Effect of Ausforming on the Macro- and Micro-texture of Bainitic Microstructures

Abstract

The reason why variant selection phenomena occur in ausforming treatments is still not known. For that reason, in this work, the effect of compressive deformation on the macro and micro-texture of a bainitic microstructure was analyzed in a medium-carbon high-silicon steel subjected to ausforming treatments, where deformation was applied at 520 °C, 400 °C and 300 °C. The as-received material presented a very weak $$\left\langle {3\, 3\, 1} \right\rangle$$ 3 3 1 fiber texture along the rod axis, due to prior thermomechanical processing. For the samples isothermally heat-treated, it was detected that the bainitic ferrite inherited a $$\left\langle {1\, 0\, 0} \right\rangle$$ 1 0 0 fiber texture from the $$\left\langle {1\, 1\, 0} \right\rangle$$ 1 1 0 fiber texture present in the prior austenite. The intensity of this transformation texture was more pronounced as the deformation temperature decreased. Also, variant selection was examined at different scales by combining Electron-Backscattered Diffraction and X-ray Diffraction. The quantification of the fraction of crystallographic variants under certain conventions for every condition revealed variant selection in samples subjected to ausforming treatments, where these phenomena were stronger as the deformation temperature was lower. Finally, some of the theories proposed so far to explain these variant selection phenomena were tested, showing that variants were not selected based on their Bain group and that their selection can be better described in terms of their belonging to packets, if these are defined according to a global reference frame. This suggests that the phenomena might have to do with the effect of deformation mechanisms on the prior austenite.

Explaining the dilatometric behavior during bainite transformation under the effect of variant selection

“…Then, Tdef was held for 15 s before samples were compressed up to 10 % at 0.04 s -1 . In previous works in this same steel and with the same austenitization and deformation conditions, it was shown that, although some stress/strain induced bainite/martensite formed during this deformation step [25], the presence of these phases did not alter the trends observed in terms of variant selection with respect to samples in which no strain induced transformations were detected [19] and, thus, their presence could be neglected. After the stress was removed, samples were held at Tdef for 10 s and cooled down to 300 °C, temperature which was held for 1 h before cooling down to room temperature.…”

“…A step size of 0.35 μm was utilized and only the ferrite (bcc) structure was considered. The XRD data and the EBSD data had also been used in two previous works [19,22] in which through characterizations was performed.…”

“…When variant selection occurs because the prior austenite is textured, stressed or plastically deformed, variants are selected based on their orientation with respect to the sample coordinate system and, thus, the final ferrite texture and the dilatometry signal may change with respect to the expected ones, becoming the dilatometry signal anisotropic -i.e. the longitudinal signal is not the same than the transverse one -and even negative along specific directions in some occurrences [19,[21][22][23] Although the causality between variant selection and the variation of the dilatometric signal has been previously assumed [21,22], it has not been thoroughly proved. He et al [24] tried to explain these phenomena, although they only analyzed 11 prior austenite grains and, even despite they claimed that they had applied the Phenomenological Theory of Martensite (PTM) for their analysis, they skipped the consideration of the shape change on which the PTM is based.…”

“…However, in some cases, not all the possible variants are formed within a prior austenite grain. This phenomenon is called variant selection and it can be due to many different reasons, including: a) the prior austenite grain size (PAGS) is smaller than a few micrometers [9][10][11][12], b) the prior austenite is textured [13]; c) the prior austenite is stressed [14,15] or plastically deformed [16][17][18][19]; d) the driving force for the transformation is low in absolute value [13] so, either the transformation temperature is low [13], the carbon content is high [20] or the concentration of the austenite stabilizer substitutional elements is relatively high [13]. Nevertheless, not all these cases can be detected at a macroscopic level.…”

“…Four different bainitic microstructures were analyzed: one of them obtained by a pure isothermal treatment and the rest of them obtained by ausforming treatments. Some of the mentioned microstructures had previously shown to lead to negative longitudinal dilatometry signals [22] and a strong variant selection phenomenon [19]. A theoretical methodology was applied to calculate the relative length changes associated to the bainitic transformation in those four different samples in order to prove the connection between the selection of crystallographic variants and the distortion of the dilatometry signal.…”