F2023_M202_A6

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Mechanical Engineering

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Jan 9, 2024

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M202 F2023 Assignment #6 Due: Dec 6th, 2023 at 4pm 1. Using the heat treatments and the TTT diagram ( eutectoid steel) shown below: Draw and Label i) the final microstructure including microconstituent(s) and phase(s) present ii) the correct phase morphology (e.g. lamellar or spherical etc.) Determine : iii) the approximate percentages of each microconstituent and phase 3 marks for each heat treatment (21marks total) *Assume that each specimen was held at 760°C sufficiently long to reach equilibrium before applying the cooling conditions shown below* a) Cool rapidly to 650 o C, hold for 1000 s, and then quench to room temperature. b) Reheat the specimen in part a) to 700 o C for 2 days (Hint Spheroidization) c) Cool rapidly to 570 o C, hold for 3000 s and then quench to room temperature. d) Cool rapidly to 375 o C, hold for 100 s, then quench to room temperature. e) Rapidly cool to 630 o C, hold for 10 s, rapidly cool to 265 o C, hold for 1000 s, and then quench to room temperature. f) Rapidly cool to 250 o C, hold for 100 s, then rapidly quench to room temperature. Reheat to 371 o C for 1 hour and slowly cool to room temperature. g) Which of the above heat treatments will give a steel with: i. the highest UTS, and why. ii. the highest ductility, and why. iii. the highest Young’s modulus of elasticity, and why. NOTE * Bainite, Tempered Martensite and Pearlite are microconstituents composed of ferrite and cementite. Martensite is considered both a microconstituent and a metastable phase*
a) b)
c) d) e) f) g) i) ii) iii)
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2 . A 1045 steel is subjected to the following heat treatments. Determine for each heat treatment (Note : you do NOT have to draw and label) i) the microconstituent(s) and phase(s) present - treat primary/proeutectoid ferrite as a distinct phase. ii) the approximate percentage (%) of each PHASE (consider only the total amount of ferrite where applicable ) in each heat treatment. 2 marks each (8 marks total) a) Heat to 900°C and hold long enough for the sample to come to equilibrium. Cool rapidly to 700°C and hold for 30 s. Quench to room temperature. b) Heat to 900°C and hold long enough for the sample to come to equilibrium. Cool rapidly to 400°C and hold for 10 s. Quench to room temperature. c) Heat to 700°C and hold long enough for the sample to come to equilibrium. Quench to room temperature and then reheat to 250°C for 5 minutes and then air cool. (Hint: 1b in first question) d) Heat to 900°C and hold long enough for the sample to come to equilibrium. Cool rapidly to 600°C and hold for 30 s. Quench to room temperature.
Weld metal HAZ Base metal 3 . A schematic of a weld is made with a hybrid laser weld is shown below. Depending on the alloy and welding procedure, the HAZ may undergo significant microstructure and property changes. If martensite is present in the HAZ, the weld is usually considered unacceptable (martensite is brittle), and many different solutions must be implemented (e.g., pre or post weld heat treatment ) to avoid its formation. Answer the following questions assuming the base metal is 4340 (left) and 1080 (right)
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4340 (2 marks each) (6 marks total) a. Assume the average cooling rate of the heat affected zone (HAZ) in the vicinity of a weld is 10°C/s. Estimate the microstructures (microconstituents and/or phase(s) and their approximate amounts) that will be present in the heat affected zone (HAZ). b. What would the answer to part a) be if the cooling rate was 0.1°C/s? c. Why would you classify 4340 as a difficult alloy to weld? 1080 ( 2 marks each) (6 marks total) a. Assume the average cooling rate of the heat affected zone (HAZ) in the vicinity of a weld is 10°C/s. Estimate the microstructures (microconstituents and/or phase(s) and their approximate amounts) that will be present in the heat affected zone (HAZ). b. What would the answer to part a) be if the cooling rate was 0.1°C/s? c. Why would you classify 1080 as a difficult alloy to weld? Q3 (2 marks) Suppose each of these alloys was welded with too high a cooling rate, and martensite formed in the HAZ. Which of the two alloys would be harder in the HAZ?
M202 F2022 Assignment #6 Due: Dec 2nd 2022 at 4pm 8
M202 F2022 Assignment #6 Due: Dec 2nd 2022 at 4pm 4. Imagine you inherit a 1970 Camaro such as the one pictured above. You manage to break the crankshaft and you are too poor to buy a brand new one. You find a deal on eBay for a used racing crankshaft ($200), but there is one catch – the crankshaft was in a fire! Luckily it wasn’t heavily oxidized or distorted but the heat treatment is no longer to specifications. You also have no idea from which alloy the crankshaft was made. After some research you find that aftermarket GMC 350 crankshafts for racing are typically made from 5140 or 4130 steel, and in some cases 4340 (if you have $3000 to spend on a crankshaft!). A 1970 Camaro Z-28 (at left) with a standard Chevy 350 Block Engine (center) with an after-market replacement crankshaft (right). Answer the following (2 marks each) (8 marks total) a. Explain if the hardness or hardenability changed for the crankshaft that was in the fire. b. Explain how you would determine if the crankshaft has high hardenability or low hardenability. 9
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M202 F2022 Assignment #6 Due: Dec 2nd 2022 at 4pm c. The original surface hardness of the crankshaft, before the fire was 55-57 HRC - to be strong, resist wear, and prevent crack initiation to prolong fatigue life. Yet, the center was tougher (e.g., a hardness between 32 and 38 HRC). For such a specification, which alloy must the crankshaft be: 5140 or 4340 steel? How do you know? d. The thickest circular section of the crankshaft is between 2.5 and 3 inches. Describe the heat treatment process to restore the required specifications for your used crankshaft as noted in part (d). 10
M202 F2022 Assignment #6 Due: Dec 2nd 2022 at 4pm 11
M202 F2022 Assignment #6 Due: Dec 2nd 2022 at 4pm 5 . Consider the heat treatment of a precipitation hardenable aluminum alloy (AA2014). The property curves as a function of aging time and temperature for this alloy are shown below. This alloy primarily forms hard CuAl 2 precipitates (the phase diagram is also shown). 4 marks each (12 marks total) a. Is it possible to heat treat a such an alloy and achieve a yield strength of 400 MPa with a ductility of 14% El? If so, explain how you would heat treat the alloy. b. What range of Copper composition would you specify for this alloy while still maintaining the ability to precipitation harden aluminum? Why? c. Imagine you are working at a company called the “Fly by Night Heat Treating”. The BOSS tells you that you have an emergency heat treating job - a small temporary structure just arrived that will only be in use for a maximum of 1 month. What heat treatment schedule is necessary to meet all the required specifications? 12
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M202 F2022 Assignment #6 Due: Dec 2nd 2022 at 4pm Assume the following: The structure is made from aluminum alloy 2014. The structure must be able of withstanding a static tension of 350 MPa, and cannot have <10%El at any time during service. The operating temperature will be a consistent 150°C. You have up to 2 days to get the component heat treated and on the truck for shipping. The furnaces you will need are at temperature already, and that your job takes precedence in the shop 13