Method of restoration of steam turbine blades

The method can be used to repair steam turbine blades with damaged edges

After removing the worn section, pulsed surfacing is performed with a metal that is more ductile than the blade material. Then, protective pads are welded with two seams, one of which is applied to the base material of the blade, and the other to the weld metal. The weld metal has high crack resistance and a melting point lower than the melting point of the base material of the blade. After surfacing and after welding the protective pads, heat treatment is performed using a heat-insulating casing. A plasma coating of silicon oxycarbide is applied to the restored blade. The method reduces the heterogeneity of the structural-phase composition of the material of the restored blade, reduces tensile residual stresses, which leads to an increase in crack resistance, corrosion and erosion resistance of the blade.

The invention relates to the field of thermal power engineering and can be used to restore steam turbine blades with damaged edges.

During operation, steam turbine blades are exposed to various destructive factors, in particular, collisions with water droplets, the influence of a corrosive environment, alternating cyclic loads. As a result, steam turbine blades are subject to various damages, in particular, erosion, corrosion, fatigue failure.

Replacing damaged steam turbine blades is a labor-intensive and expensive undertaking, since it requires removing them from the rotor, purchasing new blades, installing them on the rotor, etc. In this regard, the development of new methods that reduce the labor intensity and cost of restoring damaged steam turbine blades is an urgent task.

The main requirements for steam turbine blades after restoration are compliance of their geometric and dimensional parameters with the requirements of the drawings, a high level of mechanical and operational properties (strength, hardness, fatigue resistance, fretting resistance, erosion, corrosion and crack resistance).

A method is known for repairing a working blade of a steam turbine. When repairing a blade using this method, a plate is welded in place of the worn section of the edge.

The disadvantage of this repair method is the complexity of manufacturing and welding the plate, as well as the high level of tensile residual stresses that arise after welding the plate and welding the protective pads.

A method for restoring turbine blades is known, which includes removing the damaged material, surfacing the area to be restored, heat treatment to relieve residual stresses, and mechanical processing of the blade.

The disadvantage of this method is the use of dissimilar materials for surfacing, which on the one hand ensures a combination of plasticity of the deposited material and high hardness of its surface, but at the same time leads to the formation of a heterophase layered structure, which reduces corrosion resistance.

A method for restoring working blades of steam turbines is known, which includes removing the blades from the rotor, removing the protective pads, mechanically removing damaged edge section, multilayer surfacing of the restored edge section, furnace heat treatment, mechanical treatment of the blade and welding of stellite protective plates.

The disadvantages of this method are the need to remove the blades from the rotor, the need for furnace heat treatment after surfacing, which complicates and increases the cost of the work. Another disadvantage of this method is the lack of heat treatment after welding the protective pads, as a result of which the blade material retains high heterogeneity of the structural-phase composition and a high level of tensile residual stresses.

The closest to the proposed method is the repair method of steam turbine blades, which includes mechanical removal of the worn edge, surfacing of the restored section, mechanical processing of the blade, welding of protective pads.

The main disadvantages of this method are the occurrence of high tensile - residual stresses during surfacing and welding in the heat-affected zone and high heterogeneity of the structural-phase composition of the restored blade material. Tensile residual stresses reduce the fatigue resistance of the material, and the structural-phase heterogeneity of the material reduces its corrosion resistance, since the combination of sections with different electrode potentials forms many microgalvanic pairs.

The technical result of the proposed method is a decrease in the heterogeneity of the structural-phase composition of the material of the restored blade, a decrease in tensile residual stresses, an increase in crack resistance, fatigue limit, corrosion and erosion resistance of the restored blade, an increase in manufacturability and a decrease in the cost of the blade restoration process.

The technical result in the proposed method is achieved by the fact that in the method for restoring steam turbine blades, including the removal of protective linings and a worn section of the blade, multi-layer surfacing of the restored section, mechanical processing of the blade, welding of protective linings, unlike the prototype, the surfacing is performed in a pulsed mode.

The technical result is also achieved by the fact that in the method for restoring steam turbine blades, including the removal of protective linings and a worn section of the blade, multi-layer surfacing of the restored section, mechanical processing of the blade, welding of protective linings, unlike prototype, as the surfacing material, a ductile metal with high crack resistance and a melting point lower than the melting point of the base material of the blade is used.

The technical result is also achieved by the fact that in the method for restoring steam turbine blades, including the removal of protective pads and the worn section of the blade, multi-layer surfacing of the restored section, mechanical processing of the blade, welding of protective pads, unlike the prototype, when welding protective pads, one of the seams is applied to the base material of the blade, and the other - to the surfacing material.

