Specimen Preparation.pptx

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1. PREPARATION OF SPECIMEN FOR MICROSCOPIC EXAMINATION

2. Introduction and Purpose M e tallo g ra p hy i s t h e s t u d y o f t h e physi c a l s t r u c tu r e and components of metals, typically using microscopy . Precisi o n me t a l lur g ic a l s a m pl e p re p ar a t i o n i s a k e y s tep in performing reliable metallurgical testing . Facilitate examination and interpretation of microstructural features . Ex. To find out various phase present in material . Imp r ope p r ep a r a t i o n methods may obscu r e fe a tu r e s , an d e v e n create artefacts that may be misinterpreted.

3. Microscopic Examination is used to find Grain Size, shape and distribution Non Metallic inclusion Segregation of elements Heterogenous conditions

4. Selection of piece from main product I t i s i m p or t a nt t o s t u d y s o m e t h i n g t h a t i s r e pr e s en t a t i v e o f t h e w ho l e specimen . Cutting of that representative part must also be done very carefully . Operations such as shearing produce severe cold work, which can alter the microstructure of a sample . Abrasive cutting (sectioning) offers the best solution to eliminate these undesirable features; the resultant surface is smooth, and the sectioning task is quickly accomplished.

5. Selection of piece from main product Low-speed cut-off wheels are utilized in cases where the heat created by standard abrasive cutters must be avoided. Ample coolant and proper speed control are essential in all sectioning operations.

6. Mounting When the specimen to be tested is inexpensive and easily available, a suitable size specimen may be cut and polished for examination . Mounting of specimens is usually necessary to allow them to be handled easily. It also minimises the amount of damage likely to be caused to the specimen itself . Properties of mounting material Should not influence the specimen as a result of chemical reaction Should not impart any mechanical stresses Should adhere well to the specimen

7. Hot Mounting Press A typical mounting cycle will compress the specimen and mounting media to 4,000 psi (28 MPa ) and heat to a temperature of 350 °F (177 °C). The pressing mechanism is achieved by hydraulics . Specimens can be hot mounted (at around 200 °C) using a mounting press, either in a thermosetting plastic (e.g. phenolic resin), or a thermo-softening plastic (e.g. acrylic resin).

8. Mounting A mounted specimen usually has a thickness of about half its diameter, to prevent rocking during grinding and polishing. The edges of the mounted specimen should be rounded to minimise the damage to grinding and polishing discs. If hot mounting will alter the structure of the specimen a cold-setting resin can be used, e.g. epoxy, acrylic or polyester resin.

9. Coarse Grinding For a perfect observation sample, it must : Be free from scratches, stains and others imperfections which tend to mark the surface. Retain non-metallic inclusions. Reveal no evidence of chipping due to brittle intermetallic compounds and phases. Be free from all traces of disturbed metal . The purpose of the coarse grinding stage is to generate the initial flat surface necessary for the subsequent grinding and polishing steps.

10. Course grinding can be accomplished either wet or dry using 80 to 180 grit electrically powered disks or belts. Care must be taken to avoid significant heating of the sample. Grinding belt material is usually made of SiC paper. Rotate the specimen by 90⁰ on every grade- change

11. Fine Grinding Each grinding stage removes the scratches from the previous coarser paper . This is more easily achieved by orienting the specimen perpendicular to the previous scratches, and watching for these previously oriented scratches to be obliterated . Between each grade the specimen is washed thoroughly with soapy water to prevent contamination from coarser grit present on the specimen surface . In general, successive steps are 240, 320, 400 and 600 grit SiC and the grinding rate should steadily decrease from one stage to the next .

12. ABRASIVE BELT GRINDER Different grades of SiC paper are rolled on rollers which rotate at a specific speed. The specimen is then moved from one end to another in a uniform fashion. When shifting to a different grade, the specimen is rotated by an angle of 90⁰.

13. Polishing Polishing involves the use of abrasives, suspended in a water solution, on a cloth-covered electrically powered wheel . In intermediate polishing, SiC paper of different grades are used. Again, the specimen is rotated while switching from one grade to another. The operation is carried out on a disc with the sandpaper stretched across it. .

