NAME; BENJAMIN M. SEFORO ID; 202003798 COURSE; CCB 231 MATERIAL SCIENCE FOR ENGINEERS LAB ASSIGNMENT 2 MICROSTRUCTURAL ANALYSIS TABLE OF CONTENTS ABSTRACT INTRODUCTION OBJECTIVES ABSTRACT When analyzing the microstructure of metals, a process called metallography is carried out. According to Merriam Webster online dictionary (September 24, 1847) metallography is a study of the structure of metals and their alloys using microscopy. Since the grain sizes are too small to be observed by a naked eye, an electron microscope is used to observe these structures. Grain sizes and patterns are physical properties of materials that play a role in determining performance of materials. Before a metallic sample is observed, it has to be prepared first through sectioning, mounting, grinding, polishing and etching. After a mirror-like finish is obtained, the specimen will be ready for viewing and a photomicrograph is captured of the microstructure of the specimen. INTRODUCTION The mechanical properties of metals depend on the grain size which is observed during microscopy. This relationship between the mechanical properties of metals and their grain structure was earlier established by Hall (1951) and Petch (1953). They outlined that a decrease in grain size leads to an increase in the yield strength. During microscopy, a horizontal beam of light will be reflected by the mirror-like finish of the specimen and magnified 50 times, 100 times, 200 times and 400 times and a photomicrograph is taken. The photomicrograph will illustrate etch pits, slip lines and grain boundaries. Etch pits indicate that there was a surface defect and a slip line shows that there was slide deformation where sliding occurs between blocks of crystals within the metal. These two indicate that there were dislocations within the material which determine the plastic deformation. Grain boundaries tell whether there will be twins and cracks in the structure. OBJECTIVES The objective of this experiment is to understand the fundamental procedures in preparing a sample for microscopic observation of the microstructure of metals and to know the Importance of each step. Also, to compare the different microstructures of metals. APPARUTUS Abrasive Cut-Off Wheel Machine Specimen Mounting Press Bakelite Belt Linishing Machine Grit belts (120, 180, 240, 320, 400 and 600 grit belts) Polishing Machine PROCEDURE This section of the experiment was divided in to major parts being sample preparation: which ensures that the sample is free from scratches, stains and any other imperfections with mark the surface, retain non-metallic inclusions, have no evidence of chipping and be free from traces of disturbed metal. And also microscopic viewing. SAMPLE PREPARATION FOR MICROSCOPY SECTIONING The large sample material was shaped in to a smaller sample using an abrasive cut-off wheel in order to avoid disrupting the microstructure of the sample. The abrasive cut-off wheel was attached to a cutting machine and the sample was held using a vice. Water was applied during cutting in order to avoid heating up as this may alter the microstructure. The abrasive cut-off wheel breaks the bonding of the grains and this depends on the hardness and workability of the metal sample, the size and speed of the wheel, the power of the driving motor, the type and amount of coolant and its method of application, the amount of pressure by which the wheel is applied to the sample and the amount of vibration in the machine. During sectioning the following proceedings were followed; - The sample was placed on the table so that it was at the center of the swing of the head, The locking lever was placed in the forward position to allow the control rod to be moved to the further, The rear vice was moved closer to touch the rear face of the sample and the front vice was moved to a position closer to front face of the sample, The nuts that secure the vice section of the table were fastened, The control rod was pushed to make full contact with the sample and the locking lever was pulled down to secure the sample in place, The coolant was turned on the cool and also to clean the cutting area, Stead and moderate pressure applied until the specimen was sectioned. MOUNTING Small samples are mounted in a transparent polymer for easy handling and also to avoid chipping off the edges of the sample. A thermosetting plastic called Bakelite was used to mount the sample on using hot-mounting process. During hot-mounting, the following proceedings were followed; - Liquid soap was applied at the walls and base of the plastic cylinder cavity for easy removal, - The sample was placed at the base of the plastic cylinder cavity ensuring that it is at the center, - The Bakelite mixture was prepared and