Thixoforming : Semi-solid Metal Processing

Figure 3.17 Schematic clarification of the development of
pre-eutectic, secondary austenite by means of the calculated
phase contents of steel X210CrW12 for its standard composition.
3.4 Structural Parameter Development and Material Selectionj69
suggest that with quenching from higher temperatures metallographically lower
liquid-phase contents are being determined than expected on the basis of thermodynamic
calculations and DTA. Therefore, the following examinations are concerned
with the question of whether the solid and liquid phase contents or rather the
structural parameters above the three-phase area can be determined sufficiently
accurately with conventional analysis methods. The focus is the question of how far
the secondary austenite generated during quenching is discernible from the primary
austenite being in equilibrium during the liquid phase. Here, the regular grain
growth of the globulites is distinguishable from the irregular grain growth, where the
secondary austenite builds up in the form of dendrites on the primary grains and
forms protuberances [71].
Figure 3.17 illustrates the problem of secondary austenite formation for temperatures
of 1300 and 1350 C by means of a diagram showing the equilibrium phases of
steel X210CrW12. At the eutectic temperature, the structure exhibits an austenite
content of about 60%. If the temperature is increased, the austenite content decreases
to about 40% at 1300 C, so that theoretically Dg1300 ¼ 20% is obtained, and increases
up to a temperature of 1350 CtoDg1350 ¼ 50%. As depicted in Figure 3.16, this phase
content of secondary austenite is very hard to discern after quenching.
3.4.1.1 Metallographic Analysis of Quenched Specimens from the Three-Phase Area
Based on the thermodynamic calculations and the preliminary assumptions, it is
expected that for the material X210CrW12 the determination of the liquid-phase
fraction poses no great difficulties up to the temperature at which the material still
remains in the three-phase area (austenite–carbide–liquid). The solid phase should
occur in form of metastable retained austenite due to its high carbon content and
should be easily distinguishable from the formerly liquid phase, which is shaped as a