BTMT2074 Physical Metallurgy TTT diagrams versus CCT diagrams
BTMT2074 Physical Metallurgy
TTT diagrams versus CCT diagrams:
Their Design and Applications
YEAP YONG ZHEN
1 August 2018
Total Marks /100
A continuous cooling transformation (CCT) phase diagram is often used when heat treating steel. The figure 1 are used to represent which types of phase changes will occur in a material as it is cooled at different rates. These diagrams are often more useful than time-temperature-transformation diagrams because it is more convenient to cool materials at a certain rate than to cool quickly and hold at a certain temperature.
Figure 1: Example of continuous cooling transformation diagram
Types of Continuous Cooling Diagrams
There are two types of continuous cooling diagrams drawn for practical purposes
Type 1: This is the plot of transformation start; a specific transformation fraction and transformation finish temperature for all products against transformation time on each cooling curve.
Type 2: This is the plot of transformation start; a specific transformation fraction and transformation finish temperature for all products against cooling rate or bar diameter of the specimen for each type of cooling medium.
Isothermal transformation diagrams (also known as time-temperature-transformation (TTT) diagrams) are plots of temperature versus time (usually on a logarithmic scale). They are generated from percentage transformation-vs time measurements, and are useful for understanding the transformations of an alloy steel at elevated temperatures.
Figure 2: Example of Time-temperature-transformation diagram
An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Though usually used to represent transformation kinetics for steels, they also can be used to describe the kinetics of crystallization in ceramic or other materials. Time-temperature-precipitation diagrams and time-temperature-embrittlement diagrams have also been used to represent kinetic changes in steels.
Isothermal transformation (IT) diagram or the C-curve is associated with mechanical properties, microconstituents/microstructures, and heat treatments in carbon steels. Diffusional transformations like austenite transforming to a cementite and ferrite mixture can be explained using the sigmoidal curve; for example the beginning of pearlitic transformation is represented by the pearlite start (Ps) curve. This transformation is complete at Pf curve. Nucleation requires an incubation time.
The rate of nucleation increases and the rate of microconstituent growth decreases as the temperature decreases from the liquidus temperature reaching a maximum at the bay or nose of the curve. Thereafter, the decrease in diffusion rate due to low temperature offsets the effect of increased driving force due to greater difference in free energy. As a result of the transformation, the microconstituents, Pearlite and HYPERLINK “https://en.wikipedia.org/wiki/Bainite” o “Bainite” Bainite, form; Pearlite forms at higher temperatures and bainite at lower.
Austenite is slightly undercooled when quenched below Eutectoid temperature. When given more time, stable microconstituents can form: ferrite and cementite. Coarse pearlite is produced when atoms diffuse rapidly after phases that form pearlite nucleate. This transformation is complete at the pearlite finish time (Pf).
However, greater undercooling by rapid quenching results in formation of martensite or bainite instead of pearlite. This is possible provided the cooling rate is such that the cooling curve intersects the martensite start temperature or the bainite start curve before intersecting the Ps curve. The martensite transformation being a diffusionless shear transformation is represented by a straight line to signify the martensite start temperature.
2. Difference between TTT and CCT diagrams:
The essential difference between both the diagrams is the method of cooling. In TTT diagrams, after cooling to a transformation temperature, you keep the temperature constant until the transformation of austenite to the required transformation product (usually pearlite or bainite) is complete and then cool to the room temperature. One such process is austempering in which austenite is transformed to bainite isothermally. Below is a TTT diagram showing the austempering process.
Figure 3: Austempering process
The red lines form the transformation diagram and blue line denote the process. In this case the component which had an austenitic structure was cooled to just above Ts temperature; held at that temperature until the transformation was complete and then cooled further to the room temperature.
In CCT diagrams, there is continuous cooling i.e. there is no holding of temperature. The components are cooled at constant or varying rates. The end products are usually martensite or pearlite depending on the cooling media as well as the material of components. Fully bainitic structure cannot be obtained using continuous cooling.
Figure 4: CCT diagram
F – FerriteP – PearliteB – BainiteM – Martensite’s’ subscript denotes start temperature and f subscript denotes finish temperature.So a CCT diagrams simply gives the various transformation products which will be obtained at different cooling rates. It can be seen from the diagram that at cooling rates of more than 100 degrees celsius, ferrite and martensite will be obtained; for cooling rates between 20 and 100 degrees celsius, ferrite, bainite and martensite will be obtained and so on. These cooling rates are dependent on the cooling media.
CCT diagrams are more practical than TTT diagrams as most of the processes employ continuous cooling rather than isothermal transformation. Also it is more difficult to hold the temperature constant. It is important to note that there is not a single TTT or CCT diagram like the Iron-Carbon diagram. Different steels have different TTT and CCT diagrams. So the diagrams given here do not give data for all the steels.
In a nutshell, the difference between TTT and CCC is in one word “equilibrium”. The TTT curve and here it is important to remember that it is also called an I-T curve (isothermal transformation), is generated by rapidly cooling samples to a set temperature and then holding for increasing times to produce the resulting phases. The CCC (continuous cooling curve) shows the results in cooling that is not isothermal, and thus more representative of what happens in an actual quench.
Transformation Diagrams (CCT & TTT)
There are two main types of transformation diagram that are helpful in selecting the optimum steel and processing route to achieve a given set of properties. These are time-temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams. CCT diagrams are generally more appropriate for engineering applications as components are cooled (air cooled, furnace cooled, quenched etc.) from a processing temperature as this is more economic than transferring to a separate furnace for an isothermal treatment.
Time-temperature transformation (TTT) diagrams
Measure the rate of transformation at a constant temperature. In other words a sample is austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the rate of transformation is measured, for example by dilatometry. Obviously a large number of experiments is required to build up a complete TTT diagram.
Continuous cooling transformation (CCT) diagrams
Measure the extent of transformation as a function of time for a continuously decreasing temperature. In other words a sample is austenitised and then cooled at a predetermined rate and the degree of transformation is measured, for example by dilatometry. Obviously a large number of experiments is required to build up a complete CCT diagram.