Brazing (Soldadura fuerte)

"The American Welding Society (AWS ), defines brazing as a group of joining processes that produce coalescence of materials by heating them to the brazing temperature and by using a filler metal (solder) having a liquidus above 840°F (450°C), and below the solidus of the base metals" (lucasmilhaupt.com, 2018).

"At Brompton we braze all our steel frame parts as we feel this is the best process for our product and it allows us to make a light but durable frame; in contrast, the titanium parts are welded as this is the most suitable process for the material". (Zendesk.com, 2018).

"Brazing uses a filler metal (brass in our case), which is not the same as the base material and has a lower melting point. The component parts are not melted as with welding. This subjects the frame parts to less heat and reduces distortion, creating less stress and no altering of the physical properties of the materials. Brazing also uses less energy as the amount of heat needed is lower than welding. All this allows us to use thinner steel tubing and components, which would not be possible if we welded the steel parts because the thin material would be prone to extreme distortion or even burning" (Zendesk.com, 2018).

"A brazed joint is made in a completely different way from a welded joint. The first big difference is in temperature. Brazing doesn't melt the base metals. So brazing temperatures are invariably lower than the melting points of the base metals and, of course, always significantly lower than welding temperatures for the same base metals. If brazing doesn't fuse the base metals, how does it join them. It joins them by creating a metallurgical bond between the filler metal and the surfaces of the two metals being joined" (lucasmilhaupt.com, 2018).

"The principle by which the filler metal is drawn through the joint to create this bond is capillary action. In a brazing operation, you apply heat broadly to the base metals. The filler metal is then brought into contact with the heated parts. It is melted instantly by the heat in the base metals and drawn by capillary action completely through the joint" (lucasmilhaupt.com, 2018).

Basic steps in brazing

• Ensure fit and clearance.
• Clean metal.
• Flux prior to brazing: "Flux is a chemical compound applied to the joint surfaces before brazing. Its use is essential in the brazing process (with a few exceptions noted later.) The reason? Heating a metal surface accelerates the formation of oxides, the result of chemical combination between the hot metal and oxygen in the air. These oxides must be prevented from forming or they'll inhibit the brazing filler metal from wetting and bonding to the surfaces. A coating of flux on the joint area, however, will shield the surfaces from the air, preventing oxide formation. And the flux will also dissolve and absorb any oxides that form during heating or that were not completely removed in the cleaning process" (lucasmilhaupt.com, 2018).
• Fixturing of parts.
• Brazing the assembly.
• Cleaning the new joint: "Flux residues are chemically corrosive and, if not removed, could weaken certain joints" (lucasmilhaupt.com, 2018).

Brazing of low carbon and low alloy steel

Contain about 0.10% carbon with a fractional percentage of alloying elements such as nickel, manganese, copper and chromium. Low carbon steels have less than 0.30% carbon and low alloy steels have less than 5% total alloy content. All the BAg classifications can be used for brazing ferrous metals. Silver-base filler metals containing nickel usually provide better wettability, and are prefered for brazing certain low alloy steels where joint strength is most important. For torch brazing, the equipment would include standard oxyacetylene or propane torches. Low alloy steels are often brazed and heat-treated simultaneously using filler metals of the RBCuZn and BAg classifications. The solidus temperature of the selected filler metal must be above the austenitizing temperature recommended for the base metal (Brazing handbook, 1991). 

Fluxes for steel brazing

• Flux is required when brazing steel. Selection will depend upon the filler metal type. For example, AWS brazing flux types FB3A, FB3B and FB4 are suitable for BAg filler metals. Types FB4 and FB5 normally are used with the RBCuZn filler metals (Brazing handbook, 1991).
• It is necessary to use high temperature flux (decapante?) when using Bronze alloys for brazing (Castolin, year?).

1. https://www.castolin.com/Brazing-Consumables?page=1

Applications

Typical applications for the brazing of low carbon and low alloy steels in everyday production include components for automobiles, trucks, bicycles, motorcycles, snowmobiles, all-terrain vehicles and the like.

Tubes and pipe brazing

Most brazed joints in pipe and tubing do not need as much depth of socket insertion (overlap) as do soldered joints. Ordinarily, a depth of tube insertion three times the thickness of the tube will result in a joint as strong or stronger than the base metal (Brazing handbook, 1991).

Selection of filler materials for brazing of steel

Here is a list of filler materials for brazing steels that I have been writing.

Warning - Not recommended

• Cu-P alloy are not recommended for brazing of steels or stainless steels due to the risk of producing brittle surfaces at the joint (Castolin, year?).

References

(zendesk.com, 2018): https://brompton.zendesk.com/hc/en-us/articles/203295301-Why-are-Brompton-frame-parts-brazed-

(lucasmilhaupt.com, 2018): https://www.lucasmilhaupt.com/en-US/brazingfundamentals/brazingvssoldering/

(Brazing handbook, 1991): AWS (American Welding Society). Brazing Handbook. Fourth edition. 1991.

(Castolin, year?): Castolin: tecnología brazing. Tecnología de recargue y unión. Castolin Eutectic. Manual técnico o ficha técnica. https://www.castolin.com/sites/default/files/ckfinder/files/es-Brazing-Cat_low.pdf