CH634960A5 - Tubular heating body for domestic appliances, and a method for its production - Google Patents

Description

The invention relates to a tubular heater for household appliances, in particular for dishwashers, according to the preamble of claim 1 and a method for producing a tubular heater.

Tubular heaters are known which are made of chromium-nickel steel and accordingly have a bright, metallic, bright surface. In domestic machines, such as washing machines, dishwashers or the like, in which the tubular heating element is operated in an aggressive water, in particular in water containing saline, the tubular heating elements tend to stress corrosion cracking. The chrome-nickel steel jacket therefore protects the tubular heater against rust formation and other corrosion damage, but not against stress corrosion cracking. This is to be understood to mean that a crack can occur at points at which there are internal stresses in the tubular heating element jacket, in particular if the surface of the jacket tube is damaged, for example, by a sewing needle that has been washed with it.

It is now known that the problem of stress corrosion cracking can be solved if a nickel layer with a thickness of 5-20 nm is applied to the surface of the casing tube. The nickel layer is partially diffused into the chrome-nickel steel jacket in an annealing process at 10 900-1200 ° under protective gas. The tubular heater then has a shiny metallic surface.

There are now household appliances in which the tubular heater is constantly operated in water, e.g. B. water heater or 15 washing machines. In these devices, the nickel layer remains bare metal and does not oxidize, since the surface of the tubular heater cannot reach temperatures at which an oxidation process would occur. In other household appliances, however, the tubular heater is operated in air or - like 20 in dishwashers - during the washing process in water, but after the washing process to dry the dishes in air. With these devices, the tubular heater reaches surface temperatures of 400-600 ° C. The nickel surface is therefore oxidized, and the stronger the higher the temperature.

However, the surface temperature of the tubular heater is different over its length, due to a different heating cable of the heating coil per unit length 30, which cannot be avoided in terms of production, and due to different heat radiation from the jacket surface after the tubular heater has been installed in the device. For example, the temperature at the points of curvature of the tubular heater is generally the lowest. These temperature differences also result in different oxidation forms of the nickel layer, which can be perceived optically in the form of different tarnishing colors.

The invention is based on the object of avoiding such an uncontrolled oxidation of the nickel layer. 40 A tubular heating element is to be proposed which is extremely resistant to stress corrosion cracking.

The already during the test run of the household appliance, e.g. B. the dishwasher, occurring in the manufacturer's uncontrolled oxidation of the nickel layer of the tubular heater 45 leads to a spotty appearance. Since the tubular heater is generally visible in dishwashers, the sale of a brand-new appliance is made more difficult by the unsightly surface of the tubular heater, which suggests prior use.

The invention is based on the knowledge that a nickel layer of a certain thickness can form an oxide layer on a Chromnik stainless steel tube which is homogeneous over the entire length.

The object is achieved according to the invention by the features specified in the characterizing part of claim 1. The tubular heater according to claim 1 can be produced according to the invention by the method specified in the characterizing part of claim 3.

The dark green color of the nickel oxide layer can be achieved according to claim 4 by the nickel layer being more than 20 (im) when applied and, in contrast to the known methods, the annealing is carried out not under a protective gas but under an oxidizing medium. For example, air can be used as the oxidizing medium should be used and the temperature during annealing should preferably be between 900 and 1200 ° C. During annealing, a large part of the nickel layer diffuses into the jacket tube, so that the thickness of the nickel oxide layer is then of course substantially less than the thickness of the

applied nickel layer.

Regarding claim 8, it should be noted that annealing of the tubular heating elements in an oxidizing atmosphere has previously been avoided, since the connecting bolts oxidized - unless they were manufactured from rust-free material, but this incurred additional costs. However, since annealing in an oxidizing atmosphere is a prerequisite for the method according to the invention, the features of claim 8 represent an elegant solution for protecting the connecting bolts against oxidation.

In one embodiment of the inventive method according to claim 9, the still empty jacket tube with the applied nickel layer can initially be annealed under protective gas, resulting in a shiny metallic surface. Without this annealing process, the brittle nickel layer would flake off from the base material when compacting - also called reducing - the tubular heating element filled and bent with the heating coil and the insulating material. The annealing according to the invention in the oxidizing atmosphere is then a second annealing process. An oxidizing glow before filling the tubular heater would not be advantageous because the oxide layer could be scratched when filling and reducing.

