A COATING, AND A COATING METHOD, FOR A STEAM TURBINE AND ADJOINING STEEL SURFACES

Abstract: The present invention relates to the protection of the casing, division planes, piping, superheaters and other steel parts of a steam turbine which are subjected in a turbine plant to some corrosive and erosive wear caused by steam, the coating comprising a coating layer produced by the thermal spraying of a steel, alloyed amply with chromium and aluminum, which during the coating process oxidizes strongly in the spray, whereby large amounts of chromium and aluminum oxides are formed, which will remain inside the coating, surrounded by a steel matrix, and after the coating process there will form on the surface of the coating layer, under the oxidizing action of air, a dense chromium and aluminum oxide layer. Another object of the invention is a related coating method.

Description: A coating, and a coating method, for a steam turbine and adjoining steel surfaces The invention relates to a coating intended for protecting the interior surfaces of a steam turbine and the adjoining pipes and superheaters, the coating preventing the erosive and corrosive wear caused by steam. The invention also relates to a method for coating the interior surfaces of a steam turbine and the adjoining pipes and superheaters. As stated in Swedish Patents/Patent Applications No. 762881 and 771073, steel surfaces exposed to hot, damp steam at high pressures and velocities are subject to heavy erosive and corrosive wear. The damage caused by wear may lead to the need for fill-in and repair weldings which are difficult to carry out, and even replacement of the turbine casing and pipes. Such repair and replacement work causes long stoppages and thereby large financial losses owing to reduced production. This is the case in particular in large power plants such as nuclear power plants. For example the following coatings have been used for protecting turbine pipes: 1. Ceramic coating with a nickel-aluminum alloy in the ad- hesive layer. The thickness of the adhesive layer is 10 - 25 μm and that of the ceramic coating 50 - 250 μm. 2. Metallic so-called triple-layer coating, in which the adhesive layer is a nickel-aluminum alloy (t = 50 - 100 μm), the intermediate layer is a chromium steel (approx. 13 % Cr, with a layer thickness of approx. 200 μm), and the surface layer is a stainless or acid-resistant steel (Cr = approx. 18 %, Ni = 5 - 8 %, and Mn = approx. 8 %, with a layer thickness of appro . 200 μm) . Ceramic coatings have, for example, the following disadvan- tages: poor shock resistance, for example, when foreign bodies enter the turbine or the pipes, they may break the coating; - the thermal expansion coefficient of a ceramic coating is very low compared with that of carbon steel, so that great or rapid variations in the temperature may lead to the cracking of the coating. A crack in the coating may, in turn, lead to rapid local damage to the base material; ceramic coatings are good insulators. The coating of a turbine casing with a ceramic material could disturb intra-turbine thermal conduction and cause unexpected deformation during operation; with a ceramic coating it is difficult to coat seal surfaces which are to be machined. The coating may have a hardness higher than 1000 HV, and therefore it is difficult to machine, and, furthermore, the coating tends to crack; it is difficult to achieve a sufficient layer thickness with a ceramic coating if the coating is to be used for filling cavities. So-called triple-layer coating has functioned satisfactorily in pipe systems. However, this coating has the following disadvantages: in a triple-layer coating, each interface between the different coatings constitutes a strong barrier to thermal conduction, and so problems similar to those involved with ceramic coatings may appear in thermal conduction in the turbine casing; if it is necessary to fill cavities in coating surfaces which are to be machined, there is the risk that the machined surface will run through different layers; - if the triple-layer coating is damaged in operation, for example owing to strong local erosion, it must be repaired by first removing the old coating entirely and by then recoating the surface, layer by layer. The primary object of the present invention is to provide a coating which can be used for coating the casing, division planes, piping, superheaters and other parts of a steam turbine so that reliable and long-term protection, suitable for the conditions involved, is obtained for the steel surfaces. It is a further object of the invention that the coating work can be carried out on the site rapidly and economically, and that the coating is also well suited for the coating of surfaces which are to be machined. The coating according to the invention is characterized in that the coating comprises a coating layer which has been produced by the thermal spraying of a steel alloyed amply with chromium and aluminum, which, during the coating process, oxidizes strongly in the spray, whereby large amounts of chromium and aluminum oxides are formed which will remain inside the coating, surrounded by a steel matrix, and on the surface of the coating layer there will form, after the coating process, under the oxidizing effect of air, a dense chromium and aluminum oxide film. The coating method according to the invention is characterized in that a coating material of steel alloyed amply with chromium and aluminum is sprayed thermally onto the surface being coated, and this coating material oxidizes strongly in the spray during the coating process, whereby large amounts of chromium and aluminum oxides are formed which will remain inside the coating, surrounded by a steel matrix, and that, after the coating process, the coating which has been formed will be exposed to the oxidizing effect of air, whereby a dense chromium and aluminum oxide film will be formed on the surface of the coating. According to the invention, the coating material used is a steel preferably containing chromium 20 - 45 % by weight, aluminum 5 - 15 % by weight and molybdenum 0 - 5 % by weight, and especially preferably chromium 22 - 30 % by weight, aluminum 5 - 8 % by weight and molybdenum 0 - 3 % by weight. The coating material may be thread-like or pulverous. In the coating according to the invention, containing large amounts of chromium and aluminum oxides, the contents of chromium, aluminum and molybdenum are of the order presented abov . The chromium and aluminum oxide film in the coating accord- ing to the invention, formed under the effect of oxidation after the coating process, is strong and dense, and will prevent erosive and corrosive wear caused by wet steam. The coating according to the invention can be prepared by thermal spraying, by using flame, arc, plasma and/or supersonic spraying, but primarily arc, plasma and/or supersonic spraying, in order to obtain good adhesion of the coating to the base material. Thus there is formed in the coating according to the invention a layer with good adhesion to the base material, a steel coating which contains a large amount of oxides, and a surface layer which consists of a dense oxide film. In order that no galvanic corrosion should occur on the interface between the base material and the coating, the thickness of the coating should be at minimum 0.3 mm, preferably, however, 0.5 mm. The thickness of the coating may be up to 2.5 mm without its pealing off because of internal shrinkage of the coating. The advantages of the coating according to the invention as compared with previous ones are as follows: 1. The very dense chromium and aluminum oxide film formed on the surface of the coating provides excellent protection against corrosion and erosion. Nevertheless, the coating is very tough. If the coated surface is damaged, for example under the effect of a foreign body which has entered the turbine, the oxides inside the coating will prevent propagation of the damage. Thus the coating provides the protective effect of a ceramic coating, but it has the toughness and strength of a metal coating. 2. The adhesion of the coating to the base material is very good. When arc or plasma spraying is used, an adhesion strength greater than 60 N/mm is obtained, which is approximately double the adhesion strength of a flame- sprayed nickel and aluminum alloy. Good adhesion guarantees that the coating will not become detached by minor impacts, and that it will also be possible to coat narrow edges. Furthermore, good adhesion enables the surface to be machined. 3. The thermal expansion coefficient of the coating is close to the thermal expansion coefficient of carbon steel, so that deformation due to thermal shock and thermal expansion will not damage the coating. 4. Since the coating is made up of one single layer and it can be sprayed approx. 2 mm thick, the coating is suitable for protecting very large seal surfaces which are to be machined. 5. Although the coating contains large amounts of hard oxides, its macro-hardness is only 250 - 350 HV units, so that the coating will be easy to machine. 6. Thermal conduction will not cause problems, since the only interface hampering thermal conduction is the interface between the coating and the base material. 7. Patching of the coating is easy to perform locally, without removing all of the old coating. 8. The cobalt content of the coating is very low (approx. 0.02 %), and so the coating is highly suitable for use in nuclear power plants, also on surfaces on the active side. In a nuclear power plant the division planes and some of the turbine casing above and below the division planes were coated by arc spraying with a coating according to the invention, the analysis of which was 22 % Cr and 5 % Al. After eight years of use the turbine was opened, whereupon it was observed that the division planes were completely flawless and that the coating above and below the division planes had endured very well. On the other hand, the base material had worn off up to more than 10 mm in the area adjacent to the border of the coating.

