Thixoforming : Semi-solid Metal Processing

154j 5 Thermochemical Simulation of Phase Formation
sample, since the composition will determine at which temperature a certain fraction
of liquid is present. For steels, C will usually have the strongest influence. Therefore,
it makes sense to draw the phase diagram in Figure 5.2 as a function of C content and
not some other element. There are many possible origins of compositional variations.
Each charge will in principle have a different average composition so that billets
coming from different charges have different compositions, but even billets coming
from the same charge can have different compositions due to macrosegregation. The
centre of the billet can have a different composition from the surface region, also due
to macrosegregation or, for example, decarburizing during hot working or heat
treatment. There will also be compositional variations on a local scale due to
microsegregation. These will depend on the solidification and thermal history of
the material. In any case, the average composition of a billet should always be within
the limits defined by the standard for a particular steel grade. Large compositional
variations are also possible in parts produced by semi-solid processing due to phase
separation during processing. This is strongly dependent on the processing conditions
and the geometry of the part produced. This is discussed in detail in Chapters 6
and 9. This can lead to strongly varying properties within a produced part.
As far as phase equilibria are concerned, the influence of compositional variations
can be quantified in a straightforward manner. In the following, the influence of the
composition on a number of key points for X210CrW12 is considered. These are
linearized around the standard composition (given in Table 5.1) and given in mass%.
They are roughly valid within the composition limits given by, for example, the DIN
standard. Additionally, some indication of the influence of Mo, V, Ni and Cu is given.
The liquidus is given by:
T L ¼ 1358 C 67D%C 30D%Si 4:8D%Mn 1:3D%Cr 3:8D%W
ð 5:5D%Mo 1:9D%V 4:2D%Ni 7:8D%CuÞ ð5:1Þ
The temperature at which there is 50% liquid is given by:
T 50 ¼ 1285 C 105D%C 31D%Si 7:7D%Mn 1:4D%Cr 7:2D%W
ð 11D%Mo 6:4D%V 6:7D%Ni 12D%CuÞ ð5:2Þ
The temperature where eutectic (M7C3) formation starts is given by:
T M7C3 ¼ 1256 C 18D%C 1:7D%Si 5:3D%Mn þ 6:0D%Cr 2:8D%W
ð 9D%Mo 11D%V 1:8D%Ni 2:7D%CuÞ ð5:3Þ
The corresponding liquid fraction at that temperature is given by:
f L ¼ 38% þ 37D%C þ 9D%Si þ 1:4D%Mn þ 3:0D%Cr þ 2:2D%W
ðþ1:9D%Mo 0D%V þ 2:2D%Ni þ 3:9D%CuÞ ð5:4Þ
The (equilibrium) solidus temperature is given by:
T S ¼ 1245 C 34D%C 6:1D%Si 6:8D%Mn þ 6:9D%Cr 4:7D%W
ð 19D%Mo 17D%V 2:8D%Ni 3:7D%CuÞ ð5:5Þ