It has now been found that the tubular heating elements according to the invention are extremely resistant to stress corrosion cracking. To test for stress corrosion cracking, U-shaped tubular heating elements were clamped and tested in 42% MgCh at 145 °. The course of the crack formation was assessed according to the detectability of the cracks with different magnifications. The first cracks were visible after 122 hours in the tubular heater with a green oxide layer after 6x magnification. For comparison, a tubular heater with a non-nickel-coated tubular casing made of chromium-nickel-molybdenum steel was tested; this showed the first stress cracks after four hours under 6x magnification. The break occurred after eight hours.

To test the rust formation, tubular heaters produced according to the invention were tested using an alternating immersion test in sea water. There was practically no rust formation after four weeks. Increasing the concentration of the test equipment did not show any rust within a further two weeks.

The tests therefore show that, according to the invention, relatively low-alloyed chromium-nickel alloys of type 18/8, ie. H. 18% chromium and 8% nickel can do without disadvantages compared to higher alloyed and much more expensive steels, e.g. B. chrome nickel steel 25/20, would occur.

The consideration on which the invention is based can now be used to control the thickness of the nickel layer without the need for time-consuming and costly investigations. As long as the uniformly dark green nickel oxidation layer is achieved in series production, it is guaranteed that the thickness of the nickel layer is over 20 jx. Below 20 (x there is a sudden change in the gray to brown-black oxidation layer. A physical explanation for the effect that could be found could not be found. Examination of the X-ray fine structure of the oxidized surfaces showed only nickel oxide in addition to the background of nickel in both oxidized samples Literature has both a green and a black nickel oxide at both higher and lower temperatures.

The accompanying drawing serves to further explain the subject matter of the invention. Show it:

Figure 1 is a view of a tubular heater.

Figure 2 is a section on an enlarged scale through the tubular heater of Figure 1 in the direction of arrows I-I.

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

634960

The W-shaped tubular heater shown in Fig. 1 has a casing tube 1 made of a chromium-nickel steel, in the preferred embodiment of a steel of the type 18/8, i.e. H. with 18% chrome and 8% nickel. At the ends there are connecting bolts 2 which, as will be explained later, are optionally nickel-plated as unalloyed steel bolts. A nickel layer with a thickness of more than 20 μm, preferably of 20-30 μm, is applied to the tubular heating element jacket 1. The finished tubular heater is annealed at 900-1200 ° in air or in an oxidizing atmosphere, resulting in a dark green, uniform, good-looking oxide layer on the surface. During this annealing process, part of the nickel layer diffuses into the tubular heater jacket.

With reference to FIG. 2, the outermost layer of the tubular heating element jacket consists of a uniform nickel oxide layer 3 and a transition layer 4, which merges into the starting alloy 5, that is to say generally a type 18/8 chromnik alloy.

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

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

3 that the nickel layer has a thickness of approximately 20 | i and the transition layer 4 has a thickness of approximately 5-6 | i. 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 approximately 15 [i. The color of the oxide layer is gray to black, depending on the lighting also brown-black. The nickel layer beneath the oxide layer shows an intergranular oxidation under the dark coating, which extends to the boundary of the diffusion zone, which can be clearly seen in both cuts.

It should also be mentioned that in the sample shown in FIG. 3, the annealing process was carried out at 1080 ° C. and extended over 5 minutes.

The problem of stress corrosion cracking and possibly also rusting is particularly acute in dishwashers because it cannot be prevented that the rinsing water contains common salt. Normal, stainless steel, which is rust-free, forms an oxide layer that does not rust and rust. In addition, such a design is very sensitive to stress corrosion cracking.

The diffusion process runs in both directions, i.e. H. on the one hand the nickel layer diffuses into the chromium-nickel layer jacket - on the other hand components of the chromium-nickel alloys diffuse into the nickel layer.

