EP0575685B1 - Investment casting with wear surfaces - Google Patents

Abstract

The method according to the invention makes it possible to produce castings (1) with ceramics embedded in the manner of composites. In this method, overheated melt is poured into a ceramic moulding shell (30), integrated into the cavity of which is at least one porous ceramic body. Such a ceramic body (2), which has a fully intercommunicating pore volume (22) and the ceramic structure (26) of which forms a three-dimensional network, at least partially fills the cavity of the moulding shell. The porous ceramic body is connected to the wall of the moulding shell. In the case of refiner plates (1) for paper manufacture, the webs (10) can be reinforced in the region (20) of the beating surfaces (15) with embedded ceramic (2). Castings according to the invention can also be used for the production of turbine blades (100) with armoured tips. <IMAGE>

Description

The invention relates to an investment casting with wear surfaces

In the case of composite materials, different types of material groups are put together, in many cases a first phase being formed by a base material and a second phase — in a special spatial arrangement — being embedded in the first phase. The base material is often metallic (pure metal or alloy) and the inclusions are made of ceramic material. In the case of dispersion alloys (dispersion-hardened materials), hard particles - formed, for example, by oxides, carbides, borides or nitrides - are embedded in the metallic first phase in a finely dispersed form. In another type of particle composite, namely the so-called cermets (ceramics / metals), the first phase serves as a binder for the ceramic particles of the second phase, which, in terms of volume, makes up the main part of the composite. Cermets are manufactured using powder metallurgy. They are suitable as wear-resistant materials.

Document EP-A-0365978 discloses an investment casting process, the product of which is a composite body which is composed of a metal matrix and a reinforcing material. Document DE-A-4107416 discloses a casting process, the product of which has wear-protected parts. A multi-part mold is used in this process. The wear protection is made by means of carrier cores. These consist of a skin of hard particles that are held together with a binder. The binder is gasified or burned during the casting by the heat supplied by the hot melt.

In various mechanical processes, device components that are manufactured as castings are subjected to severe wear. Such castings can be found, for example, in refiners used in papermaking. In the refiners, the pulp dissolved in the pulper is ground between plates with sharp-edged webs, the fibers being cut and fibrillated. The ridges of the refiner plates lose height during the grinding process due to abrasion, erosion and corrosion, and the edges are rounded off by plastic deformation. In order to increase the wear resistance of the refiner plates, a reinforcement of the grinding surfaces is required: Similar to the cermets or the dispersion alloys, the webs should be reinforced with ceramics. In order to maintain the high toughness of the bar base given in the known refiners, the ceramic incorporation should be limited to the upper area of the bars at the grinding surfaces.

It is an object of the invention to provide castings that are at least partially reinforced by composite ceramic. This object is achieved by the features of the first claim. In the example of the refiner plates, the ceramic reinforcement of the cast must be provided in the webs, in the area that is worn away by the wear processes. In contrast to the cermets and dispersion alloys, where the ceramic phase is microscopically distributed within the metallic phase, the ceramic inclusions have a macroscopic character in the castings according to the invention.

The casting according to the invention is carried out by means of an investment casting process in which meltable or burnable models are used for the production of the molded shell. For the unreinforced parts of the casting wax is used with advantage for the model. The porous ceramic bodies intended for reinforcement are integrated into the cast model, the pore spaces of the ceramic bodies being filled beforehand with wax - or instead of wax with a plastic that evaporates without residue when the molded shell is fired.

The porous ceramic body, which has a completely communicating pore space and whose ceramic structure forms a three-dimensional network, can be produced in various ways. Three examples of these manufacturing processes are described below. The following ceramic structures result from these processes: 1. a pile of granular particles; 2. a ceramic structure made up of rib-like and sintered grain-like particles; 3. A ceramic structure with an open-cell foam structure.

When manufacturing refiner plates, the base material used is advantageously a stainless steel, as is customary when casting such plates (ASTM CB-30 or X 35 CrMo 17). Nickel-based or cobalt-based alloys can also be used. The ceramic material must be inert to the melt. For steel castings, for example, aluminum oxide (corundum), zirconium oxide or magnesium oxide can be used; Silicon carbide, on the other hand, is unsuitable because this substance reacts with the melt.

