From a kinetic standpoint increased cooling or "undercooling
" ([T.sub.m] - [T.sub.c]) promotes both the nucleation and crystallization process.
The operator only needs to observe the shape of the curve, and if undercooling
is present, add more grain refiner.
The eutectic undercooling
in the same alloy is increased by as much as 25 K, with an increase in MM addition up to 2 pct, and complete modification is obtained.
Thus, the depression in the eutectic temperature is mainly due to Sr modification (6[degrees]C) with undercooling
of about 2[degrees]C.
Figure 4 shows how the undercooling
changes during holding at each temperature for the base iron (carbon equivalent of 3.9[+ or -]0.1%), The slopes of the trend lines increase with higher holding temperatures, and undercooled Type D and E graphite will form earlier in the holding period.
At a high crystallization temperature, the undercooling
is relatively low and nucleation is the dominant factor determining the overall crystallization rate.
where [DELTA][T.sub.0] is the standard deviation; [n.sub.max] is the maximum density of nuclei; and [DELTA][T.sub.N] is the mean nucleation undercooling
It might be observed at high undercooling
or pressures during the melt spinning, or with special laboratory procedures such as the temperature gradient method, the use of nucleating agent, or the flow-induced crystallization technique [13, 14].
If solidification takes place at slow cooling rates in the presence of a number of potent nuclei, little undercooling
occurs and Type A graphite--thick flakes with limited branching randomly distributed and oriented throughout the iron matrix--is formed.
However, they are unable to explain the phenomena associated with modification, such as the large undercooling
observed with the addition of modifier, or the occurrence of modification with the use of superheat or a high solidification rate instead of a modifier.
where b is the layer thickness of the crysta, [sigma] is the lateral surface energy, [[sigma].sub.e] is the fold surface energy, and [DELTA]T is the undercooling
, defined by [T.sub.m sup.0] - [T.sub.c].
Because the undercooling
of ductile iron is greater than gray iron, higher inoculant additions are recommended at the rate of 0.2-0.3% for pearlitic grades, 0.3-0.5% for ferritic grades and 0.4-0.6% for thin-wall castings.