Cladding repair of high-pressure control valves

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Cladding repair of high-pressure control valves – Please continue to suggest any additional methods or better ways to improve energy efficiency in such repairs. Failure of the high-pressure control valves to open properly can severely affect the safe and stable operation of the unit. Upon disassembling and inspecting the valve, it was found that the seat of the high-pressure throttle valve had sunk by 10 mm, causing the guide shoulder of the valve disc to come out of its guide groove and preventing proper regulation of the steam flow. Given the short time available for emergency repairs on the unit and the difficulty of restoring the valve seat to its original condition on site, it was decided to use surfacing treatment to increase the height of the guide shoulder, thereby restoring the original functional capabilities of the high-pressure main steam control valve. 1 Repair plan for the high-pressure main steam control valve 1.1 Working mechanism of the valve disc guide shoulder The high-pressure main steam control valve of the N300‑16.7/537/537‑3 type turbine consists of 1 main steam valve and 2 control valves; the high-pressure control valve is used to regulate the amount of steam entering the high-pressure cylinder. During unit operation, the oil actuator serves as a mechanical lifting device that moves the valve disc’s guide shoulder up and down along the guide groove, thereby controlling the opening degree of the control valve disc. During unit operation, the high-temperature steam acting on the control valve is at 16.7 MPa and 537°C; the guide shoulder is primarily subjected to thermal stress as well as certain circumferential shear stress. The diameter of the fit between the disc and the seat of High-Pressure Control Valve No. 1 is 170 mm. The structure of its disc is shown in Figure 1, while the dimensions of the guiding shoulder are 55 mm × 30 mm × 10 mm (height × width × thickness). The valve disc is made of 20Cr3MoWVA alloy steel, and its surface has been treated with high-temperature nitriding to improve its resistance to cavitation. 1.2 The selection of surfacing materials and welding equipment is determined by the operating conditions of the control valve; the valve disc guide shoulder must possess sufficient high-temperature strength as well as a certain level of wear resistance. Given the urgent repair work on the unit, it was not possible to obtain materials that were an optimal match. In accordance with the principles for selecting surfacing materials and after analyzing the mechanical properties of various such materials, TIG-R34 (12Cr2MoWVTIB, Φ2.5 mm) welding wire, which has a composition similar to that of the base material, was chosen. The welding equipment uses the Lincoln V300-1 along with arc welding accessories ； Temperature monitoring is carried out using an American MX2 infrared thermometer. 2 Welding performance analysis is carried out using the carbon equivalent formula. The main characteristic of material 20Cr3MoWVA is its high content of carbon and alloying elements; as a result, hardened microstructures tend to form in the weld zone and the heat-affected area during welding. When the rigidity of the welded parts and the stresses at the joints are high, cold cracks are likely to occur. The valve disc that has undergone nitriding treatment has a surface hardness as high as HV900, making it highly prone to cracking during welding. 3 Cladding Process 3.1 Process Sequence: Grinding and cleaning before welding – preheating – cladding – post-weld heat treatment – post-weld turning. 3.2 Pre-welding preparation: First, use an angle grinder to thoroughly clean the area within 20 mm of the surfacing area, removing the nitrided layer; the grinding depth should be greater than 0.4 mm. Measure the hardness of the ground area to ensure that it meets the requirement of HB185–321 in the welding zone. According to the testing standard JB4730－94, the surface quality of the polished guide shoulder shall be inspected; no defects such as cracks or inclusions are allowed, and compliance with Grade I standards constitutes acceptance. Then clean the welded area of the valve disc and the area within 50 mm around it using acetone, to ensure there is no water, oil, etc ； Use sandpaper to remove dirt such as oil and rust from the surface of the TIG welding wire. 3.3 The welding process and parameters involve preheating prior to welding using an oxy-acetylene flame; the preheating temperature is 350–400°C, and a thermometer is used to measure this temperature. The welded area of the valve disc is exposed, while the other areas are covered with asbestos cloth to prevent arc burns. Process parameters: I = 80–90 A, argon flow rate of 8–10 L/min, direct current polarity, voltage range of 10–15 V, welding speed of 30–45 mm/min; the swing width per pass is ≤10 mm, and the thickness of each layer of weld deposit is ≤4 mm. Welding begins once the preheating temperature is reached; the interlayer temperature is maintained at 300–400°C. No defects are allowed to occur during welding, and special attention is paid to the start and end points of welding. Any defects found must be removed immediately using an angle grinder. To ensure the machining allowance, the surfacing dimensions are as per the drawing: the outer diameter is increased by 5 mm, the inner diameter is decreased by 5 mm, and the surfacing thickness is 13 mm. After welding one side is completed, weld the other side. After welding is completed, the area is wrapped with aluminum silicate refractory cotton for slow cooling; once the welded area reaches 100–150°C, immediate post-weld local heat treatment is carried out. For heat treatment, the material is heated to 640–660°C using a flame, and held at that temperature for 30 minutes; a temperature meter is used for monitoring. The heating rate and cooling rate should both be less than 300°C/h. There is no need to control the cooling process once the temperature drops below 300°C, and aluminum silicate fiber blankets are used to facilitate a slow cooling. 4 Post-weld inspection: After the valve disc has been surfaced and mechanically processed in accordance with the specified process, it is inspected using penetrant testing and ultrasonic testing to ensure that the welds are free from defects such as cracks, inclusions, and pores; the quality of the surfacing meets the requirements of Grade I as specified in standard JB4730–94 ； A hardness test was conducted on the machined surface, and the average hardness value was HB241, which is close to the hardness value of the original material, satisfying the performance requirements of the workpiece. 5 Conclusions on the surfacing repair of high-pressure control valves: Nearly 1 year after the welding repair of the No. 1 high-pressure main steam control valve in Unit 2 and its operation, it was disassembled for inspection during a unit overhaul; no issues were found with the shoulder of the surfaced valve disc after 5,616 hours of operation. It can be seen that the welding process of cladding 20Cr3MoWVA material using TIG-R34 argon tungsten arc welding wire, with preheating to 350–400°C, post-weld heating to 640–650°C, and holding for 30 minutes, is reasonable and feasible. It not only ensures proper regulation of steam by the high-pressure main steam control valve, but also saves tens of thousands of yuan in costs.