Thixoforming : Semi-solid Metal Processing

3.4 Structural Parameter Development and Material Selectionj71
are still connected to each other (black areas), whereas they exist isolated at a lower
threshold of 200 (light green and blue areas). The difference of the measured solidphase
fraction in the depicted image is about 5%, whereas the influence on the
average grain size (Ddm ¼ 20 mm) and the contiguity of the solid phase (DCf ¼ 0.1) is
considerably larger. Even at a threshold of 200, globulites are still connected to each
other (yellow areas), which are regarded as separate particles by the automatic image
recognition and have to be corrected by hand according to the experiences of the
metallographer.
It can be noted that the structural parameters of X210CrW12 samples quenched
from the three-phase area (austenite–carbide–cast) can be determined allowing for
diverse factors of influence, because the austenite can be distinguished relatively
clearly from the eutectic. It cannot be ruled out that the globulites grow marginally
during quenching and that, therefore, higher solid-phase contents are measured.
However, this effect is overridden by the systematic errors during structure detection.
A metallographic determination of the carbide content is not possible because the
carbides occur mostly at the phase boundaries and cannot be differentiated from the
quenched eutectic. In the determination of the structural parameters, the most
important factors of influence are the thresholds set by the user for the detection of
the different phases and the minimal particle size (number of pixels) for the
consideration of a particle. The minimum number of pixels for the consideration
of a particle can be discounted if the existing eutectic is very fine. In addition to the
evaluation-specific inaccuracy, the determined liquid phase content is also dependent
on the particular observed sample area. Therefore, the deviation of different images
of the same sample can vary widely, which is also depicted by the dispersion bars in
Figure 3.15.
3.4.1.2 Metallographic Analysis of Quenched Specimens from the Two-Phase Area
Representative structural images of the quenched samples and the determined
liquid-phase contents of the quenching experiments above the three-phase area are
summarized in Figure 3.20. The structural images show that above a temperature of
1300 C secondary austenite accumulates at some austenite particles and protuberance
formation occurs. Furthermore, at this temperature the development of fine,
secondary austenite (highlighted by arrows) starts in large liquid-phase areas. At a
temperature of 1325 C, coarse, secondary austenite is increasingly formed. If the
primary austenite grains lie close to each, other the secondary austenite adheres to the
large primary grains. If the diffusion paths become long enough in larger liquidphase
areas, the coarse secondary austenite exists isolated in the dark matrix and only
adheres to the large austenite grains in the boundary areas. This phenomenon
intensifies with rising temperature, so that a sharp boundary between primary and
secondary austenite can no longer be drawn, which becomes apparent from the
quenched sample of 1350 C. At temperatures of up to about 1325 C, the protuberance-formed
accumulating austenite and the fine secondary austenite isolated in
the liquid can be considered without large errors and disposed of during image
re-editing. At higher temperatures, the distinction is no longer possible, because on
the one hand the eutectic coarsens with increasing temperature and on the other the