Effects of Plastic Stress on Transformation Plasticity and Microstructure of a Carbide-Free Bainite Steel

Abstract

Thermal simulation experiments were conducted, the microstructures were observed by scanning electron microscope (SEM) and transmission electron microscopy (TEM), and the bainite orientation was examined by electron backscattering diffraction (EBSD). The effects of plastic stress on transformation plasticity (TP) strain were investigated. The results show that the TP strain increases nonlinearly with the applied stress. It increases slowly versus the stress in the elastic region, and there is a sharp increase in TP strain if the stress exceeds the parent austenite yield strength because of the combined effect of prior deformation and stress. As the stress continues to increase, the bainite transformation amount decreases, leading to a slower increasing rate of TP strain. In addition, it is confirmed by EBSD that the TP strain is accompanied by the variant selection in microstructures, so that the occurrence of TP is consistent with the Magee mechanism. Moreover, the TP strain is more obvious at lower transformation temperatures due to the increased bainite transformation amount and more organized bainite structure.

Effects of ausforming temperature on carbide-free bainite transformation and its correlation to the transformation plasticity strain in a medium C- Si-rich steel

“…This indicates that TP strains decrease as the bainite transformation diminishes. This finding agrees with those reported by Liu [14], who also reported a reduction of the TP strains with the holding time in a medium carbon CFB steel.…”

“…26) and small dilatation strain (δ≈0.03) as a result of the invariant plane strain (IPS) [11,12]. Due to the IPS, the volume change is always assumed to be isotropic when the bainitic reaction occurs without influences of other factors (e.g., external superimposed stress and/or deformation before transformation) [13][14][15]. The transformed bainite commonly has a Kurdjumov-Sachs, K-S, orientation relationship with the parent austenite, where 24 variants could be grown in one austenite grain [16][17][18].…”

“…The anisotropy in volume change during bainite transformation could be described in terms of transformation plasticity (TP) or, in other words, the strain associated with the anisotropic bainitic transformation. It has been reported that an increase in nucleation and growth of bainite subunits intensified the TP strains [13,14,16,19], and therefore, TP strain could be used to quantify the influence of process parameters on the level of anisotropy in bainitic transformation.…”

“…Recently Liu et al [14] investigated the effect of plastic stress applied during bainite transformation at different temperatures on the evolution of TP strains in a CFB steel with medium C content. The authors concluded that the development of the TP strain was compatible with the Magee mechanism.…”

An implicit integration scheme with consistent tangent modulus for Leblond’s model of transformation-induced plasticity in steels

“…the von Mises norm, regardless of Greenwood and Johnson's mechanism. However, for martensitic transformations under very large applied stresses, Magee's mechanism may become the dominant mode of TRIP, as shown by Liu et al[22], so that applying J 2 -plasticity for σ eq ≥ σ Y may be an oversimplification in such cases and constitutive models including Magee's mechanism may be necessary, see e.g [7]…”