Thixoforming : Semi-solid Metal Processing

3.4
Structural Parameter Development and Material Selection
In general, the characterization of the microstructure in the partial liquid state is
often carried out on quenched samples. However, in most methods for the examination
of partial liquid materials such as holding experiments, compression tests,
primary material examinations and also rheology tests, the microstructure prevailing
in the partial liquid state is of interest. Therefore, the samples have to be quenched so
quickly that the microstructure at room temperature reflects the partial liquid state.
Due to diffusion processes and phase transformations, this is often not successful so
that the microstructural changes caused by quenching have to be considered. If the
structure was changed much during quenching, the determined structural parameters
have to be corrected or can possibly not be determined at all. Therefore, exact
knowledge and examinations of the mechanisms that occur and their effects on the
structural parameters are of special importance. For structures for which an exact
determination of the structural parameters is not possible due to extremely large
changes during quenching, error-tolerance ranges have to be prepared. Within the
scope of the examinations, therefore, the steels 100Cr6 and X210CrW12 already used
for thixoforming were observed in detail to evaluate the magnitudes of error in the
determination of structural parameters of samples quenched from the partial liquid
state. These two steels were chosen to examine the behaviour of a complexly melting
steel with eutectic (X210CrW12) and a simply melting steel without eutectic (100Cr6).
The austenite in the carbon-rich steel X210CrW12 exists in a metastable state at room
temperature after quenching, whereas the steel 100Cr6 transforms martensitically.
The results of DTA examinations, thermodynamic calculations in equilibrium
(Thermo-Calc) and in consideration of diffusion-controlled processes (DICTRA) are
discussed and compared.
3.4.1
Development of the Structural Parameters of Steel X210CrW12
The examination of the structure development and the determination of the
structural parameters of steel X210CrW12 in the partial liquid state was carried out
by means of cylindrical samples (d ¼ 15 mm, h ¼ 25 mm) with a chemical composition
according to Table 3.2 at temperatures from 1200 to 1400 C. For this purpose,
they were held isothermally in an aluminium oxide pan inside a convectively
heated furnace for 30 min and subsequently quenched with water. The temperature
measurement throughout the experiments is achieved continuously by means of a
Table 3.2 Chemical composition of the examined steel
X210CrW12 (all data in mass%, except N in ppm).
3.4 Structural Parameter Development and Material Selectionj67
C Si Mn Cr Mo Ni Cu W Fe N
2.08 0.41 0.30 11.57 0.07 0.21 0.04 0.74 Rest 251