GB1587588A

GB1587588A - Tubular electrical heaters for dishwashing machines or other household appliances

Info

Publication number: GB1587588A
Application number: GB21892/78A
Authority: GB
Original assignee: ELPAG AG CHUR
Current assignee: ELPAG AG CHUR
Priority date: 1977-05-25
Filing date: 1978-05-24
Publication date: 1981-04-08
Also published as: SE7805889L; SE437108B; CH634960A5; FR2392162B1; IT1095522B; DE2723640A1; FR2392162A1; DE2723640B2; IT7823752A0

Abstract

The tubular heating body, which is provided especially for dishwashers, has an outer tube (5) consisting of chromium-nickel steel which is provided with a nickel-oxide coating (3). The nickel-oxide coating (3) is formed from a nickel coating, which is deposited onto the outer tube (5), by the tubular heating body being annealed in an oxidising atmosphere, preferably at 900 to 1200 DEG C. In this case, a transition layer (4) is formed by nickel diffusing into the chromium-nickel steel of the outer tube (5). The nickel-oxide coating (3) has either a dark green colour or a grey to browny black colour, depending on the thickness of the deposited nickel coating after annealing. Such a tubular heating body, which is operated at least at times in air, has a uniform colour of the nickel-oxide coating and is extremely resistant to stress-corrosion cracking. <IMAGE>

Description

(54) TUBULAR ELECTRICAL HEATERS FOR DISHWASHING MACHINES OR OTHER HOUSEHOLD APPLIANCES (71) We, ELPAG AG CHUR, a body corporate organised and existing under the laws of the confederation of Switzerland, of Quaderstrasse 11, CH-7001 Chur, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to tubular electrical heaters for dishwashing machines or other household appliances in which the tubular heater is operated at least at times in air.

The term 'tubular electrical heater' as used herein means a heater comprising an electrical heating element embeddd in a core of refractory material and surrounded by an outer tubular metallic sheath.

Tubular electrical heaters are known in which the sheath comprises chrome-nickel steel and which accordingly have a brightly shining, metallically smooth surface. The tubular electrical heaters have a tendency to stress crack corrosion in domestic machines such as washing machines, dishwashing machines and the like in which the tubular electrical heater is operated in aggressive water, in particular in a water containing common salt. The chrome-nickel steel jacket therefore protects the heater from the formation of rust and other corrosion damage, but not from stress crack corrosion. This term is used to indicate that a crack can occur at places at which internal stresses occur in the tubular sheath in particular if the surface of the sheath is damaged for example by a sewing needle which finds its way into the machine.

It is now known from our UK Patent Specification No. 1191 732 that the problem of stress crack corrosion can be solved if a nickel layer which is from 5 to 20 ,e in thickness is applied to the surface of the sheath. The nickel coating is partially diffused into the chrome-nickel steel sheath in an annealing process at a temperature of from 900 to 1200"C, in a protective gas.

The tubular electrical heater then therefor has a metallically smooth surface.

Now there are domestic appliances in which the tubular electrical heater is continuously operated in water, for example water heaters or washing machines. In such appliances, the nickel coating remains metallically smooth and does not oxidise, as the surface of the tubular heater cannot reach temperatures at which an oxidation process would occur. In other domestic appliances, however, the tubular electrical heater is operated in air or, as in the case of dishwashing machines, in water during the washing operation, but in air after the washing operation, for the purposes of drying the crockery. In such appliances, the tubular electrical heater reaches surface temperatures of from 400 to 600"C. The nickel surface is therefore oxidised, and the degree of oxidation increases as the temperature rises.

However, the surface temperature of the tubular electrical heater varies over its length, due to the fact that the heating element (coil) has, in each unit of length. a different heating output, which cannot be avoided in the production process, and due to different heat radiation of the surface of the jacket after the tubular heater has been fitted in the appliance. Thus, for example, the temperature at the points of curvature of the tubular electrical heater is generally lowest. These temperature differences also result in different oxidation forms of the nickel coating, as can be observed optically in the form of different oxidation tints.

The uncontrolled oxidation of the nickel coating of the tubular electrical heater, which occurs even when testing the domestic appliance, for example a dishwashing machine, in the manufacturing works, results in the tubular electrical heater having a spotted appearance. As the tubular heater is generally visible in dishwashing machines, the unattractive surface of the tubular heater, which makes it look as if the appliance has been previously used, makes it difficult to sell a factory-fresh appliance.

According to the invention there is provided a tubular electrical heater (as defined herein) for dishwashing machines or other domestic appliances in which the tubular heater is operated at least at times in air, wherein the sheath comprises a chromenickel steel with a nickel content of from 8 to 12% and a chromium content of from 8 to 25% and the sheath is provided with a nickel coating which is partially diffused into the chrome-nickel steel, the nickel coating being formed as a uniform green oxide coating which is more than 20 y in thickness.

