Lab 5 - AB5 Group D

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Dec 6, 2023

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Lab 5: Heat Treatment of Steel ME330 AB5 Group D J. Signorini, B. Goddard, D. Jhaveri, I. Jhanji, Y. Ramani 1

Table of Contents Abstract: ................................................................................................................................. 3 Introduction: ........................................................................................................................... 5 2.1 Heat Treatment Variables ............................................................................................................. 5 2.2 Heat Treatment ............................................................................................................................. 5 Procedure: ............................................................................................................................... 5 3.2 Heat Treatments ........................................................................................................................... 6 3.3 Tensile Testing ............................................................................................................................... 6 Conclusions: ............................................................................................................................ 8 Stress-Strain Curves for Various Heat Treatment Methods ................................................................. 8 Material Properties for Various Heat Treatments ............................................................................. 11 Relationship between BHN and Ultimate Strength ........................................................................... 12 Differences in Strength between Oil and Water Quenching .............................................................. 13 Physical Results of Various Heat Treatment Methods ....................................................................... 14 Acknowledgements: .............................................................................................................. 17 References: ............................................................................................................................ 17 Contribution List: ................................................................................................................... 17 2

Abstract: The microstructure of iron and steel significantly influences mechanical properties, and one way of manipulating the microstructure is through heat treatment. Heat treatment drives atomic diffusion, forming phases with distinct crystalline structure and relative concentrations of carbon. Desired phase composition, resultant microstructure and material properties are obtained by heating a material to a specific temperature and cooling it at varying rates. This lab investigates the mechanisms which form these microstructures in steel during heat treatment and quantifies the changes in hardness and tensile behavior. Above the eutectoid temperature of 727 o C, a phase called austenite (γ) is formed with face centered cubic crystal structure. This temperature is known as the eutectoid temperature as it denotes the temperature at which a single solid phase transitions into two solid phases, similar to how the eutectic temperature denotes the temperature at which a single liquid phase transitions into two solid phases. The rate of subsequent cooling determines the formation of other phases described below. Equilibrium phases are those which, under constant environmental conditions, do not change with time. For iron at standard temperature and pressure, the equilibrium phases are ferrite ) and ( ( iron carbide (Fe C, also called cementite), and their respective crystal structures are body- ₃ centered cubic and orthorhombic. Equilibrium cooling occurs slowly enough that there is sufficient time for carbon to diffuse and form these equilibrium phases, forming microstructure called pearlite, which is characterized by alternating layers of ferrite and cementite. The equilibrium cooling processes investigated in this lab are annealing and normalizing. Annealing is achieved in a furnace through very slow, controlled cooling rate of half a degree per minute in this investigation, which results in grain growth. Normalization is achieved through air cooling at a significantly faster rate, meaning less grain growth occurs, resulting in a stronger but less ductile material. Metastable phases are those in which an equilibrium state is never truly achieved, however the decay of these phases can be so slow that it is negligible for design purposes. Martensite is one such metastable phase formed during rapid, non-equilibrium cooling, and it is distinct in that it is a displacive--not a diffusion-driven--reaction. This displacement results in a unique body centered tetragonal crystal structure, which results in enhanced properties like high hardness and strength, but also brittleness. Non-equilibrium cooling can be achieved through several heat transfer mechanisms, but the most common is convective to a liquid, called quenching. The rate of cooling during quenching can be tuned both through the thermofluidic properties of the working medium and through convective enhancement, known to some as stirring. In this investigation, two rates of non-equilibrium cooling are achieved through water and a heavy-weight, high viscosity oil, creating a higher and lower cooling rate, respectively. In addition to forming martensite, quenching can cause residual stresses and microfractures due to the large temperature gradients inducing differential thermal expansion, which creates internal shearing. In addition to martensite’s pre-existing brittleness, the resultant as-quenched material 3

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properties are not very desirable for engineering applications, and thus some ductility and toughness must be reintroduced through a process called tempering. The material is held at an elevated temperature, driving partial diffusion of the martensite into the equilibrium phases of carbide and ferrite, while also inducing some creep that allows relaxation of the residual stresses. Two tempers of 400 o C and 600 o C are investigated in this lab, with elevated temperature resulting in more complete tempered martensite formation and regained ductility. Overall results show that non-equilibrium cooling results in high stiffness, but lower ductility compared to equilibrium processes. There is a limit to the relation of faster cooling rate to increased strength, as the water-quenched specimen had significantly lower ultimate strength due to internal stress fractures. 4

