Background of invention Various cast products intended to resist intensive wear are frequently manufactured by casting into a resin-bonded casting sand mold.

The casting sand can be, e. g. , a sand obtained from natural sources, such as quartz, chromite, olivine or zirconium oxide sand or, alternatively, sands produced therefrom by crushing and screening to correct grain size. The casting sand used in a foundry process typically has a grain size in the order of 0.2-0. 4 mm.

Sand binder is generally selected from the group of resins hardenable by a polymer- ization reaction or, alternatively, an inorganic binder may be selected such as water glass.

Casting into so-called green sand molds is also employed. Herein, the binders are different, conventionally the binder is a natural clay material such as bentonite.

In iron and steel foundries, one typical sand grade in resin-bonded casting sand molds is chromite sand. This sand features good durability under high temperatures.

A shortcoming of chromite sand, however, is its high price as compared to certain other sand grades. Hence, many foundries use two different sand grades in their casting molds. More specifically, e. g. , hot spots occurring close to the surface of the cast object are molded using chromite sand, while the rest of the casting sand volume

of the mold displaced at a greater distance from the hot casting and thus situated closer to the mold flask frame wall is filled with quartz sand serving as backing sand.

Natural chromite occurring in chromite sand is mineral containing oxides of chromium and iron, wherein these components are present in a certain weight percentage ratio, e. g. , 50-80 % chromium oxide and 20-50 % iron oxide. As to its crystalline structure and mineralogical type, chromite has a spinel composition, AB2X4. The main component of chromite sand is ferrochromite FeCr204, but in nature there may also be present small amounts of other oxides such as oxides of magnesium and aluminum, plus such minerals as MgCr204, FeA1204 and MgAl204.

Investigations have shown that under certain thermal treatment conditions the spinel structure of the chromite surface disintegrates, whereby the oxide ratio can be seen to change so that the chromite grain surface has a relatively greater amount of iron oxide Fe304 than chromium oxide Cr203. The surface composition of a virgin, unused chromite sand grain may be, e. g. , : Cr203 48 %, Fe304 23 %, A1203 17 %, Si02 2 %, MgO 10 %. In contrast, surface analysis of chromite sand thermally treated sufficiently long under oxidizing/reducing conditions corresponding to those in a casting mold may give, e. g. , the following data: Cr203 3 %, Fe304 95 %, A1203 1 %, Si02 1 %. The modified composition of the thin surface layer that chiefly con- tains iron oxide on the grain lowers the lower surface tension of the chromite sand grain thus promoting penetration of the molten metal into the interstices between the grains of the molded sand.

This kind of thermal degeneration in a chromite grain takes place unintentionally when the same lot of chromite sand is reused multiple times as casting sand, whereby the sand situated closest to the cast steel is heated for several hours to a temperature in excess of 1000 °C. In a casting process, this is an undesired effect inasmuch as a composite of sand and steel is not wanted to occur in a cast. Chromite sand thus eventually changes into useless waste that generally is dumped. As a result, dumping represents significant costs to foundries. In less equipped foundries, the amount of chromite sand waste may be almost equal to or even greater than the actual volume

of castings, depending on the type of casting sands, molding methods and binders used.

However, to reduce dumping and haulings costs, casting sand has traditionally been regenerated for reuse. In conjunction with a mixture of quartz and chromite sands, for instance, regeneration means that quartz sand and chromite sand are separated apart from each other, unusable grain size fractions are screened apart from the usable fractions and thermally degenerated chromite sand is removed from the undegenerated fraction of chromite sand. Recyclable sand grades and grain size fractions are returned to the casting process for reuse in packing the molds.

Plural equipment is commercially available for sand regeneration. Using a conven- tional type of commercial chromite sand screen followed by suitable classifiers such as fluidized-bed classifiers, the spent mixture of quartz/chromite sand is typically classified into four fractions: 1) dust and other fines, 2) recyclable sand chiefly comprising quartz sand, 3) chromite sand undergone a change, that is, degenerated under exposure to high temperature, and 4) undegenerated chromite sand.

