Brazing Carbon Steel and Low Alloy Steel: Explained

1. Brazing Materials

(1) Filler metals

Brazing of carbon steel and low alloy steel includes soft brazing and hard brazing. The most commonly used filler metal in soft brazing is tin-lead solder, which improves the wettability of steel with increasing tin content.

Therefore, solder with a high tin content must be used to seal the joints. The tin in tin-lead solder can form a FeSn ₂ intermetallic compound layer at the interface with the steel. To avoid the formation of this composite layer, the brazing temperature and residence time must be properly controlled.

The shear strength of carbon steel joints welded with several typical tin-lead solders is shown in Table 1.

Among them, the strength of the joint welded with filler metal with 50% w(Sn) is the highest, and the strength of the joint welded with solder without antimony is higher than that with antimony.

Table 1 Shear strength of carbon steel joints welded with tin-lead solder

When carbon steel and low-alloy steel are hard brazed, pure copper, copper-zinc and silver-copper-zinc brazing materials are mainly used. Pure copper has a high melting point and is susceptible to oxidation during brazing. It is mainly used for gas shielded brazing and vacuum brazing.

However, it should be noted that the joint gap for brazing should be less than 0.05mm to avoid problems caused by the good fluidity of copper preventing the joint gap from being filled.

Carbon steel and low alloy steel joints welded with pure copper have greater strength, with shear strength generally ranging from 150 to 215MPa and tensile strength ranging from 170 to 340MPa.

Compared to pure copper, copper-zinc brazing materials have a lower melting point due to the addition of Zn. To prevent evaporation of Zn during brazing, a small amount of Si can be added to the copper-zinc brazing material.

In addition, rapid heating methods such as flame brazing, induction brazing and immersion brazing should be used. Carbon steel and low alloy steel joints welded with copper-zinc brazing materials have good strength and ductility. For example, carbon steel joints welded with brazing material B-Cu62Zn have a tensile strength of 420MPa and a shear strength of 290MPa.

Silver-copper-zinc brazing materials have a lower melting point compared to copper-zinc brazing materials, making them easier to use for spot brazing. This kind of brazing material is suitable for flame brazing, induction brazing and furnace brazing of carbon steel and low alloy steel.

However, the zinc content should be minimized and the heating speed should be increased when using silver-copper-zinc brazing materials for furnace brazing.

Brazing carbon steel and low alloy steel with silver-copper-zinc brazing materials can obtain joints with good strength and ductility. Specific data are listed in Table 2.

Table 2: Strength of low carbon steel joints welded with silver-copper-zinc brazing materials.

(2) Brazing Agents

When brazing carbon steel and low alloy steel, brazing agents or shielding gases are required. The choice of brazing agents depends on the brazing materials and brazing methods selected.

When using tin-lead brazing materials, a mixture of zinc chloride and ammonium chloride can be used as a brazing agent or other specialized brazing agents. Residues from this type of brazing agent are generally highly corrosive, so joints must be carefully cleaned after brazing.

When using copper-zinc brazing materials for hard brazing, brazing agents FB301 or FB302 should be used, which are mixtures of borax or borax and boric acid. In flame brazing, a mixture of methyl borate and formic acid can also be used as a brazing agent, with the B2O3 vapor playing a role in deoxidizing the joint.

When using silver-copper-zinc brazing materials, brazing agents FB102, FB103 and FB104, which are mixtures of borax, boric acid and certain fluorides, can be selected. Residues of this type of brazing agent are somewhat corrosive and must be carefully removed after brazing.

2. Brazing Techniques

Several common brazing methods can be used for brazing carbon steel and low-alloy steel. When flame brazing, it is advisable to use a neutral or slightly reducing flame.

Direct heating of brazing materials and agents must be avoided. Rapid heating methods such as induction brazing and immersion brazing are very suitable for brazing quenched and tempered steel.

It is also advisable to choose quenching or a temperature lower than tempering for brazing to avoid softening the base material. When brazing low-alloy, high-strength steel in a protective atmosphere, not only is high gas purity required, but gas flow must also be used to ensure wetting and spreading of the brazing material on the surface of the base material.

Brazing agent residues can be removed chemically or mechanically. Organic brazing agent residues can be cleaned with organic solvents such as gasoline, alcohol or acetone. Residues of corrosive brazing agents such as zinc chloride and ammonium chloride should be neutralized in aqueous NaOH solution and then washed with hot and cold water.

Boric acid and borax brazing agent residues are difficult to remove and can only be resolved by mechanical methods or prolonged immersion in rising water.