Microstructural Analysis - Lab 1 | MAT E 443, Lab Reports of Materials science

Download Microstructural Analysis - Lab 1 | MAT E 443 and more Lab Reports Materials science in PDF only on Docsity! Mat E 443 - Laboratory 1: Microstructural Analysis Overview This laboratory exercise is intended to provide a brief introduction to the microstructures of ferrous alloys and the techniques of specimen preparation, optical/SEM microscopy, and quantitative microstructural analysis used to characterize them. Each laboratory team will receive a total of seven (7) specimens, sampling a variety of ferrous alloys and will perform the procedures and analyses described here. Each team will maintain a laboratory logbook and will prepare a report based on its own results. No data/analyses are to be shared between teams. Experimental Procedure Before performing this exercise, review the information in the Laboratory Safety Guidelines for this course. In addition, please read this entire document carefully before beginning any work. Test Materials Obtain one specimen of each type of test material. The materials to be examined in this laboratory exercise include four plain carbon steels, two cast irons, and two stainless steels, as listed below. 1018 Steel 1045 Steel 1075 Steel Gray Cast Iron Ductile Iron 416 Stainless 304 Stainless Preparation of metallographic specimens Section and mount each specimen. Rod-shaped specimens should be sectioned transversely so that the circular cross-section can be prepared for examination. If there is any question regarding the appropriate sectioning plane, please ask your instructor for guidance. Specimens that require mounting should be mounted in a thermosetting resin using the hot-mounting presses in the laboratory. Read the instructions for these presses prior to use. Label the specimens using a vibra-tool or punch engraver. Take care to make labels legible. All labels must include: (i) course number and semester, (ii) alloy designation and heat treatment specification, and (iii) lab team number and serial number. Additional information may be included for informational purposes, but this is better recorded in the team logbook, referenced to the specimen serial number. A typical label might be The serial numbers will increment continuously throughout the semester. Complete history information for each specimen must be included in the team logbook. 443-F07 1045 FA BISON-01 Prepare for optical microscopy using the following sequence of abrasives (or similar). Fixed abrasive grinding: 240 grit SiC paper 320 grit SiC paper 400 grit SiC paper 600 grit SiC paper Start with clean paper. If a rotating disc is used, employ a low rotation speed for grinding, and use ample lubricant (water). Hold the specimen stationary on the wheel, applying light even pressure. If a fixed paper strip is used, employ a single direction technique (rather than back and forth). Clean specimens thoroughly with a detergent solution between grinding steps. For each grit, perform two grinding operations with a 45-degree specimen rotation before the second operation. After each operation, grinding scratches should be of uniform depth and direction. Loose abrasive polishing: 15 micron alumina 5 micron alumina 1 micron alumina 0.3 micron alumina 0.05 micron alumina or silica Be sure the wheel is not contaminated. Use a medium rotation speed setting (i.e. 2-2.5). Apply light even pressure. The wheel must be lubricated, but do not flood the cloth. The wheel will rotate in a counter-clockwise direction. Hold the specimen firmly (maintain a light even pressure on the wheel) and rotate the specimen in a clockwise direction around the wheel at a frequency of 1-2 Hz. This counter-rotation will ensure that the cutting action is taking place in all directions. Each polishing step should take approximately 1-2 minutes, and specimens should be cleaned and dried between steps. If much longer times appear to be necessary, it is likely that either your wheel is contaminated or that a prior polishing step was insufficient. After the final polishing step, examine the as-polished surface at 100-500x to verify satisfactory polishing. Final polishing scratches should be invisible at 500x. When a satisfactory surface finish has been achieved, proceed to etch the specimen using an appropriate reagent. For all specimens, unless otherwise advised by your instructor, use an immersion etching technique. Appropriate chemical safety goggles and latex or nalgene gloves MUST BE WORN during the etching process. Projectile safety glasses are not adequate. Be sure the work area is clean and free of clutter. Label a beaker (300-400 ML capacity) with an appropriate label. Place the beaker in a safe but accessible location near the wet sink and fill with approximately 200 mL of the desired etchant. Be sure to replace the cap on the master container and store safely before proceeding. Fill a 500 mL beaker with clean tap water and place it in the wet sink, away from the faucet stream location. The specimen should be clean and dry. Grasp the specimen with plastic (or plastic coated) tongs. Immerse the specimen completely in the etchant, while holding the specimen in a position where the polished surface is visible. Etching times will generally be between 5 and 15 seconds but occasionally may vary well outside of this range. Five- second increments are recommended when no estimate is available. During the five second period, observe the specimen surface carefully. The specimen surface should uniformly become slightly matte. When this is observed, remove the specimen. Any time the specimen is removed (whether the etching process is complete or not) , the specimen should be immediately immersed into the beaker of clean tap water. After rinsing in this beaker under moderate agitation for 20-30 seconds, rinse the specimen in running tap water, then running DI water. Dry the specimen thoroughly, and observe at the appropriate magnification to determine whether further etching is required. Labeled specimens are to be stored in a dessicator, as directed by your instructor. Microstructural Characterization & Quantitative Analysis 1. Overall Characterization of Microstructure Observe the microstructure through the microscope eyepieces. This will provide you with the highest quality images to observe the microstructure. (Note: With digital imaging practices, it has become commonplace to observe ONLY acquired digital photos. This is NOT good practice. Use the microscope. Take some time to look at your whole specimen. Investigate details and anomalies. This will help you decide what images to record for later analysis. Compare the observed microstructures to published microstructures. Record your observations/comments in your logbook. Always record specimen ID numbers and imaging conditions in your logbook so that your comments can be traced appropriately. Before doing any detailed microstructural analysis, it is a good general practice to record the “typical” microstructure. For each specimen, obtain an image using the 10x objective and a second image using the 50x objective. These should show “characteristic” microstructures. Accordingly, be sure to choose a location that represents the typical microstructure. In addition to these images, record any features of interest or other anomalies. These would include any feature not well represented by the characteristic images. In your logbook, record any relevant notes regarding the appearance, location, or distribution of these features. If any observable features are deemed to be artifacts of the specimen preparation or imaging procedure, make note of that as well. Use a serial numbering system for all images. These numbers should permit tracing any printed image or computer image file back to your logbook entries. A good practice is to leave some space near your logbook entries to paste images, or to paste the images first and annotate with comments and pointers. 