Analyzing steel alloy filtration.

Inside This Story:

* This article examines how properly applied filtration can significantly reduce cleaning room and inspection costs, scrap rates, machining costs and customer returns.

* An overview of a number of trade-offs and other factors in considering the correct gating decision and filter application is provided.

Every steel casting producer understands that filters reduce the number of nonmetallic inclusions in steel castings. However, technical limitations of existing filter materials often result in priming problems and breakage, especially when filtering carbon and low alloy steels. In the past, this has limited the practical application of in line filtration to alloys such as stainless steels and high-chrome irons.

Inclusions, which are entrapped impurities that cause discontinuities in the metal matrix. They are caused by virtually all of the processes used to manufacture steel castings, As long as metal is poured through air and runs in a gating system that contains sand and more air it is virtually impossible to prevent inclusions from forming.

Management of the sources of inclusions, along with improved melting, pouring and molding practices including filtration, can diminish the incidence of inclusions and the casting defects they cause. This article suggests how proper filter selection and application can extend the filtration benefits of reduced cleaning room and inspection costs, lower scrap rates, lower machining costs and reduced customer returns.

Understanding Filters

Ceramic filters remove inclusions from the molten metal stream in three ways:

* coarse inclusions, too large to enter the passageways, are trapped on the face of the filter in a process known as "sieving," shown in Fig. 1;

[FIGURE 1 OMITTED]

* as pouring continues, inclusions may begin to accumulate on the filter face and form a "cake" of material that filters out even finer particles;

* inclusions in the molten metal that flow through the filter cake and into the filter substrate are attracted to the inner surfaces of the filter in a process known as "deep bed" filtration. As the metal passes from cell to cull, the entrained particles deviate from their original flow lines and make contact with the walls of the filter.

Early filtration efforts in the mid-1960s included passing molten aluminum through a packed bed of granulated refractory material. Filters commonly used in steel casting applications today include pressed and extruded ceramic shapes and reticulated foams that are able to withstand the stresses and pouring temperatures of steel. Each has its own features, benefits, restrictions and application suitability.

Mullite Strainer

Cores--Strainer cores with straight-through circular passages are pressed or extended from a mullite-alumina material (Fig. 2a).

[FIGURE 2 OMITTED]

Zirconia Foam Filters--Zirconia reticulated iceramic foam filters are produced by coating a polymer foam precursor with a zirconia-based ceramic slurry and firing it (Fig. 2b).

Carbon-Alumina Composite Foam Filters The most recent innovation in steel filtration are low thermal capacity carbon-bonded alumina composite foam filters (Fig. 2c). They are produced by coating a polymer foam precursor with a ceramic slurry which, when fired, creates a highly stable carbon-alumina matrix.

Foam filters also reduce the formation of reoxidation inclusions by "smoothing" the flow of the molten metal. As the fluid stream passes through the porous material, there is a reduction of its average velocity and a drop in pressure as measured at the entry face and again at the exit face of the filter, resulting in a more laminar metal flow exiting the filter.

These changes in average velocity and pressure occur as energy in the metal stream is dissipated. Energy loss is caused by viscous shear as the stream is separated by the cell walls in the filter and by inertial effects that result when the fluid is forced to change directions as it passes through the tortuous path created by the foam filter matrix.

By reducing the velocity and pressure of the metal stream, turbulence and splashing is minimized and the amount of molten metal surface area exposed to oxygen from the air contained within the mold cavity is reduced, thus reducing the chance for reoxidation.

Application Methods

Location and orientation of the filter in relation to the casting cavity affects filtration effectiveness and is dictated by the molding practice and the physical properties of the filter material.

For sand foundries, filter application methods can be grouped into two categories: in-line and direct pour. For inline filtration, filters are positioned in specially designed filter prints that are an integral part of a running system. In direct pour filtration, the filters are positioned in specially designed pouring basins or in insulating sleeves that replace the conventional runner system and also contribute to the feeding requirements of the casting.

The optimum method of inclusion removal with in-line filtration is to locate the filter in a specifically designed filter print positioned as close to the casting cavity as possible. Here, it intercepts both externally generated inclusions and any reoxidation and erosion inclusions formed in the gating system.

However, when filtering low alloy steels, carbon steels, and manganese steels with zirconia ceramic foam and mullite strainer core products, this optimum in-line position is not always possible without significant superheat to keep the metal hot enough until it reaches the filter. This does not always result in consistent priming and prevent freeze-off.

Therefore, these high heat capacity filters are typically located adjacent to the spree when filtering carbon and low alloy steels resulting in less than optimized filtration. With stainless steels and high-chrome irons, the higher pouring temperatures relative to liquidus and/ or greater fluidity of the molten metal will allow the positioning of zirconia foam filters and mullite strainer cores

Applications

The selection axed application steel filters depends on a variety of factors, including priming issue metal grade and cleanliness, desired pouring time, degree of filtration/flow modification required an yield objectives.

Priming Issues--To prevent the filter from freezing off in in-line applications before steady state flow begins, the first metal reaching the filter must remain well above the liquidus temperature until sufficient metallostatic pressure overcomes the resistance to initial flow through the filter (Fig. 3). This may be accomplished with additional superheat or by selecting a filter material with a lower thermal capacity such as the new carbon-bonded alumina products.

[FIGURE 3 OMITTED]

Metal Grade and Cleanliness--The chemical composition of the alloy strongly affects the filtration decision because of surface tension/fluidity effects. Elements such as chromium, nickel, carbon and silicon all have strong influences on surface tension and fluidity of steel. Stainless steels, for example, have high fluidity because of the alloying elements. The amount of slag or deoxidation products contained in the molten metal will determine the required filtration capacity of the filter.

