EP0421374A2 - Manufacturing process of casted articles with superficial wear resisting layer and product realised with the process - Google Patents

Abstract

A manufacturing process for iron products with hard, abrasion-resistant material with impregnated surface layers uses a model that dissolves during casting to produce a casting mold. A layer is applied to the model in the area to be hardened, which contains a powder of the abrasion-resistant material and a binder. When an iron melt is poured into the casting mold, the abrasion-resistant material is deposited in the surface layer of the iron product. The process is intended to create a strong bond between the abrasion-resistant material and the iron of the casting. The method is also intended to enable the use of an aqueous mortar slurry. For this purpose, a water-containing binder solution is used as the binder. An aqueous solution of polyvinyl alcohol, which is highly water-soluble and makes the use of a flammable liquid unnecessary, is particularly preferred.

Description

The invention relates to a production method for iron products with hard, abrasion-resistant material with impregnated surface layers, in which a model that dissolves during casting is used to produce a casting mold. A layer is applied to the model, at least in an area to be hardened, which layer contains a powder of the abrasion-resistant material and a binder. When an iron melt is poured into the casting mold, the abrasion-resistant material is deposited in the surface layer of the iron product.

Various methods are known by means of which iron can be coated with a hard, abrasion-resistant surface. This is done for example by flame spray coating or plasma spray coating. A disadvantage of these processes, however, is that the surface layers can flake off during the coating process and when iron products are used, and furthermore high process costs are incurred.

It is also known from US Pat. No. 4,119,459 to melt carbides into the surface by applying carbide macro particles to a workpiece and then heating the workpiece. However, it is difficult to place the carbide macro particles precisely in the desired location.

There is also a method for casting hard surfaces on workpieces in connection with the use of poly styrene models by Hansen et al, "Application of Cast-On Ferrochrome-Based Hard Surfacings to Polystyrene Pattern Castings", Bureau of Mines Report of Investigations 8942, US Department of the Interior, 1985. In this process, a paste containing a binder and the desired hard material, such as. B. tungsten carbide powder, applied to those surfaces of a polystyrene model that correspond to the wear-prone surfaces of the resulting cast.

However, this process suffers from the lack of bond between the wear-resistant layer, for example tungsten carbide, and the foam model, which is primarily due to the fact that the almost dry paste does not adequately wet the surface of the foamed synthetic resin. Therefore, the iron does not penetrate the layer before it solidifies, and instead of soaking the iron, the carbide flakes off the product. Furthermore, this process is complex and uneconomical and cannot be used effectively in large-scale production.

Furthermore, the use of non-aqueous binders in this process requires the subsequent use of a non-aqueous, refractory, thin mortar that is applied to the model to prevent contact between the molten metal and the molding sand, thereby improving the machinability and surface finish of the casting . However, using non-aqueous, refractory mortar poses a variety of safety risks and is therefore completely undesirable.

There is therefore a need for a method of impregnating an iron surface with an abrasion resistant material that overcomes, avoids, or alleviates the problems of the prior art.

The object to be achieved with the invention will be to provide a method of the type mentioned by which a strong bond between the abrasion-resistant material and the iron of the casting can be produced. The method is also intended to enable the use of an aqueous mortar slurry.

The object is achieved according to the invention in the method mentioned above in that a water-containing binder solution is used as the binder. An aqueous solution of polyvinyl alcohol, which is highly water-soluble and makes the use of a flammable liquid unnecessary, is particularly preferred. The use of a water-soluble binder enables the model to be coated with an aqueous mortar as a separating layer. Furthermore, the spalling problems can be largely avoided.

A paste is preferably produced by mixing a powder of abrasion-resistant material into the binder solution, which is then applied to the desired surface areas of the model.

According to a further embodiment of the invention, a recess or depression is made in the surface of the desired area of the model, into which either the paste is introduced or which is initially filled with the aqueous binder solution, into which the abrasion-resistant material is then sprinkled.

It is also particularly advantageous to first produce a deformable disk made of abrasion-resistant material and the binder solution by introducing the material into a mold filled with solvent and depositing it therein. This disc can then be on a ge bring the desired shape and size and is then glued to the model.

Further advantageous refinements and developments of the method according to the invention emerge from the subclaims.

The invention also provides the iron product created by one of the methods of the invention.

The invention and further advantages and advantageous developments and refinements of the invention are to be described and explained in more detail with reference to the drawing.

• Fig. 1 die Illustration eines Verfahrens zur Erhö­hung der Zeitdauer für den Kontakt zwischen dem flüssigen Metall und dem Karbid und
• Fig. 2 bis 6 Photographien verschiedener Erscheinungs­formen der vorliegenden Erfindung.

