Filtering molten iron with refractory silica cloth.
In many cases, the added cost of filtration has prevented foundries from adopting the practice because it could only be justified for expensive, high-performance castings. Until recently, U.S. foundries most commonly used two hard-fired filter types, cellular ceramic and reticulated ceramic foam. A third alternative, refractory silica cloth, first used in the Soviet Union and Europe, is now available in the U.S.
This new filter is constructed of specially treated silica fibers that can withstand pouring temperatures of more than 2900F. The stiffened refractory cloth can be cut from stock to any size or shape and is usually placed at the parting line of the mold. A matching print with a crushed bead is molded into the cope surface to hold the cloth filter tightly around its perimeter when the mold is closed.
Silica Cloth Filters
Silica cloth filters trap impurities in a manner that is unlike the filtration mechanism in mullite or alumina filters. When the molten metal reaches the cloth filter, the stiffening resin encapsulating the filter fibers decomposes, forming a carbonaceous char. This reacts with the iron to form wustite (FeO ), which in turn reacts with the silica fibers to form a layer of fayalite (2FeO-SiO).
Molten iron temperatures cause the fayalite coating to become soft and sticky so that it captures any nonmetallic inclusions that touch it, holding them onto the cloth filter. For magnesium-treated ductile iron, the fayalite absorbs the magnesium reaction products carried in the molten iron. Therefore, low-melting solid solutions are formed that further enhance the removal of inclusions, especially the magnesium dross, sulfides and silicates that are formed during the nodulizing treatment.
Hard-fired ceramic filters do not capture inclusions in this manner. A ceramic filter has a comparatively massive structure that tends to chill the first molten iron that reaches the filter. This means it must have rather coarse openings to ensure reliable passage of the molten metal.
An extruded ceramic filter, therefore, relies on the formation of a filter cake on the upstream side to remove smaller inclusions that would otherwise pass through the openings in the filter. A ceramic foam filter, on the other hand, provides more tortuous paths for the molten metal than an extruded filter and is designed to trap inclusions in the manner of a deep bed filter or a sand filter used to purify water. Nevertheless, once the cake develops on any filter, it becomes the controlling factor in relation to the size of inclusions that the filter will remove.
Because of the filtering effectiveness and smaller percentage of open area of the refractory cloth, a larger live area must be exposed to the metal flow to maintain the metal flow rate. For gray and malleable iron, the cloth filter should have a live area three-to-five times the total choke area in the gating system. For ductile iron, the ratio should be six-to-eight times.
In addition to the larger live filter area, gating systems can be designed so that multiple filtrations can be performed through the same piece of refractory cloth. The later filter steps do not need as much live area because most of the massive slag and sand inclusions already would have been stopped by the upstream stages of the filter. Multiple filtration also can be accomplished with two separate filters of different weave or hole size, tailored to the different classes of inclusions reaching the various stages.
Filters & Inoculation
Rigid cups and baskets can also be molded from the refractory cloth, so large quantities of metal can be filtered during transfer to or from holding furnaces and ladles. These cups can be adapted to a carousel-type mechanism to allow in-stream inoculation on automatic pouring lines. The inoculant can be injected directly into the cup, making the process cleaner and more efficient than spraying inoculant into the unfiltered molten stream.
The refractory cloth can also be coated with inoculants for simultaneous inmold treatment and filtration, while a second filtration step can be used to trap the reaction products created by the treatment. The inoculants or other master alloys can be placed at different locations on a multiple ingate casting for selective inoculation. A four-inch-square piece of refractory silica cloth can hold up to 20 grams of inoculant. A plastic bag attached to the filter can be used to hold larger amounts of inoculant in larger melts.
The refractory cloth resists the force generated by the flow of liquid metal by sagging in the manner of a trampoline. Therefore, it is important that the filter be adequately clamped around its perimeter to avoid filter collapse.
Ordinarily, the cloth filter will be placed horizontally between the cope and drag. However, for vertically parted molds, or where parting-line space is limited, a piece of silica cloth can be framed with core sand to fit the print of a hard-fired ceramic filter.
Another use for a sand-framed cloth filter is to fit close to the casting and act as a knock-off core to facilitate removal of the gating system. This is particularly important for ductile iron, which is otherwise so tough that the gates and risers must be cut off.
The refractory cloth filter can also be used in making investment castings because it can be incorporated into the wax pattern so that the entire perimeter of the filter becomes part of the ceramic shell of the mold. Patterns used for the expendable pattern casting process can have cloth filters built into them in a similar manner.
Cloth filters do not chip or break as easily as ceramic filters during handling and are unlikely to release damaging fragments into the casting. With their increased efficiency for inclusion capture and other operational advantages, refractory silica cloth filters can be a key ingredient for a foundry's overall quality control program.
Jay R. Hitchings Amenex Associates, Inc. West Chester, Pennsylvania