A stands for austenitization), which describes the structural transformations when steel is heated with a defined chemical composition and the existing initial structure including grain growth . Time-temperature austenitization diagrams were created for the most important steels in order to show achievable austenitization states with the lowest possible grain growth.
The temperatures at which a transformation occurs are determined with the help of a dilatometric examination. The phases present in the structure at this temperature and their volume fractions are determined by metallographic observations, e.g. B. with a light microscope determined.