AGGREGATION STRUCTURE TO PRODUCE SAID ELEMENT SUBJECT

TO WEAR"

# * * # #

FIELD OF THE INVENTION

The present invention concerns a method for the production of an element subject to wear, such as a tool used for the crushing or for the abrasion of mineral substances, masses of building debris, metal waste or other similar treatments, and an element subject to wear obtained by said method.

The present invention also concerns a temporary aggregation structure used as a base in the preliminary production steps of the element subject to wear, and a core obtained with the temporary aggregation structure.

BACKGROUND OF THE INVENTION

Several methods are known for the production of an element subject to wear, in which the element substantially comprises a metal matrix which confers great rigidity and robustness to the element, and one or more cores of ceramic material having a high resistance to abrasion.

One known method provides to make an element subject to wear by means of casting or centrifuging a molten metal material on an insert, or biscuit, made of ceramic material, disposed in a mold.

However, this type of known method does not allow to obtain elements having mechanical characteristics such as to be able to use in any application or sector, even those more exacting both in terms of stress, and also in terms of intensity and continuity of stress, and which require hardness, toughness and resistance to temperatures which cannot be obtained with the known methods.

Another known method provides to cast the molten metal material on a ceramic insert of metal oxide and/or metal carbide, which is preformed with a perforated structure made by sintering or heat pressure, so that, during the casting, the molten metal material can penetrate into the apertures and into the interstices of the insert itself.

This second type of method has, however, high production costs, in particular but not only, for the production and the pre-molding of the ceramic insert, which has to be sintered according to a desired form of use.

Moreover, since a sintering process is needed to keep the ceramic powders in a desired conformation, there is a limited possibility of molding the insert, so that conformations are made which are excessive or reduced with respect to the optimal.

This disadvantage brings, in some cases, an increase in production costs, and in other cases, a reduction in the good quality of the element made.

An element subject to wear is also known starting from powders, for the formation of titanium carbide using the heat of the metal material in the casting step of the matrix.

One purpose of the present invention is to perfect a method to obtain elements subject to wear, such as a mechanical member, an abrasion or crushing tool or similar, which have high resistance to wear, an excellent toughness and are able to resist considerable stresses, including heat stresses and prolonged stresses. Another purpose of the present invention is to perfect a method to obtain elements subject to wear, with reduced costs, greater precision in conformation of the insert and increased mechanical quality with respect to known methods.

A further purpose is to make a structure which allows to produce an element subject to wear which has great hardness and great toughness and is able to overcome the shortcomings of elements made according to the known state of the art, both in terms of production costs and in terms of mechanical quality.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purposes, a method for the production of an element subject to wear comprising a metal matrix and at least a core of hard material, provides:

- a first step in which a temporary aggregation structure is prepared having a geometric conformation coherent with the geometric conformation to be conferred on the core, and which has characteristics of being volatilized or in any case at least partly eliminated from the presence of the molten metal;

- a second step in which, on the whole internal and external surface of the temporary aggregation structure, a mixture of a binder with metal powders which contain hard elements or their precursors is uniformly distributed;

- a third step in which the core is disposed in a mold so as to only partly occupy the free volume of the mold; and

- a fourth step in which a molten metal material is cast in the mold, which metal melts the temporary aggregation structure and occupies the free volume and the volume that it makes free, both inside and outside the core, so as to anchor to the latter and thus form a single body.

According to the invention the core has a geometric conformation coherent with the requirements of the finished element, or of all the sectors of the finished element.

According to the invention, the temporary aggregation structure has a structure of intercommunicating open pores of the spongy type, disposed in the structure in a random or organized way.

In this way the mixture of liquid binder with the metal powders is made to impregnate inside the temporary aggregation structure and thus covers practically every recess present therein.

According to a first variant, the impregnation of the mixture inside the temporary aggregation structure occurs by elastically crushing the structure itself, immersing it into the liquid mixture and leaving it to expand elastically inside it. According to another variant, the temporary aggregation structure is introduced into an ambient where the vacuum is first created and then the mixture introduced.