Of these, fractions 2 and 4, that is, quartz sand and so-called regenerated chromite sand are suitable for reuse. Fractions 1 and 3 are today generally dumped.

Using the above-described sand regeneration system, foundries can dramatically reduce the amount of waste sand produced in the foundry. By way of combining the best available molding methods and binders to chromite sand regeneration, up to 90 % of the casting sand may be recycled, whereby only about 10 % of the casting sand need be dumped.

Hence, the only remaining problem to be solved is what to do with the remaining 10 % residue of degenerated chromite sand.

Advantageous use of chromite sand for forming a composite material during casting is known from the prior art. Patent publication US 3,239, 319 describes a method of casting brake shoes having a composite structure based on cast iron matrix. In one of the examples described in cited reference publication is also mentioned preheated chromite sand. According to the description of the invention, preheating is applied to prevent excessively rapid solidification of cast iron. As known in the art, heating of the constituents also promotes penetration of the matrix material into the interstices between the insoluble particles. However, the sand preheating step described in this publication cannot achieve that kind of change in the chromite sand microstructure as is described earlier in this text, because the diffusional change of the sand grain crystal structure takes several hours. Such a change of particle microstructure is mandatory for good penetration of molten steel into the interstices between the chromite sand particles that results from the lowered wetting angle between chromite sand particles and molten steel.

A further shortcoming of the invention disclosed in cited patent publication US 3,239, 319 is that the method according to the publication only produces cast articles having the reinforcing particles distributed uniformly about all sides of the finished casting. This constraint complicates or perhaps even excludes later machin- ing of the casting.

Cast iron used as the matrix metal in the composite material of cited patent publication US 3,239, 319 cannot be employed as raw material of cast parts subjected to heavy impact-type wear.

For almost a full century now, cast wearing parts intended to resist intensive impul- sive erosion for instance in jaw, gyratory, cone, impact and hammer crushers and shredders have been chiefly made of cast austenitic manganese steel that undergoes substantial work-strengthening under impacts. Generally, the composition of this kind of manganese steel is C 0.9-1. 5 %, Mn 6-25 %, Cr 0-3 %, Mo 0-1 %, and small amounts of other alloying elements such as Si and Al, possibly also Ti, Zr and certain

other elements. Other materials used in the above-mentioned crusher types, partic- ularly in impact crushers, for making their cast wearing parts are so-called white cast iron (C 2-3.5 %, Cr 10-30 %, Mo <3 %, Ni <2.5 %) and martensitic tempered steel (C 0.4-0. 6 %, Cr 0.7-3. 5 %, Ni <2.0 %, Mo <0.5 %).

Herein, the method according to the invention makes it possible to advantageously recycle generally at least 50 %, perhaps even 100 %, of the residual chromite sand fraction separated in chromite sand regeneration and conventionally considered unsuitable for recycling.

Summary of invention Now, a method described in more detail below has been found suited for reducing the amount of casting sand waste produced in a foundry and, thus, its waste disposal costs, as a byproduct of the method, for manufacturing a wear-resistant cast product and for using such a cast product. A characteristic feature of the invention is the use of thermally degenerated chromite sand waste in conjunction with manganese steel in such a fashion that localized composite material reinforced areas are formed in a manganese steel cast product.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1 and the cast products according to the invention are characterized by what is stated in the characterizing parts of claims 5 and 6. The use of the cast products according to the invention is disclosed in claims 10 and 11.

Detailed description of invention When the above-described type of chromite sand that has undergone a change in its crystal structure and properties is used either alone or blended with regenerated chromite sand, a sand grade is obtained free from the typical property of chromite sand known to prevent the penetration of molten steel into the casting sand. As a

result, this novel sand type blended with a resin binder can be used to produce such a porous core that in a desired fashion during casting can be induced to become fully impregnated with molten steel. As a result, a composite structure is obtained formed by chrome iron oxide grains embedded in a tough steel matrix. This outcome has been verified in practical tests carried out with molten manganese steel.

