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Forging - S tamping - Heat Treating
HEAT TREATMENT and METALLOGRAPHY of STEEL~| j
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January, 1925
A P r a c t i c a l C o u r s e in t h e E l e m e n t s o f
CHAPTER III—METALLOGRAPHY
PREPARATION OF SPECIMENS*
ONE of the oldest methods of testing steel is the
fracture test, in which the piece is broken, and
the exposed surface, or so-called "grain", carefully
examined. This is still widely used, and to the skilled
observer yields a surprising amount of information as
to the fineness or coarseness of true grain, presence of
blowholes, flaws, or inclusions, overheating or underheating
in heat treatment, approximate carbon content,
etc. However a highly trained eye is needed, the
results are limited at best, and the indications may
readily be misleading. For example, a piece of steel
may show a very different "grain", depending upon
whether it is broken byr gradual bending, or nicked
and broken by a sudden blow. The real grain structure
of metals is generally so small that it is invisible
to the naked eye, and can be revealed only by the
compound microscope.
In an effort to get more accurate information from
studyr of the fracture, an early scientific investigator
used the microscope to examine fractures, but little
was gained, because the surface of the break was too
irregular to permit focusing a microscope of high
power, and a clear image could not be obtained. Moreover,
the act of breaking a piece of metal will generally
distort or destroy at that point, the very structure
which it is desired to examine. Pioneers in Metallog-
*The author wishes to acknowledge his indebtedness to the
following references for information contained in this section,
and to recommend them to the student for further reading: (5)
"The Preparation of Metallographic Specimens," H. M. Boylston,
A.S.S.T. Handbook; (6) Circular of the Bureau of Standards,
No. 113—"Structure and Related Properties of Metals"; (7)
"Introduction to Physical Metallurgy." Rosenhain; (8) "The
Metallography & Heat Treatment of iron and Steel," Sauveur.
P h y s i c a l M e t a l l u r g y
The author is Chief Metallurgist, Naval Aircraft Factory,
United States Navy Yard. Philadelphia, Pa.
Copyright. 1924, by H. C. Knerr.
raphy (Sorby in England, Martens in Germany, Osmond
in France) soon found that the best way to
study the microscopic structure, or "microstructure"
of metals, was to prepare a "section", by grinding a
small flat surface, polishing it to remove all scratches,
and etching it with an acid or other reagent to reveal
the individual grains and constituents. Later it was
found that this process of polishing and etching could
be applied, to larger sections for the purpose of revealing
characteristics large enough to be seen without
the aid of a microscope, that is the "macrostructure."
Taking the specimen for metallographic examination
requires particular care and judgment, for it must
truly represent the part under examination, and the
structure must not be altered in cutting or preparing
the specimen. A record should be made of the position
of the specimen in the piece, and the relation of
the prepared surface to the direction of forging or
rolling. A sketch is helpful. In most cases a longitudinal
section yields more information than a transverse
section as it shows the effects of elongation due
to working. For a microsection the specimen should
not be larger than necessary, and may in many cases
be quite small. A section y2 inch square or round is a
good size. It is much easier to polish four pieces l/t
inch square than one piece 1 inch square. The height
dimension (perpendicular to the prepared surface)
should also be about y2 inch, for convenience in handling.
If the specimen is too high it is difficult to hold
it true in grinding and polishing, and a rounded surface
results. The polished surface must be true and
flat, otherwise it will be impossible to focus uniformly
at high magnifications, and parts of the image will be
blurred.
When the sample is not too hard or tough, specimens
are generally cut out with a hack saw. Brittle
material may be nicked on an emery wheel and broken
with a hammer. A thin alundum or carborundum
disk wheel about 3/32 in. thick, running in water to