Claims: Claims 1. A coating intended for protecting the casing, division planes, piping, superheaters and other steel parts of a steam turbine which are subjected in a turbine plant to some corrosive and erosive wear caused by hot wet steam, characterized in that the coating comprises a coating layer which has been produced by thermally spraying a steel, alloyed amply with chromium and aluminum, which oxidizes strongly in the spray during the coating process, whereby large amounts of chromium and aluminum oxides are formed which will remain inside the coating, surrounded by a steel matrix, and after the coating process there will form on the surface of the coating layer, under the oxi¬ dizing effect of air, a dense chromium and aluminum oxide film. 2. A coating according to Claim 1, characterized in that the coating is a steel which contains chromium 20 - 45 % by weight, aluminum 5 - 15 % by weight and molybdenum 0 - 5 % by weight, preferably chromium 22 - 30 % by weight, aluminum 5 - 8 % by weight, and molybdenum 0 - 3 % by weight. 3. A coating according to Claim 1 or 2, characterized in that the coating is made up of one layer the thickness of which is 0.3 - 2.5 mm. 4. A coating according to any of Claims 1-3, characterized in that the thermal spraying is carried out by the arc, plasma or supersonic method. 5. A method for coating the casing, division planes, piping, superheaters and other steel parts of a steam turbine which are subjected in a turbine plant to some corrosive and erosive wear caused by hot wet steam, characterized in that a coating material consisting of a steel alloyed amply with chromium and aluminum is sprayed thermally onto the surface to be coated, the coating material oxidizing strongly in the spray during the coating process, whereby large amounts of chromium and aluminum oxides are formed which will remain inside the coating, surrounded by a steel matrix, and that after the coating process the coating which has been formed will be exposed to the oxidizing effect of air, whereupon a dense chromium and aluminum oxide film will form on the surface of the coating. 6. A method according to Claim 5, characterized in that the coating material is a steel containing chromium 20 - 45 % by weight, aluminum 5 - 15 % by weight and molybdenum 0 - 5 % by weight, preferably chromium 22 - 30 % by weight, aluminum 5 - 8 % by weight and molybdenum 0 - 3 % by weight. 7. A method according to Claim 5 or 6, characterized in that a single-layer coating is formed the thickness of which is 0.3 - 2.5 mm. 8. A method according to any of Claims 5-7, characterized in that the thermal spraying is carried out by the arc, plasma or supersonic method.

Inventors: HELLMAN BJARNE, MAJAVA JORMA, TENKULA JAAKKO, HELLMAN, BJARNE, MAJAVA, JORMA, TENKULA, JAAKKO
Applicants: TAMPELLA TELATEK OY
Application year: 1991
Main classification: C23C4
IPC codes: C23C4, F01D5, F01D25