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mistaken for ferrite (iron jwhich also remains bright
under certain conditions. If the polished specimen is
immersed in a boiling solution of sodium picrate for
5 or 10 minutes, the cementite will be colored brown
or black, but the ferrite will be unaffected. This affords
a sure method of identifying cementite. The
reagent may be prepared as follows: Dissolve 24.5
grams of caustic soda (sodium hydroxide, NaOH,
C. P.) in 73.5 cc. water, and dissolve 2 grams picric
acid in this (Ref. 8).
Heaf tinting is an interesting method used to distinguish
certain constituents, especially phosphorus
FIG. 23.—Sulphur Print. FIG. 24.—Deep etching—segregation.
FIG. 25.—Deep etching—stresses. (Rawdon ref. 6.)
in cast iron. The specimen is first etched very lightly
in dilute nitric or picric acid, to remove the surface
film, but not to firing out the grain structure strongly,
and washed and dried. It is then slowly heated on an
electric hot plate, a bath of molten solder, or the like.
The air causes colored oxide films to appear on the
surface. These colors change as oxidation progresses,
but the parts rich in phosphorus color more rapidly,
and will therefore reach a purple or blue color while
the other portions are still yellow, brown, or red. The
specimen is cooled at this point and examined. (Ref. 8.)
A list of etching reagents for various purposes is
given in A. S. S. T. Handbook, data sheets T-7 to T-22.
forging - S tamping - Heat Treating
Rapid Method of Polishing.
January, 1925
The following method of preparing metal specimens
for microscopic examination, described by Mr.
H. B. Pulsifer, Assistant Professor of Metallurgy, Lehigh
University, ("Chem. and Met. Engineering,"
November 5, 1923), is believed worth mentioning here.
He states that this method is not only quicker, but
due to the relief polishing effects, brings out greater
details, at moderate magnifications, than the more
elaborate methods of preparation. He does not recommend
it for magnifications higher than about lOOx,
because the relief obtained is too deep for the focal
plane of the more powerful objectives. His method is
as follows:
The surface is first prepared with a file. The file
marks are ground off with flour emery on a wet wheel,
then the emery marks are removed with tripoli. This
surface is now given a fairly prolonged etch, in the
usual picric or nitric acid solution, which dissolves off
all mechanically flowed surface metal and the deeper
scratches. The specimen is then rubbed by hand, on
a thick layer of wet, levigated tripoli, until smooth.
The etching is repeated with a short attack, and the
surface again smoothed on the board with gentler
passes. A final etching should then display the structure
to advantage. The entire operation should take
3 to 8 minutes.
Dessicator.
Wrhen it is necessary to preserve specimens for
some time, after polishing or etching, they must be
protected from moisture. They may be kept in a dessicator,
which is a glass vessel having an air tight lid,
and containing a quantity of un-slacked lime or calcium
chloride to absorb the moisture from the air in
the container. Specimens wrapped in tissue paper
often remain uncorroded for days or even weeks, without
the use of a dessicator. Specimens as a rule become
corroded much more rapidly after they have
been etched. It is therefore better not to etch until
ready to examine. Slight corrosion may often be removed
by repolishing on the disk with alumina.
PART 2—MACROSCOPIC EXAMINATION
While the microscopic examination of metals gives
us the best insight into their characteristics, the effects
of various alloying elements, and the changes
which take place in heat treatment, a preliminary examination
of the grosser structure, such as can be seen
with the unaided eye or with a simple magnifying
glass, is very useful. This is called "macroscopic"
examination. It is usually made for the purpose of
revealing chemical or physical non-uniformity, segregations,
flaws, persistent ingot or casting structure,
lines of flow in forging or forming, internal flaws,
fractures, cavities, blow holes, welded areas, etc. As
in the study of microstructure, the exact method of
preparing and treating the specimen will vary with
the characteristic it is desired to bring out. The
area to be examined is of course much larger than for
a microsection, and frequently includes the full cross
section or longitudinal section of the piece. Fine polishing
is not necessary—it is generally satisfactory to
prepare a flat surface by sawing, grinding or machining
(using light finishing cuts to avoid distorting the
structure unduly) and rough polishing on fairly fine
emery cloth.