Thixoforming : Semi-solid Metal Processing

It is clear that C has by far the strongest influence, but that also Si has an appreciable
influence.
5.2.5
Enthalpy and Heat Capacity
The enthalpy of X210CrW12 around the semi-solid range is shown in Figure 5.5 and
the heat capacity is shown in Figure 5.6. The enthalpy curve reaches zero close to, but
not exactly at, room temperature for the stable state. There is some ambiguity in the
definition of heat capacity when several phases with changing compositions and
amounts are present. It makes most sense to take the heat capacity to mean the
derivative of the actual enthalpy curve [19]. Then the heat capacity is only undefined at
an invariant reaction, where it goes to infinity. In Figure 5.6, the heat capacity has not
been included in the semi-solid range. There it takes very large values, as is evident
from the slope of the enthalpy curve in Figure 5.5. It is strongly sensitive to changes in
temperature and is difficult to describe with a single equation. For practical purposes,
for example for process control (Chapter 10) or as input into a simulation program
(Chapter 6), it is then better not to use the heat capacity directly, but to use an
approximation of the following type:
DH 0:84DT þ 207Df L ð5:6Þ
where f L is the liquid fraction. If needed, a more accurate approximation can be
chosen. At low temperature the heat capacity is different depending on whether the
Figure 5.5 The enthalpy of X210CrW12 around the solid–liquid range.
5.2 Calculations for the Tool Steel X210CrW12j155