Analysis of deep hole surfacing welding process for valve body sealing surface of power station

The working condition of the power station valve is usually 540 °C high-temperature steam, so the valve body material is 25 or 12crmov, and the valve body sealing surface is made of 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 continuously opened and closed under high temperature and high pressure, with excellent wear resistance, impact resistance, oxidation resistance, corrosion resistance and cavitation resistance.

The weld metal of ercocr-a and filler wire deposited in the aws standard is characterized by a sub-cocrystal structure consisting of a network of about 13% chromium carbide eutectic in a cobalt-chromium-tungsten solid solution matrix. The result is a perfect combination of resistance to low stress wear and the toughness necessary to withstand certain program impacts. Cobalt alloys have good resistance to metal-to-metal wear, especially in high load conditions. The high alloying element content of the matrix provides excellent corrosion and oxidation resistance. Cobalt-based alloys do not undergo allotropic transformation. When the deposited metal of cobalt-based alloys is in a hot state (below 650 °C), the hardness is not significantly reduced. Only when the temperature rises above 650 °C, the hardness decreases significantly. When the temperature returns to below the hot state, its hardness returns to near the original hardness. That is, when the base material is subjected to post-weld heat treatment, the performance of the sealing surface is not lost. The power station valve is made by welding the cobalt-based alloy into the valve seat sealing surface by means of electric welding in the middle hole of the valve body. Since the sealing surface is at a deep position in the middle hole of the valve body, slag and cracks are easily generated during the welding. defect. The deep hole surfacing d802 process test was carried out according to the need of the processing test piece method. The cause of defects is found during the process test.

1 welding material surface contamination.

2 welding consumables.

3 The base metal and the filler metal contain more impurities and oil.

4 The welded body of the valve body is rigid (especially dn32 ~ 50mm).

5 Preheating and post-weld heat treatment process specifications are not properly selected.

6 welding process parameters are not properly selected.

7 improper selection of welding materials.

The main reason for the crack of the valve body of the power station in the cobalt-based alloy surfacing is that the valve body is rigid. During the welding process, the arc forms a molten pool, which is continuously melted and heated to the welded portion, and the temperature after the welding is rapidly decreased, and the molten metal solidifies to form a weld. If the preheating temperature is low, the solder layer temperature drop must be very fast. In the case of rapid cooling of the solder layer, the shrinkage rate of the solder layer is faster than the shrinkage rate of the valve body. Under this stress, the inner layer tensile stress is quickly formed between the solder layer and the base material, and the solder layer is cracked. Sharp corners are strictly prohibited when machining welded parts. The preheating temperature is too low and the heat is quickly dissipated during the welding process. If the interlayer temperature is too low, the solder layer cooling rate is too fast for the build-up material. The cobalt-based alloy itself has a high red hardness. When working at 500-700 °C, the hardness can be maintained at 300-500 hb, but its toughness is low, crack resistance is poor, and crystal cracks or cold cracks are easily formed. Preheating must be done. The preheating temperature depends on the size of the workpiece. The general preheating range is 350-500 °C. The electrode of the electrode must be kept intact before welding to avoid moisture absorption. Bake at 150 ° C for 1 h before welding and put it into the electrode holder. The arc angle r of the deep hole surfacing groove should be as large as possible under the premise of the process, generally r≥3mm. Dn10 ~ 25mm small diameter valve body can be fully welded at the bottom of the deep hole with electrode, must ensure that the interlayer temperature ≥ 250 * (2, in the middle of the arc, the arc should be slowly lifted when the arc is lifted. Workpiece welding forward furnace (250 ° C Heating to 350 ° 20 ° C, after 1.5 h of heat preservation, the welding slag is removed after each layer is welded. At the same time, the interlayer temperature is controlled to be ≥250c, and the welding slag is removed after the surfacing is completed. The valve body must be immediately after welding. Furnace (450 ° C) insulation, until the batch or the furnace is heated to 710 ± 20 ° C tempering, after 2 h of insulation, with the furnace cooling, when the furnace temperature dn ≥ 32mm valve body should be processed into a u-shaped weld bevel To solve the problem of uneven shrinkage caused by excessive rigidity when surfacing the cobalt-based alloy. Before the surfacing operation, the workpiece is cleaned, the workpiece is placed in the furnace (the furnace temperature is 250 ° C), and heated to 450-500 ° C. After 2 hours of heat preservation, the welding is carried out. The sealing surface is first welded with cobalt-based alloy welding rods, and the welding slag is removed after each layer is welded. At the same time, the interlayer temperature is controlled to be ≥250 °C, and the welding slag is removed after all the surfacing is completed. Body stainless steel electrode (high-Cr, ni-content stainless steel electrode) Fill the u-shaped groove. Valve