Introduction : 2.1 Heat Treatment Variables Alloying components, cooling rate, transformation temperature, and carbon content make up the major four factors. The microstructures and characteristics of the alloying materials change during the heat treatment process. Phase diagrams can be seen to shift as a result. The final microstructure of the sample is significantly influenced by the cooling rate. Pearlite would result from a slow cooling rate of austenite, while bainite would result from a moderate cooling rate, and martensite would result from a fast-cooling rate. We will see how a slower cooling rate causes the sample to exhibit more ductility, whereas a quicker cooling rate causes the sample to exhibit greater strength. The strength of the heated material significantly affects the sample's carbon content. When other factors are considered, carbon assumes various forms in the material's structure. The materials' strengths differ as a result. 2.2 Heat Treatment Austenitizing, annealing, normalizing, water and oil quenching, and tempering will all be covered in this lab. The first step is austenitizing, which involves heating the materials to transform them into austenite. We can undertake additional therapies thanks to this approach. The sample is transformed into ferrite and cementite during the annealing process, which involves slowly chilling the material under supervision. The microstructure shows larger grains and coarse pearlites as a result. Normalizing is comparable, but it relies on air to chill the sample down rather than a well-regulated cooling procedure. Less time is available for diffusion, leading to finer pearlites and limits changes in grain size. On the other hand, quenching creates martensite, the strongest type of microstructure. The hot sample is cooled most quickly by quenching, and water quenching accomplishes this more quickly than oil quenching. This is because water has a higher heat capacity. Quenching can also be thought of as an addition to quenching. By enhancing sample strength, quenching causes the creation of brittle material. Tempering is utilized to strike a balance between the two. This is accomplished by slightly heating the quenched material to permit some diffusion. This lessens the thermal stress created during quenching, enhancing the material's ductility while maintaining strength. Procedure: 3.1 Equipment and Material A furnace, tongs, insulated gloves, a water bath, an oil bath, a screw load frame, wedge grips and an extensometer are among the tools utilized in this lab. The 4340 Steel is initially heat treated using this equipment in a variety of ways. The samples that need to be quenched, normalized, and tempered are heated in the furnace as well as the samples that need to be annealed. The next step is to perform the heat treatment methods of normalizing, water quenching, oil quenching, and tempering (T400 and T600) using tongs, insulated gloves, water baths, and oil baths. Then, 5

4340 steel that has undergone five different types of heating treatment is put through a tensile test using the screw load frame, wedge grips, and extensometer. 3.2 Heat Treatments The samples that need to be annealed will be finished before class because the annealing heat treatment procedure takes a while. The samples that will be quenched, normalized, and tempered are currently in the oven. Heat gloves and safety eyewear must be worn when handling the tongs to remove any and all samples because they will be extremely hot. Remove the sample from the normalized samples and set it on the proper block so it can air dry. The samples that have been water quenched should be taken out of the oven, dipped vertically into the water bath within 5 seconds, and swirled there for 20 seconds. Then, it ought to be put on the brick to cool. The process for oil-quenched samples should be the same as for water-quenched samples, with the exception that the oil bath should be agitated for 2 minutes. The same oil bath technique should be followed for the tempered specimens, but the oil should be removed before the specimen is put in the appropriate oven, depending on whether it will be tempered at 400C or 600C. The samples should be prepared for part 3.3 after cooling. The gauge and grip of the samples should first be cleaned of iron oxide scale using the wire brush. The preparation should next be completed with belt sanders. Use the water buckets to chill the specimen if it becomes too warm during this process. The grip should then be cooled using the bigger belt sander so that the specimen can be held securely by the tensile grips and the hardness measurement may be performed. Finally, use the smaller belt sander to remove any scale from the gauge portion, and then use the Sharpie to identify each specimen with the appropriate heat treatment. 3.3 Tensile Testing The ASTM standard E8-81 is followed for conducting the tension testing. This specification will be applied to all specimens of 4340 steel that have undergone various heat treatments. For each specimen, the Rockwell hardness must first be determined. Because the variety of microstructures in this lab calls for both B and C scales, one must be careful to use the appropriate one. The next step is to measure each specimen's original gauge diameter and average the three measurements for that specimen to obtain the average area. When determining the curvature for the hardness adjustment, the grip diameter should also be considered. The specimen should then be secured in the screw load frame, and the standard tensile testing process should be followed, with the exception that the extensometer should only be balanced, not the load. The specimen should then be monitored until failure before the Control program is started at a rate of 4.0 mm/min. Following failure, the specimen should be examined, the fracture surface marked, and the final neck diameter measured to record the final area. The tempered specimens should be taken out of the oven after all other heat treatments have been tested, and the same cleaning and loading procedures should be followed for them. After completing all tensile tests, the fracture surface will be examined under the stereomicroscope. Then, join the lab group to examine the heat-treated plain carbon steels' microstructures, which will be displayed on a projector. 6

