Thixoforming : Semi-solid Metal Processing

50j 3 Material Aspects of Steel Thixoforming
The examined steels solidify in one-phase (100Cr6) or multi-phase form
(X210CrW12). For the steel 100Cr6, the results of thermodynamic calculations
under equilibrium terms (Thermo-Calc) and with diffusion (DICTRA) show slight
differences, so that during solidification a low segregation aptness exists for this
steel [18]. Due to its relatively large semi-solid interval, the steel 100Cr6 should be in
general suitable for thixoforging and thixocasting. For the steel X210CrW12, a
comparison of equilibrium calculations and calculations with diffusion shows that
a tendency to segregate exists until the completion of solidification, so that the M7C3
carbide is formed, as indicated in Figure 3.5a. The steep incline of the liquid phase
concentration up to 40% suggests that this steel could be rather more suitable for
thixocasting than for thixoforging, due to its lower temperature sensitivity with
higher liquid-phase fraction [18].
A further crucial factor for the position and size of the semi-solid interval of
different aluminium alloys and steels is the variation of the chemical composition
within the specified boundary concentrations of the alloy standards [18, 20]. In
steels, the elements carbon and chromium in particular have a crucial influence on
the liquid-phase progression (Figure 3.5b). For the steel X210CrW12, the intervalsize
for thixocasting thus decreases in the extreme case from 50 to 5 K, so that a
batch with disadvantageous composition is no longer processable in the partial
liquid state [18].
Figure 3.5 Liquid-phase fracture as a function of temperature for
100Cr6, HS6-5-3 and X210CrW12 steels. The solid lines show the
diffusion evaluation by DICTRA and the dashed lines show the
equilibrium evaluation by Thermo-Calc (a). Equilibrium evaluation
of the determined liquid phase fraction as a function of
temperature with maximum and minimum composition of
alloying elements corresponds to the valid standards (b) [18].