Since the thermal expansion of ceramic and steel is different, there were concerns about the production of the composite material according to the invention. The tests that were carried out against these concerns, however, showed that the ceramic structures used in combination with cast steel remained intact - at least without any disturbing damage.

Fig. 1

einen Ausschnitt aus einer Refinerplatte, gemäss Erfindung,

Fig. 2

einen Ausschnitt aus der Formschale und dem Modell für den Guss der Refinerplatte gemäss Fig.1,

Fig. 3

eine Variante zur Formschale der Fig.2,

Fig. 4

ein Hilfsmittel für die Herstellung eines ersten porösen Körpers,

Fig. 5

ein Haufwerk von keramischen Körnern,

Fig. 6

das in Gussstahl eingelagerte Haufwerk der Fig.5,

Fig. 7

eine offenzellige Schaumstruktur,

Fig. 8

eine Keramikstrukur aus gerippeartig angeordneten und zusammengesinterten kornartigen Partikeln,

Fig. 9

ein keramisches Gerippe mit offenzelliger Schaumstruktur

Fig. 10

einen Ausschnitt eines Gussstücks mit eingelagerter Keramikstruktur gemäss Fig.9,

Fig. 11

das gleiche Gussstück wie in Fig.10 nach teilweiser Erosion der Oberfläche und

Fig. 12

eine Turbinenschaufelspitze mit Panzerung.

The investment castings according to the invention can also be used in the production of, for example, bearing parts or seals. In the present description, the invention is mainly dealt with in connection with the refiner plates mentioned. The invention is explained in detail below with reference to the drawings. Show it:

Fig. 1

a section of a refiner plate, according to the invention,

Fig. 2

a section of the molded shell and the model for the casting of the refiner plate according to FIG. 1,

Fig. 3

a variant of the molded shell of Figure 2,

Fig. 4

an aid for the production of a first porous body,

Fig. 5

a pile of ceramic grains,

Fig. 6

the pile of cast iron stored in Fig. 5,

Fig. 7

an open-cell foam structure,

Fig. 8

a ceramic structure made up of rib-like and sintered grain-like particles,

Fig. 9

a ceramic structure with an open-cell foam structure

Fig. 10

a section of a casting with embedded ceramic structure according to Figure 9,

Fig. 11

the same casting as in Fig.10 after partial erosion of the surface and

Fig. 12

a turbine blade tip with armor.

A refiner plate 1 with webs 10 is shown in detail in FIG. The web surface 15 (the grinding surface) is subjected to wear during the grinding of the fibrous material. According to the invention, the wear areas of the webs 10 consist of a steel / ceramic composite 20, in which a porous ceramic body 2 is embedded in cast steel. The base body 11 of the refiner plate 1 and the web base areas have no embedded ceramic; this maintains the high toughness of the cast material in the web roots, where large bending moments occur during milling.

The cross-sectional drawing in FIG. 2 shows part of the molded shell 30 (including a cast model with a base body 11 'and a wax-filled ceramic body 20'). The casting is advantageously carried out in the position shown in order to obtain a good filling of the pore space of the ceramic body 2 by the melt. If a ventilation duct 12, see FIG. 3, is provided below the webs 10, the air displacement from the pore spaces improves. (The poured channel 12 is, of course, removed when the casting is reworked.) The molding shell 30 is heated before casting to at least a temperature customary for precision casting (around 1150 ° C.). The melt, whose liquidus temperature is between around 1300 and 1400 ° C, is overheated. As experience has shown, the pore spaces of the ceramic bodies 2 fill completely thanks to the overheating before the solidification of the melt occurs.

FIGS. 4, 5 and 6 relate to the production of a first porous ceramic body 2 Ceramic grains 21 (for example corundum, diameter between 0.8 and 1.2 mm) are poured in with a drain 32. A pile 2 'with a pore space 22 is formed. This pore space 22 is filled with a free-flowing binder (eg silicate binder) and then emptied again via the outlet 32, bridges 21a of the binder material being formed between the grains. By drying the binder, a coherent ceramic body 2 is obtained, which can be removed from the trough 31 and solidified by firing.

