Patent Grant
9393613
This application claims the priority of Chinese patent application No. 201410396737.X, filed on Aug. 13, 2014, which is incorporated herewith by reference.
1. Field of the Invention
The present invention belongs to the field of metallurgical technology, in particular relates to a method for manufacturing hollow ingot for retaining ring of large generator by electroslag remelting.
2. The Prior Arts
It is determined by the constitution of primary energy source in China that coal-fired power plants are predominant, and this is the biggest challenge for China in controlling greenhouse gas emissions. Therefore, using less coal to generate more electric power and using coal in a clearer and more effective way have become the strategic issues related to the sustainable development of China's national economy, wherein one of the most important ways is to develop supercritical and ultra supercritical generating units with high capacity and high parameters so as to increase the thermal efficiency of the generating units. Retaining ring of a steam turbine generator is a critical component for thermal power generator and is a ring-shaped forging used for fastening winding coils on both ends of a generator rotor. The retaining ring is also used for preventing the coils on both ends of a driving motor rotor from flying off under centrifugal action. The retaining ring works in the environmental conditions of high speed, high stress, corrosion and high magnetic field, therefore, the quality of the retaining ring is in direct relation to the safe operation of the whole generating unit, and the retaining ring is thus one of the most critical components of a steam turbine generator unit. Meanwhile, the higher the capacity of the generating unit is, the larger the size of the retaining ring is required.
The traditional manufacture method for retaining ring is to use a solid ingot as forging material, and the solid ingot needs to be processed into a hollow ingot first. The manufacture process comprises the steps of manufacturing a solid ingot through electric furnace melting and electroslag remelting; heating the solid ingot to a forgeable temperature range; upsetting and punching with a 10,000-ton open forging press to make the solid ingot into a hollow ingot with a certain inside diameter; inserting a mandril into the hollow ingot; and stretching the hollow ingot while hole expansion. In order to ensure the interior quality of the ingot, the forging deformation at each time needs to be slight. However, the ingot is extremely prone to cracking during repeated-heating slight-deformation free forging. Forging has to be stopped at any time when cracks are found, and the cracks can only be removed after the ingot is cooled. After the cracks are removed, the ingot needs to be heated to the forgeable temperature range again to proceed with the forging, and the preparation of the hollow ingot is not completed until the inside and outside diameters both reach required dimensions.
The traditional method for manufacturing hollow ingot for retaining ring has complex process and low forging yield, and a large amount of material is wasted during punching process. As multiple heating and deformation processes are used, the internal structure of the ingot is prone to change, and forging cracks are prone to form but difficult to remove. Repeated cooling and heating have a serious impact on the quality of the product, thus it is difficult to prepare hollow ingot for supersized retaining ring, and product accuracy and material uniformity cannot be ensured.
In consideration of the shortage of the existing method for manufacturing hollow ingot for retaining ring, the present invention provides a method for manufacturing hollow ingot for retaining ring of large generator by electroslag remelting process. The present invention uses an electroslag remelting device composed of two power supplies, water cooled T-shaped internal and external molds and electrode lifting devices, to manufacture a hollow ingot for retaining ring of large generator with uniform composition and good surface quality by adjusting the slag composition for withdrawing type electroslag remelting and controlling the operation of electroslag remelting.
The method for manufacturing hollow ingot for retaining ring of large generator by electroslag remelting of the present invention comprises the following steps:
According to the method, the liquid metal level detecting device, the consumable electrode lifting devices and the withdrawing device are controlled by a computer control system; at the beginning of the withdrawing, the level of the liquid metal is controlled by matching the lowering speed of the consumable electrode with the withdrawing speed; when the level of the liquid metal is higher than the position of liquid metal level detecting device, increase the withdrawing speed; when the level of the liquid metal is lower than the position of liquid metal level detecting device, reduce the withdrawing speed; control the level of the liquid metal at the just right position of the liquid metal level detecting device.
According to the method, the two parts with the same size of the consumable electrode assembly are symmetrical and are connected in parallel and distributed at both sides of the cross beam.
The slag contains 35%-40% of CaF2, 30%-35% of CaO, 10%-15% of Al2O3, 1%-5% of MgO and 10%-15% of SiO2 by weight percentage.
According to the method, the upper segment of the T-shaped external mold and the water-cooled bottom plate are power supplied by connecting the upper segment of the T-shaped external mold and the water-cooled bottom plate respectively with two terminals of a secondary output end of one transformer through a copper bar flexibly to form a high current loop; the electrodes and the water-cooled bottom plate are respectively connected with two terminals of the other transformer through a copper bar flexibly to form another high current loop.
Hollow ingot for retaining ring of large generator prepared according to the method has the outside diameter of 650 to 900 mm and the inside diameter of 450 to 500 mm.
Each of the consumable electrode lifting devices is fixed on a frame carriage; when one consumable electrode assembly is to be replaced, the former consumable electrode lifting device is removed by a frame carriage from the remelting position, and the next consumable electrode lifting device is moved by next frame carriage to the remelting position for electroslag remelting of the next consumable electrode assembly.
