Time Temperature Transformation Diagrams
Time Temperature Transformation Diagrams
Time has influence on phase transformation. The rate of transformation is dependent on the time and the temperature. Phase diagrams describe equilibrium microstructural development which is achieved at extremely slow cooling or heating conditions. The phase diagrams give no information on the time which is taken to form a phase as well as on shapes, size and distribution of the phase. Hence, there exist an importance of kinetics.
The temperature of transformation controls the nature of decomposed product (of austenite) which in turn decides the resultant properties of steel. Hence, the study of the effect of the transformation temperature on the nature of the decomposed product is of great importance. The kinetics of austenitic transformation can be studied best at a constant temperature rather than by continuous cooling. The constant temperature transformation is also referred to as isothermal transformation.
Time-Temperature-Transformation (TTT) diagram is also known as isothermal transformation diagram, sigmoidal diagram, C-curve, orS-curve of the steel. It shows the time needed for austenite to begin to transform, to proceed halfway, and to be completely transformed at any constant temperature in the range covered by the curves.
TTT diagram of a steel can be regarded as a kind of map which charts the transformation of austenite as a function of temperature and time and permits approximation of how the steel responds to any mode of cooling from the austenitic state. These are plots of temperature versus time (normally on a logarithmic scale).
The TTT diagram has been developed by Davenport and Bain in 1930 and has been a break-through in understanding the kinetics of bulk phase transformations. Davenport and Bain were the first to develop the TTT diagram of eutectoid steel. They determined pearlite and bainite portions whereas Cohen later modified and included Ms (martensite start) and Mf (martensite finish) temperatures for martensite. There are number of methods used to determine TTT diagrams. These are salt bath techniques combined with metallography and hardness measurement, dilatometry, electrical resistivity method, magnetic permeability, in situ diffraction techniques (X-ray, neutron), acoustic emission, thermal measurement techniques, density measurement techniques and thermodynamic predictions. Salt bath technique combined with metallography and hardness measurements is the most popular and accurate method to determine TTT diagram.
TTT diagrams are generated from percentage transformation versus time measurements, and are useful for understanding the transformations of an alloy steel at higher temperatures. TTT diagrams are accurate only for phase transformations in which temperature of the steel is held constant through-out the duration of the reactions, which means these reactions are isothermal. This is why, a TTT diagram is also called isothermal (constant temperature) transformation diagram. When steel in the austenitic state is held at any constant temperature lower than the minimum at which its austenite is stable, it transforms with time. The course of isothermal transformation can be represented by plotting percentage of austenite transformed against corresponding elapsed time at constant temperature. For a given steel austenitized in a particular way, information given by a series of such curves, each determined at a different constant temperature, can be summarized in a single diagram, as shown in the lower portion of Fig 1.
Fig 1 Isothermal and TTT diagrams
Austenite is stable above eutectoid temperature 727 deg C. When steel is cooled to a temperature below this eutectoid temperature, austenite is transformed into its transformation product. TTT diagram relates the transformation of austenite to time and temperature conditions. Hence, the TTT diagram indicates transformation products as per temperature and also the time needed for complete transformation. In Fig 1, curve 1 is transformation begin curve while curve 2 is the transformation end curve. The region to the left of curve 1 corresponds to austenite. The region to the right of curve 2 represents the complete transformation of austenite to pearlite. The interval between these two curves indicates partial decomposition of austenite into pearlite.
For a better understanding of heat treatments of steel, a TTT diagram has been a requirement. Using the TTT diagram the phase transformation sequence for microstructures has been estimated in terms of temperature and time.
A TTT 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 normally 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 are also being used to represent kinetic changes in steels.
TTT diagram is associated with mechanical properties, micro-constituents / microstructures, and heat treatments in carbon steels. Diffusional transformations like austenite transforming to a ferrite and cementite mixture can be explained using the TTT diagram.
The iron-carbon (Fe-C) phase diagram does not show time as a variable and hence the effects of different cooling rates on the structures of steels are not revealed. Moreover, equilibrium conditions are not maintained in heat treatment. Although, the iron-carbon equilibrium diagram reveals on the phases and corresponding microstructures under equilibrium conditions, but several useful properties of the steels can be achieved under non-equilibrium conditions, e.g., variable rates of cooling as produced during quenching and better transformation of austenite into pearlite and martensite. For each steel composition, there is a different TTT diagram.
