Download Guideline Macro Micro Etching PDF

TitleGuideline Macro Micro Etching
Tags Welding Manmade Materials Metals Abrasive
File Size30.9 KB
Total Pages5
Document Text Contents
Page 2

insure equal metal removal from the entire surface by not allowing prolonged polishing in any
one direction.

4.1.2 Fine Polishing

Fine polishing is often done using a water coolant and silicon carbide abrasive paper mounted on
a rotating disc. The two-wheel unit in the laboratory can be equipped with rough polishing wheel
and a final polishing wheel. Wheel speed for final polishing has to be set at a second position
(about 1150 rpm). A small stream of water is directed at the center of the rotating wheel.

4.1.3 Final Polishing

Final polishing is very similar to that used for rough polishing. Aluminum oxide is the most
popular final polishing abrasive for ferrous and non-ferrous materials. It is also possible to use
chromium oxides for soft materials.

The abrasive particles used in final polishing are generally carried on a napped or short pile cloth
such as billiard cloth or "microcloth." Most polishing cloths can be obtained cut to size and
coated with an adhesive backing. The adhesive back eliminates the need for mechanical
clamping.


CAUTION!

Be sure that you supply the rotating wheel with continuous flowing water. This step is taken to
insure cool surface during polishing.

4.2 Etching of specimens

Temperature - 160 to 180 °F.


Polished metal specimens usually show no structural characteristics. Etching of the metal surface
is done to make visible the crystalline structure of the metal and to produce optical contrast
between the various constituents. Etching is done by exposing the cleaned and polished
specimen surface to suitable etching solution such as those described in Table 1. These etching
reagents are powerful, hazard and must be handled with care.


CAUTION!

Do not forget: etchants are composed of organic and inorganic acids, alkalis or other complex
substances.


If the specimen is not sufficiently etched after the first application the etching process may be
repeated.

If the specimen is overetched it must be repolished prior to re-etching. Immediately following
the etching the specimen should be washed in warm water to stop the etching, then immersed in
alcohol and finally dried in a blast of warm air. Rapid drying is important to prevent water spots.

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Microscopic Examination - Reveal s
maximum detail in pearlite, untempered
and tempered martensite, and bainite;
reveals undissolved carbide particles in
martensite; differentiates ferrite,
martensite, and massive carbide by
coloration; differentiates bainite and fine
pearlite; reveals carbide particles in grain
boundaries of low-carbon steel and
wrought iron - Polished surface.

Iodine
Tincture,
USP

15 to 60
minutes

Macroscopic Examination - works better
with steel is 180 to 200 °F. Slow but
available from Pharmacy

Iodine and
Potassium
Iodide

ß 1 part of
powdered
iodine
(solid
form),

ß 2 parts of
powdered
potassium
iodide,

ß 10 parts of
water, all
by weight.

30 to 60
minutes

The solution should be used at room
temperature, and brushed on the surface
to be etched until there is a clear
definition or outline of the weld.


Hydrochloric
Acid

ß 1 part HCL
Hydrochlor
ic Acid
(Muriatic
Acid)

ß 1 part
Distilled
Water

15 to 60
minutes

Macroscopic Examination - Segregation,
porosity, hardness penetration, cracks,
inclusions, dendrites, flow lines, soft
spots, structures, weld examination. - Sa-
cut, machined or average ground surface.
Use at or near Boiling Point of Acid
Solution.

Ammonium
Persulfate

ß 1 part of
ammonium
persulfate

ß 9 parts of
water, by
weight

Applied by vigorously rubbing the
surface to be etched with a piece of
cotton saturated with the solution. The
etching process should be continued until
there is a clear definition of the structure
in the weld.





5. Acceptance

For acceptable qualification, the test specimen, when inspected visually, shall conform to the
following requirements:
Fillet Welds. Fillet welds shall have the following:

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