Zhanjiang Power Plant No. 2 to No. 1 high-pressure steam turbine control valve to open failure, seriously affect the unit safe and stable operation. To the disintegration of the valve, it was discovered high-pressure Control Valve seat sink 10 mm, leading to valve plate from the guide groove-oriented convex shoulder, unable to conduct the normal adjustment process steam. In view of Temporary repair crew a short time, the valve seat sink on-site hard to recover, decided to adopt the surfacing treatment, increase the orientation convex shoulder height, to reach the main steam pressure regulating valve to restore the original functional.
A high-pressure main steam valve repair program
1.1 The valve disc-oriented working mechanism of convex shoulder
Unit No. 2 for the manufacture of Dongfang Steam Turbine Works (N300-16.7/537/537-3 type) steam turbine, and its main steam pressure regulating valve is a main steam valves and 2 control valve composed of high-pressure regulating valve is used to regulate the amount of high-pressure steam into the cylinder. Unit run-time, the oil motive for mechanical lifting equipment, so that valve disc oriented convex shoulders move up and down along the guide slot, the control valve disc opening. Unit run-time, high-temperature steam control valve 16.7 MPa, 537 ℃, oriented primarily to withstand thermal stress convex shoulders and some weeks to shear stress.
No. 1 high-pressure regulating valve with the valve disc and valve seat diameter of 170 mm, the valve plate structure shown in Figure 1, oriented convex shoulder size 55 mm × 30 mm × 10 mm (H × W × thickness). Valve disc material used 20Cr3MoWVA alloy steel, in order to improve the resistance to cavitation valve disc performance, its surface temperature nitriding treatment conducted.
1.2 surfacing materials and welding equipment selection
According to the working conditions of control valve, the valve plate oriented convex shoulder it is necessary to ensure that there are sufficient high-temperature strength, but also to meet some wear. In view of crew repairs, and can not be procured to the best matching materials, taking into account the principles of the choice of surfacing materials, as well as the mechanical properties of various surfacing materials, analysis, materials selection and parent material similar to TIG-R34 (12Cr2MoWVTIB, Φ2.5 mm) wire. Using welding equipment
Lincoln V300-1, and arc welding supporting tools; temperature monitor the use of the United States MX2 infrared thermometer.
2 Welding Performance Analysis
According to carbon equivalent formula, the material 20Cr3MoWVA the main characteristics of carbon and alloy elements in more, welding and heat treatment weld areas prone to hardening body rigidity and joints while welding pieces of the stress is large, easily lead to cold cracking.
After nitriding treatment of the valve plate, the surface hardness up to HV900, welding cracks easily.
3 Welding Technology
3.1 Process Route
Welding pre-polishing cleaning - preheat - surfacing - post weld heat treatment - after welding turning.
3.2 The pre-welding
First, with the angle to the grinding wheel to remove a thorough clean-up parts of 20 mm surfacing within the nitrided layer, grinding depth should be larger than 0.4 mm, measurement of grinding parts of the hardness value, and to ensure that welding area to achieve the requirements of HB185 ~ 321 . According to JB4730-94 testing standards, check the orientation convex polished surface quality of the shoulder should not have cracks, defects such as slag, reaching the standard for qualified Grade Ⅰ.
And then cleaned with acetone valve plate weld and the surrounding area within 50 mm to ensure that water and oil; argon arc welding wire with sandpaper remove surface oil and rust spots and other dirt.
3.3 welding process and parameters of
Using oxyacetylene flame heating method of preheating before welding, preheating temperature of 350 ~ 400 ℃, with the thermometer measurement of preheating temperature. Valve plate welded parts exposed, other parts of cotton coated with stone to avoid arc burn. Parameters I = 80 ~ 90 A, argon gas flow rate 8 ~ 10L/min, DCEN voltage range 10 ~ 15 V, welding speed 30 ~ 45 mm / min, Every swing width ≤ 10 mm, the thickness of each layer surfacing ≤ 4 mm. Preheat temperatures after welding, the temperature control between layers 300 ~ 400 ℃, the welding process does not allow any defects, focusing on inspection cited, to close arc Office, found flaws with the angle grinder to remove immediately. To ensure the machining allowance, according to Figure surfacing size, diameter increased 5 mm, diameter reduction of 5 mm, piled high 13 mm. Side of the surfacing after the completion of surfacing on the other side.
Welding is complete, fire-resistant cotton coated with silicate slow cooling, welding parts of the cold to 100 ~ 150 ℃, local heat treatment immediately after welding. Heat treatment using flame heated to 640 ~ 660 ℃, heat 30 min, with the thermometer monitoring, heating rate and cooling rate were <300 ℃ / h, cooling to below 300 ℃ may from time to control, cooling blanket coated with aluminum silicate fiber slow cooling.
4 weld inspection
In accordance with process route to complete the surfacing and mechanical disc valves processed by penetrant inspection and ultrasonic testing inspection, there is no weld cracks, slag, porosity and other defects in the quality of welding quality has reached JB4730-94 standard grade Ⅰ requirements; in the processing surface hardness testing, the average hardness value of HB241, the original parent material hardness values similar to the use of performance to meet the workpiece.
5 Conclusion
2, Unit 1, the main steam pressure regulating valve repair welding and running nearly a year after the dissolution of the unit when the overhaul inspection, welding of the valve disc convex shoulders run through the 5 616 h did not find any problems. Can be seen with the TIG-R34 argon arc welding wire welding material used 20Cr3MoWVA preheated 350 ~ 400 ℃, after welding, 640 ~ 650 ℃, heat welding process 30 min treatment is reasonable and feasible. It not only solves the main steam pressure regulating valve to adjust the normal pairs of steam, while also saving money on the yuan.
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