pre lab 4 maurs

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Apr 3, 2024

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Lab 4: Heat Treatment of Steel Date(s) Experiment Performed: 11/24/2020 Course section: 9 Lab instructor: Professor Gloekler Experiment performed: 11/19/2020 Mauricio Zaragoza
2 Abstract This lab compares the properties of heat treated 4140 steels. The purpose of this lab is to show the processes that material science engineers. Engineers need to understand how heat treatments affect different types of metals in order to correctly implement the steels in their design/building. Four specimens of 0.50 in. Diameter 4140 Quenched and Tempered, fully annealed, and normalized specimens were tested with a Rockwell hardness tester and the Universal Testing Machine. The results of this test showed that the Quenched and Tempered specimens had higher UTS values (at about 150 and 147 ksi). The breaking strength of the quenched and tempered specimens were also the higher than the other specimens. This means the Q&T specimens were not able to withstand as much strain before breaking as the other specimens. The Normalized specimen had the next highest UTS at 105 ksi. The breaking strength was lower than the Q&T specimens (11815 psi) meaning that the specimen can withstand more strain before fracture occurs. The specimen with the lowest UTS was the full annealed specimen (88.8 ksi). This specimen also had one of the lowest breaking strengths at 12818 psi, meaning it can withstand the most strain without fracture occurring. The hardness of the specimens was tested using the Rockwell hardness tester. For the Fully Annealed specimen the B-scale was used and a value of 72.2 was achieved. For all the other specimens the C-scale was used. For Quenched and Tempered A, the hardness was 25 and for Quenched and Tempered By the hardness was 30.1 and lastly, for the normalized specimen the hardness was found to be 7.2. Author’s Keywords
3 Introduction In our lab four we will be discussing and introducing the heat treatment of steel. The definition of heat treatment can be defined as the process in which a metal is heated to a certain temperature which is then followed by a cooling prosses (these vary on the metal being treated) which would in turn change the metals structure from a microscope standpoint for the uses desired properties. This process is imperative from an engineering perspective due to the fact that engineering properties are improved by heat treatment methods to help enhance structural components to endure whatever operating conditions it may come across. The goal of this experiment is to test the physical properties of steel through both Annealing (This softens the metal and increases its electrical conductivity) and tempering (this hardens the steel but also makes it much more brittle), we did this by testing the mechanical properties of our steel samples by testing a steel sample after the quenching and tempering process and a separate sample that is fully annealed and comparing the two. Procedure In our lab Heat treatment of steel, we had different procedures we had to come across. We were given 3 tensile test specimens of a hard enable steel and each would be conditioned in their own way. The first one would be conditioned to fully annealed and heated to austenite region and slowly be cooled in furnace. The second one would be quenched and heated to austenite region and quenched to RT in oil. When it comes to the actual testing and procedures we have to think and come across the measurements. The specimen’s length and diameter were measured using a caliper. The length was measured by marking 2 inched from each end of the specimen and measuring the length between the two points. The diameter equaled about 0.505 inches which was the standard diameter for all the specimens, some groups had slightly different diameters, which is okay. After the specimen in broken the final diameter and length readings are taken using the caliper. A clamping system may need to be used to align the specimen to get the final length. Then the hardness testing, the Rockwell hardness of each steal was measured using the B or C scaled depending on the specimen. The specimens were placed horizontally on the stand
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4 underneath the indenter. The indenter control wheel was turned clockwise until the indenter contacted the specimen. Once in contact with the specimen the control wheel was turned carefully and slowly until the small pointer on the gage pointed directly over the dot above it. A dial at the base of the machine was rotated until the large pointer was adjusted to match the corresponding indicator arrow. The pedal at the lower front of the machine was pressed to engage the load and once the load was done, the lever at the bottom right of the machine was pulled counterclockwise. This was repeated 3 times and an average value was used. Using this data and graphs provided the tensile strength can be found. Finally, the Tensile test’s, are samples were tested using the Universal Testing Machine with the electronic extensometer. An extensometer was attached to the specimen at the points marked earlier in the lab, measuring the strain as the initial length. The lab instructor slowly loaded the material and the strain was measured using an extensometer. Once the material is close to the breaking point the extensometer must be removed or it can be damaged in the breaking process. Most machines will have a warning signal to remove the extensometer. After the extensometer is removed the lab instructor continues to put load on the specimen until it breaks. When it comes down to tempering and quenching the sample must meet 100 0 F for at least one hour. Once all of these steps are finished can see if your steel is hardened. Results For the Elastic Modulus and 0.2% Offset Yield Strength, graphs (in appendix) were used to find the value. For UTS and Breaking strength data from the graphs and calculations (in appendix) were used to achieve the values. All calculations for the data below will be found in the Appendix section. The values from Table 1 were found experimentally in Lab 3. For the conversions from ksi/psi to GPa/MPa a sample calculation can be found in the appendix; other than that conversion calculators were utilized online.
