Soldering, Brazing, Welding and Pipe Joining: Heat, Filler and Preparation
What this domain tests
This domain tests joining methods by heat and by mechanical means. Expect questions on temperature ranges, capillary action, flux, filler metals and the preparation needed for a sound joint.
Core rules and concepts
- Soldering uses a filler that melts below 450°C; brazing uses a filler above 450°C; welding fuses the parent metal.
- Capillary action draws molten solder into a closely fitted copper joint; the gap must be clean and uniform.
- Use lead-free solder for potable water. Flux cleans the surface but must be applied sparingly and flushed after joining.
- Prepare copper by cutting square, deburring, cleaning with abrasive and assembling dry before heating.
- PPR and HDPE are joined by heat fusion, not solder. GI is joined by threaded fittings. PVC/CPVC use solvent cement.
- Hot-work safety: fire watch, extinguisher, shielding and ventilation.
Worked example
A copper joint leaks after soldering. The likely causes are a wet pipe, insufficient heat, too much flux or a dirty surface. The correct method: cut square, deburr, clean both surfaces, apply a thin flux film, assemble, heat evenly until the solder melts on contact and is drawn into the gap by capillary action, then wipe and cool. The reasoning: solder does not fill a dirty or wet gap, and overheating burns the flux.
Traps that cost marks
- Heating a pipe that still contains water.
- Using too much flux and leaving corrosive residue.
- Applying solder before the joint reaches temperature.
- Using the wrong filler for potable water.
- Soldering a PPR joint instead of heat-fusing it.