poured into the plastic cylinder cavity, - The set-up was left for an hour for the Bakelite to solidify. GRINDING The sample, (now a mounted piece) is then filed using a belt linishing machine to create a flat surface subsequent for polishing. During this step, the following was carried out; - The motor was turned on, - The water was allowed to flow steadily creating a thin film, The 120 grit belt was used at first, the face of the sample was placed on the exposed side of the belt gentle securing it in place with hands, Moderate pressure was applied while moving the sample slowly left and right, Grinding is continued until parallel scratches are achieved on the face of the sample, After parallel scratches are achieved, the sample is rinsed with tap water and dried with a paper towel, The same proceedings are carried with the 180, 240, 320, 400 and 600 grit belts. POLISHING In this part the sample was abraded with an abrasive suspension on a cloth-covered rotating wheel. The abrasive suspension was Aluminum powder and water mixture and the polishingcloth was a nylon cloth. This will leave the face of the sample with a mirror-like surface. It is important to ensure thorough washing of the sample and the operators before proceeding to polishing, even between each polishing step. During this step, the following was carried out; - At first, a small amount of water was applied to the wheel, and it was cleaned off with fingers, - Abrasive suspension of 5 micrometers was applied to the wheel, - The sample was then placed face down on the wheel while gently gripping it. Then it was moved in a circular motion against the direction of the wheel while applying small pressure to it, - After some time, it was held in one position to ensure parallel scratches that will clearly indicate that the damage from grinding was removed, - The sample was then examined under a microscope to make sure that the scratches are the same size, - The same procedure was repeated with a 0.3 micrometer abrasive suspension, - The sample was then washed in methanol and dried for the final step. ETCHING Etching allows grains to be seen under a microscope as it attacks different grains. Since grains differ in angles and composition, etching will lead to different appearance on them. The etchant used was Nital, which is a mixture of nitric acid and alcohol. During this step, the following proceedings where followed; - The sample was placed in the etchant for some time, - Then it was rinsed with water. Now the sample was an ideally prepared metallogical sample, I was ready for microscopic viewing. MICROSCOPIC VIEWING The sample was then placed under a microscope and a microphotography was captured of the microstructure of the sample. RESULTS AND ANALYSIS The different microstructure of metals observed where, ferrite, pearlite and martensite. FERRITE MICROSTRUCTURE - They have FERRITE MICROSTRUCTURE PEARLITE MICROSTRUCTURE They have round plain grey grains, They are ductile They are soft They have little carbon in composition Grain boundaries marked with black lines, Their grains are lined with scratch-like marks They are made from combined ferrite grains They are brittle and hard They contain a lot of carbon in composition Grain boundaries marked with white lines, MARTENSITE MICROSTRUCTURE They have no grain boundary in sight Their grains appear needle-shaped instead of being round They result from rapid cooling of steel They are very brittle and hard Brittleness depends on the time of cooling. DISSCUSSION From the results, it can be observed that the microstructure of steel differs due to the different carbon composition which leads to either brittleness when the carbon is too much or ductility when the carbon is limited. Another factor that affects the microstructure of steel is the cooling time: when the steel is given a much longer cooling time, it becomes more ductile as the grain boundaries have enough time to strengthen whereas a shorter cooling time will lead to brittleness since the grain boundaries did not have enough time to strengthen. During sample preparation, the following precautions were observed. The coolant was made available during sectioning and grinding to avoid heating up which may alter the grain structure of the sample. The plastic cylinder cavity was cleaned before placing the sample to avoid dirty sticking on the face of the sample thus denting the face. During polishing the sample had to be held firm to avoid accidents. After the sample was prepared, it was not touched on the face to be viewed as it was susceptible to damage. REFERENCES Research gate online website on Metallography RECOMMENDATIONS I would recommend that the same experiment be repeated with a different carbon to iron composition alloy.
