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Brazing welding of carbon steel, stainless steel and cast iron

The main welding method of carbon steel and stainless steel is fusion welding, but brazing welding shows its superiority in the occasions of dissimilar metals, complex and precise workpieces, and large deformation due to the thermal influence of fusion welding. Brazing is used for welding and repair welding of cast iron, which can avoid white structure, so it is also of practical significance.

According to the different uses of carbon steel, stainless steel and cast iron weldments, brazing welding temperature, joint performance, and production costs, the recommended solders include tin-lead solders, silver-based solders, copper-based solders, and nickel-based solders.

Tin-lead solder is the main solder used for soldering carbon steel and stainless steel. The strength of the brazing welding joint is shown in Table Strength of Low Carbon Steel and Stainless Steel Soldered Joints. In the arc repair welding of cast iron, the air tightness can be improved by covering the surface with tin-lead solder.

When brazing welding with hard solder, copper-based solder and silver-based solder are usually used. Copper-based solders include pure copper, brass and copper-nickel alloys, which are inexpensive and suitable for mass production. Among them, brass brazing material is mainly used for brazing carbon steel and cast iron, while brazing stainless steel is easy to cause “self-cracking” phenomenon of stainless steel, so it is not recommended to use. Pure copper and copper-nickel solder are mainly used for stainless steel brazing, which has good heat resistance, but the high brazing temperature is easy to make the base metal grain grow. Silver-based solder is the most commonly used solder for brazing carbon steel and stainless steel. It has low melting point, good wettability and easy operation. The effect of brazing cast iron with silver-based solder is better, but its use is limited due to its high cost. The strength of low carbon steel and stainless steel brazing joints is shown in Table Strength of mild steel and stainless steel brazed joints.

Strength of Low Carbon Steel and Stainless Steel Soldered Joints
Strength of mild steel and stainless steel brazed joints

When brazing carbon steel and cast iron with soft solder, zinc chloride or zinc chloride ammonium chloride aqueous solution can be used as brazing flux, and hydrochloric acid can also be used as brazing flux when brazing galvanized iron sheet. However, when brazing stainless steel with soft solder, more active zinc chloride hydrochloric acid solution or phosphoric acid solution must be used as the flux.

When brazing steel and cast iron with copper-based brazing material, dehydrated borax or borax-boric acid mixture can be used as brazing flux, but when brazing stainless steel, because of their insufficient activity, 200# flux containing calcium fluoride should be used. When brazing with silver-based solder, QJ101, QJ102 and other fluxes containing potassium fluorine borate or potassium fluoride are used.

Before brazing welding, the brazing part should be carefully cleaned. For cast iron, in order to remove the graphite that hinders the flow of the solder, the graphite on the surface can be burned off with the oxidation flame of oxyacetylene. When brazing with tin-lead solder, preheat the workpiece first, then add zinc chloride type flux, and finally heat the workpiece to the brazing temperature and fill it with solder. For carbon steel, flame brazing with brass filler metal is easy to operate; but when brazing cast iron, especially thin-walled workpieces, care must be taken to control the temperature, otherwise the base metal may melt. For larger workpieces, sprinkle a layer of flux on the cleaned brazing surface, then put the workpiece into a furnace (coke oven can be used) to heat, when the workpiece is heated to about 800C, add supplementary flux, and heat the workpiece with flame When the brazing temperature is reached, scrape the edge of the joint gap with the solder to melt the solder and flow into the gap. Flame brazing carbon steel, stainless steel and cast iron with silver-based brazing material is not difficult to operate.

Brazing welding of copper and copper alloys

Copper and copper alloys are usually divided into three categories: pure copper, brass and bronze.

