The principle of electroplating process of gate valve is discussed

The principle of electroplating process of gate valve is discussed

The main cause of cracking of power plant valve bodies in cobalt base alloy spray welding is usually high valve stiffness. In the welding operation, the arc generates a solubilization pool, which continues to melt and warm the welding position, and the temperature drops rapidly after welding, and the molten metal condenses to produce welding. If the heating temperature is low, the welding layer temperature must be reduced rapidly. Under the premise 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 such stress, the welding layer and the original material quickly form an internal tensile stress, and the welding layer cracks. The working condition of the power station valve is generally 540℃ high temperature steam, so the main material of the gate valve is 25 or 12crmov, valve body.. The working condition of power station valve is generally 540℃ high temperature steam, so the main material of gate valve is 25 or 12crmov, and the raw material of valve body spray welding is cobalt-base alloy d802(sti6) welding wire.
d802 matches edcocr -A in gb984 specification, which is equivalent to ercocr -A in aws.
d802 raw materials can be continuously opened and closed from ultra-high pressure and high temperature work, with excellent wear resistance, impact resistance, oxidation resistance, corrosion resistance and cavitation resistance.
The weld metal of ErCoCr-A electrode and filler wire cladding in Aws specification is characterized by a subeutectic mechanism consisting of about 13% chromium cementite eutectic network distributed in the Cochromium-tungsten ion crystal substrate. The result is a perfect blend of the raw material’s resistance to low stress damage and the toughness necessary to resist the impact of certain types of process flow.
Cobalt alloy has good resistance to metal – metal wear, especially scratch resistance under high load.
The strong alloy composition in the substrate can provide better corrosion resistance and oxidation resistance.
When the molten metal of cobalt-based alloy is in the warm state (within 650℃), its strength does not decrease significantly. Only when the temperature rises above 650℃, its strength will decrease significantly. When the temperature returns to the normal temperature state, its strength will return to the initial hardness.
In fact, when the original material carries out post-welding heat treatment, the surface performance is not easy to damage. The valve of the power station should be sprayed with cobalt-based alloy at the middle hole of the valve body to make the high pressure gate valve face by arc welding. Because the face is in the deep part of the middle hole of the valve body, the spray welding is most likely to cause defects such as welding noddle and crack.
The process test of shallow hole spray welding d802 was carried out by producing and processing samples as required. The reason of easy deviation is found out in the process test link.
① Welding material surface environmental pollution.
② Welding materials absorb moisture.
③ The original material and filler metal contain more impurities and oil stains.
④ The welding position stiffness of valve body is large by electric welding (especially dn32 ~ 50mm).
(5) The technological standard of heating and post-welding heat treatment is unreasonable.
The welding process is not reasonable.
⑦ welding material selection is unreasonable. The main cause of cracking of power plant valve bodies in cobalt base alloy spray welding is usually high valve stiffness. In the welding operation, the arc generates a solubilization pool, which continues to melt and warm the welding position, and the temperature drops rapidly after welding, and the molten metal condenses to produce welding. If the heating temperature is low, the welding layer temperature must be reduced rapidly. Under the premise 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 such stress, the welding layer and the original material quickly form an internal tensile stress, and the welding layer cracks. Bevel angles should be prohibited when producing welding positions.
The heating temperature is too low, and the heat is released quickly during welding operation.
The solid layer temperature is too low, the welding layer refrigeration speed is too fast for the spray welding raw materials.
The welding material cobalt base alloy itself has a high red hardness, when working at 500 ~ 700℃, the strength can maintain 300 ~ 500hb, but its ductility is low, crack resistance is weak, easy to produce crystal cracks or cold cracks, so it needs to be heated before welding.
The heating temperature depends on the size of the workpiece, and the general heating range is 350-500℃.
Welding electrode coating should be kept intact before welding to prevent moisture absorption.
During welding, the cake is baked at 150℃ for 1h and then put into the welding wire insulation cylinder.
The arc r Angle of shallow hole spray welding weld should be as large as possible, generally r≥3mm, if the process permits.
dn10 ~ 25mm caliber valve body can be welded through from the bottom of the shallow hole with welding wire, to ensure that the solid layer temperature ≥250*(2, in the middle of the arc, arc to slow speed mentioned welding wire.
