Abnormal Grain Growth Behavior of Ferrite in Pure Iron with Cold Torsion

This study systematically examined the abnormal grain growth (AGG) behavior of ferrite in pure iron subjected to cold torsion followed by annealing. Continuous shear strain in the radial direction of the cylindrical test specimen was introduced by cold torsion. Two torsion angles 1300 ° and 3700 ° were applied, and the resulting microstructural change was characterized through hardness distribution, optical microscopy, electron backscatter diffraction (EBSD), and inclusion particle analysis. Annealing at 1123 K revealed that AGG consistently occurred at regions with a shear strain of approximately 1.5, independent of the torsion angle or radial position. The abnormal grains formed in a circumferential chain, reaching diameters of 6 mm and 2 mm for 1300 ° and 3700 ° torsion, respectively. The recovery and recrystallization behavior were influenced by the shear strain. The emergence of AGG was successfully interpreted using the defect-model proposed by Hillert, which defines AGG conditions based on the balance between the driving force of grain growth and the pinning force exerted by dispersed particles.