Brazing of Cemented Carbide
Cemented carbides are nine kinds of metal carbides of IVa, Va and VIa groups in the periodic table of elements, and iron group metals such as Fe, Co and Ni are prepared by powder metallurgy. The carbide phase gives the alloy high hardness and wear resistance, while the bonding phase gives the alloy a certain strength and toughness.
According to the composition, cemented carbides 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 carbides.
According to its application scope, cemented carbides can be divided into four categories: cemented carbide cutting tools, cemented carbide dies, cemented carbide measuring tools and wear-resistant parts, and cemented carbides for mining petroleum geology.
Generally speaking, WC-Co cemented carbides are widely used in cutting tools, metal drawing dies, stamping dies, measuring tools and wear-resistant parts for mining machinery and geological exploration; WC-TiC-(NbC)-Co alloys are mainly used for steel cutting; WC-TiC-(NbC)-Co alloys are mainly used for cutting parts of high hardness materials.
Although other types of cemented carbides have made great progress in recent years and achieved great success in some special applications, due to the excellent comprehensive mechanical properties of WC-Co (YG) cemented carbides, these cemented carbides are the most widely used and used cemented carbides in industry.
1. Problems encountered in cemented carbide brazing
The brazing property of cemented carbide is poor. This is because the cemented carbide has a high carbon content, and the untreated surface often contains more free carbon, which hinders the wetting of solder. In addition, cemented carbide is easy to oxidize to form oxide film at brazing temperature, which also affects the wettability of solder. Therefore, the surface cleaning before brazing is very important to improve the wettability of brazing filler metal on cemented carbide, and measures such as copper or nickel plating can be taken when necessary.
Another problem in cemented carbide brazing is that joints are prone to cracks. This is because its linear expansion coefficient is only half of that of low carbon steel. When cemented carbide is brazed with the matrix of this kind of steel, great thermal stress will occur in the joint, which will lead to joint cracking. Therefore, crack prevention measures should be taken when cemented carbide is brazed with different materials.
2. Surface treatment before brazing
Before brazing, the oxide, grease, dirt and paint on the surface of the workpiece must be carefully removed, because the melted filler metal can not wet the surface of the parts that have not been cleaned, nor can it fill the gap between the joints. Sometimes, in order to improve the solderability of the base metal and the corrosion resistance of the brazed joints, the parts must be pre-coated with a metal layer before brazing.
(1) Oil and oil pollution can be removed by organic solvents.
Commonly used organic solvents are alcohol, carbon tetrachloride, gasoline, trichloroene, dichloroethane and trichloroethane. In small batches of production, the zeros can be soaked in organic solvents and cleaned. The most widely used in mass production is degreasing in the vapor of organic solvents. In addition, satisfactory results can be obtained by cleaning in hot alkali solution. For example, steel parts can be degreased by immersing in 10% caustic sodium solution at 70-80 C, copper and copper alloy parts can be cleaned in 50 g trisodium phosphate, 50 g sodium bicarbonate and 1 L water solution, and the solution temperature is 60-80 C. The degreasing of parts can also be carried out in detergent and carefully cleaned with water. When the surface of the part can be completely wetted by water, it shows that the grease on the surface has been removed. For small parts with complex shape and large quantity, it can also be cleaned by ultrasonic wave in special grooves. Ultrasound oil removal efficiency is high.
(2) Removal of oxides
Before brazing, the oxides on the surface of parts can be carried out by mechanical, chemical and electrochemical methods. When cleaning by mechanical method, we can use files, metal brushes, sandpaper, grinding wheels, sandblasting and so on to remove zero: oxide film on the surface. Files and sandpaper cleaning are used for single piece production. The grooves formed during cleaning are also beneficial to the wetting and spreading of solder. In batch production, grinding wheel, metal brush, sand blasting and other methods are used. The surface of aluminium, aluminium alloy and titanium alloy should not be cleaned by mechanical method.
(3) Metal plating on base metal
The main purpose of metal plating on the base metal surface is to improve the solderability of some materials and increase the wettability of the solder to the base metal; to prevent the interaction between the base metal and the solder from having adverse effects on the quality of the joint, such as preventing cracks and reducing brittle intermetallic compounds at the interface; to act as the solder layer to simplify the assembly process and improve productivity.
3. Brazing materials
(1) Brazing tool steels and cemented carbides with brazing fillers usually use pure copper, copper-zinc and silver-copper brazing fillers.
Pure copper has good wettability to all kinds of cemented carbides, but it needs brazing in hydrogen reducing atmosphere to get the best effect. 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 shear strength of the joints brazed with traditional pure copper is about 150 MPa, and the plasticity of the joints is high, but it is not suitable for high temperature work.