The invention also provides a method of producing a tubular electrical heater as set forth in the preceding paragraph, wherein a nickel coating is applied to the sheath of the tubular electrical heater and the sheath is annealed in air or in another oxidising atmosphere until a green nickel oxide coating which is uniform and homogenous over the entire length of the sheath is produced, the unoxidised nickel base of the nickel oxide coating being partially diffused into the sheath.

The green oxide layer, which makes the heater of an attractive appearance compared to the above-mentioned prior art heater having a spotted appearance, is produced if the nickel layer is applied in a thickness of more than 20y, preferably from 20t1 to 30!1, and if, in contrast to the known methods, an annealing process is not performed under a protective gas, but in an oxidising medium, for example air, preferably at a temperature of from 900 to 12000C.

It has been found that tubular electrical heaters embodying the invention are extremely resistant to stress crack corrosion.

For the purposes of testing for stress crack corroion, tubular electrical heaters which were bent into a U-shape were stressed and tested at 145"C in 42% MgCI ,. The development of crack formation was judged according to the recognisability of the cracks under different magnification. In the case of a tubular heater with a green oxide layer, the first cracks were observed after 122 hours, under magnification of a factor of 6. For the purposes of comparison, a tubular electrical heater with a sheath tube which was not nickel plated, comprising chrome-nickel molybdenum steel was tested; under a magnification factor of 6, this tubular heater exhibited the first stress cracks after 4 hours. Rupture occured after only 8 hours.

For the purposes of testing, the formation of rust, tubular electrical heaters embodying the invention were tested in seawater by means of an alternate immersion test. Virtually no rust attack could be found after 4 weeks. Even an increase in the concentration of the testing agent gave no rusting within a further 2 weeks.

The tests therefore show that, in heaters embodying the invention, it is possible to manage with relatively lowly alloyed chrome-nickel alloys of the type 18/8, that is to say, 18% chromium and 8 nickel, without suffering from disadvantages in comparison with more highly alloyed and substantially more expensive steels, for example 25/20 chrome-nickel steels.

The principle upon which the present invention is based can be utilised for the purposes of controlling the thickness of the nickel layer, without time-consuming and expensive experiments being necessary for this purpose. As long as a uniformly dark green nickel oxidation layer is produced in series production, it is certain that the thickness of the nickel layer is more than 20,u.

Below a thickness of 20,to, there is an abrupt change to a grey to brownish-black oxidation layer. Thus, any thin spots in the coating, which could lead to subsequent stress crack corrosion, will be immediately evident to the eye. It was not possible to find a physical explanation for the effect which occurs. Examination of the X-ray microstructure of the oxidised surfaces resulted, in both oxidised samples, in only nickel oxide, beside the substratum of nickel. According to the literature, both a green and also a black nickel oxide exists both at higher and also at lower temperatures.

The invention will now be further described, by way of illustrative and nonlimiting example, with reference to the accompanying drawings, in which: Figure 1 shows a tubular electrical heater embodying the invention; Figure 2 is a view on an enlarged scale of the tubular electrical heater of Figure 1, in cross-section taken along line I-I in Figure 1; Figure 3 shows a polished section through the outermost layer of the sheath of the tubular electrical heater of Figures 1 and 2; and Figure 4 is a view corresponding to Figure 3 of the outermost layer of the sheath of a tubular electrical heater not in accordance with the invention.

The tubular electrical heater shown in Figure 1, which is bent into a W-configuration, includes an outer tubular metallic sheath 1 comprising a chrome-nickel steel, which in the preferred embodiment is a steel of type 18/8, that is to say containing 18% of chromium and 8% of nickel. Provided at the ends of the heater are connection pins 2 which, as is also described hereinafter, can be in the form of unalloyed steel pins, possibly nickel-plated. A nickel coating which is more than 20u in thickness, and preferably from 20 to 30,u in thickness. is applied to the sheath 1. The finished tubular electrical heater is annealed at a temperature of from 900 to 1200"C in air or in another oxidising atmosphere, and a dark-green, uniform oxide coating of good appearance is produced on the surface.In the course of the annealing process, a part of the nickel coating diffuses into the sheath 1.

Alternatively, the sheath 1, before it is filled, may also be provided with the specified nickel coating and annealed under a protective gas; when this is done, a metallically smooth surface is produced. After the tube has been filled and reduced, a second annealing operation is performed in an oxidising atmosphere, as described above.

Referring to Figure 2, the outermost layer of the sheath 1 comprises a uniform nickel oxide layer 3 and a transition layer 4 which merges into the starting alloy 5, generally a chrome-nickel alloy of type 18/8.

In the polished section of the final product after annealing, as shown in Figure 3, the starting material used was a tubular electrical heater to which a nickel coating of about 30ay in thickness was applied. The polished section shown in Figure 3 shows that the nickel coating is about 20,u in thickness, and the transition layer 4 is about 5 to 6,u in thickness. The nickel oxide coating is of a uniform, pleasant, dark-green colour.

