Thixoforming : Semi-solid Metal Processing

86j 3 Material Aspects of Steel Thixoforming
In contrast, the two cast exhibit a fine-grained, globular structure without noteworthy
intra-globular liquid phase content.
3.5.2
Concluding Appraisal of TiN Doping
In conclusion, it can be noted that the rolled primary material, in which the carbides
exist long-stretched and chain-like, should not be used for thixoforming, due to the
disadvantageous melting behaviour and the increased segregation tendency. Whereas
the hot-rolled material exhibits coarse-grained and cloudy melting behaviour with
a high content of intra-globular liquid phase, the two cast states show fine-grained,
globular melting behaviour without noteworthy intra-globular phase contents. In the
solid state, an influence of the TiN particles on the austenite grains could be detected.
Because the grain-fining effect of the TiN particles is lost in the partial liquid state and
the TiN particles have a negative influence on the dynamic mechanical component
properties due to their angular morphology, TiN doping should not be applied. The
examinations of the laboratory melts show that a structural regulation is unnecessary
by means of TiN particles, because the as-cast material exhibits a rather fine-grained,
globular structure. Concerning the primary material selection, it can be generally
noted that primary material for the production of low-segregation components
should not contain pronounced carbide banding because these (and also very
homogeneous material due to the self-blocking–remelting effects) lead to a complex,
non-globular melting behaviour with high intra-globular liquid-phase contents.
Similar results for examinations of steel HP9/4/30 can be found in the literature [63].
3.6
Microstructure Analysis and Material Properties
In the partial liquid state, steels exhibit a multiphase structure. Due to their
equilibrium concentrations, sometimes highly different element contents between
the liquid and solid phases can occur, which lead to different phase transformation
behaviours. Resulting from these conditions, an isolated observation of both structural
areas and their interaction concerning the possible mechanical properties is
necessary. If the steels are continuously cooled or quenched from the partial liquid
state and subsequently aged, this can lead to new, complex structures. In this section,
the results of the examination and development of adaptive heat-treatment strategies
exploiting the former partial liquid state are presented and the potential is assessed by
means of the two steels X210CrW12 and 100Cr6.
3.6.1
Structural Changes and Properties of the Tool Steel X210CrW12
For the experimental examination of the transformation behaviour, cold-rolled rods
of steel X210CrW12 with a diameter of 56 mm and a composition of 2.12% C, 12.12%