Inoculation is the process of adding silicon alloys to molten iron. This helps change the graphite distribution, improve mechanical properties, and reduce chilling tendencies. Adding graphite or ferrosilicon to molten iron also has these benefits, without changing the composition much. Two irons with the same composition can behave differently, depending on whether one is inoculated. Researchers have studied the mechanism of inoculation but haven’t reached a clear conclusion.
The purpose of inoculants is to increase the number of nuclei in molten iron. This helps graphite precipitation start with minimal undercooling, reducing the tendency to form white iron (chill). It also leads to a more uniform microstructure and improved mechanical properties, such as better machinability.
Inoculation helps in several ways. The main benefit is controlling chill in parts of castings that cool quickly, like thin sections or edges. Inoculation can also improve tensile strength, especially for low-carbon equivalent (CE) irons. However, low-CE irons are more prone to carbide formation, so inoculation helps reduce this problem. Extended storage of molten iron also makes it more susceptible to chill. This is worsened by high-temperature holding and electric melting methods.
Types of Inoculants
Graphite and ferrosilicon alloys are commonly used to inoculate gray iron. Graphite should be highly crystalline for the best results. Amorphous carbon forms like coke are not effective. Graphite is usually combined with ferrosilicon, as inconsistent results can happen when graphite is used alone.
Ferrosilicon alloys contain around 50-75% ferrosilicon and carry reactive elements like aluminum, barium, calcium, magnesium, or rare earths. These reactive elements can cause dross to form, depending on their amount. The amount of dross is related to the number of reactive elements in the alloy. There are different types of inoculants, such as standard, intermediate, high-potency, and stabilizing, each with varying effectiveness in controlling chill. (Refer Below Table)
Inoculants with calcium reduce chill but have limited impact compared to strontium or calcium-cerium alloys. It’s important to add the right amount—too much inoculant can lead to issues like dross, hydrogen pinholes, and shrinkage. Aluminum, found in all ferrosilicon inoculants, should be kept low to avoid hydrogen pinholes. (Refer Below Image)
Fig. General Classification of Inoculants showing Chill Reduction in Iron with a Carbon Equivalent of 4.0
Stabilizing inoculants promote pearlite and graphitization, helping produce high-strength castings with less chill. These are usually more difficult to dissolve and aren’t recommended for mold addition.
Read Also: Matrix Structure in Grey Iron
Evaluating Inoculants
To evaluate the effectiveness of an inoculant, various tests can be done. Chill bars or wedges can be poured to measure chill depth. Eutectic cell count, which shows the nucleation state of the melt, is another method. However, chill depth and cell count do not always correlate. For example, some inoculants reduce chill without increasing cell count much.
The microstructure and mechanical properties of the iron also reveal the success of inoculation. If type A graphite is present and mechanical properties meet desired levels, inoculation is effective. Cooling curves can also help evaluate inoculants, as the amount of undercooling indicates their effectiveness.
Addition Methods
Ladle inoculation is the most common method. In this method, the alloy is added to the metal as it flows into the pouring ladle. A small amount of metal is kept in the bottom of the ladle before adding the inoculant to ensure proper mixing. The alloy should not be added to an empty ladle or a full ladle, as it may cause problems.
Inoculant levels typically range from 0.15% to 0.4%, depending on the potency of the inoculant. If graphite alone is used, the level is about 0.1% to 0.2%. Adding too much inoculant can increase costs and lead to defects. All additions should be measured accurately, and proper metal temperatures help ensure good results. The inoculant’s effect fades over time, with significant loss occurring within the first 5 minutes after addition. Inoculants should be added as late as possible to minimize fading.
Late Inoculation Methods
Stream and mold inoculation are late methods that help reduce fading. Stream inoculation involves adding the alloy to the metal as it flows from the pouring ladle to the mold. This ensures even treatment of the last metal entering the mold. For stream inoculation, uniform particle size is important, and graphite is not recommended. (Refer Below Image)
Fig. Schematic Showing the Principl of Stream Inoculation
Mold inoculation places the alloy in the mold, such as in the pouring basin or runner system. This method can use crushed material, bonded powder, or precast slugs. Mold inoculation is often used in addition to ladle inoculation.
Late inoculation offers several advantages over ladle inoculation. It helps reduce fading, ensures more consistent results, and prevents carbide formation in thin sections, eliminating the need for heat treatment.
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