الفهرس 
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Abstract
As often happens, the invention of ductile ( nodular ) iron was a matter of change, which occurred in the course of a program to develop a Ni Hard Iron containing no nickel or chromium. Explanations and hypotheses describing the mechanism of nodular graphite formation came later. While a number of hypothetical possibilities ( nodular ) graphite in cast iron, insufficient evidence is available to substant-tiate any of these. Even though as extensive as those for gray iron because of the short history of this material, four principal hypotheses have been advanced.
1)Nucleation from inoculants following the salt like carbide theory or high silicon concentration which has been applied in the case of gray iron.
2)Nucleation from products of nodularizing additions such as magnesium or cerium in combination with post inoculants.
3)Nucleation within magnesium vapor bubbles and ignoring the effect of post inoculants.
4)Nucleation sites from microscopic SI02 particles.
These are reduce to silicon and (co) carbon monoxide bubbles by carbon in the iron. Graphite nodules form on the interior of the bubbles.
The investigators who advanced these hypotheses have each presented some data for their individual cases and, as is evident, considerable disagreement exists. It may be that nucleation can occur from a number of different sources simultaneously. The following paragraphs are a further discussion of these possibilities and various aspects of graphite nodule growth.
To critically examine the factors which brings about the formation of nodular graphite in iron-silico-carbon alloys requires an understanding of the nucleation of graphite in liquie and its subsequent growth. Formation of stable graphite nuclei in molten iron is a relatively involved procedure, i.e., the positioning of a sufficient number of carbon atoms with graphite crystallographic arrangement onto a solid particle or nucleus in the melt.
The flake graphite in gray iron forms along the A-axis or a horizontal plane while spheroidal graphite in ductile iron forms along the c-axis or nucle vary. When they are numerous and effective, only a relatively small degree of undercooling occurs. When the particles are few and ineffective the undercoolin increases. An increasing amount of undercooling will tend to result in the formation of iron carbide (Fe3C) rather than stable graphite during the solidification of both gray and ductile iron.
The particles or nuclei which bring about nucleation in molen ductile or gray iron with a minimum of undercooling includes a number of materials.
Similar materials are effective for both gray and ductile iron , although the melt history in each case favors the presence of certain types of inoculants. The more effective sources of nuclei or particles or heterogeneous nucleation of graphite from molten iron appear to include : salt like carbides ( calcium, aluminum, silicon carbide and barium ) from commercial inoculants, graphite particles, sulfide which forms during magnesium treatment and boron nitride. The localized high-silicon areas resulting from addion of high silicon inoculants such as 75% ferrosilicon also may cause the precipitation of additional graphite nuclei. Ppost-inoculation of nodular iron provides sufficient effective nuclei or particles for graphite nucleation and the suppression of carbide formation.
In the case of ductile iron, the degree of inoculation or nucleation establishes the number of graphite particles present in the molten iron, however, the form of graphite ( flake or spheroid ) is generally determined by the conditions of growth. As previously stated, it has been shown that in both liquid and solid iron flake graphite growth in the direction of the A-axis of the graphite in crystal whereas spheroidal graphite grows in the C-axix direction. In the liquid state, it appears that the degree of undercooling prior to inoculation influences nodule size to some extent.
The growth of graphite as a flake or spheroid is thought by many to be determind by a series of kinetic factors resulting from the absorption and incorporation of surface-active foreign atoms atgraphite-metal interface. The presence of element which combine with oxygen, hydrogen, nitrogen and sulfur influences the surface tension or interfacial energies of the metal as a part of overall mechanism of graphite formation. Cerium and magnesium favor the growth of spheroidal graphite in the liquid by this mechanism. Free or uncombined sulfur, oxygen, hydrogen and nitrogen favor the growth of flake graphite.
The influence of atoms of atoms from these and other surface-active elements determines whether spheroidal graphite, compacted graphite, flake graphite or combinations are obtained. Lists of elements which tend to promote and retard the formation of spheroidal graphite are as follows :
Retard PROMOTE
Sulfur
Oxygen drogen
Nitrogen
Lead
Titanium
Arsenic
Magnesium
Cerium
Calcium
Potasium
Lithium
Sodium
Beryllium
Yttrlfur
Variations in the relative atomic activity of these elements on liquid metal surface emergies occur not only from iron to iron but can also exist at different locations in the same iron. Combinations of spheroidal-compcted-flake structures of ductile ironoccur when the relative activity of the subject