Copper-zinc solder is the most commonly used solder for brazing tool steel and cemented carbide. In order to improve the wettability of solder and the strength of joints, alloying elements such as Mn, Ni and Fe are often added to the solder. For example, the addition of W (Mn) 4% in B-Cu58ZnMn makes the shear strength of brazed Cemented Carbide Joint maintain 220-240 MPa at room temperature of 300-320 MPa: 320 degrees C. The shear strength of brazed joints can reach 350 MPa by adding a small amount of CO on the basis of B-Cu58ZnMn, which has high impact toughness and fatigue strength, and significantly improves the service life of cutting tools and drilling tools.
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 brazing. In order to improve the wettability of solder and the strength and working temperature of joints, alloying elements such as Mn and Ni are often added to the solder. For example, B-Ag50CuZnCdNi solder has excellent wettability to cemented carbide, and brazed joints have good comprehensive properties.
In addition to the three types of solders mentioned above, Mn-based and Ni-based solders, such as B-Mn50NiCuCrCo and B-Ni75CrSiB, can be selected for cemented carbides working above 500 degrees C and with high joint strength requirements. For brazing of high speed steel, special brazing filler metals matching brazing temperature and quenching temperature should be selected. One kind of brazing filler metals is ferromanganese filler metals, mainly composed of ferromanganese and borax. The shear strength of brazed joints is generally about 100 MPa, but the joints are prone to cracks. The other type is special copper alloys containing Ni, Fe, Mn and Si. It is not easy to crack when brazing joints with ferromanganese filler metals. The shear strength of grain can be increased to 300 MPa.
4. Brazing process
Tool steel must be cleaned before brazing. The machined surface need not be too smooth to facilitate the wetting and spreading of materials and flux. The surface of cemented carbide should be sandblasted before brazing, or polished with silicon carbide or diamond grinding wheel to remove excessive carbon on the surface, so as to wet the solder during brazing. Cemented carbide containing titanium carbide is difficult to wet. Copper oxide or nickel oxide paste is deposited on its surface in a new way and baked in a reducing atmosphere to transfer copper or nickel to the surface, thus increasing the wettability of the strong solder.
Brazing of carbon tool steels is best carried out before or at the same time as quenching. If the solder is brazed before quenching, the solidus temperature of the solder used should be higher than the quenching temperature range, so that the solder still has a high enough strength when reheated to the quenching temperature without failure. When brazing and quenching are combined, solder with solidus temperature close to quenching temperature is selected.
The composition range of alloy tool steel is very wide. The suitable brazing filler metal, heat treatment process and the technology of combining brazing and heat treatment process should be determined according to the specific steel grades, so as to obtain good joint performance.
The quenching temperature of high-speed steel is generally higher than that of silver-copper and copper-zinc solders. Therefore, it is necessary to quench before brazing and braze during or after secondary tempering. If quenching must be done after brazing, only the special brazing filler metal mentioned above can be used for brazing. Coke oven is suitable for brazing high speed steel tools. When the brazing filler metal melts, the cutting tool is taken out and pressured immediately. The excess brazing filler metal is extruded, then oil quenched, and tempered at 550-570 C.
When brazing cemented carbide inserts with steel tool rods, it is advisable to increase the gap between the brazing seams and apply plastic compensating gaskets in the brazing seams, and slow cooling after welding to reduce the brazing stress, prevent cracks and prolong the service life of the cemented carbide tool assemblies.
5. Cleaning after brazing
Most of the flux residues corrode the brazed joints and hinder the inspection of the brazed joints, so they need to be cleaned up. Flux residues on weldments are first washed with hot water or with a general slag removal mixture, and then pickled with a suitable pickling solution to remove the oxide film on the base tool rod. But be careful not to use nitric acid solution to prevent corrosion of brazing metal. Residues of organic solders can be wiped or cleaned by gasoline, alcohol, acetone and other organic solvents. Residues of zinc oxide and ammonium chloride are highly corrosive and should be cleaned in 10% NaOH solution, then washed with hot or cold water. Residues of borax and boric acid solders are usually solved by mechanical method or by long-term immersion in boiling water.
6. Inspection of brazing quality
The inspection methods of brazed joints can be divided into non-destructive inspection and damage inspection. The following are mainly nondestructive testing methods:
(1) Appearance inspection.
(2) Coloring test and fluorescence test. These two methods are mainly used to inspect micro-cracks, pore, porosity and other defects which can not be found by appearance inspection.