PREPARATION OF SPECIMEN FOR MICROSCOPIC EXAMINATION

Deep Patel : U13ME235

B. Bharath : U13ME236

 

B. Tech II, IV Semester

 

 MATERIALS SCIENCE & METALLURGY

DEPARTMENT OF MECHANICAL ENGINEERING

Introduction and Purpose Metallography is the study of the physical structure and

components of metals, typically using microscopy.

Precision metallurgical sample preparation is a key step in performing reliable metallurgical testing.

Facilitate examination and interpretation of microstructural features. Ex. To find out various phase present in material.

Improper preparation methods may obscure features, and even create artefacts that may be misinterpreted.

Brass

Cu 60, Zn 40 (wt%)

Selection of piece from main product It is important to study something that is representative of the whole

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.

Al-Mg alloy(a) cast condition showing

constituent particles in the grain boundaries

(b) ECAP (equal-channel angular pressing) condition showing shear bands (a part of it is marked by a white bar) introduced into the matrix

(c) elongated grains in the shear band

(d) fairly unchanged grains in the matrix, which is similar to the extruded condition

Selection of piece from main product Abrasive cutting (sectioning) offers the best solution to eliminate these

undesirable features; the resultant surface is smooth, and the sectioning task is quickly accomplished.

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.

Abrasive cutting wheel material

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

Mounting 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).

If hot mounting will alter the structure of the specimen a cold-setting resin can be used, e.g. epoxy, acrylic or polyester resin.

Hot Mounting PressA 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.

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.

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.

• 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

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.

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.

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.

Double Disc Polishing Machine

For fine polishing, this machine is used with a napped cloth fixed atop it. Diamond particles or Al2O3 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.

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.

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

Polishing Cloth 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.

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.

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

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

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

a) Polished but unetched surface gives a clean image but no details about the microstructure of the specimen

b) Etched surface: When the specimen has grains with same orientations, only the grain boundaries are visible.

c) Etched surface: When the specimen has grains oriented differently, each grain reacts differently to give varying colours.

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.

Sample material Etchant Time

Wrought Iron 5% HNO3 in Alcohol 30 sec – 1 min

Cast Iron2% HNO3 in

alcohol or 5% picric acid10 – 30 sec

Tempered high carbon steel 1% in HNO3 alcohol 5 – 15 sec

Low alloy steel 10% HNO3 in alcohol Upto 1 min

Stainless Steel 10% HNO3 in alcohol 5 – 40 min

High Speed Steel10g of Potassium ferrialdehyde+10g KOH in

10ml water20 sec – 6 min

Cu and its alloys 10% soln. of Ammonium Sulphate in water -

Magnesium Alloys 2 – 4 % soln. of HNO3 in alcohol -

Aluminum 2% HF (conc.) + 25% HNO3 in water Swab for 15 sec.

30 min 90 min 240 min

Electrolytic Etching In electrolytic (anodic) etching, electrical

potential is applied to the specimen using an external circuit

During electrolytic etching, positive metal ions leave the specimen surface and diffuse into the electrolyte

Typical examples are platinum, graphite and stainless steels.

Process of oxidizing a sample in a furnace. This induces oxidation of surface features at different rates, to reveal various features.

Coloration of the surface takes place at different rates according to the reaction characteristics of different elements

The observed interference colours allow the differentiation of phases and grains

Heat tinting

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.

Video

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