Thixoforming : Semi-solid Metal Processing

Figure 3.31 BSE image (a), austenite (light) and ferrite or carbide
distribution (dark), (b) and orientation distribution of a specimen
quenched from 1425 C (c).
3.4 Structural Parameter Development and Material Selectionj81
partial liquid 100Cr6. Furthermore, the liquid-phase determination is complicated by
the superimposed martensite development during quenching.
3.4.3
Concluding Assessment of the Microstructure Parameter Determination
For the determination of the structural parameters and the phase contents in the
partial liquid state, thermodynamic calculations and quenching experiments were
conducted on steels X210CrW12 and 100Cr6. For steel X210CrW12, the characterization
of the microstructure was considered to be difficult above the eutectic
temperature, because liquid phases transform into secondary austenite and are
difficult to discern from the primary phase. The same limitation holds even more
strongly for systems without eutectic components.
For steel X210CrW12, a clear distinction between primary and secondary austenite
seems to be possible only with reservations. It cannot be discerned whether the
globulites grew in a homogeneous way or merely formed protuberances. At a
quenching temperature of 1350 C, the fraction of secondary austenite increases
markedly, whereas in comparison with the situation at 1300 C considerably more
secondary austenite with fine, dendritic formation in the eutectic is observable. This
dendritic austenite and also the austenite protuberances can be definitely attributed
to a former liquid phase whereas it is not evident whether an even globulite growth
took place during quenching. It can be assumed that during quenching of steel
X210CrW12, three different growth types of the secondary austenite can be discerned.
During structural evaluation, the uneven growth (formation of protuberances)
and the dendritic growth can be considered metallographically. In contrast, the
homogeneous grain growth cannot be quantified either with conventional image
analysis or by means of the microprobe and EBSD analysis methods.
In comparison with the results for steel X210CrW12, no conclusion can be drawn
concerning the structure in the partial liquid state due to the higher process temperatures
and the, therefore, higher permeability of the elements and the transformation
behaviour during the solidification of steel 100Cr6. Furthermore, the low