Hard alloy is a general term of alloy prepared by powder metallurgy, which is composed of 9 kinds of metal carbide of IVa, Va and VIa group in the periodic table and iron group metals such as Fe, Co and Ni. The carbide phase gives the alloy high hardness and wear resistance, while the bond phase gives the alloy certain strength and toughness.
According to the composition, hard metals can be divided into five categories: tungsten carbide, titanium carbide, coated hard metals, steel - based hard metals and other hard metals.
Hard alloy according to its application range can be divided into: hard alloy cutting tool, hard alloy die, hard alloy measuring tools and wear-resistant parts, and mining petroleum geology with hard alloy four categories.
Generally speaking, the application of WC-Co hard alloy is more extensive, can be used in cast iron, non-ferrous metal and alloy cutting tools, metal drawing die, stamping die, measuring tools and mining machinery and geological exploration wear-resistant parts; WC-Ti-Co alloy is mainly used for steel cutting; WC-TiC- (NbC) -Co alloy is mainly used for high hardness material parts machining.
Although other types of hard alloys have made great progress in recent years, and have achieved great success in some special applications, but because of the excellent comprehensive mechanical properties of WC-Co series (that is, YG class) hard alloys, this kind of hard alloy is the most widely used in industry, the largest consumption of a class of hard alloys.
1. Problems encountered during brazing of hard alloy
The brazing ability of hard alloys is poor. This is because the carbide has a higher carbon content, and the uncleaned surface often contains more free carbon, which prevents the wetting of the filler metal. In addition, the hard alloy is easy to oxidize and form oxide film at brazing temperature, which will also affect the wetting of the filler metal. Therefore, surface cleaning before brazing is very important to improve the wettability of the brazing filler metal on the hard alloy. If necessary, measures such as surface plating of copper or nickel can also be taken.
Another problem in brazing hard alloys is that the joints are prone to crack. This is because its coefficient of linear expansion is only half that of mild steel. When the hard alloy brazes with the matrix of this kind of steel, it will produce a lot of thermal stress in the joint, which leads to the cracking of the joint. Therefore, hard alloy and different materials brazing, should try to take anti-crack measures.
2. Surface treatment before brazing
The oxide, grease, dirt and paint on the workpiece surface must be carefully removed before brazing, because the melted filler metal cannot wet the surface of the uncleaned part, nor can it fill the joint gap. Sometimes, in order to improve the brazing properties of the base material and to improve the corrosion resistance of the brazing joint, the parts must be pre-coated with a metal layer before brazing.
(1) Remove oil stains Oil stains can be removed with organic solvents
Common organic solvents are alcohol, carbon tetrachloride, gasoline, trichloride, dichloroethane and trichloroethane. Small batch production can be soaked in organic solvent clean. Degreasing in vapors of organic solvents is the most widely used in mass production. In addition, satisfactory results can also be obtained by cleaning in hot alkali solution. For example, steel parts can be immersed in 10% caustic soda solution of 70-80 ℃ degreasing, copper and copper alloy parts can be cleaned in 50g trisodium phosphate, 50g sodium bicarbonate and 1L water solution, solution temperature of 60 ~ 80°C. Parts can also be degreased in detergent after degreasing carefully cleaned with water. When the surface of the part can be completely wet by water, it indicates that the surface grease has been removed. For complex shape and large number of small parts, can also be cleaned in a special tank with ultrasonic. Ultrasonic degreasing efficiency is high.
(2) Remove oxides
Before brazing, the oxides on the surface of the parts can be carried out by mechanical, chemical and electrochemical etching methods. When cleaning mechanically, files, metal brushes, sandpaper, grinding wheels, sand blasting can be used to remove the oxide film on the surface. The file and sandpaper cleaning is used for single production, and the groove formed when cleaning is conducive to the wetting and spreading of the filler metal. Mass production with grinding wheel, metal brush, sand blasting and other methods. The surface of aluminum and aluminum alloy and titanium alloy should not be cleaned mechanically.
(3) Metal plating on the base metal surface
The main purpose of plating metal on the base metal surface is to improve the brazing properties of some materials and increase the wetting ability of the filler metal to the base metal. To prevent the interaction between base metal and filler metal on the joint quality of adverse effects, such as prevent crack, reduce the interface to produce brittle intermetallic compounds; As a filler metal layer to simplify the assembly process and improve productivity.
3. Braze materials
Brazing tool steel and hard alloys usually use pure copper, copper zinc and silver copper solder.
Pure copper has good wettability to all kinds of hard alloys, but the best results can be obtained by brazing in reducing atmosphere of hydrogen. At the same time, due to the high brazing temperature, the stress in the joint is larger, which leads to the increase of crack tendency. The joint brazed with traditional pure copper has a shear strength of about 150MPa and a high ductility, but it is not suitable for high temperature operation.
