S2020-HW5-Solutions-

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Oregon State University, Corvallis *

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322

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Mechanical Engineering

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Feb 20, 2024

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Spring 2020 1/6 MATS 322 Homework #5 Due May 13 th (Wed) at 7 pm (Submit PDF through Canvas-gradescope ) Name: ___________________________________ Student ID #: _____________________________________ Please use this file as a template for your HW assignment and write your answers in the blank space after each question. From Lab For tensile testing, nine 1018 steel tensile bars were homogenized at 760°C and then each was subjected to one of the following heat treatments: x water quenching (WQ) x water quenching followed by a 1 hour anneal at 200°C (WQ + 200) x water quenching followed by a 1 hour anneal at 300°C (WQ + 300) x water quenching followed by a 1 hour anneal at 400°C (WQ + 400) x water quenching followed by a 1 hour anneal at 500°C (WQ + 500) x water quenching followed by a 1 hour anneal at 600°C (WQ + 600) x Oil quenching, (OQ) x Air cooling, (AC) x Furnace cooling (FC) 1. Below (next page) is TTT diagram. This is useful for determining expected microstructures when cooling from above the eutectoid. Neatly and clearly draw the process path for the steel for each cooling process (WQ, OQ, AC, FC). You’ll need to estimate as best you can the time to quench to room temperature based on the lab videos.
Spring 2020 2/6 2. What is the expected microstructure of the WQ, OQ, AC, and FC samples? Your description of each should include the expected phases and an estimate of the phase fraction of each. Solution HT Microstructure Phase Fraction WQ: 100% Martensite 100 % Martensite OQ: 50 % Martensite + 50% Pearlite 50% Martensite, 48.8% Ferrite, 1.2% Fe3C AC: 100% fine Pearlite 98% alpha 2%Fe3C FC: 100% coarse Pearlite 98% alpha 2%Fe3C ࠵? ࠵? = ࠵? ࠵? − ࠵? ࠵? ࠵?࠵?3࠵? ࠵? ࠵? ࠵? − ࠵? ࠵? ࠵?࠵?3࠵? = 0.18% − 6.7% 0.05% − 6.7% = 98.05% Commented [BPM1]: Okay if it’s 100% Martensit e Commented [BPM2]: Range 96%-99% alpha
Spring 2020 3/6 3. What phases are expected in the WQ specimens annealed at 200, 300, 400, 500, and 600°C? Solution WQ + 200: 100% Tempered Martensite WQ + 300: 100% Tempered Martensite WQ + 400: 100% Tempered Martensite WQ + 500: 100% Tempered Martensite WQ + 600: 100% Tempered Martensite 4. How do you expect the microstructure the annealed 1018 steel bars to vary with anneal temperature? Solution Sphere-like precipitates (cementite) for the tempered Martensite is going to be coarse with increasing the annealing temperature. 5. Martensite is on the TTT diagram, but isn’t on the phase diagram of steel. How do you explain the formation of martensite in steel? Solution By the rapid cooling (quenching) of the austenite form of iron, carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe 3 C).
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