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Inoculation Mechanisms: Part Two :: Total Materia Article
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Inoculation Mechanisms: Part Two
Inoculation can be summarized as te addition of silicon alloys during te latter stages to molten iron provides control in “chill” areas of castings e.g. thin sections and corners of te casting.
Covered in this article are te main processing factors to take notice of and also te main methods of inoculation including, ladle, in-stream and in-mould inoculation.
Inoculation is defined as te late addition of certain silicon alloys to molten iron to produce changes in graphite distribution, improvements in mechanical properties, and a reduction of te chilling tendency that are not explainable on te basis of composition change with respect to silicon. Graphite, added alone or in combination with ferrosilicon, will also produce these changes without significantly altering te chemistry of te iron. It is recognized that two irons with te same apparent composition can have dramatically different microstructures and properties if one is inoculated and te other is not. A great deal of research has been dedicated to determining te mechanics behind inoculation. Although many theories exist no conclusions have been reached regarding possible mechanisms.
Irons are inoculated for various reasons. Te primary reason is to control chill in areas of castings that experience rapid solidification, such as in thin sections, at corners, and along edges. Tensile strength can be improved through inoculation. This is particularly true for te low-CE irons, which are selected for applications requiring a higher tensile strength (tensile strength decreases as te CE increases). However, low CE irons are also te grades that are most susceptible to carbide formation.
Inoculation helps overcome this problem by minimizing te chill forming tendency of te iron, thus allowing low CE irons to be poured in thin sections. Irons that are stored in holding furnaces or in pouring systems for an extended period of time are also more susceptible to chill formation. This susceptibility can be attributed to te reduction in nuclei in te melt that takes place during extended holding. This effect is accelerated if holding occurs at high temperatures. Melting method influences white iron formation; electric melted irons are generally more prone to carbide formation than cupola melted irons.
Te principal inoculation mechanisms are quite different in grey and ductile irons. In grey iron, a stable oxide will be te primary nuclei for manganese sulphide precipitation that again will nucleate graphite flakes of good type A form. In ductile iron, a sulphide is te nuclei for complex silicates that again will nucleate a high number of graphite nodules. Te same inoculant materials can however be used successfully in both type of irons, since te reactive elements such as Ca, Ba, Sr and Al are all strong oxide, sulphide and silicates formers in both grey or ductile irons.
Te inoculant fading effect is connected to diffusion rate, growth and coarsening, and a general reduction in te number density of micro-inclusions as nucleation sites for graphite. In order to obtain a sound and reproducible iron production process some critical inoculation factors will have to be controlled properly. For grey iron one should pay special attention to te following factors:
1) Te Mn/S ratio should be maintained at te same level every time and sulphur should preferentially be kept at minimum 0.05%.
2) Aluminum is found to be an important part of te nucleus core and should be adjusted and kept at controlled levels every time. Recommended residual Al level in grey iron is 0.005% - 0.01% for optimum inoculation effectiveness.
3) There should be a certain oxygen level in te base iron from fresh metal processing. Te use of some rusty raw materials may assist in providing a good oxygen potential.
4) Pouring time after inoculation should be minimized in order to keep fading losses under control.
5) Use an inoculant with defined chemical composition and sizing.
Cast iron may be inoculated by several methods such as:
Iron is inoculated by adding inoculant to te metal as it is transferred from te furnace to te pouring ladle. Te turbulence quickly dissolves te inoculant and evenly disperses it throughout te molten bath (Figure 1)
In many automatic pouring operations, inoculation is done in-the-stream (Figure 2)
Inoculants may also be added as a preformed insert placed in te pouring basin of a mould or as granulated inoculant placed in te gating system. (Figures 3a and 3b)
In-stream and in-the-mould inoculation techniques offer little inoculation fades, and generally require less inoculants material to provide te desired results.
Figure 1: Ladle inoculation
Figure 2: In stream inoculation
Figure 3a: In te mould inoculation
Figure 3b: In te mould inoculation
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Inoculation Mechanisms: Part One
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