Foundrymen have long recognized the value of adding special additives to gray and ductile cast irons to modify structure and control properties. While graphite growth mechanisms are better understood today than just a few years ago, comprehensive theories have yet to be established and accepted. Nevertheless, efforts to comprehend and control the inoculation and nodularizing processes have resulted in a variety of products and treatment techniques. This paper endeavors to summarize the present status of the alloys used to modify cast iron.
A very diverse product line of inoculants exists in today's market. While most inoculants are based on ferrosilicon and contain some aluminum and calcium, they may also contain barium, titanium, strontium, rare earths, or other elements.
The method of addition, as well as the location and time of addition of special alloys to cast iron, plays a prominent role in determining the addition levels and the effectiveness of a particular treatment. For example, inoculants may be added to the ladle, or later in the process as the metal stream enters the mold, or in the mold itself. Inoculants used in the ladle are normally added in the stream entering the ladle and are usually granular in nature. In late inoculation, on the other hand, the inoculant may be added as a powder, an encapsulated powder, or a bonded or cast insert. Today, the bulk of inoculants are added in the ladle. However, due to metallurgical and economical considerations plus production restrictions, the practice of late inoculation has grown in recent years.
The nodularizing treatment for ductile iron is commonly accomplished by ladle treatment with magnesium-ferrosilicon alloys. A variety of other additives, including pure magnesium, is finding use. Recently, particularly in the automotive industry, many foundries have begun to convert to the Inmold Process.
Efforts are currently being made within the foundry industry to produce an intermediate iron to gray and ductile iron, known as compacted graphite iron. At least one additive is available commercially for this purpose.