The product workpiece was heated in the furnace (250℃) to 350 10 20℃ before welding. After 1.5h of heat insulation, the welding was carried out.
At the same time control the solid layer temperature ≥250c, spray welding all the end of the welding scar. After welding, the valve body must be immediately put into the furnace (450℃) for heat insulation and insulation. When the temperature of the batch or the welding temperature of the furnace is quenched to 710±20℃, the heat insulation and insulation are held for 2h and then refrigerated with the furnace. When the temperature control dn is greater than 32mm, the valve body should be welded into a u shape first to solve the problem of uneven elasticity caused by too much stiffness after spraying welding of cobalt-based alloy. Before the spray welding operation, the product workpiece is cleaned, the product workpiece is put into the furnace (temperature control is 250℃), heated to 450 ~ 500℃, heat insulation and hold for 2 hours, and the welding is announced.
First, spray weld the surface with cobalt-based alloy welding wire, and finish the scar welding of each layer. At the same time, control the temperature between layers ≥250℃, and spray weld the scar after all the end.
Then replace the martensitic stainless steel wire (high cr, ni relative content stainless steel wire) to weld the U-shaped weld. After the electric welding of the valve body is completed, it will be put into the furnace immediately (450℃) for heat insulation and heat preservation. After the completion of the electric welding of this batch or furnace, the temperature will be raised to 720±20℃ for quenching.
The heating rate is 150℃/h, and the heat insulation is kept for 2 hours.
Electroplating tank contains two electrical levels, the general product workpiece as the cathode, switching power access after the construction of electrostatic field between the two aspects, under the influence of electrostatic field metal ions or thiocyanogen root to the cathode transfer, and near the cathode surface to produce the so-called double layer, In this case, the ion concentration around the cathode is smaller than that in the region avoiding the cathode, which may lead to long-distance ion transfer.
Metal positive ions or thiocyanogen released by the release of complex ions, according to the double layer and arrive at the cathode surface to generate oxidation reaction to form metal molecules.
Electroplating process electroplating history is relatively early, the surface treatment process at the beginning of the research and development is mainly to meet people’s corrosion prevention and ornament must.
In recent years, with the development of industrialization and science and technology, the continuous development of new production processes, especially the emergence of some new coating materials and composite plating technology has greatly expanded the application field of the surface treatment process, and made it become an indispensable part of the surface engineering design.
Electroplating process is one of the metal electrodeposition technologies. It is a process of obtaining metal alluvium on solid surface by electrolysis. Its purpose is to change the surface characteristics of solid raw materials, improve the appearance, improve the corrosion resistance, wear resistance and friction resistance, or prepare metal cladding with special composition characteristics. Give unique electrical, magnetic, optical, thermal and other surface characteristics and other process properties.
Generally speaking, The process of metal electrodeposition on the cathode is composed of the following processes 1) The heat transfer process of the pre-plated positive ions or their thiocyanogen roots in the lithium battery electrolyte to the cathode (product work piece) surface or the surface of the transfer due to the concentration difference 2) the surface conversion process of the metal positive ions or their thiocyanogen roots on the surface of the electric level and in the liquid layer near the surface of the oxidation reaction process , such as the conversion of thiocyanogen ligand or the reduction of coordination number 3) photocatalytic process metal ions or thiocyanogen on the cathode to obtain electrons, into metal molecules 4) new phase formation process that is to form a new phase, such as the formation of metal or aluminum alloy. Electroplating tank contains 2 electrical levels, general product workpiece as the cathode, switching power supply access after the construction of an electrostatic field between the two aspects, under the influence of electrostatic field metal ions or thiocyanogen root to the cathode transfer, and near the cathode surface to produce the so-called double layer, then the cathode surrounding ion concentration is less than the ion concentration in the area to avoid the cathode, It could lead to long-distance transfer of ions.
Metal positive ions or thiocyanogen released by the release of complex ions, according to the double layer and arrive at the cathode surface to generate oxidation reaction to form metal molecules.
The difficulty of charge and discharge of positive ions at each point on the cathode surface is not the same. At the node and acute Angle of the crystal, the current intensity and electrostatic action are much larger than other positions of the crystal. At the same time, the molecular unsaturated fat located at the crystal node and acute Angle has a higher adsorption capacity. And here the charge and discharge at this site form the lattice constant of molecules into the metal. The preferred charging and discharge site of this positive ion is the eye of the coated metal crystal.
