What Does Brazing Mean?
Brazing is a heat fusion method of joining pipes for trenchless construction and rehabilitation jobs using a molten filler metal, which has a melting point above 450°C or 840°F but below the melting temperature of the joining parts.
As the filler metal liquefies, it distributes itself by capillary action between the closely fitted surfaces of the joint.
The brazing process differs from welding where high temperatures are used to melt the base metals together. Brazing can be carried out in an atmosphere with air, combusted fuel gas, ammonia, nitrogen, hydrogen, noble gasses, inorganic vapors, and vacuum.
The heating source can be from a torch, furnace, or induction coil. A good brazed joint is achieved when the filler and parent material are metallurgically compatible.
For brazing, the pipe joints have to be designed to include a gap into which the filler material will be distributed by capillary action. The filler material used to seal the connection between the two pipes often consists mainly of tin. However, since tin is a weaker metal, it is combined with copper, bismuth, silver, or nickel to strengthen it and help maintain structural integrity. Brazed joints work well for pipes transporting material of moderate temperatures.
Brazing can also be used for joining dissimilar metals and for both critical and non-critical applications due to the high-integrity joints produced.
Brazing is a reliable method, however, to prepare a good quality joint, care has to be taken to ensure that the joining parts are properly cleaned and proper brazing procedures are followed.
Trenchlesspedia Explains Brazing
Alloys for brazing are made of a mix of copper and phosphorus or silver mixed with other elements. The brazing process is especially useful for pipe fittings that are subject to high stresses.
It is necessary to follow proper brazing procedures to ensure that the end result is long lasting and problem free. Below are some important steps to successful brazing as indicated by The Fabricator :
- Good fit and proper clearance
For capillary action to be effective there should be proper clearance between the base metals so that the molten filler is well distributed. The typical clearance ranges from 0.001 to 0.005 inches (0.025 to 0.13 mm) but the optimum joint gap for most filler metals is 0.0015 inches (0.04 mm). When joining dissimilar metals, it is necessary to take into account their co-efficient of thermal expansion.
For new pipe parts, cleaning may not be necessary but when repairing, often metals are coated with dirt, corrosion, oil, or grease. This will form a barrier that will prevent proper brazing procedure resulting in an improper joint. Chemicals used for cleaning should be compatible with the base metals and residual chemicals should be completely removed.
A chemical compound called flux is applied to the joints before brazing as oxide formation is accelerated when metal is heated. Oxides prevent the filler metal from properly bonding to the base metals. Flux also helps dissolve and absorb oxides that remain after cleaning or that may be formed during the brazing process.
The joining parts should be held in the proper position for the capillary action to work effectively during the heating and cooling cycles. During brazing, both metals in the assembly should be uniformly heated to ensure they reach brazing temperature at the same time. Next, the brazing metal is deposited at the joints, a portion of which will be melted by the heated assembly and drawn by capillary action through the entire joint area.
As brazing fluxes are corrosive, it is essential to clean the assembly by removing the flux residues and oxide scales formed during brazing.