SE437108B - PROCEDURE FOR THICKNESS LAYOUT OF NICKEL LAYER APPLIED ON HEATING ELEMENT COATS - Google Patents

Description

10 15 20 25 30 35 7805889-8 2 of a temperature-resistant organic dye.

GB-A-523 751 discloses a method for achieving coming from a black surface on a nickel chromium iron alloy, there nickel is the main constituent, whereby in oxidizing atmospheres before the alloy is heated to a relatively high temperature.

Examples of such an alloy are 70 - 80% nickel, 11 - 15% chromium, 5 - 10% iron. In this procedure is thus applied on the mainly nickel containing the alloy nothing additional nickel layers. _ The resistance of heating elements with applied and possibly undiffused nickel layer or nickel oxide layer is dependent on the layer along the entire surface of the heating element uniform and defined thickness. While in steel details one unproblematic layer thickness testing can be performed using of magnetic fields, this fails at current austenitic steel with a nickel content between 8 - 12% and a chromium content of 8 - 25%. If by nickel plating, eg through grease traces due to manual contact, in some places of the surface of the heating element the nickel layer becomes too thin, the stress corrosion will to start right at this place. It through the application of nickel layer intended extension of the life of the heating element to about 10-fold compared to non-nickel-plated values. element is thus achieved only if it is ensured that indeed the entire surface is homogeneously nickel-plated.

You can with grinding images and control under a microscope randomly check the nickel coating. This procedure However, the inconvenience causes the heating element to is disturbed and that a place where the nickel layer is examined may be completely in order while the layer in another of the same element may be subject to errors which are not veiled. Leaving, however, a faulty series of pipe heating elements the factory can only after one or more years with management of the life of the elements, it is revealed whether the production have been affected by errors or not. in The invention has been given the object of achieving one method by means of which series in a simple manner the play of an applied to a tubular heating element and ' any nickel layer diffused into the mantle surface can be visually enchanted. 10 15 20 25 30 35 7805889-8 3 This task is solved in accordance with the patent claim characteristic part.

According to the new procedure, the nickel layer can thus thickness is assessed without being required for this purpose time consuming and costly studies.

The present invention is thus based on the insight that nickel layer of the type in question with a certain homogeneity thickness of chromium-nickel steel tubes can form an oxide layer such as is homogeneous over its entire length, ie has a uniform colored oxide layer.

As long as in series production, a similar shaped dark green nickel oxide layer, it is known that the nickel layer thickness is above 20 / u. Under 20 / u occurs suddenly a cover to gray to brownish black oxide layer. Any physical calic explanation for the behavior has so far could not be given. An examination of the oxidized surfaces X-ray fine structure shows in addition to the base of nickel at both oxidized samples nickel oxide only. According to pure exists at both higher and lower temperatures as well a green as a black nickel oxide.

It has now been established that tubular heating elements according to the invention are extremely resistant to stress corrosion sion, ie stress cracking. For control of voltage corrosion was examined U-shaped bent tubular heating elements under bias in 42% MgCl 2 solution at 145 ° C.

The course of crack formation was assessed after the observations of the cracks. at different magnifications. Regarding heating elements with green oxide layer could at six times magnification the first cracks are found after 122 hours. As a comparison examined a tubular heating element with non-nickel plated mantle tubes of chrome-nickel molybdenum steel. The same showed at six times magnification of the first voltage cracks already after four hours. After eight hours, a crime had already occurred.

To control the formation of rust, heating elements were tested according to the invention by a gear immersion test in seawater.

After four weeks, it could practically not be ascertained any rust formation. Nor increased sample media concentration gave rcst formation within another two weeks. vaosass-8 4 10 l5 20 25 30 35 g Fig. 1 The invention will now be further explained in connection to figures shown in the accompanying drawing. Thereby showing: a view of a tubular heating element Fig. 2 is a section on a larger scale through the heating element according to FIG. l in the direction of the arrows I-I Figs. 3 and 4 show grinding images through the outermost part of the heating element layer.

The W-shaped bent heating element shown in Fig. 1 . has a mantle tube 1 of chrome-nickel steel, namely according to a preferred embodiment of an 18/8 steel, i.e. with 18% chromium and 8% nickel. At the ends of the heating element there are connection bolts 2 which, as explained below, as unalloyed steel bolts, if applicable, are also nickel-plated. On heating elements the mantle l is applied a nickel layer with a thickness of more than 20 P, 20-3Qu is preferred. The completed element annealing gas at 900-1200 ° C in air or in an oxidizing atmosphere, giving a dark green, uniform, attractive appearance oxide layer on the surface. During this annealing process diffuses a part of the nickel layer into the heating element jacket.

Alternatively, the as yet unfilled jacket pipe l provided with said nickel layer and annealing gas under shielding gas.

This gives a metallic glossy surface. After filling the tube and reduction takes place a second annealing in oxidizing atmosphere in the manner described above.