5 Tables Table 1: Properties for 4140 Fully Annealed (FA) steel Property Experimental Value (FA) Literature Value % Diff. Elastic Modulus (ksi) 58000 29700 64 0.2% Offset YS (ksi) 60.2 UTS (ksi) 100 95 5.1 Breaking Strength (ksi) 80 % Elongation 27.9 25.7 8.2 % Reduction in Area 55 56.9 3.3 Hardness (HRb) 84.5 92 9 Table 2: Properties for 4140 As-received Property Experimental Value (Norm/AR) Literature Value % Diff. Elastic Modulus (ksi) 34000 29700 13.5 0.2% Offset YS (ksi) 97.9 UTS (ksi) 114 148 25.9 Breaking Strength (ksi) % Elongation 15.3 17.8 15.1 % Reduction in Area 35 48.2 31.7 Hardness (HRc) 73 32 78 All of the percent differences seem really low. The lowest are % Elongation, % Reduction in Area, and Yield Strength. This may be due to the fact that there is less steps in getting these results and measurements of this manor are pretty simplistic to make. Breaking Strength did not seem to have any literature values and this may be due to it being different for every specimen.
6 The largest % difference was Elastic Modulus and this may be due to error in creating and interpreting the graph. As for Hardness this could be due to user error on the Rockwell machine. Table 3: Properties for 4140 QT Steel Property Experimental Value (QT) Literature Value % Diff. Elastic Modulus (ksi) 32000 29700 7.4 0.2% Offset YS (ksi) 129 UTS (ksi) 154 136 12 Breaking Strength (ksi) 109 % Elongation % Reduction in Area Hardness (HRc) Figure 1: Figure 1 – Complete engineering stress-strain Normalized, Quenched and Tempered, Fully Annealed and 4140 AR.
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7 Figure 2: Figure 2 – Stress-strain to 0.025 strain to determine elastic modulus and yield strength for 4140 FA steel.
8 Figure 3: Figure 3 – Stress-strain to 0.025 strain to determine elastic modulus and yield strength for 4140 AR steel.
9 Figure 4: Figure 4 – Stress-strain to 0.025 strain to determine elastic modulus and yield strength for 4140 Norm steel.
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10 Figure 5: Figure 5 – Stress-strain to 0.025 strain to determine elastic modulus and yield strength for 4140 QT steel. Discussion Heat treatment is the process of heating and cooling metals to change their microstructure and to bring out the physical and mechanical characteristics that make metals more desirable. The temperatures metals are heated to, and the rate of cooling after heat treatment can significantly change metal's properties. The most common reasons that metals undergo heat treatment are to improve their strength, hardness, toughness, ductility, and corrosion resistance. Common techniques for heat treatment are annealing, quenching and quench and tempering. Annealing is a form of heat treatment that   brings a metal closer to its equilibrium state. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is   slow-cooled. Then quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process
11 stops   the cooling process from altering the metal's microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does. Next is tempering is to heat a treatment   used to improve hardness and toughness in steel as well as to reduce brittleness. The   process   creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals. The fully annealed specimen was the only specimen to have a different elastic modulus and it was 30000ksi versus all the others which had an elastic modulus of 20000 ksi. This could have been due to graphing error or measurement error. I did not expect all the specimens to have the same elastic modulus, but this makes sense since they are all the same material just heat treated different. Heat treatment does not necessarily affect the Elastic Modulus, but the strength and ductility of the specimens. The specimens in the other experiments also had the same elastic modulus. The as- received specimen from lab 3 can be found in the results section. The UTS of the quenched & tempered was a lot higher than the UTS of the fully annealed specimen. The breaking strength for the quenched & tempered was a lot higher than the fully annealed. This means that the fully annealed was able to be put under a larger strain before finally breaking. Thus, the quenched & tempered can withstand a higher force but cannot withstand a high strain like the fully annealed specimen can. The fully annealed specimen was extremely like the as-received specimen. I did not expect the quenched & tempered to be so drastically different from the fully annealed. Nor did I expect the as-received specimen to by fully annealed, but there was not this much comparison of the same specimen in Lab 3 as in Lab 4. Heat Treatment Phase Present Constituents Present How Microstructure is formed As quenched BCT Martensite When austenite is rapidly cooled, the face centered cubic structure distorts into a body centered tetragonal. This is a phase change, occurs very quickly once the austenite cools enough.
