Thixoforming : Semi-solid Metal Processing

74j 3 Material Aspects of Steel Thixoforming
Figure 3.23 Carbon, chromium, tungsten and silicon contents of
the line-scan measurements of specimens quenched from
1290 C (a) and 1330 C (b).
boundaries (Figure 3.23). The calculated depletion of silicon within the liquid phase
is also confirmed. In contrast to this, the transition of chromium is somewhat
flattened. However, the grains exhibit diverse average element contents. Thus, the
chromium and carbon contents in the left grain are about 7% (8.85–8.29%) and
(1.32–1.23%) higher, respectively, than in the middle grain of the line scan, whereas
the silicon and tungsten contents are approximately the same in the different grains.
One reason for the differences in the chromium and carbon contents can be found in
the local geometric circumstances. Another reason could be that even after a 30 min
holding time the equilibrium state is still not reached due to some inhomogeneity of
the rolled primary material and locally higher element contents still exist [71].
In the sample quenched from 1330 C, the difference in element content of
different grains is smaller, which can be ascribed to the higher diffusion rate at higher
temperatures. Whereas the carbon content changes erratically at the grain boundary,
the chromium content exhibits a more flattened transition from austenite to eutectic
than for the sample quenched from 1290 C. The difference within one grain is to
some extent smaller than the difference between different grains, so that a differentiation
of the accumulated austenite is no longer possible by means of microprobe
examinations. The tendency decreases with increase in temperature because of
increasing diffusion.
Figure 3.24 shows enlarged the chromium and carbon contents in the neighbourhood
of a liquid-phase area. It can be assumed that in the area primary austenite