LU88246A1 - Method for manufacturing composite wearing parts and parts produced by this method - Google Patents

Description

METHOD FOR MANUFACTURING COMPOSITE WEAR PARTS AND PARTS MADE BY THIS METHOD

The present invention relates to a method of manufacturing a composite wear part, in particular a hoop or wear segment of vertical or cylindrical mills comprising inserts and a body, in which said body is cast around the inserts arranged in a mold as well as wearing parts produced by this process, in particular grinder hoops.

Many wearing parts, for example in the field of shredders, are subjected to high mechanical stresses in the mass and to high wear by abrasion on their working surface. It is therefore desirable that these parts have a high abrasion resistance and a certain ductility in order to be able to withstand mechanical stresses such as impacts. In addition, a certain ductility is also required if these parts must possibly be machined. However, it is well known that these properties cannot be reconciled. Admittedly, it is possible to choose a compromise between these two opposite properties, but this must necessarily be done at the expense of wear resistance or ductility.

To avoid such a compromise, it is known to produce bimetallic parts, the part exposed to abrasion, consisting of chromium cast iron with high abrasion resistance, is supported by a more ductile steel core. This reduces wear on the part, while avoiding breakage of the core.

Several processes are known for manufacturing such composite or bimetallic parts.

Thus, it is known to produce bimetallic wear parts by welded assembly. If, in theory, the welded assembly does not present limits at the level of the morphology of the components to be assembled, in practice, such limits exist and they depend on the welding process used. In addition, all welding processes applied to fragile materials require perfect control of their heating and cooling cycle as well as very precise positioning of the surfaces to be assembled. As a result, the welded joint is a difficult technique to implement, expensive and not very versatile.

From US patent US-2,155,215, it is known to manufacture bimetallic wear parts by casting the support of the part around inserts arranged in the mold. The mass of the inserts must be calculated in such a way that the latter are heated above their critical temperature by the cast metal. The inserts are kept inside their housing in the part due to the superior thermal contraction of the metal forming the body of the part. However, during the casting of the metal forming the body of the part, there is a risk of displacing the inserts in the mold. This is particularly the case when the inserts are small and / or have a relatively small base relative to their height.

European patent application EP-0 476 496 describes a method in which longitudinal inserts, comprising at least one projecting radial rib, are arranged against each other on the circumference of a mold with axial symmetry. This way of operating ensures that the inserts are not displaced by the casting of the second metal. However, this method can only be used for relatively large inserts in an axially symmetrical mold.

The object of the present invention is to propose a method of manufacturing composite wearing parts making it possible to maintain the inserts in their position during the casting of the second metal, without the inserts having to be in direct contact with the neighboring inserts.

To achieve this objective, the present invention provides a method of manufacturing a composite wearing part comprising inserts and a body in which the body is poured around the inserts arranged in a mold and which is characterized in that said inserts arranged in said mold are fixed to a surface of a support, said surface constituting a surface forming part of the mold.

The inserts can be fixed to this surface by mechanical means, such as screws, rivets or circlips, by adhesive means, such as high temperature ceramic glues, or by welding.

To facilitate positioning and fixing, the support can be perforated at the places where the inserts will be placed.

The support, for example in sheet metal, can be coated with a refractory before the inserts are fixed.

After the solidification of the cast metal, the support can be removed, for example by machining, abrasion, fusion, etc., if the operating conditions of the part require it.

If the wear part has a composite wear surface, the support advantageously has a first surface which, in the mold, at least partially delimits the wear surface, and the inserts are fixed directly to this first mold surface.

The inserts can advantageously be subjected to a homogenization heat treatment and / or coated with a refractory before being fixed to the support.

The inserts can be of varied shape, for example: cylindrical, frustoconical, polygonal or can have the shape of a truncated pyramid. They can also include at least one re-entrant radial groove and / or at least one projecting radial rib. The inserts are therefore anchored in the body of the part by their particular geometric shape.