As can be seen from Fig. 2, the heating element the outermost layer of the mantle of a uniform nickel oxide layer 3 and a transition layer 4, which merges into the starting alloy 5, thus generally a carbon nickel alloy of the type 18/8.

In the grinding image of the final product shown in Fig. 3 after annealing, a tubular shape was used as starting material heating element, to which an approximately 30 / p thick has been applied nickel layer.

In the embodiment shown in Fig. 4, it had before annealing, the nickel layer applied a thickness of slightly below 15 p. The grinding pattern according to Fig. 3 now shows that the nickel layer has a thickness of about 20 p and the transition layer 4 a thickness of about 5-6 p. The nickel oxide layer has an evenly beautiful dark green color. 10 15 20 25 30 35 40 5 7805889-8 In the embodiment shown in Fig. 4, the oxide layer 3 of smaller thickness compared to that shown in Fig. 3 the embodiment, namely a thickness of only l5 p. The oxide layer is gray to black, depending on lighting also brownish black. The the nickel layer lying below the oxide layer shows below it dark coating an intercrystalline oxidation, which extends to the boundary of the diffusion zone and which is visible clearly in both grinding images.

It can further be added that in the case shown in Fig. 3 The sample annealing process took place at 1080 ° C for 5 minutes.

The problem of stress corrosion and its occurrence cases of rust formation are very acute especially in dishwashers, because you can not prevent the water from containing common salt. Normally in itself stainless chrome-nickel steel without nickel plating forms an oxide layer which rusts. In addition such an embodiment is very sensitive to stress corrosion.

The diffusion process proceeds in both directions, ie. on the one hand, the nickel layer diffuses into the chromium-nickel layer mantle and on the other hand diffuses constituents from the chromium-nickel alloys into the nickel layer.

Resistance to stress corrosion is excellent at both embodiments, provided that a nickel layer at all is available. Missing the same, e.g. p.g.a. a manufacturing defect, or is it too weak, e.g. only 4-5 y, roasts the oxide, which, however, can only be ascertained after prolonged ning time. The oxide layer's stated color change from green to dark gray resp. gray-brown now allows in terms of manufacturing a simple control of the thickness of the nickel layer. The green the oxide layer is thus preferred, because it, as long as it is present, it is ensured that the key layer during application has a thickness, which largely excludes stress corrosion.

If the application thickness of the nickel layer is checked otherwise way or if for other reasons during manufacture it is safe set that the nickel layer will always have a certain thickness, for saving nickel, thinner can also be used nickel layer.

Oxidizing annealing processes on completed heating elements ment has so far been avoided, as the connecting bolts of unalloyed steel is subjected to a strong oxide formation, so that they are no longer suitable as connection bolts. Uses 10 15 20 25 30 7805889-8 ON one instead of unalloyed steel bolts such of stainless nickel steel, these must also be used to achieve a satisfactory positioning spot weld is released from the oxide layer.

To solve this problem, it is preferred that the jacket pipe 1 and the connecting bolts 2 are made low longer. It was used normal unalloyed steel bolts, which may be nickel-plated. The one in highly compressed magnesium oxide the part of the connection bolt is during the annealing process protected against oxidation. After annealing, the excess lengths are cut on the jacket pipe 1 and the oxidized end of the connection bolt, so that the non-oxidized connection bolts 2 are exposed.

In summary, it can thus be stated that in the effect of the diffusion process with the formation of the nickel oxide the layer can be ascertained two different processes, which at one base alloy of 18% chromium and 8% nickel turns over fairly exactly at 20lp nickel layer, which takes place in a shape that corresponds against the grinding images according to Fig. 3 resp. Fig. 4. Both surface shapes however, are extremely stable against stress corrosion. Which preferred form depends, on the one hand, on aesthetic considerations - dark gray / black / resp. green surface - and on the other hand of heating intended use of the product.

In itself, it is more economical to nickel-plate it empty yet unobstructed jacket pipe for the heating element. However an annealing procedure must then be performed during shielding gas. Without this annealing process, at compact no - also called reduction - of the filled and bent the tubular heating element the brittle nickel layer can crack detached from the basic material. The completed heating element then annealed a second time in an oxidizing atmosphere to provide the oxide layer. An oxidizing annealing before tube heating the filling of the element is not an advantage, however, because the oxide layer can be scratched during filling and reduction, so that the end product has a less appealing appearance.

Claims (1)

7805889-8 PATENT REQUIREMENT Method for thickness assessment of nickel layers applied to heating element jacket pipes of chromium-nickel steel with 8 - 12% nickel content and 5 ~ 28% chromium content, characterized in that the tubular heating elements are annealed in air or oxidizing atmosphere ° C to 1200 ° C, the surface of the heating element jacket tube obtaining a green, gray or brownish-black oxide layer, where the colors correspond to different thickness ranges.