Iron-carbon phase diagram shows the ideal phases when (i) the system is held at a temperature for sufficiently long time, and (ii) all the diffusions are complete to ideal lattice position. But it is not the practical situation, since the steel is normally heated or cooled at a definite rate. Hence, all the changes are either not started or not completed. The practical situation of phase transformation of steel is studied by a TTT diagram where both the time and temperature i.e., the rate of cooling is taken in consideration.
A TTT diagram is a plot of temperature versus log of time for a steel for a definite composition. The diagram determines when transformations begin and end for an isothermal (constant temperature) phase transformation of a previously austenitized steel alloy. The TTT diagram is used to determine when transformations begin and end for an isothermal heat treatment of a previously austenitized steel. The family of C-shaped curves at different temperatures are used to construct the TTT diagrams.
TTT diagrams give the kinetics of isothermal transformations. They represent specific thermal histories for the given microstructure. They indicate a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved. The aim of TTT diagrams is to determine the type of structure for and portion in the curve and to achieve specific properties. An accurate calculation of the TTT diagram is considered necessary for promoting the development of steel.
The temperature of transformation controls the nature of decomposed product (of austenite) which in turn decides the resultant properties of steel. Hence, the study of transformation temperature effect on the nature of decomposed product is of much importance. The kinetics of austenitic transformation can be studied best at a constant temperature rather than by continuous cooling. The constant temperature transformation is also referred to as isothermal transformation which is studied by the following experiment.
A number of small samples are taken from the steel under consideration. These samples are heated to pre-determined austenitizing temperature and are held at this temperature for a sufficiently long time so as to achieve a homogeneous austenite. These austenitized samples are transferred quickly to another bath maintained at a constant temperature below eutectoid temperature, selected for the study of kinetics of transformation. These samples are taken out one by one from the sub-critical temperature bath after different time intervals and are quenched immediately. The quenching of samples results in the formation of martensite from the untransformed austenite. By this technique, the quantity of transformed austenite can be determined as a function of time at constant temperature. The quantity of transformed austenite increases by allowing samples to remain in constant temperature bath for longer time. After a particular time, all the austenite gets transformed to an aggregate of ferrite and cementite at a given temperature. Fig 1a shows the effect of time on the amount of transformed austenite for a given transformation temperature T. Fig 2 shows the isothermal transformation curves for transformation of austenite to pearlite (i.e., eutectoid steel containing 0.76 % carbon).
Fig 2 Isothermal curves and TTT diagram for transformation of austenite for steel
The effect of time-temperature on the microstructure changes of steel can be shown by the TTT diagram. These diagrams are extensively used in the assessment of the decomposition of austenite in heat-treatment of steels.
It is clear from the figure that the transformation of austenite does not start immediately on quenching austenitized sample to a constant temperature bath. Transformation of austenite to ferrite-cementite mixture (pearlite) occurs after a definite time (equals to t1 of Fig 2a). This time during which transformation does not proceed is known as incubation period. The magnitude of incubation period provides a qualitative idea about the relative stability of supercooled austenite. Smaller incubation period corresponds to lesser stability of austenite.
Fig 2a has one important limitation, i.e., it only correlates the quantity of transformed austenite with transformation time for a constant temperature. Both time and temperature of austenitic transformation have considerable impact on the nature and morphology of transformed product. Hence, a diagram which can include all the three parameters, i.e., time, temperature and transformation, is of great importance, especially in the heat treatment process. Such a diagram is known as a TTT diagram.
For the construction of the TTT diagram for a steel, a large number of small samples of the steel (say, eutectoid steel) are taken. These samples are treated in a way similar to that already mentioned for the study of isothermal transformation of austenite. The only difference now is that the same process is repeated a number of times at varying transformation temperatures instead of a single temperature. The results are shown in Fig 1b. The quantity of transformed austenite at different time periods for different transformation temperatures can be known in this way. The temperature T1 is higher than T2, T3, T4, T5, – – – – so on, and is near to the eutectoid temperature.
It can be analyzed from Fig 1b that the higher the transformation temperature, the more is the incubation period and time needed for completion of the transformation. Incubation period and transformation time decrease with the lowering of transformation temperature. However, after a particular temperature (corresponding to T4 of Fig 2b), the decreasing trend is reversed and both incubation period and transformation time increase again with further lowering of transformation temperature. The minimum that is observed in the incubation period can be described below.