With the present invention the molten metal material penetrates both into the interstices created by the interconnected holes and also into those which are generated by the elimination of the temporary aggregation structure, enveloping at least partly the metal powders, or in any case keeping them in the reticular position originally provided and defined.

According to a variant, if the hard elements or their precursors are carbon or even include carbon, they achieve reticular structures with increased hardness or hard particles by a chemical-physical reaction in contact with the molten metal material.

According to this variant, a structure is made with a core which has continuity but with variations in hardness in a reticular form defined by the metal material cast.

Making the temporary aggregation structure in spongy material with intercommunicating open pores, such as a reticule, entails both costs and production times which are considerably lower than known solutions in which the whole core, and not only its support, is made by sintering powders of oxides and/or metal carbides.

According to a first formulation of the invention, the reticular structure of the communicating holes can be random.

According to a another formulation the reticular structure can develop in an organized way according to three or more axes.

A further advantage of the solution according to the present invention is given by the possibility of conforming the core in a more simple and precise way compared with known solutions, so as to guarantee great precision in obtaining the hard zones of the element subject to wear.

Moreover, with the invention, the conformation of the core can be made easily.

It is also possible to provide in the space the density of the products or of the hard compounds even varying the type of the temporary aggregation structure, or by combining structures with different holes.

The temporary aggregation structure, as we said, is made of a material which in the presence of the heat of the molten metal material evaporates or is transformed into material of very reduced sizes.

According to a variant, at the end of the casting step a heat treatment step is provided, in which the element subject to wear is subjected to at least a heat treatment in order to confer determinate mechanical and structural characteristics on it.

The metal material with which the matrix is made is advantageously iron based, even if this characteristic is not essential for the present invention. In the case of material with an iron base, it is advantageously manganese steel, martensitic or others. According to a variant it is chromium cast iron or other similar material.

According to another variant of the present invention, before the casting of the metal material is carried out, both the sand mold and also the internal core can be kept at an ambient temperature and do not have to be heated, thus allowing a considerable reduction in costs in setting up and feeding the heating apparatuses.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a three-dimensional view of an element subject to wear according to the present invention;

- fig. 2a is a cross section of a sand mold in a first step of the method to make the element in fig. 1 ;

- fig. 2b is an enlarged schematic section of a part of the temporary aggregation structure;

- fig. 3a is a cross section of a sand mold in a second step of the method to make the element in fig. 1 ;

- fig. 3b is an enlarged schematic section of the same part of fig. 2b,of the element subject to wear obtained.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings.

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF

EMBODIMENT

With reference to fig. 1, a method according to the present invention for the production of an element 10 subject to wear, such as a mechanical member, an abrasion or crushing tool or similar, comprising a core 12, or panel, of hard material and a metal matrix 14, provides a step of preparing and molding the core 12, in which a temporary aggregation structure 17 is prepared on which a mixture of a liquid binder and metal powders which contain hard elements or their precursors, such as for example titanium, chromium, tungsten, molybdenum or others in a single or combined form, are aggregated.

The mixture is uniformly aggregated both on the internal surface and also on the external surface of the temporary aggregation structure 17, which has an open intercommunicating pore structure, of the spongy type.

In this case, the aggregation of the mixture to the temporary aggregation structure 17 occurs by means of an elastic compression of the temporary aggregation structure 17 inside a mixing bath.

From this condition the compression of the temporary aggregation structure 17 is released so that the mixture penetrates into the pores of the temporary aggregation structure 17, distributing itself in a substantially uniform way on the temporary aggregation structure 17 and inside the open and intercommunicating pores.

The mixture can consist of two or more metal powders, according to different percentages of mix in weight so as to obtain, on each occasion, a core 12 having determinate characteristics of toughness, heat dilation, resistance to abrasion and others, depending on the type of application for which the element 10 is intended. The temporary aggregation structure 17 is made, in this case, with polymeric foams, although it cannot be excluded that it can be made of any other similar or comparable material, which evaporates in the presence of the heat of the molten metal material.

