DE7716584U1 - TUBULAR RADIATOR FOR DISHWASHER - Google Patents

Description

A 8302

ELPAG AG CHUR
Quaderstraße 11, CH-7001 Chur / Switzerland

Tubular heating elements for dishwashers

The innovation concerns a tubular heating element for dishwashers or other household appliances in which the tubular heater is operated at least temporarily in air and which has a jacket tube that is provided with a nickel coating, which is partially diffused into the chrome-nickel steel.

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Tubular heating elements are known which are made of chrome-nickel steel and, accordingly, have a bright, shiny, metallic surface own. In household machines such as washing machines, dishwashers or the like. In which the tubular heater is in an aggressive Water, in particular in a water containing table salt, is operated, the tubular heating elements tend to stress corrosion cracking. The chrome-nickel steel jacket thus protects the tubular heating element against rust formation and other corrosion damage, but not against stress corrosion cracking. Below it is to be understood that a crack can occur, in particular, at places where there are internal stresses in the tubular heater jacket if the surface of the jacket pipe is damaged, for example by a sewing needle that was washed along with it.

It is now known that the problem of stress corrosion cracking has been solved can be achieved if a nickel layer with a thickness of 5 - 20 μ is applied to the surface of the jacket pipe. The nickel layer is partially diffused into the chrome-nickel steel jacket in an annealing process at 900 - 1200 under protective gas. The tubular heater then has a metallic bright surface.

There are now household appliances in which the tubular heater is constantly in Water is operated, e.g. B. water heaters or washing machines. In these devices, the nickel layer remains metallically bright and oxidized not, as the surface of the tubular heating element does not assume any temperatures where an oxidation process would occur. In other household appliances, however, the tubular heating element is operated in air or - as in dishwashers - in water during the washing process, but after the washing process to dry the dishes in air. With these devices, the tubular heating element reaches surface temperatures from 400 - 600 ° C. The nickel surface is therefore oxidized, and the more so, the higher the temperature.

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However, the surface temperature of the tubular heater is above its Length differently, due to a production-wise unavoidable different heating power of the heating coil per unit of length and due to the different heat radiation of the jacket surface after the tubular heating element has been installed in the device, For example, the temperature at the points of curvature of the tubular heater is generally the lowest. Because of these temperature differences there are also different forms of oxidation of the nickel layer, visually in the form of different tarnishing colors can be perceived.

The innovation is now based on the task of such an uncontrolled Avoid oxidation of the nickel layer. A tubular heater is to be proposed that is extremely resistant against stress corrosion cracking.

The already in the test run of the household appliance, z. B. the dishwasher, Uncontrolled oxidation of the nickel layer of the tubular heating element occurring in the manufacturer's works leads to a blotchy appearance. Since the tubular heating element is generally visible in dishwashers, the sale of a brand-new device is unsightly Surface of the tubular heater, which suggests prior use, made more difficult.

The innovation is based on the knowledge that a nickel layer A certain thickness can form an oxide layer on a chromium steel tube, which is homogeneous over the entire length.

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The tubular heater according to the innovation is accordingly characterized in that the nickel layer is a uniformly green-gray oxide layer is trained.

The green oxide layer is achieved when the nickel layer is in a Thickness of more than 20μ, preferably 20-30μ, is applied and if, in contrast to the known methods, an annealing process is not carried out under protective gas, but under an oxidizing medium, e.g. air, at 900 - 1200 °.

It has now been found that the tubular heater according to the innovation are extremely resistant to stress corrosion cracking. To test for stress corrosion cracking, tubular heating elements bent in a U shape were clamped and tested in 42% MgCU at 145. The course of the crack formation became different after the cracks were recognizable Judged magnification. In the case of the tubular heater with a green oxide layer, the first were magnified after 122 hours To recognize cracks. For comparison, a tubular heater with a non-nickel-plated jacket tube made of chromium-nickel molybdenum steel was used checked; this showed the first stress cracks under 6x magnification after four hours. The break occurred after eight hours.