The inserts are advantageously executed in a metal alloy having a high resistance to wear, for example, in chromium cast iron containing complex carbides in chromium (Cr), niobium (Nb), vanadium (V) and / or molybdenum (Mo). The inserts can also be made of molten tungsten carbides and infiltrated by brazing based on nickel (Ni) or copper (Cu).

The body of the part, for its part, is advantageously made of a metal alloy combining good mechanical properties with an average resistance to wear, for example in chromium cast iron with carbon content (C) <2.1%, in Gs perlitic or bainitic cast iron, martensic steel or manganese steel (Mn).

The average temperature of the inserts being lower than the average temperature of the material forming the body of the part during its solidification, the material forming the body contracts more than the inserts during cooling after casting of the body of the part. In this way, the inserts are held in position by the contraction forces exerted by the body of the hoop.

The mass ratio of the inserts / total mass of the part is advantageously between 5 and 40%.

The mold, and therefore the part manufactured according to the method of the present invention, can, for example, be cylindrical or frustoconical. The cylindrical shape can either be a complete cylinder or added segments.

The part may have a bore which may be either cylindrical or conical. In the case of added cylinder segments, the mounting face on the bore can be flat so as to allow easier machining of the part and, in this case obviously, the shaft on which the segments will be mounted is polygonal.

According to an advantageous embodiment of the present invention, the distribution of the inserts is regular and forms a network whose mesh is a centered hexagon. The arrangement and the distance between the inserts can be chosen so that the structure formed by the ductile metal gives the assembly a high resistance to mechanical stresses. Other distributions of the inserts are also possible, such as for example in the form of a network whose mesh is a square or a centered square. Tests have shown that, for hoops for mills, the diameter of the inserts is advantageously between 10 and 100 mm and that the optimal spacing between the inserts is between 2 and 40 mm.

According to another preferred embodiment, the resistance to wear, the diameter of the inserts, the distribution of the inserts and the density of the inserts can be adapted to the local intensity of the abrasion to avoid irregular wear along the generator and thereby eliminate costly periodic rectification operations.

According to yet another embodiment, the arrangement of the inserts in the shredders of the grinders is adapted so that the different hardness of the metals promotes the appearance of macro-roughness which reduces the slip between the material to be ground and the surface of grinding so as to increase the throughput and reduce wear on the parts.

It is obvious that the method described in this invention can also be used for other types of composite parts whose body of the part is cast. Other particularities and characteristics will emerge from the description of some advantageous embodiments, presented below, by way of illustration with reference to the drawings in which: - Figure 1 schematically represents a cylindrical crusher; - Figure 2 schematically shows a vertical mill; - Figure 3 shows the details of a grinder at the grinding level; - Figure 4 shows a radial section through one half of a mold in which are placed inserts; - Figure 5 illustrates in axial section a grinding hoop in its mold; - Figure 6 shows a distribution of the inserts whose mesh is a centered hexagon; - Figure 7 shows a distribution of the inserts whose mesh is a centered square; - Figure 8 illustrates a distribution of the inserts whose mesh is a square; - Figure 9A shows a cylindrical insert; - Figure 9B shows a section along line IX-IX 'of Figure 9A; - Figure 10A shows a square base insert; - Figure 1OB shows a section along line X-X 'of Figure 10A; - Figure 11A shows an insert with a hexagonal base; - Figure 11B shows a section along line XI-XI 'of Figure 11A; - Figure 12 shows an example of distribution of the inserts around the periphery of a hoop.

A first advantageous application for the implementation of composite parts will now be described with reference to a cylindrical mill as shown diagrammatically in Figure 1. Such mills are, for example, used to grind ores, coal or clinker. They essentially consist of two cylindrical rollers 21, 22 which rotate in opposite directions. The material to be ground 23 is introduced through a feed channel (not shown). It is gripped between the two cylinders 21, 22 and ground under high pressure. The ground material 24 then falls on a conveyor belt which discharges it to a storage place for example.