It shows:

• Fig. 1 is an illustration of a method for increasing the time for the contact between the liquid metal and the carbide and
• 2 to 6 photographs of various aspects of the present invention.

The present invention can be applied to the casting of any known iron product. However, it is particularly preferred for cast iron, in particular spheroidal graphite cast iron or gray cast iron.

Any suitable material can be used with respect to the destructible model used in the present invention. However, foamed polystyrene (EPS) and polymethyl methacrylate (PMMA) are preferred. PMMA is the most suitable because it is less sensitive with regard to the formation of undesirable carbon vacancies during casting and less problems with flaking.

In the present invention, a hard, abrasion-resistant material with a particle size of about 15 microns to about 1.5 mm or more is preferred. The particle size is preferably between about 140 and about 548 microns (30 mesh) and more preferably between about 140 and about 149 microns (100 mesh). Since carbon imperfections are easier to form when using powders with a finer particle size, for example 200 mesh (74 microns), coarser powder is preferred, but without fine powder being considered completely unsuitable.

The particles are also usually spherical, for example to improve fluidity, but the particle shape is not critical.

Regarding the choice of hard, abrasion resistant material, any known hard state material can be effectively used, such as e.g. B. tungsten carbide, chromium carbide and the like or a mixture thereof. It has been shown that the use of an abrasion-resistant material with sufficient wettability with regard to the cast iron used effectively reduces the chipping problems known from the prior art. Where ductile iron is used as the metal to be cast, spherical or sharp-edged tungsten carbide or a eutectic mixture of WC and W₂C or other carbides such as chromium carbide are preferred, while aluminum is the least suitable.

It has also been shown that the wettability of the tungsten carbide is increased when the carbon content of the powder is less than stoichiometric required (e.g. less than 6.5 percent by weight for WC). It is therefore particularly advantageous to use substoichiometric carbon, spherical tungsten carbide powder with a carbon content of approximately 4% and also a eutectic mixture of W₂C and WC (commercially available under the generic name "crushed carbid") in connection with ductile iron.

A solution of polyvinyl alcohol (PVA) is preferred as the binder, since it is highly water-soluble and makes the use of a flammable liquid, such as alcohol, unnecessary. PVA also evaporates quickly without leaving carbon residue on the particles, increasing the wetting effect of the molten metal so that the metal can penetrate the carbide particle crosslinking in a fluid manner. The binder preferably contains a solution of PVA and water with a concentration of more than 5 percent by weight of PVA, preferably between about 9.5 and about 10.5 percent by weight of PVA.

The method is particularly used to deliver a casting that has abrasion resistant material at a particular location (or locations) using a destructible model of the desired cast product. A destructible model of certain shape and size (which depend on the desired cast product) can be made by any of the known methods. In particular, some successful methods for making destructible models are described in U.S. Patents 4,093,018, 4,462,453 and 4,691,754.

A mass of the abrasion resistant particles and the binder solution, which consists of a mixture of water and PVA is made by mixing the particles into the binder solution. The paste is then applied to the surface of the model, for example by spreading or in a similar manner, to those places where it is desired to impregnate the iron surface with the abrasion-resistant material. If necessary, a binder solution consisting of water and PVA can also be used.

After the paste, which contains the hard, abrasion-resistant particles, is applied to the desired locations on the destructible model and the paste has dried completely at room temperature or preferably at elevated temperatures up to a maximum of 60 ° C. for several hours, a ceramic slurry can be used in a known manner as a separating layer can be applied to the entire model to prevent contact between the liquid metal and the sand mold, which can improve both the machinability and the surface finish of the product.

Previous attempts to use an aqueous ceramic slurry at this stage of the process failed because the application of aqueous sludge to a foam model which carries a layer of carbide and a previously used binder causes an undesired dissolution of the binder in the aqueous slurry, which leads to the detachment of the carbide layer leads. However, the use of a PVA binder according to the present invention eliminates this problem and allows the use of an aqueous ceramic slurry, provided the simple precautions described above are taken. By using aqueous sludges according to the present invention, the safety risk associated with the conventional non-aqueous sludges can also be overcome.

Various methods can be used to apply the ceramic slurry to the model, e.g. B. Brush the surface using a brush or air spray the slurry. However, under the conditions of mass production, direct immersion of the model in the sludge is considered the most effective method.

It has also been found that the problems of dissolving the binder in the aqueous sludge can be further reduced if the model is quickly withdrawn from the sludge, then excess sludge is swept away from the model and the model is immediately fed to a hot air oven which is required for complete drying preferably held at a temperature of about 50 ° C for a few hours.

