Sample Mid-term Exam 2

ENG 152 - Engineering Materials Science The College of New Jersey

Part I. Please select correct answers for the following questions. (20 points) 1. Steel is an alloy of which of the following elements? (a) iron and silicon (b) iron and carbon (c) iron and chromium (d) iron and copper 2. Ferrite and martensite are (a) both equilibrium solutions of carbon and iron (b) an equilibrium solution and a non-equilibrium solution, respectively, of carbon and iron (c) a non-equilibrium solution and an equilibrium solution, respectively, of carbon and iron (d) both non-equilibrium solutions of carbon and iron. 3. What is the hardest microstructure for the iron-carbon system? (a) pearlite (b) ferrite (c) bainite (d) martensite 4. Which of the following conditioning methods does not depend on diffusion? (a) tempering (b) precipitation hardening (c) water quenching (d) annealing 5. AISI 1080 steel is in equilibrium at 850oC. What is the structure? (a) austenite (b) austenite and cementite (c) austenite and ferrite (d) pearlite and cementite 6. The graphite in malleable cast iron has the following form: (a) flakes (b) rosettes (c) nodules (d) needles 7. Please identify the eutectoid reaction from the following. S denotes the solid phase, and L denotes the liquid phase. 8. The carbon content in AISI 4020 steel is: (a) 0.2wt%C (b) 2wt% C (c) 0.40wt% C (d) 4wt% C 9. An iron-carbon alloy of eutectoid composition is cooled from 1200oC, and it is known that the formation of microstructure depends on the time-temperature path. Which of the following microstructures will not be formed through the cooling process? (a) martensite (b) pearlite (c) bainite (d) tempered martensite

cool

heatL S1 + S2

cool

heatS1 S2 + S3

cool

heatS1 + L S2

cool

heatL S

(a) (b)

(c) (d)

cool

heat

cool

heatL S1 + S2

cool

heat

cool

heatS1 S2 + S3

cool

heat

cool

heatS1 + L S2

cool

heat

cool

heatL S

(a) (b)

(c) (d)

10. From Figure 9.20, the composition for Mg2Pb is (a) 67.wt% Pb (b) 81wt% Pb (c) 6.7wt% Pb (d) 95wt% Pb Part II. Please solve the following problems, and show all the necessary details for full credits. 11. (20 points) Consider a 40wt% Sn-60 wt% Pb alloy. From the given phase diagram, determine the following:

(a) the composition of each phase at 250oC, (b) the composition and mass fraction of each phase at 150oC. (c) sketch microstructure for the two cases.

12. (20 points) From the iron-carbon phase diagram, determine: (a) the phase(s), and % composition for 0.25wt% C-99.75wt% Fe @ 1000°C and

726°C (b) the phase(s), and % composition for 1.5wt% C-98.5wt% Fe @ 728°C, and

726°C. (c) the mass fraction for each phase and sketch microstructure for

0.25wt%C-99.75wt% Fe @ 726°C, and 1.5wt% C-98.5wt% Fe @ 726°C.

13. (10 points) The microstructure of an iron-carbon alloy consists of proeutectoid cementite and pearlite; the mass fractions of these microconstituents are 0.2 and 0.8 respectively. Determine the concentration of carbon in this alloy and the mass fractions of total ferrite and cementite at 726oC (just below the eutectoid temperature).

14. (20points) Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, sketch and label time-temperature paths on the diagram to produce the following microstructure:

(a) 100% fine pearlite, (b) 90% martensite and 10% austenite, (c) 50% coarse pearlite and 50% bainite, (d) 50% bainite and 50% martensite.

15. (10 points) An engineer choose to use an alloy steel (4340) in two applications. The requirements are: Case I, 1400MPa or above for tensile strength and 40% or above for ductility (%RA), Case II, 1500MPa or above for yield strength and 45% or above for ductility (%RA). Oil-quenching followed by tempering is to be used to treat the steel. Determine whether it is possible to satisfy the design requirement by using this heat treatment method. If so, describe the tempering temperature. If not, explain why.

16. (10 points, Extra credit) Consider the case of a cylindrical nucleus of radius r and length r. Based on the expression of the total free energy change for nucleation, determine the critical radius r*.