Thixoforming : Semi-solid Metal Processing

Figure 5.4 Composition profiles of X210CrW12
after quenching at 300 K s –1 from 1285 C (50%
liquid) simulated using DICTRA. The dashed
vertical line shows the original solid–liquid
interface before quenching. The dashed
horizontal lines show the C and Cr
that, much higher cooling rates are needed, which cannot be realized in a normal
quenching experiment. Here, the already present austenite simply continues to grow
and it will be practically impossible to distinguish the original austenite from the
austenite formed during quenching. There is a change in growth morphology [5], but
this does not produce a sharp boundary. The eutectic formed during quenching is
much finer than the eutectic formed during normal cooling, so that liquid
fractions below about 40% can be evaluated fairly accurately. DTA is also frequently
used to evaluate liquid fraction as function of temperature (see Chapter 3). It is far
from trivial, however, to evaluate the fraction of liquid from the DTA signal even if the
instrument is well calibrated and the measurement is carefully done, since the
DTA signal depends on many factors which are difficult to determine and control
(see, e.g., [18]).
5.2.4
Origins and Consequences of Compositional Variations
5.2 Calculations for the Tool Steel X210CrW12j153
contents before quenching. The composition
profiles for the eutectic region are not realistic
and are therefore shown with dotted lines. The
M7C3 carbide is to the right, starting at about
18.3 mm.
If temperature is used to control the heating (or cooling) of the material to the
appropriate semi-solid state, it is necessary to know accurately the composition of the