Brazing of tool steels and cemented carbides
1. Brazing material
(1) pure copper, copper zinc and silver copper solder are usually used for brazing tool steel and cemented carbide. Pure copper has good wettability to all kinds of cemented carbides, but the best effect can be obtained only by brazing in hydrogen reducing atmosphere. At the same time, because of 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 brazed with pure copper is about 150 MPa, and the plasticity of the joints is high, but it is not suitable for high temperature work.
Cu Zn brazing filler metals are the most common solder for brazing tool steels and cemented carbides. In order to improve the wettability of solder and strength of joints, alloy elements such as Mn, Ni and Fe are often added in brazing filler metals. For example, W (Mn) 4% is added to B-Cu58ZnMn, so that the shear strength of brazed cemented carbide joints can reach 300-320 MPa at room temperature and can still maintain 220-240 MPa at 320%. The shear strength of the brazed joint can reach 350 MPa, and the impact toughness and fatigue strength can be improved by adding a small amount of Co on the basis of B-Cu58ZnMn. The service life of the cutting tools and rock drilling tools can be significantly improved.
The lower melting point of silver-copper brazing alloy and the smaller thermal stress produced by brazing joint are beneficial to reduce the cracking tendency of cemented carbide during brazing. In order to improve the wettability of the solder and increase 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 solders, Mn-based and Ni-based solders, such as B-Mn50NiCuCrCo and B-Ni75CrSiB, can be used for cemented carbides working above 500 C and requiring higher joint strength. For the brazing of high speed steel, special brazing filler metals should be selected which match the brazing temperature and quenching temperature. This kind of brazing filler metals can be divided into two types. One is ferromanganese type, mainly composed of ferromanganese and borax. The alloy welded joint is not easy to crack, and its shear strength can be increased to 300MPa.
(2) the choice of flux and protective flux should be matched with the base metal and the solder selected. When tool steel and cemented carbide are brazed, borax and boric acid are the main fluxes, and some fluorides (KF, NaF, CaF2, etc.) are added. FB301, FB302 and FB105 brazing alloys are used for Cu Zn brazing filler metals, and FB101 ~ FB104 brazing flux is used for Ag Cu brazing filler metals. The borax flux is mainly used when brazing high-speed steel with special solder.
Gas shielded brazing can be used to prevent the oxidation of tool steel during brazing heating and avoid cleaning after brazing. The protective gas can be either inert or reductive, requiring that the dew point of the gas should be lower than - 40 C. The cemented carbide can be brazed under the protection of hydrogen, and the dew point of the required hydrogen should be lower than - 59 C.
2. Brazing technology
Tool steel must be cleaned before brazing, and machined surfaces need not be too smooth for wetting and spreading of materials and fluxes. The surface of cemented carbide should be sandblasted before brazing, or grinded with silicon carbide or diamond grinding wheel to remove excessive carbon on the surface so as to be wetted by brazing filler metal. The cemented carbide containing titanium carbide is difficult to wet. The wettability of the brazing filler metal is increased by applying copper oxide or nickel oxide paste on the surface of the cemented carbide and baking it in reducing atmosphere.
Brazing of carbon tool steel is best done before or at the same time as the quenching process. If brazing is carried out prior to quenching, the solids temperature of the solder used should be higher than the quenching temperature range, so that the solder still has enough strength to prevent failure when it is reheated to the quenching temperature. When brazing and quenching are combined, the solder with the solidus temperature approaching to the quenching temperature is selected.
The composition range of alloy tool steel is very wide. The suitable filler metal, heat treatment process and the technology of combining brazing and heat treatment process should be determined according to the specific steel grade to obtain good joint properties.
The quenching temperature of HSS is generally higher than that of Ag-Cu and Cu-Zn solders, so it is necessary to quench before brazing and braze during or after secondary tempering. If quenching is necessary after brazing, only the special solder should be selected for soldering. Coke oven is suitable for brazing high speed steel tools. When the brazing filler metal is melted, the cutting tool is taken out and pressured immediately, the excess brazing filler metal is extruded, then oil quenched, and tempered at 550 ~ 570%.
When brazing cemented carbide inserts with steel tool rods, it is advisable to increase the gap between the brazing seams and apply plastic compensation gaskets in the brazing seams, and slow cooling after brazing to reduce brazing stress, prevent cracks and prolong the service life of cemented carbide tool assemblies.
After fiber welding, the brazing flux residue on the welding piece is first washed with hot water or cleaned with a general slag removal mixture, and then pickled with a suitable pickling solution to remove the oxide film on the base cutter rod. But be careful not to use nitric acid to prevent corrosion.