The composite wear parts can also be advantageously implemented in a vertical roller mill as shown diagrammatically in FIG. 2. Such shredders essentially consist of a rotary track 30 on which grinding rollers 32 operate. The material to be ground is introduced through a central feed channel 34 and falls on the track 30 where it is crushed and ground between this track and the rollers 32. As shown in more detail in Figure 3, the ground material is seized , at the periphery of the track 30, by an ascending current of hot air and separated at the same time under the effect of gravity and of a separator 36 according to the particle size. To avoid friction between the rollers 32 and the track 30, the rollers 32 must have a frustoconical shape as shown in Figure 2. It is also possible to provide rollers 40, as in the embodiment of Figure 3, with a convex running surface, the track 38 having a corresponding concave annular surface.

The grinding rollers are, in general, constituted by a frustoconical or cylindrical annular hoop, mounted on a hub. On the one hand, they must have sufficient resistance to wear caused by grinding and, at the same time, be able to be machined to be mounted on the hub. The known hoops are generally cast in Ni-hard cast iron or in chromium cast iron and then machined with great precision before being mounted on their hub.

In use, the wear of such a hoop is uniform over the circumference of the hoop. On the other hand, wear is generally variable, along the same generator, the ends, especially the peripheral ends, wear out less quickly than the central part. In addition, there is a gradual polishing of the work surface, with the consequence of an increased risk of slippage between the hoop and the material to be ground. As a result, the profile of the working surfaces changes and the backlash adjustment system no longer makes it possible to restore optimal grinding conditions. In addition, as the outer surface is polished, the sliding between the material to be ground and the surface of the hoop accelerates wear and reduces the flow rate, especially if the material to be ground is wet.

To overcome these drawbacks, it is proposed to produce the mill frets with inserts as shown in FIG. 4.

A support on which inserts 6 are mounted by means of screws 8 follows the external shape of a mold.

The mold is limited on the inside by a cylindrical wall 10 so as to define a cylindrical bore 12. The metal to be cast will fill the volume 1_4 to form the body of the grinding hoop.

Figure 5 shows an axial section of a grinding hoop cast in the mold of Figure 4. The inserts 6 are anchored in the body 14 of the hoop by a geometric shape specially studied. In addition, the average temperature of the inserts being lower than the average temperature of the material forming the body of the part during the solidification of the latter, the material forming the body contracts more than the inserts during cooling after casting of the body of the room. In this way, the inserts are also held by the pressure stresses exerted by the material forming the body of the hoop on the inserts. The mold is shown in dotted lines.

Examples of different modes of distribution of the inserts on the hoop are shown in Figures 6, 7 and 8. Thus, in Figure 6, we see a unitary mesh in the form of a centered hexagon. The unit cells shown in Figures 7 and 8 have the shape of a centered square, respectively a square. It will be noted that, due to the fixing of the inserts, the difference between the latter may be significant, which makes it possible to give the assembly a high resistance to mechanical stresses.

Figures 9A, 10A and 11A show different forms of inserts. The geometric shape of the inserts 6 is important for the life of the hoop and for its mechanical stability. The inserts 6 shown comprise, in the illustrated cases, two re-entrant radial grooves which reinforce the anchoring of the inserts in the body of the part.

Figures 9B, 1OB and 11B show three preferred cross-sectional shapes of the inserts: circular, square or hexagonal respectively. The shape of the base surface of the inserts is important because it determines, with the diameter of the inserts and their spacing, the percentage of the wear surface formed by the material of the inserts, respectively by the material of the hoop body.

By the work of the hoop, preferential wear of the ductile alloy is caused and grooves between the inserts are formed. These grooves grip the material to be ground and increase the yield of the hoop. The diameter, shape and spacing of the inserts is chosen according to the friction characteristics of the material used, its particle size and its angularity.

Figure 12 shows an example of a particular distribution on the working surface of the hoop. Larger diameter 6 '' inserts are placed in the middle of the work surface while smaller diameter 6 'inserts are located at the peripheral ends. This particular distribution of the inserts is advantageous because the wear of a hoop is, in general, not regular. In fact, the wear of a hoop is circumferentially uniform. However, wear is generally variable along the same generator. Indeed, the peripheral ends wear less quickly than the central part. It follows that the profile of the working surfaces changes and that ultimately, the backlash system no longer allows to work in optimal conditions. The frets must then be dismantled and rectified.

