ME Metallurgy and Phase Diagrams 2 — Questions and Answers
Question 1: Tempering of martensite is performed to:
- Increase hardness and brittleness
- Reduce brittleness while accepting some reduction in hardness (Correct answer)
- Convert martensite fully to austenite
- Increase carbon content of the steel
Correct answer: Reduce brittleness while accepting some reduction in hardness
Tempering reheats quenched martensite to a temperature below the eutectoid to allow some carbon diffusion, reducing brittleness while sacrificing some hardness.
Question 2: Austenite in iron-carbon alloys has which crystal structure?
- Body-centered cubic (BCC)
- Body-centered tetragonal (BCT)
- Face-centered cubic (FCC) (Correct answer)
- Hexagonal close-packed (HCP)
Correct answer: Face-centered cubic (FCC)
Austenite (gamma-iron) has an FCC crystal structure, which can dissolve significantly more carbon (up to 2.14 wt%) than the BCC ferrite phase.
Question 3: In the TTT (Time-Temperature-Transformation) diagram, the 'nose' of the curve represents:
- The martensite start temperature
- The temperature and time of fastest austenite decomposition (Correct answer)
- The upper bainite transformation zone
- The eutectoid temperature
Correct answer: The temperature and time of fastest austenite decomposition
The nose of the TTT curve is where decomposition of austenite is fastest because both the thermodynamic driving force and atomic diffusivity are balanced.
Question 4: Bainite microstructure in steel is produced by isothermal transformation of austenite at temperatures:
- Above the eutectoid temperature
- Between the nose of the TTT curve and the martensite start temperature (Correct answer)
- Below the martensite finish temperature
- At the martensite start temperature only
Correct answer: Between the nose of the TTT curve and the martensite start temperature
Bainite forms by isothermal transformation of austenite at temperatures between the pearlite nose and the martensite start (Ms) temperature.
Question 5: Which alloying element is added to steel primarily to improve hardenability?
- Sulfur
- Lead
- Chromium (Correct answer)
- Phosphorus
Correct answer: Chromium
Chromium (and elements like molybdenum, nickel, and manganese) shift the TTT curve to the right, improving hardenability by allowing martensite formation at slower cooling rates.
Question 6: Stainless steel derives its corrosion resistance primarily from:
- High carbon content forming a protective carbide layer
- A minimum of about 10.5 wt% chromium forming a passive Cr2O3 surface film (Correct answer)
- Nickel additions that repel moisture
- Molybdenum forming a thick oxide scale
Correct answer: A minimum of about 10.5 wt% chromium forming a passive Cr2O3 surface film
Stainless steel requires at least ~10.5 wt% Cr to form a stable, self-repairing chromium oxide (Cr2O3) passive film that prevents oxidation and corrosion.
ME Metallurgy and Phase Diagrams 2
Tempering of martensite is performed to: