Microstructure & Properties of Steels: Correlation with Equilibrium Diagram, Lab Reports of Materials science

Download Microstructure & Properties of Steels: Correlation with Equilibrium Diagram and more Lab Reports Materials science in PDF only on Docsity! LAB III - CORRELATION OF MICROSTRUCTURE WITH THE EQUILIBRIUM DIAGRAM PRINT THIS AND TAKE IT TO THE LAB Objective The objective of this lab is the understanding of the correlation of the chemical composition and temperature with the microstructure and properties of certain materials. Specially prepared samples will be analyzed under the microscope and their structure will be correlated to the chemical composition of the sample. In addition, hardness values will help characterize the properties of each sample. Procedure Small cubes of 1018, 1045 and 1095 steel will be provided in the annealed and water-quenched form. Their hardness will be measured and compared. Use HRB or HRC, but remember to transform all values to DPH for your report. The microscopes will be used for analyzing the structure of samples made of the same steels as the tested ones. Iron - Cementite phase diagram Atomic Percent Carbon 40D 10 16 e a 4 Weight Percent Carbon A SIMPLE APPROACH FOR THE IRON CEMENTITE PHASE DIAGRAM Figure 1 In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase diagram is shown above (Figure 1), up to around 7 % Carbon. This is a fairly complex phase diagram but, as we are only interested in the steels part of the diagram we can make a few simplifications. The γ phase is called austenite. Austenite is a high temperature phase and has a Face Centered Cubic (FCC) structure (which is a close packed structure). The α phase is called ferrite. Ferrite is a common constituent in steels and has a Body Centered Cubic (BCC) structure (which is less densely packed than FCC). Fe3C is called cementite and lastly (for us), the "eutectic like" mixture of α+ cementite is called pea lite. Figure 2 Figure 3 We are considering steels, and therefore only need to look at the Fe-C phase diagram up to around 1.4 %C. Looking at the phase diagram, all alloys up to 1.4 % C must cool through the γ (austenite) phase. So we will consider alloys from below around 1000 °C, as illustrated above in Figure 2, and magnified in Figure 3 (above). Figure. 9 The microstmctures of steels vary considerably with carbon content. With increasing amounts of carbon, the amount of hard, brittle, cementite increases. This variation in micro structure leads to significant changes in engineering properties, as shown in the figure. For example, strength increases with carbon content up to the eutectoid composition but then starts to drop as a grain-boundary network of brittle cementite is formed. Phase diagrams allow us to understand why the properties of steels change with differing carbon content and enable us to make steels with the properties we require. TRANSFORMATIONS DEFINITIONS: Note that "new" here describes a phase with different composition when compared to the previous phase. Eutectic: A liquid phase that transforms into two or more intimately mixed solid phases, depending upon the number of components of the system. L(4.3 % C) austenite (2.08 % C) + Fe3C (6.67 % C) Eutectoid: A single solid transformation into two or more intimately mixed solids, depending upon the number of components of the system. 1148°C austenite (0.8 % C) ferrite (0.02 % C) + Fe3C (6.67 % C) 1148°C Monotectic: A liquid phase that transforms into another liquid and a solid. Peritectic: A liquid phase reacted with a solid phase to form a new solid phase. Liquid (0.53 % C) + T (0.09 % C) T(0.17 % C) Peritectoid: Two solid phases reacting to form a single new phase. Microconstituent: A microstructural feature that is made up of various solid solutions, and/or intermetallie compounds, (e.g. Pearlite) α Ferrite (or lower ferrite): A solid solution of one or more elements (usually carbon) in BCC iron which exists below 912 °C. 1148°C Austenite: An interstitial solid solution of one or more elements in FCC iron, with carbon being the typical solute. It has a higher solid solubility for carbon than a ferrite. Cementite: A compound of iron and Carbon, called iron carbide, with the chemical formula being Fe3C. It has orthorhombic structure and is hard and brittle. δ Ferrite (or upper ferrite): An interstitial solid solution of carbon in BCC iron with a larger lattice constant when compared to a ferrite.lt is formed above 1395 °C. Martensite: A metastable interstitial solid solution of carbon in BCC (or BC tetragonal) iron. It is highly strained with % C grater than 0.02 and is formed by a diffusionless process. Pearlite: A lamellar eutectoid structure formed upon slow cooling from austenite that is made up of alternating plates of a ferrite and cementite (Fe3C). Proeutectoid ferrite: (α') sometimes called primary **, forms above the eutectoid temperature in the γ + α region, typically on the austenitic grain boundaries. Proeutectoid cementite: (Fe3C’ ) sometimes called primary Fe3C, forms above the eutectoid temperature in the α+Fe3C region, typically on the austenitic grain boundaries. AREA MEASUREMENT METHOD The area measurement method is a very simple way to determine the amount of each distinguishable constituent in an image. This method is used also, for the calculation of areas under plots and for determining areas in general, especially when appropriate softwares were not available for this purpose. The method consists basically of drawing a grid in the image (can be done with a transparency over the original, sparing the document) and counting how many squares are mainly filled by the constituent you're looking for. Check out the examples below: Figure 10 represents the constituent B in the image. The white area is A. Then, Figure 11 shows the same image with a mesh. Figure 11 Figure 12 The total amount of squares on the mesh equals 80. The number of squares where black is dominant is about 27. This means that the constituent B is approximately 34 % of the total. SAMPLES DESIGNATION Alloys: 2-0.18%C, 0.39 %Mn 3-0.47%C, 0.88 %Mn 7-1.01%C, 0.39 %Mn Heat treatments: A - Austenitized at 870 °C for two hours and furnace cooled. W - Austenitized at 870 °C for two hours and water quenched.