EP1581779B9 - Cooling element, particularly for furnaces, and method for producing a cooling element - Google Patents

Abstract

The invention relates to a cooling element, particularly for use in walls of furnaces that are subjected to high levels of thermal stress, and to a method for producing a cooling element. The cooling element is comprised of cast copper or of a low-alloyed copper alloy and is provided with coolant channels, which consist of tubes cast inside the copper or the copper alloy and which are placed inside the cooling element. In order to create a cooling element with an improved material bond on the contact surfaces between the cooling tube and the metal cast around it and thus with an increased heat transfer, the invention provides that the tubes of the coolant channels are provided with an electrolytic coating on the exterior thereof. The use of copper tubes has been shown to be particularly advantageous, and the coating of the tube exteriors thereof ensues in an electroplating bath.

Description

The invention relates to a cooling element, in particular for use in walls of highly thermally stressed furnaces, consisting of cast copper or a low-alloyed copper alloy with arranged in its interior coolant channels made of cast copper in the copper or copper alloy copper pipes.

1. a) Fertigung des Rohres einschließlich aller gewünschten Krümmungen, Abzweigungen und dergleichen Strömungsstrukturen,
2. b) innerhalb einer Gießform Umgießen der Rohre mit geschmolzenem Kupfer oder Kupferlegierung bei vorzugsweise gleichzeitiger Kühlung der Rohrinnenwandungen,
3. c) Abkühlen der Kupferschmelze.

Furthermore, the invention relates to a method for producing a cooling element provided in its interior with coolant channels formed from copper pipes, in particular for use in walls of thermally highly stressed furnaces, with the steps

1. a) fabrication of the pipe including all desired bends, branches and the like flow structures,
2. b) casting the tubes within a casting mold with molten copper or copper alloy with preferably simultaneous cooling of the tube inner walls,
3. c) cooling the copper melt.

Such cooling elements are usually arranged between the shell and the lining of a furnace, often for use behind the refractory lining, including the cooling elements to the cooling system of the furnace, for. Example of a pyrometallurgical furnace, connected. The surfaces of these cooling elements can, as for example in the EP 0 816 515 A1 is described, provided on the side facing the furnace interior with additional webs or grooves or honeycomb depressions, so as to allow a better bond with the refractory lining of the furnace or a good adhesion of the furnace process resulting and due to the intense cooling by the cooling elements solidifying slag or metal to protect the cooling element from chemical attack and erosion. The use of the cooling elements is usually in the form of cooling plates in the furnace walls or the ceiling or the hearth area of cylindrical or oval shaft furnaces. Also used are such Cooling elements also in pig iron blast furnaces, in electric arc furnaces, direct reduction reactors and melter gasifiers. Further areas of use for the cooling elements are burner blocks, nozzles, pouring troughs, electrode clamps, taphole blocks, hearth anodes or dies for anode forms.

Basically, a high degree of heat dissipation is aimed at the cooling elements, whereby both the service life of the cooling elements can be improved and it is avoided that thermal peak loads of the furnace process, especially during dynamic operation, lead to destruction of the cooling element.

In cooling elements with cast-in tubes as coolant channels, in addition to a good, loss-free as possible flow guidance, a good heat transfer from the cast metal of the cooling element to the cooling liquid flowing in the tubes is desired. In the above already named EP 0 816 515 A1 For this purpose, an improved bond between pipe and potting compound is proposed in such a way that a part of the thick-walled copper pipes is melted when encapsulating with the liquid copper, but this, since the tube and melt substantially equal melting point due to their material equality, with considerable process engineering difficulties. With a relatively cold cast, there is a risk that the tube will not weld sufficiently to the cast-in metal. The consequence of this is a very large heat transfer resistance between pipe and Umgussmetall. If, conversely, the casting temperature is increased, it is hardly possible to avoid localized dissolution and melting through of the pipes, or at least impressions of the cross section of the pipes, even when using thick-walled pipes. A composite body produced in this way is unusable for use in an oven.

