Thixoforming : Semi-solid Metal Processing

62j 3 Material Aspects of Steel Thixoforming
following hot deformation. Despite the comparatively high content of retained
austenite of these steels, degrees of hardness of 63–68 HRC are reached [46].
The multiphase structures of tool steels consist of a highly alloyed matrix, in which
carbides of different chemical compositions and of different types are included. The
carbides have a significant influence on the mechanical properties. Their impact is
determined by their constitution, amount, size, shape and arrangement. In general,
the chromium-rich carbide M 7C 3 is considerably harder than the pure iron carbide
Fe3C, whereby the chemical composition of the carbides can fluctuate over a wide
range [45]. During annealing at temperatures from about 100 C, first low-alloyed
carbides precipitate in several steps, and with increasing annealing temperature also
high-alloyed carbides precipitate, because of which the martensite hardness decreases
continuously [45].
3.3.1
Tool Steel X210CrW12
The tool steel X210CrW12 (1.2434) belongs to group 2 of the alloyed steels for cold
working. This ledeburitic cold working steel is in its usual structural constitution a
low-dimension changing, air-hardening, martensitic chromium steel with high wear
resistance. Its application area comprises cutting tools and punching tools, tools for
chipless forming and generally wear-resistant tools and components. Figure 3.11
shows a pseudo-binary section of the four-component system Fe–Cr–W–C. The steel
X210CrW12 precipitates bar-shaped M7C3 primary carbides from the liquid already
during solidification due to its high chromium content of 12%, whereas close to
equilibrium no carbides of the type M23C6 or M3C appear. The great hardness of the
chromium-rich M7C3 carbide leads to a hardness of 63–68 HRC even with retained
austenite contents of >20% and, therefore, to an excellent wear resistance which,
Figure 3.11 Pseudo-binary phase diagram for the Fe–Cr-W–C system (after [4]).