Repair scheme of surfacing welding for the hottest

2022-08-02
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Repair plan for surfacing of high pressure control valve

the opening failure of No. 1 high pressure control valve of No. 2 turbine in Zhanjiang Power Plant has seriously affected the safe and stable operation of the unit. The valve was disassembled and inspected. It was found that the valve seat of the high-pressure governing valve sank by 10 mm, resulting in the guide shoulder of the valve disc separating from the guide groove, and the steam could not be adjusted normally. In view of the short temporary repair time of the unit, it is difficult to recover the valve seat sinking on site, so it is decided to adopt surfacing treatment to increase the height of the guide shoulder, so as to restore the original function of the high-pressure main steam control valve. 1 high pressure main steam regulating valve repair scheme 1.1 working mechanism of valve disc guide shoulder unit 2 is a (N.7/537/type) steam turbine manufactured by Dongfang turbine works. Its high pressure main steam regulating valve is composed of one main steam valve and two regulating valves. The high pressure regulating valve is used to regulate the steam inlet of the high pressure cylinder. When the unit is running, the hydraulic servo motor acts as a mechanical lifting device to move the guide shoulder of the valve disc up and down along the guide groove to control the opening of the regulating valve disc. When the unit is running, the high-temperature steam of the control valve is 16.7 MPa, 537 ℃, and the guide shoulder mainly bears thermal stress and certain circumferential shear stress. The development diameter of the valve disc and valve seat of No. 1 high pressure regulating valve is 170 mm. The structure of the valve disc is shown in Figure 1, and the size of the guide shoulder is 55 mm × 30 mm × 10 mm high × wide × Thick). The valve disc is made of 20cr3mowva alloy steel. In order to improve the cavitation resistance of the valve disc, its surface is subject to high-temperature nitriding treatment. 1.2 the selection of surfacing materials and welding equipment shall be based on the working conditions of the regulating valve. The guiding shoulder of the valve disc shall not only ensure sufficient high-temperature strength, but also meet certain wear resistance. In view of the emergency repair of the unit, the best matching materials cannot be purchased, tig-r34 (12Cr2MoWVTiB) with similar material to the base metal is selected according to the selection principle of surfacing materials and the analysis of the mechanical properties of each surfacing material, Φ 2.5 mm) welding wire. The welding equipment adopts Lincoln V and argon arc welding supporting tools; American MX2 infrared thermometer is used for temperature monitoring. 2. Welding performance analysis according to the carbon equivalent formula, the main characteristics of material 20cr3mowva are that it contains more carbon and alloy elements. During welding, hardening structure is easy to appear in the weld and heat treatment area. When the weldment is rigid and the joint stress is large, it is easy to produce cold cracks. After nitriding treatment, the surface hardness of the valve disc is as high as hv900, which is easy to produce cracks during welding. 3 surfacing process 3.1 process route grinding and cleaning before welding preheating surfacing heat treatment after welding turning after welding. 3.2 preparation before welding: firstly, use the angular grinding wheel to grind and thoroughly remove the nitrided layer within 20 mm of the surfacing part. The grinding depth shall be greater than 0.4 mm. Measure the hardness value of the grinding part and ensure that the welding area meets the requirements of hb185 ~ 321. According to JB inspection standard, the surface quality of the polished guide shoulder shall be checked to ensure that there are no cracks, slag inclusions and other defects, and it is qualified if it meets class I standard. Then use acetone to clean the welding part of the valve disc and within 50mm around it to ensure that there is no water, oil, etc; Remove the oil stain, rust spot and other dirt on the surface of argon arc welding wire with sandpaper. 3.3 the welding process and parameters shall be preheated before welding by means of oxyacetylene flame heating. The preheating temperature shall be 350 ~ 400 ℃, and the preheating temperature shall be measured by a thermometer. The welding part of the valve disc is exposed, and other parts are covered with asbestos cloth to avoid electric arc burn. Process parameter I = 80 ~ 90 a, argon flow rate 8 ~ 10L/min, DC positive connection, voltage range 10 ~ 15 V, welding speed 30 ~ 45 mm/min, swing width of each pass ≤ 10 mm, surfacing thickness of each layer ≤ 4 mm. Welding shall be started after the preheating temperature is reached, and the interlayer temperature shall be controlled at 300 ~ 400 ℃. No defects are allowed during welding. The starting and ending arc shall be mainly inspected, and the defects found shall be removed immediately with an angular grinder. In order to ensure the machining allowance, the outer diameter shall be increased by 5 mm, the inner diameter shall be reduced by 5 mm, and the stacking height shall be 13 mm according to the surfacing dimensions in the drawing. Weld one side and then weld the other side. After welding, aluminum silicate refractory cotton shall be used to cover the slow cooling, and the welding part shall be cooled to 100 ~ 150 ℃, and local post welding heat treatment shall be carried out immediately. The heat treatment shall be heated to 640 ~ 660 ℃ by flame for 30 min. the temperature shall be monitored by a thermometer. The heating rate and cooling rate shall be less than 300 ℃/h. cooling to less than 300 ℃ may not be controlled. During cooling, aluminum silicate fiber blanket shall be used for coating and slow cooling. 4 post weld inspection: after the surfacing and machining of the valve disc are completed according to the process route, through the penetrant flaw detection and ultrasonic flaw detection, the weld is free of defects such as cracks, slag inclusions and air holes, and the surfacing quality meets the requirements of class I of JB standard; The hardness test is carried out on the machined surface. The average hardness value is hb241, which is close to the hardness value of the original base metal and meets the service performance of the workpiece. Conclusion the No.2 extruder Market in China is developing rapidly. After the No.1 high pressure main steam control valve of the unit was repaired by welding and operated for nearly one year, it was disassembled for inspection during the overhaul of the unit. No problem was found in the overlaid valve disc shoulder after 5616 hours of operation. It can be seen that the tig-r34 argon arc welding wire is used for surfacing 20cr3mowva material, and the approximate value of the elastic limit of the material without obvious linear section in the preheating 350 ~ 400 ℃ apparent elastic limit stress-strain curve is adopted. The welding process of 640 ~ 650 ℃ after welding and heat preservation for 30 min is reasonable and feasible. It not only solves the problem that the high-pressure main steam regulating valve regulates the steam normally, but also saves tens of thousands of yuan

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