SK156394A3 - Process for producing a protective coating on heat stressed metal walls - Google Patents

Abstract

A process for producing a protective coating on walls subject to attack by hot gases in a predetermined temperature range, which are made of metal and a predetermined basic material, in combustion plants, heat exchangers or similar installations, in which a powder of metallic, carbide, oxycarbide or silicide materials or mixtures thereof are applied to the metal walls using the plasma jet process. The invention proposes that: a) the surface of the wall is roughened; b) the basic material of the wall is activated; and c) immediately afterwards the powder is applied at room temperature and in atmospheric conditions by the plasma jet process; being d) the composition of the powder selected beforehand so that the stress as a function of the temperature in the unstressed state (at room temperature) found with the aid of the coefficients of heat expansion of the basic material and test-pieces for the transition region between the basic material and the applied coating produced from various powders gives tensile stresses of between 50 and 800 N/mm2 and preferably between 500 and 800 N/mm2, which is reduced to 0 or exhibits slight compression stresses in the predetermined temperature range.

Description

Technical field

BACKGROUND OF THE INVENTION The present invention relates to a process for producing a protective layer on walls of metal base material, preferably combustion devices or heat exchangers, attacked by hot gases, in particular flue gases, in which powdered metal, carbide, oxidoceramic powder is applied to the previously cleaned metal walls. or

pure substances or mixtures these substances. BACKGROUND OF THE INVENTION these protection layers are deposit for example at cooling the walls boilers on waste

heat, to steel converters. These walls are subject to particularly high loads. On one side, about 1400 ° to 1800 ° C flows hot flue gases loaded with ash and slag particles along, while on one of the other sides saturated steam pressures of about 20 to 80 bar. The walls of the steam-cooled tube have internal pressure gradients of up to 2 bar / min.

DE 23 55 532 C2 discloses a method of welding powders and alloys to a pre-heated metal substrate pretreated by sand blasting, in which the metal substrate is heated to at least 100 to 60 ° C.

650 ° C. In both the electrode welding process and the powder welding or flame spraying with subsequent melting, the base material heats up quite strongly when the protective layer is applied, resulting in an undesirable structural change, especially in the flame spraying the melting temperature varies depending on the sprayed powder used. between 980 and 1060 ° C. In addition, due to the introduction of high heat, the coated walls are stretched. Problems and additional costs can then arise in the construction of these walls due to dimensional inaccuracies. When the protective layers are later applied by these known methods, the temperature-dependent stresses cannot react in the sense of elongation, but lead to surface cracks in the built-in wall elements, especially in the weld area. In the case of welding, the protective layer has a thickness of about 8 to 10 mm and, in the case of flame spraying, 1 to 2 mm.

DE-AS 26 30 507 furthermore discloses a method for producing protective coatings on workpieces against corrosion by hot gases and / or mechanical wear, in which a coating powder is applied by means of a plasma vacuum on this vacuum spraying method to produce a vacuum in a working process. accessible from the outside and realized This is not the case with larger walls, such as the walls of a waste heat boiler

-possible; · 'stnexama in material. When you have a sizeable chamber that linen. built - in

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a species-like method in which the problems mentioned do not occur and, in particular, the elongation of the workpieces and the stresses in the crack-forming base material are avoided.

A solution to this object according to the invention is disclosed in the characterizing part of claim 1. Claims 2 to 8 comprise meaningful additional steps of the method.

In the method according to the invention, not only the surface of the walls is roughened under the plasma spraying method under atmospheric conditions before application of the powder, but also the wall base material is activated by high-purity noble corundum so as to create defects in the metal grid, thereby increasing adhesion forces. Immediately thereafter, rather than eliminating the lattice defects, atmospheric plasma spraying conditions occur again, then a wall powder is then applied in a manner, the surface of which retains the room.

approximately average temperature

The powder composition is determined according to the base material present and the later operating conditions, in particular the predetermined temperature ranges. According to the invention, tensile stresses of 50 to 800 N / mm ² are to be applied to the transition region between the base material and the applied layer in a no-load state, i.e. at room temperature, preferably between 500 and 800 N / mm ² , this is substantially reduced to 0 within a predetermined temperature range or there are slight pressure stresses within this range. These voltage / level states. The attached Fig. 1 is ensured by calculating both the thermal expansion coefficients of the base material and the samples of workpieces made of different powders. The calculation can be checked according to DIN 50121.

By means of the method according to the invention, for example, a protective shock-insensitive and easily repairable layer against corrosion by hot gases and / or mechanical wear can be produced on straight or arched walls of combustion plants, heat exchangers, in particular on steel convectors of waste heat boilers.

It has been found that a final layer thickness of 0.1 to 0.5 mm, preferably 0.15 to 0.25 mm, is sufficient to prevent wear which would be worthwhile even after a significantly longer period than hitherto possible. mention. A 80 kW plasma spraying device with an internal powder supply has proven particularly suitable for the application of such a protective layer. In this case, a powder having a grain size of less than 75 [mu] m, preferably 20 to 40 [mu] m, is used. With this powder it is possible to apply in particular a very thin layer which fulfills the condition of insensitivity to heat shock and corrosion resistance by hot gases. and avoids a large self-stress caused by the laminar layer construction of the process. The total layer is preferably produced at at least two passes.

