Transformation textures during diffusional α→γ→α phase transformations in ferritic steels

The crystallographic transformation textures of a microalloyed low carbon steel were studied using X-ray and neutron diffraction techniques. Both with X-rays and with neutron radiation special equipment allowed to conduct in situ texture measurements at high temperatures, i.e, the determination of the texture evolution during rather than subsequent to annealing. The texture measurements were carried out in the ferritic regime at room temperature, after transformation into austenite at 950 degreesC, and after back transformation into ferrite. Furthermore, the temporal evolution of texture was studied by measurement of textures in the alpha+gamma two phase region with progressing transformation. The alpha --> gamma transformation degraded the texture sharpness, while during the alpha --> gamma transformation the texture intensity decreased only slightly. Qualitatively the final ferrite texture after alpha --> gamma --> alpha transformation was inherited from the initial ferrite texture. Based on the experimental data transformation textures were calculated using the Kurdjumov-Sachs relation and compared with the experimental transformation textures. Differences between experimental and calculated transformation textures were obtained and attributed to variant selection. For the alpha --> gamma transformation variant selection was associated with both dislocations and residual stresses interacting with strain caused by the volume change during transformation. The employed variant selection model depends on the grain orientation through the history of the initial grain. The variant selection during the gamma --> alpha transformation is proposed to be caused by crystallographic constraints.