To test the rust formation, tubular heating elements according to the innovation tested by means of an alternating immersion test in seawater. Practically no rust was found after four weeks. Even an increase in the concentration of the test agent did not result in any rusting within a further two weeks.

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The tests show that, according to the innovation, relatively low-alloy chromium-nickel alloys of the type 18/8, ie. H. 18% chromium and 8% nickel can get by without disadvantages compared to higher alloyed and much more expensive steels, e.g. B. Chrome-nickel steel 25/20.

The idea on which the innovation is based can now be used for this purpose be able to control the thickness of the nickel layer without the need for time-consuming and costly investigations. As long as the evenly dark green nickel oxidation layer is in series production is achieved, it is ensured that the thickness of the nickel layer is over 20 μ. Below 20 μ occurs suddenly Envelope in the gray to brown-black oxidation layer. A physical explanation for the occurring effect could not be found will. An examination of the X-ray fine structure of the oxidized surfaces showed, in addition to the background of nickel, in both oxidized surfaces Samples nickel oxide only. According to the literature, there is both a green and a lower temperature at both higher and lower temperatures a black nickel oxide.

The accompanying drawing serves to explain the subject further the innovation. Show it:

Fig. 1 is a view of a tubular heater;

Fig. 2 is a section on an enlarged scale through the pipe

heater of Figure 1 in the direction of arrows I-I;

3 and 4 micrographs through the outermost layer of the tubular heating element jacket.

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The tubular heating element shown in FIG. 1c, which is bent in a W-shape, has a jacket tube 1 made of a chromium-nickel steel, specifically in the preferred embodiment made of a steel of the 18/8 type, ie with 18% chromium and 8 % nickel. Connection bolts 2 are provided at the ends, which, as will be explained later, are designed as unalloyed steel bolts, optionally nickel-plated. A nickel layer with a thickness of more than 20 μ, preferably 20-30 μ, is applied to the tubular heater jacket 1. The finished tubular heating element is annealed at 900 - 1200 in air or in an oxidizing atmosphere, resulting in a dark green, even, good-looking oxide layer on the surface. During this annealing process, part of the nickel layer diffuses into the tubular heating element jacket.

Alternatively, the not yet filled jacket tube 1 can also be used with the specified Nickel layer and annealed under protective gas; this results in a shiny metallic surface. After filling and reducing the tube, a second annealing is carried out in an oxidizing atmosphere, as described.

Referring to Fig. 2, there is thus the outermost layer of the tubular heater jacket from a uniform nickel oxide layer 3 and from a transition layer 4, the alloy in the starting 5, so in general a chrome-nickel alloy of type 18/8 passes over.

In the micrograph of the end product after annealing shown in FIG. 3, a tubular heating element was used as the starting material, on which a nickel layer was applied in a thickness of about 30 μ .

In the embodiment shown in FIG. 4, the nickel layer applied before the annealing had a thickness of slightly less than 15 μm.

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On the micrograph according to FIG. 3 it can now be seen that the nickel layer a thickness of about 20 μ and the transition layer 4 has a thickness of about 5 - 6 μ. The nickel oxide layer has a uniform, beautiful dark green color.

In the embodiment shown in FIG. 4, the oxide layer 3 has a smaller thickness compared to the embodiment shown in Fig. 3, namely a thickness of about 15 μ. The color of the oxide layer is gray to black, depending on the lighting, also brown-black. The nickel layer underneath the oxide layer shows an intergranular oxidation under the dark layer, which up to The limit of the diffusion zone is enough, which is clearly recognizable in both sections is.

It should also be mentioned that in the sample shown in FIG Annealing process

extended.

Annealing process was carried out at 1080 C and lasted for 5 minutes

The problem of stress corrosion cracking and possibly also the Rusting is particularly acute in dishwashers because it cannot be prevented that the washing water contains table salt. Normal, Chromium-nickel steel, which is actually rustproof, forms an oxide layer that rusts without nickel-plating. In addition, such a design is against Stress corrosion cracking very sensitive.