2. Quantitative Microstructural Analysis a. Determination of Average Grain Size (ASTM E0112) One of the most basic measurable microstructural features is the average grain size, a quantity that has a well documented effect on mechanical properties. It is a general feature of most metals that both strength and toughness increases with decreasing grain size. For example, the empirically developed Hall-Petch relationship states that the strength of a material is inversely related to the square root of the grain size. In this section, the average grain size of the test materials will be measured. Read Lab Instructions, “Standard methods for the measurement of average grain size”. For the 1018 steel, use all three methods from ASTM E0112 to estimate (measure) the average grain size. b. Volume fraction of pearlite (ASTM E562) The amount of pearlite in plain carbon and alloy steels is directly related to the composition of the material, the temperature and duration of the austenitization treatment, and the cooling rate from the austenitization temperature. The fraction of pearlite, as well as the fineness of the lamellar structure itself, influences the mechanical properties of the steel. In this section, the fraction of pearlite in the 1018, 1045, and 1075 steels, will be measured to reveal the effect of carbon content. Using a polished and etched specimen of each test material, measure the fraction of pearlite by following the procedure below. Using the television monitor and a transparent 6x6 grid with 36 points (i.e. N=36), determine a suitable magnification for your volume fraction measurements. Record a typical image at this magnification, for your records. For each of the three specimens (1018, 1045, 1075), use the point-count method to estimate area fraction on 40 fields of view. Use a uniform distribution of locations across your specimen. For each field, count and record the number of points (ni) lying within a pearlite colony. Using a spreadsheet, compute: o the point fraction (ni/N) for each field of view This fraction is also denoted as nn. It can be proved that the point fraction is an estimate of the area (and volume) fraction. VAN VAN o the cumulative mean value of nn, which is an estimate of VV, o the cumulative standard deviation for NN o the 95% confidence interval for VV Plot the distribution of NN. c. Pearlite Spacing The growth of the two-phase eutectoid constituent, pearlite, is a diffusive process. As such, the lamellar spacing that is observed depends on the temperature(s) at which the transformation takes place. At high temperatures (low undercooling below the A1), the driving force is rather low, and the growth rate is low relative to the diffusion “speed”. This condition gives rise to a large pearlite spacing. At low temperatures (high undercooling below the A1), the high driving force gives rise to a high growth rate, relative to the diffusion “speed”, and the pearlite spacing is finer. Thus, pearlite spacing varies inversely with the cooling rate from austenite temperatures. THe objective of this section is to measure the pearlite spacing in a 1075 steel. Using the metallographic microscope, record three characteristic images which reveal the lamellar structure of the pearlite. Using the television monitor and a transparent overlay, employ the intercept method for determine the lamellar spacing of pearlite. For each specimen, obtain at least ten (10) measurements, where each measurement includes at least six (6) lamellae. Analyze your results and examine how the average pearlite spacing varies with carbon content. d. Hardness and Microhardness Ferrous alloys are often designed for strength properties such as yield strength, tensile strength, and hardness. Hardness is a particularly useful property because it generally reflects the strength of a material and is very easily measured. Throughout this course, hardness will be used as a way to assess microstructural changes. In this laboratory exercise, you will perform Rockwell hardness tests and Vickers microhardness tests. For each plain carbon steel (1018, 1045, 1075, 1095) test material, use the UNMOUNTED portion of the specimen and measure the hardness using the Rockwell B and Rockwell C tests. Take at least six (6) measurements for each specimen. Record your results in your logbook. For the MOUNTED 1045 steel specimen, obtain at least 5 microhardness measurements from a pearlite colony and at least five measurements from ferrite grains. Record your results in your logbook. Obtain images of the areas selected, showing the indentation. For each test material, use the MOUNTED specimen and measure the Vickers hardness using a series of microindentations across the specimen diameter. Perform two profiles for each specimen, roughly at right angles to each other. Each profile should contain approximately 20 measurements. e. Graphite microstructure in cast irons (ASTM A247) Graphite morphology is of critical importance to the properties of cast iron. Compare the graphite morphologies in the gray iron and ductile iron specimens to those in ASTM standard A247. Record your observations in your logbook. Summary of Activities The table below summarizes the various activities to be performed in this exercise. Alloy>> 1018 1045 1075 GI DI 304 SS 416 SS General Microstructure X X X X X X X Average Grain Size X Pearlite Volume Fraction X X X Pearlite Spacing X Hardness / Microhardness X X X Graphite morphology X X Reporting All reporting of laboratory activities will be done using a web-based report format, as described by your instructor. Please consult the Laboratory Reporting Instructions for detailed requirements and supplemental instructions regarding format. The report should a comprehensive and detailed description of your activities and findings. All micrographs should serve to support and enhance your discussion points as you describe the analysis techniques and measurements.