ME Metallurgy and Phase Diagrams 1 — Questions and Answers
Question 1: On the iron-iron carbide (Fe-Fe3C) phase diagram, what is the carbon content at the eutectoid composition?
- 0.022 wt% C
- 0.76 wt% C (Correct answer)
- 2.14 wt% C
- 4.30 wt% C
Correct answer: 0.76 wt% C
The eutectoid point in the Fe-Fe3C system occurs at 0.76 wt% C and 727°C, where austenite transforms to pearlite upon slow cooling.
Question 2: Pearlite is a microstructure consisting of alternating lamellae of:
- Austenite and ferrite
- Ferrite and cementite (Correct answer)
- Martensite and bainite
- Ledeburite and ferrite
Correct answer: Ferrite and cementite
Pearlite forms by the eutectoid reaction and consists of alternating layers of alpha-ferrite and cementite (Fe3C).
Question 3: Martensite in steel is formed by:
- Slow furnace cooling of austenite
- Rapid quenching of austenite, preventing diffusion-based transformation (Correct answer)
- Tempering of bainite at high temperature
- Prolonged holding at the eutectoid temperature
Correct answer: Rapid quenching of austenite, preventing diffusion-based transformation
Martensite forms by a diffusionless shear transformation when austenite is quenched too rapidly for carbon to diffuse, trapping carbon in a body-centered tetragonal structure.
Question 4: The Gibbs phase rule for a system in equilibrium is expressed as:
- F = C + P - 2
- F = C - P + 2 (Correct answer)
- F = C × P - 2
- F = C + P + 2
Correct answer: F = C - P + 2
The Gibbs phase rule F = C - P + 2 relates degrees of freedom (F), components (C), and phases (P) at equilibrium.
Question 5: In a binary phase diagram, the lever rule is used to determine:
- The temperature of a phase transformation
- The relative amounts (fractions) of each phase in a two-phase region (Correct answer)
- The crystal structure of each phase
- The diffusion coefficient between two phases
Correct answer: The relative amounts (fractions) of each phase in a two-phase region
The lever rule applies the principle of mass balance to calculate the weight fractions of each phase present in a two-phase region of a phase diagram.
Question 6: Normalizing a steel involves:
- Heating to the austenite region and quenching in water
- Heating to the austenite region and air cooling (Correct answer)
- Heating to just below the eutectoid temperature and slow furnace cooling
- Rapid heating followed by immediate quenching in oil
Correct answer: Heating to the austenite region and air cooling
Normalizing heats steel into the austenite phase field and then allows it to air cool, producing a fine, uniform microstructure.
ME Metallurgy and Phase Diagrams 1
On the iron-iron carbide (Fe-Fe3C) phase diagram, what is the carbon content at the eutectoid composition?