Effect of Double Hit Hot Deformation on the Evolution of Dynamically Transformed Ferrite
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Effect of Double Hit Hot Deformation on the Evolution of Dynamically Transformed Ferrite K. Chadha1 · C. Aranas Jr.2 · M. Jahazi3 Received: 8 July 2020 / Accepted: 11 August 2020 © The Korean Institute of Metals and Materials 2020
Abstract Double-hit hot compression tests were carried on medium-carbon low-alloy steels using Gleeble 3 800® thermomechanical simulator. The experiments were performed at strain rates of 0.25 and 0.5 s−1 and temperatures of 1150 and 1200 °C with interpass times of 5, 15, and 25 s. The onset of critical stresses for dynamic transformation (DT) for both first and second hit were detected using the double-differentiation method. It was found that the critical stress for DT increased with a decrease in temperature and an increase in strain rate. The presence of dynamically transformed ferrite was observed and quantified using electron-backscatter diffraction, kernal average misorientation, and grain boundary maps. Then, a thermodynamic analysis was carried out using JmatPro software. A method of determining the change in Gibbs energy during DT phenomenon is proposed for double hit deformation. Keywords Dynamic transformation · Thermomechanical processing · Double-hit hot compression · Thermodynamics
1 Introduction Components made of high strength steels used for critical applications such as turbine shafts and gears are manufactured by forging. The manufacturing process starts with an as-cast ingot, which is heated in the temperature range well above the Ac3 of the steel. The cast ingot, characterized by microstructural and chemical inhomogeneities, is then open die forged through multiple passes to give the as-cast ingot a final shape. The ultimate property of the forged ingot is reliant on the chronicle of the deformation parameters such as temperature, strain rate, interpass time, stress etc. Understanding the evolution of the phases during the hightemperature deformation process is essential. During this deformation process, various dynamic and static softening processes occur, such as dynamic transformation (DT), * K. Chadha [email protected] 1
Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
2
Department of Mechanical Engineering, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
3
Department of Mechanical Engineering, École de Technologie Supérieure (ÉTS), Montréal, QC H3C 1K3, Canada
dynamic recrystallization (DRX) and static recrystallization (during interpass time) [1]. The occurrence of DT was first studied by Yada et al. [2, 3] in the 1980s. They explored the progress of DT under both laboratory testing conditions and pilot rolling mill trials. Later, they were able to follow the phenomenon in real-time when they deformed steel samples via torsion testing in an X‐ray diffraction apparatus. Since then, several researches have been made on the transformation of DT, both using in situ and ex situ techniques [4–8]. The peak in the flow curve is usually a good indication that a softening process occurred. H
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