Oxygen-acetylene flame spray welding

       Spray welding is to reheat the preheated self-dissolving alloy powder coating to 1000-1300 ℃, so that the particles are melted, slag is formed and floated to the surface of the coating, and the generated borides and silicides are dispersed in the coating, so that the particles Good bond with the substrate surface. The final deposit is a dense metal crystalline structure and forms a metallurgical bonding layer of 0.05-0.1 mm with the matrix. Its bonding strength is about 400 MPa, and it has good impact resistance, wear resistance and corrosion resistance, and has a mirror-like appearance.



advantage

Compared with the spray coating, the advantages of the spray coating are significant. However, due to the local heating of the matrix during the remelting process, the temperature reaches 900 °C, which will cause a large thermal deformation. Therefore, the scope of application of spray welding has certain limitations. The parts and materials suitable for spray welding are generally: ①Consumable parts subject to impact load, requiring high surface hardness and good wear resistance, such as sandblaster blades, crusher tooth plates, excavator bucket teeth, etc.; ②Geometry Large-scale vulnerable parts with relatively simple shapes, such as shafts, plungers, sliders, hydraulic cylinders, chute plates, etc.; ③ Low carbon steel, medium carbon steel (with carbon content below 0.4%), manganese, molybdenum, and vanadium total < 3% structural steel, nickel-chromium stainless steel, cast iron and other materials.



Self-fluxing alloy powder for spray welding



The self-fluxing alloy powder is an alloy with nickel, cobalt and iron as the base material, in which an appropriate amount of boron and silicon elements are added to act as a deoxidizing and slag-forming welding flux, and at the same time, it can reduce the melting point of the alloy, and is suitable for oxyacetylene flame. Remelting.



There are many varieties of domestic self-fluxing alloy powders, nickel-based alloy powders have strong corrosion resistance, oxidation resistance can reach 650 ° C, and strong wear resistance; the biggest feature of cobalt-based alloy powders is good red hardness, which can be used in 700 ℃ maintains good wear resistance and corrosion resistance; iron-based alloy powder wear resistance is better than the other two types.



Spray welding process



The process procedure of spray welding is basically the same as that of spraying, the difference is that a remelting procedure is added in the powder spraying process. There are one-step spray welding and two-step spray welding. Before construction, attention should be paid to: ①The surface of the workpiece has a carburized layer or a nitrided layer, which must be removed during pretreatment; ②The preheating temperature of the workpiece is 200-300 °C for general carbon steel and 350-400 °C for heat-resistant austenitic steel. Use neutral or weak carbon flames for preheating flames. In addition, after the spray coating is remelted, the thickness is reduced by about 25%, and this amount should be taken into account when measuring in a hot state after spray melting.



One-step spray welding. One-step method is to spray a section and then melt a section, spray and melt alternately, and use the same spray gun to complete. Optional medium and small spray guns. After the workpiece is preheated, spray a 0.2mm protective layer first, and seal the surface tightly to prevent oxidation. The spray melting starts from one end, with a spray distance of 10-30mm, and the protective layer is locally heated in order until it melts and starts to wet (it cannot flow). ), then spray powder, and repeat with melting, until the predetermined thickness is reached, the surface appears "mirror" reflection, and then expands forward until the surface is completely covered with the spray welding layer. If the thickness is insufficient at one time, it can be thickened repeatedly. The one-step method is suitable for spray welding of small parts or small areas.



Two-step spray welding method. The two-step method is to complete the spray coating first and then remelt it. Both spraying and remelting use high-power spray guns, such as SpH-E spraying and welding dual-purpose guns, so that the alloy powder is fully melted in the flame, and a plastically deformed deposit layer is produced on the surface of the workpiece. Use a weak carbon flame when spraying iron-based powders, and use a neutral or weak carbon flame when spraying nickel-based and cobalt-based powders.



The thickness of each layer of powder spraying is less than 0.2mm, and the thickness of remelting can be achieved by repeated spraying. Generally, it can be remelted at 0.5-0.6 mm. If the spray welding layer is required to be thick, and one remelting cannot meet the requirements, it can be sprayed and remelted several times.



