Dilatometric and microstructural response of variant selection during α′ transformation

Abstract

Variant selection during displacive transformation is popularly monitored by assessing the pole figure or orientation density function of the bulk texture or pole figures from single prior austenite grains. It can also be done by measuring the physical orientation of α′ (bainite or marteniste) plates in the microstructure. It is known that when variants of α′ form randomly, i.e. there is no variant selection, the shear strain associated with each α′ plate gets canceled and transformation strain becomes equal to the volume strain. Under this condition, orientation of sheaves of α′ also shows random distribution. In this work, it has been shown how transformation strain and orientation of sheaves change when variant selection occurs. Mathematical models have already been developed to calculate the transformation strain under various loading conditions; extensive experiments have been conducted during the course of this work to show how the nature of externally applied stress influences the transformation strain. In addition to this, it has also been shown that the variant selection might leave its trace in the microstructure by affecting the alignment of the sheaves under externally applied stress and strain. Mathematical models have been developed to show that variant selection has negligible influence on physical orientation of α′ plates when transformation occurs under external stress, but prior plastic deformation can dictate the orientation to a great extent.

Microstructure Evolution at Different Cooling Rates of Low Carbon Microalloyed Steels

“…As a consequence, displacive transformations, such as bainite and martensite, have been discussed in detail along the last years [6].…”

“…To detect the phase transformation temperatures, dilatometry tests are performed in these materials, as it is frequently used to study phase transformation temperatures of different steel grades [5,6]. In particular, in this work the samples of both steels were tested from austenite phase field at the maximum heating temperature of 950 °C and different cooling rates between 0.03 °C/s -8.21 °C/s accordingly to industry requirements.…”

“…The carbide precipitates (or nano precipitates) promote: a) the interstitial C removal from the matrix; b) modification on the γ transformation kinetic; c) promotion of grain refinement; and d) control of crystallographic texture. The different types of carbides that could be formed in the structure include M 23 C 6 and MX (M = Nb, V, Ti and X = C and/or N) [4].…”

CRITICAL ASSESSMENT 1: Outstanding issues in crystallographic variant selection in displacive transformations

“…The changing nature of transformation strain with changing nature of external stress has been described with the help of mathematical models, 40 supported by experimental evidence. 23 However, no report has been found on the nature and amount of transformation strain due to variant selection caused by plastic deformation.…”

“…As described by many researchers, [41][42][43] martensite or bainite has a tendency to form in 'packets' with typical cystallographic features that are capable of playing a critical role in variant selection 23,44 or cleavage fracture. 42,45,46 However, a clear definition of the 'packets' is vital.…”