Thixoforming : Semi-solid Metal Processing

68j 3 Material Aspects of Steel Thixoforming
Figure 3.15 By means of Thermo-Calc and DTA (10 K min 1 )
calculated and metallographically determined liquid-phase
contents with fluctuation margin (with A ¼ austenite, K ¼ carbide,
S ¼ cast, according to Thermo-Calc calculation).
thermo-element placed within the sample s centre. A temperature of 200 C after a
cooling time of 7 s was reached, so that cooling rates of 100–200 K s 1 were realized.
Figure 3.15 shows the metallographically established liquid-phase contents of the
quenched samples, denoted by the error bars. Above the three-phase area (above
about 1250 C), the samples exhibit a constant liquid-phase content of about 30%,
where the metallographic determination was carried out by measuring the secondary
austenite and the eutectic areas. Figure 3.16 shows exemplarily the approach and the
problems in the determination of the phase contents. The results in Figure 3.15
Figure 3.16 Micrograph of a sample quenched from 1350 C. In
the partial image (a) the completely light phase exhibits a volume
content of 72.5% while the detected areas in the partial image (b)
show a content of 55%. The different colours symbolize the
different grain-size grades. The enlarged partial image (c) clarifies
the difficulty in distinguishing between primary and secondary
austenite.