ProblemSet3 - Problem set #3, 2011, due Thursday, December...

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Unformatted text preview: Problem set #3, 2011, due Thursday, December 1 3.1 Phase equilibria and structure 65 1) The Fe-rich portion of the Fe-Fe3C phase diagram is shown below. Tapping of melt 1535 d Liquid 1400 1300 liquid g 1200 1100 Temperature (°C) Liquid Austenite g Austenitization for hardening, normalizing and full annealing 1000 910 Ae3 800 Liquid g 700 Fe3C Acm g 0.025% Ferrite a Fe3C g ag Ae2 Ae1 a Fe3C Spheroidizing Process or sub-critical annealing 600 500 Fe3C Cementite Fe3C Nitriding Hypo-eutectoid steels a 0 Hyper-eutectoid steels Cast irons Fe3C 0.8 1 2 3 4 5 6 7 % w/w carbon Suppose Phase diagram (A, Fe–C system (dotted wt.% C are iron–graphite Figure 2.20 three samplesfor theB, and C) with 1 lines represent each equilibrated at 1000°C. equilibrium). a) Sketch and label the microstructure of for the samples at 1000°C. copper-rich end of this diagram. Copper can dissolve up to 40% w/w of zinc and cooling of any alloy in this range will produce an extensive primary solid solution (fcc-α). By contrast the other primary solid solution (η) is extremely limited. A special feature of the diagram is the presence of four intermediate phases (β, γ , δ, ε). Each is formed during freezing by a peritectic reaction and each exists over a range of composition. Another notable feature is the order–disorder transformation which occurs in alloys containing about 50% zinc over the temperature range 450–470◦ C. Above this temperature range, bcc β-phase exists as a disordered solid solution. At lower temperatures, the zinc atoms are distributed regularly on the bcc lattice: this ordered phase is denoted by β . Suppose that two thin plates of copper and zinc are held in very close contact and heated at a temperature of 400◦ C for several days. Transverse sectioning of the diffusion couple will reveal five phases in the sequence α/β/γ/ε/η, separated from each other by a planar interface. The δ-phase will be absent because it is unstable at temperatures below its eutectoid horizontal (560◦ C). Continuation of diffusion will eventually produce one or two phases, depending on the original proportions of copper and zinc. Iron–carbon system Problem set #3, 2011, due Thursday, December 1 3.2 b) Sample A is quenched in oil from 1000°C to room temperature. Sketch and label the microstructure of sample A after quenching. c) Sample B is cooled from 1000°C to 730°C, equilibrated at 730°C, and then quenched in oil from 730°C to room temperature. Sketch and label the microstructure of sample B after quenching. d) Sample C is cooled slowly from 1000°C to room temperature. Sketch and label the microstructure of sample C when it has reached room temperature. e) Rank the three samples in terms of their hardness from highest to lowest after each has reached room temperature. Briefly justify this ranking. Problem set #3, 2011, due Thursday, December 1 3.3 2) The mass fraction of eutectoid cementite in an Fe-C binary alloy is 10.9%. Is it possible to determine the total composition of the alloy? If so, what is the composition? If not, explain why. 3) Briefly describe why fine pearlite is harder and stronger than coarse pearlite, which in turn is harder and stronger than spheroidite. 4) The figure below shows the isothermal transformation (TTT) diagram for an FeC binary alloy of the eutectoid composition. Sketch and label time-temperature paths on this diagram to produce the following microstructures: a) 100% coarse pearlite, b) 50% martensite and 50% austenite, 410 Physical Metallurgy and Advanced Materials c) 50% coarse pearlite, 25% bainite, and 25% martensite. 800 Austenite A1 Temperature ( C) 600 Pearlite AFC Ferrite Cementite 400 Bainite Ms 200 50% martensite on quenching to this temperature 90% martensite at this temperature 10 102 103 104 Time (s) 105 (a) 800 A3 A1 800 Austenite 700 AF 600 AFC 400 Ferrite Cementite rature ( C) rature ( C) 600 500 400 A3 A1 0% transformed Austenite 100% transformed ...
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This note was uploaded on 12/05/2011 for the course MSE 4100 taught by Professor Hennig during the Fall '11 term at Cornell University (Engineering School).

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