Chilling Tendency and Chill of Cast Iron

182
The chilling tendency CT can be calculated from Eq.
(7) as a function of the chemical composition of the
cast iron, Ns and b, listed in Table 3, the mean temperature
of the metal just after pouring, Ti≈1260℃, and any
relevant thermophysical data (shown in Table 1).
Table 4 shows results for plate-shape castings reported
in the literature [4] with various chemical compositions
and wall thicknesses, as well as the number of
nodules and the cementite fraction. In these cases, the
nucleation coefficients b and Ns are not known, so CT
is calulated using Eq. (8). The results in Table 4 show
that in melts I and II, chill starts to occur in walls with
thicknesses between 3 and 6 mm, while in melt III chill
starts to occur for wall thicknesses between 1.5 and 2
mm. scr must be calculated using Eq. (1) to compare
these results with the theoretical predictions. These
calculations assumed that a=0.11 J/(cm 2 ·s 1/2 · ℃ )
and Ti=1250℃ with other information taken from Table
1. The results in Table 4 show that in melt I the
Tsinghua Science and Technology, April 2008, 13(2): 177-183
change of the wall thickness, s, from 6 mm to 3 mm is
closely linked to the change in the number of nodules
from 588 to 1039 mm −2 . As a result, an average nodule
count of NF,cr=813 mm −2 was used in this work. Similar
determinations were made for melts II and III with NF,cr
values of 945 and 1959 mm −2 , respectively. scr for
melts I, II, and III were then 3.4 mm, 3.1 mm, and
1.8 mm as shown in Table 4. In addition, comparison
of s and scr in Table 4 indicates that the predictions
from the theoretical analysis are in good agreement
with the experimental determinations of the wall thickness
at which chill occurs. Table 4 shows the CT for
the three melts calculated using Eq. (8) and data in Table
1. The results in Table 4 show that as the chilling
tendency increases from 0.34 to 0.68 s 1/2 ·℃ –1/3 , the
critical wall thickness, scr, increases from 1.8 to
3.4 mm.
Table 4 Chemical composition, wall thicknesses, nodule count, cementite fraction, and chilling tendency for three
ductile cast iron melts
Melt
No.
C/% Si/% P/%
I 3.40 2.70 0.046
II 3.45 2.91 0.044
III 3.31 4.42 0.051
4 Conclusions
Wall thickness (mm)
Experimental s Calculated scr
(1) A simple theoretical analysis is presented which
predicts the chilling tendency and chill in flake graphite
and ductile cast iron based on
The chemical composition of the casting which
for flake graphite cast iron is characterized by ∆Tsc,
fγ, T, µ, Ncr or Ns, and b, and for ductile cast iron is
characterized by ∆Tsc, D, Tl, Ncr or Ns and b;
The inoculation and spheroidization method, the
superheating temperature, and bath holding
Nodule count
(NF/mm −2 )
Fraction of
cementite (%)
CT/( s 1/2 ·℃ –1/3 )
6.0 588 0.0 0.68
3.4
3.0
1039 9.0 —
2.0 1380 24.0 —
1.5
1311 34.0 —
6.0 854 0.0 0.60
3.1
3.0
1037 7.3 —
2.0 1100 24.0 —
6.0 1127 0.0 —
3.0
1726 0.0 —
2.0 1890 0.0 0.34
1.8
1.5
2027 9.3 —
times as characterized by Ncr or Ns and b;
Selected thermophysical data of the metal and
mold material as well as the metal temperature
just after pouring into the mold.
(2) Theoretical calculations of the width of the total
chill in wedges for flake graphite and of the critical
wall thickness for nodular cast iron compare well with
experimental data.
(3) The absolute values of the chilling tendency of
cast iron are calculated to range from 0.3 to
1.1 s 1/2 ·℃ –1/3 .