With decrease in the isothermal transformation temperature, the austenite becomes more unstable, and hence, the driving force for the austenite to pearlite transformation increases. Hence, the rate of nucleation increases. However, with decrease in transformation temperature, the rate of diffusion, which is an exponential function of temperature, decreases. Transformation rate depends on the overall effect of the rate of nucleation and rate of diffusion. The temperature at which the incubation time is minimum (T4 in Fig 2b) is the one below which the increase in nucleation rate by decrease in temperature is more than offset by decrease in the diffusion rate as a result of decrease in temperature. As a result, any further decrease in temperature increases the incubation time.
As already stated, below a particular transformation temperature, the rate of diffusion becomes practically insignificant and if transformation temperature is lowered below this limit, a diffusion less product, namely, martensite, is formed. Formation of martensite takes place instantaneously at a particular transformation temperature. From the result of Fig 2b, another diagram (i.e., Fig 2c) can be constructed with time and temperature as abscissa and ordinate, respectively. How this diagram is constructed is shown in Fig 1.
Fig 3a, Fig 3b, and Fig 3c show the TTT diagrams for eutectoid, hypo-eutectoid, and hyper-eutectoid steels, respectively. A common feature of these TTT diagrams is that pro-eutectoid phase (ferrite for hypo-eutectoid and cementite for hyper-eutectoid steels) separates out in upper temperature region. For hypo-eutectoid steels, ferrite starts separating out from the austenite as soon as austenite is cooled below the upper critical temperature (A3). The quantity of pro-eutectoid ferrite decreases as austenite is under-cooled more and more below the upper critical temperature. After a certain degree of under-cooling, austenite transforms directly to pearlite. On further cooling, there is no surplus ferrite. Similarly, cementite is separated out in hyper-eutectoid steels from austenite on cooling below the upper critical temperature (Acm). The quantity of cementite decreases with increased degree of super-cooling and finally reduces to zero when austenite is cooled below a particular temperature.
Fig 3 TTT diagrams for eutectoid, hypo-eutectoid, and hyper-eutectoid steels
Shape and position of curves of the TTT diagram – The form of each of the curves constituting the TTT diagram and their position with respect to the time axis depend upon the composition and grain size of the austenite which transforms. Certain alloying elements, or combinations of alloying elements, change the form of the curve in a characteristic way. In effect, this permits classification of steels on the basis of the type of curve. For present purposes, it suffices to state that, with few exceptions, an increase in alloy content or in grain size of the austenite always retards isothermal transformation (moves the curve toward the right) at any temperature higher than about 482 deg C, that is, above what has been called the ‘nose’ or ‘knee’ of the beginning curve. This retardation is reflected in the higher hardenability of steel with higher alloy content or larger austenite grain size. Indeed, it is normally recognized that response of a steel to any specified heat treatment which involves transformation of austenite is largely, if not entirely, determined by those factors which influence the time needed for isothermal transformation, and hence, the shape and position of the curves which comprise the TTT diagram.
Material used – Each diagram contains sufficient information to identify the steel to which it pertains with respect to principal elements of its composition, austenitizing temperature employed, and normally the austenite grain size established at that temperature. Samples are prepared in such a way that a representative area of the entire cross-section is examined, with no efforts being made to minimize possible segregation by discarding certain portions in the cross section. As a result, the TTT diagrams are believed to be reasonably representative of austenite transformation as it occurs in commercial grades of steel.
Conventions for constructing the TTT diagrams – The TTT diagrams are normally drawn upon a uniform-size chart having a linear scale of temperature drawn vertically and a logarithmic scale of time drawn horizontaIIy. The logarithmic time scale is used in conformance with weII-established practice in order to encompass both the very short and extremely long-time intervals encountered. Time intervals of 1 minute, 1 hour, 1 day, and 1 week are shown for convenience in locating familiar reference points on the basic logarithmic scale of time in seconds. The basic temperature scale is normally in degrees C. The significance of the various lines, numbers, and symbols comprising the diagram proper is discussed below under each appropriate subheading.
The ‘As’ (austenite start) and ‘Af’ (austenite finish) temperatures, represented by horizontal lines near the top of the diagram, correspond respectively to the lower and upper limit of the so-called critical range. Since these temperatures are limiting or ceiling temperatures for isothermal transformation, they are a significant feature of the diagram. For the determination of the ‘As’ and ‘Af’ temperatures, samples are heated to and held for a relatively long time at each of a series of temperatures in the neighbourhood of the austenite start and austenite finish temperatures. The ‘As’ temperature is chosen as that temperature at which a trace quantity of austenite forms in the ferrite matrix and does not increase perceptibly in quantity when the holding time is doubled. Hence, ‘As’ denotes the maximum tempering temperature which can be used without forming a considerable quantity of austenite in the particular steel being considered.
