Thixoforming : Semi-solid Metal Processing

82j 3 Material Aspects of Steel Thixoforming
austenite stability complicates the structural examination due to the solved carbon
and chromium contents in steel 100Cr6 and the associated martensitic transformation
during quenching.
Concerning the liquid-phase determination, it can finally be noted that it should be
resorted to liquid-phase contents calculated thermodynamically and detected by
means of DTA measurements, because metallographically determined values cannot
be used. A determination of the structural parameters such as the grain form and the
contiguity on quenched samples is not possible especially with higher liquid phase
contents, but is already problematic with medium or low liquid-phase contents.
3.5
Melting Behaviour
In this section, the results of investigations concerning the influence of carbide
distribution and the effect of titanium nitride on the melting behaviour of steel
100Cr6 are described, because examinations of thixoforged damper brackets within
the EU project Thixocomp exhibited very high segregation of solid and liquid phases
for long flow lengths during thixoforging. The reason for this was a coarse-grained,
cloudy melting of the rolled primary material with high intra-globular liquid-phase
contents. Damper brackets generated from the fine-grained, globular melting
material HS6-5-3 exhibited a considerably lower segregation tendency. To examine
the primary material influence and the specific adjustment of the fine-grained,
globular structure, the rolled state was, therefore, compared with the cast states (with
and without titanium doping).
3.5.1
Thermodynamic Preliminary Considerations and Microstructural Examinations
Concerning the Structural Regulation of Steel 100Cr6 by Means of TiN Particles
To determine the necessary amount of titanium for the formation of a sufficient
amount of TiN particles, thermodynamic calculations were executed with a standard
composition of 1.00% C, 0.35% Si, 0.30% Mn, 1.50% Cr and 50 ppm N. Figure 3.32
shows that with a liquid-phase content of 50% about 300 ppm of Ti can be solved,
whereas at the solidus temperature only 50 ppm of Ti can be solved. Because 1000 ppm
(0.1%) Ti would be necessary to form TiN in the pure melt, titanium is unsuitable as a
grain development addition for rheo-processes. The calculations show, furthermore,
that addition of 50–200 ppm of Ti should be sufficient for the microstructural
adjustment. Higher contents would lead to an early development of TiN particles in
the cast or in the partial liquid state and would, therefore, have no growth-hampering
influence on the austenite grain size. The calculations show, furthermore, that under
the given circumstances no unwanted formation of CrN is to be expected.
Based on the thermodynamic calculations, three different laboratory melts were
generated in a vacuum-induction furnace. As input -materials, rolled primary
material and sponge titanium were used. A laboratory melt without alloying of