308A Metal Types and Properties 3 — Questions and Answers
Question 1: What type of sheet metal is required for kitchen exhaust hoods handling grease-laden vapours?
- Galvanized steel
- Minimum 18-gauge stainless steel (Type 304 or 316) for grease resistance and fire safety (Correct answer)
- Aluminum
- Copper
Correct answer: Minimum 18-gauge stainless steel (Type 304 or 316) for grease resistance and fire safety
Kitchen exhaust hoods require stainless steel (typically Type 304 or 316) at a minimum of 18 gauge for grease resistance, cleanability, fire resistance, and compliance with fire codes. Galvanized steel is not permitted.
Question 2: What is 'G90' coating designation on galvanized steel?
- The gauge of the steel
- The zinc coating weight — 0.90 ounces of zinc per square foot of sheet (both sides combined) (Correct answer)
- The grade of the steel
- The guaranteed lifespan in years
Correct answer: The zinc coating weight — 0.90 ounces of zinc per square foot of sheet (both sides combined)
G90 indicates the zinc coating weight is 0.90 ounces per square foot (total for both sides). This is the most common coating weight for HVAC ductwork, providing good corrosion protection for normal environments.
Question 3: Why is Type 316 stainless steel specified instead of Type 304 for certain sheet metal applications?
- Type 316 is cheaper
- Type 316 contains molybdenum which provides superior resistance to chloride and chemical corrosion (Correct answer)
- Type 316 is lighter weight
- Type 316 is easier to form
Correct answer: Type 316 contains molybdenum which provides superior resistance to chloride and chemical corrosion
Type 316 stainless steel contains 2-3% molybdenum, which provides significantly better resistance to chlorides, acids, and other corrosive chemicals. It is specified for marine, chemical, and coastal environments.
Question 4: What is 'terne-coated' steel and what was it historically used for?
- Steel coated with copper
- Steel coated with a lead-tin alloy, historically used for roofing and architectural sheet metal (Correct answer)
- Steel coated with zinc
- Steel coated with aluminum
Correct answer: Steel coated with a lead-tin alloy, historically used for roofing and architectural sheet metal
Terne coating is a lead-tin alloy applied to steel. Historically used for roofing, it provided excellent solderability and corrosion resistance. Modern terne alternatives use tin-zinc coatings due to lead health concerns.
Question 5: What physical property of metal is described by 'ductility'?
- Resistance to heat
- The ability to be stretched, formed, or drawn into shapes without fracturing (Correct answer)
- The weight per cubic foot
- The electrical conductivity
Correct answer: The ability to be stretched, formed, or drawn into shapes without fracturing
Ductility is the ability of a metal to be deformed (stretched, bent, drawn, or hammered) without breaking. Highly ductile metals like copper, aluminum, and mild steel are well-suited for sheet metal forming operations.
Question 6: What is the effect of cold rolling on sheet metal properties?
- It makes the metal thicker
- It increases hardness and tensile strength while reducing ductility and producing a smoother surface finish (Correct answer)
- It makes the metal softer
- It has no effect on properties
Correct answer: It increases hardness and tensile strength while reducing ductility and producing a smoother surface finish
Cold rolling passes sheet metal through rollers at room temperature, which increases hardness and strength through work hardening, improves surface finish and dimensional accuracy, but reduces ductility.
308A Metal Types and Properties 3
What type of sheet metal is required for kitchen exhaust hoods handling grease-laden vapours?