Observation of .GAMMA..RAR..ALPHA. Transformation in Ultralow-carbon Steel under a High Temperature Optical Microscope.

A study of the α ↔ γ transformation in pure iron: rate variations revealed by means of thermal analysis

“…Next to grains grown to large dimension, there are often fine grains apparently recrystallized in a later stage. Similar microstructures were reported previously for pure Fe, Fe-Co and Fe-Mn alloys and low carbon steel [6,7,9]. After one heating and cooling cycle the grain size of ferrite increased from 91 µm to 276 µm and to higher values for more cycles.…”

“…This should be mainly due to the permanence of the sample at high temperature after the α↔γ transformation since it has been reported that the grain size decreases after the reverse transformation [9]. The ferrite grain size is thought to be representative of the average austenite grain size [6] consistent with direct observation through the temperature range of transformation [7]. …”

“…It should also been admitted that, although high purity samples were employed in all these analyses, the inevitable impurities they contain, including interstitials, can be segregated locally at grain boundaries either as solute atoms or even ultra fine precipitates causing pinning of the boundaries during their movement. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 The average values of the transformation rate estimated in this work are one order of magnitude lower than the average values reported from direct observation of the frames taken in high temperature microscopy of the transformation in a extra-low carbon steel [7] but fall within the range of those collected in the literature for Fe and Fe-alloys [11]. However, the outcome of the present work is that there is a range of transformation rates and that an absolute value can hardly be determined.…”

“…Since the driving force is of the order of 10-100 J·mol -1 at the given undercoolings [1,6], the interface velocity, v, is estimated in the range from 10 -4 to 10 -6 ms -1 . Direct observation of the interface movement in a low C and low N steel provided values of v from 1·10 -4 ms -1 to 9·10 -4 ms -1 [7].…”

“…During the last decade there has been continuous interest in experimental and theoretical studies on the α↔γ transformation in Fe and Fe alloys due to its relevance in steel processing [1][2][3][4][5][6][7][8][9][10][11]. Papers have appeared in relation to the kinetics of such transformation [1,3,4], the topology of grains [2], the mobility of the interface between the two phases, dilatation and enthalpic effects.…”