Analysis of Deep Hole Overlay Welding Process on Sealing Surface of Valve Body in Power Station (HY-industry technical centre)

• Working condition of power station valves is usually 540℃ high temperature steam, so the valve body material is stellite25 or 12crmov, and the valve body sealing surface surfacing material is cobalt-based alloy stellite6 electrode. The stellite6 material can continuously open and close at high temperature and high pressure, and has excellent wear resistance, impact resistance, oxidation resistance, corrosion resistance and cavitation resistance.

• Weld metal deposited by stellite6 welding wire and filler wire is characterized by a hypoeutectic structure composed of about 13% chromium carbide eutectic network distributed in the cobalt-chromium-tungsten solid solution matrix. The result is a perfect combination of low-stress wear resistance and the toughness necessary to resist a certain process impact. Cobalt alloy has good resistance to metal-metal wear, especially scratch resistance under high load conditions. The high content of alloying elements in the matrix can provide excellent corrosion resistance 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 decrease is not obvious. 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 close to the original hardness. That is, when the base material is subjected to post-weld heat treatment, the performance of the sealing surface will not be lost. Power station valves are welded with a cobalt-based alloy in the middle hole of the valve body by electric welding to process the seat sealing surface. Because the sealing surface is in the deep position of the middle hole of the valve body, slag inclusion and cracks are easy to occur during surfacing. defect. According to the needs, the process test of deep hole surfacing (Deep Hole Overlay Welding Process)  stellite 6 is carried out by processing the test piece method. During the process test, the reasons for the defects were found out.

1. Surface contamination of welding consumables.

2. Welding consumables absorb moisture.

3. There are many impurities and oil stains in the base metal and filler metal

4. Welding part of the 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 cobalt-based alloy surfacing of the power station valve body is the rigidity of the valve body. In the welding process, the arc forms a molten pool, which continuously melts and heats to the welding part, and the temperature drops rapidly after welding, and the molten metal solidifies to form a weld.

If the preheating temperature is low, the temperature of the solder layer must drop quickly. In the case of rapid cooling of the welding layer, the shrinkage rate of the welding layer is faster than the shrinkage rate of the valve body. Under the action of this stress, the welding layer and the base metal will quickly form an internal tensile stress, which will tear the welding layer. Sharp corners should be strictly prohibited when processing welding parts. The preheating temperature is too low, and the heat is quickly dissipated during the welding process. If the interlayer temperature is too low and the welding layer cooling rate is too fast, it is very unfavorable for the surfacing material. The cobalt-based alloy of welding material itself has high red hardness. When working at 500～700℃, the hardness can maintain 300～500hb, but its toughness is low, the crack resistance is poor, and it is easy to form crystal cracks or cold cracks, so before welding Must be preheated. The preheating temperature depends on the size of the workpiece, and the general preheating range is 350-500℃.

Electrode coating must be kept intact before welding to avoid moisture absorption. Bake at 150℃ for 1h before welding, then put it into the welding rod insulation cylinder. For deep hole surfacing welding (Deep Hole Overlay Welding Process) , the arc r angle of the groove should be as large as possible under the premise of the technological permission, generally r≥3mm. dn10～25mm small diameter valve body can be fully welded with welding rod aFt the bottom of deep hole, and the interlayer temperature must be ≥250*(2, the arc is closed in the middle, and the welding rod should be slowly lifted when the arc is closed. The workpiece is welded to the furnace (250℃) ) Heat to 350 + 20 ℃, heat preservation for 1.5 hours, and then apply welding, remove the welding slag after each layer is welded. At the same time, control the inter-layer temperature ≥ 250c, and remove the welding slag after the surfacing is completed.

After the valve body is welded, it must be kept in the furnace (450℃) immediately. After the batch or this furnace is finished, the temperature will be heated to 710±20℃ and tempered, and the temperature will be cooled with the furnace after 2h. When the furnace temperature dn≥32mm, the valve body should be overlaid The groove is processed into a U shape to solve the problem of uneven shrinkage caused by excessive rigidity during surfacing of cobalt-based alloys. Before the surfacing operation, clean the work piece, put the work piece into the furnace (furnace temperature is 250 ℃), heat it to 450-500 ℃, keep it for 2 hours, and put it out of the furnace for welding. Firstly use cobalt-based alloy electrode to surfacing the sealing surface, and remove the welding slag after each layer is welded. At the same time, the temperature between the layers must be controlled to be ≥250℃, and the slag should be removed after the surfacing is completed. Then use austenitic stainless steel electrode (stainless steel electrode with high Cr and Ni content) to weld the U-shaped groove completely. After the welding of the valve body is completed, immediately enter the furnace (450℃) for heat preservation. After the welding of this batch or this furnace is completed, it will be heated to 720±20℃ for tempering. The heating rate is 150℃/h, and the temperature is kept for 2h and then cooled with the furnace.

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