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.