The control system is a two-stage computer control system composed of a PLC and an industrial control computer.
The present invention is based on the traditional electroslag remelting technology and creatively adopts the key technology and processes of T-shaped current supply mold, two power supplies for heating and supplying power, water cooled internal and external molds, frame carriage electrode lifting device and liquid metal level detecting device to manufacture hollow ingots directly by electroslag remelting; the method of the present invention has the following advantages:
As the method of the present invention adopts T-shaped molds, filling ratio is increased, the length of the electrode is reduced, and meanwhile, hollow ingots of different sizes can be prepared by remelting through replacing the internal and external molds with different sizes; as two power supplies and current supplying mold technology are used, the quality of the internal and external surfaces of hollow ingot can be ensured when exchanging the electrodes; as combined type water cooled internal and external molds are used, the hollow ingot has the advantages of compact structure, uniform composition, good thermoplasticity and lower compression ratio for processing; the consumable electrodes can be exchanged repeatedly by moving each frame carriage carrying an electrode lifting device so as to reduce the length requirements for the consumable electrodes; as the liquid metal level detecting device is used, the level of the liquid metal in the mold can be monitored in real time, thus to match corresponding withdrawing speed with the melting rate of the consumable electrode; as premelting slag with a certain composition is used, the problems of slag and steel leakage can be avoided, and meanwhile, the quality of the internal and external surfaces of the hollow ingot can be ensured.
According to the present invention, molten slag is added to the annular space formed by the internal mold, the external mold and water-cooled bottom plate on the withdrawing device, the end part of each consumable electrode is inserted into the annular space. When multiple consumable electrodes connected in parallel form a power supply circuit together with the slag and the water-cooled bottom plate through a high current loop, there is a current output from the transformer and passing through the molten slag to make the end part of each consumable electrode heat and melt gradually; liquid metal drops pass through the slag bath and enters the metal pool; as the metal pool is cooled by the internal and external molds, the liquid metal solidifies gradually and forms a hollow ingot. When the height of the hollow ingot reach a certain value, withdrawing starts; the withdrawing speed shall be matched with the melting rate of the consumable electrode, and the level of the liquid metal in the mold is monitored by the liquid metal level detecting device.
Theoretical analysis and experimental results show that as the hollow ingot prepared by electroslag remelting is compulsorily cooled by water with the internal and external molds, the solidification quality of the hollow ingot can be effectively controlled, and large temperature gradient makes the liquid metal solidify quickly under the action of two opposite solidification fronts, therefore refined solidification structure of the ingot is ensured, transverse plasticity and toughness as well as the indexes such as anisotropy, fracture toughness, notch sensitivity and low cycle fatigue are significantly improved, and the ingot has the advantages of high cleanliness, compact structure, uniform composition, good thermoplasticity and lower compression ratio for processing; the hollow ingot prepared by electroslag remelting is directly used for producing large retaining ring forging, therefore the heating number for forging can be reduced greatly, crack formation during forging can be significantly alleviated, and grain growth caused by repeated heating can be avoided; when the hollow ingot prepared by the method of the present invention is used as the material for producing retaining ring of large generator, punching process can be saved, heating number and forging number can be reduced, forging cracks can be reduced, interior quality of the ingot can be ensured, and the production cost and production cycle of retaining rings with large caliber can be substantially reduced; compared with the method of using a solid ingot as forging material to prepare a retaining ring forging, the method of directly using a hollow ingot to prepare a retaining ring forging of the same size can save material cost by at least 15%, heating cost by at least 50%, and forging cost by at least 30%.
The fluorite used in the embodiments of the present invention is fluorite powder with the weight purity not less than 97% or fluorite granules with the weight purity not less than 97%; the weight content of CaO in lime used is not less than 92%; the weight purity of Al2O3 in industrial alumina used is not less than 99%; the weight purity of MgO in fused magnesite used is not less than 97%; the weight purity of SiO2 in silica used is not less than 98%; the above materials are prepared into slag by a method disclosed according to a low-pollution energy-saving slag system for preparing hollow ingot through withdrawing-type electroslag remelting with the Chinese patent application number of 201310239951.X.
The molten steel level detecting device used in the embodiments of the present invention is MLOC-2M liquid metal level detecting device manufactured by Elmet-Roll, Ukraine.
The consumable electrode with the diameter of 160 mm used in the embodiments of the present invention is produced by an EAF+LF+VD process and rolling.
The electroslag remelting hollow ingot mold used in the embodiments of the present invention has the following sizes: 650 to 900 mm for the diameter of the T-shaped external mold, and 450 to 500 mm for the diameter of the internal mold.