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Conclusions: Stress-Strain Curves for Various Heat Treatment Methods Figure 1. Annealed 1045 Steel Stress vs Strain Curve Figure 2. Cold-Rolled 1045 Steel Stress vs Strain Curve Figure 3. Normalized 1045 Steel Stress vs Strain Curve Figure 4. Water Quenched 1045 Steel Stress vs Strain Curve 7

0 0.05 0.1 0.15 0.2 0 500 1000 1500 2000 4340 Steel Stess vs Strain Curves Annealed Normalized Tempered @600 Tempered @350 Oil Quenched Water Quenched Strain (mm/mm) Stress (MPa) Figure 5. 4340 Steel Stress vs Strain Curve for all Heat Treatments Heat treatments are crucial for altering the microstructures of metals which affect key mechanical properties – such as ductility and strength. Different heat treatments have varying effects on these properties and can also influence the strain hardening after yielding. Above, Figures 1 – 4 depict the stress-strain curves for annealed, cold-rolled, normalized, and water quenched 1045 steels. In Figure 5, the stress-strain curve for annealed, normalized, tempered (at 350C and 600C), oil quenched, and water quenched 4340 steels are shown. Before beginning any of the heat treatment techniques mentioned above for steels, the carbon inside of the steel needs to be dissolved into FCC iron. This is done by heating up the steel and holding it at a certain temperature (typically around 850C but can vary based on the alloy) so that austenite can nucleate and grow new crystals. This process is called austenitizing. Annealing is the process of cooling the austenitized steel at a controlled rate in a furnace. Usually, annealing causes materials to have large grain sizes and coarse perlite with a lower strength and higher ductility. If the steel was air cooled after austenitizing, the process is called normalizing. Because the specimen is air cooled, there is less time for diffusion and grain growth than annealing. This results in a fine pearlite and uniform microstructure. Typically, normalized 8

steels have a higher strength and toughness and a lower ductility compared to annealing. This can be observed in Figure 5, comparing the annealed to normalized stress-strain curves. The 4340 annealed has a much lower strength but is much more ductile than the 4340 normalized. Quenching is the process of rapidly cooling austenitized steel by submerging it into a quench media. If the cooling rate from quenching is fast enough, the transformation between austenite to ferrite and iron carbide does not occur and instead martensite is formed. In this lab, two mediums were used: oil and water. Water has a higher heat capacity and conducts heat from the surface of the steel faster than oil. This causes the cooling process to be very fast, however, because of its severity, it can distort or crack the material. Oil quenching is a significantly less severe medium which still transfers heat a bit slower but does not cause significant distortions in the material. Overall, water quenched steels tend to be harder and more brittle, while oil quenched steels tend to have a more moderate hardness and ductility. Looking at Figure 5 again, this can be confirmed by comparing the two quenched curves. It was additionally observed that while the strength of both quenched steels is much higher than other heat treatment methods, they are significantly more brittle because of the rate at which cooling is conducted. Tempering follows quenching and involves reheating the martensite to below the eutectoid temperature and help for several hours. This allows carbides to precipitate and residual stresses caused by rapid cooling to be relived. This process forms tempered martensite. Regular martensite formed after quenching is very strong, however, brittle as well. By tempering martensite, the strength is reduced while the ductility is significantly improved, making the material more usable in applications. This can be seen comparing the two quenched curves in Figure 5 to the two tempered curves. The strength of the tempered steel maintains a strength greater than annealing and increases the ductility from quenching. Different heat treatment methods also cause different amounts of strain hardening after yielding. Based on the results shown in the Figures above, quenching leads to the most significant strain hardening because the rapid cooling process locks dislocations in place and promotes the formation of martensite (which is strong but brittle). This is followed by normalized steel, 350C tempered, and 600C tempered. Annealing minimized the strain hardening which can be because it promoted the formation of a fine-grained, equiaxed, microstructure which leads to increased ductility but reduced hardness/strength. 9

