Reticulated Filtration of
Large Ferrous Casts and Castings

David Shane Heckman
Vesuvius Hi-Tech Ceramics
Tel. : (607) 587-9146

Introduction

Metal filtration in the ferrous industry is not new. Utilizing silicon carbide filters with iron and zirconia filters with steel applications, metal filtration has been available for many years. These filters make it possible to remove slag, inclusions, and other melt contaminants that cause defects, while creating a more laminar flow of metal throughout the mold cavity. Yet, the size of these filtered castings has been limited to the smaller end of the spectrum ( < 1000 pounds).

Application of these materials to larger castings may now be accomplished in a variety of ways. First, manufacturing advancements have been made enabling an increase in the overall dimensions of these products. This increase in size can lead to an increase in the metal volume a single filter can accommodate. Secondly, filters are now available with an undulating surface to increase surface area. This increase in surface area can lead to increased capacities and flow rates for a fixed cross sectional area. And lastly two methods utilizing precast refractory shapes are being used to filter large volumes of metal. These precast shapes, known as the spider gate and carousel filter systems, enable the use of multiple filters in a space efficient manner.

This paper will describe the techniques to apply filtration systems to large ferrous pours. The benefits including reduction in rework and lead times involved with the manufacture of these castings will also be discussed.

Filter Description

Reticulated Zirconia Filters

Reticulated zirconia filters are comprised of zirconia utilizing magnesium oxide as its stabilizing agent. The ability to make this ceramic structure from high performance oxides has allowed these materials to be optimized for the filtration of ferrous alloys up to and including temperatures of 3200 degrees F (1). The benefits associated with partially stabilized zirconia are temperature capability, chemical inertness, mechanical strength, and thermal shock resistance.

In the past products were limited to a size range of up to 6 inch round and square filters. This equates to recommended capacities of approximately 700 to 1000 pounds and 900 to 1250 pounds, respectively. With recent manufacturing advancements, zirconia filters are now being produced to 10 inch round and square sizes, with large rectangular filters also being realized (see figure 1). These filters equate to recommended capacities of approximately 2150 to 3000 pounds and 2750 to 3850 pounds for steel and twice that for iron, respectively.

As a general rule, ferrous filtration applications start from 10 ppi and increase for more desired filtration.

While these materials have benefit for many different ferrous alloys, more cost-effective materials have been employed with iron castings.

Reticulated Silicon Carbide Filters

Reticulated silicon carbide filters allow the filtration of ferrous alloys up to 2650 degrees F. Utilizing an oxide bonded silicon carbide, mechanical / thermal strengths have increased to produce larger filters. This provides an increase in the resistance to thermal creep, allowing to these filters to withstand the longer pour times associated with large ferrous casts.

High Surface Area Filters

Reticulated filters are now available with an undulating surface geometry (see figure 2). This configuration allows more metal to pass before the flow rate is restricted, resulting in an increased capacity. Improvements realized in pour time and capacity have been up to 30%.

These filters have been found to provide exceptional benefit to extremely drossy metal pours that have a tendency to blind off the surface of the filter, and can be produced in any filter chemistry desired.

*Fig. 1 – Reticulated ceramic Filters

*Fig. 2 – High Surface Area Filters

Carousel Filtration System

The carousel filtration system is a precast mullite shape, containing a hexagonal arrangement of filters and is implemented with the standard hollowware utilized in large bottom fed castings. The concept of the invention is to allow tangential flow of metal into the ring/channel in front of the reticulated ceramic filters. A convexity is incorporated at the top and bottom of the channel to promote the segregation of the low-density particles allowing the filters more deep-bed-filtration. Also, this tangential flow facilitates the prevention of coarse impurities approaching the filter surface. The metal then finally penetrates the filter and exits the carousel entering into a refractory gating system or directly into the mold cavity. (See figure 3)

* Fig. 3 – Carousel Filtration system

These products can be used in singular or multiple configurations, with a linear increase of pouring rate and capacity. For these applications, the carousel can be provided in a left and right version, allowing a symmetric installation of the gating system. Different carousel sizes may also be chosen.

Filter Selection

Filter selection is made on a casting by casting basis. Variables, such as pouring temperature, casting size, gating system, desired pouring rate, pouring equipment, deoxidization practice and alloy will influence the filter application.

Typical drossy metal capacity recommendations are 25 pounds per square inch, with flow rates of 2.2 pounds per second per square inch. More fluid alloy capacities may range up to 50 pounds per square inch, with flow rates of 3.0 pounds per second per square inch. These recommendations vary and change with alloy chemistry and metal cleanliness.