Oxygen-free copper in pure copper is easy to braze; but when oxygen-containing copper is heated in a hydrogen-containing gas, hydrogen embrittlement will occur. At this time, hydrogen and copper oxide react to form water vapor, which generates high pressure and ruptures the interior of copper. The brazing performance of brass is better, but because zinc is easy to evaporate, it cannot be brazed in vacuum; at the same time, for brass with high lead content (greater than 3%), lead will seriously damage the wettability of solder, and lead Alloying with solder tends to produce brittle joints. Phosphor bronze, silicon bronze and white copper in bronze have better brazing properties, but aluminum bronze has increased the difficulty of brazing due to the formation of stable alumina on the surface. It should be noted that many cold-worked copper alloys, such as brass, phosphor bronze and white copper, are prone to thermal cracks and stress cracks when they are heated locally.

Almost all brazing methods are suitable for brazing copper and copper alloys, but flame brazing is the easiest.

Solder and Flux

The solder used for brazing copper and copper alloys includes tin-lead solder, cadmium-based solder and zinc-based solder, among which tin-lead solder is the most widely used. When brazing copper with tin-lead, cadmium-based and zinc-based solders, brittle intermetallic compounds are easily formed at the interface between the solder and the base metal. The formation of these compounds depends on the brazing temperature and time. The higher the brazing heating temperature and the longer the heating time, the greater the brittleness of the brazing joint. Practice has proved that when the tin-lead solder is lower than 300°C and the cadmium-based solder is lower than 400°C, the heating time is not long and the compound layer is very thin, which has little effect on the performance of the joint. The joint strength of brazing copper and brass with solder is shown in Table Joint strength of solder brazing copper and H62 brass. The heat resistance of tin-lead solder brazing joints is poor, and it is usually only used for brazing joints whose working temperature is lower than 150°C. If a higher working temperature is required, lead-silver solder and cadmium-based solder should be selected. Hot solder.

Joint strength of solder brazing copper and H62 brass

When brazing copper and copper alloys with brazing filler metals, brass filler metals, copper-phosphorus filler metals and silver filler metals can be used. Brass solder has a high melting point and is difficult to master. Copper-phosphorus solder has a low melting point, and because phosphorus can reduce copper oxide to act as a flux, copper-phosphorus solder does not need flux when brazing copper. However, when brazing copper alloys, since phosphorus cannot fully reduce the oxides formed by alloying elements, it should be used in conjunction with brazing flux. Silver solder has a low melting point and good wettability. When brazing copper and copper alloys, joints with good comprehensive properties can be obtained. The joint strength of brazing copper and brass with hard solder is shown in Table Joint Strength of Brazing Copper and H62 Brass with Hard Solder.

Brazing welding process

Preparation before brazing welding

When soldering copper and copper alloys, various fluxes shown in Table Joint strength of solder brazing copper and H62 brass. can be used. When brazing, borax, borax-boric acid and QJ301 can be used as brazing flux when brass solder is used, and potassium fluoroborate boric anhydride, QJ101 and QJ102 can be used as flux when silver solder is used. When brazing aluminum bronze, in order to improve the removal of aluminum-containing oxides, 10%~20% (mass fraction) aluminum flux can also be added to these fluxes.

Joint Strength of Brazing Copper and H62 Brass with Hard Solder

Before brazing welding copper and copper alloys, the surface of the weldment must be cleaned. Use tools such as scrapers, fine files, emery cloths and fine wire brushes to remove oil and scale on the surface. If there is still oil on the surface after cleaning, it must be cleaned with gasoline or alcohol. This method is inefficient and labor-intensive, and is suitable for small batch production.

For mass production or high surface quality requirements, it is best to use chemical methods for cleaning.

A typical cleaning process is as follows:

  •  Pure copper: soak in sulfuric acid with a mass fraction of 5%.

  • Brass and cupronickel: first soak in sulfuric acid with a mass fraction of 5%, and then soak in a mixture of sodium dichromate with a mass fraction of 2% and sulfuric acid with a mass fraction of 3%.

  • Silicon bronze: first dipped in hot sulfuric acid with a mass fraction of 5%, then dipped in a mixture of hydrofluoric acid with a mass fraction of 2% and sulfuric acid with a mass fraction of 3%, and finally in a 2% mass fraction of sulfuric acid Sodium dichromate and 5% sulfuric acid mixed solution for leaching.

