Stress induced transformation to bainite in Fe–Cr–Mo–C pressure vessel steel

Abstract

The kinetics of the bainitic transformation in a polycrystalline Fe-Cr-Mo-C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the transformation behaviour, with clear evidence that the stress tends to favour the development of certain crystallographic variants of bainite, even though the stress may be well below the single phase yield strength. It is concluded that the transformation is influenced significantly by stresses as low as 45 MN m-2, even though the effect may not be obvious in metallographic studies. The results are analysed and discussed in terms of the mechanism of the bainite transformation.MST/1394

Properties of fine-grained steels generated by displacive transformation

“…Note that TRIP is not unique to martensite. Bainite [10] and Widmanstätten ferrite [11] also exhibit the phenomenon given their similar shape deformations. These transformations all are displacive, i.e., they are accompanied by a shape deformation which is an invariant-plane strain with a large shear component and hence show pronounced transformation-induced plasticity under appropriate circumstances.…”

Calculation of crystallographic texture due to displacive transformations

“…That displacive transformations produce highly anisotropic strains when variant selection is significant has been demonstrated experimentally [37][38][39]. • An important outcome of the fact that transformation strains can be calculated using the crystallographic set of martensite is that such strains can be exploited as an alternative or supplemental method of assessing texture.…”

Mathematics of Crystallographic Texture in Martensitic and Related Transformations

“…7 for uniaxial tension and compression. That displacive transformations produce highly anisotropic strains when variant selection is significant has been demonstrated experimentally [28][29][30]. Fig.…”

“…It would be reasonable to assume that there is strong variant selection when the ratio of ΔG MECH /ΔG is large [29]. This turns out to be the case as illustrated in Fig.…”

“…We begin therefore with an introduction to this theory. [19][20][21][22][23][24][25][26][27][28][29][30][31] Crystallographic Theory of Martensite Structure of the Interface Any process which contributes to the formation of martensite cannot rely on assistance from thermal activation. There must therefore exist a high level of continuity across the interface, which must either be coherent or semi-coherent.…”