Thixoforming : Semi-solid Metal Processing

72j 3 Material Aspects of Steel Thixoforming
Figure 3.20 Structures and determined liquid-phase contents of
quenched samples above the three-phase area, where fine,
secondary austenite is highlighted by the arrows.
globulites become increasingly smaller, which makes the differentiation more
complicated.
Due to the large difference (>20 vol.%) between the metallographically determined
and the DTA and Thermo-Calc designated phase contents, it has to be assumed that
in addition to the described local, irregular increase in secondary austenite, a smooth
globulite growth occurs in all directions in space. This mechanism is more pronounced
at higher temperatures, so that it cannot be neglected as with lower liquidphase
contents. An examination of the differently formed austenite is impossible by
means of conventional metallography.
Because conventional examination methods meet their limits for materials with
fine, low-contrast structures, where a differentiation by means of grey-scale areas is
impossible, texture analysis and analysis of the element distribution provide methods
to solve many problems [62]. Therefore, microprobe examinations and EBSD
(Electron Beam Scattering Detection) measurements were executed to achieve a
differentiation of primary austenite and regularly growing secondary austenite.
3.4.1.3 Microprobe Examinations of the Quenched X210CrW12 Specimens
In addition to the metallographic examination, two samples quenched from 1290 and
1330 C were examined with a microprobe to differentiate between primary and
secondary austenite by means of element distribution. X-ray intensity distribution
images with a step range of 2 mm show that the austenite grains for these two
temperatures exhibit nearly identical element content intensities (Figure 3.20). In
Figure 3.21, the intensities of the elements Si, W, Cr and C are displayed. The
examination shows that no gradients giving information about the former phase