Desired Pouring Time--The pouring time is affected by the rate at which the metal flows through the filter. Depending on the alloy being cast, a certain minimum filter area is required to achieve the desired mold-filling rate. Typically, this filter frontal area is four to six times the choke area.

Filtration Effectiveness--Foam is an extremely efficient inclusion remover. Inclusions as small its several microns can be effectively trapped. The ability of the filter to intercept inclusions and minimize turbulence to prevent reoxidation is dependent upon the frontal area, thickness and pour size of the filter.

While filters finer than 10 ppi (pores per in.) would allow even further efficiencies in metal filtration, priming issues limit their use. In some cases. massive amounts of entrained inclusions can actually block the filter and halt the flow of metal, resulting in short pours and/or misrun castings.

Yield--In-line filtration may improve casting yield by reducing the need for the traditional long, nonpressurized runners with high gating ratios such its 1:2:2 or 1:4:4 that reduce metal velocity and related turbulence, allowing entrained inclusions to "float out." Using foam filtration, lower ratios can be safely applied, such as 1:1.1:1.2, with a potentially significant impact on yield.

Optimizing Filtered Gating Designs

Depending upon the casting buyer's specifications and cost considerations, the optimum filtered gating design may be the one that produces the best quality steel casting (fewest inclusions), or it may be the one that produces the host yield. Simulation programs available today can be used to model alternate gating designs and avoid the expense of trial-and-error casting engineering and optimize the system before any castings are made.

A number of trade-offs and other factors should be considered in reaching the correct gating decision:

Priming vs. Positioning--As suggested earlier, the best quality is obtained witch the filter or filters are positioned as near to the ingate as possible. However, this has been difficult with strainer cores and zirconia-based ceramic foam filters because of printing difficulties.

With the recent introduction of low heat capacity carbon-bonded alumina foam, filters can be positioned at the ingates and will normally prime consistently within the normal range of of steel pouring temperatures (Fig. 4). Optimized gating design and filter positioning freedom also can improve directional solidification by permitting temperature gradients within the mold cavity to be equalized

[FIGURE 4 OMITTED]

Casting Cleanliness vs. Yield--The greatest yield (casting weight to pour weight) is obtained when metal is poured directly through a top riser into the casting cavity. However, turbulence, reoxidation and erosion as the metal impacts the casting cavity wails and or cores generate more inclusions than are normally acceptable.

Using a direct pour unit with a flow modifying team filter in the riser sleeve eliminates most of these casting cleanliness issues. However, allowing the metal stream to drop excessive distances beyond the filter or to impact cores will still break up the metal stream and generate reoxidation inclusions and erosion defects. Utilizing the filtered direct pour unit as a side riser avoids most of these issues and further increase the quality, with only a slight loss in yield over the top riser application.

Filter Material Considerations

The filter material plays a major role in the performance and application of the filter.

Chemical Composition--Chemical composition determines if the filter will be vulnerable to attack by slag, deoxidation products and other aggressive chemicals in the molten metal that can weaken or chemically attack the filter.

Chemical composition also affects the filter strength and its resistance to thermal shock. Thermal shock resistance is the ability of a solid to withstand sudden temperature changes without cracking or breaking. When molten steel initially contacts a filter, the filter temperature rises from room temperature to as much as 3050F (1676C) almost instantaneously and the filter must be able to resist that shock without breaking or cracking.

Thermal Capacity/Specific Heat--The thermal or heat capacity of the filter refers to the quantity of the heat it absorbs from the molten metal before the filter reaches the same temperature as the metal. It is a function of the density or mass of the material and its "specific heat."

A filter with a higher heat capacity such as zirconia ceramic foam or a mullite strainer core will extract or absorb more heat from the molten metal passing through it, making the high-heat capacity filter more difficult to prime. Because of lower heat capacity and lower priming temperature, the newly developed carbon-bonded alumina products minimize priming problems, especially with carbon and low alloy steels.

Strength--The filter structure must be strong enough to resist the force of the molten metal stream as it falls from the ladle and/or passes through the gating system. If strength is inadequate, the filter may crack or break, allowing filter particles and unfiltered metal to reach the casting.

Creep Resistance--Creep is a change in dimension of a solid object through exposure to physical stress or thermal stress. With greater creep resistance, filters are able to withstand the increased physical and thermal stresses caused by constant exposure to molten steel for longer time periods, allowing the filtration of larger castings.

Filtration Advances

Continuing improvements in new filter materials and advanced application techniques are improving filter performance and extending the benefits of filtration to a new range of steel casting applications that was not achievable in the past. Steel foundries can now use these advances to increase casting quality without the need to increase energy and ancillary manufacturing costs.

This improved casting quality will lead to reduced internal scrap and customer returns, taster lead times and lower overall manufacturing costs, making foundries more efficient and help keep them more competitive in this aggressive global market.

For More Information

"Filtering Basics: Who, What, Where, Why and How," K.Adams, E.J. Williams and S. Kannan, MODERN CASTING, March 2002, p.19-21.

"Filters: The Hows and Whys," M.J. Jacobs, Engineered Casting Solutions, Fall 200], p.50-51.

Eric Morgan is a production application manager for feeding and filtration products at Foseco Metallurgical, Inc., Cleveland, Ohio, with 38 years experience in the steel foundry industry.