The solid state phenomenon of austenite precipitation from ferrite occurs at some point during the thermal processing of nearly all steels. Austenitization in pure iron is expected to be controlled by processes which occur at the migrating austenite/ferrite interfaces. An analytic expression which accounts for these processes has been proposed which generally follows the transition state theory for thermally activated processes. The velocity of an interface controlled by this mechanism should be very fast (for pure iron, a velocity of 100s of μm/s in a temperature range from about 915°C to 940°C has been measured), will be linear with temperature, and is not time dependant. This model for interface-reaction controlled migrating interfaces has been found to be consistent with observations in pure iron, and in interstitial free steels. The morphology of austenite precipitates during the interface reaction controlled transformation suggests that this phase transformation is a massive transformation with incoherent interfaces and no partitioning of solute atoms. The mobility of interface reaction-controlled transformation boundaries reported in the present and previous investigations have been discussed in further detail.
The morphology of austenite precipitates, with regard to the appearance of the migrating interfaces and the initial location of carbon in the microstructure, have been found to be consistent with the massive transformation in pure iron. This can he shown in binary iron-carbon alloy and in a set of carbon steels which contain various amounts of e.g. manganese, chromium, and nickel.
The mobility of partitionless, massive transformation interfaces has been found generally to range over 6 orders of magnitude, and is a few to several orders of magnitude larger in pure iron than in Fe-C or Fe-C-X steels. If the transformation can be made to occur in the single phase austenite region for an alloy, the interface mobility may increase significantly at long growth distances, about 1-2 orders of magnitude greater than the initial mobility. The two most important variables that affect the mobility of an interface have been determined to be the activation enthalpy (which is roughly equivalent to an activation energy) and activation entropy required for atoms to jump across an interface. The data in the present investigation have been fit to the interface reaction model by varying the activation enthalpy, through a range from about 175–250 kJ/mol. This is fairly consistent with a known range of expected activation enthalpies for atomic migration in iron, from the grain boundary self-diffusion (∼160 kJ/mol) to the bulk self diffusion of iron (250 kJ/mol). A similar variation in activation entropy, from about 10 to 150 J/inol, would also fit the reported data for this range of activation enthalpies. The variation of mobility does not seem to be predicted by the presence of any individual alloying element. The other components of mobility, which include jump frequency, boundary thickness, and molar volume, are not expected to vary enough to have a significant effect.
The occurrence of a massive, partitionless transformation in steel with ferrite and eutectoid structure has significant implications on carbon redistribution during austenitization, and thus the subsequent decomposition structure after annealing. This may be useful in the refinement or development of novel heat treating procedures, such as induction hardening, rapid annealing, or thermal cycling. (Abstract shortened by UMI.)