1. Brazing Materials
(1) Brazing Materials
Pure copper, copper-zinc, and silver-copper brazing materials are commonly used for brazing tool steel and hard alloys. Pure copper has good wetting properties for many hard alloys, but the best results are obtained during brazing in a hydrogen reducing atmosphere.
However, due to the high brazing temperature and the resulting high stress on the joint, there is a greater tendency for cracking. Pure copper brazed joints have shear strength of about 150MPa and higher ductility, but are not suitable for high temperature applications.
Copper-zinc brazing materials are the most commonly used brazing materials for tool steel and hard alloys. To improve the wetting properties of the brazing material and the strength of the joint, alloying elements such as Mn, Ni, Fe, etc. are used. are often added to the brazing material.
For example, adding 4% Mn into B-Cu58ZnMn can achieve a shear strength of 300-320MPa at room temperature for brazing hard alloy joints, which can still maintain 220-240MPa at 320°C.
Adding a small amount of Co to B-Cu58ZnMn can increase the shear strength of the welded joint to 350MPa and improve its impact resistance and fatigue resistance, significantly increasing the service life of cutting tools and rock drilling tools.
Silver-copper brazing materials have a lower melting point, resulting in lower thermal stresses at brazed joints and reducing the tendency to crack.
To improve the wetting properties of brazing material and increase the strength and working temperature of the joint, alloying elements such as Mn, Ni, etc. are used. are also often added to the brazing material. For example, the brazing material B-Ag50CuZnCdNi has excellent wetting properties for hard alloys, and the brazed joints have good overall performance.
In addition to the above three types of brazing materials, for hard alloys that work at temperatures above 500℃ and require high joint strength, Mn and Ni-based brazing materials such as B-Mn50NiCuCrCo and B-Mn50NiCuCrCo can be used. Ni75CrSiB.
For brazing high-speed steel, specialized brazing materials must be selected that match the brazing temperature with the quenching temperature.
These brazing materials can be divided into two types: iron manganese-based brazing materials, mainly composed of iron manganese and borax, with a shear strength of the welded joint generally around 100MPa, but prone to cracking; and special copper alloys containing Ni, Fe, Mn and Si, which can prevent cracking and increase the shear strength of the joint to 300MPa.
(2) Brazing Agents and Shielding Gases
The selection of brazing agents must be compatible with the base material to be brazed and the brazing materials chosen. For brazing tool steels and hard alloys, borax and boric acid are the main brazing agents, with the addition of some fluorides (KF, NaF, CaF2, etc.).
Copper-zinc brazing materials are used with FB301, FB302 and FB105 brazing agents, while silver-copper brazing materials are used with FB101-FB104 brazing agents. Borax brazing agents are primarily used for specialized brazing of high-speed steel.
To prevent oxidation of tool steel during brazing and avoid the need for post-brazing cleaning, gas shielded brazing can be employed. The shielding gas can be an inert gas or a reducing gas, with the requirement that the dew point of the gas be below -40°C. The hard alloy can be welded under hydrogen protection, with the required hydrogen gas dew point below -59°C.
2. Brazing Techniques
Tool steel must be cleaned before brazing. The surface does not need to be very smooth for mechanical processing, to facilitate the wetting and spreading of the material and brazing agent.
The surface of the hard alloy must be sandblasted before brazing, or polished with silicon carbide or diamond wheels to remove excess carbon and facilitate wetting by the brazing material.
Hard alloys containing titanium carbide are difficult to wet. A new method is to apply copper oxide or nickel oxide paste to the surface and bake it in a reducing atmosphere to transfer copper or nickel to the surface, thereby increasing the wetting properties of the brazing material.
Brazing carbon tool steel is best done before or simultaneously with the quenching process.
If brazing is performed before the quenching process, the solidus line temperature of the brazing material must be higher than the quenching temperature range to ensure that the joint still has sufficient strength when reheated to the quenching temperature without failure.
When brazing and quenching are carried out together, brazing materials with solidus line temperatures close to the quenching temperature should be selected.
The composition range of tool steel alloy is wide, and the appropriate brazing materials, heat treatment processes, and techniques for combining brazing and heat treatment must be determined based on the specific steel type to achieve good joint performance.
The quenching temperature of high-speed steel is generally higher than the melting temperature of silver-copper and copper-zinc brazing materials. Therefore, quenching must be carried out before brazing, and brazing must be carried out during or after secondary tempering.
If quenching needs to be carried out after brazing, only the specialized brazing materials mentioned above may be used. When brazing high-speed steel tools, a coke oven is suitable.
After the brazing material is melted, the tool must be immediately removed and pressed to extrude excess brazing material, followed by quenching and tempering in oil at 550-570°C.
When welding hard alloy blades to steel tool shanks, it is advisable to increase the brazing gap and apply plastic compensating shims to the joint to reduce the cooling rate after brazing, thereby reducing the brazing stress, preventing the formation of cracks and extending the useful life of hard parts. alloy tool components.
After brazing, the brazing agent residues on the joint should be rinsed with hot water or general flux mixture, followed by acid pickling with appropriate acid solution to remove the oxide film on the base material. However, nitric acid solution should not be used to prevent corrosion of the welded joint metal.