Similarly, ‘A’ denotes the maximum temperature at which a barely detectable amount of ferrite can exist in a hypo-eutectoid steel. In eutectoid and hyper-eutectoid steels, the ‘A’ temperature is only slightly higher than ‘As’ and is of relatively little practical significance. Hence, only the ‘As’ is given on the diagrams for such steels. On some of the diagrams the ‘As’ and ‘Af’ temperatures are noted as ‘estimated’. This indicates that these temperatures are calculated according to an empirical formula designed to estimate ‘As’ and ‘Af’.
Martensite formation is represented by a horizontal line, labelled ‘Ms’, appears on the TTT diagram. This line indicates the temperature at which martensite starts to form on quenching from the austenitizing temperature. Upon further cooling below this temperature, more and more martensite forms. The percentage of austenite transformed to martensite as cooling progresses is indicated on the diagrams by arrows pointing to the temperatures at which the austenite is half transformed (M50) and is 90 % transformed (M90). These particular percentages of martensite have no special significance and are used merely to convey some idea of the progress of transformation of austenite to martensite as cooling continues below Ms.
The temperature for 90 % martensite, rather than that for some higher percentage, is chosen since these measurements become increasingly less reliable with higher percentages of martensite, and since some of the steels can retain an appreciable percentage of austenite, the precise quantity being dependent upon several complex factors. In several diagrams, the data on martensite formation are obtained by direct measurement using a metaIIographic technique. When such was the case, the Ms, M50, and M90 appear without a qualifying note. In others, these temperatures are calculated as per an empirical formula developed for this purpose, and the Ms, M50, and M90 symbols are designated as ‘estimated temperature’. It is to be noted that these are not to be construed as highly precise temperatures, for in some cases the composition of the austenite is either not known exactly (because of undissolved carbides) or the composition is not within the range to which the empirical formula applies.
Curves of the TTT diagram – Starting from the left of the diagram, the first curve encountered, extends from near the ‘Ar’, ‘Acm’, or ‘As’ temperature down to the line labelled Ms. This so-called beginning line is drawn through points representing the time needed at each temperature level investigated for a measurable quantity of austenite to transform. In its simplest form the beginning line has a ‘C’ shape with a minimum time value at a temperature normally in the neighbourhood of 538 deg C. Alloying elements, especially those of the carbide-forming type, such as chromium and molybdenum, cause the beginning curve to assume a more complex shape.
The percentage of transformation product necessary for a measurable beginning depends upon the sensitivity of the technique used in following the progress of transformation. In the majority of the curves around 0.1 % transformation serves as the basis for locating the beginning line. In all but a few diagrams which represent eutectoid steels, the second curve from the left. which starts in the neighbourhood of ‘As’ and extends down to around 482 deg C where it merges with the beginning line, represents the beginning of transformation to ferrite-cementite aggregate (pearlite in its broadest sense) in the range of temperature where the first product of austenite transformation is either pro-eutectoid ferrite or pro-eutectoid cementite.
An exception to the above statement occurs in the diagrams in which the appearance of the microstructure in the range 538 deg C to 482 deg C prevents reliable location of the lower portion of the line. In these diagrams, a cross-hatched zone is normally drawn to indicate uncertainty of the point at which it merges with the beginning line.
The broad curve farthest toward the right represents the time needed at each temperature for the last trace of austenite to transform. This curve approaches but can never cross ‘As’. It extends from near ‘As’ down to below ‘Ms’. A sample quenched below ‘Ms’, transforms, at least in part, to martensite during cooling and hence strictly isothermal transformation of all of the austenite is impossible below ‘Ms’. The portion of the austenite which reaches any temperature below ‘Ms’, in time transforms isothermally to what for all practical purposes can be regarded as bainite. The time needed is indicated by the portion of the ending line extended below the ‘Ms’, horizontal. This portion of the ending line is normally shown dashed since some uncertainty exists as to its correct location, reliable measurement being relatively difficult in this region.
In some of the higher alloy steels, a portion of the ending curve lies beyond the range of the chart, but it can be logically assumed that the ending line is continuous since austenite is unstable at all temperatures below ‘As’ and in time presumably transforms. In certain steels, the time needed for austenite to transform completely below ‘Ms’, and at temperatures in the neighbourhood of 482 deg C is far beyond the duration of ordinary heat treatments.