  • Aluminum bronze: It needs to be continuously dipped in the following two solutions: ① a mixture of hydrofluoric acid with a mass fraction of 2% and sulfuric acid with a mass fraction of 3%; ② sodium dichromate with a mass fraction of 2% and a mass fraction of 5% sulfuric acid mixture. Repeat washing until clean.

The weldments cleaned by the above method must be rinsed in running water and dried or dried at 110 ~ 120 ℃.

Brazing operation

Because copper has good thermal conductivity, it must be heated with a powerful torch when flame brazing. The entire joint should be heated evenly. For large and complex parts, preheating should be carried out. The preheating temperature is 450~600C. The heating flame should use a neutral flame or a reducing flame with a small amount of excess acetylene. When using brass solder, heating with an oxidizing flame can reduce the evaporation of zinc.

When brazing with ordinary silver solder, heat the joint to orange red (600~700%C), then heat the end of the solder wire, and dip it in powdered flux, and smear it on the brazing surface (paste can also be used) Flux, or dip the weldment in the flux solution first), the heated flux melts and spreads along the joint gap. If some parts of the base metal are blackened and not stained with flux, it means that the oxide film here has not been removed, and the flux should be applied again until it is completely wetted and filled, then the solder can be added and the joints should be heated continuously. Make the solder flow into the deep gap. The heating flame moves back and forth along the entire joint to heat the joint evenly until the solder oozes out at the other end of the gap and forms a smooth fillet. Small weldments can be rotated and heated once to complete the brazing, while large weldments must be brazed in sections (that is, the next section is brazed after one section is brazed).

If copper-phosphorus solder is used to braze copper, no flux is required. As long as the joint is heated to orange, you can rub the solder on the brazing surface to melt it by the heat conduction of the base metal. The molten solder quickly wets and fills the joints.

Copper and copper alloy flame brazing should pay attention to the following points:

  • The brazing heating time should be as short as possible to avoid excessive oxidation of the joints.

  • It is not possible to directly heat the solder wire with a flame, but the weldment should be heated so that the solder melts when it touches the weldment, wets and fills the joint. The flame should not be heated against the melted solder and flux, so as to avoid overheating of the solder and flux, causing volatilization of certain components and oxidation of the solder to deteriorate the performance of the joint. If the weldment is not heated enough to fill the gap with the brazing filler metal, the base metal near the brazing seam can be heated with a flame, and the heat is transferred to the brazing filler metal to be heated and melted, and the entire gap is filled under the action of capillary suction and brazing flux .

  • After the parts are brazed, do not move them immediately, so as not to cause the brazing seam to shift due to the unsolidified brazing filler metal.

Post-solder cleaning

After brazing with flux, there are residual slag and oxide film on the surface of the weldment. These substances are easy to absorb moisture in the air and cause corrosion of the parts. Therefore, the residue and oxide film must be removed after brazing. There are many cleaning methods, which can be mechanical or chemical, depending on the specific situation.

Borax and boric acid flux used for brazing are glassy after brazing, have low solubility in water, and are difficult to remove. They are generally removed by mechanical methods (such as sandblasting). It is best to put the weldment in water before it is completely cooled after brazing, and the thermal shock generated will crack the residue and make it easy to remove. But this method cannot be used for brazed joints that are sensitive to thermal shock. In addition, the weldment can also be dipped in a potassium dichromate solution with a mass fraction of 2% to 3% at a higher temperature (70~90°C) for a longer period of time.

Brazing flux for silver brazing containing more potassium fluoroborate or potassium fluoride will not form glassy slag, and the weldment can be boiled in water or 10% citric acid hot water solution to remove slag. Zinc chloride and ammonium chloride type solder can be removed with hot water. However, for the zinc chloride type solder paste prepared with vaseline, the residual grease on the joint surface should be removed with an organic solvent, and then soaked in hot water.