2.1. Mathematical Approach
The description and evaluation of the plastic material properties presented below is based on the following assumptions:
The material behaves isotropic regarding all mechanical properties.
Strain hardening effects are negligible compared to the phase transformation effects.
Compared to the plastic deformations, the elastic, thermal and transformation strains are negligible small.
In the Norton–Bailey constitutive model [
14], in the double logarithmic depiction the stress–strain rate relation is linear, which can be formulated as
or as
in which
and
are material parameters specified for a reference strain rate
and as a function of temperature
. Equation (
2) is valid for a wide range of strain rates and materials and documented in several data collections [
8,
9]. Under the premise that all isothermal flow curves
intersect in one characteristic point
, Schmicker et al. [
7] bypass the determination of
for every temperature. Therefore, knowing the point
C, the Norton–Bailey exponent can be expressed as
as a feature in the so-called consistently assessed Carreau fluid model. The strain rate sensitivity typically increases with increasing temperatures and to produce higher strain rates, higher stresses are required, which is ensured by
To distinguish heating and cooling, the flow properties during continuous cooling are subsequently denoted by an apostrophe. Two effects are taken into account for.
Firstly, a heat treatment effect, in which the material either hardens or softens due to microstructural changes. Rapid quenching causes the formation of martensite, which achieves more than twice the hardnesses than ferrite and pearlite. In annealing processes on the other hand, the cooling is typically slow to avoid this transformation.
Secondly, a transformation inertness that causes the transformation to shift to other temperature ranges depending on the cooling rate. The quicker the cooling process, the less is the time for the carbon diffusion processes. If the diffusion can not take place at all, the carbon become trapped in a body-centered tetragonal lattice configuration below martensite start temperature. In
Figure 1 it is also seen that even for very slow cooling, the transformation starts well below
.
Assuming that
during cooling is not necessarily identical to
at heating, but similarly shaped, the two curves are linked by adding offset parameters
and
for prior discussed transformation effects, depending on
.
The hardening factor
can be interpreted as a vertical scaling of
to account for the heat treating effect as prior applied by Rößler et al. [
15]. For its evaluation the linear relation in-between the yield strength
and the Vickers hardness
H [
16,
17]
is utilized, in which it is physically reasonable that
b is zero. It should be mentioned that for other hardness scales the correlation can be non-linear. At room temperature,
can be expressed as the proportion
in which
is the initial hardness corresponding to
and
the hardness after cooling with a specific rate. To couple the hardening to the actual transformation, sigmoid function
limits hardening to the lower temperature range. The parameters
and
are either found in the CCT diagram (
Figure 2) or can be experimentally determined by indentation and dilatometric testing. The use of the
temperature is a recommendation for a free value of this equation, because it guarantees
, which must always be fulfilled for mathematical reasons.
To shift
horizontally due to a transformation inertness, the delay temperature
is similar defined as
. It is worth mentioning that
and
actually start to raise above
to compensate discontinuities of
introduced by large
for martensitic transformations, for instance.
Concerning Equation (
3), the characteristic intersection point
has to be re-evaluated, too, to satisfy (
4) for
. To maintain the same high strain rate senstivity at high temperatures around the melting point
and assuming that the low sensitivity for low temperatures will not change, point
is identified using
To estimate hardnesses not documented in the CCT diagram, law of mixture
as presented by Ion et al. [
18] can be applied, in which
are the phase fractions of ferrite, pearlite, bainite, martensite, and austenite. The calculation of the phase fraction might be done numerically using evolution equation
by Leblond and Devaux [
19].