ENB231-T1-S1-2010

ENB231-T1-S1-2010 - III-I..- Examination Paper SEMESTER:...

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Unformatted text preview: III-I..- Examination Paper SEMESTER: FIRST SEMESTER EXAMINATIONS 2010 UNIT: ENBZ31 MATERIALS AND MANUFACTURING 1 - THEORY 1 DURATION OF EXAMINATION: PERUSAL: 10 MINUTES WORKING: 2 HOURS EXAMINATION MATERIAL SUPPLIED BY THE UNIVERSITY: EXAMINATION BOOKLETS — TWO (2) PER STUDENT EXAMINATION MATERIAL SUPPLIED BY THE STUDENT: WRITING IMPLEMENTS CALCULATORS - ANY TYPE INSTRUCTIONS TO STUDENTS: Students are prohibited from having mobile phones or any other device capable of communicating information (either verbal or written) in their possession during the examination NOTES MAY BE MADE QNLX ON THE EXAMINATION PAPER DURING PERUSAL TIME SECTION A - ALL FOUR (4) QUESTIONS ARE TO BE ATTEMPTED SECTIQN — ALL FOUR (4) QUESTIONS ARE TO BE ATTEMPTED ATTEMPT EACH SECTION IN A SEPARATE EXAMINATION BOOKLET ALL QUESTIONS ARE OF EQUAL VALUE Queensland University of Technology GUT Gardens Point All 8 questions are to be attempted All Questions have Equal Marks Use a Separate Answer Book for each Section SECTION A QUESTION 1 Solidification and Microstructures (a) Briefly describe the specimen preparations needed in order to examine the microstructure of a metallic sample using optical microscope. (b) The equations for the excess free energy of a small solid nucleus of radius (r) in a molten metal and the critical radius (r*) for the formation of nuclei are given below. AG —4 2 4 3G T — 7” 7‘37” v (1) 27 T r* = S] m AH f AT Using the data in Table l for copper, calculate the size of the critical radius and the number of atoms in the critical nucleus when solid copper forms by homogeneous nucleation. Table 1. Homogeneous cooling of copper. Lattice Crystal Freezing Heat of Interface energy Undercooling parameter structure temperature fusion (ysl) (AT) (a) (Tm) 0.3615nm FCC 1085°C i1625J/cm3 177x10’7 J/cmz 236°C QUESTION 2 Heat Treatments of Steels (a) Explain the formation of martensite from austenitic eutectoid steel? In your answer include the process of martensite formation, its microstructure and mechanical properties. Sketch the hardness of martensite with increasing carbon content and compare this to that of pearlitic steel. (b) Describe the processes of martempering and austempering and explain the advantage of each process over conventional tempering process. QUESTION 3 Surface Hardeningpf Steels (a) What are the differences between carburizing and nitriding? In your answer include processing procedures, the requirements for the steel used in each process and where each process is generally applicable. (b) Why heat treatment is useful after carburizing low—carbon plain carbon steel. Briefly explain the heat treatment stages. ENB231T1.101 cont/... QUESTION 4 Phase Diagram of Steel Using the FC-FC3C equilibrium phase diagram in Figure 2: (a) Estimate the carbon content in a steel containing 50% pearlite and 50% ferrite. Show details of how you arrive at your answer. (b) Describe the microstructure that would be obtained and the phase proportions if this alloy is heated to 730°C (just above the eutectoid temperature). 1000 Austenite, 7 Temperature 800 Ferrite, 0L 727C 600 400 .l l—r—I—I—1—,—| 0.00 0.40 0.80 1.20 1.60 2.00 Figure 2: Fe-Fe3C Equilibrium Phase Diagram. Composition of Fe3C is 6.7%C. END OF SECTION A ENB231T1.101 cont/... SECTION B QUESTION 5 Forging Process (a) Define the concept of flow stress of a material as applicable to the forging process. What is its relationship to the strain hardening of the material? Mention the material properties that influence the flow stress of a material. (b) A cylindrical part is warm upset forged in an open die. The initial diameter is 45 mm and the initial height is 40 mm. The height after forging is 25 mm. The coefficient of friction at the die—work interface is 0.20. The work material has a flow curve defined by a strength coefficient of 600 MPa and a strain hardening exponent of O. 12. Determine the force in the operation (i) just as the yield point is reached (yield at strain = 0002), (ii) at a height of 35 mm, and (iii) at a height of 25 mm. Use: Yf = K8" QUESTION 6 Rolling (a) Describe rolling in the context of the bulk deformation processes? (b) A single-pass rolling operation reduces a 20 mm thick plate to 18 mm. The starting plate is 200 mm wide. Roll radius = 250 mm and rotational speed = 12 rev/min. The work material has a strength coefficient = 600 MPa and a strength coefficient = 0.22. Determine (i) roll force, (ii) roll torque, and (iii) power required. Use: F = LWYave P = M . (60,000)kW QUESTION 7 Deep Drawrng (a) What is springback in sheet metal bending? How can springback be controlled/compensated? (b) A sheet metal with a thickness of 3 mm has a limiting drawing ratio of 2.4. Calculate the largest circular blank that could be used successfully in deep drawing a cylindrical cup with an internal diameter of 75 mm. Also calculate the punch force required when the sheet metal has an ultimate tensile strength of 530 MPa. Use: LDR = D0 /D,., F = 7rDito(UTS)[LDR — 0.7] QUESTION 8 Extrusion Process (a) Explain the different ways in which changing the die angle affects the extrusion process. (b) Assuming an ideal drawing process, what is the smallest diameter to which a 100-mm diameter rod can be drawn? (c) What is the dead metal zone in an extrusion process? Briefly comment on the grain flow patterns with respect to dead metal zone and the possible defects the phenomenon can induce in an extruded part. END OF SECTION B END OF PAPER ENB231T1.101 ...
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This note was uploaded on 11/03/2011 for the course EN 40 taught by Professor Mcgregor during the Three '10 term at Queensland Tech.

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ENB231-T1-S1-2010 - III-I..- Examination Paper SEMESTER:...

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