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1. Preparation of Specimens for Metallographic Examination 2.

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Presentation on theme: "1. Preparation of Specimens for Metallographic Examination 2."— Presentation transcript:

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2 Preparation of Specimens for Metallographic Examination 2

3 Metallography

4 INDEX Objectives Introduction Metallography Questions 4

5 Objective To prepare the specimens surfaces to be examined for their microstructure study by the microscope. To learn and to gain experience in the preparation of metallographic specimens. 5

6 Introduction Metallography is basically the study of the structures and constitution of metals and alloys, using metallurgical microscopes and magnifications, so that the physical and mechanical properties of an alloy can be related to its observed microstructure. It provides information about the specimen under investigation, including the size and shape of the grains (crystallites), the presence of micro defects (such as segregation, hair cracks, and nonmetallic inclusions), and the nature and distribution of secondary phases. 6

7 Metallography Cutting Grounding – emery paper (240, 300, 400, 600) Polishing (0.5, 0.1, 0.05μ) Etching –Nitol/ Kellers Solution Microscopy

8 Grounding grinding is a subset of cutting, as grinding is a true metal cutting process. Abrasive :The use of Premium SiC abrasive paper is the most efficient and practical technique for grinding metallic metallographic specimens. Although many qualities of silicon carbide are readily available, only the premium grade SiC powder provides the most consistent results and highest grinding rates. Each grain of abrasive functions as a microscopic single- point cutting edge and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.) 8

9 Soft non-ferrous metals - Initial grinding is recommended with 320 grit SiC abrasive paper followed by , 600 and, 800 grit SiC paper. because These materials are relatively soft they do not easily break down the SiC paper. The initial grinding with 320 grit is generally sufficient for minimizing initial deformation and yet maintaining adequate removal rates. For extremely soft materials such as tin, lead and zinc it is also recommended that the abrasive paper be lightly coated with a paraffin wax. The wax reduces the tendency of the SiC abrasive to embed into the soft specimen. Grinding Soft non-ferrous metals

10 Ferrous metals - are relatively easy to grind with the depth of deformation being a major consideration. 120 grit SiC abrasives provide a good initial start with subsequent use of 240 or 320, 400, 600 and 800 grit SiC. Super alloys - are generally of moderate hardness but have extremely stable elevated temperature characteristics and corrosion resistance. the procedures for preparing super alloys is very similar to that for most non-ferrous metals. Grinding Ferrous metals

11 Sample Preparation The purpose of this practice is to understand how to prepare and interpret metallographic samples systematically. Gather information about chemical composition, heat treatment, processing, phase diagram. Cut representative sample. Mount sample, grind and polish. Examine un etched sample. Etch lightly and examine again. Etch further if necessary. Compare with microstructure expected from equilibrium phase diagram

12 Spheroidal Graphite Cast Iron

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14 Abrasive Material 14 Materials used for the abrading particles are:  garnet: commonly used in woodworking garnetwoodworking  emery: commonly used to abrade or polish metal emery  aluminium oxide: perhaps most common in widest variety of grits; can be used on metal (i.e. body shops) or wood aluminium oxide  silicon carbide: available in very coarse grits all the way through to micro-grits, common in wet applications silicon carbide  alumina-zirconia: (an aluminium oxide–zirconium oxide alloy), used for machine grinding applicationszirconium oxide  chromium oxide: used in extremely fine micron grit (micrometre level) papers chromium oxidemicrometre  ceramic aluminum oxide: used in high pressure applications, used in both coated abrasives, as well as in bonded abrasives. ceramic aluminum oxide

15 Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant reflection Aluminum Oxide (0.5, 0.1, 0.05μ) Polishing

16 PROCESS 16 The specimen must 1. Be free from scratches, stains and others imperfections which tend to mark the surface. 2. Reveal no evidence of chipping due to brittle inter metallic compounds and phases. 3. Be free from all traces of disturbed metal. 4. The specimen has to be grounded with the help of abrasive papers. 5. Polishing enhances the surface and makes it suitable to observe its grain structure under Microscope.

17 Grain The micro structure of many metallic or ceramic materials consists of many grains. A grain is portion of the materials within which the arrangement of the atoms is nearly identical but the orientation or crystal structure of atoms are different. 17

18 A Grain Boundary The surface that separates the individual grains is a narrow zoon in which the atoms are properly spaced. One method of controlling the properties of a material is by controlling the grain size. aluminum oxide and titanium dioxide (Al 2 O 3. TiO 2 ) 18

19 Surface Defects Surface defects are the boundaries, or plans, that separate a material into regions. Each region may have the same crystal structure but different orientation 19

20 Industrial etching The surface is than exposed to chemical attack or ETCHING, with grain boundaries being attacked more aggressively than reminder of the grain to reveal the microstructure. Light from an optical microscope is reflected or scattered from the sample surface depending how the surface is etched 20

21 Etching: Sample material EtchantCompositionRemarks Carbon steel(usually 2%) (nitric acid) HNO ml Ethyl alcohol 100ml Few seconds (15 Sec) Carbon steelPicric AcidPicric acid 4g Ethyl alcohol 100ml Few seconds (15 Sec) AluminumHydrofluoric acidHF (conc.) 0.5ml H 2 O 99.5ml Swab for 15 sec. 21

22 Surface Finish and Surface Roughness (Original Magnification 50x) 22

23 REFERENCES L= /ISU=9/ART=688/MediaObjects/MEDIUM_11661_2011_68 8_Fig26_HTML.jpg L= /ISU=11/ART=749/MediaObjects/MEDIUM_11661_2011_74 9_Fig1_HTML.jpg gr11.jpg 23


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