Metallurgical dependencies also play a major role in the use of copper melts. Copper melts tend to absorb gases. Hydrogen and oxygen in particular have a disruptive effect on the casting process. The duration of the melting time and possibly the superheating temperature also play a role and can vary from melting process to melting process. Hydrogen and oxygen are in equilibrium with each other, which is why low hydrogen contents are set at high oxygen contents and vice versa. Since the solubility of hydrogen in solid copper is much lower than in liquid copper, it can be deduced that the solubility for hydrogen decreases significantly with decreasing temperature. The transition from the liquid to the solid phase of the copper melt has an extremely large reduction in solubility for hydrogen, one generally speaks of a solubility jump when falling below the liquidus temperature, this is about 3.5 ml of hydrogen per 100 g of copper melt.

For the absorption capacity of a melt for gases also the temperature and the pressure play an essential role. The pouring of a hydrogen-containing molten copper in the presence of oxygen in the form of copper oxide on the pipe surface is problematic because it forms during casting by the atmospheric oxygen due to the extremely rapid pipe heating by the melt. Due to the solubility jump in the transition of the melt from its liquid to the solid state of the released hydrogen reacts with the copper oxide by this is reduced and causes the resulting water vapor gas porosity of the casting. In terms of process technology, this can be remedied with vacuum degassing, which, however, represents an additional expense. Alternatively, a targeted oxygenation can achieve a shift of the water-oxygen balance in the direction of oxygen, and thus a removal of the hydrogen. Following the oxidizing treatment of the melt, the oxygen content must be deliberately reduced by a deoxidizing treatment of the melt in the ladle. Due to this, however, complicated two-stage metallurgical treatment of the copper melt, a reaction with the oxygen of the copper oxide of the cast-copper pipes can no longer lead to an undesirable formation of water vapor and thus gas bubbles within the melt.

By the contact of a highly heated copper melt with a copper tube arranged in the casting mold, as already described, a mechanical weakening of the copper tube occurs. The tube tends to be pushed in at those places where a higher metal column is loaded. To eliminate this difficulty is in the DE-PS 726 599 discloses passing gases or liquids through the tubes during casting under increased backpressure, said backpressure being approximately equal to the deformation resistance of the tube at the softening temperature. But even with the use of this method, oxidation of the tube on its outer surfaces during the casting process can not be avoided.

Different alternatives in the choice of material of the potted tubes are in the US 6,280,681 described. In addition to the possibilities, but also the limits of the use of pipes made of steel, stainless steel and copper, a type of cooling elements is also described the pipes are used in a material referred to as "Monel" in the trade. This material has a copper content of 31% and a nickel content of 63%. Furthermore, it is described in this document that one can not only use copper tubes to achieve a good bond, but also tubes made of Cu-Ni alloys such as UNS C 71500 with a copper content of 70% and a nickel content of 30% , These tubes have the advantage of higher thermal stability during casting due to their higher melting point and can often be produced without simultaneous passage of cooling water through the tubes during and after casting. With such pipes, the risk of breakthroughs of copper melt into the pipe interior can be significantly reduced. To maintain a free pipe diameter, these are filled with sand prior to casting, so as to maintain the pipe cross-section and to avoid collapse of the pipe. Unfortunately, the said tubes of Cu-Ni and Ni-Cu alloys have a significantly poorer thermal conductivity than copper tubes, which can dissipate significantly less heat during later operation as a cooling element, and it can come to thermal overloads areas of the furnace wall in particular. In addition, alloys of nickel and copper are much stronger, which makes them less forgeable and bendable. In critical areas such as narrow 180 ° bends, the use of preformed sheets requires much more welds to be made, which, apart from the higher manufacturing costs, increases the risk of later leakage.

Furthermore, there is the already described danger of increased gas porosity due to water vapor formation, which also degrades the casting quality, limits the heat dissipation and thus reduces the heat conduction, since the gas bubbles act as insulators in the casting. Another disadvantage is the different thermal expansion coefficient of the metals involved. It comes to compressive and tensile stresses on the embedded in the mold tube, which can lead depending on the shape of the tube to a locally poorer bond between the tube and the cast-copper and thus in turn to a deterioration of the heat conduction.

