Thixoforming : Semi-solid Metal Processing

Figure 9.9 Middle temperature courses of the
steels 100Cr6, 42CrMo4 and X210CrW12.
Change of the electromagnetic qualities at
780 C. As in the case of aluminium, the
temperature course is very flat according to
admission to the SSM region and hardly allows
9.3 Thixocasting of Steel Alloysj321
the assignment of the liquid phase content. The
end of the dissolution of the chromium carbides
becomes clear in the hold step. The energy set
point courses show characteristic differences in
the transition in the hold step (around 300 s) and
in the power.
With the help of some heating trials, an energy set point curve is successful. This
curve consists in principle of four periods: start-up, fast heating, reduced ferritic
phase and homogenization phase (Figure 9.9). Following the short start-up phase, the
performance can be increased rapidly to achieve the maximum converter system
voltage because of the good coupling of the steel to the coil in the ferromagnetic area
(for 1.7225: 70 s).
After the Curie temperature in the outer billet shell has been exceeded, the power
must be reduced slightly (300 s) in phase three. With this power, the bolt is heated to
the semi-solid range since both the amount for the heating of the billet and the share of
the melting heat must be found. The time for switching over to the homogenization
phase has to be determined when the bolt reaches its desired liquid phase content but
also largely keeps its form. A temperature compensation is realized by heat conduction
in this hold step (Figures 9.10). The middle billet temperature is constant and merely
the overheating of the outer shells is used to warm the inside and bring the liquid state
contents into line. The tray drives out automatically for all alloys after 420 s.
In the early years, the pre-material billet had the dimensions f 55 160 mm and a
shot weight of 3.5 kg. The billet was moved into the coil recumbently in a tray. Due to
the high energy content of the high-carbon semi-solid steel billet at temperatures
around 1300 C and also the occurrence of high-temperature corrosion, new solutions
for the tray had to be developed. The new framework of the tray was constructed
of magnetically soft stainless-steel sheets to protect the reheating unit from melting.
The billet holder was lined with a specifically designed ceramic insulation insert,
which actually carries the semi-solid billet (Figure 9.11). Additionally, a cover with an
integrated inert gas supply was designed.
It turned out to be disadvantageous that a scale had formed up to the triggering of
the shot by oxidation and cooling at the bolt surface and that the liquid state content
was not completely homogeneous.