The model can subsequently be used to form a casting mold and subjected to a known metal casting process. A sand casting process was e.g. B. described in the previously cited article by Hansen et al.

In the case of metal casting, it has been found that with an increase in the time during which the abrasion-resistant material comes into contact with the liquid metal, the tendency for the material to flake off decreases. One method to increase contact time is to use a superheated liquid metal. The liquid metal is overheated to a temperature above the liquidus line. To ensure proper overheating, the metal is heated to a temperature that is preferably about 250 to 320 ° C above the liquidus line. As a result, the solidification time and thus also the time for the penetration of the metal into the carbide layer is extended, so that a non-flaking surface layer can form.

Another method for increasing the contact time between metal and carbide is to increase the casting volume and thus the ratio between the casting volume and the carbide area. In other words, the casting volume is chosen so that the ratio of the casting volume to the area of the abrasion-resistant material is sufficient to ensure an increased contact time between the liquid metal and the abrasion-resistant material during the casting process. This will be explained in more detail with reference to FIG. 1.

As can be seen from Fig. 1, the abrasion-resistant layer 1 is much less likely to flake off in casting A than in casting B because the larger volume of metal 2 in A requires a longer time to solidify. It has accordingly been found that when casting thin sections, for. B. B, expanding the cast beyond the required size (as indicated by the dashed lines) increases the time of contact between the carbide and the molten metal, thereby reducing the likelihood of chipping.

According to an alternative embodiment of the invention, at least one depression or depression can be formed in the foam model before the composition with the abrasion-resistant particles is applied. These depressions can be produced by a conventional mechanical method, such as milling, drilling or the like. The depressions or troughs preferably have a depth of about 0.5 mm to about 3.0 mm, depending on the part or the amount of wear required.

The recess or trough can be filled with the mass containing the hard, wear-resistant particles, which ensures the exact position in the resulting casting.

Instead of introducing the mass into the depression, the binder can first be introduced into the depression, complete wetting of the foam surface being achievable. The wear-resistant material consisting of isolated parts can then be poured into the recess, where it settles and the recess is closely lined. Excess PVA water binder can then be wiped off with a suitable absorbent material. If desired, you can let the wear-resistant layer dry before coating with the ceramic slurry, e.g. B. at room temperature, but preferably at elevated temperatures, particularly preferably at about 60 ° C.

The model is then coated with the sludge and the casting with the metal is carried out in the manner already described.

According to a further embodiment of the invention, disks are made from a powder of wear-resistant material and a binder with the aid of molds and then divided into required formats.

First, the wear-resistant material consisting of individual parts and the PVA water binder are mixed in a mold and evenly distributed. Excess binder can be removed using a suitable absorbent material. The disc is then allowed to dry under suitable conditions, the disc partially setting. The disk is preferably dried over a period of about 45 to about 75 minutes, particularly preferably over 60 minutes, in an oven or the like which is kept at a temperature between about 60 and 65 ° C., particularly preferably at 60 ° C. This makes the disc strong enough to be handled and cut into the desired pieces.

After the disc is cut into pieces of the desired shape and size, or after holes have been drilled in the discs, as shown in Fig. 2, the cut pieces are dried under conditions which allow immediate use or storage for later use . Drying is preferably carried out in a temperature range between approximately 60 ° C. and approximately 65 ° C., particularly preferably at 60 ° C., and over a period of 8 to 24 hours, particularly preferably 24 hours.

Preferably, a completely dried slice is softened on a non-flat surface before use. This is done e.g. B. by exposing the disc to steam for about 15 to 25 seconds.

If the disks are deformable, they can be bent around a cylinder, as shown in FIG. 3. The deformed disks are then adhered or otherwise attached to the surface of the destructible model in a manner that does not adversely affect the casting of the desired product. As shown in Fig. 4, the pane can be glued to the destructible model using an aqueous solution of PVA or other suitable adhesive. The aqueous PVA binder solution described above is particularly preferred as the adhesive material.

The resulting destructible models with the hard, wear-resistant particles are then used to produce castings as described above. Examples of cast products according to the present invention are shown in FIGS. 5 and 6.

The method is particularly advantageously used in mass production. For example, the An Using the last-described exemplary embodiment, the manufacturing process for the disks (for example the formation of the disks from the particles and the binder) can be carried out at a location which is remote from the location of the casting process. This is an essential aspect for efficient mass production.