The ceramic body 2 is filled with wax and built into the casting model for the production of the molded shell 30. The wax is removed from the surface 23 of the ceramic body 2, so that a direct connection between the grains 21 and the molded shell 30 is created. After the wax 35 has melted out, there is a connection between the ceramic body 2 and the molded shell 30. During casting, a surface 23 is formed with the metallic phase 35 filling the pore space 22, from which tips of the grains 21 emerge.

A second porous ceramic body 2 is produced by means of an open-cell foam structure 50 made of plastic, see FIG. 7. (For the sake of clarity, only one net layer 55 lying on the surface of the foam structure 50 and a second net layer 55 'lying underneath are shown in FIG. 7. The two net layers are connected to one another by invisible rungs. The average distance between adjacent ones Mesh layers can be equated with the average cell diameter, the cell diameter is in the range of around 1 to 5 mm 50 applied. The diving and subsequent sanding must be carried out several times - for example three times. The resulting rib-like agglomerate of ceramic grains is sintered together by firing, the plastic of the original foam structure 50 evaporating at the same time. In FIG. 8, this ceramic body 2 is again shown as in FIG. 7 with two net layers 25 and 25 '(on an enlarged scale), the ceramic grains being drawn only for the upper net layer 25, however somewhat too large.

The third ceramic body 2 shown in FIG. 9 consists of a commercially available ceramic structure, which is used, for example, as a filter for molten metal. The production of this ceramic framework with an open-cell foam structure is known for example from CH-PS 679394. FIG. 9 shows a section through the walls of the ceramic foam structure 26. Thanks to openings 27 in the walls, the cells form a completely communicating pore space 22.

The oblique image in FIG. 10 shows the ceramic structure 26 of FIG. 9, the pore space of which is filled with the metallic phase 28 (or with wax or plastic) and forms the composite body 20. When this composite body 20 is used in refiner plates, because of the greater resistance of the ceramic 26, the metallic phase 28 is washed out on the web surfaces, as indicated in FIG. 11. The relief-like surface structure probably has an improved fibrillating effect when the pulp is ground.

In the case of blades for gas turbines, it is advantageous if their tips are armored. 12 shows a turbine blade tip 100 with armor 102, which is designed as a casting according to the invention. The Armor 102 is formed by a casting zone with embedded ceramic, while the main part 101 of the blade is an unreinforced casting (for example the product of a method with directional solidification).

Claims (8)

1. A precision casting with wear surfaces (15) intended for abrasive operations, in whose zone (20) porous ceramic members (2) are intercalated composite-fashion in a metallic phase (28, 35), each ceramic member having a structure (26) in the form of a cohesive three-dimensional mesh, with a pore space (22) comprising communicating cells having diameters in the range from about 1 to 5 mm, and wherein the ceramic material forms part-surfaces which are exposed in the region of the wear surfaces.
2. A casting according to claim 1, characterised in that the ceramic material forms projections at least at the wear surfaces, said projections projecting from the surface formed by the metal phase.
3. A casting according to claim 1 or 2, characterised in that the ceramic structure of at least one of the porous ceramic members (2) forms a ceramic skeleton (26) having an open-cell foam structure.
4. A casting according to claim 1 or 2, characterised in that the ceramic structure of at least one of the porous ceramic members (2) is in the form of an accumulation of intercalated particles (21) in the form of grains whose diameters are about 1 mm.
5. A casting according to claim 1 or 2, characterised in that the ceramic structure (25, 25') of at least one of the porous ceramic members (2) is built up from particles in the form of grains which are arranged skeleton-fashion, and which are sintered together and whose diameters are about 0.1 mm.
6. A casting according to any one of claims 1 to 5, characterised in that the ceramic phase (26) consists essentially of corundum, zirconium oxide or magnesium oxide and the metallic phase (28, 35) consists of steel, a nickel-based or cobalt-based alloy.
7. A refiner plate (1) for paper manufacture, which is a casting according to any one of claims 1 to 6, characterised in that beating surfaces (15) are formed by webs (10) and in that the webs (10) are reinforced with intercalated ceramic (2) in the region (20) of the beating surfaces.
8. A turbine blade (100) having a blade tip with cladding, which is a casting according to any one of claims 1 to 6, characterised in that a cladding (102) at the blade tip is formed by a cast zone having intercalated ceramic.

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