A study of nonisothermal austenite formation and decomposition in Fe-C-Mn alloys

Abstract

Experiments using ahot-stage confocal scanning laser microscope (CSLM) havebeen carried out to observe phase transformations in two steels: Si-killedr esulfurized Fe-0.38w tp ct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite coloniesa nd subsequently in the ferrite phase. The measured advance rates of the g /pearlite front were roughly twicet hoseo ft he g / a front and both interfaces were found to be curved. The g /pearlite migration ratew as found to be in qualitativeagreement with publishedrate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstä tten ferrite laths precipitate as distinct colonies from the existing allotriomorphicf errite phase but then also at MnS precipitates. The electron backscatterdiffraction (EBSD) analysis showed that all of the laths in ap articular colony exhibit similar orientation to onea nother but as lightly different orientation than the parent allotriomorph, supporting as ympathetic nucleation mechanism.T he growth rate of the laths was found to vary widelywithin arange of 1.5 to 11 m m/s. The ferrite formation is finally halted by impingement with other advancing fronts. Ther esults are presented in ap henomologicald iscussion, with some quantitativea nalysis of the transformation kinetics.

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.…”

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.…”

The Austenite/Ferrite Front Migration Rate during Heating of IF Steel

“…The relationship between these two processes generally means that the faster the migration rate of the interface, the less likely it is to be smoothed out before it can be observed. In previous studies of CSLM observations, interface migration has been observed for a range of transformations in steels: Widmanstätten ferrite precipitation 4,5) ; austenite to d-ferrite transformation, 1,6) austenite to pearlite 4) and ferrite plus pearlite to austenite. 5) In many of these cases, the surface structure is not preserved after the transformation, due to either rapid surface diffusion and high temperatures or sub- Table 2(b).…”

“…Significant improvement to current understanding of austenite formation is possible using in situ observational techniques [1][2][3] to study the behavior of migrating interfaces, in terms of rate, evolving morphology, and effect of initial structure, during the phase change. One of these techniques, more recently developed by Emi 2) for high temperature observation of metals, is the hot stage Confocal Scanning Laser Microscope, [4][5][6][7] (CSLM), which will be utilized in the current investigation.…”