The method according to the invention can be used to produce iron products for a wide range of applications. In particular, engine parts such as camshafts or eccentric rollers, agricultural equipment, field order tools, brakes, etc. can be manufactured. Products made in accordance with the present invention are superior to known products because the bond between the wear resistant particles and the iron is more effective here. Furthermore, the use of non-aqueous sludges can be dispensed with in the method according to the invention, as a result of which the associated safety risks are eliminated.

In order to describe the present invention and its advantages in more detail, special exemplary embodiments are given below, which are, however, only exemplary and do not constitute any restriction.

EXAMPLES Example 1: Process (A) and (B) for producing an iron product according to the present invention.

(A) A PMMA model with a carbide disc attached is made by first removing the carbide and the PVA can be mixed in a rectangular shape and spread evenly. The excess binder is then removed using a suitable absorbent paper.

The disk and the mold are dried in an oven at a temperature of 60 ° C for 60 minutes to partially cure the binder. This will make the disc firm enough to continue using and cutting into pieces.

The partially hardened disc is cut with a sharp cutting edge into pieces of the desired shape and size, as shown, for example, in FIG. 2. These pieces are dried at 60 ° C for an additional 24 hours and then glued to the model using the PVA binder to form the desired model, as shown, for example, in FIG. 4.

Then a mold is made in a known manner by z. B. the model is embedded in a cast iron box in bound or unbound sand. The corresponding procedure is described in more detail on page 3 of the article by Hansen et al cited above.

The desired metal, such as ductile iron, is poured into the mold in a liquid state, which leads to evaporation of the model. The model gas exits through the sand and the liquid metal fills the cavity left by the model. The metal then hardens and forms an iron product in which a wear-resistant layer is impregnated.

(B) In a PMMA model, several depressions with a depth of 0.5 mm are inserted in those places where the wear-free layers are to be created. A binder, which consists of an aqueous solution with 10 percent by weight PVA, is poured into the wells.

Crushed carbide particles are then introduced into the wells and can settle. The excess binder is wiped off and the layer is dried in a warm air oven at 60 ° C for 6 hours.

The dried model is then immersed in an aqueous ceramic slurry and swung out to remove excess sludge. Now the model is immediately placed in a warm air oven, where it is dried at 50 ° C for 16 hours.

Then, in the same manner as in Example 1 (A), a mold was made and the iron product was cast.

Example 2: Testing of samples made in accordance with the present invention.

Several sample pieces in accordance with the present invention containing ductile iron and various hard materials were cast using a PMMA model. These sample pieces are described in more detail in Table 1. TABLE 1 material Mesh size Wettability 1.GTE angular toilet (1) 40/80 wetted by DI 2.GTE spherical toilet (1) 40/80 wetted by DI 3.GTE spherical toilet (1) 100/200 wetted by DI 4. Macrocryst., WC (2) 40/80 wetted by DI 5. Macrocryst., WC (2) 100/140 wetted by DI 6. Macrocryst., WC (2) 140/200 wetted by DI 7. Macrocryst., WC (2) 200/325 wetted by DI 8. Macrocryst., Toilet (2) 325/15 micron wetted by DI 9. Kenface, WC + 6w / 0Co (3) 40/80 wetted by DI 10. KS-12, WC + 12w / 0Co (4) 100/140 wetted by DI 11. KS-12, WC + 12w / 0Co (4) 140/200 wetted by DI 12. Chrome. Carbide (5) 60/120 wetted by DI (1) Excellent wettability (2) Good wettability (3) wettability is lower than that of Macrocrystalline WC (4) Wettability is the same as that of Macrocrystalline WC (5) Excellent wettability, carbide tends to dissolve in cast iron

Macrocrystalline, Kenface and KS-12 are trademarks of Kennametal, Inc. for tungsten carbide compositions.

These sample pieces according to the present invention, identified with sample numbers 1 to 18, were evaluated using rubber wheel rubbing tests with dry sand.

