Soldering Flux and Brazing Materials: Effective Usage and Storage

Use and Storage of Flux and Re-drying to Prevent Moisture Absorption

Use and Storage of Flux

Flux should be protected from moisture, contamination, and impurities, and its particle size should be maintained. The following points should be noted regarding the use and storage of flux:

1) Fused flux does not absorb moisture, simplifying packaging, transportation, and storage processes. Non-fused flux is highly hygroscopic, which is the main cause of porosity in weld metal and hydrogen-induced cracking.

Therefore, flux that has been dried before leaving the factory should be placed in moisture-proof containers and sealed, and care should be taken to prevent damage during transportation. Various types of flux should be stored in dry warehouses at temperatures ranging from 5 to 50°C and should not be placed in high-temperature or high-humidity environments.

2) When the particle size of flux is less than 0.1mm or greater than 2.5mm, it generates a large amount of dust, affecting environmental hygiene and should not be used during welding. When the particle size of flux exceeds 2.5mm, it does not effectively shield the air to protect the weld metal, and it also adversely affects the transition of alloy elements.

Therefore, care should be taken during the storage, transportation, and recovery of flux to prevent agglomeration or pulverization, in order to avoid any adverse effects on the welding process.

3) Flux should be kept clean and pure. Undisinfected or unfused flux can be reused multiple times, but it should not be contaminated by rust, oxide scale, or other foreign substances, and slag and fines in the flux should also be removed. Flux contaminated with oil or other substances should be discarded.

4) Appropriate stacking height: The height at which flux is stacked during welding is directly proportional to the pressure on the surface of the weld pool.

If the stack is too high, the weld surface will have large ripples and unevenness, resulting in “pimples.” The generally used vitreous flux should be stacked at a height of 25 to 45mm, and when welding at high speeds, the flux should be stacked lower, but not too low, as excessive exposure of the arc will result in a rough weld surface.

The Moisture Absorption and Re-drying of Flux

Similar to welding rods, flux products leaving the factory have been dried and packaged using moisture-resistant materials. However, during the storage of flux, it also adsorbs some moisture. The moisture absorption of flux is influenced by the storage environment’s temperature and humidity, as well as the manufacturing process and composition of the flux.

Under the same temperature and humidity conditions, the moisture absorption curves of smelting flux, high-temperature sintering flux, and low-temperature sintering flux (also known as bonding flux) produced using different manufacturing processes can be seen in Figure 14-7.

When moisture-absorbing flux is used for submerged arc welding, pockmarks may appear on the weld bead, and it may even cause porosity. During the welding process, a “popping” sound occurs, and the surface formation of the weld bead deteriorates.

When welding high-strength steel using moisture-absorbing flux, the diffusion of hydrogen in the weld seam increases, which can easily lead to weld cold cracking, posing a hidden danger to structural safety.

For moisture-absorbing flux, re-drying must be carried out before use, and the drying temperature and time should be differentiated based on the type of flux. Smelting flux is mostly glassy or vitreous, and it is not prone to moisture absorption or deterioration. Even if left for a long time, it will only adsorb a small amount of moisture.

As shown in Figure 14-7, when placed under high-temperature and high-humidity conditions of 30°C and 90% humidity for 5 days, the moisture absorption is only below 0.1%. Despite the small amount of moisture absorption, it still increases the diffusion of hydrogen in the weld seam. Therefore, re-drying and strict management of the flux are essential. To remove adsorbed moisture, drying at temperatures above 250°C is sufficient.

The moisture absorption characteristics of high-temperature sintering flux are similar to those of smelting flux, as shown in Figure 14-7. Hence, the re-drying and management of high-temperature sintering flux can also follow the requirements for smelting flux. Low-temperature sintering flux is similar to low hydrogen type welding rods, as they both use water glass as a binder.

Therefore, their moisture absorption characteristics are also similar, and they will absorb significant moisture after being placed in a humid environment for a long time. To prevent moisture absorption, the flux should be packaged in sealed iron drums.

Due to the different alkalinity and uses of flux, the re-drying procedures also vary. Table 14-4 lists the drying process parameters commonly used domestically and internationally as a reference.

Additionally, when welding important high-strength steel structures, the properly dried flux should be stored in a heat preservation box at a temperature of 120 to 150°C, taken out as needed, and its usage time should be limited (usually 4 hours). Flux that exceeds the specified usage time needs to be re-dried before use.

Figure 14-7 Moisture Absorption Curves for Various Types of Fluxes
Figure 14-7 Moisture Absorption Curves for Various Types of Fluxes

Use and Storage of Brazing Materials

Brazing materials mainly consist of brazing filler metal and flux. The brazing filler metal serves as the filling material during brazing, joining the workpieces through the melted filler metal.

The wetting of the molten filler metal to the base metal during brazing primarily depends on the action of the flux. Therefore, both the brazing filler metal and the flux are crucial components during the brazing process, and their usage and storage measures are essential for the brazing process.