In addition, the technical result is achieved by the fact that in the method for restoring steam turbine blades, including the removal of protective pads and the worn section of the blade, multi-layer surfacing of the restored section, mechanical processing of the blade, welding of protective pads, unlike the prototype, after surfacing and after welding of the protective plates, heat treatment is carried out by applying on the blade of the heat-insulating casing heated during surfacing or welding and holding the blade in it until it cools completely.

Further, the technical result is achieved by the fact that in the method of restoring steam turbine blades, including the removal of protective pads and the worn section of the blade, multi-layer surfacing of the restored section, mechanical processing of the blade, welding of protective pads, unlike the prototype, a plasma coating of silicon oxycarbide (SixCyOz) is applied to the blade.

The achievement of the technical result is explained by the following.

The surfacing in the proposed method is performed in a pulsed mode, which reduces the amount of heat introduced into the blade and shortens the time the blade is at an elevated temperature. The use of the pulse mode allows obtaining a fine-grained structure of the deposited metal due to the improvement of its crystallization conditions and, as a result, its higher strength, hardness and erosion resistance.

During operation, the restored blades are subject to alternating deformations, as a result of which various structural defects and microcracks occur in the deposited material, leading to fatigue failure of the blade. In order to increase the resistance of the restored blades to crack formation, the proposed method uses a ductile metal with high crack resistance and a melting point lower than the melting point of the base material of the blade as the deposited material. Such properties of the deposited material create conditions for filling (the so-called "healing") of the emerging microdefects, and also prevent the occurrence of cold cracks.

In the proposed method, when welding protective pads, one of the seams is applied to the base material of the blade, and the other - to the deposited material. This allows to reduce residual stresses in the welded joint due to their relaxation in the more plastic deposited material.

When restoring steam turbine blades by surfacing, it is necessary to perform post-welding heat treatment to reduce the structural-phase heterogeneity of the restored blade material, reduce tensile residual stresses, and prevent warping. Furnace heat treatment complicates and increases the cost of restoring the blade. In the proposed method, heat treatment is carried out due to the heat accumulated by the blade material during surfacing and welding of protective plates. To do this, immediately after surfacing, as well as immediately after welding of the protective plates, a heat-insulating casing, for example, asbestos or ceramic, is applied to the blade and the blade is kept in it until it cools completely. In this case, due to a decrease in the cooling rate of the blade, residual stresses relax. This method of heat treatment allows heat treatment of steam turbine blades without removing them from the rotor. This significantly improves the manufacturability and reduces the cost of the restoration process.

The subsequent application of a plasma coating of silicon oxycarbide increases the erosion resistance of the blade material by increasing the hardness of its surface and the endurance limit of the restored blade by creating compressive residual stresses in its surface layer.

Restoration of a steam turbine blade using the proposed method makes it possible to reduce the heterogeneity of the structural-phase composition of the material of the restored blade, reduce tensile residual stresses, increase crack resistance, endurance limit, corrosion and erosion resistance of the restored blade, improve manufacturability and reduce the cost of the blade restoration process.

Experimental studies were conducted on samples of 20X13 steel, simulating blades restored in various ways. The results of fatigue and erosion tests of samples processed using different restoration technology options are presented in the table.

The test results show that heat treatment using the proposed method has the same effect on fatigue resistance as furnace heat treatment, increasing the endurance limit of samples by 17%. Application of a plasma coating of silicon oxycarbide leads to an increase in the endurance limit by 17-33% and erosion resistance by 8-20%.

In terms of the combination of erosion resistance and fatigue resistance characteristics, the optimal blade restoration option is one that includes sequential surfacing, heat treatment, welding of protective pads, heat treatment, and application of a plasma coating of silicon oxycarbide. In this case, from the technical and economic points of view, it is advisable to carry out heat treatment using the proposed method (by applying a heat-insulating casing to the blade and keeping it in it until it cools completely).

The results of the studies confirm that the proposed method provides the stated technical result and its advantages over analogues.

1. A method for restoring steam turbine blades, including removing protective pads and a worn section of the blade, multilayer surfacing of the section to be restored, mechanical processing of the blade, welding of protective pads and heat treatment, characterized in that surfacing is performed in a pulsed mode with a metal that is more plastic than the blade material, and welding of protective pads is performed with two welds, one of which is applied to the base material of the blade, and the other - to the weld metal.
2. The method according to paragraph 1, characterized in that a metal with high crack resistance is used as the surfacing material and a melting point lower than the melting point of the base material of the blade.
3. The method according to claim 1 or 2, characterized in that the heat treatment after surfacing and after welding of the protective pads is carried out by applying a heat-insulating casing to the blade heated during surfacing or welding and holding the blade in it until it has completely cooled.
4. The method according to any of claims 1-3, characterized in that a plasma coating of silicon oxycarbide is applied to the restored blade.