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19.1. BRAZING OF GRAPHITE TO
HASTELLOY N
W. J. Werner
ra phite-to-
Studies were continued to develop methods for
brazing large graphite pipes to Hastelloy N. At the
present time, three different joint designs are being
tested for circumventing the differential thermal expansion
problem associated with joining the two materials.
Concurrently, two different brasing techniques
are under development for joining graphite to
graphite and to Hastelloy N. In addition to wettability
and flowability of the brazing alloy on graphite,
the brazing technique development takes under
consideration the application-oriented problems of
service temperature, joint strength, compatibility
with the reactor environment, and braze stability
under irradiation in neutron fluxes
Joint Design
The first joint design, which has been reported
previously,' is based on the incorporation of a
transition material between the graphite and
Hastelloy N that has an expansion coefficient between
those of the two materials. Molybdenum and
tungsten are two applicable materials, and, in addition,
they possess adequate compatibility with
the reactor system. The transition joint design is
illustrated in Fig. 19.1. The design incorporates
a IOo tapered edge to reduce shear stresses arising
from thermal expansion differences.
The second design is the same as the first except
that the transition portion of the joint is deleted
and the graphite is joined directly to the Hastelloy
'MSH Program Serniann. Progr. Rept. Feb. 28, 1966,
ORNL-2936, p. 140.
236
Fig. 19.1. Transition Joint Design with 10-deg
Tapered Edges to Reduce Sheor Stresses.
right the materials are grophite, molybdenum, and
tiastelloy N.
r-
. ,~ .
From left to
. ,.-,
GRAPYITE COO0 CLEARGhZE
HASTELLO'I lil
ORNL-OWG 61-11846
Fig. 19.2. Schematic lllustrution of a Direct Hustelloy
N -t o-Gro ph i te Joint.
N. This joint is, of course, more desirable from
the standpoint of simplicity. In addition, the graph-
ite is in compression, which is highly desirable for
a hrittle material. The amount of compressive strain
induced in a 31/2-in.-OD by 1/2-in.-wall graphite tube
using this design is approximately 1%, which is
postulated to be an acceptable compressive strain
for high-density, low-permeability graphite.