1. Brazing material
(1) Brazing of carbon steel and low alloy steel includes soft brazing and hard brazing. The widely used solder in soft soldering is tin lead solder. The wettability of this solder to steel increases with the increase of tin content, so the solder with high tin content should be used for sealing joints. Fesn2 intermetallic compound layer may be formed at the interface between tin and steel in tin lead solder. In order to avoid the formation of compound in this layer, the brazing temperature and holding time should be properly controlled. The shear strength of carbon steel joints brazed with several typical tin lead solders is shown in Table 1. Among them, the joint strength brazed with 50% w (SN) is the highest, and the joint strength welded with antimony free solder is higher than that with antimony.
Table 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 metal during brazing. It is mainly used for gas shielded brazing and vacuum brazing. However, it should be noted that the gap between brazed joints should be less than 0.05mm to avoid the problem that the joint gap cannot be filled due to the good fluidity of copper. The carbon steel and low alloy steel joints brazed with pure copper have high strength. Generally, the shear strength is 150 ~ 215mpa, while the tensile strength is distributed between 170 ~ 340mpa.
Compared with pure copper, the melting point of copper zinc solder decreases due to the addition of Zn. In order to prevent Zn evaporation 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. The joints of carbon steel and low alloy steel brazed with copper zinc filler metal have good strength and plasticity. For example, the tensile strength and shear strength of carbon steel joints brazed with b-cu62zn solder reach 420MPa and 290mpa. The melting point of silver copper station solder is lower than that of copper zinc solder, which is convenient for needle welding. This filler metal is suitable for flame brazing, induction brazing and furnace brazing of carbon steel and low alloy steel, but the content of Zn should be reduced as much as possible during furnace brazing, and the heating rate should be increased. Brazing carbon steel and low alloy steel with silver copper zinc filler metal can obtain joints with good strength and plasticity. The specific data are listed in Table 2.
Table 2 strength of low carbon steel joints brazed with silver copper zinc solder
(2) Flux: flux or shielding gas shall be used for brazing carbon steel and low alloy steel. The flux is usually determined by the selected filler metal and brazing method. When tin lead solder is used, the mixed liquid of zinc chloride and ammonium chloride can be used as flux or other special flux. The residue of this flux is generally highly corrosive, and the joint should be strictly cleaned after brazing.
When brazing with copper zinc filler metal, fb301 or fb302 flux shall be selected, that is, borax or the mixture of borax and boric acid; In flame brazing, the mixture of methyl borate and formic acid can also be used as brazing flux, in which B2O3 vapor plays the role of film removal.
When silver copper zinc brazing filler metal is used, fb102, fb103 and fb104 brazing fluxes can be selected, that is, the mixture of borax, boric acid and some fluorides. The residue of this flux is corrosive to a certain extent and should be removed after brazing.
2. Brazing technology
The surface to be welded shall be cleaned by mechanical or chemical methods to ensure that the oxide film and organic matter are completely removed. The cleaned surface shall not be too rough and shall not adhere to metal chips or other dirt.
Carbon steel and low alloy steel can be brazed by various common brazing methods. During flame brazing, neutral or slightly reducing flame should be used. During operation, direct heating of filler metal and flux by flame should be avoided as much as possible. Rapid heating methods such as induction brazing and dip brazing are very suitable for brazing of quenched and tempered steel. At the same time, quenching or brazing at a temperature lower than tempering should be selected to prevent softening of base metal. When brazing low alloy high strength steel in protective atmosphere, not only high purity of gas is required, but also gas flux must be used to ensure the wetting and spreading of filler metal on the surface of base metal.
The residual flux can be removed by chemical or mechanical methods. The residue of organic brazing flux can be wiped or cleaned with gasoline, alcohol, acetone and other organic solvents; The residues of strong corrosive flux such as zinc chloride and ammonium chloride shall be neutralized in NaOH aqueous solution first, and then cleaned with hot and cold water; Boric acid and boric acid flux residues are difficult to remove, and can only be solved by mechanical methods or long-time immersion in rising water.
Post time: Jun-13-2022