The prior art further includes a cooling element, as this in the DE-PS 1386 645 is described. In this cooling element, the tube to be encapsulated is not in the mold from the beginning, but first the copper melt for the production of copper block is placed in the mold, and then immersed the prefabricated tube in this melt, at the same time the tube inner walls are cooled. In the event that tube and melt are made of different metals, the attachment proposed an additional layer on the outside of the tube, said additional layer consists of a further, third metal, which can be applied, for example, galvanically on the tube. Which metals can be suitable for such purposes, remains open.

The invention has for its object to provide a cooling element in particular for use in walls of thermally highly loaded stoves, which is characterized at the interfaces between the cooling tube and Umgussmetall by an improved composite material and thus an increased heat transfer. Furthermore, a method is proposed, with which such a cooling element can be produced.

To solve a cooling element with the features of claim 1 is proposed.

To solve the subtask of providing a suitable method for the production of such cooling elements method, a method having the features of claim 3 is proposed.

According to the invention, therefore, the tubes to be cast in the manufacture of the cooling element are previously coated by electroplating with a suitable metal layer, this metal layer on the one hand brings no deterioration, but rather an improvement of the heat transfer with it, so has a very good specific heat conduction. On the other hand, the galvanically applied metal layer leads to advantages in the passivation of the pipe outside against oxidation effects during casting, further improves the adhesion between the pipe and Umgussmetall as a result of bordering adjusting diffusion processes. It is thus an immediate connection between the Umgussmetall and the cast-tube allows, the heat transfer is greatly improved and the cast-tube body promotes the subsequent use of the cooling element, for example, in an industrial furnace, a good cooling effect.

Of particular advantage are, in particular, the diffusion processes which occur in the outermost layer of the electrolytic coating after it comes into contact with the cast-in molten copper. These diffusion processes lead to a significantly improved adhesion of the cast metal to the pipe, combined with a virtually lossless heat transfer. Since a thin alloy layer is formed at the interface between the electrolytic coating of the tube and the cast-copper, the bonding surface in this region is almost corrosion-resistant.

In a preferred embodiment of the cooling element is proposed that the tubes are copper tubes, and the coating is a galvanic nickel coating. According to the method, this is achieved by coating the outside of the pipe in a galvanic nickel bath, the thickness of the layer thus formed being between 3 and 12 .mu.m, preferably between 6 and 10 .mu.m.

Nickel is characterized by a relatively good thermal conductivity, in addition, nickel has a density comparable to copper and a very similar atomic diameter. The melting point of nickel at 1453 ° C is significantly higher than the melting point of copper at 1083 ° C, which prevents or delays melting of the electrolytic nickel layer during filling of the liquid copper. In experiments, it has been found that the high melting point of the nickel protects the nickel plating layer of the tube from attack by the melt, such as an additional tube. At the same time, the high heat energy causes diffusion processes to take place between the galvanic nickel layer and the copper encapsulation, which lead to a significantly better adhesion of the encapsulation to the copper tube. The formation of a thin alloy layer at the interface between the tube and the encapsulant, the connection surface is corrosion resistant, here in particular affects the complete solubility of the copper for nickel and the approximately equal atomic diameter positive. After completion of the casting and the solidification of the copper, the nickel of the galvanic nickel layer in this region is barely detectable. Here, too, the long cooling time after the solidification of the copper up to the end of the diffusion processes at about 400 ° C, which at least makes up depending on the size of the cast cooling element 4 to 8 hours.

With regard to the thickness of the nickel layer electroplated on the outside of the pipe, the optimum seems to exist between 6 and 10 μm.

