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well as in the tube-to-header joints
of the manifold section. The tube-to-
header joint,s could be repaired by
rebrazing, but the leaks in the body
of the unit are almost impossible to
repair successfully.
Examination of the completed unit,
revealed several probable causes for
the leaks and excessive distortion.
This examination has led to techniques
for overcoming these flaws in future
brazing operations. The tube-to-fin
leaks were thought to be caused by
excessive use of Nicrohraz alloy,
with the resultant dissolving away of
the tube walls. The bad distortion
a1 so tended to aggravate this situ-
ation by allowing puddling of the
molten brazing alloy. The formation
of "hot spots" in the body of the
assembly was also thought to he a
prevalent condition. A very rapid
heating rate was also an important
factor in the warpage.
Techniqucs that are planned for
incorporation in further brazing
operations are the use of lesser
quantities of Nicrobraz, the use of
several aspirators to promote more
even hydragen flow between the €ins,
the build-up of the whole assembly off
the can hottom to overcome the drastic
initial heating rate, and the use of
a slightly lower brazing temperature.
Brazing of Copper to Inconef, The
need for a Satisfactory brazing alloy
for joining copper fins to Inconel
tubing has been emphasized. The
resultant braze should have relatively
high strength at 1500°F and should
also be a diffusion barrier against
capper penetration into the Inconel
during service. It seems likely that
some alloy other than a copper-base
alloy would best fill these require-
ments. Nicrobrazing at 1900°F is very
promising, but the alloy does not
completely me1 t at this temperature.
It is hoped, however, that the use of
other alloys very similar in compo-
sition to Nicrobraz but of somewhat
PERIOD ENQING DECEMBER 10, 1952
lower melting point w i l l result in a
satisfactory brazed joint.
In view of the possibility that it
might be advisable to use copper-base
alloy for the production of these
tube-to-fin joints, several such
alloys are being investigated. Small
m e l t s of Cu-Re, Cu-Si, Cu-Sn, and
Cu-Mn alloys have been prepared and
are to be tested for flowability
char act eri s ti c s , oxidation resistance ,
and room- and elevated-temperature
tensile strengths.
EVALUATION TESTS OF BRAZING ALLOYS
G. M. Slaughter Y. G. Lane
C. E. Schubert
Metallurgy Division
The results of recent high -tempera -
ture oxidation tests and static cor -
rosion tests in sodium hydroxide,
sodium, and lithium are summarized
in Table 12.4. The high -temperature
oxidation tests were conducted in
a stream of moist air at 1500°F on T
specimens of Inconel and type 316
stainless steel brazed with the
various alloys. The samples tested
in molten sodium hydroxide at 1500°F
were A nickel T j oints brazed with
these alloys. Brazed Inconel and
type 316 stainless steel T joints were
used as specimens for the corrosion
tests in sodium and lithium. The
tensile strength of several brazed
joints has been examined at, room and
1500" F temper a tures.
Oxidation of Brazing Alloys. Ex-
amination of metallographic sections
of the high -temperature oxidation
specimens showed the 60% Pd-40% N i
alloy to be the most oxidation re-
sistant of the brazing alloys tested.
This alloy was unattacked when in
contact with the base metals tested;
the Inconel joint is shown in Fig.
12.5. An example of severe oxidation
is shown in Fig. 12.6, which is the
photomicrograph ofatype 316 stainless
steel joint brazed with the 64% Ag-33%
L65