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Heat treatment and metallic materials

Which correctly describes the difference between a TTT (isothermal transformation) diagram and a CCT (continuous cooling transformation) diagram?

  1. AA TTT diagram shows transformation when the steel is quenched to a fixed temperature and held there, while a CCT diagram shows transformation during continuous cooling
  2. BA TTT diagram shows transformation during continuous cooling, while a CCT diagram shows transformation when the steel is held at a fixed temperature
  3. CThe two contain the same information, with the vertical and horizontal axes swapped
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Answer: A. A TTT diagram shows transformation when the steel is quenched to a fixed temperature and held there, while a CCT diagram shows transformation during continuous cooling

A TTT diagram shows the start and finish times of transformation when austenitized steel is quenched to a given temperature fast enough to avoid transformation and then held at that temperature; it is used to design isothermal treatments such as austempering. When the temperature falls continuously, as in actual quenching, the CCT diagram applies, and its transformation start line is shifted to longer times and lower temperatures than in the TTT diagram. The choice assigning continuous cooling to the TTT diagram and isothermal holding to the CCT diagram swaps the two definitions. Both are separate diagrams with temperature on the vertical axis and time (logarithmic) on the horizontal axis, and cooling faster than the nose gives martensite.

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