Thixoforming : Semi-solid Metal Processing

Figure 5.7 The density of X210CrW12. The dashed curve shows
the density in the case when austenite is retained metastably
below its stable range.
5.2 Calculations for the Tool Steel X210CrW12j157
state there is usually a rich spectrum of particles, grain boundaries and so on where
the liquid phase can nucleate on heating. In X210CrW12, the most favourable
locations are at the phase interfaces between austenite and M7C3. In addition,
these phases react to form the liquid phase. The initial globule size can therefore
be expected to correspond the distance between M7C3 particles, which usually means
that the globules will be fairly small. In order to investigate the melting process
further, the melting process was simulated using DICTRA (see also [20]). The result
is shown in Figure 5.8. The heating was started at 1100 C, either with a completely
equilibrated sample or a sample solidified and cooled to 1100 C, that is, still showing
considerable microsegregation. The heating rate was 240 K min –1 . There are some
points worth noting. The melting curve is not the same as the solidification curve, but
is fairly close if the just solidified sample is reheated. The temperature where the first
liquid appears (incipient melting) depends strongly on the initial state of the sample
and, for a well-homogenized sample can even be considerably above the equilibrium
solidus. This melting curve is different from the solidification curve, also concerning
the temperature and liquid fraction where M7C3 is completely dissolved. The melting
is complete very close to the equilibrium liquidus temperature. This is independent
of the initial state and heating rate within some limits. Heating rates of 5–500 K min –1
were tested. However, if the heating rate is increased further or if the microstructure
is considerably coarser, the slope of the melting curves will decrease due to limited