Thixoforming : Semi-solid Metal Processing

70j 3 Material Aspects of Steel Thixoforming
Figure 3.18 Quenched structure after a holding time of 30 min at
1200 C (a), 1225 C (b) and 1250 C (c).
fine eutectic and also easily measurable. Due to the high temperature dependence up
to liquid-phase contents of about 40%, this area represents only a small temperature
interval. By means of quenching experiments, these assumptions could be confirmed
experimentally up to a temperature of about 1250 C. At 1200 C, a completely
austenitic matrix with finely distributed carbides occurs (Figure 3.18). Above about
1225 C, the material starts to melt locally at a few grain boundary triple points (b),
which fits well with the determined solidus temperature of 1230 C from the
DTA. Because of the inhomogeneous primary material, first areas with liquid phase
form already locally at temperatures below the expected solidus temperature. In
Figure 3.18c, the structure of an sample quenched from 1250 C is exhibited. At this
temperature, the material is close to the upper boundary of the three-phase area. In
the quenched structure, a retained austenite content (light) of about 70% can be
determined, which is in good agreement with the value from DTA of 66%. The in part
strong dependence with the use of different thresholds for the same micrograph is
depicted in Figure 3.19, where the solid phase particles are assigned to different
grain-size grades, that is underlaid darker. At a lower threshold of 160, many particles
Figure 3.19 Structural evaluation of a sample at 1250 Cat
different lower grey-value thresholds of 160 (a), 180 (b) and 200
(c) for the solid phase.