In particular, these sample pieces were compared with comparison samples according to sample numbers 19 to 21, which contain 1020 steel, 1080 steel (hardened and tempered) and 1080 steel (hardened). The results are shown in Table 2. TABLE 2 Calculation of volume losses in dry sand rubber wheel abrasion tests: Material: cast metal melting core made of ductile iron with tungsten carbide and chrome carbide Pattern No. Reinforcement material Sample No. Initial weight gm Final weight gm Weight loss gm Density gm / cm³ Volume loss cm³ average Volume loss cm³ 1 MC 1 99.4599 99.3885 0.0714 10.87 6,564 7,521 2nd MC 2nd 93.1229 93,048 0.0749 10.87 6,886 3rd MC 3rd 100.3228 100.2237 0.0991 10.87 9,111 4th CR3C2 1 85.7913 857356 0.0557 7.05 7,896 7,971 5 CR3C2 2nd 86.4831 86.4241 0.059 7.05 8,364 6 CR3C2 3rd 81.7488 81.6948 0.054 7.05 7,655 7 GTE-WC, sharp. 1 101.7786 101.7226 0.056 11.95 4,686 5,648 8th GTE-WC, sharp. 2nd 93.9399 93.8593 0.0806 11.95 6,744 9 GTE-WC, sharp. 3rd 89.5642 89.4983 0.0659 11.95 5,514 10th GTE-WC, spherical. 1 94.5713 94.5287 0.0426 12.77 3,335 2,834 11 GTE-WC, spherical. 2nd 89.2265 89.2028 0.0237 12.77 1,855 12 GTE-WC, spherical. 3rd 92,6073 92,565 0.0423 12.77 3,312 13 Kenface 1 95,413 95.2824 0.1304 7.54 17,294 20,119 14 Kenface 2nd 94,278 94.1113 0.167 7.54 22,148 15 Kenface 3rd 93,545 93.3769 0.1683 7.54 22,320 16 KS-12 1 95,813 95.5581 0.2549 9.32 27,346 28,119 17th KS-12 2nd 95,558 90.9325 0.2397 9.32 25,716 18th KS-12 3rd 93,321 93.0284 0.2917 9.32 31,294 19th Steel 1020 1 96.2164 95.45 0.7664 7.85 97,518 97,518 20th Steel 1080 (Q + T) 1 33.57 21st Steel 1080 (only Q) 1 24.57

The results show that spherical WC has the highest abrasion resistance (last column) of all carbide types tested, which is an order of magnitude higher than that of hardened and tempered steel. It also shows that although the spherical toilet was the best, all the sample pieces according to the invention were also good.

Even if the invention has only been described on the basis of a few exemplary embodiments, many different alternatives, modifications and variants which fall under the present invention will become apparent to the person skilled in the art in the light of the above description.

Claims (16)

1. Production process for iron products with hard, abrasion-resistant material with impregnated surface layers, in which a model of the desired iron product that dissolves during casting is used to produce a casting mold, to which a layer is applied, at least in an area to be hardened, which contains a powder from the contains abrasion-resistant material and a binder, wherein when an iron melt is poured into the casting mold, the abrasion-resistant material is deposited in the surface layer of the iron product, characterized in that a water-containing binder solution is used as the binder. 2. The method according to claim 1, characterized in that a paste is applied to at least one surface area of the model, which contains powder of an abrasion-resistant material and the binder solution. 3. The method according to claim 1, characterized in that in the areas to be hardened in the surface of the model at least one recess or depression is inserted, in which the water-containing binder solution and the abrasion-resistant material is introduced. 4. The method according to claim 3, characterized in that the recess or trough has a depth of about 0.5 mm to about 3 mm. 5. The method according to any one of claims 1, 3 and 4, characterized in that a deformable disc is produced, which contains abrasion-resistant material and binder solution and is produced from the at least one piece of the desired shape and size, which on at least one area to be hardened the surface of the model is applied. 6. The method according to claim 5, characterized in that the piece with the binder solution is glued to the surface of the model. 7. The method according to any one of claims 1 to 6, characterized in that the model is coated with a ceramic slurry before the production of the casting mold. 8. The method according to claim 7, characterized in that the ceramic slurry has an aqueous base. 9. The method according to any one of claims 1 to 8, characterized in that the iron product contains cast iron, in particular ductile cast iron or gray cast iron. 10. The method according to any one of claims 1 to 9, characterized in that the abrasion-resistant material contains spherical tungsten carbide, sharp-edged tungsten carbide, chromium carbide, a eutectic mixture of WC and W₂C or a mixture thereof. 11. The method according to claim 10, characterized in that the abrasion-resistant material comprises a further additional element, preferably cobalt. 12. The method according to any one of claims 1 to 11, characterized in that the model which dissolves when cast contains polystyrene or polymethyl methacrylate. 13. The method according to any one of claims 1 to 12, characterized in that the binder contains an aqueous solution of polyvinyl alcohol. 14. The method according to claim 13, characterized in that the polyvinyl alcohol is contained in the binder in a proportion of more than 5% by volume, preferably in a proportion of 9.5 to 10.5% by volume. 15. The method according to any one of claims 1 to 14, characterized in that the volume of the model dissolving during casting is chosen such that the ratio of the casting volume to the area of the abrasion-resistant layer with which the model is impregnated is large enough to to provide increased contact time between liquid metal and abrasion resistant material during casting. 16. Iron product with an abrasion-resistant layer, which is produced by a method according to any one of claims 1 to 15.

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