The line labelled ’50 %’ and located between the beginning and ending lines represents the time needed at each temperature for transformation of half of the total austenite. It is included to give. some idea as to the progress of transformation and is especially useful in regions of a diagram in which the beginning and ending lines are not parallel. The principal curves of the TTT diagram are normally drawn as broad lines, not only so that they stand out among fainter coordinate lines but also to emphasize that their exact location on the time scale is not highly precise even for the particular steel sample represented. Portions of these lines are frequently shown as dashed lines to indicate a much higher degree of uncertainty.
Hence, all portions of lines extending to the left of the 2-second coordinate are dash-lines since for times less than about 2 seconds reliable and accurate measurements are not possible by the methods normally used. In this connection, it is to be recognized that the TTT diagram is designed to represent the overall pattern of transformation in a particular composition and particularly in regions in which transformation occurs rapidly and not to be regarded as always being a summary of a complete set of highly precise quantitative measurements.
The principal fundamental difficulty is that even a very small piece of steel needs some appreciable time interval to cool throughout to the temperature of the isothermal bath. The order of magnitude of this time interval is influenced by several factors including (i) the cross-section of the sample, (ii) the agitation it receives when immersed in the isothermal bath, and (iii) the composition, volume, and temperature of the isothermal bath. When quenching in a lead-alloy bath such as is normally used in determining a TTT diagram, rapid movement of the sample through the bath is especially desirable since mechanical stirrers are relatively ineffective in agitating such a heavy liquid.
As a result, an accurate evaluation of the time to reach bath temperature after immersion is rarely feasible. When transformation begins within a few seconds and proceeds rapidly as in the ‘nose’ region of a plain carbon steel, the time needed for the sample to reach the temperature of the bath is a considerable portion of the total time needed for transformation. An additional difficulty arises from the circumstance that heat generated by transformation (recalescence) can prevent a sample from ever quite reaching bath temperature until after transformation is completed.
Despite the above limitations, a beginning line even in the ‘nose’ region of a rapidly transforming steel can be located with sufficient accuracy for several practical purposes. This is possible since accumulated knowledge of the kinetics of isothermal transformation makes it possible to rationalize the entire reaction from a limited number of measurements. The method of plotting isothermal data first proposed by Austin and Rickett is especially useful in estimating a beginning time from measured data for longer times. It is also true that the beginning curve has a characteristic ‘C’ shape which is modified in a predictable way by certain alloying elements.
Since a large number of TTT diagrams, including several for steels which transform slowly enough to permit accurate direct measurement at all temperature levels, are now available, difficulty in getting accurate direct measurements within a limited temperature range need not prevent construction of a reasonably reliable ‘nose’ region for the TTT diagram of a rapidly transforming steel. A given TTT diagram, even if constructed from a complete set of highly precise measurements, is truly accurate only with respect to transformation of the particular sample of steel used in its determination. Other samples of the same grade of steel can vary appreciably in the exact time needed for transformation to begin and to end at each temperature.
In practice, isothermal data are normally used in connection with the heat treatment of pieces of steel very much larger than the small specimens used in developing a TTT diagram. Although it appears that the mass of the sample does not per se appreciably influence transformation rates provided the difference in cooling time (from immersion to attainment of thermal equilibrium with the isothermal bath) at the centre of a large, as compared to a small, piece of steel is taken into account, it frequently happens that the large piece encompasses a higher range of composition because of segregation. Hence, portions of the large piece can begin to transform somewhat sooner and finish transformation somewhat later than is indicated by the TTT diagram. Hence, the usefulness of a TTT diagram is not seriously impaired by failure to get a highly precise measurement of the beginning time at all temperature levels.
Considerable judgment is frequently needed in constructing a TTT diagram from experimental data, and equal judgment is needed in its interpretation with respect to conditions different from those under which it is being determined. The experienced user does not read into a TTT diagram an unduly high degree of accuracy, nor condemn it since it is not always based upon a complete set of highly precise measurements. The use of a dash-line to the left of the 2-second coordinate has been explained as representing a relatively high degree of uncertainty as to the exact location of the line in this region. In some examples, other portions of a beginning or an ending line can appear as a dash-line since the number or kind of measurement does not serve to locate the dashed portion with quite the same certainty realized elsewhere.
