Cct Curve
A Continuous Cooling Transformation (CCT) curve is a metallurgical diagram used for steel heat treatment, plotting how austenite transforms during continuous cooling in a specific steel composition. It shows transformation start, fraction transformed, and finish as functions of time and temperature, revealing whether cooling produces ferrite, pearlite, bainite, or martensite, and predicts the resulting microstructure and hardness.
On the shop floor, CCT curves are less a graph pinned to the wall and more the logic behind heat-treat specifications. When a batch of 4140 or 4340 parts is quenched, the cooling medium—air, oil, polymer, or water—is selected so the actual cooling path lands on the desired region of the CCT map for that grade. The curve tells the heat treater whether the section will transform to bainite or martensite, and what hardness to expect. Welders use the same idea to anticipate heat-affected zone structure, since a fast cool after welding can produce hard, brittle martensite instead of a tougher microstructure. Section size matters: a thick flange cools more slowly than a lab coupon, so relying on small-specimen data overpredicts hardening on heavy parts. In production, operators apply CCT indirectly through austenitizing temperature, soak time, and quench severity, then verify results by checking hardness and microstructure against the predicted values.
What does the “nose” of a CCT curve represent?
It is the region where diffusional transformations begin fastest; if the actual cooling path misses the nose sufficiently fast, the steel is more likely to bypass ferrite/pearlite formation and form harder transformation products.
Why do transformations shift to lower temperatures in CCT diagrams compared to TTT diagrams?
Because during continuous cooling, the material spends less time at each temperature, delaying the start of diffusion-controlled transformations relative to isothermal testing.
Can a CCT curve predict hardness?
Yes; CCT diagrams are used to predict the resulting microstructure and hardness after continuous cooling, making them a primary production tool for quench and temper processes.