Power station valve sealing surface deep hole welding process

Power station valves operating conditions are usually 540 ℃ high temperature steam, so the valve body material selection 25 or 12CrMoV, the valve sealing surface surfacing material selection cobalt-based alloy D802 (STI 6) electrode. D802 corresponds to the model EDCoCr-A in the GB 984 standard, which is equivalent to ERCoCr-A in AWS. D802 material can be opened and closed in a row under high temperature and pressure work, with excellent wear resistance, impact resistance, oxidation resistance, corrosion resistance and cavitation resistance. The ERCoCr-A wire and filler wire cladded weld metal in the AWS standard is characterized by hypoeutectic microstructure consisting of approximately 13% of a matrix of chromium carbide eutectic in a cobalt-co-tungsten solid solution matrix. The result is a perfect combination of the material's resistance to low stress wear and the toughness necessary to withstand some sort of procedural impact. Cobalt alloys have a good resistance to metal-to-metal wear, especially in high load conditions, scratch resistance. The high alloying element content of the matrix provides excellent resistance to corrosion and oxidation. Cobalt-based alloy does not change allotrope. When the deposited metal of Co-based alloy is in the hot state (below 650 ℃), the decrease of hardness is not obvious. Only when the temperature is increased above 650 ℃, the hardness decreases obviously, When the temperature returns below the hot state, its hardness returns to near the original hardness. That is, when the base metal after heat treatment, the performance of the sealing surface will not be lost. Power station valve is in the middle part of the valve body welded by welding method of cobalt base alloy processed into the valve seat sealing surface, the sealing surface at the valve body in the middle hole deep position, easy to produce slag and cracks in the surfacing defect. According to the need to use the method of processing specimens deep hole surfacing D802 process test. In the process of testing to identify the causes of defects. ① welding surface contamination. ② welding consumables. ③ base metal and filler metal contains more impurities and oil. ④ valve body parts rigid welding (especially DN32 ~ 50mm). ⑤ preheating and welding heat treatment process specifications improper selection. ⑥ welding process parameters improper selection. ⑦ improper selection of welding materials. Power station valve body in the cobalt-based alloy surfacing cracks mainly due to the valve body rigidity. During the welding process, the arc forms a weld pool, which continuously melts and heats the welding part, and the temperature of the weld drops rapidly after the welding, and the molten metal solidifies to form a weld. If the preheating temperature is low, the temperature of the bonding layer must drop rapidly. In the case of rapid cooling of the weld layer, the shrinkage rate of the weld layer is faster than the shrinkage of the valve body. Under such stress, the weld layer and the parent material quickly form an internal tensile stress and the weld layer is cracked. In the processing of welding parts should be strictly prohibited sharp corners. Preheating temperature is too low, the heat in the welding process quickly distributed. The interlayer temperature is too low, the cooling rate of the coating is too fast for the surfacing material is very unfavorable. Welding consumables cobalt-based alloy itself has a very high red hardness, working at 500 ~ 700 ℃, the hardness can maintain 300 ~ 500HB, but its low toughness, crack resistance is poor, easy to form a crystal crack or cold crack, so before welding Must be preheated. Preheating temperature depends on the size of the workpiece, the general preheating range to take 350-500 ℃. Welding electrode coating must be kept intact, to avoid moisture absorption. Baking 1h at 150 ℃ before welding into the insulation tube. Deep groove surfacing groove arc R angle as far as possible in the process allows the use of larger values, generally R ≥ 3mm. DN10 ~ 25mm small diameter valve body can be used at the bottom of the deep hole full of welding, must ensure that the interlayer temperature ≥ 250 * (2, in the middle of the arc, arc welding should be slowly lifting the electrode. ) Heated to 350 ten 20 ℃, insulation 1.5h after welding, welding slag after each layer is removed. At the same time control the interlayer temperature ≥ 250 "C, after the completion of welding to remove slag. Welding body must be immediately after welding Into the furnace (450 ℃) insulation, until the batch or the furnace welding temperature is raised to 710 ± 20 ℃ tempering, heat 2h after the furnace cooling, when the furnace temperature <250 ℃ air cooled to room temperature for subsequent processing. 32mm valve body should be processed into a U-shaped groove bevel, to solve the welding of cobalt-based alloy caused by excessive rigidity caused by the shrinkage of non-uniform problem.In the surfacing operation, the workpiece clean, the workpiece into the furnace The furnace temperature is 250 ℃), heated to 450 ~ 500 ℃, heat 2h, baked furnace welding. Coated with cobalt-based alloy welding rod sealing surface, after each layer to remove welding slag slag, at the same time to control the interlayer temperature ≥ 250 ℃ , After the completion of surfacing to remove welding slag. Re-austenitic stainless steel electrode (high Cr, Ni content of stainless steel electrode) U-shaped groove welding full. Immediately after the valve body welded into the furnace (450 ℃) insulation, until the batch or the furnace after welding is warmed to 720 ± 20 ℃ tempering. Heating rate 150 ℃ / h, after 2h With the furnace cooling, when the furnace temperature <250 ℃ air cooled to room temperature for subsequent processing (Figure 3) This process improvement has no effect on the performance of the product, with the U-groove can completely eliminate the body in the welding of cobalt Alloy when producing internal stress, and austenitic stainless steel welding consumables large tensile strength, high elongation, thus reducing the welding cracks, improve the product pass rate.Non-destructive test results show that the passing rate of 95 % Or more. More Keyword Search: Valve

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