Vacuum brazing for carbide and carbon steel and stainless steel

---

The hard alloy is made of the hard compound of refractory metal and the bonding metal through powder metallurgy process. It is an alloy material with high hardness. It is characterized by high hardness, high strength, good toughness, wear resistance and heat resistance, especially its high hardness and excellent wear resistance, making it basically remain unchanged even at a temperature of 500°C and still have a very high hardness at 1000°C.

According to composition and performance characteristics, commonly used cemented carbide can be divided into three categories: tungsten-Cobalt, tungsten-Titanium-cobalt, and tungsten-Tantalum (niobium). Tungsten-cobalt and tungsten-Titanium-Cobalt cemented carbide are most widely used in production.

Brazing of tool steel and cemented carbide is mainly used for connecting cutting tools, measuring tools, molds, mining tools, and integral cutting tools. The tool handle is generally made of medium carbon steel or low alloy steel (such as 40Cr) with a carbon mass fraction of 0.4%~0.6%.
This kind of workpiece is subjected to considerable stress when working, especially compression bending, impact or alternating load, so it requires high strength and reliable quality of the joint.

Secondly, for tool steel, its structure and performance must not be affected by the brazing process, especially the hot annealing, high temperature oxidation and decarburization, etc., in order to ensure the cutting performance and working reliability, it is required that the brazing temperature and quenching temperature should be compatible, in order to get the maximum hardness and wear resistance when cutting.

For cemented carbide, its linear expansion coefficient is very different from that of ordinary steel. Cemented carbide is about 6×10-6/℃, and steel is 12×10-6/℃. The difference of linear expansion coefficient results in great stress after brazing, which is an important reason for carbide to crack. This is one of the main problems in cemented carbide brazing, so measures must be taken to reduce brazing stress. Therefore, measures should be taken to reduce brazing temperature, preheat brazing before brazing and slow cooling after brazing, use good brazing alloy, add compensation gasket or sandwich welding piece, and improve the structure of brazing joint.

Carbide flame and induction brazing have become relatively common welding methods, especially when the carbide brazing tool is heated to over 800 degrees Celsius in the air, the surface of the carbide brazing tool begins to oxidize to form a loose oxide layer, accompanied by decarburization. When heated to 950 c to 1100 C, the surface layer of cemented carbide brazing tools undergoes rapid oxidation, which reduces the mechanical properties of cemented carbide.

Choice of carbide and carbon steel, stainless steel brazing filler metal
According to the melting point and brazing temperature, the brazing alloy and brazing alloy can be divided into three types: high temperature solder, normal temperature solder and low temperature solder. Brazing solder above 1000℃ is called high temperature solder, such as red copper and 106 solder, etc. The brazing temperature of medium temperature solder is 850 ~ 1000℃, such as H62, H68 brass solder, etc. Low temperature solder, silver base solder.

Advantages of vacuum brazing:
1) Vacuum brazing The heating temperature of brazing is low, which has little influence on the microstructure and properties of the base material;
2) Vacuum brazing brazing joint is smooth and smooth, with beautiful appearance;
3) The deformation of vacuum brazing welding pieces is small, especially the brazing method using uniform heating (such as brazing in vacuum brazing furnace), the deformation of welding pieces can be reduced to the minimum, easy to ensure the dimensional accuracy of welding pieces;
4) In some brazing methods, dozens or hundreds of brazing joints can be welded at one time, with high productivity;
5) Vacuum brazing can realize the connection between heterogeneous metals or alloys, metals and nonmetals.

In the vacuum brazing of cemented carbide and steel, the base material is uniformly heated, and the joint strength is high after brazing.

Selection of filler metals for vacuum brazing of cemented carbide and steel
1. High-temperature solder: red copper.
2. Medium temperature solder: high strength brazing solder.

Preparation for brazing: mechanical or chemical cleaning of the surface to be welded to ensure the thorough removal of oxide film and organic matter. After cleaning the surface should not be too rough, do not adhere to metal particles or other dirt.

Cleaning after welding:
The residue of the flux can be removed by chemical or mechanical means.

The residue of organic brazing agent can be wiped or cleaned with gasoline, alcohol, acetone and other organic solvents:

The residue of strong corrosive brazing agent, such as zinc chloride and ammonium chloride, should be neutralized in NaOH aqueous solution first, and then cleaned with hot and cold water.

The residue of boric acid and borate brazing agent is not easy to remove and can only be solved by mechanical method or long time immersion in boiling water.

Learn More : Vacuum Brazing Furnace