As the eyes expand along the crystal, a layer of monatomic growth is formed connected by an external economic ladder. Because the lattice constant surface of the cathode metal contains a ground stress broadened by lattice constant forces, the atoms gradually attached to the cathode surface occupy only the part that is continuous with the molecular structure of the substrate metal (cathode), regardless of the difference in lattice constant geometry and specifications between the substrate metal and the coating metal. If the molecular structure of the coating metal is too different from that of the substrate, the growth crystallization will be the same as the molecular structure of the foundation, and then gradually change to its own relatively stable molecular structure. The molecular structure of electroalluvium depends on the crystallographic characteristics of the accumulated metal itself, and the organizational structure depends on the preconditions of the electrocrystallization process to a certain extent. The compactness of the alluvium depends entirely on the ion concentration, exchange current and surface surfactant, and the crystal size of the electrocrystal depends largely on the surface surfactant concentration.
Two, single metal plating process single metal plating refers to the plating solution with only a kind of metal ions, after plating to form a single metal coating method.
Common single metal plating processes mainly include hot dip galvanizing, copper plating, nickel plating, stainless steel plating, tin plating and tin plating, etc., which can not only be used as steel parts and other anti-corrosion, but also have the function of decoration design and improve the characteristics of malleability.
The standard electrode potential of zinc is -0.76v. For steel substrate, zinc coating is a subanodic oxidation coating, which is mainly used to avoid corrosion of steel. Electrogalvanizing process is divided into two categories: physical hot dip galvanizing and hot dip galvanizing without cyanide.
Physical hot dip galvanizing is characterized by good plating function in aqueous solution, smooth and delicate coating, wide use, plating solution is divided into micro cyanide, low cyanide, medium cyanide and high cyanide several classes.
But because the substance is toxic, in recent years has tended to choose micro cyanide and no cyanide plating solution.
Cyanide-free plating solution includes acid zinc phosphate plating solution, salt plating solution, potassium thiocyanate plating solution and hingeless fluoride plating solution.
1. Partial alkali hot dip galvanizing coating crystal fine, good gloss, plating solution level and deep plating ability are good, allow the use of current intensity and temperature range is wide, small corrosion on the system.
It is suitable for parts with complicated electroplating process and coating thickness above 120μm, but the current strength of plating solution is relatively low and toxic.
The following aspects should be paid attention to in the plating solution configuration and plating process: 1} strictly control the concentration of each component in the plating solution.
The concentration value of each component of high cyanide hot-dip galvanized water solution (moll/L} should be maintained as :2) pay attention to the solution in the bath, sodium hydroxide and gas related components.
When the sulfide composition exceeds 50~100g/L, the conductivity of the plating solution is reduced, and the anodic oxidation passivation treatment must be used in freezing method (the refrigeration temperature is -5℃, the duration is above 8h, the potassium carbonate concentration value is reduced to 30~40g/L). Or ion exchange method (adding sodium carbonate or barium hydroxide deposition in the plating solution) to be treated. 3) anodic oxidation application of cold rolled steel plate (zinc content of 99.97%) should pay attention to the anodic oxidation sleeve, to avoid the anode mud floating in the plating solution, so that the coating is not smooth.
4) The sensitivity of physical hot-dip galvanized solution to residue is relatively small, and its allowable content is: copper 0.075 — 0.2g/L, lead 0.02 — 0.04g/L,0.05 — 0.15g/L, tin 0.05 — 0.1g/L, chromium 0.015 — 0.025g/L, Impurities in iron 0.15g/L· plating solution can be solved in the following ways: Add 12.5-3g /L sodium sulfide, so that it can form sulfide precipitate with iron and lead and other key metal positive ions to remove: Add A little zinc powder, so that copper and lead can be replaced at the bottom of the tank to remove: can also plug solution, cathode current strength is 0.1-0.2 A/cm2.
2 partial alkali zinc phosphate hot dip galvanized partial alkali zinc acid th hot dip galvanized bath composition is simple, convenient to use, fine and bright coating, coating is not easy to fade, small corrosion of the system, sewage treatment is also very easy.
But the plating solution of homogeneous plating level and deep plating ability than the plating solution is poor, the current intensity is low (70%~80%), coating over a certain thickness ductility improvement.