In a further embodiment of the method, it is proposed that the copper pipes are coated only after the production of the desired pipe shape. Thus, first of all, the manufacture of the copper tube including all desired bends, branches and similar flow structures takes place. Only then will the Copper pipes electrolytically nickel-plated on the outer side of the pipe in a galvanic bath. If, in contrast, the copper pipe is already nickel-plated before the various deformation processes are carried out, it turns out that the nickel layers change greatly due to the heating in the region of, for example, the arcs and radii of the pipe, and thus does not subsequently establish a uniform bond with the metal casting.

With a further embodiment of the method, it is proposed that the tube outer sides are mechanically blasted before the coating, preferably by blasting with a coarse glass grain. Before electroplating is a strong pickling, d. H. Staining required. Furthermore, it is advantageous if the coated tube outer sides are degreased prior to encapsulation of the tubes, preferably by cleaning with acetone.

The finished in their geometry copper tubes are first blasted with coarse glass grain, so as to achieve a rough as possible and thus large surface with the result of a good pre-cleaning and activation of the tubes. Subsequently, the electrolytic coating of the tube outside in the galvanic nickel bath then takes place. Due to the previously activated by decapitation surface good adhesion of the nickel layer is achieved. During the subsequent installation of the copper pipes in the molding box of the casting mold, attention should be paid to a non-greasy surface, whereby the cleaning of the pipes with acetone is recommended. Then, the sprue of the liquid copper takes place in the casting mold. Based on the previously cleaned surface, any oxidation of the pipe surfaces during casting could be avoided. A deterioration of the network is prevented in this way. Even a slight oxidation of the nickel surface does not appear to be detrimental to the incoming fusion and the ongoing diffusion processes.

The results of tests carried out show that rapid cooling from the liquid state as a result of very intensive cooling of the copper tubes charged with cooling water during and after the casting process is also possible. Normally, such intensive cooling adversely affects the composite quality. By contrast, when using galvanized copper pipes, good quality castings could be obtained even with strong cooling performance of the water passing through the pipes. It can therefore speak of a robust, relatively insensitive to variations in the process parameters casting process.

With a further embodiment of the cooling element according to the invention it is proposed that the tubes are not copper tubes, but copper-nickel tubes with a copper content of 30 to 70% and a nickel content of 20 to 65%, wherein the electrolytic coating is a copper coating is.
Accordingly, a suitable method for producing such a cooling element method is characterized in that the tubes used are copper-nickel tubes with a copper content of 30 to 70% and a nickel content of 20 to 65%, and that the coating of the tube outer sides done in a galvanic copper bath.

In the following Table 1, the results are summarized on the basis of a total of eleven performed samples, which also comparative samples were tested without electrolytic finishing. The test was carried out using infrared heat measurements (thermographic analysis) and subsequent shear tests: Table 1 Sample no. material layer edition X-ray results 1 Monel 400 (NiCu 63/31) coppers 9 μm Composite good; worse in the bow 2 Monel 400 nickel 9 μm Composite very bad (NiCu 63/31) 3 copper nickel 3 μm Gas bubbles; Composite good 4 copper nickel 6 μm No bubbles; Composite very good 5 copper nickel 9 μm Very light bubbles; Composite very good 6 Monel 400 Without finishing - Total gassed (NiCu 63/31) 7 Monel 400 (NiCu 63/31) Without finishing - Pretty gassed 8th copper Without finishing - Composite very good 9 copper Without finishing - Composite very good; slight bubbles in a partial area 10 CuNi Without finishing - Composite rather bad (10Fe1Mn) 11 CuNi Without finishing - Composite rather bad (10Fe1Mn)

Therefore, the best results were shown in Samples Nos. 4 and 5, each of which used a copper tube with galvanic nickel plating, the layer thickness being 6 μm for Sample No. 4 and 9 μm for Sample No. 5. A good composite also shows the sample No. 3 with a reduced nickel layer of 3 microns. However, the tests carried out using the parallel method using a tube "Monel 400" also show a good bond between pipe and casting compound, only in the area of the pipe bend showed the shear tests performed worse results.

