Widening of Laths in Bainite

Abstract

Units of bainite in Fe-C alloys from the upper temperature range inherit their shape from Widmansta¨tten plates of ferrite, which are lathlike. The thickness increases by long-range diffusion of carbon and the length by short-range diffusion of carbon from the advancing edge of the tip. Both have been studied extensively and are fairly well understood. Widening growth seems to have been much neglected, but a study of some aspects of widening is now presented. The present report is the last one in a series of four morphological studies of bainite, isothermally formed in Fe-C alloys with 0.3 or 0.7 mass pct carbon, mainly in the upper temperature range. It contains a number of morphological observations made on cross sections of packets of bainite, and it elucidated a number of interesting questions about bainite and resulted in some proposals. The ferrite plates in a packet are nucleated as a group on a grain boundary, not each one separately on the side of a prior plate. Lengthening occurs by advancement of a short edge that is formed in close contact to the grain boundary. Widening of laths does not start spontaneously. It is initiated by a modification of the structure of the long edge of the lath. When it then moves, the lattice of the new ferrite is rotated relative to the ferrite formed by lengthening and the habit plane is different. In a section through the length direction, it is difficult to recognize what part of ferrite has formed by widening growth. Furthermore, it is proposed that the individual plates in a microstructure, previously used to illustrate subunits formed by repeated nucleation, were nucleated on a hidden grain boundary.

Revealing the Unexpected Two Variant Pairing Shifts Due to Temperature Change in a Single Bainitic Medium Carbon Steel

“…It can also be noted that if N-W would have been used, which have 12 possible crystallographic orientations, variant pairs belonging to the same Bain group in the present work instead should have been considered as a single variant. The shift in misorientation within the ferrite plates in the same packet has previously been presented by Yin et al [14] where it was described as a rotation of the bainitic plate during widening. It is also noted that there is a deviation from the theoretical and experimental misorientation angles for V1-V4, V1-V8 and V1-V6, decreasing with increasing temperature, see Table III, as previously reported.…”

“…The bainitic ferrite formed will thus, according to this hypothesis, be supersaturated with carbon from which carbides can form within the ferrite or if the formation of carbides is prevented by e.g., a Si addition, the carbon will diffuse into the residual austenite. [7,8,11] On the other hand, according to the diffusional hypothesis the first stage of the bainitic transformation occurs through the formation of thin bainitic ferrite plates that grow parallel to each other, constituting a packet, [13][14][15] while the second stage can be regarded as an eutectoid transformation, which can be either cooperative or degenerate in its nature as discussed by Hillert, [1,16] Aronson [17] and more recently by Yin et al [13] The cooperative eutectoid growth is characterized by ferrite and cementite growing together forming a lamellar structure if the carbon content is high enough. They grow preferentially at an angle to the primary ferritic plate and, moreover, both ferrite and cementite are in contact with the parent austenite during growth.…”

Effect of Carbon Content on Variant Pairing in Bainitic Low Alloy Steel

“…[13][14][15][16] However, the microstructural difference observed at intermediate austempering temperatures correspond well to the observed difference in variant pairing with no clear dominant variant pair even if a slight increase in V1-V2 can be observed for the three steels with higher carbon content. Even if the complexity at the intermediate temperatures not is fully understood nor the mechanisms behind it, it is here clearly shown that all the changes in the bainitic microstructure appears gradually with austempering temperature which is consistent with Yin et al [24,42,44,45] Thus, definitions such as upper and lower bainite is not sufficient to describe the bainite structure over the full austempering temperature range since there is a region in-between that needs to be addressed. Further, a comparison between earlier published data and the data presented within this work was made by comparing the deviation angle for CPP and CPD between austenite and ferrite, Figure 7(a).…”