Thixoforming : Semi-solid Metal Processing

3.3
Alloying Systems
Mainly highly alloyed materials are suitable for thixoforming, because of their broad
melting interval and their reduced solidus temperatures. Therefore, model alloys
were chosen, which belong to the group of tool steels. According to the temperature of
their main application field, these are divided into cold, warm and high-speed
working steels (EN ISO 4957) [45]. The division of cold working steels into three
groups takes place according to their carbon content and the resulting microstructure:
hypoeutectoidic, hypereutectoidic and hypoeutectoidic–ledeburitic (Figure 3.10). In
the diagram, it can be seen that for a chromium content of about 13 mass% a
ledeburitic eutectic can be already formed with a carbon content of 0.8 mass%. The
probably most essential property of cold working steels is hardness, which is
dependent on the alloying elements and the microstructural components. The
hardness is influenced by the matrix, which is characterized by the amounts of
martensite and retained austenite and also by the included carbides, for which the
constitution and the arrangements are relevant. The attainable hardness as a function
of the tempering temperature serves essentially as characteristic attribute [46].
In hypoeutectoidic steels (0.4–0.7%), the carbon is nearly completely dissolved in
the matrix during holding at hardening temperature and the accomplishable hardness
increases with rising C content. However, for steels in this group, it remains
below 62 HRC, whereas the retained austenite content is low. In hypereutectoidic
steels (0.8–1.5%), the carbon content is chosen to be so high that on the one hand the
full martensite hardness of at least 64 HRC is reached, but on the other still up to
about 5 vol.% unsolved carbides remain in the matrix. In hypoeutectic or ledeburitic
steels, the C content is over 1.5%, so that towards the end of the solidification a
carbide eutectic is formed, which will nearly completely be spit out during the usually
Figure 3.10 Division of cold working steels in three groups by
means of a schematic Fe–C diagram (a) and the resulting
structural constitution (b). [46].
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