Copper - zinc filler metal is the most commonly used filler metal for brazing tool steel and hard alloy. In order to improve the wettability and joint strength of the filler metal, alloy elements such as Mn, Ni and Fe are often added to the filler metal. For example, 4% w(Mn) is added to B-Cu58ZnMn, so that the shear strength of the carbide brazed joint can reach 300 ~ 320MPa at room temperature, and can maintain 220 ~ 240MPa at 320°C. By adding a small amount of Co on the basis of B -- Cu58ZnMn, the shear strength of brazed joints can reach 350MPa, with high impact toughness and fatigue strength, and the service life of cutting tools and drilling tools can be significantly improved.
The melting point of silver copper solder is low and the thermal stress generated by brazing joint is small, which is beneficial to reduce the cracking tendency of hard alloy. In order to improve the wettability of the filler metal and increase the strength and working temperature of the joint, alloy elements such as Mn and Ni are often added to the filler metal. For example, B-Ag50CuZnCdNi filler metal has excellent wettability to hard alloy, and the brazing joint has good comprehensive performance.
In addition to the above 3 types of brazing metal, for the hard alloy working at 500°C and higher joint strength requirements, Mn base and Ni base brazing metal can be used, such as B-Mn50NiCuCrCo and B-Ni75CrSiB, etc. For the brazing of high speed steel, the brazing temperature should be selected to match the quenching temperature of the special filler metal, this filler metal is divided into two categories, one is ferromanganese type filler metal, mainly composed of ferromanganese and borax, the brazed joint shear strength is generally about 100MPa, but the joint is prone to crack: The other is the special copper alloy containing Ni, Fe, Mn and Si, with which the joint brazed is not easy to crack, and its shear strength can be increased to 300MPa.
(2) brazing agent and protective gas
The choice of brazing agent shall be in accordance with the base material to be welded and the filler metal selected. When brazing tool steel and hard alloy, the brazing agent used is mainly borax and boric acid, and some fluoride (KF, NaF, CaF2, etc.) is added. FB301, FB302 and FB105 brazing agents should be used for copper and zinc brazing materials, and FB101 ~ FB104 brazing agents should be used for silver copper brazing materials. When special filler metal is used to braze high speed steel, borax is mainly used.
In order to prevent oxidation of tool steel during brazing heating and avoid cleaning up after brazing, gas shielded brazing can be used. The protective gas can be inert gas or reducing gas, and the dew point of the gas should be lower than -40℃. Hard alloys can be brazed under the protection of hydrogen. The dew point of hydrogen required should be below -59℃.
4. Brazing process
Tool steel must be cleaned before brazing. The machined surface need not be too smooth to facilitate wetting and spreading of materials and brazing agents. The surface of hard alloy should be sandblasted before brazing, or polished with silicon carbide or diamond grinding wheel, to remove excessive carbon on the surface, in order to be wetted by brazing filler metal. The hard alloy containing titanium carbide is more difficult to wettability, through a new way on the surface of the copper oxide or nickel oxide paste, and baked in a reducing atmosphere to make the copper or nickel transition to the surface, so as to increase the wettability of the solder.
The brazing of carbon tool steel is best done before or at the same time as the quenching process. If brazing is performed prior to the quenching process, the solid line temperature of the filler metal used shall be above the quenching temperature range so that the solder will remain strong enough to not fail when reheated to the quenching temperature. When brazing and quenching are combined, the solder whose solid-phase line temperature is close to quenching temperature is selected.
The composition range of alloy tool steel is very wide, and the appropriate filler metal, heat treatment process and technology combining brazing and heat treatment process should be determined according to the specific steel, so as to obtain good joint properties.
The quenching temperature of high speed steel is generally higher than the melting temperature of silver copper and Cu-Zn brazing metal, so it is necessary to quench before brazing and braze during or after secondary tempering. If quenching must be carried out after brazing, only the above-mentioned special filler metal can be used for brazing. It is more appropriate to use coke furnace when brazing high speed steel tool. When the filler metal is melted, the cutting tool is taken out and pressurized immediately, the excess filler metal is extruded, and then the oil is quenched, and then tempered at 550 ~ 570℃.
When the carbide blade is brazed with the steel tool rod, it is advisable to increase the gap of the brazing joint and apply the plastic compensation gasket in the brazing joint, and slow cooling after welding, in order to reduce the brazing stress, prevent the crack, and prolong the service life of the carbide tool assembly.
5. Cleaning after brazing
Most of the brazing residue corrodes the brazing joint, and also prevents the inspection of the brazing joint and needs to be removed. The brazing residue on the weldment is rinsed with hot water or cleaned with a general desagging mixture followed by pickling with a suitable acid wash to remove the oxide film on the base cutter bar. But be careful not to use nitric acid solution, in case of corrosion of brazing metal. The residue of organic soft brazing agent can be wiped or cleaned with organic solvents such as gasoline, alcohol and acetone. Zinc oxide and ammonium chloride residue is very corrosive, should be cleaned in 10%NaOH solution, and then washed with hot or cold water, borax and boric acid brazing residue is generally solved by mechanical method or in boiling water for a long time.
6. Inspection of brazing quality
The inspection methods of brazed joints can be divided into nondestructive inspection and damage inspection. The following are the main methods of nondestructive testing:
(1) Appearance inspection.
(2) Staining test and fluorescence test. These two methods are mainly used to check the defects such as tiny cracks, pores and porosity that can not be found by appearance inspection.
