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352 Fbrging-Stamping - Heat Treating
periods are probably not necessary. In fi\e hours the
casting strains should be removed and the casting attain
the maximum growth (before machining).
A few words should be said concerning the uniformity
in strength and hardness of the alloy in any
given condition, whether as cast or as heat-treated.
In Table I is given the range between minimum and
maximum in ultimate strength and in Brinell hardness
in the three specimens to a treatment. In one case
the variation in strength was only 700 lb. per sq. in.,
an in another it was as high as 12.800 lb. per sq. in.,
which is seen to be a broad range, because no test specimen
was weighted in this compilation ii it possessed
an abnormal fracture. When the range in ultimate
strength was calculated as a percentage of the ultimate
strength, using those values for strength which lay
between 29.000 and 39,000 lb. per sq. in. (Table I), it
was found that the average range was sonic 13 per cent
of the average ultimate strength. Since the average
ultimate strength was about 34.000 lb. per sq. in., the
average range was about 4.500 lb. per sq. in. Ten per
cent would probably be a fair allowable ratio of range
for ultimate strengths falling between 18.000 and
28.000 lb. per sq. in.
Boiling water is to be preferred to air as a quenching
medium from the standpoint of uniformity of tensile
properties. Of the six heat treatments in Group
1 which did not meet the ratios stated above (excluding
Molds 24 and 25, taken into the melting zone) four
comprised air-quenching and two quenching in boiling
water. Four of six molds air-quenched and but two of
six quenched in boiling water were not uniform. In
Group 2 only two of the 10 molds quenched in boiling
water did not meet the standard.
Metallography.
The metallography of this alloy is illustrated in
Figs. 4 to 19 inclusive.
As sand-cast the alloy is shown in Figs. 4 and 5 at
low magnification and in Figs. 11 to 13. inclusive, at
higher magnification. Fig. 4 represents the average
structure and Fig. 5 that in a segregated area, both
unetched. Besides large quantities of roughly triangular,
sometimes filigreed. pinkish-white patches of
CuAI., (light colored particles in Figs. 11 and 12) there
were a moderate quantity of watery gray needles and
filigreed areas of one iron-bearing compound, the
needles generally cutting through CuAI., (Figs. 11. 12.
13. 14, 16. 17, 18, and 19). A considerably smaller
amount of harder, purplish skeletons and other nondescript
particles of a second iron-bearing constituent
were also present, being especially prominent in segregated
areas (dark gray in Figs. 5, 11. 12, 13. 15, 18,
and 19). These two iron-bearing constituents were
often intimately associated (Figs. 11, 12 and 13). Unetched,
the difference in color between the two was
not very sharp, but etching with the nitric acid quench
often heightened this color contrast, and also turned
the Cua'M;, brown or black, as shown in Fig. 13, under
oil immersion at 1.000 diameters. It was found at
times that this quench did not always define between
needles and skeletons in color I Fig. 15); that it revealed
a duplex condition in the particles of the skeletons
themselves; and that it resolved apparentlv solid
particles of CuAL into what might be taken for a true
eutectic structure, provided the specimen was not
etched too deeph. In addition to these constituents
there were Mack oxide films and a few other small
October, 1925
black particles, some of which were blue Mg.,Si tarnished
during polishing as shown in Fig. 11. Pinholes
were numerous.
Heat treatment affected but two of these compounds,
the CuAI, and the iron-bearing skeletons. The
specimens soaked at 850 deg. F. before quenching were
not distinguishable from the alloy as cast, although
the tensile and hardness properties were different. In
the specimens treated at 925 deg. F for 5 hours,
whether quenched in boiling water or in air, both
structures, which were similar except for a greater
degree of segregation in the latter (Figs. 6 and 7),
exhibited partial breaking up of the network of CuAI,.
by solution (Fig. 14). The continued soaking at 925
deg. F. for 96 hours caused even more severe attack
upon the network structure (Fig. 8). Though some
of the triangular patches of excess CuAI, still persisted,
the rounded form (Fig. 16) was predominant. In
this condition of treatment, the aluminum-rich solid
solution is probably saturated with dissolved CuAL,.
The progress of the solution of CuAI,, may be followed
in Figs. 11, 14, and 16, the metallographs representing
in order triangularity, necking-down, and rounding.
When the alloy was treated at 1000 deg. F. (and at
1025 deg. F., undoubtedly), not only was an incipient
grain growth noted, but the CuAlz assumed a filigree
or lace-work form (Fig. 17) conspicuous in the specimen
quenched in boiling water (Fig. 9), but less so in
that quenched in air (Fig. 10), in which the particles
were coarser. This filigree structure in itself would
not be indicative of burning—for it is often observed
in the 92 Al-8 Cu alloy as cast—but large grain size
and numerous small circular or semi-elliptical areas of
CuAL intimately associated with one or both of the
iron-bearing constituents, occurring apparently in the
center of grains (Figs. 9 and 17. lower left), are usually
tell-tale. In the burned alloy, too, one of the ironbearing
constituents, evidently the purple compound.
underwent disintegration from the skeleton to the
rounded form depicted in Figs. 18 and 19 commingled
with the lighter-colored CuAI.. The iron-bearing
needles did not seem to be affected by the treatment.
It is possible that the purple constituent contains copper.
Particles of silicon were identified in the specimen
heated at 1000 deg. F for 5 hr. and air quenched.
New Manual Contactor
The CR-1049 manual contactor recently placed on
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Both case and handle can be locked in the open
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Silver contact tips are used on all 'sizes with the exception
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In designing the enclosing cases, consideration
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