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Home  Filtration technology  Gating system designing  Specifics of steel filtration

Specifics of steel filtration

Steel casting is a specific field of application of the ceramic foam filters. Compared with other filtered alloys, steel is specific in several aspects, mainly a very high temperature of melting of these alloys (up to 1700 ˚C) and the related high price of refractories (zircon, carbon) used for manufacture of ceramic foam filters, a number of different production technologies and last but not least a high variety of the cast materials from the point of their chemical composition and from the point of liquid metal features (high differences in viscosity and width of the two-phase zone, i.e. tendency to alloy freezing). The factors named above break down the issue of steel filtering into several groups.

 

Carbon steels

The requirement for a high casting speed to avoid metal freezing on the filters is the common factor of carbon steel filtration.

 

Stainless steels

With respect to low carbon content (usually 0.1-0.15 wt.%, and up to 0.05-0.07 wt.%) use of the filters is restricted to the zircon filters, otherwise moderate metal carburizing during filtration could take place, causing local metallurgical defects. Stainless steels are characterized by very high casting temperatures ranging from 1600 ˚C to 1700 ˚C.

 

Manganese alloys

Filtration of alloys with a high manganese content (Hadfield steel) is a specific field. With respect to their foundry features these alloys are “ideal” for direct casting. It is above all the capability of metal feeding from the riser to high distances. If the filter is inserted into the riser, the thermal node originally created by the gate outlet is eliminated and risering may be avoided. On the other side, the Hadfield steel is characterized by a narrow two-faze zone range and is therefore susceptible to freezing and to creation of cold laps in the casting. Therefore when casting, the filter must take the minimum possible heat from the melt; that’s why carbon-based filters are ideal.

 

Direct casting

Direct casting is a popular technology enabling to reduce costs for manufacture of castings materially. The main advantage consists in higher utilization of the liquid metal. This is facilitated both by progressive materials of the exothermic extensions and by arrangement of the gating system itself, i.e. metal filling (and gating system heating) takes place through the point of the last solidification, the so called riser. Filter is the necessary integral part of the  direct heating method; on the one side the filter eliminates undesirable inclusions and on the other side reduces dynamic metal impact and prevents mould erosion. In the predominant majority of cases the ceramic foam filters are used for direct casting, because they are capable to withstand complex and demanding conditions during casting (dynamic effect of the falling metal, often very high filter capacity, etc.).
Direct casting is used with success for the castings weighing tens of kilograms up to several tons (2 t). In most cases the castings do not have a complex geometry or are of plate shape only (milling/baffle plates, grids, covers, etc.)

 

Investment casting

Use of the filters in the technology of investment casting is a separate chapter in steel casting filtration. Practically all available alloy types are used here. Use of the filters is also restricted by shell annealing before the casting operation itself, i.e. the carbon filters cannot be used, because they would be “burnt” during annealing in the oxidation atmosphere. In most cases the filter is placed in the pouring basin and the metal is cast directly on the filter. Filter size is determined rather by the sprue cup size; filter of corresponding capacity would be too small. In some cases the filter can be placed close to the casting, mainly in case of the clusters with a higher number of castings.

 
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