Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
1-4 MACROSTRUCTURE REVEALED BY MACHINING
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MACROSTRUCTURE 41
The macrostructure of certain metals and alloys can be revealed by machining.
This was first shown by Dewrance in 1927, but no details were provided.
Subsequently, Ljunggren showed that the grain structure of soft iron was revealed
when the surface was scribed with closely spaced ruled lines just as if it had been
etched [26]. Ljunggren also showed that the macrostructure of relatively pure lead
was revealed by planing with a microtome. Best results were obtained with the
knife blade inclined at an angle of about 4.5° (see Fig. 109, Ref. 26).
Clarebrough and Ogilvie used machining to study the macrostructure of pure
lead [27]. The samples were annealed to produce an average grain size of about 5
mm. Orthogonal cuts were made with a high-speed steel microtome with a depth
of cut of 0.001 in. Examination of the cut surfaces revealed transverse marks
extending across some grains in a direction perpendicular to that of the cut. Grain
boundaries were revealed by a change in pitch of these marks. Maximum contrast
was obtained when a grain with strong markings was adjacent to a grain without
marks. Etch pit techniques, which were used to determine the orientations of
grains with strong markings and those without marks, showed that grains with a
[100] direction close to the direction of machining formed strong surface marks
while grains with a [111] direction close to the direction of machining did not
produce marks.
Hanson and Pell-Walpole state that the machining method is the best method
for revealing the macrostructure of cast bronzes [28]. They recommend using a
sharp, square tool 0.01 in across at the tip, with a depth of cut of 0.01 in and a feed
of 0.01 inch.
1-5 THE FRACTURE TEST
Examination of test sample fractures is a well recognized, simple test for evaluating
the quality of metals. Indeed, such tests have been conducted since the
production of metals first began. In this section, the use of macroscopic examination
of sample fractures to evaluate the macrostructure and microstructure of
quality control specimens is reviewed.
The breaking of test pieces for examination can be a very crude operation, or
it can be carefully controlled in test machines. The simplest procedure is to
support the sample on its ends and strike the center with a sledgehammer. In the
fracturing of hardened steel discs, a mold can be designed to support the specimen
edges, while a top cover is used to locate a chisel over the center of the specimen.
The chisel is struck with a sledgehammer to make the break. The mold prevents
the broken pieces from striking personnel in the area. If the fracture is desired at a
particular spot, it is useful to nick the specimen at the desired spot, and a fracture
press is a very useful tool for such work. One end of a specimen can also be placed
in a sturdy vise and the specimen struck on the other end. Body-centered cubic
metals are occasionally refrigerated in dry ice or liquid nitrogen to facilitate