Moreover, the temporary aggregation structure 17 has a geometric reticule conformation coherent with that which is to be given to the core 12, so as to precisely maintain the metal powders in determinate zones of the volume of the mold 16 and therefore of the element 10 once casting has been carried out.

In this case the mixture provides to use suitable glues, advantageously from 1% to 3% in weight, with respect to the metal powders provided.

As shown schematically in fig. 2a, in this condition each segment of the temporary aggregation structure 17 is externally enveloped by the mixture of powders 13, kept together and in aggregation to the temporary aggregation structure 17 by the layer of glue 15.

In the preliminary molding step of the core 12, spacer elements 18 are provided, in one piece or a single body, which are disposed uniformly on the external surface of the temporary aggregation structure 17.

In the following first step, the temporary aggregation structure 17 with the mixture of aggregated powders is inserted inside the sand mold 16 for casting, so that the spacers 18 are stably positioned in corresponding lateral walls 22 of the mold 16.

The spacers 18 have substantially a double advantage: they confer on the temporary aggregation structure 17 a self-bearing characteristic, avoiding the need for a bearing framework inserted at the center of the temporary aggregation structure 17, with the advantage of reducing the production costs and times; they define a correct position of the temporary aggregation structure 17, determining a free volume around the core 12 inside the mold 16.

The molten metal material is then cast, through a casting channel not shown in the drawings, so as to penetrate inside the interstices of the spongy structure of the temporary aggregation structure 17, substantially eliminating the latter and taking its place, so as to envelop the powders or possibly react with them.

This condition determines the amalgamation of the core 12 inside the matrix 14 forming a single body of the two parts, in which there is a structural continuity but with variations in hardness in correspondence with the reticular disposition of the powders, according to the spongy conformation of the temporary aggregation structure 17.

Simply to give an example, in fig. 3a a hypothetical shaping of the core 12 inside the matrix 14 is shown by a line of dashes, while the section signs, which have been deliberately extended, go to ideally define a communion zone between the core 12 and the matrix 14.

In fig. 3b the same sectioned part as in fig. 2b is shown, but after the casting of the metal material making up the matrix 14.

As can be seen from a comparison, the metal material 14 has completely taken the place of the temporary aggregation structure 17 and of the layer of glue 15. The position of the powders 13 remains, instead, reticular and substantially unchanged according to the disposition originally defined by the temporary aggregation structure 17.

Advantageously the sand of the mold 16 is made up of olivine, that is, iron and magnesium silicate, which does not develop free silica, and does not therefore cause silicosis, and is particularly suitable for the casting of molten metal material.

The temporary aggregation structure 17 can also be temporarily attached to the mold 16 by means of attachment elements 24, such as nails, screws or similar, which are disposed between the temporary aggregation structure 17 and the walls 22 in order to firmly anchor the temporary aggregation structure 17 in the position defined by the spacers 18.

Both the temporary aggregation structure 17 and the mold 16 are at ambient temperature before casting is carried out.

The molten metal material is, in this case, a mix of martensitic steel. Alternatively chromium cast iron is used.

In this case, the element 10 is cooled slowly in the mold to a temperature of less than 300°C, this in order to reduce internal tensions, it is then dug out and subjected to hardening at about 950-l,100°C, preferably at 1,000°C, for a determinate period of time, depending on the thickness of the element 10, and cooled in forced air, or in water or according to other known methods. In a preferential solution, during hardening the element 10 is progressively heated for about 10 hours up to 950-l,100°C, following a determinate temperature ramp, and then kept at temperature for about 2-6 hours.

After cooling the element 10 is worked, in order to carry out flattening, leveling or other workings so that it can then be assembled on a crushing member, such as for example the rotor of a mill. The element 10 shown in the drawings has a substantially parallelepiped shape for example, but it is clear that this shape is not limiting for the present invention, because it depends on the subsequent application of the element 10.

It is clear that modifications and/or additions of steps or parts may be made to the method, the temporary aggregation structure 17 and the element subject to wear 10 as described heretofore, without departing from the field and scope of the present invention.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of method for the production of an element subject to wear, element subject to wear and temporary aggregation structure for the production of the element subject to wear, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.