CNC Ferrous & Non-Ferrous Material Properties 2 — Questions and Answers
Question 1: Which heat treatment process increases the surface hardness of low-carbon steel by introducing carbon into the outer layer?
- Annealing
- Case hardening (carburizing) (Correct answer)
- Stress relieving
- Normalizing
Correct answer: Case hardening (carburizing)
Carburizing diffuses carbon into the surface of low-carbon steel, allowing the outer layer to be hardened while the core remains tough.
Question 2: What is the primary alloying element that makes stainless steel corrosion resistant?
- Nickel
- Molybdenum
- Chromium (Correct answer)
- Manganese
Correct answer: Chromium
Chromium (minimum 10.5%) forms a passive oxide layer on the steel surface that prevents corrosion.
Question 3: Which aluminum alloy series is primarily alloyed with copper and is commonly used in aerospace applications?
- 1xxx series
- 2xxx series (Correct answer)
- 5xxx series
- 6xxx series
Correct answer: 2xxx series
The 2xxx series aluminum alloys use copper as the primary alloying element and offer high strength, making them popular in aerospace structures.
Question 4: Gray cast iron is easy to machine primarily because of which material characteristic?
- High ductility and elongation
- Graphite flakes that act as chip breakers and lubricants (Correct answer)
- High tensile strength
- Uniform carbide distribution
Correct answer: Graphite flakes that act as chip breakers and lubricants
The graphite flakes in gray cast iron interrupt the chip and act as a built-in lubricant, reducing tool friction and producing short, manageable chips.
Question 5: Titanium alloys are significantly more difficult to machine than aluminum primarily because of:
- Higher density causing vibration
- Low thermal conductivity concentrating heat at the cutting edge (Correct answer)
- Very high ductility causing built-up edge
- Tendency to spark during cutting
Correct answer: Low thermal conductivity concentrating heat at the cutting edge
Titanium's poor thermal conductivity traps cutting heat at the tool tip, accelerating tool wear and requiring lower speeds.
Question 6: Which property describes a material's ability to be drawn into wire without breaking?
- Malleability
- Ductility (Correct answer)
- Toughness
- Hardness
Correct answer: Ductility
Ductility is the ability of a material to undergo plastic deformation in tension (wire drawing) before fracture.
Question 7: A CNC machinist is selecting a material that must resist wear but also absorb impact without fracturing. Which combination of properties is most important?
- High hardness and low ductility
- High hardness and high toughness (Correct answer)
- Low tensile strength and high ductility
- High compressive strength and low hardness
Correct answer: High hardness and high toughness
Wear resistance requires hardness while impact resistance requires toughness; balancing both is critical for applications like dies and gears.
CNC Ferrous & Non-Ferrous Material Properties 2
Which heat treatment process increases the surface hardness of low-carbon steel by introducing carbon into the outer layer?