Effect of Double Hit Hot Deformation on the Evolution of Dynamically Transformed Ferrite

K Chadha,C Aranas,M Jahazi

doi:10.1007/s12540-020-00855-x

Abstract

Double-hit hot compression tests were carried on medium-carbon low-alloy steels using Gleeble 3800® 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.

Highlights

Components made of high strength steels used for critical applications such as turbine shafts and gears are manufactured by forging
The results show that the stress levels increase with an increase in strain rate; these stresses decrease with increasing deformation temperature
It can be observed from the double differential curves of the first hit deformation that critical stresses required for initiation of dynamic transformation (DT) at 1200 °C are lower than that of the critical stresses at 1150 °C for both strain rates

Summary

Introduction

Components made of high strength steels used for critical applications such as turbine shafts and gears are manufactured by forging. Understanding the evolution of the phases during the high-temperature deformation process is essential. During this deformation process, various dynamic and static softening processes occur, such as dynamic transformation (DT), 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. Several researches have been made on the transformation of DT, both using in-situ and ex-situ techniques [4,5,6,7,8]

Methods

Results

Conclusion