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Brazing of carbon steel and low alloy steel

1 brazing sex

The brazing properties of carbon steel and low alloy steel depend largely on the type of oxide formed on the surface of the material. As the temperature rises, four types of oxides, y-fe2o3, a-fe2o3, Fe3O4 and FeO, are formed on the surface of carbon steel.
 These oxides are porous and unstable except Fe3O4, which can be easily removed by brazing agent and reduced by reducing gas. Therefore, carbon steel has good brazing property.

For low alloy steel, if the content of alloy elements is quite low, the material on the surface of the oxide is basically iron oxide, at this time of the low alloy steel with carbon steel as brazing. If the increase in the number of alloy element contains, especially, such as A1 and Cr is easy to form stable increase in the number of elements of the oxide, can make the brazing of low alloy steel, then should choose larger flux or active low dew point of shielding gas for brazing.


Brazing of carbon steel and low alloy steel


2 brazing material

(1) brazing of solder carbon steel and low alloy steel includes soft brazing and hard brazing. The most widely used solder in soft soldering is tin lead solder. The wettability of this solder to steel increases with the increase of tin content. The fer-sn intermetallic compound layer may be formed on the interface between tin and steel in the solder metal. In order to avoid the formation of the compound layer, the brazing temperature and thermal insulation time should be controlled appropriately. Several typical tin lead solder brazing of shear strength of carbon steel joints as shown in table 1, among them with w (Sn) for 50% of the highest solder brazing joint strength, do not contain antimony welding of solder joint strength is higher than of antimony.

For hard brazing of carbon steel and low alloy steel, pure copper, copper zinc and silver copper zinc are mainly adopted. High melting point of pure copper, easy to oxidize the base material when brazing, mainly used for gas shielded brazing and vacuum brazing. However, it should be noted that the clearance of brazing head should be less than 0. 05mm, in order to avoid the problem that the joint clearance cannot be filled due to the good fluidity of copper. Carbon steel and low alloy steel joints welded with pure copper brazing have high strength, generally the shear strength is within the range of 150 ~ 215MPa, and the tensile strength is between 170 ~ 340MPa.

Compared with pure copper, the melting point of the filler metal is reduced by the addition of Zn. In order to prevent the evaporation of zinc when soldering, on the one hand, can add a small amount of Si in copper zinc filler metals, on the other hand must adopt the method of rapid heating, such as flame brazing, induction brazing and dip soldering, etc. Carbon steel and low alloy steel joints welded with copper and zinc solder have good strength and plasticity. For example, carbon steel joints welded with b-cu62zn solder have tensile strength of 420MPa and shear strength of 290MPa. The melting point of silver, copper and zinc solder is lower than that of copper and zinc solder. The solder is suitable for flame brazing, induction brazing and furnace brazing of carbon steel and low alloy steel. High strength and good plasticity can be obtained by welding carbon steel and low alloy steel with silver and copper needle solder.

(2) when brazing carbon steel and low alloy steel, the brazing agent or protective gas shall be used. The brazing agent is usually determined by the selected solder and brazing method. When using tin-lead solder, the mixture of zinc chloride and ammonium chloride can be used as brazing agent or other special brazing benefits. The soldering flux residues are generally highly corrosive and should be cleaned strictly after brazing. No soldering agent is required for vacuum brazing.

Used for brazing copper zinc filler metals, should choose FB301 or FB302 solder (see JB/T6045-92 use brazing flux, similarly hereinafter), namely, or a mixture of borax and boric acid, borax in flame brazing, also can use a mixture of boric acid methyl ester with formic acid as flux, which is B2O3 steam stripping effect.

FBl02, FBl03 and 1FBl04 brazing agents, i.e. a mixture of borax, boric acid and some fluoride, can be selected when silver, copper and zinc solders are used. The residue of this kind of soldering agent is corrosive and should be cleaned after soldering.

3 brazing technology

Mechanical or chemical cleaning of the surface to be welded to ensure complete 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.

Carbon steel and low alloy steel can be brazed by various common brazing methods. For flame brazing, neutral or slightly reducing flame should be used, and direct heating of solder and brazing agent should be avoided. Induction brazing, dip brazing and other rapid heating methods are very suitable for the brazing of tempered steel. At the same time, it is advisable to choose the quenching or lower tempering temperature for brazing to prevent the softening of the base material. When brazing low alloy and high strength steel in protective atmosphere, it not only requires high purity of gas, but also must be equipped with gas brazing agent to ensure the wetting and spreading of filler metal on the surface of the base material.

The solder residue can be removed chemically or mechanically. Organic solder residue available petrol, alcohol, acetone and other organic solvents to wipe or wash: zinc chloride and ammonium chloride and so on the corrosive solder residue, should be neutral in the NaOH aqueous solution, and then clean with hot and cold water; The residue of boric acid and borate soldering agent is not easy to be removed, and can only be solved by mechanical method or by soaking in boiling water for a long time.

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