Table 2 below shows the test results of the thermographic examination by thermal image evaluation: Table 2 Test results of the thermographic examination (thermal image evaluation) Cooling through 1.8 m ³ / h water flow rate and 6 bar pressure from about 175-180 ° Celsius TEMPERATURES IN ° CELSIUS Sample No. After 10 sec After 30 sec After 60 sec After 120 sec After 200 sec 1 168.8 159.9 143.5 116.2 89.4 2 173.2 167.4 157.7 131.8 100.8 3 165.7 145.1 124.4 92.0 64.7 4 165.3 144.4 122.2 88.9 62.8 5 163.9 143.2 119.1 86.7 59.7 6 176.4 172.6 167.2 155.0 123.7 7 174.1 169.7 163.7 152.6 135.5 8th 166.6 158.2 133.4 103.2 71.8 9 168.0 157.5 141.2 110.2 79.7 10 177.2 171.1 172.3 165.9 144.4 11 179.0 176.8 172.6 159.3 125.6

Finally, the following Table 3 gives the results of the shear tests carried out, specifying the shear strength τ in N / mm ² for the four material combinations copper without nickel plating, copper with nickel plating, Monel 400 without copper layer and Monel 400 with electrolytic copper layer. The particularly good results when using a nickel-plated copper tube and a copper-plated tube made of Monel 400 are obvious: Table 3 Results of the shear test in N / mm ² Example results: copper without Ni layer 4.5 copper with Ni layer 20.7 4-5 times by optimal nickel coating Monel 400 without Cu layer 4.8 Monel 400: with Cu layer 27.4 5-6 times by optimal copper coating

The summarized in Tables 1, 2 and 3 sample and Scherergebnissen is the in Fig. 1 underlying sample body. The tube takes a U-shaped course through the casting, with a projecting from the casting inlet and a drain. In the experiments were used in each case tubes with an outer diameter of 33 mm, and an inner diameter of 21 mm, the dimensions of the cast block were 360 mm / 200 min / 80 mm. The pipe dimensions indicate that the wall thickness of the pipes used in the casting trials was 6 mm in each case.

The thus prepared specimens were heated in an annealing furnace, during the subsequent cooling with a defined amount of water and a defined pressure thermographic images were taken with the help of an infrared camera.

Claims (8)

1. Cooling element, in particular for use in walls of furnaces subjected to high thermal stress, composed of cast copper or a low-alloy copper alloy with coolant ducts arranged in its interior comprising pipes cast in the copper or the copper alloy, characterised in that the pipes of the coolant ducts are copper pipes, which have an electroplated coating on the outside.
2. Cooling element according to claim 1, characterised in that the thickness of the coating amounts to between 3 and 12 µm, preferably between 6 and 10 µm.
3. Process for the production of a cooling element provided with coolant ducts formed from pipes in its interior, in particular for use in walls of furnaces subjected to high thermal stress, with the steps

   a) producing the pipe including all the required bends, branches and similar flow structures,

   b) casting molten copper or a low-alloy copper alloy around the pipes within a die, preferably with simultaneous cooling of the pipe inside walls,

   c) cooling the copper melt,

   characterised in that copper pipes are used and during the production of the pipes at least those regions of the pipe outer surfaces, around which the copper or the copper alloy are later cast, are electrolytically coated.
4. Process according to claim 3, characterised in that the pipes are only coated after the required pipe structure has been produced.
5. Process according to claim 3 or 4, characterised in that the pipe outer surfaces are blasted mechanically before coating, preferably by abrasion blasting with coarse glass grains.
6. Process according to one of claims 3 to 5, characterised in that the coated pipe outer surfaces are degreased before casting around the pipes, preferably by cleaning with acetone.
7. Process according to one of claims 3 to 6, characterised in that the pipes used are copper pipes, and that the coating of the pipe outer surfaces takes place in a nickel plating bath.
8. Process according to one of claims 3 to 7, characterised in that the thickness of the electroplated coating amounts to between 3 and 12 µm, preferably between 6 and 10 µm.

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