F22_Homework_3_Solutions_updated

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University of California, Berkeley *

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108

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Mechanical Engineering

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Apr 3, 2024

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ME108, Fall 2022 Homework 3 Prof. Grace O’Connell All Homework assignments are individual, but you may work with other students. Please provide the names of other students that you worked with here (submit with your Answer to question 1): 1. List at least three classifications of steels and describe the general properties and applications for each. Low, medium and high alloy. The carbon concentration increases from low to high. With the increase in carbon content, there is an increase in strength but a decrease in ductility or the amount a material can elongate before fracture. Low can be used in autos, bridges, towers. Medium: crank shafts, hammers, gears. High: drills, saws, turbines 2. Cold working a metal by rolling it to a lesser thickness or hammering it introduces a large number of dislocations into the crystal structure. Would you expect the yield strength to be affected by this and if so, should it increase or decrease and why? In a similar manner, how would you expect the elastic modulus to be affected and why? When a metallic material is deformed by cold working, strain hardening occurs and additional dislocations are introduced into the structure. Very large increases in strength may be obtained in this manner. The ductility of the strain hardened metallic material is reduced. However, we would not expect any changes in the elastic modulus (slope of the stress-strain relation), which is related to the linear stress-strain response prior to yielding.

ME108, Fall 2022 Homework 3 Prof. Grace O’Connell 3. Watch the heat treatment video on bCourses >> Modules >> Lab videos (~5 minutes) and answer the following sub-questions. a. How does oil quenching change the results of the heat treatment? Describe the potential impact on the grain structure. Oil quenches slower, resulting in increased ductility as some diffusion of the carbon occurs. b. What is the phase change during heat treatment from the first phase? During phase 1, at that high temperature we will still have austenite. Then when we quench, we get hard and brittle martensite. c. What is the purpose of the second heat treatment phase? The second heat treatment phase does not go to as high temperatures as the first, so it is to temper and add ductility. 4. A specimen of A36 steel is heated to 760 o C and held for 1 hour, then cooled to 600 o C (2-minute hold) before quenching to room temperature. On the TTT diagram below draw a solid line for that heat treatment. a. For this heat treatment, identify the final microstructure. See solid line on figure; pearlite is the final structure (alternating layers of ferrite and cementite, ductile) b. Design a new heat treatment protocol that will increase the overall strength of the specimen while maintaining ductility. Plot your heat transformation protocol on the TTT diagram above, using a dashed line. See dashed line Multiple solutions exist, but students will need to cool and hold such that the line will go between the blue dashed line and solid green line before quenching. c. For your new protocol, provide the final expected microstructure, cooling times, and temperature.

ME108, Fall 2022 Homework 3 Prof. Grace O’Connell 5. Which of the following characteristics distinguishes the stainless steels from other steel types? a. They are more corrosion resistant. b. They are stronger. c. They are more wear resistant. d. They are more ductile. 6. For each one, state whether the following will occur during an annealing process and why. a. Stresses may be relieved. b. Ductility may increase. c. Toughness may increase. d. All of the above. There are five different types of annealing processes discussed in class (Stress Relief, Process Anneal, Spheroidizing, Full Anneal, and Normalizing) (Lecture 9, slide 9). Each process serves its own purpose.

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ME108, Fall 2022 Homework 3 Prof. Grace O’Connell 7. In Spring of 2020, ME 108 students performed hardness tests on non-heat treated (using Rockwell B scale; HRB) and heat treated A36 steel (using Rockwell C scale; HRC). These measurements are shown in the table below and available here: https://docs.google.com/spreadsheets/d/11ev8RAG1XseqG_YDwuwUGrv8mwIh7YQsJJ n-YE6Bz9k/edit?usp=sharing Hardness Results Team Number A36 (HRB) A36 Heat Treated (HRC) 1 83.83 35.67 2 83 35 3 83.73 34.07 4 81.67 35.67 5 81.33 34.67 6 82.83 32.5 7 83.33 34 8 80.33 35 9 80.67 34.33 10 82.67 33.33 11 83 37 12 81.67 35.33 13 82.83 36 14 80.33 36 15 83.13 36 16 83.17 36.5 17 82.7 35.5 18 83.83 32 19 80.33 34.33 20 79.7 32.3 21 83.7 37.3 22 83.73 36.13 23 80.67 34.83 24 83.63 37.6 25 83 35.33 26 80.83 35.77 27 83.33 36 28 81.33 35.67 29 83.83 36.5 Using these values, perform the following analysis: a. Convert measurements to a common scale. Conversions from Rockwell to Brinell hardness can be found online (or you can use this table: https://www.engineersedge.com/hardness_conversion.htm ; you may need to interpolate values) b. Plot the average and standard deviation for each group on a bar graph.

ME108, Fall 2022 Homework 3 Prof. Grace O’Connell c. What are the differences between the Rockwell B & C scales? Indenter: Rockwell B scale uses a 1/16-inch diameter ball indenter while C uses a cone shaped diamond tip. . Load: B uses a 100-kg load, C uses a 150-kg load. Uses: example: The Rockwell C scale is good for measuring hardened steel. Rockwell B is better for softer steel or relatively hard aluminum d. What type of heat treatments could induce the change seen in the data? The material is much harder, suggesting increases in strength seen by quenching or quenching and tempering. e. What other mechanical properties do you expect to change with heat treatment, and why? How would you expect these mechanical properties to differ relative to a non-heat-treated sample (increase or decrease) and why? A heat treatment that increases strength like that will also likely decrease the ductility of the material, but we have little information with regards to elongation to determine if the martensite has been tempered or not. Elastic modulus will not change much. Fracture strength will likely increase, as well as yield stress. 8. In your own words, describe the advantages and disadvantages of low alloy steels versus cast iron. Low alloy steels can be worked through multiple manufacturing processes, while cast iron is primarily limited to casting or wrought deformation. Being able to use casting for cast iron is advantageous for making complex designs (e.g., engine block). Cast iron has a lower melting point than low alloy steel, which could be a concern in certain applications

ME108, Fall 2022 Homework 3 Prof. Grace O’Connell

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