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Material Properties for Various Heat Treatments Table 1. Properties for all Heat Treatments of 4340 Steel Annealed Normalized Tempered @ 600C Tempered @ 350C Oil Quenched Water Quenched Hardness (BHN) 90.67 281.5 230.43 297.97 396.14 251.04 σ 0.2% ( MPa ) 390.490 775.995 962.365 1464.890 1424.050 1729.925 σ ultimate ( MPa ) 672.791 1694.361 1065.8 1620.062 2012.922 1957.030 % Elongation 25.239 15.491 36.219 19.034 4.189 1.752 Table 2. Properties of 1018 CR Steel with Varying Treatments As Received Normalized Annealed Spheroidized Water Quenched Rockwell Hardness # 100.3 (R) 78 (Rb) 67.4 (Rb) 52.1 (Rb) 42.3 (Rc) BHN 227.90 148.52 121.17 90.33 382.40 UTS (MPa) 727.93 517.13 469.23 341.60 1462.97 Table 3. Properties of 1045 HR Steel with Varying Treatments As Received Normalized Annealed Spheroidized Water Quenched Oil Quenched at 500C Oil Quenched at 350C Rockwell Hardness # 93.4 (Rb) 91.4 (Rb) 82.6 (Rb) 66.8 (Rb) 57.9 (Rc) 28.4 (Rc) 37.5 (Rc) BHN 199.62 192.10 162.23 119.78 566.57 269.40 338.82 UTS (MPa) 729.50 696.01 649.70 466.00 444.19 976.83 1220.95 In Table 1 above, several properties of heat treated 4340 Steel are given. These properties corroborate with the explanation of Figure 5 where these heat treatments were plotted on a stress-strain curve. The Harness for the samples above were adjusted based on the curvature of the surface where necessary and were additionally converted from the Rockwell scale to the Brinell scale. The Annealed sample was converted from HRB to BHN, and the additional 5 samples were converted from HRC to BHN. The formulas used in the conversions are given below. 10

BHN = 33.22 e ( 0.0192 R B ) BHN = 131.7 e ( 0.0252 R C ) Relationship between BHN and Ultimate Strength 0.000 50.000 100.000 150.000 200.000 250.000 300.000 350.000 400.000 450.000 500.000 0 500 1000 1500 2000 f(x) = 3.45 x Ultimate Stength vs Hardness 4340 Anealed 1018 CR - Annealed 1045 HR - Annealed 4340 Normalized 1018 CR - Normalized 1045 HR - Normalized 4340 Tempered @ 600C 4340 Tempered @ 350C 4340 Oil Quenched 1045 HR - OQ @350 1045 HR - OQ @500 4340 Water Quenched 1018 CR - Water Quenched 1045 HR - Water Quenched 1018 CR - As Received 1045 HR - As Received 1018 CR - Spheroidized 1045 HR - Spheroidized UTS = 3.45 * BHN Linear (UTS = 3.45 * BHN) BHN Ultimate Stress (MPa) Figure 6. Ultimate vs Hardness Linear Plot of 4340, 1018, and 1045 Steels Figure 6 above plots the ultimate tensile strength (UTS) against the Brinell hardness (BHN). The following legend can be used to better understand the graph: o Square = Annealed o Diamond = Normalized o X = Tempered o + = Spheroidized o Orange, Red = 4340 Steel o Yellow = 1018 Steel 11

