Critical cell count
Total chill W
a cast iron with 3.31 %C, 2.42 %Si, 0.046 %P for a Ti 1250o
C, a = 0.11 J/cm2 s1/2 o C and M = 0.3 cm. •
Pouring temperature—it can be assumed that the higher the pouring temperature, Tp
the higher the
both, the cell and nodule counts (Figs. 15a,c). Thus, the higher the magnitude of a, and the lower the cast- ing modulus, M the stronger is the effect of the tem- perature, Ti
crease in the temperature, Ti temperature, Ti on the resultant N and Nn
Relation between undercooling and cell or nodule count
Over the years, many attempts have been made to use cool- ing curves for controlling the graphite nucleation tendency in liquid iron and as a result the nodule or eutectic cell count. From a theoretical analysis of cast iron solidification, it is found that at the onset of eutectic solidification, ∆Tm graphite iron (Fig.2) and ∆Tm,n
in flake in ductile iron are related to
the cell or nodule count by: • Flake graphite cast iron:4,49
Equation 24 • Ductile cast iron:1,2 values.
Among other things, the applications of cast iron depend strongly on the chilling tendency index (CT) of the iron melt. Castings made of cast iron with a high chilling tenden- cy index develop chill zones consisting of cementite eutec- tic. As a result, the casting properties are not homogeneous and the casting machinability is severely impaired due to the high hardness of the cementite constituent. This is particu- larly important in the production of thin-wall castings. Al- ternatively, during the production of castings made of white cast iron or containing a chilled layer, a relatively small CT will promote the formation of gray iron. In turn, this leads to low hardness and to a reduction in the thickness for the chill layer resulting in poor wear properties.
Figure 17 shows graphically the predictions of Eqn. 25. From this figure, it is apparent that the minimum tempera- ture for the solidification of graphite eutectic, Tm a constant cooling rate) on the nodule count Nn values above Nn,cr cal ∆Tm low Tc
, the solidification of cementite eutectic is dominant. Accordingly, the critical nodule count, Ncr,n
can be given by ∆Tsc , Tm
falls in between Ts = Ts
– Tc Equation 25
These equations show that for a constant cooling rate Q, Qn (and constant wall thickness, s) as the cell count, N or nod-
ule, Nn count decreases, ∆Tm 48 or ∆Tm,n increase. Estimations
Similar arguments are valid for ductile iron. From a theoretical analysis for cast iron solidification1-4
chill forms can be obtained from Eqn. 24 for ∆Tm total chill (Fig. 18), below which, the chill is formed is given by found that the critical wall thickness, scr International Journal of Metalcasting/Summer 10 = ∆Tsc
. At temperatures be- under which a
and Tc . it is and the width, W of the
depends (at . Hence, for and the criti-
. Calculations on the effect of an in- lead to a reduction in
Figure 17. Predicted effect of nodule count on the solidification temperature Tm
for graphite eutectic for =
Figure 18. Test wedges, based on the ASTM A367-55T Standard.
based on thermo-physical data (Table 1) and on Eqns. 24 and 25 are shown in Fig.16. In particular, notice from these figures that the experimental results are in good agreement with the curves predicted using the theoretical analysis.
Influence of Cell or Nodule Count on the Chilling Tendency Index
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