The diffusion process goes in both directions; H. on the one hand the nickel layer diffuses into the chromium-nickel layer jacket - on the other hand constituents of the chromium-nickel alloys diffuse into the nickel layer.

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The resistance to stress corrosion cracking is common to both embodiments excellent provided a nickel coat exists at all. If this is not available, e.g. B. by a Error in production, or is uie too weak, e.g. B, only 4 -5 μ strong, then the oxide rusts, but only after a long period of use is to be determined. The observed color change of the oxide layer from green to dark gray or gray-brown is now ready for production a simple control of the thickness of the nickel layer. Accordingly, the green oxide layer is preferred because it achieves the same it is also ensured that the nickel layer has a thickness when applied that largely excludes stress corrosion cracking. If the The thickness of the nickel layer is checked in a different way or if you are certain of other reasons during production, that the nickel layer will always have a certain thickness, then in order to save nickel it is also possible to use thinner nickel layers to be worked.

Oxidizing annealing processes in completed tubular heaters were Avoided so far, since connection bolts made of unalloyed steel scale so much that they are no longer suitable as connection bolts. Used instead of unalloyed steel bolts, those made of stainless nickel steel must also be used for the purpose of subsequent production a perfect spot welded joint to be freed from the oxide layer.

In order to solve this problem, it is preferred that the jacket tube 1 and the connecting bolts 2 are made somewhat longer. It will then normal unalloyed steel bolts are used, which can be nickel-plated if necessary. The one lying in high-density magnesium oxide Part of the connecting bolt is protected from oxidation during the annealing process. Then after annealing the excess lengths of the jacket pipe 1 and the scaled end of the connecting bolt

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cut off so that the non-scaled connecting bolts 2 are exposed.

In summary, it can be stated that in the interaction of the diffusion process with the formation of the nickel oxide layer, two different processes can be determined Change the base alloy 18% chromium and 8% nickel almost exactly with 20 μ nickel application, in shapes according to the micrographs according to FIG. 3 and FIG. 4. However, both forms of the surface are extremely stable against stress corrosion cracking. Which shape preferred depends on the one hand on aesthetic aspects - dark gray / black or green surface - and on the other hand of the intended area of application of the tubular heating element.

In itself it is more economical to use the empty and still unbent casing pipe of the tubular heater to be nickel-plated. However, an annealing process under protective gas must then be carried out. Without this The annealing process would reduce the brittle nickel layer when compressing - also known as reducing - the filled and bent tubular heating element The base material flakes off. The completed tubular heater is then annealed a second time in an oxidizing atmosphere to achieve the oxide layer. An oxidizing glow before filling the tubular heater however, it is not advantageous because the oxide layer can be scratched when it is filled and reduced, giving it an unattractive appearance of the end product.

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Claims (4)

'. j I ft I I "I" ill ·> I M I <claims for protection 1. Tubular heating element for dishwashers or other household appliances, in which the tubular heating element is operated at least temporarily in air, and which has a jacket tube made of chromium-nickel steel with a nickel content of 8-12 % and a chromium content of 8-25%, which is provided with a nickel coating is, which is partially diffused into the chromium-nickel steel, characterized in that the nickel layer is designed as a uniform green / gray / brown-black oxide layer. 2. tubular heating element according to claim 1, characterized in that that the nickel layer has a thickness of 20-30μ when applied, and that the nickel oxide layer is dark green. 3. tubular heater according to claim 1, characterized dadurc h, that the nickel layer has a thickness of less than 20 μ when applied, and that the nickel oxide layer is gray to brown-black, so that the Surface the oxidation layer of a non-nickel-coated chrome nickel Istahlrohrheizkörper is similar. 4. tubular heater according to claims 1 to 3, characterized in that that the jacket pipe is made of chrome-nickel steel with 18% chrome and 8% nickel. 8302 7716584 24.1177