BTMT2074 Physical Metallurgy
TTT diagrams versus CCT diagrams:
Their Design and Applications
YEAP YONG ZHEN
1 August 2018
Total Marks /100
Continuous Cooling Transformation (CCT) Diagrams
Figure 1: Continuous cooling transformation diagram
Continuous Cooling Conversion (CCT) phase diagrams are commonly used for heat treatment of steel. From Figure 1, these charts are used to indicate which types of phase transitions occur when materials are cooled at different rates. These plots are generally more useful than time-temperature maps because it is more convenient to cool the material at a rate than to cool it quickly and maintain it at a certain temperature.In continuous cooling transformation from martensite to pearlite takes place at a range of temperature.
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
Time-temperature-transformation (TTT) diagram
The isothermal conversion map (also known as the time-temperature conversion (TTT) map) is a plot of temperature versus time (usually expressed on a logarithmic scale). They are produced by a percent conversion – time measurement and are useful for understanding the transformation of alloy steels at elevated temperatures.
The isothermal conversion map is only valid for a particular material composition and is only effective if the temperature remains constant during the transition and is cooled strictly to this temperature. Although commonly used to represent the transformation kinetics of steel, they can also be used to describe the crystallization kinetics in ceramics or other materials. Time – Temperature – Precipitation and Time – Temperature – The embrittlement diagram has also been used to indicate the dynamics of the steel.
The isothermal transformation (IT) or C curve is related to the mechanical properties of carbon steel, trace components/microstructures and heat treatment. The diffusion transformation of austenite to cementite and ferrite mixtures can be explained by a sigmoidal curve; for example, the onset of pearlite transformation is represented by a pearlite onset (Ps) curve. This conversion is done on the Pf curve. Nucleation requires incubation time. As the temperature decreases from the liquidus temperature to the maximum at the bulge or nose of the curve, the nucleation rate increases and the microcomponent growth rate decreases. Thereafter, the decrease in the diffusion rate due to the low temperature offsets the influence of the increased driving force due to the large difference in the free energy. Due to this transformation, trace components of pearlite and bainite are formed; pearlite is formed at a higher temperature, and bainite is formed at a lower temperature.
When quenched below the temperature of the Eutectoid, the austenite is slightly too cold. When more time is given, stable trace components can form: ferrite and cementite. When the atoms rapidly diffuse after forming a phase of pearlite nucleation, coarse pearlite is produced. This transformation is done at the pearlite completion time (Pf).
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
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 a 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. Below is a CCT diagram:
Figure 4: CCT diagram
F – FerriteP – PearliteB – BainiteM – Martensites 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.
Plotting of TTT and CCT diagramsI) Time-Temperature-Transformtion diagram
TTT (Time-Temperature-Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.
Cooling rates in the order of increasing severity are achieved by quenching from elevated temperatures as follows: furnace cooling, air cooling, oil quenching, liquid salts, water quenching, and brine. If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to complete the transformation may be found.
In Figure 5 the area on the left of the transformation curve represents the austenite region. Austenite is stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation)
Figure 5: Example of TTT diagram
Transformation of austenite to pearlite is not linear as shown at constant temperature i.e600°C. Initially the rate of transformation is slow, then it increases rapidly, and finally it slows down towards the end.
II) CCT Transformation Curve
In practice, structural steels are strengthened through microstructural transformations which take place under continuous cooling conditions rather than isothermal conditions. In continuous cooling heat treatments, the material is cooled from above the A3 temperature to room temperature continuously; the transformation from austenite to pearlite and/or martensite therefore occurs over a range of temperatures instead of at a single temperature, resulting in a more complex microstructure.
The products formed by continuous cooling procedures can be predicted using a continuous cooling diagram which differs from the isothermal transformation diagram in that the beginning and the end of the transformation are generally shifted to lower temperatures and longer times. Different rates of cooling, and hence different microstructures are achieved through the use of different cooling media, as shown in Fig. Essentially as the cooling rate is increased the hardness and the strength of the steel increases.
Rapidly cooling or quenching, the austenite at a rate equal to or greater than the critical cooling rate will bypass the knee of the TTT curve and result in the formation of martensite. However, martensite is too hard and brittle for most purposes and it are usually transformed to the equilibrium phases using a low-temperature heat treatment called tempering. The resulting microstructure is not lamellar like that of pearlite but contains many dispersed carbide particles. Tempering causes the strength and hardness of the martensite to decrease, while the ductility and impact properties are improved. By selecting the appropriate tempering temperature, a wide range of properties can be obtained.
