What is ttt diagram

Time-temperature conversion diagram

in the Time-temperature conversion diagram (ZTU diagram, in English TTT, "Time Temperature Transformation"), the structure development can be followed at different temperature profiles during hardening. ZTU charts are created using a quenching dilatometer. In principle, a distinction is made between the isothermal and the continuous ZTU diagram. In addition to the material designation (including chemical composition), the austenitizing conditions must also be recorded.

The isothermal ZTU diagram

Schematic representation of an isothermal time-temperature conversion diagram. A warming transformation from state B to A is shown. The red line indicates the temperature profile of the warming over time. The percentages indicate the portion of the time-dependent transformation; Both states are present as a mixture within the transformation.

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 conversion points are recorded. In addition, the achievable hardness is usually noted at the end of the cooling curve.

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)[1], which describes the structural transformations when heating steel 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 conversion occurs are determined with the help of a dilatometric examination. The phases present in the structure at this temperature and their volume proportions are determined by metallographic observations, e.g. with a light microscope.

See also

Iron-carbon diagram, heat treatment

Individual evidence

  1. ↑ Temperature and time-dependent transformation behavior