The ratio of steel to chrome iron oxide in a composite structure thus obtained can be, e. g. , 50/50 or 30/70. The optimal composition ratio can be adjusted compatible with the intended use of the composite material, whereby high steel content accentuates the toughness of the matrix material in the composite structure and, conversely, high content of chromite sand increases the hardness of the composite structure.

A porous insert core needed in the formation of the composite material is prepared from chromite sand and a binder using the same molding technique as is employed in the preparation of a solid core from casting sand. First, chromite sand and binder are blended and packed into a flask of suitable size and shape, wherein they are allowed to harden to form an insert core. However, the surface of the molded insert core is not rammed by any means in order to offer the molten steel maximally easy penetration into the pores of the chromite sand insert core during casting.

In products to be cast, the desired composite portion in the material of the cast wearing part is formed by way of placing an insert core of binder-hardened ferro- chromite oxide sand into the casting mold. As the molten manganese steel is being poured into the mold, the interstices between the oxide sand grains of the porous insert core become filled with the molten steel, whereby a portion of composite structure comprising a steel matrix embedding the ferrochromite grains is created into the cast product.

The amount of the composite material portion in a cast product may be quite small.

Herein, the composite material portion in a cast product refers to such a specific portion of the overall volume of a finished casting wherein a composite of steel and chromite sand is present. In a major number of cast products related to the present

invention, the portion of composite material needed may be very minute. As an example, the dimensions of the insert core required for forming the composite material portion can be as small as 25x25x100 mm.

In the casting mold of the wearing part, the insert core is located on those surfaces of the wearing part to be cast that during the use of the part will be subjected to the heaviest erosion. An insert core may be placed directly against the inner surface of the mold cavity, whereby the insert core will be situated immediately on the surface of the wearing part, or alternatively, the insert core is displaced inward at a small dis- <BR> <BR> tance from the mold surface with the help of, e. g. , a chaplet or spacer plates. Instead of conventional core fixing method, other means may alternatively be used herein, e. g. , a screw, nail or glue.

Furthermore, the regenerated chromite sand may be classified so that only selected fractions thereof are taken for reuse in the insert core. Fines may be removed from the reclaimed chromite sand with the help of a fluidized-bed classifier or a screen, for instance. Removal of the finest fraction from the chromite sand makes its possible to produce a more porous insert core, whereby the sand-steel ratio in the composite material portion of the casting can be adjusted toward a higher steel content.

The composite structure formed by the chromite casting sand and manganese steel on the areas of the wearing part exposed to the heaviest erosive wear can substantially improve the resistance of those areas to abrasive and scratching erosion inasmuch as the hardness of chromium iron oxide is in the order of 1000-1400 HV. Depending on the degree of working hardening, manganese steel respectively has only a hardness of 250-600 HV.

Durability tests have shown an improvement of 50 % to 200% in the service life of a wearing part when comparing at a test crusher plant operated under standardized conditions the service life of wearing parts made from manganese steel alone with the service life offered by a manganese steel wearing part that is locally strengthened by a composite structure of chromite sand impregnated with manganese steel.

In the preparation of the insert core, at least 20 %, optimally in excess of 50 % of the chromite sand that is used therein is chromite sand degenerated thermally when being recycled as casting sand.

The invention is not limited to the reuse of degenerated chromite sand alone obtain- able as foundry waste, while this material is a cost-advantageous source of degener- ated chromite sand. Namely, thermal degeneration of spinel-structured chromite sand may be accomplished also artificially by way of keeping for a longer time under a controlled atmosphere and elevated temperature.

While the invention is neither limited to applications related to manganese steel alone, as far as known today, the effect utilized in the invention is optimally appli- cable in conjunction with this steel grade. Nevertheless, to a certain degree, the effect can promote melt penetration into the chromite sand insert core for other steel grades, too.