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Super-austenitic
stainless steels are special grades of austenitic stainless steels alloyed with
higher concentrations of chromium and nickel in addition to the presence of
relatively higher contents of nitrogen, molybdenum, and copper 1–3.  Due
to its excellent combination of alloying elements, these steels often possess superior
mechanical properties and greater corrosion resistance compared to ordinary
grades of austenitic stainless steels which facilitates its broad applicability
in thermonuclear and chemical industries. 4–7. Thermo-mechanical processing is extensively
employed for manufacturing of complex parts and shapes of the alloy that are used
for various industrial applications 8–11. The evaluation of forming load has
cardinal importance in forming industries for designing various forming
components. The forming load often depends upon flow behavior, geometry of the
deformation and friction between workpiece and die interface 12–16. Therefore, formulation of a suitable constitutive relation to
predict the elevated temperature flow behavior has gained much attention
nowadays.  The constitutive flow behavior of
polycrystalline alloys is often found to be very complex and chiefly depends on
various processing parameters like temperature, strain, and strain-rate 17–20. Several constitutive models viz.
physically based 12,21–30, phenomenological 13,19,31–39 and empirical/semi-empirical 2,40–42 models have been developed by researchers
in the past for predicting flow behaviour of different grades of metals and
alloys following hot deformation.

Amongst the physically
based/phenomenological relationships,
Johnson-Cook (JC) 31 and Zerilli-Armstrong (ZA) 21 are widely accepted and extensively used in many commercial
metal forming simulation software .  The
JC model considers only the individual infleunce of processing parameters viz. isotropic
hardening, thermal softening and strain-rate hardening 31. Although, this model has widely been employed in the flow
prediction, it often fails when there is a change in flow mechanism 32,43,44. On the other hand, ZA model was often
preferred for low-temperature deformation below 0.6 Tm , where Tm is the melting
temperature of the alloy 45,46. The ZA model is often giving better prediction than JC
model as couples the effect of processing parameters such as
temperature and strain-rates 12,32,44. However, the
flow prediction employing this model often gives inaccurate results at higher
temperatures (>0.6 Tm), and lower strain rates 47. In view of this, a modified ZA (M-ZA) model was proposed by
Samantaray et al. 32 for predicting the flow behavior in  high temperatures and wide strain-rate domain.
This was accomplished by neglecting the athermal part of flow-stress and
incorporating the synergistic effects of strain-rate and temperature as well as
strain and temperature 32. The M-ZA model has been successfully applied by various
researchers for a various grades of materials 48–50.

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The objective of the
present work is to develop a suitable phenomenological constitutive model for
predicting high temperature flow characteristics of a super-austenitic
stainless steel in a wide range of TMP domain with good accuracy and
reliability. Before contemplating on developing a new model, we have first
assessed the applicability of the JC and M-ZA model for predicting the flow response
of the studied alloy. The individual and coupled effects of various process
parameters viz. strain, strain rate and temperature on the flow behaviour of
the alloy under investigation have been carefully evaluated. Based on this
observation and evaluation, a novel revised ZA (R-ZA) model has been proposed
and the predictability of the proposed model has been critically compared with
the existing JC and M-ZA models

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