Isothermal transformation diagram

Consider again the iron-iron carbide eutectoid reaction 

Eutectoid reaction for the iron-iron carbide system 

In interpreting this diagram, note first that the eutectoid temperatme [727°C (1341 °F)] is indicated by a horizontal hne at temperatures above the eutectoid and for all times, only anstenite exists, as indicated in the figure. The austenite-to-pearhte transformation occurs only if an alloy is supercooled to below the eutectoid as indicated by the cmwes, the time necessary for the transformation to begin and then end depends on temperature. The start and finish curves are nearly parallel, and they approach the eutectoid line asymptotically. To the left of the transformation start curve, only austenite (which is unstable) is present, whereas to the right of the finish curve, only pearUte exists. In between, the austenite is in the process of transforming to pearlite, and thus both microconstituents are present. 

For iron-carbon alloys of other compositions, a proeutectoid phase (either ferrite or cementite) coexists with pearhte, as discussed in Section 9.19. Thus, additional 

Figore 10.14 Isothermal transformation diagram for a eutectoid iron-carbon alloy, with superimposed isothermal heat treatment curve (ABCD). Microstructures before, during, and after the austenite-to-pearlite transformation are shown. 

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Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

In austempering the article is quenched to the desired temperature in the lower bainite region, usually in molten salt, and kept at this temperature until transformation is complete (Fig. 22). Usually, the piece is held twice as long as the period indicated by the isothermal transformation diagram. The article may then be quenched or air-cooled to room temperature after transformation is complete, and may be tempered to lower hardness if desired. 

The figure below shows the isothermal transformation diagram for a coarse-grained, plain-carbon steel of eutectoid composition. Samples of the steel are austenitised at 850°C and then subjected to the quenching treatments shown on the diagram. Describe the microstructure produced by each heat treatment. 

Figure 5.30. Schematic drawing showing the construction of an isothermal transformation diagram from measurements of the progress of the transformation at various constant temperatures. This may be done, for instance, by metallographic examination of several specimens, quenched from the 7-field quickly enough to miss the nose of the C-curve and then isothermally annealed for various length of time. Notice that curves for the transformation of different samples may be shown on the same diagram and that more complex trends may be observed in real diagrams of specific alloys. In the example reported, Ms is the temperature at which the alloy will begin to show the martensitic transformation, Mf indicates the temperature below which no additional martensite forms.

The effect of this extrapolation can be seen in isothermal diagrams. Figure 7.8 is the isothermal transformation diagram for a 1050 steel. Note that proeutectoid ferrite must form before cementite for transformation temperatures above about 600 °C in accordance with Figure 7.8. The agreement is not perfect because in addition to 0.50% C, the 1050 steel contains 0.91 % Mn, which lowers the eutectoid temperature and composition. 

The net effect is that the overall transformation rate has a maximum somewhat below 600 °C, as shown in Figure 11.7. The time for transformation is inversely related to the transformation rate. Isothermal transformation diagrams (or TTT curves) are plots of the time required for transformations to occur. Because time is reciprocally related to rate, isothermal transformation diagrams have a C shape, as shown in Figure 11.8. 

Isothermal transformation of a 1080 steel. The left-hand line is for the observable start of the reaction and the right-hand line is for the essential completion of the reaction. Data from Atlas of Isothermal Transformation Diagrams (Pittsburgh U.S. Steel, 1951). 

Isothermal Transformation Diagram. To separate the effects of transformation temperature from those of heat flow, it is essential to understand the nature of the transformation of austenite at a given, preselected temperature below the A. Information needed includes the starting time, the amount transformed as a function of time, and the time for complete transformation. A convenient way to accomplish this is to form austenite in specimens so thin (usually about 1-mm thick) that heat flow is not an issue, rapidly transfer the specimens to a Hquid bath at the desired temperature, and foUow the transformation with time. The experiment is repeated at several other transformation temperatures. On the same specimens, the microstmcture and properties of the transformation products can be assessed. These data can be summarized on a single graph of transformation temperature versus time known as an isothermal transformation (IT) diagram or, more usually, a time—temperature—transformation (ITT) diagram. A log scale is used for... 

Wever F et al Atlas zur Warmebehandlung der Stable . Max-Planck-Institut fiir Eisenforschung. Verlag Stahleisen, Diisseldorf 1954/56/58. Atlas of isothermal transformation diagrams . US Steel Corp., Pittsburgh, USA 1951 and supplement 1953. 

The assumption made in an isothermal transformation diagram such as Figure 14.3 is that the temperature can be lowered to a particular holding temperature fast enough so that no transformations take place during the time it is being lowered. The temperature is then held constant for a period of time to allow the transformation to take place, hence the term isothermal. Examples of isothermal transformations are shown in Figure 14.4. 

Figure 10.13 Demonstration of how an isothermal transformation diagram (bottom) is generated from percentage transformation-versus-logarithm of time measurements (top). [Adapted from H. Boyer (Editor), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977. Reproduced by permission of ASM International, Materials Park, OH.)...

Several constraints are imposed on the use of diagrams like Figure 10.13. First, this particular plot is valid only for an iron-carbon alloy of eutectoid composition for other compositions, the curves have different configurations. In addition, these plots are accurate only for transformations in which the temperature of the alloy is held constant throughout the duration of the reaction. Conditions of constant temperature are termed isothermal thus, plots such as Figure 10.13 are referred to as isothermal transformation diagrams or sometimes as time-temperature-transformation (or T-T-T) plots. 

Figure 10.16 Isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy A, austenite C, proeutectoid cementite ...

Figore 10.18 Isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, including austenite-to-f)earlite (A-P) and austenite-to-bainite (A-B) transformations. 

The time-temperature dependence of the bainite transformation may also be represented on the isothermal transformation diagram. It occurs at temperatures below those at which pearlite forms begin-, end-, and half-reaction curves are just extensions of those for the pearlitic transformation, as shown in Figure 10.18, the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition that has been extended to lower temperatures. All three curves are C-shaped and have a nose at point N, where the rate of transformation is a maximum. As may be noted, whereas pearlite forms above the nose [i.e., over the temperature range of about 540°C to 727°C (1000°F to 1341 °F)], at temperatures between about 215°C and 540°C (420°F and 1000°F), bainite is the transformation product. 

Consider an alloy of eutectoid composition that is very rapidly cooled from a temperature above 12TC (1341°F) to, say, 165°C (330°F). From the isothermal transformation diagram (Figure 10.22) it may be noted that 50% of the austenite will immediately transform into martensite as long as this temperature is maintained, there will be no further transformation. 

Figure 10.23 Isothermal transformation diagram for an alloy steel (type 4340) A, austenite B, bainite ...

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