Stress or strain induced martensitic and bainitic transformations during ausforming processes

Abstract

The so-called ausforming treatment consists in plastically deforming a fully austenitized steel below the recrystallization stop temperature, prior to either a martensitic or a bainitic transformation. Although this procedure is envisioned as a way to improve the mechanical response of the attained microstructure, it is not without its drawbacks, as the possibility of phase transformations occurring during the deformation step. When such step is applied at relatively higher temperatures than those of the aimed microstructure, the presence of those softer phases could be rather detrimental for the final microstructure. The current study aims to further study those phase transformations to analyze whether they are induced via either stress or strain and to identify the temperature ranges in which they tend to occur, so that they can be avoided or used to improve the final microstructure properties in the future. A final evaluation on the impact of these induced transformations on the final microstructure when deformation is followed by either cooling or an isothermal treatment is also performed.

Future Trends on Displacive Stress and Strain Induced Transformations in Steels

“…The appearance of the metastable phases named in this section as a product of SIT/DIT mainly depends on the chemical composition of the steel and on the deformation temperature. Hence, whereas the formation of stress or strain induced α is very common in many different microstructures [23,25,35,[49][50][51][52][53][54][55][56], ε is mainly found in austenitic steels [23,25,39,48,52], although it has been sporadically detected in multiphase steels [47]. The formation of α B has not been reported, to the authors' knowledge, at room temperature, although it has been found in fully austenitized steels [35] and in multiphase steels [1,3,57] deformed at high temperatures.…”

“…Depending on the chemical composition of the steel, a bainitic microstructure can contain cementite, martensite, and/or retained austenite, apart from αB. Figure 1a,b show two micrographs showing the granular bainite and plate-like bainite microstructures that were formed in a medium carbon high silicon steel by a DIT at 520 °C and 400 °C, respectively [35]. [35], Copyright (2021) with permission from Elsevier; (b) strain induced (plate-like) bainite formed at 400 • C in the same medium carbon high silicon steel surrounded by a martensite matrix, after [35]; (c) In-situ TEM micrograph taken during traction test showing the direct γ →α transformation observed while straining a 304 stainless steel at −32 • C, where the white arrows point the interface front and the straight white line is the invariant line; reprinted from [37], Copyright (2021) with permission from Elsevier; (d) in-situ TEM micrograph taken during traction test showing a α embryo on a band of ε in a 304L steel deformed at −196 • C, reprinted from [38], Copyright (2021) with permission from Taylor & Francis.…”

“…, and B s (σ Y ) [35,64]. For a given temperature, the critical stress that must be applied so that a α , ε or a α B SIT starts is called σ crit−α −SIT , σ crit−ε−SIT and σ crit−α B −SIT , respectively.…”

“…Regarding the nucleation of α B , although it is known that is promoted by the presence of a larger amount of defects or slip bands [99] generated during plastic deformation [31], it has not been reported how the nucleation condition found in Equation 2is modified because of the increase of nucleation sites. Further information on the calculation of the M α d and B d critical temperatures or energies can be found in [35]. Again, it is considered that α and α B DIT can happen below the M α s (σ Y ) and B s (σ Y ) temperatures, respectively, as seen in Figure 2 because, even though the α and α B transformations are thermodynamically possible below those temperatures if plastic deformation is not applied, the application of plastic strain enables to further stimulate both transformations.…”

“…Concerning the second case (SIT/DIT happening in thermomechanical treatments), there are many scenarios that have not received much attention, especially the ones involving the formation of bainite. For instance, although the bainite SIT/DIT has been studied under constant stress [30][31][32][33][34], the formation of bainite while straining fully austenitized steels [35,36] has not been deeply assessed.…”