Prior to plasma spraying, the treated wall surface is roughened and activated with a noble corundum, preferably a high clarity, white noble corundum.

Furthermore, it has been found to be advantageous in the method according to the invention to heat the surface by a plasma beam and in this molten particle only to about 40 ° C, to a maximum of 60 ° C. This avoids, in particular, the elongation of the walls.

Preferably, a powder containing nickel alloy is used.

It has been shown that the plasma coating should take place no later than 45 minutes, preferably no later than 30 minutes after the wall surface has been activated.

Finally, the temperature at which the walls treated with the protective layer are subjected to stress can be in the range between 300 and 1800 ° C, preferably between 500 and 1000 ° C.

Overview of the figures in the drawing

FIG. 1 is a stress-temperature diagram, for example, the stress behavior in the transition region of the base. the material and the applied protective layer in the temperature range between

0 to 1800 ° C. The basis for this are the measured, linear linear coefficients of thermal expansion of the two material partners; Fig. S shows the course of the thickness of the NiCrBSi alloy layer in the boiler waste heat, when testing more than 13300 batches.

DETAILED DESCRIPTION OF THE INVENTION

In the state in which the coated wall surfaces of the waste heat boiler converter are not stressed, tensile stresses corresponding to values above 600 N / mm ² are in the transition region between the base material and the coating material.

In the working state of the coated wall surface of the waste heat boiler converter, the sprayed layer is suddenly attacked by the high temperatures of the molten steel spraying high out of the converter and the hot slag. The diagram illustrates the course of stress, at which the neutral stress region will run at about 700 ° C, and above 700 ° C, pressure stresses will be created in the transition region to prevent peeling or cracks in the layer. By means of the wall tubes of the waste heat boiler, usually cooled with water, the stress condition in the compressive state is slowly re-established as a result of the stress, i.e. the stress-line indicated in the diagram will run in the opposite direction. Only an example of a temperature-dependent voltage curve is shown in FIG.

....... For other stress areas, the so-called 0-state can naturally take place at? 00 ° C even at 400 ° C or at 800 ° C.

On a normally stressed 1000 ° C metal pipe of a waste heat boiler, a highly wear-resistant protective layer according to the invention was applied by plasma spraying by

(a) the wall surface is first roughened and activated by high-purity noble corundum; and

(b) a powder containing nickel alloy is then applied directly at room temperature and under atmospheric conditions after plasma spraying, wherein:

c) NiCrBSi coatings were applied to the pipes of the heat-resistant boiler of the most heat-resistant steel, in particular of St 35.8 15 Mo 3, by plasma spraying.

Even after prolonged operation, the wear values of these coated pipes were much lower than those of the unprotected pipe. Combining the NiCrBSi layer with an even more wear-resistant Cr2C3 layer exhibited a similar wear behavior throughout the time, especially after reaching a final layer thickness of 0.15 to 0.35 mm, there was no significant reduction in layer thickness over a long period of time ( compare the following figure).

6a

PL · / ^ 6 ^ 9 c?

Claims (9)

PATENT CLAIMS Method for producing a protective layer on walls of metallic material, preferably combustion devices or heat exchangers, attacked by hot gases, in particular flue gases, in which, by means of a plasma spraying method, metal walls are applied to pre-cleaned metal walls to form a protective layer of metallic powder , carbide, oxidoceramic or siliceous materials, or a mixture of these materials, characterized in that a / the surface of the walls is roughened and activated by high-purity noble corundum, and / or then sprayed directly at room temperature and atmospheric conditions by plasma spraying, wherein c / the powder composition is selected first such that the stresses determined by the thermal expansion coefficients of the base material and of the samples of the workpieces made of different powders on the transition area between the base and the deposited layer are taken as a function of unstressed / room temperature a tensile stress of between 50 and 800 N / mm ² , preferably between 500 and 800 N / mm ² , which substantially decreases in the temperature range of 300 to 1800 ° C, preferably 600 to 1000 ° C essentially at 0 or exhibits slight compressive stresses. Method according to claim 1, characterized in that the applied protective layer has a final thickness of 0.1 to 0.5 mm, preferably 0.15 to 0.25 mm. Method according to claim 1, characterized in that it applies 80 kW by means of an internal powder guide device. 1 or 2, a protective spray layer is in plasma with 4. 1 to 3, preferably a layer is used. Method according to at least one of the claims, characterized in that a powder having a grain size of less than 75 µm, 20 to 40 µm for the application of the Method according to at least one to 1 to 4, characterized in that the protective layer is produced on at least the courses. Method according to at least one of the preceding claims, characterized in that the surface of the walls is roughened and activated with noble corundum, preferably highly pure corundum, before the plasma spraying. Method according to at least one of the preceding claims, characterized in that the surface of the walls is heated by a plasma beam and the powder particles melted therein are only heated up to about 45 ° C, to a maximum of 60 ° C. Method according to the preceding claims, characterized in that a nickel alloy is used. at least one of which is characterized by a powder containing more than 45% more realistic Process according to claim 1, characterized in that it is atmospheric 1, plasma deposition is performed at the latest, preferably at the latest 30 minutes after activation of the wall surface. Method according to claim 1, characterized in that the stresses of the wall are in the range of 300 to 1800 ° C, preferably 600 to 1000 ° C.