Remelting is a key step in the two-step process and takes place immediately after spraying. Use a high-power soft flame of neutral flame or weak carbonizing flame, the spray distance is about 20 ~ 30mm, the angle between the flame and the surface is 60° ~ 75°, starting from about 30mm away from the coating, properly grasp the remelting speed, and apply the coating. The layer is heated until the coating has a "mirror" reflection, and then the next part is remelted.



Remelting should prevent over-melting (ie, mirror cracking), flow of coated metal, or excessive local heating to oxidize the surface. When multi-layer remelting, the former layer is cooled to about 700 ℃, and the surface slag is removed, and then the secondary spray melting is performed. Remelting should not exceed 3 times.



cooling of the workpiece. Workpieces of medium-low carbon steel, low-alloy steel, thin welding layers, and simple-shaped cast iron are naturally cooled in air. For iron castings with thick welding layers and complex shapes, alloy steel parts with large manganese, copper and vanadium content, and parts with high chilling properties, they should be buried in a lime pit to cool slowly.



the difference

The difference between spraying and spray welding



The bonding of the sprayed layer and the sprayed-welded layer with the base metal is different. When the nickel-clad aluminum is heated by the spraying flame beam, an exothermic chemical reaction occurs, and a micro-metallurgical bond is formed on the surface of the carbon steel with sand blasting and rust removal to Sa3 level and RZ>50μm. The bottom layer and the working layer produce a mechanical bonding coating with "anchor hook" effect, while the bonding between the spray welding layer and the substrate is purely a metallurgical bonding coating.



Different spraying materials, spray welding requires the use of self-fluxing alloy powder, while spraying does not require high self-fluxing properties of the powder, and it is not necessarily a self-fluxing alloy powder. Various self-fluxing alloy powders can be used for both spray welding and It can be used for spraying, but the spraying powder does not have self-fluxing and can only be used for spraying and not for spray welding.



The heating conditions of the workpiece are different. In the process of spraying and spray welding, the preheating temperature before spraying is different, the workpiece is affected by heat differently, and the structure and performance of the workpiece after spraying are also different.



The densities of the coatings are different, the sprayed layer is dense, while the sprayed layer has a few pores.



The ability to bear the load is different. Generally, the spray coating can withstand large-area contact, and it is mostly used on the working surface with lubricated conditions, the mating surface and other working conditions with less force, but the spray coating can withstand greater impact force. , extrusion stress or contact stress, etc.



The spray welding process should be used in the following situations



(1) The surface load of various carbon steel and low alloy steel workpieces is large, especially the impact load. For workpieces with a bonding strength of 350-450N/mm2 between the coating and the substrate, the spray welding hardness is HRC150≤65, and the coating thickness is from 0.3 to After grinding, the surface roughness of the spray-welded layer can reach Ra0.4-0.1μm or more.



(2) For use in corrosive media, the coating is required to be dense and free of pores.



(3) The original design of the surface of the workpiece adopts processes such as quenching, carburizing, nitriding, and hard chrome plating, which requires high hardness on the surface.



⑷The working environment of the workpiece is harsh, such as strong abrasive wear, erosion wear, cavitation and so on.



⑸ Oxygen-acetylene alloy powder spray welding process is suitable for surface strengthening or repairing of various carbon steel and low alloy steel parts, but some characteristics of parts material should be noted. When the difference in expansion coefficient is larger than 12×10-6/℃ and greater than 12×10-6/℃, this process should be used with caution to avoid cracks. When the content of molybdenum and molybdenum is more than 3%, the total content of aluminum, magnesium, cobalt, titanium, molybdenum and other elements is more than 0.5% or the sulfur content in the steel is large, it will also bring difficulties to spray welding, because these materials interact with oxygen. It is easy to form a dense and stable oxide film, which blocks the wetting effect of the molten alloy on the substrate. During remelting, the liquid alloy will roll down like a "sweat bead". Therefore, when using the spray welding process, you should pay attention to this. The adaptability of the process to the substrate material being sprayed.



⑹ Metal materials that can be spray welded without special treatment:



① Carbon structural steel with carbon content ≤ 0.25%.



② Alloy structural steel with total content of Mn, Mo, V, Cr, and Ni < 3%.



③ 18 - 8 stainless steel, nickel stainless steel, gray cast steel, malleable cast iron, ductile iron, low carbon pure iron, red copper.



