Introduction
The final property of the forged ingot is dependent on the chronicle of the deformation process. The forging process begins with an as-cast ingot which has microstructural and chemical inhomogeneities. Understanding the evolution of the phases during the high-temperature deformation process is very essential. In the manufacturing industry, more than one hit is applied to give the as-cast ingot a final shape. During this deformation process, various dynamic softening processes occur, amongst which are dynamic transformation (DT) and dynamic recrystallization (DRX)[1]. The occurrence of DT was first reported by Yada et al.[2, 3] in the 1980s. They investigated 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 researchers have since studied both the forward and reverse transformations till date [4-8].
The initiation of DT and DRX usually occur at a particular strain known as the critical stress (σc) and their related strains are known as critical strain (εc) [9]. As DT and DRX are flow softening mechanisms, they usually lead to a reduction in the net work‐hardening rate as related to the values anticipated when only dynamic recovery is taking place. However, the flow softening is relatively insignificant at the critical strains since very minute volume fraction of ferrite and recrystallized austenite form during this stage. Therefore, the flow stress further increases beyond these critical strains until the softening due to the effect of DT and DRX is balanced by the strain hardening taking place simultaneously in the undeformed material. After this, steady state stress is achieved, which is the final balance between the rates of hardening and softening [10].
The peak in the flow curve is usually a good indication that a softening process occurred. However, it does not provide any evidence about the exact initiations of DT or DRX. Such critical stress can be successfully determined using the double differential technique developed by Poliak and Jonas [11, 12]. The method involves the determination of the critical stress (and its corresponding strain) from the point of inflection in a plot of the strain hardening rate as a function of the stress. Since most of the extensive analysis of critical strains in the literature involves single hit experiments, in the current work, the double differentiation technique is applied to the flow stress data of double hit deformed medium carbon low alloy steel, processed at two temperatures, two strain rates and one interpass time. The dependence of critical stresses and strains on temperature and strain rate to initiate DT and DRX (complemented with microstructural analysis) are presented.