Time-temperature-transformation diagram, iron-carbon

Figure 6.4 The time-temperature-transformation diagram of the iron-carbon system, beginning at the composition of austenite...

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. 

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

In this discussion of the microstructural development of iron-carbon alloys, it has been assumed that, upon cooling, conditions of metastable equilibrium have been continuously maintained that is, sufficient time has been allowed at each new temperature for any necessary adjustment in phase compositions and relative amounts as predicted from the Fe-FejC phase diagram. In most situations these cooling rates are impracti-cally slow and unnecessary in fact, on many occasions nonequilibrium conditions are desirable. Two nonequilibrium effects of practical importance are (1) the occurrence of phase changes or transformations at temperatures other than those predicted by phase boundary lines on the phase diagram, and (2) the existence at room temperature of nonequilibrium phases that do not appear on the phase diagram. Both are discussed in Chapter 10. 

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. 

ConcBpt Check 10.3 Make a copy of the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure 10.22) and then sketch and label on this diagram a time-temperature path that will produce 100% fine pearlite. 

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