Fields of the TTT diagram – Each field on the diagram above ‘Ms’, is labelled to indicate the phases observed in samples austenitized and then quenched and held isothermally within the time-temperature limits of each field. The region above the ‘Af’ temperature and to the left of the beginning line is labelled ‘A’ for austenite which is presumed to have existed in this region since samples treated within the time-temperature limits of this field are entirely martensitic when quenched to room temperature. In a few of the diagrams, the austenitizing treatment does not dissolve all carbides in austenite and this is indicated on each of such diagrams.
The region labelled A+F or A+C which lies between the beginning line and the intermediate broad line represents the time-temperature region in which austenite and a pro-eutectoid phase are observed. The latter is ferrite (F) in a hypo-eutectoid steel and cementite (C) in a hyper-eutectoid steel. This field is, of course, missing in a eutectoid composition. The A+F (or A+C) field extends from near ‘Af’ (or ‘Acm’) normally down to around 482 deg C where the field is pinched out because of the merging of its two boundary lines.
The field labelled A+F+C–which is bounded at the right by the ending line, at the left by the right-hand boundary of the A+F (or A+C) field at higher temperatures, and by the beginning line at lower temperatures, extends from ‘As’ or somewhat above, down to ‘Ms’. Samples held at any constant temperature for a time period within the limits of the A+F+C field are observed to contain the three phases namely (i) austenite (observed at room temperature as martensite), (ii) ferrite, and (iii) cementite. Either ferrite or cementite can exist separately as a pro-eutectoid constituent and in addition the two are normally intimately associated with each other in the form of an aggregate constituent. The latter is classified as pearlite at higher temperatures and bainite at lower temperatures, and at intermediate temperatures both pearlite and bainite can form.
The labelling of fields on the basis of phases formed avoids the necessity of classification of all micro-constituents resulting from austenite transformation at constant temperature and hence simplifies the diagram. The field to the right of the ending line is labelled F+C to indicate that only ferrite and cementite are present, all austenite having been converted by the transformation process to these phases.
Hardness after transformation – At the right-hand edge of several of the diagrams a series of HRC (hardness Rockwell C scale) numbers indicates the hardness of a sample held only long enough at each temperature to transform all of the austenite, measured at room temperature. In all these steels hardness increases as the transformation temperature decreases, although in the intermediate region in the neighbourhood of 538 deg C, there is frequently an inversion in this overall trend.
Microstructure – In practically all steels hardenable by heat treatment, the character of the ferrite-cementite aggregate is determined primarily by the temperature at which it has formed. There is the same normal sequence of microstructures ranging in appearance from coarse lamellar at the higher temperature to fine acicular at the lower levels. Regardless of differences in composition, familiarity with this sequence in only a few steels makes it possible merely by examining the TTT diagram for any steel to make a reasonably good prediction as to its microstructure at each transformation temperature level.
Characteristic differences in the microstructure exist between steels of markedly different composition, but these differences are more readily taken into account when the TTT diagram is available for comparison with those of more familiar steels. Hence, the presence of pro-eutectoid ferrite in the microstructure is indicated by an ‘A+F’ field on the TTT diagram. For a particular austenite grain size, the relative quantity of pro-eutectoid ferrite is roughly proportional to the temperature difference between ‘As’ and ‘Af’.
The character of the ferrite-cementite aggregate is primarily determined by transformation temperature so that the difference in its appearance among different steel compositions is normally less than that which results from a difference in transformation temperature of little more than 38 deg C. In general, acicular aggregates normally classified as bainite form from the neighbourhood of the ‘nose’ temperature (the lower ‘nose’ if there happen to be two) down to ‘Ms’. Microstructures formed in several alloy steels, particularly those containing strong carbide-forming elements such as chromium, molybdenum and vanadium, are somewhat different from those in plain carbon steel, yet the same general trend is common to all with modifications indicated by the TTT diagram.
It is normally true that two different steels with similar TTT diagrams also have similar microstructure at corresponding temperature levels, and hence quite similar mechanical properties when heat treated alike. When it is necessary to discontinue a particular composition which has long been successfully used, it is a sound rule to select a substitute which has a TTT diagram as nearly as possible like that of the old one. If this can be done, very little modification of heat-treating practice is needed when the new composition is substituted for the old.
Application of TTT diagrams to heat treatment – The TTT diagrams have gained great importance from heat treatment point of view. This is because of the simple reason that these diagrams are extremely useful as they give information about the hardening response of steels and the nature of transformed products of austenite at varying degrees of supercooling. These diagrams have been of great practical importance to some special heat treatment processes such as austempering and isothermal annealing. In practice, however, transformation during heat treatment occurs by continuous cooling, and not isothermally. Hence, TTT diagrams have limited applications. For majority of the heat treatment processes, these diagrams are useful only qualitatively, and not quantitatively. Some of the application of the TTT diagrams are described below.
