1. The characteristic of carbide
Cemented carbide is a high-productivity tool material with excellent properties such as high hardness, high strength, wear resistance and good red hardness. Used to make knives, molds, measuring tools, mining tools and various parts with wear resistance as the main performance, and are widely used in mechanical processing, geological prospecting, mining and other industrial fields.
Cemented carbide is expensive and has poor toughness, making it difficult to produce products with larger sizes and complex shapes. Moreover, many parts do not need to be made of cemented carbide as a whole when in use, so it is of important use value to connect cemented carbide with steel with good toughness, high strength, excellent processing performance, and low cost.
The brazing method is currently the main welding method for cemented carbide and steel. In the past ten years, some new welding methods such as sintered diffusion welding, tungsten inert gas shielded welding, laser welding, etc. are also being actively researched and explored. It may be widely used in the welding of cemented carbide.
2. Brazing
Ⅰ The brazing property of cemented carbide is poor. This is because the carbon content of cemented carbide is relatively high, and the uncleaned surface often contains more free carbon, which hinders the wetting of the solder. In addition, cemented carbide is easy to oxidize at the brazing temperature to form an oxide film, which will also affect the wetting of the brazing filler metal. Therefore, the surface cleaning before brazing is very important to improve the wettability of the solder on the cemented carbide. If necessary, measures such as copper plating or nickel plating on the surface can also be taken.
Ⅱ Another problem in hard alloy brazing is that the joints are prone to cracks. This is because its coefficient of linear expansion is only half of that of low-carbon steel. When the cemented carbide is brazed with the base of this type of steel, it will generate a lot of thermal stress in the joint, which will cause the joint to crack. Therefore, when the cemented carbide is brazed with different materials, measures should be taken to prevent cracking.
3. Brazing materials
Cemented carbide usually uses pure copper, copper-zinc and silver-copper solder.
ⅰPure copper: Pure copper has good wettability to various cemented carbides, but it needs to be brazed in a reducing atmosphere of hydrogen to get the best results. At the same time, due to the high brazing temperature, the stress in the joint is larger, which leads to an increase in the tendency of cracks. The shear strength of the joint brazed with pure copper is about 150MPa, and the joint plasticity is also high, but it is not suitable for high temperature work.
ⅱ Copper-zinc solder: Copper-zinc solder is the most commonly used solder for cemented carbide. In order to improve the wettability of the solder and the strength of the joint, alloy elements such as Mn, Ni, Fe, etc. are often added to the solder. For example, W (Mn) 4% is added to B-Cu58ZnMn to make the shear strength of hard alloy brazed joints reach 300-320MPa at room temperature: 220-240MPa can be maintained at 320°C. Adding a small amount of Co on the basis of B-Cu58ZnMn can make the shear strength of the brazed joint reach 350MPa, and has higher impact toughness and fatigue strength, which significantly improves the service life of cutters and rock drilling tools
ⅲSilver-copper solder: The melting point of silver-copper solder is low, and the thermal stress generated by the brazing joint is small, which is beneficial to reduce the cracking tendency of hard alloy brazing. In order to improve the wettability of the solder and increase the strength and working temperature of the joint, alloy elements such as Mn and Ni are often added to the solder. For example, B-Ag50CuZnCdNi solder has excellent wettability to cemented carbide, and the brazing joint has good overall performance.
4. Preparation before welding
① Before welding, check whether the cemented carbide has cracks, bends, or unevenness defects. The brazing surface must be flat. If it is a spherical or rectangular hard alloy brazing surface, it should also conform to a certain geometric shape to ensure good contact between the alloy and the substrate to ensure the brazing quality.
② Sandblasting the cemented carbide. If there is no sandblasting equipment, you can hold the cemented carbide in your hand and grind off the oxide layer and black brand letters on the brazing surface on the rotating green silicon carbide grinding wheel. If the oxide layer on the brazing surface of the cemented carbide is not removed, the brazing filler metal will not easily wet the cemented carbide. Experience has proved that if there is an oxide layer or black brand letters on the brazing surface, it should be sandblasted. Otherwise, the brazing filler metal will not easily wet the cemented carbide, and there will still be obvious black letters in the brazing seam, which will reduce the brazing area. Desoldering phenomenon.
③ When cleaning the hard alloy brazing surface, it is best not to use chemical mechanical grinding or electrolytic grinding, etc., because they all rely on the bonding agent (cobalt) that corrodes the surface layer of the hard alloy to speed up the grinding or improve the grinding. It is efficient, and after the cobalt on the surface of the cemented carbide is corroded, it is difficult for the solder to wet the cemented carbide again, which is likely to cause desoldering. Under special circumstances, when the cemented carbide brazing surface must be treated by the above method or wire EDM, the treated cemented carbide can be sandblasted or the surface layer can be ground with a silicon carbide grinding wheel. The cemented carbide after sandblasting can be cleaned with gasoline or alcohol to remove oil stains.
④ Check carefully whether the trough shape on the steel substrate is reasonable before brazing, especially for the cemented carbide workpieces of easily cracked grades and the cemented carbide workpieces with large brazing surfaces, strict requirements should be imposed. The knife groove is also sandblasted and cleaned to remove oil stains. When the cleaning volume is large, the alkaline solution can be used to boil for 10 to 15 minutes. For high-frequency or copper-dip brazing multi-blade tools and complex measuring tools, it is best to boil them with saturated borax aqueous solution for 20-30 minutes, take them out and dry them before welding.
