Analysis of deep hole hardfacing technology for sealing surface of valve body in power plant

The working condition of power station valve is usually 540 ℃ high temperature steam, so the valve body material is 25 or 12CrMoV, and the valve body sealing surface surfacing material is cobalt base alloy d802 (STI 6) electrode D802 corresponds to the model edcocr-a in GB 984, which is equivalent to ERCoCr-A in AWS D802 material can be opened and closed continuously under high temperature and pressure It has excellent wear resistance, impact resistance, oxidation resistance, corrosion resistance and cavitation resistance The weld metal deposited by ERCoCr-A welding wire and filler wire in AWS standard is characterized by hypoeutectic structure composed of about 13% chromium carbide eutectic network distributed in the matrix of CO CR w solid solution The result is a combination of low stress wear resistance and the toughness necessary to resist a program impact Cobalt alloy has good resistance to metal to metal wear, especially under high load The high content of alloying elements in the matrix can provide corrosion resistance and oxidation resistance The hardness of the deposited metal of cobalt base alloy does not decrease obviously when it is in the hot state (below 650 ℃) Only when the temperature rises above 650 ℃, the hardness decreases obviously When the temperature returns below the hot state, the hardness returns to the original hardness In other words, when the base metal is subject to PWHT, the performance of sealing surface will not be lost The valve seat sealing surface of power station is made of cobalt base alloy by electric welding in the middle hole of valve body Because the sealing surface is deep in the middle hole of valve body, the defects such as slag inclusion and crack are easy to occur during the welding According to the need, the deep hole surfacing d802 process test is carried out by using the method of processing test piece In the process of process test, the causes of defects are found out ① The surface of welding material is polluted ② Moisture absorption of welding materials ③ There are many impurities and oil stains in the base metal and filler metal ④ The welding position of valve body is rigid (especially dn32-50mm) ⑤ Improper selection of preheating and post weld heat treatment process specifications ⑥ Improper selection of welding process parameters ⑦ Improper selection of welding materials, etc The main reason for the cracks in the valve body of power station during the hardfacing of cobalt base alloy is the high rigidity of valve body In the process of welding, the weld pool is formed by electric arc, which is continuously melted and heated to the welding position However, the temperature drops rapidly after welding, and the molten metal solidifies to form the weld If the preheating temperature is low, the temperature of welding layer will decrease rapidly Under the condition of rapid cooling of welding layer, the shrinkage rate of welding layer is faster than that of valve body Under the action of this stress, the welding layer and base metal form an internal tensile stress, which will crack the welding layer Sharp corners shall not be allowed when processing welding parts The preheating temperature is too low, and the heat is quickly dissipated during the welding process Too low interpass temperature and too fast cooling rate of welding layer are very unfavorable for surfacing materials The cobalt base alloy has high red hardness When it works at 500-700 ℃, its hardness can keep 300-500hb, but its toughness is low, its crack resistance is poor, and it is easy to form crystal crack or cold crack, so preheating must be carried out before welding The preheating temperature depends on the size of the workpiece Generally, the preheating range is 350-500 ℃ Before welding, the electrode coating must be kept in good condition to avoid moisture absorption Bake at 150 ℃ for 1H before welding, and then put it into the electrode insulation barrel The larger value of the arc R angle of the groove for deep hole hardfacing should be adopted as far as possible under the premise of the technological tolerance, generally R ≥ 3mm Dn10-25mm small diameter valve body can be fully welded with welding rod at the bottom of the deep hole, and the interlayer temperature must be ensured to be ≥ 250 * (2 In the middle of the arc, the welding rod shall be lifted slowly when the arc is stopped The workpiece shall be heated to 350-20 ℃ in the furnace (250 ℃) before welding, and the welding shall be carried out after 1.5h of heat preservation, and the welding slag shall be removed after each layer of welding At the same time, the interlayer temperature shall be controlled ≥ 250C, and the welding slag shall be removed after all the overlaying welding is completed After welding, the valve body must be put into the furnace (450 ℃) for heat preservation immediately After the welding of this batch or this furnace is completed, the temperature shall be raised to 710 ± 20 ℃ for tempering After the heat preservation for 2h, the valve body shall be cooled with the furnace When the furnace temperature DN ≥ 32mm, the welding groove shall be processed into a U-shape to solve the problem of uneven shrinkage caused by excessive rigidity during the welding of cobalt base alloy Before the surfacing operation, the workpiece shall be cleaned The workpiece shall be put into the furnace (furnace temperature is 250 ℃), heated to 450-500 ℃, kept warm for 2h, and then put out of the furnace for welding Firstly, the sealing surface shall be overlaid with cobalt based alloy electrode, and the welding slag shall be removed after each layer of welding is completed At the same time, the interlayer temperature shall be controlled ≥ 250 ℃, and the welding slag shall be removed after all the overlaying is completed Then austenitic stainless steel electrode (stainless steel electrode with high Cr and Ni content) is used to weld the U-shaped groove The valve body shall be put into the furnace (450 ℃) for heat preservation immediately after welding, and the temperature shall be raised to 720 ± 20 ℃ for tempering after welding of this batch or this furnace The heating speed is 150 ℃ / h, and it is cooled with the furnace after holding for 2h.