Quenching and tempering – The most common method of hardening steel by heat treatment consists of heating to a temperature at which the steel becomes austenitic and then cooling fast enough, normally by quenching in a liquid such as water or oil, to avoid any transformation of the austenite until it reaches the relatively low-temperature range within which it transforms to the hard, martensitic microstructure. The minimum rate of cooling necessary is related to the location with respect to the time scale of the ‘nose’ of the TTT diagram.
In a TTT diagram, showing a quench and temper type of heat treatment, the cooling curves as drawn lie to the left of the ‘nose’ and hence indicate full hardening on quenching. One of the curves represents cooling at the surface of a quenched piece of steel, whereas the other curve represents cooling at the centre of the same piece. Locations between surface and centre are, of course, cool at intermediate rates. The TTT diagram has no bearing on the tempering operation unless the austenite-to-martensite transformation is incomplete, as sometimes happens. In this case, retained austenite normally transforms during tempering to the transformation product indicated by the TTT diagram.
Martempering – In martempering heat treatment process, the steel is quenched into a bath at a temperature in the neighbourhood of ‘Ms’, and held in the bath until the centre of the piece reaches bath temperature, after which it is removed and allowed to cool in air. Again, if complete hardening is to occur, austenite is required to cool with sufficient rapidity to avoid transformation at the ‘nose’ of the TTT diagram. Since it shows the ‘Ms’, temperature, the TTT diagram is useful in selecting the optimum bath temperature for martempering and in estimating how long the steel can be held in the bath without forming bainite.
Austempering – Austempering is a hardening process based upon isothermal transformation of austenite to bainite. Hence, the TTT diagram, or at least its lower portion, is not only useful but almost indispensable. In an ideal austempering treatment, austenite is transformed isothermally, or nearly so. The TTT diagram shows the time needed for austenite to transform and hence the minimum duration of the austempering treatment. The TTT diagram is also useful in planning austempering treatments since it shows the temperature range within which bainite forms and the hardness of bainite as a function of temperature.
Other applications to hardening – Special hardening treatments, or minor variations of regular hardening practice, can be based upon the specific pattern of austenite transformation for a particular steel. Hence, in high carbon steel, there is opportunity for variation in the hardening cycle. When austenite has cooled below the ‘nose’ of the TTT diagram, it inevitably transforms to martensite or at least to moderately hard bainite. Steels containing certain alloying elements or combinations of alloying elements can have a TTT diagram of such nature that unique hardening treatments are feasible. In such diagrams, there can be a lower as well as an upper ‘nose’ separated by a region of very slow transformation.
Annealing or softening – The aim of the heat treatment in the foregoing examples has been to harden steel, but it can be equally important to know how to avoid hardening. In this case, the curve of the TTT diagram representing completion of transformation is the important one. For example, in conventional annealing in which steel initially in the austenitic state is slowly and continuously cooled, the TTT diagram in conjunction with the cooling curve indicates the approximate temperature range in which transformation occurs and when slow cooling can be safely discontinued. It is also possible to estimate in advance a cooling rate which allows austenite to transform completely in a temperature range sufficiently high to develop the desired soft microstructure without unnecessary expenditure of time.
In several alloy steels, there is a pronounced minimum in the ending line of the TTT diagram at a relatively high temperature. Assuming that the transformation produced at this temperature is satisfactory, as is frequently the case, advantage can be taken of the time-temperature coordinates of this minimum to design a short annealing cycle. This is accomplished by cooling the steel initially in the austenitic state as rapidly as convenient to the temperature of the minimum in the ending line and then holding it approximately at this temperature for the time needed to transform austenite completely. Subsequently, the steel can be cooled in any convenient manner.
Effect of alloying elements on TTT diagram -Almost all alloying elements, except cobalt, decrease both the tendency for and the rate of decomposition of austenite. The reason for this is obvious for austenite stabilizing elements. Ferrite stabilizers do the same job by forming carbides. Alloy carbides are more stable than cementite, and hence they retard the diffusion of carbon which in turn decrease the rate of decomposition of austenite. Strong carbide formers have more pronounced effect on the retardation of austenite decomposition than the weak carbide formers. Fig 4 shows different types of TTT diagrams for alloy steels.
