In this article we will discuss about transformation of continuous cooling of austenite and its CCT diagram.
Continuous Cooling Transformation of Austenite:
When austenite is cooled to A1, its free energy is equal to the free energy of low temperature transformation products and thus, transformation does not take place at A1. During continuous cooling further, austenite becomes metastable when super-cooled to temperatures below A1 and transforms to more stable phases.
The transformation temperature, Ar gets gradually lowered as the rate of cooling is increased as illustrated in Fig. 3.12. At slower cooling rates, austenite may transform to pearlite, but at faster cooling rates it may transform to bainite (in alloy steels) or martensite, or may transform to complex aggregates of these phases.
Fig. 3.12 (b) illustrates summary of data obtained on continuous cooling at increasingly faster rates. This Fig. and its (a) part illustrates gradual lowering of temperature of transformation as cooling rate is increased, which means the product is a finer ferrite-carbide aggregate with lowering of transformation temperature.
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Curve, V1 represents slow cooling as in a furnace (i.e. annealing), where transformation occurs close to A1 to get coarse-pearlite. Curve V2 represents cooling as expected in air-cooling (i.e., normalising) with the formation of pearlite (finer than in V1). The cooling rate represents oil-quenching—still faster cooling to result in still finer-pearlite.
Beyond a certain rate of cooling, such as in V4, fine pearlite (normally called nodular pearlite) starts to form but the insufficient time, in the upper range of transformation, prevents the completion of this transformation, the remaining austenite then transforms to martensite at lower temperatures. Under such cooling, the transformation occurs in two’ stages and in two temperature ranges [3.12 (&)], and is called split transformation. The microstructure of the steel, which transformed in this manner, consists of very fine pearlite (nodular pearlite) and martensite as illustrated in Fig. 3.13 (a).
Bainite is not usually formed in the continuous cooling of plain carbon steels. However with some alloying elements present, the structure could consist of fine pearlite (nodular pearlite-which in old but wrong terminology is also called troostite), bainite and martensite (Fig. 3.13 b). At very high rates of cooling, as Vc, diffusion based transformation of austenite becomes entirely impossible, and the transformation does not occur until a low temperature, Ms is reached, and then martensite forms.
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The minimum cooling rate, Vc (Fig. 3.12 a) at which all the austenite is super-cooled to Ms and then produces a fully martensitic structure is called the critical cooling rate. Critical cooling rate is different for different steels and is dependent on the composition and the grain size of the steel. Diagrams like 3.12 (b) provide useful information on the behaviour of steel during continuous cooling but lack details.
Continuous Cooling Transformation Diagrams (CCT):
Most of the industrial heat treatments of steels are carried out by continuous cooling, rather than by isothermal transformations. Such informations cannot be obtained directly from TTT diagram, and thus, it became necessary to develop diagrams that represent the transformation of austenite on continuous cooling at various rates.
These diagrams are called continuous cooling transformation (CCT) diagrams actually a TTT diagram can be easily determined needing relatively simple experimental set up and metallographic equipment. For these reasons, though the usefulness of CCT diagram was realised quite long back, most of the early work on the transformation in steels has been done on determining the TTT diagrams.
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Efforts were made to derive CCT diagram from the available TTT diagrams. Two methods, one the graphical method of Grange and Kiefer, and the other due to Avrami rule made assumptions, which are not completely valid, were adopted but proved unsatisfactory, though reasonable success is achieved in getting the transformation start curve in the ferrite-pearlite reaction range.
Fig. 3.14 illustrates a derived CCT diagram for eutectoid steel and its relationship to TTT curve. Generally, continuous cooling shifts the beginning of the austenite transformation to lower temperatures and for longer times.
This is explained as- Cooling curve ‘C’ (Fig. 3.14) intersects at (a) the beginning of the pearlite transformation curve of TTT curve at 650°C and at 6 seconds time, i.e., 6 seconds are required to nucleate pearlite isothermally at 650°C. A continuously’ cooled piece may be considered to have remained above 650°C for almost the entire 6 seconds.
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As the time required to start the pearlitic transformation is longer at temperatures above 650°C (incubation period increases with rise of temperature above the nose of the curve), than it is at 650°C, thus, enough incubation has not taken place in 6 seconds of time in a continuously cooled specimen for pearlite to start forming, and requires more time.
As in continuously cooling an increase in time is associated with a drop in temperature, the point at which transformation actually begins thus lies to the right and below the point (a) in Fig. 3.14 and is shown as (b). Thus, Fig. also shows that bainitic reaction in the steel does not appear to take place on continuous cooling as illustrated by cooling curve A because the specimen stays in the bainitic region for too small a time to allow any significant amount of bainite to be formed.