The device for preparing hollow ingot for retaining ring of large generator by electroslag remelting in the embodiments of the present invention has the structure shown in
The electroslag remelting hollow ingot mold is composed of a T-shaped external mold and an internal mold; the internal mold is composed of a water cooled sleeve 3 and a cross beam 4 on the top of the water cooled sleeve 3, the cross beam 4 is fixed on the upper flange of the T-shaped external mold, and a cooling water channel 7 is arranged in the water cooled sleeve 3 for allowing cooling water to flow; the T-shaped external mold comprises an upper segment 1 and a lower segment 2, between which an insulation blanket 5 is arranged and which are respectively installed with a water cooled device; the lower segment 2 of the T-shaped external mold is also installed with a liquid metal level detecting device 6.
The liquid metal level detecting device 6 is 60 to 80 mm below the insulation blanket 5.
The water-cooled bottom plate 17 is arranged at the bottom of the electroslag remelting hollow ingot mold; the water-cooled bottom plate 17 is respectively connected with one terminal of each transformer (transformer I 19 and transformer II 20) through a copper bar, wherein the other terminal of one transformer (transformer I 19) is connected with a stub 12 through a copper bar, and the other terminal of the other transformer (transformer II 20) is connected with the upper segment 1 through a copper bar.
Each consumable electrode lifting device comprises a lifting device driving motor 14, an electrode cross arm 13 and a stub 12; the other terminal of the transformer I 19 is connected with the stub 12 through a copper bar; a consumable electrode assembly is arranged below the stub 12, and the consumable electrode assembly is composed of a consumable electrode assembly right part 8 and a consumable electrode assembly left part 9 which have the same size and are symmetrically distributed at both sides.
Each consumable electrode lifting device is fixed on a frame carriage which comprises a frame cart base 21 and a frame carriage driving motor 15.
The withdrawing device comprises a withdrawing device bottom plate 24 and a withdrawing device driving motor 18.
The withdrawing device driving motor 18, the lifting device driving motor 14, the frame carriage driving motor 15 and the liquid metal level detecting device 6 are all connected with a control system 16 through wires.
When one consumable electrode assembly is to be replaced, the former consumable electrode lifting device is removed by a frame carriage from the remelting position, and the next consumable electrode lifting device is moved by next frame carriage to the remelting position for electroslag remelting of the next consumable electrode assembly.
A computer is used as a control system, and a PLC used as a lower computer and an industrial control computer used as an upper computer form a secondary monitoring system.
The diameter of the T-shaped external mold is 650 mm, and the diameter of the internal mold is 450 mm; the amount of molten slag used is calculated under the condition of ensuring that the height of the slag bath is 260 to 280 mm, i.e. 240 to 260 kg of a single heat. The voltage is 63 to 68 V (the process set value of voltage), the current is 18 to 20 kA (the process set value of current), the melting rate is 800 to 970 kg/h, and the withdrawing speed for hollow ingot is 10 to 12 mm/min.
The consumable electrode for electroslag remelting is made of 1Mn18Cr18N steel.
The internal surface and the external surface of the hollow ingot by electroslag remelting have good quality without slag ditch, scab, lap, fold, slag inclusion, etc. After vertically and horizontally dissecting electroslag hollow ingot, polish the surfaces, and no obvious macro defects are found. After corrosion, the macrostructure of the electroslag hollow ingot is shown in
From comparison of the chemical composition of the consumable electrode and the sampled chemical composition of the head and tail of hollow ingot in Table 1, it can be seen that elements of the head and tail have less deviation and are distributed equally. In addition, the burning loss of carbon, silicon and manganese elements is low, but harmful elements are reduced significantly, in particular, the desulfurization capacity is strong, the average desulfurization ratio is about 50%, but the other elements are essentially unchanged, which fully exert the advantages of the electroslag remelting process.
The slag contains 40% of CaF2, 30% of CaO, 15% of Al2O3, 1% of MgO and 14% of SiO2 by weight percentage;
The electroslag remelting method is the same as the embodiment 1, and the differences are:
The size of the hollow ingot is 900/500 mm, the internal surface and the external surface of the hollow ingot have good quality without obvious slag ditch, scab, etc., and the total weight content of non-metallic inclusions is reduced by 62%.
The slag contains 39% of CaF2, 34% of CaO, 14% of Al2O3, 3% of MgO and 10% of SiO2 by weight percentage;
The electroslag remelting method is the same as the embodiment 1, and the differences are:
The size of the prepared hollow ingot is 900/200 mm, the internal surface and the external surface of the hollow ingot have good quality without obvious slag ditch, scab, etc., and the total weight content of non-metallic inclusions is reduced by 65%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014 1 0396737 | Aug 2014 | CN | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20110214830 | Holzgruber | Sep 2011 | A1 |
| Number | Date | Country |
|---|---|---|
| WO 0161060 | Aug 2001 | AT |
| 409729 | Oct 2002 | AT |
| 103334015 | Oct 2013 | CN |
| Entry |
|---|
| EPO machine translation of AT 409729 B, Oct. 25, 2002. |
| Number | Date | Country | |
|---|---|---|---|
| 20160045952 A1 | Feb 2016 | US |
