Thixoforming : Semi-solid Metal Processing

Figure 5.2 Calculated temperature versus mass% C phase
diagram for X210CrW12. The dashed lines show the allowed
interval for C.
temperature. More details about microstructure evolution of semi-solid processed
X210CrW12 are given in Chapter 3 and by Uhlenhaut et al. [17].
5.2.2
Solidification
5.2 Calculations for the Tool Steel X210CrW12j151
X210CrW12 forms austenite as the primary phase during solidification and, when
about 40% liquid remains, an austenite þ M7C3 eutectic forms. The cooperation
between austenite and M7C3 is not very good so the eutectic structure becomes rather
irregular and, depending on detailed conditions, more or less coarse M7C3 particles
can be found in the microstructure. The liquid fraction as a function of temperature
calculated with different methods is shown in Figure 5.3. The eutectic formation
starts at the knee. For the DICTRA simulation a cell size of 20 mm and a cooling rate of
20 K min –1 were used. The liquid fraction is given as mole fraction. It would be better
to use volume fractions, but in order to do this molar volume data are needed, which
are not generally available in thermodynamic databases. In the case of steels, volume
data have only very recently become available (in TCFE4 [11]). Mole fractions can in
most cases be assumed to be reasonably close to volume fractions, more so than
weight fractions, which can also be easily calculated. In Figure 5.3, The DICTRA
simulation can be assumed to be the most realistic. The equilibrium curve is
very close to the DICTRA simulation above 20% liquid and the modified Scheil
calculation, assuming that C is a fast-diffusing element, is practically coincident with