14. Double Disc Polishing Machine For fine polishing, this machine is used with a napped cloth fixed atop it. Diamond particles or Al 2 O 3 is suspended in distilled water in a light slurry. The disc is rotated and the specimen is held with mild pressure to absolutely remove scratches

15. Polishing Following the final 600 grit fine-grinding stage, the sample must be washed and carefully dried before proceeding to the first polishing stage . Beginning with 25-micron suspended aluminium oxide particles (suspended in water) on a Nylon-cloth, the final fine-grinding surface layer resulting from the previous grinding procedure should be completely removed with a rotation rate of 150-200 rpm . Wash the specimen and move on to finer suspended particles on separate cloth.

16. Polishing Cloth The final polishing stage with 1-micron suspended aluminium oxide or diamond particles should be carried out on a separate polishing wheel at a slower speed of 100 - 150 rpm using a napped cloth. After 1 or 2 minutes a properly polished specimen should have a mirror-like surface free of scratches There are three types of polishing clothes; Woven, Non-Woven and Flocked. Woven cloths offer ‘hard surface’ polishing properties and guarantee flat pre- polishing, without deterioration of the edges.

17. Non-woven cloths, are used on very hard materials for high precision surface finishing such as glass, quartz, sapphire and semi-conductors. The Flocked cloths, guarantee a super-polished finish. The polishing duration must be as short as possible, to avoid inclusions from being extracted.

18. The grinding and polishing procedure for steel sample preparation is as given below

19. Etching Metallographic etching is the process of revealing microstructural details that would otherwise not be evident on the as-polished sample. Etching is not always required as some features are visible in the as-polished condition such as porosity, cracks and inclusions, for eg, in grey cast iron. Properties revealed by etching grain size Segregation shape, size, and distribution of the phases mechanical deformation

20. The specimen after polishing needs to be properly washed and cleaned with distilled water and after proper drying, the etching reagent is applied by various methods. Types of Etching: Chemical Etching Electrolytic etching Heat tinting

21. a) Polished but unetched surface gives a clean image but no details about the microstructure of the specimen Etched surface: When the specimen has grains with same orientations, only the grain boundaries are visible. Etched surface: When the specimen has grains oriented differently, each grain reacts differently to give varying colours.

22. Chemical Etching This typically involves immersing the sample in an etchant such or swabbing the surface with an etchant. The etchant selectively corrodes microstructural features. Immersion time or etching time is highly dependent on the system and in most cases requires experience. The reactivity of a grain is dependent on the orientation of its microstructure. Deeper etches are preferred for low magnification examinations, while shallow etches are preferred for higher magnification etches.

23. Sample material Etchant Time Wrought Iron 5% HNO 3 in Alcohol 30 sec – 1 min Cast Iron 2% HNO 3 in alcohol or 5% picric acid 10 – 30 sec Tempered high carbon steel 1% in HNO 3 alcohol 5 – 15 sec Low alloy steel 10% HNO 3 in alcohol Upto 1 min Stainless Steel 10% HNO 3 in alcohol 5 – 40 min High Speed Steel 10g of Potassium ferrialdehyde+10g KOH in 10ml water 20 sec – 6 min Cu and its alloys 10% soln. of Ammonium Sulphate in water - Magnesium Alloys 2 – 4 % soln. of HNO 3 in alcohol - Aluminum 2% HF (conc.) + 25% HNO 3 in water Swab for 15 sec. 30 min 90 min 240 min

24. Final step After etching process, the specimen needs to be washed again in distilled water to remove any excess reagent present on it . If not washed, under microscopic observation, there might be aberrations in the colour of the sample . Also, slow and continuous reaction for a long time may take place because of which we cannot use the sample for proper microscopic observation. Cleaning can also be done by placing a drop of spirit and drying it. After washing, it can be dried using a low power blower . Finally, the specimen is ready for observation under microscope.

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