Thixoforming : Semi-solid Metal Processing

78j 3 Material Aspects of Steel Thixoforming
Figure 3.27 By means of Thermo-Calc and DTA (10 K min 1 )
calculated liquid-phase contents of steel 100Cr6.
Figure 3.27 shows the liquid-phase contents calculated using Thermo-Calc and
DTA. The difference between the two curves is continuously increasing due to the
influence of the heating rate, the difficulty in evaluating the continuously increasing
DTA signal and the extensive scattering of several measurements of the same
material due to local inhomogeneities (carbon enrichments).
3.4.2.1 Metallographic Analysis of Quenched Specimens of Steel 100Cr6
For the examination of the structure of steel 100Cr6 in the partial liquid state,
quenching experiments were conducted for which the samples were embedded in
glass to avoid oxidation. As an example, the water-quenched structures from 1350,
1375, 1400 and 1425 C after a holding time of 20 min are depicted in Figure 3.28. The
samples quenched from 1350 C exhibit some light areas, at which the material
already starts to melt locally due to the enriched alloying elements at the grain
boundaries (a). The quenched formerly partial liquid structures show that no conclusion
can be drawn about the structural conditions in the semi-solid state, because
the structures look approximately the same over the complete semi-solid interval.
Independent of the quenching temperature, a martensitic formation with lighter
grain boundary areas is discernible with minor amounts of retained austenite between
the needles. The martensite needles are somewhat coarser at lower temperatures,
where at the retained austenite content seems to increase due to the higher carbon
Figure 3.28 Microstructures of 100Cr6 specimens quenched from 1350, 1375, 1400 and 1425 C.