Fig 4 Different types of TTT diagrams for alloy steels
Since pearlitic transformation involves diffusion of both carbon and metallic atoms, the effect of alloying elements is much more pronounced in pearlitic region. The effect is less pronounced in bainitic region since bainitic transformation involves diffusion of carbon atoms only. The TTT diagrams for alloy steels can broadly be classified into four types as shown in Fig 4.
The first type of TTT diagram (Fig 4a) is similar to that of carbon steel. There is practically no difference in the pattern of austenite decomposition in the presence of non-carbide forming elements. However, in the presence of carbide forming elements, super-cooled austenite decomposes to a mixture of ferrite and carbides rather than to an aggregate of ferrite and cementite.
The second type of TTT diagram (Fig 4b) differs from the remaining TTT diagrams as it consists of two minima with respect to the stability of austenite. The upper bay (at higher temperature) corresponds to the transformation of austenite to pearlite, whereas the lower bay corresponds to the transformation of austenite to bainite. Very few steels show such a TTT diagram. The two types of TTT diagrams discussed above are, in general, observed for low alloy steels.
The third type of TTT diagram (Fig 4c) is peculiar in the sense that bainitic region is not present. This implies that bainite cannot be formed in such steels. Such a TTT diagram is obtained, in general, for high alloy steels, especially those in which the start of martensitic transformation temperature has been shifted to sub-zero region. In such steels, stable austenitic structure is achieved at room temperature.
The fourth type of TTT diagram (Fig 4d) does not show pearlitic bay. Here, under normal cooling conditions, either bainite or martensite is formed.
Continuous cooling transformation – The TTT diagram is useful in planning heat treatments and in understanding why steel responds as it does to a particular heat treatment, but it cannot be used directly to predict accurately the course of transformation as it occurs during continuous cooling. It is possible, however, to derive from the TTT diagram another time-temperature-transformation diagram which while not highly accurate, is of considerable aid in bridging the gap between isothermal and continuous cooling transformation. This diagram is referred to as the continuous cooling transformation diagram (CCT diagram). It is necessary to derive only a few CCT diagrams in order to demonstrate their relationship to the TTT diagram. Once the fundamental difference between the two types of transformation diagrams is recognized, it is possible to interpret more rationally any TTT diagram with respect to continuous cooling conditions.
The TTT diagrams have gained high importance from heat treatment point of view. This is because of the simple reason that these diagrams are extremely useful as they give information about the hardening response of steels and the nature of transformed products of austenite at varying degrees of super-cooling. These diagrams have been of high practical importance to some special heat treatment processes such as austempering and isothermal annealing.
In practice, however, transformation during heat treatment occurs by continuous cooling, and not isothermally. Hence, TTT diagrams have limited applications. For the majority of the heat treatment processes, these diagrams are useful only qualitatively, and not quantitatively. A diagram, which can correlate transformation, temperature, and time during continuous cooling, is of real value for heat treatment process. Continuous cooling transformation (CCT) diagrams can be obtained by a technique which is similar to that for TTT diagrams except that, in the case of CCT diagrams, points of start and end of austenitic transformation are recorded on continuous cooling.
For the construction of CCT diagram for a eutectoid steel, a large number of small samples are heated above the lower critical temperature (A1) to get a completely austenitic structure. From this temperature, samples are cooled at a constant cooling rate, and points corresponding to start and finish of pearlite are determined. By repeating the same process at various cooling rates, different sets of start and end points for pearlitic transformation are obtained. On joining start and end points, two curves, similar to those in TTT curves, corresponding to start and end of transformation, are obtained. Hence, a CCT diagram is obtained.
CCT diagram for eutectoid carbon steel – In Fig 5, a CCT diagram has been derived and superimposed on the TTT diagram of a eutectoid carbon steel, chosen for this purpose because of its relative simplicity. The diagram shows, in principle at least, how the TTT diagram through the medium of a CCT diagram derived from it, can be correlated with a typical heat treatment which involves austenite transformation as it occurs during continuous cooling.
Fig 5 CCT diagram for eutectoid steel
Transformation on continuous cooling – In the heat-treatment operations involving continuous cooling from the austenitic condition, transformation occurs over a range of temperatures rather than at a single constant temperature, and hence the final structure is a mixture of isothermal transformation products. The TTT diagram, particularly the examination of isothermal microstructures incidental to its construction, aids greatly in classifying the microstructure of steel transformed during continuous cooling. If the TTT diagram is at hand, it is possible to visualize at what stage of the cooling cycle different structures are formed. This facilitates changes in heat treatment necessary to get more of the desirable and less of the undesirable structures.
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