This agrees with the general fact about bainitic transformation that the rate at which bainite forms rapidly decreases with the falling temperatures. The presence of a gap in CCT diagram is another significant difference as illustrated in Fig. 3.14 as well as 3.17. This gap represents a temperature range in which no transformation occurs on cooling. This may be due to the carbon enrichment of austenite on cooling because of the formation of ferrite at higher temperatures.
Continuous cooling transformation kinetics can be obtained by measuring changes in some physical property which changes due to the transformation. The change in specific volume, magnetic permeability, and microstructure alongwith the changes in hardness can be used. Commonly, CCT curves are determined by dilatometry, hardness measurement and quantitative metallography.
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1. Dilatometry:
Dilatometry (supplemented with metallography and hardness measurements) commonly uses a quenching dilatometer in which changes in length and temperature with time of a standard sample are simultaneously recorded. It is now a well-established method for drawing CCT diagrams. Fig. 3.15 illustrates a schematic dilatometer the sophisticated one can detect a change in length of 10-4 mm. The cooling of the sample can he monitored right from using gas quenching (giving higher cooling rates) to furnace cooling (giving slower cooling rates). The highest cooling rate should be also to produce 100% martensite.
Fig. 3.16 (a) illustrates various cooling rates. For one such cooling rate of the specimen in dilatometer, says (d), Fig. 3.16 (b) illustrates a dilation curve. In this Fig., OP illustrates contraction in length (thermal) in austenitic range, PQ corresponds to the change in length during transformation, and QR to the contraction of the fully transformed steel. The transformation starts at T1, and the cumulative proportion of transformed product at a temperature T2 is given by yz/xz. Thus for a particular cooling rate (d), the temperature at which the transformation begins and produces 10%, 20% 50% 80%, 100%, can be obtained. Similar information for other cooling rates can be obtained and plotted to get CCT curves of Fig. 3.14 and 3.17. Metallography and hardness measurements at the end help to know more about transformation products.
2. End Quench Test:
As the universally used Jominy end quench hardenability test provides a series of cooling rates along its length, as illustrated in Fig. 3.14 and 3.17, because the specimen is water- quenched only at one end, and thus, the cooling rate is maximum at that end and drops with increasing distance from the end, it has also been used to obtain information for the continuous cooling diagram.
The cooling rates at various locations of a Jominy specimen have been measured by attachment of thermocouples, the four of which have been superimposed on the lower part of Fig. 3.14 and 3.17. With decreasing cooling rate, or increasing distance from the quenched end, the austenite transforms to micro-structures containing increasingly greater quantities of pearlite. The decreased hardness associated with the replacement of martensite by pearlite with the decreasing cooling rate is also shown in Fig. 3.14 and 3.17.
A number of Jominy test pieces are first end-quenched for a series of different times, and then quenched by complete immersion in water to ‘freeze’ the already transformed structures.
Micro-structures at the point where cooling rates are known are subsequently examined and the amount (%) of transformation can be measured by quantitative microscopy Hardness measurement is done at the end of each specimen at each investigated point. Fig. 3.14 and 3.17 curves can thus be obtained.
One important aspect of CCT curves is that there takes place depression of Ms temperature as cooling rates become slower, which is due to the rejection of carbon into austenite as ferrite or bainite structures form on cooling, The untransformed austenite has increasingly higher carbon contents and thus, has decreasing Ms temperatures.
3. Other Methods of Presenting Data:
The most common method, as illustrated in Fig. 3.14 and 3.17, of presenting the data from continuous cooling is a plot of temperature on the ordinate and time, on a logarithmic scale on the abscissa. In another important method, abscissa has size of pieces (bar diameters) plotted and not time.
A heat-treater finds it very useful as it helps to estimate the micro-structure to be present in the centre of the bar of a given diameter of steel in air-cooled, oil-quenched and water-hardened state. Each such CCT curve accompanies a plot of hardness with bar diameters in the as-cooled condition.
Fig. 3.18 illustrates such a continuous cooling transformation diagram of a steel (C = 0.38%%; Mn = 0.70; Si = 0.20%). The abscissa has three scales of bar diameter, one for air- cooling, the second for oil quenching and the third for water quenching. A vertical line drawn for a given bar diameter illustrates the micro-structures to be obtained at the centre of that bar diameter. For example,
This method of plotting CCT diagrams allows a direct assessment of possibility to produce different types of micro-structure in centre of bar diameters, and also various hardness values connected with the micro-structures. Thus, these diagrams have immense importance.