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Introduction to TTT and CCT Diagrams


This 20-page document covers fundamental issues on the continuous cooling transformation. There are two forms of transformation diagrams, such as the time-temperature transformation (TTT) and the continuous cooling transformation (CCT) diagrams, which are commonly used by metallurgists to assess the microstructure produced during heat treatment, and provide a judgment of the hardenability of a steel.

The time-temperature-transformation diagram, represents the time-temperature relationship of austenite decomposition. It is a sort of kinetic phase diagram, where temperature is plotted as the ordinate on a linear scale and concentration, as the abscissa, is replaced by the time on a logarithmic scale. It thus denotes the times required to start and complete austenite transformation into various microstructures under constant-temperature conditions.

The TTT diagrams can be well used in (1) providing a better understanding of the transformation of austenite; (2) determining a suitable temperature for isothermal heat treatment of steel as isothermal annealing, martempering, and austempering are desired; (3) ausforming; and (4) providing an approximate indication of the hardenability. Usually, the longer the time taken for the start of transformation, the greater the hardenability.

Most heat treatment of steels is carried out by continuous cooling rather than by isothermal transformation from the austenitizing temperature to room temperature.It is, therefore, necessary to consider the continuous cooling transformation diagram, which represents the progress of austenite transformation as a function of cooling rate. This allows us to predict the microstructure and associated mechanical properties of quenched, normalized, and annealed steels obtained by continuous cooling from the austenitizing temperature.

In many cases, a CCT diagram can be derived from its TTT counterpart for a eutectoid steel. Four cooling curves for a given austenitizing temperature corresponding to several positions along the Jominy end-quench bar are superimposed on the CCT diagram. The cooling rate should be slow enough to allow complete transformation of austenite into coarse pearlite. To be noted is that, those derived CCT diagrams from their counterpart TTT diagrams often do not give satisfactory  results, and hence the experimentally determined CCT diagrams should be used.

Modified continuous cooling transformation diagrams with individual bar diameter on the abscissa instead of transformation times have recently been adopted by some manufacturers. These diagrams provide an estimate of the microstructure comprising major phases that will form at the centers of bars of the given diameters for a wide variety of engineering steels in air-cooled, oil-quenched, and water-quenched conditions. Moreover, the microstructure at positions different from the center can be inferred. These diagrams show the hardness of the microstructure so produced. Sometimes hardness values after tempering are also indicated.

These CCT diagrams usually refer to an average composition within a particular steel grade. Variations in chemical composition within a specification range can sometimes result in a marked variation in microstructure and accompanying mechanical properties. Moreover, critical ranges of bar diameter are usually observed where slightly faster or slower cooling rates produce a drastic change in the predominant microstructure.

From the correlation between Jominy hardenability and bar diameter, it is possible to determine the position of Jominy end-quench test results, in terms of hardness versus equivalent bar diameter, in relation to the center position of an oil- and water-quenched bar accompanying each CCT diagram. The additional applications of this modified CCT diagram include

  • the study of continuous heat treatment cycles for thin sections such as wire or strip;
  • the design of safe heat treatment cycles for sections that require retarded cooling after hot-working in order to avoid low-temperature reaction products;
  • prediction of machinability from expected microstructures and hardness values;
  • investigation of mass effect; and
  • assistance in the calculation of critical quenching rates for complex-shaped and varying sections.

There are also limitations for it. Accuracy of the experimental determination of CCT diagrams may be affected by prior heat treatment and undissolved carbides and noninclusion of the effect of agitation in the air-cooling, oil-, and water-quenching media, etc.
 

Information
The Full Document Is Available as Metal Pass Consulting Reference Material. For Metal Pass Work Areas in Consulting, Please Visit www.metalpass.com/consulting. For Metal Pass Software Supply, Please See www.metalpass.com/supply.
 

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