⑺ Metal materials that need to be preheated at 250-375 ℃ and need to be cooled slowly after spray welding.



① Carbon structural steel with carbon content > 0.4%;



② Alloy structural steel with total content of manganese, molybdenum, vanadium and nickel >3%;



③ Alloy structural steel with chromium content ≤ 2%;



⑻ Metal materials that need isothermal annealing after spray welding:



① Martensitic stainless steel with chromium content ≥ 13%;



② Nickel-molybdenum alloy structural steel with carbon content ≥ 0.4%.



After the spray welding process is determined, the one-step or two-step spray welding process is selected according to the following conditions:



(1) The workpiece needs to be partially repaired, and the heat input at the spray welding place is not allowed to be very large. For example, the repair of local scars on the guide rails of various machine tools should use a one-step spray welding process;



(2) If the surface of the workpiece is complex or irregular, such as sprocket, gear tooth surface, screw feeder, etc., one-step spray welding process should be used;



(3) It is difficult to heat large workpieces as a whole, such as locomotives, mine wheels, etc., and one-step spray welding process should be used;



⑷ The general shaft parts that can be rotated on the machine tool should use the two-step spray welding process;



⑸ The hardness of the obtained coating should be as close as possible to the surface hardness of the original design. For example, if the original design adopts the quenching or chemical treatment process to make the surface hardness reach about HRC ≥ 55, the so-called "hard coating" powder should be used, such as Ni15, Ni60, Fe65 or Wc composite powder;



⑹ Non-matching surfaces with strong wear, such as the impeller, shell, rock loader shovel teeth of sediment pumps, helical surfaces of screw feeders, etc., should use high hardness such as Ni15, Ni60, Fe65 or Wc composite powder;



⑺ For workpieces that need to be processed, but cannot be used on lathes and grinders, and can only be processed by hand with tools such as files, such as the repair of local scars on the guide rail surface of the machine tool, only low-hardness spray powders such as SH?F103, Ni15, etc. can be used;



⑻ The difference between spray welding process and arc surfacing welding: the combination between the spray welding layer and the substrate is a dissolution-diffusion metallurgical bond, while the surfacing is a fusion metallurgical bond. During the spray welding process, the substrate does not melt, but the spray welding layer There is a dissolution effect between the substrate and the substrate, and there is a diffusion and mutual dissolution zone between the two. Due to the insoluble matter in the substrate, the spray-welded layer will not be diluted by the substrate material, so the dilution rate is extremely low, which can ensure the good performance of the spray-welded layer. , The surfacing welding matrix is melted, and the dilution rate of the surfacing layer is high. It is possible to ensure the performance of the welding layer if the surfacing welding is very thick, and the edges and corners of the parts are difficult to guarantee. Undercuts and edge collapse are common, but spray welding does not occur. defect.



Characteristics of spray welding of cast iron parts



(1) Cast iron is an iron-carbon alloy with a carbon content of more than 2%. Cast iron commonly used in industry has a carbon content of 2.5 to 4%, a silicon content of 1 to 3%, and a small amount of manganese, sulfur, phosphorus, etc. Among them, gray cast iron is the most used. Due to the high content of cast iron, low strength, sensitivity to temperature changes, and local heating during welding repair, the temperature difference is large, and the cooling speed is fast, which brings difficulties to the welding repair of cast iron. The weldability of cast iron is poor, and the following problems are prone to occur during welding repair:



(2) The white mouth structure is easy to appear in the welding repair part. The white mouth structure is hard and brittle, and it is difficult to perform mechanical processing after welding, and it is easy to cause cracks;



(3) Cracks are easy to occur, and thermal stress cracks are prone to occur due to uneven heating and cooling rates during welding repairs; in addition, there are many impurities such as sulfur and phosphorus in cast iron, which are also prone to cracks at welding repairs;



⑷ Pores and slag inclusions, because of the high carbon content in cast iron and many impurities, and due to the fast cooling rate during the welding repair process, the gas and some oxides have no time to precipitate and float, so pores or cavities are formed in the weld area. slag, the use of oxygen-acetylene flame alloy powder one-step spray welding, can satisfactorily solve the above problems;



