Thixoforming : Semi-solid Metal Processing

66j 3 Material Aspects of Steel Thixoforming
the warm transformation, diffusion annealing at temperatures beneath the solidus
(1150–1200 C) is necessary. The holding times here are strongly dependent on the
component dimensions [58, 59].
Calculations concerning the disintegration, segregation and element contents of
steels with 1%C and 1.5% Cr during solidification show segregation fractions of
Is(Cr) ¼ 3 4. The segregation fraction is calculated according to Equation 3.10 from
the maximum and minimum carbon contents:
IsðCrÞ ¼ cmax=cmin ð3:10Þ
The segregation fraction increases with rising C content up to maximum 1.5%
carbon and decreases again above this critical value, due to the beginning of carbide
formation. As soon as the carbon content at which carbides can form has been
reached, further addition leads to premature development of eutectic with less
chromium content during solidification. Therefore, the segregation fraction of the
element chromium decreases, so that no development of eutectic carbides results for
steel 100Cr6 [60].
For steel 100Cr6, furthermore, the risk exists that at temperatures from 1140 to
1160 C the material becomes overheated and the grain boundary areas start to
melt [59]. The reason for this is the local enrichment with chromium and carbon in
the surroundings of dissolved, ledeburitic carbide grains, because relatively high
chromium and carbon contents remain in their surroundings after their dissolution
(Figure 3.14) [61].
Important process-relevant material characteristics of the steel 100Cr6 such as
heat-transmission coefficient, dilation coefficient, heat capacity and thermal conductivity
from room temperature up to fusion heat are currently determined by
means of basic experiments. Furthermore, flow curves in the partial liquid state were
established with case-compression experiments [4, 50].
Figure 3.14 Pseudo-binary phase diagram for the Fe–Cr–C system.