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llll Foiling Stamping- HeatTieating
acids started. Deep etching was accomplished by
heating a piece of steel of definite size for a given time.
in a solution of concentrated hydrochloric acid.
It was found that by duplicating deep acid-etching
conditions as nearly as possible in each text, we were
able to make excellent comparisons between samples.
In these etching tests it was noted that steels of different
analyses varied in solubility and that the rate
of solubility varied according to the amount of mechanical
work which had been done upon the piece.
Different annealing temperatures on the same steel
caused differences in solubility. It was noted that
steels made by the acid open hearth, basic open hearth
or electric furnace methods could be distinguished one
from the other, due to differences of solubility of steels
of similar analysis. It was with great ease that samples
taken from forgings made from sand-cast and
chill-cast ingots could be distinguished. These points
are brought out simply to show the wide range and
the possibilities of comparison of similar samples subjected
to the deep acid etch. Figs. 5, 6, 8 and 9 show
the results of some deep etching tests made on bars
cored from forgings. Fig. 5 shows a bad surface condition
with considerable pitting following the acid
etch. The physical test properties of this specimen
is shown in Table 1, text 1064. The elastic limit,
elongation and reduction of area are below the specifications.
Photomicrographs of this specimen are
shown in Figs. 3 and 4. It will be noted that these
three different methods of test lead to the
same verdict, that is, that the steel was
dirty and contained many sonims.
Fig. 6 shows a specimen with less pronounced
pits than those of Fig. 5. In order
to establish whether or not these pits were
in the center of the specimen, as well as on
the surface, the same specimen was cut
transversely and longitudinally and then
deeply etched. After etching, the specimens
were closely examined and it was found that
where a fissure or pit existed on one segment
there was a corresponding pit on the
matching segment. Pitting occurred on interior
sections as well as on the surface.
Fig. 7 shows a bar of rolled wrought iron
after it has been deeply etched. The rough
fluted condition of the surface is due to the
fact that the slag lines or inclusions in the
bar have been eaten away by the acid, thus
revealing the piling.
Fig. 8 shows a sound bar of steel after
it has been deeply etched. With the .exception
of the fissure at the extreme upper end
of the bar. this specimen shows no pitting or
fissures. The crack at the top of the bar
was produced by mechanically breaking the
specimen, which upon deeply etching was
enlarged to a fissure. Such hair line cracks
which open up as fissures during deep acid-
etching may be produced by extreme internal
stress, or by impact. It has always been
found that these fissures occur at right
angles to the line of stress in the material
and are apparently always intercrystalline as
compared with intracrystalline cracks which
are inherent in the material due to conditions
of manufacture.
March, 1925
Some time subsequent to making these tests we
were having difficulty in hardening certain areas of a
gas engine cam shaft. These soft areas always appeared
in the same location on the shaft regardless of
the method or condition of heat treatment. Deep
etching of numerous specimens cut from different
cam shafts revealed the conditions as shown in Fig.
10. The parting line of the forging die was at the
part of this cam which was giving difficulty in hardening.
In the forging of a cam shaft, clue to the nature
of^its shape, the material flows in a horizontal direction
until it meets the closed faces of the die, or until
the flash chills. The material then flows perpendicularly
upward and downward to fill the die. If ample
curvature is not provided, or the dies are not brought
down with the minimum thickness of flash, a number
of fiber-like ends of worked-material are left on the
forging at the parting of the dies. The mechanical
working of steel between dies tends to move nonmetallic
particles toward the parting line of the die,
resulting in a concentration of these inclusions at the
point where the best metal is wanted. Further examination
of these specimens showed that it was evident
that the trimmer die was not functioning properly
and was tearing the metal in shearing, resulting in
voids or fissures at the parting line of the dies. To
properlv carburize and harden such material is of
course quite impossible.
FIG. 5—Photograph of a deeply etched bar showing many deep pits on the
surface. FIG. 6—Photograph of a deeply etched bar showing fewer
surface pits and cracks. Fig. 7—Photograph of a deeply etched rolled
wrought iron bar showing a very rough surface resulting from the
etching away of the elongated slag inclusions. FIG. 8—Photograph of a
deeply etched sound bar showing an absence of pits and cracks with
the exception of a minor fissure at the top of the bar which was caused
by mechanically breaking the bar. FIG. 9—Photograph of a deeply
etched bar showing very deep surface pits; photomicrograph of this
bar is shown in Fig. 3.