12 Quenched & Tempered a ferrite + Fe 2 C cementite Tempered Martensite Martensite is heated up, giving enough energy for carbon atoms in the BCT structure to diffuse, resulting in very fine Fe 2 C spheres in a matrix of a ferric. Annealed a ferrite + Fe 2 C cementite Primary a + Pearlite Austenite is cooled very slowly. This allows enough time for pearlite to form, but less than annealing. Resulting in a very fine bands are resulting to ferric and cementite. Normalized a ferrite + Fe 2 C cementite Primary a + Pearlite The austinite is air cooled. This allows enough time for pearlite to form, but less than with annealing, resulting in very fine bands of ferrite and cementite. As Quenched Microstructure is martensite, a super-carbon-saturated steel solution. It is extremely hard and brittle and has almost no ductility. Successful formation of martensite depends on how rapidly the steel was cooled. Quenched and Tempered Microstructure has extremely small spheres of cementite in a matrix or martensite. Very fine structure has a very large amount of phase boundaries and can be very hard and less brittle then as quenched. Material properties depend on size of cementite spheres which may vary based on tempering temperature and time.
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13 Annealed The thick bands of pearlite formed because minimal phase boundaries compared to other possible microstructures. This results in more ductility and lower hardness. The thicker bands of pearlite form when steel is cooled very slowly. These properties vary with heat treatment because the treatment is changing the microstructures of the steel. With the change of the microstructures the properties that are affected by these microstructures also change. The trends are what is expected for quenched & tempered specimens. They are not exact with the literature value and this could be due to measurement or some other type of error. The results are consistent with the literature trends, they follow the same slope pattern just not at the exact same spot. 400 600 800 1000 1200 0 50 100 150 200 250 0 20 40 60 80 100 120 140 160 Literature Data vs Tempering Temperature TS YS % EL % RED AREA BHN/10 TS (QT) YS (QT) %EL (QT) %RA (QT) BHN/10 (QT) Tempering Temperature ksi BHN/10 % 4140 STEEL                 ksi ksi     Bhn/10   Tem p F TS YS % EL % RA Hardnes s   400 236 212 10 41 46.7   600 217 200 11 43 43.5
14   800 186 173 13 49 38   1000 150 132 17 57 31.5   1200 118 102 22 64 24.5               Experiment Values               Tem p TS YS %EL %RA Hardnes s NORM 1600 105.5 66.5 21.63 44.8 7 FA 1500 88.8 49 22.62 46.6 72.2 Q&TA 1000 150.8 136 17.43 57.1 25 Q&TB 1000 147.41 134 19.1 58.4 31.5 To put the quenched specimen of 4140 into annealed condition it would have to be heated into the austenite (γ) region and then cooled to the α + Fe C region. The microstructure after annealing is ferrite and pearlite. The part should be machined in the annealed state because it is not as hard and more ductile. After undergoing the final quench and tempered process the specimen’s hardness and ductility increases making machining more difficult and harder on tools. In order to have a good working chisel the properties required are high strength and hardness. The chisel should contain some ductility to avoid brittle fracture but not so much that it cannot cut. To get those properties first anneal (and shape chisel) then quench and temper to get a high strength with some ductility. Conclusion In conclusion from our lab 4 Heat treatment of steel, is a very important topic to our engineers. Engineers need to understand how heat treatments can affect different types of metals in order to correctly implement the steels in their design/building. Heat treatment   is   the process   of heating the   metal, holding it at that temperature, and then cooling it back. During   the process,
15 the   metal   part will undergo changes in its mechanical properties. This is because the high temperature alters the microstructure of the   metal. There are five basic heat-treating processes: hardening,   case hardening, annealing,   normalizing, and   tempering. Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling. The heat treatment of steels is important for understanding what different properties of steel. As an engineer student we came across that it is very important to understand the heat treatment of steel because when it comes down to different types of materials we will need to know the different types of reactions when heat is implied. References Matweb.com. (2018). AISI 4140 Steel, normalized at 870°C (1600°F), air cooled, 13 mm (0.5 in.) round. Matweb.com. (2018). MatWeb - The Online Materials Information Resource. Appendix % Elongation = L f L 0 L 0 L f = Final Length L 0 = Final length % Reduction = A 0 A f A 0 A f = Final Area A 0 = Initial Area
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16 % Difference = | V 0 V f | ( V 0 + V f ) 2 *100 V f = Literature Value V 0 = Experimental Value