1. Brazing material
(1) Brazing tool steels and cemented carbides usually use pure copper, copper zinc and silver copper brazing filler metals. Pure copper has good wettability to all kinds of cemented carbides, but the best effect can be obtained by brazing in the reducing atmosphere of hydrogen. 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 joint brazed with pure copper is about 150MPa, and the joint plasticity is also high, but it is not suitable for high-temperature work.
Copper zinc filler metal is the most commonly used filler metal for brazing tool steels and cemented carbides. In order to improve the wettability of the solder and the strength of the joint, Mn, Ni, Fe and other alloy elements are often added to the solder. For example, w (MN) 4% is added to b-cu58znmn to make the shear strength of cemented carbide brazed joints reach 300 ~ 320MPa at room temperature; It can still maintain 220 ~ 240mpa at 320 ℃. 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 high impact toughness and fatigue strength, significantly improving the service life of cutting tools and rock drilling tools.
The lower melting point of silver copper brazing filler metal and the smaller thermal stress of brazed joint are beneficial to reduce the cracking tendency of cemented carbide during brazing. In order to improve the wettability of the solder and improve the strength and working temperature of the joint, Mn, Ni and other alloy elements are often added to the solder. For example, b-ag50cuzncdni solder has excellent wettability to cemented carbide, and the brazed joint has good comprehensive properties.
In addition to the above three types of brazing filler metals, Mn based and Ni based brazing filler metals, such as b-mn50nicucrco and b-ni75crsib, can be selected for cemented carbide working above 500 ℃ and requiring high joint strength. For the brazing of high-speed steel, the special brazing filler metal with the brazing temperature matching the quenching temperature should be selected. This filler metal is divided into two categories: one is ferromanganese type filler metal, which is mainly composed of ferromanganese and borax. The shear strength of the brazed joint is generally about 100MPa, but the joint is prone to cracks; Another kind of special copper alloy containing Ni, Fe, Mn and Si is not easy to produce cracks in the brazed joints, and its shear strength can be increased to 300mpa.
(2) The selection of brazing flux and shielding gas brazing flux shall match with the base metal and filler metal to be welded. When brazing tool steel and cemented carbide, the brazing flux used is mainly borax and boric acid, and some fluorides (KF, NaF, CaF2, etc.) are added. Fb301, fb302 and fb105 fluxes are used for copper zinc solder, and fb101 ~ fb104 fluxes are used for silver copper solder. Borax flux is mainly used when special brazing filler metal is used to braze high-speed steel.
In order to prevent oxidation of tool steel during brazing heating and to avoid cleaning after brazing, gas shielded brazing can be used. The protective gas can be either inert gas or reducing gas, and the dew point of the gas shall be lower than -40 ℃ Cemented carbide can be brazed under the protection of hydrogen, and the dew point of hydrogen required shall be lower than -59 ℃.
2. Brazing technology
The tool steel must be cleaned before brazing, and the machined surface need not be too smooth to facilitate the wetting and spreading of materials and brazing flux. The surface of cemented carbide shall be sand blasted before brazing, or polished with silicon carbide or diamond grinding wheel to remove excessive carbon on the surface, so as to be wetted by brazing filler metal during brazing. Cemented carbide containing titanium carbide is difficult to wet. Copper oxide or nickel oxide paste is applied on its surface in a new way and baked in a reducing atmosphere to make copper or nickel transition to the surface, so as to increase the wettability of strong solder.
The brazing of carbon tool steel should preferably be carried out before or at the same time as the quenching process. If brazing is carried out before the quenching process, the solidus temperature of the filler metal used shall be higher than the quenching temperature range, so that the weldment still has high enough strength when reheated to the quenching temperature without failure. When brazing and quenching are combined, the filler metal with solidus temperature close to quenching temperature shall be selected.
Alloy tool steel has a wide range of components. Appropriate brazing filler metal, heat treatment process and the technology of combining brazing and heat treatment process should be determined 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 braze during or after secondary tempering. If quenching is required after brazing, only the above-mentioned special brazing filler metal can be used for brazing. When brazing high-speed steel cutting tools, it is appropriate to use coke furnace. When the brazing filler metal is melted, take out the cutting tool and immediately pressurize it, extrude the excess brazing filler metal, then carry out oil quenching, and then temper it at 550 ~ 570 ℃.
When brazing the cemented carbide blade with the steel tool bar, the method of increasing the brazing gap and applying plastic compensation gasket in the brazing gap should be adopted, and slow cooling should be carried out after welding to reduce the brazing stress, prevent cracks and prolong the service life of the cemented carbide tool assembly.
After fiber welding, the flux residue on the weldment shall be washed with hot water or general slag removal mixture, and then pickled with appropriate pickling solution to remove the oxide film on the base tool rod. However, be careful not to use nitric acid solution to prevent corrosion of brazing joint metal.
Post time: Jun-13-2022