Vacuum brazing of carbon steel and low alloy steel

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1. Brazing Materials

1. 1 Solder

The brazing of carbon steel and low alloy steel includes soft brazing and brazing. The solder that is widely used in soft soldering is tin-lead solder. The wettability of this solder to steel increases with the increase of tin content. Therefore, it is advisable to use solder with high tin content for sealed joints. The tin and steel in the tin-lead solder may form a FeSnz intermetallic compound layer at the interface. To avoid the formation of this layer compound, the soldering temperature and holding time should be properly controlled. The shear strength of several typical carbon steel joints brazed by tin-lead solder is shown in Table 1. Among them, the joint strength of solder brazed with w (Sn) of 50% is the highest. The welded joints have higher strength than those containing antimony.

Tab 1 Shear strength of carbon steel joints brazed with tin-lead solder

When brazing carbon steel and low-alloy steel, pure copper, copper-zinc and silver-copper-zinc brazing filler metals are mainly used. Pure copper has a high melting point and is easy to oxidize the base material during brazing. It is mainly used for gas shielded brazing and vacuum brazing. But it should be noted that the brazed joint gap should be less than 0.05mm, so as to avoid the problem that the joint gap cannot be filled due to the good fluidity of copper. Carbon steel and low alloy steel brazed with pure copper.The head has high strength, the general shear strength is 150~215MPa, and the tensile strength is distributed between 170~340MPa.Compared with pure copper, copper-zinc brazing filler metal reduces the melting point of the brazing filler metal due to the addition of Zn. In order to prevent the evaporation of Zn during brazing, on the one hand, a small amount of Si can be added to the copper-zinc solder; on the other hand, rapid heating methods must be used, such as flame brazing, induction brazing and dip brazing. Both carbon steel and low-alloy steel joints brazed with copper-zinc brazing filler metal have good strength and plasticity. For example, The tensile strength of the carbon steel joint brazed with B-Cu62Zn solder is 420MPa, and the shear strength is 290MPa. The melting point of the silver-copper station solder is lower than that of the copper-zinc solder, which is convenient for brazing. This brazing filler metal is suitable for flame brazing, induction brazing and furnace brazing of carbon steel and low-alloy steel, but the Zn content should be reduced as much as possible when brazing in the furnace, and the heating rate should be increased.

1.2 Flux

Brazing of carbon steel and low-alloy steel requires the use of flux or shielding gas. The flux is often determined by the selected solder and brazing method. When tin-lead solder is used, a mixture of zinc chloride and ammonium chloride can be used as a solder or other special solder. The residue of this flux is generally very corrosive, and the joints should be strictly cleaned after brazing. When using copper-zinc solder for brazing, FB301 or FB302 flux should be used, that is, borax or a mixture of borax and boric acid; in flame brazing, a mixture of methyl borate and formic acid can also be used as the flux. B203 steam is used to remove the film.

When silver-copper-zinc solder is used, FB102, FB103 and FB104 solders can be selected, that is, a mixture of borax, boric acid and certain fluorides. The residue of this flux has certain corrosiveness and should be cleaned up after brazing.

2. Brazing Technology

Use mechanical or chemical methods to clean the surface to be welded to ensure that the oxide film and organic matter are completely removed. The cleaned surface should not be too rough, and metal crumbs or other dirt should not be adhered. Carbon steel and low alloy steel can be brazed by various common brazing methods. For flame brazing, it is advisable to use a neutral or slightly reductive flame. During operation, avoid direct flame heating of the brazing filler metal and flux. Induction brazing and dip brazing and other rapid heating methods are very suitable for quenched and tempered steel. For brazing, at the same time, it is advisable to choose a temperature lower than quenching or tempering to prevent softening of the base material. When brazing low-alloy high-strength steel in a protective atmosphere, not only the purity of the gas is required, but also the gas flux must be used to ensure the wetting and spreading of the solder on the surface of the base metal. The residue of flux can be removed by chemical or mechanical methods. The residue of organic flux can be wiped or cleaned with organic solvents such as gasoline, alcohol, acetone, etc.: The residue of strong corrosive flux such as zinc chloride and ammonium chloride should be neutralized in NaOH aqueous solution first, and then hot and cold water Cleaning: The residues of boric acid and boric acid flux are not easy to remove and can only be solved by mechanical methods or long-term immersion in boiling water.