In the arrangement shown in Figure 4, the nickel coating applied before the annealing operation was a little under 15, in thickness. The oxide layer 3 is thinner than in the embodiment of the invention shown in Figure 3, namely, it is about 15zt in thickness. (The fact that the oxide layer 3 is slightly thicker than the original nickel layer is due to the absorption of oxygen during oxidation and to chromium from the sheath merging with the nickel layer during annealing). The oxide layer is grey to black in colour, and, according to the lighting, also brownish-black. Under the dark covering, the nickel layer below the oxide layer exhibits oxidation which is present in an intercrystalline form and which extends to the limit of the diffusion region which is clearly visible in both ground sections.

It should also be mentioned that, in the product embodying the invention shown in Figure 3, the annealing process was performed at a temperature of 1080"C and extended over a period of 5 minutes.

The problem of stress crack corrosion and possibly also rusting is very acute in dishwashing machines, as it is not possible to prevent the washing water from containing common salt. Normal chrome-nickel steel which is rust-proof per se, forms, without any nickel plating, an oxide layer which rusts. In addition, such a construction is very sensitive to stress crack corrosion.

The diffusion process extends in both directions, that is to say, on the one hand the nickel coating diffuses into the surface of the chrome-nickel and on the other hand components of the chrome-nickel alloys diffuse into the nickel coating.

In products embodying the invention, resistance to stress crack corrosion is excellent, assuming that the nickel coating is present. If the nickel coating is not present, for example due to an error in production, or if it is excessively thin, for example only from 4 to 5y in thickness, then the oxide rusts, but this is only established after a long period of operation. However, the colour change noted in the oxide coating from green, to dark-grey or to greyish-brown, provides a simple manner of checking the thickness of the nickel coating in the production process, merely by visually inspecting it. When the uniform green oxide coating is achieved, it is certain that the nickel coating applied is of a thickness which substantially excludes stress crack corrosion.

Oxidising annealing processes on finished tubular electrical heaters were hitherto avoided as connection pins of unalloyed steel (connected to the heating element of the heater) suffer much severe scaling that they are no longer suitable for use as connection pins. If, instead of unalloyed steel pins, rust-proof nickel steel is used, the oxide layer must also be removed from such pins, for the purposes of subsequently making a satisfactory spot welded joint.

In order to solve this problem it is preferred for the sheath 1 and the connection pins 2 to be made somewhat longer. Normal unalloyed steel pins which can possibly be nickel-plated are then used. The part of the connection pin which lies in a core of refractory material in the form of highlycompressed magnesium oxide is protected from oxidation during the annealing process. After the annealing operation, the excess lengths of the sheath 1 and the scaled end of the connection pins are then cut off, so that the unscaled connection pins 2 are exposed.

In itself it is more economical for the empty and still unbent sheath of the tubular electrical heater to be nickel plated. However, an annealing process must subsequently be performed, under a protective gas. Without this annealing process, when the filled and bent tubular electrical heater is compacted, also referred to as reduced, the brittle nickel layer would peel off the base material. The finished tubular electrical heater is then annealed a second time in an oxidising atmosphere, for the purposes of producing the oxide layer. An oxidising annealing operation before the tubular electrical heater is filled is not advantageous however, because when the tubular heater is filled and reduced, the oxide layer may be scratched, thus resulting in an unpleasant appearance on the final product.

Claims

WHAT WE CLAIM IS:

1. A tubular electrical heater (as defined herein) for dishwashing machines or other domestic appliances in which the tubular heater is operated at least at times in air, wherein the sheath comprises a chromenickel steel with a nickel content of from 8 to 120:, and a chromium content of from 8 to 25% and the sheath is provided with a nickel coating which is partially diffused into the chrome-nickel steel, the nickel coating being formed as a uniform green oxide coating which is more than 201 in thickness.

2. A tubular electrical heater according to claim 1, wherein the nickel coating upon application is from 20 to 301 in thickness.

and the nickel oxide coating is dark green.

3. A tubular electrical heater according to claim 1 or claim 2, wherein the sheath comprises chrome-nickel steel containing 18% chromium and 8% nickel.

4. A tubular electrical heater substantially as herein described with reference to Figures 1, 2 and 3 of the accompanying drawings.

5. A method of producing a tubular electrical heater according to any one of claims I to 4, wherein a nickel coating is applied to the sheath of the tubular electrical heater and the sheath is annealed in air or in another oxidising atmosphere until a green nickel oxide coating which is uniform and homogenous over the entire length of the sheath is produced, and unoxidised nickel base of the nickel oxide coating being partially diffused into the sheath.

6. A method according to claim 5, wherein the sheath is annealed at a temperature of from 900 to 12000C.

7. A method according to claim 5 or claim 6, wherein, during the annealing process, connection pins are disposed in the sheath partially embedded into the refractory core, and, after the annealing operation, the ends of the sheath are cut so that the connection pins are exposed.

8. A method according to claim 5, claim 6 or claim 7, wherein the sheath, prior to its being filled, is provided with a nickel coating and annealed in a protective gas, providing a metallically smooth surface, and subsequently, after the tube has been filled and reduced, the said annealing operation is effected in air or in another oxidising atmos phere, for the purpose of producing the uniform green oxide coating.

9. A method according to claim 5, substantially as herein described.