Table 14-4 Process Parameters for Drying Various Types of Fluxes

type of fluxgradedrying process
smelting fluxHJ130,HJ131,HJ150Around 2502
HJ152Around 3502
HJ211350 ± 101
HJ230Around 2502
HJ252Around 3502 (cool to below 100°C after baking)
HJ330Around 2502
HJ360Around 2502
HJ430,HJ431,HJ433Around 2502
sintering fluxSJ101300-3502
SJ203around 2502
SJ401around 2502
SJ607,SJ608,SJ608A 300~3502

Use of brazing materials

When using brazing materials, the following issues should be noted:

1.Avoid excessive flow of molten brazing material in the brazed joint to prevent uneven melting of the base material and brazed joint structure.

2.If the brazing material is relatively fine compared to the base material, it should be placed in a stable position (such as in a groove) to prevent it from rolling away due to its low heat capacity. If there are significant differences in mass between different parts of the base material, the brazing material should be placed against the heavier component.

3.When the primary heat source for brazing is through radiant heat transfer (such as in flame automatic brazing lines and furnaces), precautions should be taken to prevent premature melting and rolling of the base material before reaching the brazing temperature due to radiant heating.

4.Mixing chloride-based brazing flux with anhydrous acetone to form a paste, then using it to adhere the brazing material to the desired location and applying a small amount of flux paste on top can help reduce the aforementioned issues during the brazing process.

Storage of brazing materials

1.The brazing flux should be stored in a container (such as a barrel) that does not affect its performance, and it should be sealed without any signs of leakage. Each container should be labeled with the manufacturer’s name, trademark, type of brazing flux, and date of manufacture, and should have a quality certificate.

2.The outer packaging of liquid brazing flux should bear the label “flammable liquid,” following the specific operations as outlined in the national standard GB/T15829-2008 “Classification and Performance Requirements of Soft Brazing Flux.” During transportation, it should be protected from light, heat, vibration, and impact.

3.Brazing flux should be stored in a cool place at a temperature of 5 to 35°C, and its effective storage period is six months.

4.The surface of brazing materials is highly susceptible to reacting with the ambient atmosphere to form a corrosion film, mainly composed of various oxides (which may also include chlorides, sulfides, carbonates, etc.), which will significantly affect the brazing performance of the brazing materials. Therefore, the brazing materials must be stored in a sealed container.

Safety Precautions for Brazing Filler Metal and Flux

During the use of brazing materials, especially flux, proper ventilation and protection against toxic substances are essential. The brazing materials contain certain toxic substances that are easily volatile when heated, such as Cd, Be, Zn, and Pb.

Fluxes contain fluorides, chlorides, and borides. Therefore, it is necessary to take proper protective measures when using brazing materials to prevent contamination of the brazing environment and harm to the operator’s health.

When cleaning parts and brazing materials before brazing, the use of cleaning agents (such as acids, alkalis, chlorinated hydrocarbons, and other organic solvents) also requires strict protective measures to ensure that the environment is not contaminated by toxic substances.

Typically, effective protective measures include indoor ventilation, which can exhaust the toxic fumes and volatile atmospheres produced during the brazing process outdoors, effectively ensuring the health and safety of the operator. When brazing metals and brazing materials contain toxic metals such as Cd, Be, Zn, Pb, as well as when fluxes contain fluorides, strict and effective protective measures must be taken.

1.Beryllium (Be) has high application value in the atomic energy, aerospace, and electronics industries, but it is highly toxic. Therefore, it is best to braze beryllium and beryllium oxide in a closed ventilation system with purification devices that meet specified standards before being discharged outdoors.

2.Cadmium (Cd) is usually added to brazing materials to improve brazing processability. It easily volatilizes upon heating and can enter the human body through the respiratory and digestive tracts, causing acute poisoning. Therefore, apart from brazing in a closed ventilation system, efforts should be made to minimize the use of Cd.

3.Lead (Pb) is a primary component of soft brazing materials. When heated to 400-500°C, it generates a large amount of lead vapor and lead oxide in the air. Lead vapor exposure typically leads to chronic poisoning. To protect against lead vapor, the maximum allowable concentration of lead smoke in the workshop air is regulated at 0.03mg/m3, and lead dust at 0.05mg/m3.

4.Zinc (Zn) and its compound ZnCl2 volatilize during brazing, producing zinc fumes. Inhalation of these fumes can cause metal fume fever. Therefore, contact with fumes must be prevented, and personal protective equipment and good ventilation must be used. When the skin comes into contact with ZnCl2 solution, the affected area should be thoroughly washed with plenty of water.

5.When using flux containing fluorides, brazing must be conducted in ventilated conditions or with personal protective equipment. When using fluoride-containing flux for dip brazing, the exhaust system must ensure that the environmental concentration is within the specified range, with the current national maximum allowable concentration set at 1mg/m3.

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