⑸ The melting point of the alloy powder used in spray welding is lower than the melting point of the matrix. During remelting, the cast iron matrix does not melt, there is no dilution problem of the spray welding layer, and there is no semi-melting area, so the correct spray welding will not make the welding repair area. The white mouth structure is produced, which is easy to process, and because the matrix does not melt, it naturally controls the impurities such as sulfur and phosphorus contained in the matrix to melt into the spray welding layer, which is beneficial to prevent the occurrence of cracks;



⑹ One-step spray welding has less heat input to the substrate, and the substrate is less affected by heat, which is conducive to reducing thermal stress, thereby effectively controlling thermal stress cracks; less heat input, local spray welding repairs for parts with high dimensional accuracy It is unique. At the same time, the use of oxygen-acetylene flame heating is slower than electric welding, which is also beneficial to prevent cracks and deformation;



⑺ The alloy powder contains strong deoxidizing elements boron and silicon, which not only protects other elements in the powder from oxidative burning, but also reduces the oxides on the surface of the substrate by boron and silicon elements, preventing pores and slag inclusions;



⑻ The structure of the spray-welded layer is compact and flat, the forming is good, and there is no undercut phenomenon. It can be used with only a small amount of processing, material saving and high efficiency; the commonly used spray-welded cast iron parts are SH F103, nickel-based alloy powder Ni15, etc., iron-based The alloy powder has a high melting point, a large brittleness, and a large influence on the matrix, and the effect is poor.



⑼ The main purpose of preheating the workpiece during spray welding is to remove the moisture on the surface of the workpiece, and to generate a certain thermal expansion to reduce the temperature difference, thereby reducing thermal stress, which is conducive to improving the bonding strength of the coating and ensuring the quality of the spray welding layer. Generally, the steel temperature is 250-300 ℃ , 450-500 ℃ for austenitic stainless steel, 350-400 ℃ for nickel-chromium stainless steel, 250-300 ℃ for low alloy steel and cast iron, generally small workpieces and easily oxidized steel preheating temperature should be lower; spray welding layer The thickness is based on the thermal expansion and contraction characteristics of the workpiece after spraying, and the shrinkage after remelting is about 25-30%. Therefore, when determining the thickness of the sprayed layer, in addition to considering the machining allowance and the diameter of the workpiece before spraying, the shrinkage must be reduced. Taking into account the thickness of the spray layer, the thickness of the spray layer before remelting = (the thickness of the spray layer + machining allowance) ÷ (1-0.3);



The spray welding process cannot be used in the following cases



(1) Materials below the melting point of alloys, such as aluminum and its alloys, magnesium and its alloys, brass and bronze;



(2) The workpiece is a slender shaft or a very thin plate that is not allowed to deform;



(3) The original design of the workpiece has high requirements, and the metallographic structure is not allowed to have any changes;



⑷ Nickel-chromium-molybdenum alloy steel with high hardness;



⑸ Martensitic high chromium steel with chromium content > 18%.



(PS: The outer diameter of the long shaft is more than 28 mm, and the length within 8 meters can be used for high-precision spray welding)



The main points of spray welding of iron castings



①In vehicles and mechanical equipment, there are quite a lot of parts made of cast iron, and various problems will inevitably occur in the process of manufacturing or use. The use of oxygen-acetylene flame spray welding is not only an effective method for strengthening cast iron parts, but also for repairing Ideal means for various defects and damages of castings (such as blisters, pores or wear and other damages in use);



② Spray welding is mostly used for repairing local defects of iron castings, and the defects vary in size and depth, so one-step spray welding is suitable. The heat input is generally selected as QH-1/h, QH-2/h, QH-4/h, etc.;



③ During spray welding, due to the poor weldability of cast iron parts, nickel-based alloy powder (Ni-B-Si series) should be preferred. , The nickel-based powder spray welding layer with low carbon content has low hardness and good plasticity, which can relax the spray welding stress and help prevent cracks. At this time, it is extremely important for those who are not skilled in operation;



④ When selecting the spray welding specification, factors such as cast iron material, defect size, and working conditions should be considered. On the premise of ensuring the necessary flame energy, the heat input to the substrate should be minimized, and the lower limit of oxygen and acetylene gas pressure should be taken. , When spraying and remelting, adjust the spraying and melting distance appropriately and control the heat input;



⑤Welding to repair local small defects, such as pores and sand holes, can not be repaired before spray welding.