similar cooling rates in a casting section of approximately 10 mm, it is found that the cell count in inoculated iron is 2 - 4 times higher than in the base iron. Moreover, the nodule count in ductile iron is 50 - 200 times larger than the cell count in flake graphite iron.
Melt Graphite Nucleation Potential From the published literature,20
it is evident that unlike
other alloys, liquid cast iron is unique due to the strong crystallographic anisotropy of graphite. During the melt- ing of gray cast iron, weak molecular bonds between graphite layers are easily broken while layers with strong covalent bonds remain stable21 peratures of up to 2000° scopic graphite particles20
C (3632°
in the molten iron at tem- F). As a result, submicro-
are commonly present in liquid
cast iron due to incomplete graphite dissolution. Hence, cast iron melts can be considered as a liquid containing carbon dispersed as a graphite suspension, with particle sizes ranging20,22
from 1 nm to 1000 nm. Moreover, differ-
ent particles of various sizes which act as graphite nucle- ation substrates are found to be present in cast iron melts. These substrates are in continuous permanent movement and thus mutually interacting. Hence, at increasing times, processes such as chemical reactions, coagulation, coa- lescence, flotation, etc. all affect the number and size of these substrates.
The number of graphite undissolved particles and the number of nucleation sites from substrates of a given size can be de- scribed by the following size distribution function.23
Equation 4 Where; Ns
are the number of all of the graphite nuclea- tion sites in the melt per unit volume and the average site size, respectively and l is the substrate size of the incom- pletely dissolved graphite particles.
and la shown in Fig.2a.
Accordingly, this function can be considered as the graph- ite nucleation potential during the solidification of cast iron and for two potential levels n(l) and ni
(l) it is schematically
The iron melt contains a distribution of graphite particles, but not all of the substrate sites take an active role in the nuclea- tion process. The minimum substrate site size, l from which a nucleus of graphite can grow is determined by (see Fig. 3):
Equation 5
Where; θ is the wetting angle and r* graphite nucleus
is the critical size of a
Table 1. Selected Thermophysical Data
International Journal of Metalcasting/Summer 10
37
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81