The judicious choice of shape, dimensions, hardness and distribution of the inserts makes it possible to obtain minimum total wear (in gr / τ of ground material), to generate a wear surface having a profile favorable to the process and reduce the frequency of interventions to rectify the generator.

Claims (19)

1. A method of manufacturing a composite wear part (2) comprising inserts (6) and a body (14) in which said body (14) is poured around the inserts in a mold, characterized in that said inserts (6) arranged in the mold are fixed on a surface of a support (4), said surface constituting a surface forming a part of said mold. 2. A method of manufacturing a composite wear part (2) according to claim 1 characterized in that the inserts (6) are fixed to the surface of the support (4) by mechanical means, by adhesive means, or well by welding. 3. A method of manufacturing a composite wear part according to claim 2 characterized in that said surface of the support (4) is provided with a refractory. 4. A method of manufacturing a composite wear part according to any one of claims 1 to 3 characterized in that the support (4) is removed after solidification of the cast metal. 5. A method of manufacturing a composite wear part according to claims 1, 2 or 3 characterized in that the support (4) is provided with positioning holes for the inserts (6). 6. A method of manufacturing a composite wearing part according to any one of claims 1 to 5 characterized in that the inserts (6) are subjected to a homogenization heat treatment and / or provided with a refractory. 7. A method of manufacturing a composite wear part (2) according to any one of claims 1 to 6 characterized in that the inserts (6) have a cylindrical, frustoconical, polygonal shape or the shape of a trunk pyramid. 8. A method of manufacturing a part (2) of composite wear according to any one of claims 1 to 7 characterized in that the inserts (6) comprise at least one recessed radial groove (16) and / or at least a protruding radial rib. 9. A method of manufacturing a composite wear part (2) according to any one of claims 1 to 8 characterized in that the inserts (6) are made of a metal alloy having a high wear resistance , for example, in chromium cast iron containing complex chromium carbides, niobium, vanadium and / or molybdenum. 10. A method of manufacturing a composite wear part (2) according to any one of claims 1 to 8 characterized in that the inserts (6) consist of molten tungsten carbides infiltrated by a nickel-based solder or copper. 11. A method of manufacturing a composite wear part (2) according to any one of claims 1 to 10 characterized in that the body (14) is made of a metal alloy combining good mechanical properties with average resistance for wear, for example, in chromium cast iron with C content <2.1%, in pearlitic or bainitic Gs cast iron, in martensic steel or in manganese steel. 12. A method of manufacturing a part (2) of composite wear according to any one of claims 1 to 11 characterized in that the mass ratio of the inserts / total mass of the part is between 5 and 40%. 13. A method of manufacturing a composite wear part (2) according to any one of claims 1 to 12 characterized in that the mold is cylindrical, frustoconical or else formed from added cylinder segments. 14. A method of manufacturing a composite wear part (2) according to any one of claims 1 to 13 characterized in that the distribution of the inserts (6) on the wear strip is regular, for example in the form of a network whose unit cell is a centered hexagon, a centered square or a square. 15. Method according to any one of claims 1 to 14 characterized in that the wear part is a grinding roller. 16. The method of claim 15 characterized in that the diameter of the inserts (6) is between 10 and 100 mm. 17. The method of claim 15 characterized in that the spacing of the inserts (6) is between 2 and 40 mm. 18. The method of claim 15 characterized in that the resistance of the inserts (6) to wear, their diameter, their distribution and the density of inserts, expressed in inserts / m ^, are adapted as a function of the intensity of abrasion along the generator. 19. Cylindrical or conical grinding roller comprising inserts (6) integrated radially into the part, characterized in that the inserts (6) are completely surrounded by the body and are held in place by their studied geometric shape and by the contraction pressures exerted by the body (14) on the inserts (6) which are not in contact with each other.