308A Metal Types and Properties 1 — Questions and Answers
Question 1: What is the most commonly used metal for HVAC ductwork fabrication?
- Copper
- Galvanized steel (Correct answer)
- Stainless steel
- Aluminum
Correct answer: Galvanized steel
Galvanized steel (steel coated with zinc) is the most widely used material for HVAC ductwork. The zinc coating provides corrosion resistance, and the steel provides structural strength at a reasonable cost.
Question 2: What is the purpose of the zinc coating on galvanized steel?
- To make it look shiny
- To provide sacrificial corrosion protection — the zinc corrodes before the base steel (Correct answer)
- To increase the weight
- To make it easier to weld
Correct answer: To provide sacrificial corrosion protection — the zinc corrodes before the base steel
Zinc acts as a sacrificial anode, corroding preferentially before the underlying steel. Even if the coating is scratched, the zinc continues to protect nearby exposed steel through galvanic (cathodic) protection.
Question 3: What gauge system is used for sheet metal thickness in Canada, and how does the gauge number relate to thickness?
- Higher gauge numbers mean thicker metal
- Lower gauge numbers indicate thicker metal — 16 gauge is thicker than 24 gauge (Correct answer)
- All gauge numbers represent the same thickness
- Gauge is measured in pounds per square foot
Correct answer: Lower gauge numbers indicate thicker metal — 16 gauge is thicker than 24 gauge
The gauge system is inversely related to thickness. Lower numbers are thicker (16 gauge = 0.0598") and higher numbers are thinner (26 gauge = 0.0179"). This system is used throughout North America.
Question 4: What type of stainless steel is most commonly used in sheet metal work and what makes it 'stainless'?
- Type 100, containing lead
- Type 304 (18-8), containing at least 18% chromium and 8% nickel that form a passive oxide layer (Correct answer)
- Type 500, containing copper
- Type 700, containing tin
Correct answer: Type 304 (18-8), containing at least 18% chromium and 8% nickel that form a passive oxide layer
Type 304 (also called 18-8) stainless steel contains minimum 18% chromium and 8% nickel. The chromium forms a thin, self-healing chromium oxide layer on the surface that resists corrosion.
Question 5: What is the primary advantage of using aluminum sheet metal for certain applications?
- It is the strongest metal available
- It is lightweight (about 1/3 the weight of steel) and naturally corrosion-resistant (Correct answer)
- It is cheaper than all other metals
- It is the easiest metal to weld
Correct answer: It is lightweight (about 1/3 the weight of steel) and naturally corrosion-resistant
Aluminum weighs approximately one-third as much as steel and forms a natural oxide layer that provides excellent corrosion resistance. These properties make it ideal for architectural, marine, and food-processing applications.
Question 6: What does the term 'temper' refer to in sheet metal materials?
- The temperature of the metal during installation
- The hardness and strength condition of the metal, determined by the degree of cold working or heat treatment (Correct answer)
- The colour of the metal
- The age of the metal
Correct answer: The hardness and strength condition of the metal, determined by the degree of cold working or heat treatment
Temper describes the mechanical condition of sheet metal — how hard, strong, and formable it is. Tempers range from dead soft (fully annealed) to full hard (extensively cold worked), affecting both strength and formability.
308A Metal Types and Properties 1
What is the most commonly used metal for HVAC ductwork fabrication?