o ⚹ = Oil Quenched o Circle = Water Quenched o – = As Received o Blue, Purple = 1045 Steel In addition to the BHN and UTS values, a “rule of thumb” for steel line is additionally plotted in Figure 6. This best fit line follows the relations listed below: UTS ( MPa ) = 3.45 ×BHN Analyzing Figure 6 based on the heat treatment method – annealed, spheroidized, normalized (for 1018/1045), and oil quenched (for 1018/1045) are very similar to the rule of thumb. The tempered (for 4340) and water quenched had the most disparity from the rule of thumb. Another valuable characteristic of the plot is that the 4340 Steel data points are all significantly above the best fit line which indicates that there could have been some human error in the creation of the test samples or in the physical testing of them. Some examples of human error could be not quenching the samples fast enough or allowing the samples to cool for too long while the oven door is open (which also can create an uneven heat distribution for a period). All heat treatments, other than water quenched, for 1018 and 1045 steel follow very closely to the trend. Differences in Strength between Oil and Water Quenching Oil and water quenching are two mediums used to cool down heated samples, however, they do so at much different rates. Water has a much higher heat transfer coefficient than oil meaning it can cool the sample significantly faster. The difference in cooling rates yield samples with varying characteristics. One such difference is their microstructure. The water quenching tends to produce a harder, more brittle sample compared to oil quenching. Table 1 does not support this claim, however, this likely can be attributed to error when recording the data. Additionally, the strength of our samples will change with different cooling rates. The rapid cooling of the water quenched sample resulted in many small, fine grains. This allows the sample to be able to withstand more stress than the oil quenched sample, as evident in the elastic region shown in figure 5. The steeper slope in the elastic region shows it takes more stress to cause deformation, indicating a greater strength. In summary, both methods will produce different properties in our steel sample. Water quenching tends to have small, fine grains resulting in a hard and strong material. Oil quenching produces coarser grains which gives the sample a moderate balance of hardness and strength. 12

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Physical Results of Various Heat Treatment Methods Figure 7. Spheroidized 1045 Steel Microstructure The spheroidized 1045 steel microstructure consists of a soft ferrite matrix with spherical carbide particles. Spheroidization primarily affects the machinability of steel. The soft ferrite matrix provides increased ductility, while the spheroidal carbides contribute to the dispersion strengthening mechanism. This dispersion hinders dislocation movement and provides resistance to deformation, contributing to the material's improved machinability and resistance to wear. Figure 8. Water Quenched 1045 Steel Microstructure This microstructure mainly consists of martensite, characterized by a lath or needle-like structure. The rapid cooling rate during quenching suppresses the formation of pearlite and traps carbon atoms within the iron lattice. The primary strengthening mechanism in this case is solid- solution strengthening. The rapid cooling traps carbon atoms in the interstices of the iron lattice, leading to the formation of the hard and brittle phase called martensite. This phase offers high strength and hardness due to the limited movement of dislocations, providing increased wear resistance and improved mechanical properties. 13

Figure 9. Normalized 1045 Steel Microstructure This microstructure consists of fine pearlite and ferrite, achieved through heating the steel above the critical range followed by cooling in still air. This process refines the grain structure and creates a uniform distribution of pearlite. The main strengthening mechanisms at play here are grain refinement and precipitation hardening. Grain refinement improves the strength and toughness of the steel by reducing the grain size, which hinders dislocation movement. 14

Figure 10. Annealed Steel Tensile Test Fracture Surface Figure 11. Normalized Steel Tensile Test Fracture Surface Figure 12. Oil Quenched Steel Tensile Test Fracture Surface Figure 13. 600C Temper Tensile Test Fracture Surface Acknowledgements: We would like to thank groups D and C in lab section AB5 for collecting tensile data of different heat-treated specimens. 15

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References: "HEAT TREATMENT OF STEEL - ME330 Lab 5 Manual." (n.d.): Print. 2022 Board of Trustees of the University of Illinois Contribution List: Dhruv Jhaveri – Introduction and Procedure Ishaan Jhani – Scaffolding, Conclusions, Formatting Data, Formatting Report, Question 1, 2, 3 Yogesh Ramani – Formatting Report, Table of Contents, Conclusions, Question 5, Jacob Signorini – Excel data, question 4, abstract & general formatting Ben Goddard – Abstract, Fracture Surface images 16