Thermo-Module-problem-set_v3

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University of Wisconsin, Madison *

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Mechanical Engineering

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Dec 6, 2023

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CALPHAD Lab First step is to download Thermocalc. To get most recent Thermocalc demo go here: https://www.thermocalc.com/academia/students/educational-package-for- students/ Useful links: 1. Thermocalc help: file:///Applications/Thermo-Calc-academic- 2017b.app/Contents/Resources/HTML5/Content/Welcome.htm 2. Save projects often. Mine tend to freeze and I need to restart the Thermo-calc software. 3. Use “Project” pane features to reuse settings and “Clone” feature to perform similar actions. Right clicking gives options. All of problem 1 and 2 can each be in their own projects, giving two projects that contain the whole lab. 1. Martensitic stainless steels offer a combination of high mechanical strength and corrosion resistance. To increase the hardness of the surface, the process of “carburization”, or annealing in the presence of a carbon-rich gas, can be performed, which will increase the carbon concentration in the steel near the surface. However, the increase in hardness must be balanced against a loss of corrosion resistance. This exercise will help understand the reasons for this trade-off. The most basic martensitic stainless steel is Type 410, which has a nominal composition of (by mass percent) Cr 13.5 C 0.15. Fe 86.35 Usually when specifying steel compositions this would be written Fe-13.5Cr- 0.15C, where composition is written prior to each element, and the Fe composition is omitted since it is the main constituent. 410 steels may also include a small amount of Mn which is primarily used to combine with and remove oxygen impurities; it can be neg lected for the purposes of this exercise. Carburization causes an increase in carbon concentration near the surface, leading to the formation of chromium-rich carbide phases within the FCC matrix, including M 23 C 6 , M 7 C 3 , and M 3 C (where M stands for metal). M 23 C 6 and M 7 C 3 increase hardness, but the formation of M 3 C (also called cementite) reduces strength. In the following, you can label phases in picture by right-clicking but it takes a lot of time as you need to do each one separately so don’t worry about labeling them unless I ask explicitly. 1.1. Using Thermo-calc with the database FEDEMO do the following. 1.1.1. Plot an isopleth of this steel for mass percent carbon varying from 0-5 mass-%. An isopleth keeps all compositions constant except one alloying element and the host, in this case C and Fe, respectively. You can do this with the “Property Model” tool and select “Phase Diagram” in the “Equilibrium Calculator” to do this. Put plot below as Figure 1.1.1. Please

label the phases that occur at 1230K. Note that the FCC Fe phase forms at this temperature. 1.1.2. Plot the phase fractions as a function of mass percent carbon at 1230 K in the range 0-5 mass-% carbon. You can do this with “Property Model” in the “Equilibrium Calculator”. Put plot below as Figure 1.1.2. 1.2. Using the results of part (a), at a temperature of 1230 K, determine the range of mass percent carbon that allows M 23 C 6 and/or M 7 C 3 to form, but prevents cementite from forming (carburization will not raise C content dramatically

which is why you can limit your search to <5 mass % C). Put text below as Response 1.2.  From about 0.2% C to 3.5% C. 1.3. Corrosion resistance in stainless steels is significantly enhanced by the presence of Cr in the FCC matrix phase. After carburization, it has been found that corrosion resistance at the surface decreases. Explain why this occurs using a plot of Cr content of the FCC phase versus carbon concentration. Again use a temperature of 1230 K and up to 5 mass % C. You can get Cr content as the output variable with a “Property Model” in the “Equilibrium Calculator” but you must set the output in the “Plot Renderer Y-axis tab. Put plot below as Figure 1.3. Put text below as Response 1.3.  The corrosion resistance decreases because as the carbon concentration increases, the Cr content in FCC matrix decreases as seen in the plot. 2. Kinetics of carbide precipitation in steels. Open example: P_03_Precipitation_Fe- C-Cr_TTT_Cementite-M7C3-M23C6.tcu. You can find this under File/Example Files/Precipitation Module – TC-Prisma. 2.1. First, write down the composition of this example. Put text below as Response 2.1.  Fe-12Cr-0.1C 2.2. Perform a calculation at thermodynamic equilibrium (use the “Project Pane” to find the preset “Equilibrium Calculation”) to determine

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what phases are present at 1000K. Put the plot below as Figure 2.2. Put text below as Response 2.2. Note that the point of this is to understand where our kinetic simulation should end up when we run for long enough time, which is equilibrium. Figure 2.2  BCC_A2 and M23C6 are present at 1000K 2.3. Calculate a TTT diagram and determine how long it takes to form M23C6 at 1000K (try to be accurate within a factor of 2x or so but don’t worry about being precise) (use the “Project Pane” to find the preset “Precipitation Calculation”). Keep all the precipitate phases even though we are just asking about M23C6 as they could influence the behavior of M23C6. Set the temperature step size to 50 to make the calculation faster. Set all the interfacial energies to be “user defined” (somewhat strangely, this calculates defaults and please use these defaults. This gives a value of 0.252 J/m2 for M23C6). Set the max time to 10 5 seconds. Put the TTT plot below as Figure 2.3. Put text below as Response 2.3. Note that this calculation can take 2- 10min to run depending on the computer.

Figure 2.3  About 860 seconds. 2.4. Change the interfacial E by ~10% (reduce by 0.02 J/m2) for M23C6 to see impact on time. Determine how long it takes to form M23C6 at 1000K with the new interfacial energy (try to be accurate within a factor of 2x or so but don’t worry about being precise)? This demonstrates how sensitive the results are to the exact interfacial energy. Why do you think the results are so sensitive? Put plot below as Figure 2.4. Put text below as Response 2.4.

Figure 2.4  About 43.4 seconds with the new interfacial energy. 2.5. Perform an isothermal annealing simulation at 1000 K for10 5 s. Use the original interfacial energies, Adjust plot renderer settings to get clear plots of average radius and number density and volume fraction vs. time for all 3 precipitates. Set axes to log if needed to see all data. Put plots below as Figure 2.5a (radius), 2.5b (number density), and 2.5c (volume fraction). Please put results for each precipitate type on the same plot, e.g., Figure 2.5a should have radius of cementite, M7C3, M27C6). What is happening over long times to these precipitates? Put text below as Response 2.5. Figure 2.5a

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