ORNL-1771 - Oak Ridge National Laboratory

was used with type-430 stainless-steel-clad copper,
os shown in Fig. 7.8. In this case, the intergranular
penetration was completely through the tube wall.
The effect oaf the intergranular penetration on the
physical properties of the tubing has not yet been
determined, but it is believed that it would be ad-
visable to use a brazing alloy that does not react
with the iron-base cladding material.
The tube-to-fin joints shown in Figs. 7,9 and
7.10 are, respectively, type-310 stainless-steel -clad
and type-430 stainless-steel-clad copper fins that
were brazed ivith Coast Metals alloy 52 to lnconel
and then exposed to static air for 400 hr at 1500°F.
A negligible amount of dilution and a relatively
minor amount of boron diffusion into the lnconel
tubing occurred.
Although low-melting-point Nicrobraz (LMNB) has
been used successfully to braze all the proposed
cladding materials to Inconel, its relatively high
Fig. 7.7. Type-310 Stainless-Steel-Clad Copper
fins Brazed to Inconel Tubing with 88% M1-12% P
Alloy. Note dilution of fin and intergranular pene-
tration into tube wall. As pofished. SOX. Re-
duced 15%.
PERIOD ENDING SEPTEMBER ?O, 1954
flow point cf 1925°F is so close to the melting
point of copper that it cannot be used with ease.
Since the oxidation resistance of the Coast Metals
alloy 52 is comparable to that of LMNB and the
effect of boron diffusion is the same for both alloys,
the Coast Metals alloy was selected for the fabri-
cation of the first high-conductivity-fin sodium-to-air
radiator.
The problem of edge protection of the clad copper
fins has been resolved. Aluminum is applied to the
edges of fins by dip coating or spraying, or it is
painted on as aluminum powder in (I Nicrobraa
cement slurry. The fin is then heated at 80OoC for
approximately '/2 hr. The diffusion of the aluminum
into the copper core results in an aluminum bronze
which exhibits excellent resistance to oxidation
when exposed to static air for 500 hr at 1500°F.
It appears that the depth of bronzing, which con-
3
tinues during oxidation testing, is about /32 to
in. in 500 hr. The feasibility of simultaneously
bronzing large numbws of fins is to be determined.
Fig. 7.8. Type-430 Stainless-Steel-Clad Copper
Fins Brazed to lnconel Tubing with 88% Ni-I2% P
Alloy. Note dilution of the fin and severe inter-
granular penetration into the tube wall. A5 pol-
ished. 50X. Reduced 19%.
119