A Study of Diffusion- and Interface-Controlled Migration of the Austenite/Ferrite Front during Austenitization of a Case-Hardenable Alloy Steel

Abstract

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.

The Effect of Ultrafast Heating on Cold-Rolled Low Carbon Steel: Formation and Decomposition of Austenite

“…The initial microstructure has an impact on the kinetics of austenite formation, as pointed out in earlier works [11][12][13]. Experimental studies [14,15] of the anisothermal formation of austenite in ferrite-pearlite aggregates revealed different kinetics of the γ/α interface migration towards pearlite, in comparison to its advance into proeutectoid ferrite. Dilatometric data [16] are in agreement with the different kinetics of austenite growth in pearlite and proeutectoid ferrite.…”

“…The carbon diffusion distance is considered to be on the order of one-half of the pearlitic interlamellar spacing. This assumption, first proposed by Zener for the growth of pearlite [26], has been accepted and applied to the modeling of austenite formation in pearlitic steels [12,13] [15] confirmed that the kinetics of the γ/α interface into pearlite is~2 times faster than its movement into proeutectoid ferrite. Figure 1a show that, in samples heated at 150 • C/s to 704 • C, the austenite is forming at the α/θ interfaces, preferentially at triple junctions of ferritic grains.…”

“…However, there is a remarkable kinetic difference between one and the other [15]. The kinetic of the austenite growth into the proeutectoid ferrite is slower compared to the advance of austenite into pearlite, since the carbon must now diffuse longer distances than the interlamellar spacing of pearlite.…”

“…Dilatometric data [16] are in agreement with the different kinetics of austenite growth in pearlite and proeutectoid ferrite. The massive formation of austenite during heating was experimentally measured using in situ techniques [15,17]. The change in the austenite formation mechanism, a consequence of the ultrafast heating rates, will thus have an impact on the microstructure on quenching.…”

Austenite Nucleation and Growth Observed on the Level of Individual Grains by Three-Dimensional X-Ray Diffraction Microscopy

“…The change in transformation mode from diffusion controlled to massive was observed by Schmidt et al, [26,27] who studied the austenite formation from ferrite-pearlite microstructure during continuous heating using hot-stage confocal microscopy. During the experiments, under conditions above the T 0 temperature, the growth rate increased drastically and it was claimed that the interface-reaction-controlled growth mechanism was responsible for the transformation.…”