⑤ Wipe the brazing filler metal with alcohol or gasoline before use, and cut it into shape according to the brazing surface. When brazing general carbide tools or molds, the thickness of the brazing filler metal is about 0.4-0.5mm, and the size is similar to the brazing surface. When heating with a coke oven, the brazing filler metal can be increased appropriately. When brazing hard alloy multi-blade tools, measuring tools and other workpieces, the area of the brazing sheet should be reduced as much as possible. Generally, the brazing sheet can be cut to about 1/2 of the brazing surface. The brazing material sheet is reduced to 1/3 or less of the brazing surface. Reducing the solder can make the appearance of the workpiece after welding more beautiful and more convenient for sharpening.
5. Flux and shielding gas
Flux: The choice of flux should match the base material to be welded and the selected brazing filler metal. When brazing tool steel and cemented carbide, the main flux used is borax and boric acid, and some fluorides (KF, NaF, CaF2, etc.) are added. Copper-zinc solder is matched with FB301, FB302 and FBl05 solders, and silver-copper solder is matched with FBl01~FBl04 solders. When brazing high-speed steel with special brazing filler metal, borax flux is mainly used.
Shielding gas: In order to prevent the oxidation of tool steel during the brazing heating process and to avoid cleaning after brazing, gas shielded brazing can be used. The protective gas can be an inert gas or a reducing gas. The dew point of the gas should be lower than -40°C. Cemented carbide can be brazed under the protection of hydrogen, and the required hydrogen dew point should be lower than -59°C.
6. The brazing process
The surface of cemented carbide should be sandblasted before brazing, or polished with silicon carbide or diamond grinding wheels to remove excessive carbon on the surface so that it can be wetted by the brazing filler metal during brazing. Cemented carbide containing titanium carbide is more difficult to wet. Coating copper oxide or nickel oxide paste on its surface and baking it in a reducing atmosphere makes copper or nickel transition to the surface, thereby enhancing the solder’s performance. Wettability.
When welding cemented carbide tools, uniform heating of the tool shaft and cemented carbide sheet is one of the basic conditions to ensure the welding quality. If the heating temperature of the cemented carbide sheet is higher than that of the shank, the molten solder will wet the cemented carbide sheet but cannot wet the shank, and the joint strength will be reduced. When the alloy sheet is cut along the solder layer, the solder will not Destroyed, and detached with the alloy sheet. The traces of the cutter support face milling cutter can also be seen on the welding layer. If the heating speed is too fast and the temperature of the tool holder is higher than that of the alloy sheet, the opposite phenomenon will occur.
The placement sequence and mutual position of flux, brazing filler metal and cemented carbide have a direct effect on the quality of brazing. The correct placement method is: put the brazing filler metal on the slot, sprinkle the brazing flux, then put the cemented carbide, and sprinkle a layer of brazing flux on the top surface of the cemented carbide along the side weld. In this way, it is easy to control the brazing temperature during brazing, and reduce the excess brazing material adhering outside the weld.
※The key points of the operation technique of hard alloy and steel oxygen-acetylene brazing are as follows:
① In order to prevent the carbide blade from decarburization or overburning during the brazing process, a carbonized flame should be used.
② The brazing temperature is about 1000℃, that is, the hard alloy blade is bright red when heated. If the blade is dark red or bright white, it cannot be brazed, because the temperature of the former is too low and the temperature of the latter is too high, and overburning has occurred.
③ The welding torch heats the knife body repeatedly from left to right, from right to left, and from top to bottom, so that the knife body and the blade are evenly heated.
④ When brazing, the distance between the welding nozzle and the tool bar is about 50mm, and the inclination angle of the welding nozzle and the end of the tool bar is 110º, which can ensure the effective use of flame heat and heating balance. During the brazing process, the flame should always cover the entire brazing part and isolate it from the air to prevent oxidation or porosity.
⑤ The brazing speed should be determined according to the size of the blade. Brazing of 40 steel and YT15 cemented carbide turning tools should be completed within 1 minute as far as possible, which can effectively prevent the cemented carbide from over-burning or decarburization.
⑥ After brazing, heat the blade part with flame, and then slowly move the welding tip away to cool the weldment slowly to prevent cracks.
During the brazing process, the brazing temperature of the workpiece must be properly controlled. If the brazing temperature is too high, it will cause the oxidation of the weld seam and the evaporation of the zinc element in the zinc-containing brazing filler metal; if the brazing temperature is too low, the weld seam will be thicker due to the poor fluidity of the brazing filler metal, and there will be a large number of pores in the weld seam. And slag inclusion, which is the main cause of desoldering. The brazing temperature should be 30-50°C higher than the melting point of the brazing filler metal. At this time, the brazing filler metal has good fluidity and permeability and is easy to penetrate the entire weld. After the solder is melted, the hard alloy is moved reciprocatingly along the slot 2 to 3 times with a red copper pressure rod to remove the slag in the weld. The moving distance is about 1/3 of the length of the cemented carbide.
The cooling rate after brazing is one of the main factors affecting brazing cracks. When cooling, the surface of the cemented carbide sheet produces instantaneous tensile stress, and the tensile stress of the cemented carbide is much lower than the compressive stress. Especially for YT60, YT30, YG3X and other hard alloys with larger brazing area and small matrix but larger hard alloy workpieces, more attention should be paid to the cooling rate after brazing. Usually, the workpiece is immediately inserted into the lime tank or charcoal powder tank after welding, so that the workpiece is slowly cooled. This method is simple to operate, but cannot control the tempering temperature. If conditions permit, the workpiece can be placed in a furnace at 220-250℃ and tempered for 6-8h immediately after brazing. The use of low-temperature tempering treatment can eliminate part of the brazing stress, reduce cracks and extend the service life of cemented carbide tools.
