Brazing of tool steel and hard alloy
Tool steels usually include carbon tool steels, alloy tool steels and high speed steels, while cemented carbides are powder sintered from carbides (such as WC, TiC, etc.) and bonded metals (such as Co, etc.). The brazing technology of tool steel and cemented carbide is mainly used in the manufacture of cutting tools, moulds, measuring tools and mining tools.
The main problem in tool brazing is that its microstructure and properties are easily affected by brazing process. If the brazing process is improper, high temperature annealing, oxidation and decarbonization will easily occur. Such as high speed steel W18Cr4V quenching temperature is 1260-1280 ℃, to avoid the occurrence of these problems, ensure that have the greatest hardness and wear resistance, when cutting requirements must be compatible with quenching temperature brazing temperature.
The brazing property of cemented carbides is poor. This is because of the high carbon content of cemented carbide, the uncleaned surface often contains more free carbon, which impedes the wetting of the filler metal. In addition, the cemented carbide is easy to oxidize to form oxide film at brazing temperature, which will affect the wetting of the filler metal. Therefore, the surface cleaning before brazing is very important to improve the wettability of the filler metal on the hard alloy.
Another problem in cemented carbide brazing is that joints are prone to crack. This is because its linear expansion coefficient is only half of that of low carbon steel. When cemented carbide is brazed with the base of this kind of steel, great thermal stress will be generated in the joint, resulting in the joint cracking. Therefore, crack prevention measures should be taken when brazing cemented carbides with different materials.
2. Brazing materials
Brazing tool steels and hard alloys usually use pure copper, copper zinc and silver copper brazing alloys. Pure copper has good wettability to all kinds of hard alloys, but the best effect can be obtained by brazing in hydrogen reducing atmosphere. At the same time, due to the high brazing temperature, the stress in the joint is large, which leads to the increase of crack tendency. The shear strength of the joints using pure copper brazing is about 150MPa, and the plasticity of the joints is also high, but it is not suitable for high temperature work.
Brazing brazing alloy is the most commonly used brazing filler for tool steel and cemented carbide. In order to improve the wettability and joint strength of the solder, Mn, Ni, Fe and other alloy elements are often added in the solder. B - Cu58ZnMn, for example with w (Mn) added 4%, makes the shearing strength of the cemented carbide brazing welding head at room temperature up to 300 ~ 320 mpa: at 320 ° C can still maintain a 220-240 mpa. The addition of a small amount of Co on the basis of BCu58ZnMn can make the shear strength of the brazing joint reach 350MPa, and has a high impact toughness and fatigue strength, significantly improving the service life of the tool and drilling tool
The melting point of silver copper solder is low and the thermal stress of brazing joint is small, which is helpful to reduce the cracking tendency of cemented carbide brazing. In order to improve the wettability of the solder and improve the joint strength and working temperature, Mn, Ni and other alloy elements are often added in the solder. For example, BAg50CuZnCdNi filler metal has excellent wettability to hard alloy and brazing joint has good comprehensive performance.
Apart from the above three types of filler metals, for work in more than 500 ° C and the joint strength to demand higher carbide, can choose Mn and Ni based solder, such as B - Mn50NiCuCrCo and B - Ni75CrSiB. For the brazing of high-speed steel, the special filler metal with brazing temperature matching with quenching temperature should be selected, as shown in table 3. The brazing alloy is divided into two categories: one is ferromanganese solder, which is mainly composed of ferromanganese and borax. The brazing joint's shear strength is generally about 100MPa, but the joint is prone to crack; the other is a special copper alloy containing Ni, Fe, Mn and Si. The brazing joint is not easy to crack, and its shear strength can be increased to 300MPa.
(2) the choice of brazing agent and shielding gas brazing agent shall be matched with the base metal to be welded and the selected filler metal. When brazing tool steel and cemented carbide, borax and boric acid are the main brazing agents, and some fluoride is added (KF, NaF, CaF2, etc.). Brazing agents FB301, FB302 and FBl05 for cu and zn filler metals and FBl01 ~ FBl04 for ag - cu filler metals. Borax brazing agent is mainly used in brazing high speed steel with special filler.
Gas shielded brazing can be used to prevent tool steels from being oxidized during brazing heating and from being cleaned after brazing. Shielding gas can be inert gas, also can be a reducing gas for gas should be lower than dew point to 40 ° C. Hard alloy under hydrogen protection for brazing, the dew point of hydrogen required should be lower than 59 ° C.
3. Brazing technology
Tool steels must be cleaned before brazing, and the machined surface need not be too smooth to facilitate wetting and spreading of the solder and brazing agent. The hard alloy surface should be sandblasted before brazing, or polished with silicon carbide or diamond grinding wheel to remove excessive carbon on the surface, so as to be wetted by the filler metal during brazing. Hard alloys containing titanium carbide are difficult to be wetted. Wettability of the filler metal is enhanced by coating the surface with a paste of copper oxide or nickel oxide and baking in a reducing atmosphere to transfer the copper or nickel to the surface.
Brazing of carbon tool steels is best done before or at the same time as the quenching process. If brazing is carried out before the quenching process, the solid phase line temperature of the solder shall be higher than the quenching temperature range, so that the welding parts are still strong enough to avoid failure when reheated to the quenching temperature. When brazing and quenching are combined, the filler metal with solid phase line temperature close to quenching temperature is selected.
Alloy tool steels have a wide range of compositions, and it is necessary to determine the appropriate filler metal, heat treatment procedure and the technology of combining brazing and heat treatment procedure according to the specific steel type, so as to obtain good joint performance.
The quenching temperature of high speed steel is generally higher than the melting temperature of silver copper and copper zinc solder, so it is necessary to quench before brazing and brazing during or after the second tempering. If quenching must be carried out after brazing, only the aforementioned special filler metal shall be selected for brazing. Brazing of HSS cutting tools when used coke oven is more appropriate, after the solder melts, immediately remove the cutting tool and pressure, squeeze out excess solder, and then to oil quenching, and then in 550 ~ 570 ℃ tempering.
When brazing cemented carbide blade with steel tool rod, it is advisable to increase the brazing gap and apply plastic compensation gasket in the brazing joint, and conduct slow cooling after welding, so as to reduce the brazing stress, prevent crack generation and prolong the service life of cemented carbide tool assembly.
After brazing, the brazing agent residue on the welding piece is washed with hot water or general slag removal mixture, followed by pickling with appropriate pickling solution to remove the oxide film on the matrix blade. But be careful not to use nitric acid solution to prevent corrosion of the brazing metal.