Carousel filters are currently produced in three sizes. The smaller utilizes six 3x3x1 inch filters, has a flow rate of 65 –100 pounds per second, and a capacity of 2200 pounds. The medium utilizes six 4x4x1.25 inch filters, has a flow rate of 130 – 260 pounds per second, and a capacity of 4400 pounds. The larger carousel utilizes six 6x6x1.25 filters, has a flow rate of 220 – 400 pounds per second, and a capacity of 8800 pounds. These recommendations are for fluid alloys. Again, changes in metal chemistry will result in changes to the above recommendations.

Low Range Pours (1000 pounds – 5000 pounds)

Pours in this size range can show extreme benefit from the large reticulated filters produced today. Horizontal and vertical orientation in the gating system provides the most productive means of filtering these casting. Strategic filter placement in conjunction with a sound gating system will allow for the best combination of yield and quality.

When looking at the gating system, the number of contacts to the casting in conjunction with the runner system will provide the most influence over the location of filters and the number of filters applied. After location is selected, pour rate and capacity influences the filter size selected for the casting (see figure 4).

When a higher flow rate or higher capacity is desired, without gating alteration, the high surface area filter may be applied. These filters are applied in the same manner as the regular reticulated filters.

Carousel filtration may also be a cost-effective way of producing a profitable low range casting. The ability to tie into existing hollowware systems make it an excellent choice for bottom fed castings. These systems may also be implemented vertically or horizontally. (Further description of the carousel will take place in the carousel implementation section)

*Fig 4 – Gating systems implementing numerous filters

Midrange Pours (5000 pounds – 10000 pounds)

With increasing pour rates and capacities, there is an increased necessity to move toward the high surface area and carousel filters. Multiple high surface area filters may be used near the lower end of the spectrum, but the carousel filter is most applicable in this range. The carousel filtration system again will tie into hollowware gating systems easily, providing an effective mode of filtration for this range.

The spider gate filtration system may also be applied to ingot pours or castings with multiple contact points.

Highrange Pours (10000 pounds +)

These large pours, will require the use of the spider gate and/or carousel filtration systems. As the amount increases, multiple systems may be required to comply with the expected pour rates and capacities. Systems have seen up to 76,000 pounds of metal poured, and are not limited with multiple units.

Filter Implementation

Reticulated Filters and High Capacity Filters

These filters are typically implemented in a vertical or horizontal fashion. In either case a blank, a removable object to mimic the filter size, is used to insert the filters. This may be accomplished by adjusting the pattern for sand systems. The blank placement should be near the parting line to assist with installation. Filters should be placed as near to finished product as possible.

Vertically placed filters are inserted perpendicular to the parting line. Choosing a filter with 4 times the surface area to the cross sectional area of the initial runner system will allow the proper flow of metal without choking. Care should be taken to blow out all passageways with an air hose before assembling the cope and drag.

Horizontally placed filters are also best implemented at the parting line with a blank system. In conjunction with a filter, placement of an overhead reservoir can assist with the trapment of slag and larger oxide inclusions. Less dense oxides collect in the reservoir, assisting the filter to maintain a more consistent flow and capacity throughout the pour. (See figure 5).

*Fig 5 – Horizontal implementation of Reticulated Ceramic Filters

Carousel Filters

The carousel filters are implemented along with the hollowware system of a bottom fed casting. Male and female ends match existing hollowware to allow easy implementation. The assembly of the carousel unit completes with filters and spacers placed in the interior chamber and sealing the unit (see figure 6). During the assembly process sand must be packed carefully around the extremities of the carousel to allow for external support (see figure 7). If the carousel is not supported properly, a failure in the casting process may occur.

*Fig 6 – Attachment of carousel filters

* Fig 7 – Sand compaction with two carousel filters

Oxide Inclusion Removal

On further examination of the reticulated filters after pouring, one can inspect for the oxide inclusions that have been removed. The findings can be produced with the use of X-Ray Diffraction, Electron Microscopy, and Spectroscopy (Fig. 8 & 9). With this information, one can make determination on where inclusion development may begin and take steps to reduce these occurrences. This, along with the cooperative efforts of filtration, will lead to the most cost-effective production of ferrous castings.

*Figure 8 – Electron Microscope Photo of Ceramic
Filter material with oxide inclusions sintered on

*Figure 9 – EDS Analysis of oxide inclusions

Conclusions

No longer are the benefits of filtration restricted to casting sizes smaller than 1000 pounds. The use of reticulated ceramic filters provides a cost-effective way to produce large steel castings, while offering the ability to shorten turn around time.

References

1) J.M Morris, S. Sahu, and U. Sievers, Advanced Reticulated Ceramic Metal Filters and Performance Results from Select Steel Foundries, American Foundryman’s Society Annual Meeting (1989)

2) J M Morris & Truett Sweeting, Metal Filtration for Bottom Pour Ingots, Steel Times, Pgs. 454, 456

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