308A Welding and Soldering for Sheet Metal 1 — Questions and Answers
Question 1: What type of welding process is most commonly used for joining sheet metal in HVAC fabrication?
- Shielded Metal Arc Welding (SMAW/Stick)
- Gas Metal Arc Welding (GMAW/MIG) (Correct answer)
- Submerged Arc Welding (SAW)
- Electroslag Welding (ESW)
Correct answer: Gas Metal Arc Welding (GMAW/MIG)
GMAW (MIG) welding is the most common process for sheet metal fabrication because it offers good speed, produces clean welds on thin materials, and is relatively easy to learn and control on gauges used in HVAC work.
Question 2: What is the primary difference between soldering and brazing?
- There is no difference
- Soldering uses filler metals that melt below 840°F (450°C), while brazing uses filler metals that melt above 840°F (450°C) (Correct answer)
- Brazing is only for copper
- Soldering requires more heat than brazing
Correct answer: Soldering uses filler metals that melt below 840°F (450°C), while brazing uses filler metals that melt above 840°F (450°C)
The defining difference is temperature: soldering occurs below 840°F (450°C) and brazing above. Brazing produces stronger joints than soldering due to the higher temperatures and stronger filler metals used.
Question 3: What flux is commonly used when soldering galvanized sheet metal?
- No flux is needed
- Zinc chloride (killed acid) flux or muriatic acid-based flux (Correct answer)
- Borax flux
- Silver brazing flux
Correct answer: Zinc chloride (killed acid) flux or muriatic acid-based flux
Zinc chloride (killed acid) flux or muriatic acid-based flux is used for soldering galvanized steel. The acid flux removes the zinc oxide layer and promotes wetting of the solder on the galvanized surface.
Question 4: What is the correct soldering technique for a flat lock seam?
- Apply solder only to the outside surface
- Heat the seam evenly, apply flux, and draw the solder through the entire seam by capillary action (Correct answer)
- Use a torch to melt the base metal
- Apply cold solder and heat it afterwards
Correct answer: Heat the seam evenly, apply flux, and draw the solder through the entire seam by capillary action
Proper soldering technique involves heating the seam to the correct temperature, applying flux to clean the surfaces, and allowing capillary action to draw the molten solder through the entire joint for a complete seal.
Question 5: What safety hazard is specifically associated with soldering galvanized steel?
- Electric shock
- Zinc oxide fumes that can cause metal fume fever (Correct answer)
- Hydrogen gas explosion
- Cyanide gas release
Correct answer: Zinc oxide fumes that can cause metal fume fever
Heating galvanized steel releases zinc oxide fumes which can cause metal fume fever — a flu-like illness with chills, fever, and body aches. Adequate ventilation or respiratory protection is essential.
Question 6: What is the correct shielding gas for MIG welding mild steel sheet metal?
- Pure oxygen
- 75% argon/25% CO2 (C25) or pure CO2 (Correct answer)
- Pure nitrogen
- Acetylene
Correct answer: 75% argon/25% CO2 (C25) or pure CO2
The most common shielding gas for MIG welding mild steel is 75% argon/25% CO2 (C25 mix), which provides a stable arc, good penetration, and minimal spatter. Pure CO2 is a less expensive alternative.
308A Welding and Soldering for Sheet Metal 1
What type of welding process is most commonly used for joining sheet metal in HVAC fabrication?