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Migrating austenite/ferrite interfaces in the ferrite regions of an alloy steel, containing 0.20 wt pct C, 0.87 wt pct Mn, and 0.57 wt pct Cr, with a ferrite/pearlite microstructure have been observed during austenitization by a high-temperature confocal scanning laser microscope in order to determine the mechanisms of transformation. The samples were subjected to isothermal (790°C to 850°C) and nonisothermal (0.5°C to 20°C/s) temperature profiles. The kinetic rates extracted from the observations were compared to models for long-range diffusion-controlled and interface reaction-controlled migration. The transition between the two mechanisms was found to occur at T 0 , which defines the temperature and composition at which a partitionless phase transformation is possible. Occurrence of the partitionless, interface-controlled transformation was confirmed by an analysis of carbon distribution and microstructure before and after a sample was subjected to a particular thermal profile. The mobility of such interfaces was found to be in the range 1.6AE10 )13 to 2AE10 )12 m 4 AEJ )1 AEs )1 , which is consistent with previous studies on interface-controlled migration of the reverse reaction, a to c, during cooling of dilute substitutional iron alloys. The diffusioncontrolled migration, at temperatures below T 0 , was found to occur in two stages: an initial stage, at which the growth rate can be predicted by a semi-infinite diffusion model; and a second stage, at which the growth slows more rapidly, possibly due to the overlap of diffusion fields.

Continuous cooling and isothermal dilatometry was performed for a binary Fe-0.3C alloy and a ternary Fe-0.3C-1.0Mn alloy at slow (< 0.1 oC/s) cooling rates and isothermally at temperatures below the equilibrium eutectoid reaction temperature but above the bainite start temperature (625 to 715 oC). Some of the test conditions produced an unusual morphology in which fine scale ‘sub-grains’ are decorated with carbide, with additional discrete carbide particles inside the ‘sub-grains’. A detailed investigation into the network carbide formation indicates formation during austenite decomposition, as opposed to a post lamellar transformation coarsening or spheroidization reaction, but only for select temperatures, and apparently only during isothermal conditions.

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