Cemented carbide is a general term for alloys prepared by powder metallurgy by 9 kinds of metal carbides of the IVa, Va and VIa groups in the periodic table and Fe, Co, Ni and other ferrous group metals. The carbide phase gives the alloy a high hardness and wear resistance, while the bonding phase gives the alloy a certain strength and toughness.
According to the composition, cemented carbide can be divided into five categories: tungsten carbide-based cemented carbide, titanium carbide-based cemented carbide, coated cemented carbide, steel-based cemented carbide and other cemented carbide.
Cemented carbide can be divided into four categories according to its scope of application: cemented carbide cutting tools, cemented carbide molds, cemented carbide measuring tools and wear-resistant parts, and cemented carbide for mining petroleum geology.
In general, WC-Co carbide is widely used, can be used for cast iron, non-ferrous metals and their alloys cutting tools, metal tensile molds, stamping dies, measuring tools and mining machinery and geological exploration wear-resistant parts; WC-Ti-Co alloys are mainly used for steel cutting processing; WC-TiC- (NbC)-Co alloys are mainly used for cutting and processing of high-hardness material parts.
Although other types of cemented carbide have made great progress in recent years and have achieved great success in some special applications, due to the WC-Co series (that is, YG class) cemented carbide has very excellent comprehensive mechanical properties, this type of cemented carbide is currently the most widely used and the largest amount of cemented carbide in the industry.
1. Problems encountered when brazing cemented carbide
The brazability of cemented carbide is poor. This is because cemented carbide has a higher carbon content, and unpurgeed surfaces tend to contain more free carbon, thus hindering the wetting of the filler. In addition, cemented carbide is easy to oxidize to form an oxide film at the temperature of brazing, which will also affect the wetting of the brazing material. Therefore, the surface cleaning before brazing is important to improve the wetting properties of the brazing material on cemented carbide, and measures such as copper plating or nickel plating can be taken if necessary.
Another problem in carbide brazing is that joints are prone to cracks. This is because its linear expansion coefficient is only half that of mild steel, and when cemented carbide is brazed with the matrix of this type of steel, it will generate a large thermal stress in the joint, resulting in cracking of the joint. Therefore, when brazing cemented carbide with different materials, anti-cracking measures should be tried.
2. Surface treatment before brazing
Oxides, greases, dirt and paints must be carefully removed from the surface of the workpiece before brazing, because the melted filler cannot wet the surface of the part that has not been cleaned, nor can it fill the joint gap. Sometimes, in order to improve the brazing properties of the base metal and the corrosion resistance of the brazing head, the part must also be pre-plated with a certain metal layer before brazing.
(1) Oil pollution can be removed by organic solvent
Commonly used organic solvents are alcohol, carbon tetrachloride, gasoline, trichloroene, dichloroethane and trichloroethane. Snacks can be cleaned in organic solvents in small batch production. The most widely used in mass production is degreasing in steam from organic solvents. In addition, cleaning in a hot alkali solution can also obtain satisfactory results. For example, steel parts can be immersed in 70-80 ° C 10% caustic soda solution degreasing, copper and copper alloy parts can be cleaned in 50g of trisodium phosphate, 50g of sodium bicarbonate plus 1L of water solution, the solution temperature is 60 ~ 80 ° C. The degreasing of parts can also be carefully washed with water after degreasing in detergents. When the surface of the part can be completely wet with water, it indicates that the surface grease has been removed. Small parts with complex shapes and large quantities can also be ultrasonically cleaned in special slots. Ultrasonic degreasing efficiency is high.
(2) Remove oxides
Before brazing, the oxides on the surface of the part can be carried out using mechanical methods, chemical etching methods and electrochemical impregnation methods. Mechanical cleaning can be used to remove zeros such as file, metal brush, sandpaper, grinding wheel, sandblasting, etc.: the oxide film on the surface. Among them, file and sandpaper cleaning are used for single-piece production, and the grooves formed during cleaning are also conducive to the wetting and spreading of the filler. In mass production, grinding wheels, metal brushes, sandblasting and other methods are used. The surface of aluminum and aluminum alloys and titanium alloys should not be used by mechanical cleaning method.
(3) The surface of the base metal is coated with metal
The main purpose of plating metal on the surface of the base metal is to improve the brazability of some materials and increase the wetting ability of the brazing material to the base metal; to prevent the interaction between the base metal and the brazing material from adversely affecting the quality of the butt, such as preventing cracks and reducing the brittle intermetallic compound produced at the interface; as a brazing layer to simplify the assembly process and improve productivity.
3. Brazing material
(1) Brazing tool steel and cemented carbide are usually made of pure copper, copper zinc and silver copper brazing.
Pure copper has good wetting properties for all kinds of cemented carbide, 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 large, resulting in an increase in crack tendency. The shear strength of the joint using traditional pure copper brazing is about 150MPa, and the plasticity of the joint is also high, but it is not suitable for high temperature work.