Applications of TTT diagrams
Martempering : This heat treatment is given to oil hardenable and air hardenable steels and thin section of water hardenable steel sample to produce martensite with minimal differential thermal and transformation stress to avoid distortion and cracking. The steel should have reasonable incubation period at the nose of its TTT diagram and long bainitic bay. The sample is quenched above MS temperature in a salt bath to reduce thermal stress (instead of cooling below MF directly).Surface cooling rate is greater than at the centre. The cooling schedule is such that the cooling curves pass behind without touching the nose of the TTT diagram. The sample is isothermally hold at bainitic bay such that differential cooling rate at centre and surface become equalise after some time. The sample is allowed to cool by air through MS -MF such that martensite forms both at the surface and centre at the same time due to not much temperature difference and thereby avoid transformation stress because of volume expansion. The sample is given tempering treatment at suitable temperature.
Austempering: Austempering heat treatment is given to steel to produce lower bainite in high carbon steel without any distortion or cracking to the sample. The heat treatment is cooling of austenite rapidly in a bath maintained at lower bainitic temperature (above Ms ) temperature (avoiding the nose of the TTT diagram) and holding it here to equalise surface and centre temperature and till bainitic finish time. At the end of bainitic reaction sample is air cooled. The microstructure contains fully lower bainite. This heat treatment is given to 0.5-1.2 wt%C steel and low alloy steel. The product hardness and strength are comparable to hardened and tempered martensite with improved ductility and toughness and uniform mechanical properties. Products donot required to be tempered.
Isothermal annealing: Isothermal annealing is given to plain carbon and alloy steels to produce uniform ferritic and pearlitic structures. The product after austenising taken directly to the annealing furnace maintained below lower critical temperature and hold isothermally till the pearlitic reaction completes.. The initial cooling of the products such that the temperature at the centre and surface of the material reach the annealing temperature before incubation period of ferrite. As the products are hold at constant temperature i.e. constant undercooling) the grain size of ferrite and interlamellar spacing of pearlite are uniform. Control on cooling after the end of pearlite reaction is not essential. The overall cycle time is lower than that required by full annealing. : Isothermal annealing heat treatment superimp
Patenting: Patenting heat treatment is the isothermal annealing at the nose temperature of TTT diagram. Followed by this the products are air cooled. This treatment is to produce fine pearlitic and upper bainitic structure for strong rope, spring products containing carbon percentage 0.45 %C to 1.0%C. The coiled ropes move through an austenitising furnace and enters the salt bath maintained at 550°C(nose temperature) at end of salt bath it get recoiled again. The speed of wire and length of furnace and salt bath such that the austenitisation get over when the wire reaches to the end of the furnace and the residency period in the bath is the time span at the nose of the TTT diagram. At the end of salt bath wire is cleaned by water jet and coiled.
Application of CCT diagram
The overall continuous cooling transformation kinetics can be readily described by the continuous cooling transformation (CCT) diagram. The CCT diagram is constructed by plotting a series of cooling curves onto a temperature against time diagram and then connecting the transformation start temperatures and transformation finish temperatures with separate lines. The CCT diagrams containing the quantitative data pertaining to the dependence of steel structure and hardness on temperature and time of the supercooled austenite transformations are used for determination of the structure and hardness of the quenched, normalised, or fully annealed steels. Locations and shapes of the supercooled austenite transformations’ curves, plotted on the CCT diagrams, depends mostly on the chemical composition of the steel, extent of austenite homogenising, austenite grain size, as well as on austenitising temperature and time.
During transformation of Austenite to Martensite or Pearlite it is impossible to change the temperature from 727°C to 500°C in zero time and from 500°C to 0°C. So, for recommendation, try to decreasing the furnace temperature to 100°C in order to rapid cooling as compare to set 500°C, then once the furnace turns 500°C then only set back to 500°C. Other than that, specimen should be small in nature and care should be taken and special care should be taken during their rapid transfer between baths. The recommendation of solving this problem is using robot hand to handle it. Lastly, Austenite to Martensie transformation does not occur at the room temperature and the material should be cooled below room temperature after transformation. In order to solve this problem, the recommendation is placed the specimen at chiller room.
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.
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.
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