The Effect of Retained Work Hardening on the Driving Force for Dynamic Transformation
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The high-temperature deformation of steels is known to induce the austenite to ferrite phase transformation at temperatures above the Ae3.[1,2] This transformation may affect the mechanical properties of the final product following hot deformation and any subsequent processing. For example, this mechanism contributes to the production of high-strength steels with fine-grain microstructure, as shown in References 3 and 4. However, despite the benefits of dynamic transformation (DT), consideration of this phenomenon is often neglected in the thermomechanical modeling of hot forming processes such as rolling or forging.[5,6] This is because of the complex nature of DT, and/or the limited understanding of the exact effects of multi-pass deformation on its occurrence.
K. CHADHA and J.G. SPRAY are with the Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada. Contact e-mail: [email protected] C. ARANAS JR. is with the Department of Mechanical Engineering, University of New Brunswick, Fredericton, NB E3B 5A3, Canada. D. SHAHRIARI and M. JAHAZI are with the Department of Mechanical Engineering, E´cole de technologie supe´rieure (E´TS), 1100 Notre Dame Street West, Montreal, QC H3C 1K3, Canada. Manuscript submitted May 21, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
To date, several papers have provided in situ evidence of dynamic transformation (DT) employing neutron diffraction and synchrotron techniques.[7,8] There are also thermodynamic models to support the occurrence of this type of phase transformation.[9–11] These models show that either (1) the stored energy of dislocations or (2) transformation softening act as driving forces to induce the phase transformation. The former cannot explain the dynamic phase transformation that takes place well above the Ae3 temperature (100 C or more above the Ae3),[1,2,12] while the latter does support the transformation. Although there are numerous experimental and theoretical studies on DT, the effect of multipass deformation on the driving force to initiate DT has not been explored despite the fact that most of the industrial hot deformation operations are multi-pass by nature. The retained stress after each pass affects the driving force, and therefore needs to be taken into consideration. The present work provides a method to incorporate the retained stress in the calculation of driving force during multi-pass deformation. An as-cast medium-carbon low-alloy steel was employed for this purpose. The composition of the steel used for the experiments is shown in Table I, along with its para-equilibrium temperature. These values were calculated using the FactSage thermodynamic software.[13] Materials were provided by Finkl Steel-Sorel. Cylindrical specimens were machined from the central region of the as-cast ingot with diameters and heights of 10 and 15 mm, respectively. Hot compression tests were performed using a Gleeble 3800 thermomechanical simulator following the procedures described in ASTM E209. The schematic diagram for double-hit co
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