Copper-zinc brazing is the most commonly used brazing tool steel and cemented carbide. In order to improve the wettability of the filler and the strength of the joint, mn, Ni, Fe and other alloying elements are often added to the filler. For example, B-Cu58ZnMn is added w(Mn)4%, so that the shear strength of the cemented carbide brazing head reaches 300~320MPa at room temperature: it can still maintain 220-240MPa at 320°C. Adding a small amount of Co on the basis of B-Cu58ZnMn can make the shear strength of the brazing head reach 350MPa, and has high impact toughness and fatigue strength, which significantly improves the service life of the tool and the rock drilling tool.
The melting point of silver copper brazing is low, and the thermal stress generated by the brazing head is small, which is conducive to reducing the cracking tendency of cemented carbide brazing. In order to improve the wettability of the filler and improve the strength and working temperature of the joint, mn, Ni and other alloying elements are often added to the filler. For example, B-Ag50CuZnCdNi filler has excellent wetting properties on cemented carbide, and the brazing head has good comprehensive properties.
In addition to the above three types of fillers, for cemented carbide that works above 500 °C and has high joint strength requirements, Mn-based and Ni-based fillers can be selected, such as B-Mn50NiCuCrCo and B-Ni75CrSiB. For the brazing of high-speed steel, the brazing temperature should be selected to match the special brazing material with the quenching temperature, which is divided into two categories, one is manganese ferro-type brazing, mainly composed of ferromanganese and borax, the shear strength of the brazed joint is generally about 100MPa, but the joint is prone to cracks: the other is a special copper alloy containing Ni, Fe, Mn and Si, and the joint brazed with it is not easy to produce cracks, and its shear strength can be increased to 300MPa.
(2) Brazing agent and protective gas
The choice of filler should be in line with the base metal and the selected filler. When tool steel and cemented carbide are brazed, the flux used is mainly borax and boric acid, and some fluoride (KF, NaF, CaF2, etc.) is added. Copper-zinc brazing compound is formulated with FB301, FB302 and FB105 brazing agent, and silver copper brazing material is formulated with FB101~FB104 brazing agent. When using special brazing brazing high-speed steel, borax brazing agent is mainly used.
In order to prevent oxidation of tool steel during brazing heating and to eliminate cleaning after brazing, gas shielded brazing can be used. The shielding gas can be an inert gas or a reducing gas, requiring that the dew point of the gas should be lower than -40 °C. Cemented carbide can be brazed under the protection of hydrogen, and the dew point of the required hydrogen should be lower than -59 °C.
4. Brazing process
Tool steel must be cleaned before brazing, and the machined surface does not have to be too smooth to facilitate wetting and spreading of materials and solders. The surface of cemented carbide should be sandblasted before brazing, or sanded with silicon carbide or diamond grinding wheel to remove excess carbon on the surface so that it can be wet by the solder during brazing. Cemented carbide containing titanium carbide is more difficult to wet, and the wettability of strong brazing material is increased by applying copper oxide or nickel oxide paste to its surface in a new way and baking in a reducing atmosphere.
Brazing of carbon tool steels is best carried out before or at the same time as the quenching process. If brazing is performed before the quenching process, the solid phase line temperature of the solder used should be higher than the quenching temperature range so that the weldment remains strong enough when reheated to the quenching temperature without failure. When brazing and quenching are combined, a filler with a solid-phase line temperature close to the quenching temperature is selected.
The composition range of alloy tool steel is very wide, and the suitable brazing, heat treatment process and the combination of brazing and heat treatment processes 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 brazing, so it is necessary to quench before brazing and brazing during or after secondary tempering. If it must be quenched after brazing, only the aforementioned special brazing material can be used for brazing. When brazing high-speed steel tools, it is more appropriate to use a coke oven, when the filler melts, take out the cutting tool and immediately pressurize, extrude the excess filler, and then oil quench, and then temper at 550 ~ 570 ° C.
When brazing cemented carbide blades and steel tool holders, it is advisable to increase the gap between the brazing seam and apply a plastic compensation gasket in the brazing seam, and slowly cool after welding to reduce the brazing stress, prevent cracks, and extend the service life of the cemented carbide tool components.
5. Cleaning after brazing
Most of the brazing agent residues have a corrosive effect on the brazing head, and also hinder the inspection of the brazing seam, which needs to be removed. The solder residue on the weldment is first rinsed with hot water or with a general slag removal mixture, followed by pickling with a suitable pickling solution to remove the oxide film on the base knife holder. But be careful not to use nitric acid solutions to prevent corrosion of the brazed metal. The residue of organic soft brazing agent can be wiped or cleaned with organic solvents such as gasoline, alcohol, acetone; the residues of zinc oxide and ammonium chloride are very corrosive, should be cleaned in a solution of 10% NaOH, and then washed with hot or cold water, and the residue of borax and boric acid brazing agent is generally solved by mechanical methods or long-term immersion in boiling water.
6. Inspection of brazing quality
The inspection method of brazing welding head can be divided into non-destructive testing and destruction inspection. The following are mainly the methods of non-destructive testing:
(1) Visual inspection.
(2) Coloring test and fluorescence test. These two methods are mainly used to check for defects such as tiny cracks, stomata, and looseness that cannot be found due to visual inspection.
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