PREPARATION OF SPECIMEN
— Specimen preparation or polishing is necessary to study its micro-structure, because the metallurgical microscope discussed earlier makes use of the principle of reflection of light (from the specimen) to obtain the final image of the metal structure.
— Satisfactory metallographic results can be obtained only, when the specimen has been carefully prepared. Even the most costly microscope will not reveal the metal structure if the specimen has been poorly prepared.
— A properly prepared metal specimen
* is flat,
* does not contain scratches,
* is nicely polished, and
* is suitably etched.
Procedure: Steps Involved. The procedures for preparing the specimen for both macro and micro-examination is the same, except that in the latter case the final surface finish is more important than in the former.
- Selection of specimen. When investigating the properties of a metal or alloy, it is essential that the specimen should be selected from that area (of the alloy plate or casting) which can be taken as representative of the ‘.’.hole mass.
- Cutting of the specimen. After selecting a particular area in the whole mass, the specimen may be removed with the help of a saw, a trepanning tool, an abrasive wheel, etc.
- Mounting the Specimen. If the specimen is too small to be held in hand for further processing, it should be mounted [Fig. 42.2. (a)] in thermoplastic resin or some other low melting point alloy.
- Obtaining Flat Specimen Surface
— It is first necessary to obtain a reasonably flat surface on the specimen. This is achieved by using a fairly coarse file or machining or grinding, by using a motor driven emery belt.
5. Intermediate and Fine Grinding
— Intermediate and fine grinding is carried out using emery papers of progressively finer grade.
The emery papers should be of very good quality in respect of uniformity of particle size.
Four grades of abrasives used are: 220 grit, 320 grit, 400 grit and 600 grit (from coarse to fine); the 320 grit has particle sizes (of the silicon carbide) as about 33 microns and 600 grit that of 17 microns (1 micron = 10~4 cm).
— The specimen is first ground on 220 grit paper, so that scratches are produced roughly at right angle to those initially existing on the specimen and produced through preliminary grinding or coarse filing operation.
Having removed the primary grinding marks, the specimen is washed free of No. 220 grit.
— Grinding is then continued on the No. 320 paper, again turning the specimen through 90° and polishing until the previous scratch marks are removed.
— The process is repeated with the No. 400 and No. 600 papers. Grinding with the No. 200, No. 320, etc., papers could be done in the
(a) The specimen may be hand-rubbed against the abrasive paper, which is laid over a flat surface such as a piece of glass plate.
(b) The abrasive paper may be mounted on the surface of a flat, horizontally rotating wheel and the specimen held, in the hand, against it [Fig. 42.2(b)].
In either case, the surface of the abrasive paper (with a water proof bases) shall be lubricated with water so as to provide a flushing action to carry away the particles cut from the surface.
6. Rough Polishing
— A very small quantity of diamond powder (particle size about 6 microns) carried in a paste that is oil-soluble is placed on the
nylon cloth-covered surface of a rotating polishing wheel (.imilar to oneofFig.42.2(ft)].
The lubricant used during the polishing operation is a specially prepared oil.
The specimen is pressed against the cloth of the rotating wheel with considerable pressure and is moved around the wheel in the direction opposite to rotation of the wheel to ensure a more uniform polishing action.
7. Fine Polishing
— The polishing compound used is alumina (Al203) power (with a particle size of 0.05 microns) placed on a cloth covered rotating wheel. Distilled water is used as a lubricant.
— Fine polishing removes fine scratches and very thin distorted layer remaining from the rough polishing stage.
— Even after fine polishing, the granular structure in a specimen usually cannot be seen under the microscope; because grain boundaries in a metal have a thickness of the order of a few atom diameters at best, and the resolving power of a microscope is much too low to reveal their presence.
In order to make the grain boundaries visible, after fine polishing the metal specimens are usually etched.
Etching imparts unlike appearances to the metal constituents and thus makes metal structure apparent under the microscope (See micro-structure in Fig. 40.5)
— Before Etching, the polished specimen is thoroughly washed in running water.
Then, the etching is done either by
(i) immersing the polished surface (of the specimen) in the Etching Reagent or by
(ii) rubbing the polished surface gently with a cotton swab wetted with the Etching Reagent.
— After etching, the specimen is again washed thoroughly and dried.
— Now, the specimen can be studied under the microscope.
Etching Reagents for Microscopic Examination
|Sl. No.||Types of etchant||Composition||Uses|
|1||Nital||(i)Cone, Nitric acid(ii)Absolute methyl alcohol||2CC98CC||For etching steels, gray cast iron and black heart malleable iron|
|2||Acid ammonium per-sulphate||(i)Hydrochloric acid(ii)Ammonium per-sulphate(iii)Water||10CC10gms |
|For Etching stainless steel|
|3||Ammonia hydrogen peroxide||(i)Ammonium hydroxide (0.88)(ii)Hydrogen peroxide(iii)Water||50CC20-50CC|
|The best general etchant for copper brasses and bronzes|
|4||Dilute hydrofluoric acid||(i)Hydrofluoric acid(ii)water||0.5CC99.5CC||A good general etchant for Al and its alloys (apply by swabbing)|
|5||Keller’s reagent||(i)Hydro fluoric acid(ii)HCL(iii) HNO3|
|For (immersion) etching of Duralumin type alloys|
|6||Mixed nitric and acetic acid||(i)nitric acid(ii)Glacial acetic acid||50CC50CC||For Etching Nickel and Monel metal|