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Computer guided inoculation simplifies CGI production.

Compacted graphite iron's image is taking on a new luster after an uneventful four decades as a casting material. A unique material with a fortunate combination of properties midway between gray and ductile irons, it was considered just an inferior grade of ductile iron because of its lower tensile properties. Production difficulties also held it back.

Like a bit player suddenly discovered for the starring role, it is finally getting the attention long denied. It is vying aggressively with aluminum and other cast irons for a place in the high stakes race for durable, lightweight, low-cost castings for the huge motor vehicle market.

Aluminum and ductile iron offer selected advantages of weight reduction and strength, but each has disadvantages. Aluminum is more costly, requires heavier sections (more space) and generally must use cylinder liners for wear resistance. Ductile iron cannot be cast into complex shapes, such as cylinder blocks, due in part to shrinkage, machining difficulties, poor thermal conductivity, damping and wear characteristics.

Compacted graphite iron (CGI) has the physical and metallurgical properties that combine the best of those of gray and ductile irons. Its tensile strength, elongation and fatigue strength are close to ductile iron, its damping and thermal conductivity rival gray iron, and it exceeds gray and ductile iron in its resistance to thermal shock. Its graphite structure contains no more than 20% of the nodular form of graphite normal to ductile iron and none of the flake graphite that is the mark of gray iron. The graphite morphology of CGI has a vermicular, or worm-like, shape.

The principal drawback to a wider use of CGI has been the difficulty in controlling its graphite formation subsequent to final inoculation. Too little inoculant can produce the flake graphite of gray iron; too much forms excess nodular graphite, or ductile iron. Inoculants can be added prematurely to the melt prior to pouring, resulting in inoculation fade. They also can be added too late and form excess nodular graphite. This costly uncertainty can be controlled by a new process available from SinterCast, Inc. of Auburn Hills, Michigan.

Computerizing Inoculation

The company is marketing the Backerud process, a computerized, precise inoculation system. Developed by AFS award-winning Swedish metallurgist, Lennart Backerud, its uses computer controls and microprocessors to calculate and dispense the exact amounts of inoculants required for a particular heat.

Because melt variations, treatment, timing and chemistry affect a casting's microstructure, inoculation must be controllable to assure good CGI at minimum cost. Figure 1 illustrates the transition of graphite development during inoculation, showing the small window during which high quality CGI can be formed.

Although this 2-1/2-minute process uses a sophisticated technology, it can be installed quickly and economically. It is simple and logical, involving melt sampling, analysis, corrective action and pouring.

A furnace melt is transferred to a holding furnace for chemistry and temperature adjustments, the base iron is treated with magnesium and ferrosilicon material and deslagged. Sampling probes with thermocouples are dipped into the base treatment melt, removed and allowed to cool. The cooling information registered by the probe is amplified and relayed by a sampling processor to the patented process control computer. The system is represented in the schematic shown in Fig. 2.

While the computer is calculating, the melt is transferred into a pouring ladle and transported to the casting area. At the pouring station, the ladle is placed under a lid equipped with a wire feed unit. The computer sends inoculation data to the wire feeder for any final inoculation adjustments before pouring begins. The system's computer also sends data back to the base treatment station, bringing the following melt closer to desired microstructure target.

System feedback accuracy is said to reduce the final pouring treatment to 5-10% of the total treatment required.
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Title Annotation:compacted graphite iron
Author:Bex, Tom
Publication:Modern Casting
Date:Oct 1, 1992
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