The resistance to stress corrosion cracking is excellent in the two embodiments, provided that a nickel layer is present at all. If this is not available, e.g. B. due to a manufacturing error, or it is too weak, e.g. B. only 4-5 | i strong, then the oxide rusts, but this can only be determined after a long period of use. The change in color of the oxide layer from green to dark gray or gray-brown now provides a simple way of checking the thickness of the nickel layer in terms of production. Accordingly, the green oxide layer is preferred, since when it is achieved it is also ensured that the nickel layer has a thickness during application which largely precludes stress corrosion cracking. If the application thickness of the nickel layer is controlled in another way or if one is safe during the manufacture for other reasons

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is that the nickel layer always has a certain thickness, so that two different processes are fixed in the nickel oxide layer, then in order to save nickel it can also be thinned with those that are worked with a basic alloy of 18% less nickel layers. Chromium and 8% nickel fairly exactly at 20 (i. Nickel application

Oxidizing annealing processes in the case of completed pipe heating changes, namely in shapes corresponding to the micrographs, have previously been avoided, since connecting bolts made from s according to FIG. 3 or FIG. 4. Both forms of the surface are so hard on unalloyed steel that they act as connecting however extremely stable against stress corrosion cracking. Which bolts are no longer suitable. If one uses instead of shape, on the one hand aesthetic unalloyed steel bolts depend on those made of stainless nickel steel, so considerations - dark gray / black or green surfaces also have to be used for the subsequent production of a surface - and on the other hand perfect spot welding connection from the oxide layer io on the intended area of application the tubular heater.

be freed. As such, it is more economical to empty and empty

In order to solve this problem, it is preferred that the material of the tubular casing of the tubular heating element be nickel-plated. There must be a longer tube and the connecting bolts 2, but then an annealing process under protective gas. Normal unalloyed steel bolts will then be carried out. Without this annealing process, the would be turned, which can be nickel-plated if necessary. The part of the connecting bolt-bent tubular heater located in high-15 compressions - also called reducing - of the filled and compressed magnesium oxide, the brittle nickel layer of the zens is protected against oxidation during the annealing process. Flake off the base material. The finished tubular heater

After the annealing, the excess lengths are then annealed a second time under an oxidizing atmosphere to the jacket tube 1 and the scaled end of the connection to achieve the oxide layer. An oxidizing glow bolt is cut off, so that the non-scaled 20 before filling the tubular heater is not advantageous.

Expose connecting bolts 2. adheres because the oxide layer wears out when filling and reducing

In summary, it can be stated that it can be scratched, so that there is an unattractive appearance of the interaction of the diffusion process with the final product.

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1 sheet of drawings

Claims (9)

634960 1. Tubular heating element for household appliances, in particular for dishwashers, in which the tubular heating element is operated at least temporarily in air, and which has a tubular casing (1) made of a chromium-nickel steel with a nickel content of 8 to 12% and a chromium content of 8 to 25%, which is provided with a nickel layer 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 heater according to claim 1, characterized in that the casing tube (1) consists of chromium-nickel steel with 18% chromium and 8% nickel. 2nd 3. A method for producing a tubular heater according to claim 1, characterized in that a nickel layer is applied to the tubular casing (1) of the tubular heater and that the tubular heater is annealed in an oxidizing atmosphere until a homogeneous and uniform nickel oxide layer (3) over its entire length. is formed, which is partially diffused into the casing tube (1) with its non-oxidized nickel base. 4. The method according to claim 3, characterized in that the nickel layer is applied with a thickness of 20 to 30 | o.m and that a dark green color of the nickel oxide layer (3) is achieved during annealing. 5. The method according to claim 3, characterized in that the nickel layer is applied with a thickness of less than 20 (xm, that a gray to brown-black color of the nickel oxide layer (3) is achieved during annealing, so that the surface of the nickel oxide layer (3) is similar to that of a non-nickel coated tubular steel radiator. 6. The method according to any one of claims 3 to 5, characterized in that air is used as the oxidizing atmosphere. 7. The method according to any one of claims 3 to 6, characterized in that the annealing takes place at a temperature between 900 and 1200 ° C. 8. The method according to any one of the claims 3 to 7, characterized in that connecting bolts (2) partially embedded in insulating material are inserted into the jacket tube (1) in front of the bulb and that after the annealing, the ends of the jacket tube (1) are trimmed in such a way that that the connecting bolts (2) are exposed. 9. The method according to any one of claims 3 to 8, characterized in that the jacket tube (1) is annealed with the applied nickel layer before filling with insulating material and heating coil under a protective gas, resulting in a bright metallic surface and that the annealing in the oxidizing Atmosphere after filling and reducing.