Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF SELECTIVE GRAIN GROWTH IN
NICKEL-BASE SUPERALLOYS BY
CONTROLLED BORON DIFFUSION
B GROUND O~ THE INVENTION
The present invention relates to nickel-based
alloys having relatively great strength at high tempera-
ture and generally referred to (e.g. U.S. Patent Re28,681
issued to Baldwin on January 13, 1976) as superalloys,
and, in particular, to articles with different grain sizes
in different portions of the article.
The literature shows that, for nickel-based
superalloy systems, grain size has a definite effect on
mechanical properties. In general, increasing grain size
has the effect of increasing high temperature creep rup-
ture properties. The effect of increasing grain size
decreases the total grain boundary area and thereby re-
duces the propensity of grain boundary failure mechanisms
to occur.
Conversely, decreasing grain size enhances such
properties as high cycle fatigue due to the increased
fracture energy required to propagate cracks by enabling
the increased grain boundary area to retard dislocation
movement. In addition, such properties as impact strength
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are enhanced by decreasing grain size.
Grain size control in superalloys is generally
considered critica~ in the manufacture of most turbine
hardware. Difficulties, however, are routinely experi-
enced by forging, casting and powder forming processes
insofar as grain size and uniformity are concerned. The
literature shows that increased carbon additions to nickel-
based alloys generally aid grain size control in forged
products.
0 Complex cooling schemes and insulated molds de-
signed to control cooling rates of cast alloys have been
employed for grain size control. Powder metallurgical
component structures are generally restricted by the
initial particle size. A dual grain structure for im-
proved properties is described in U.S. Patent 3,741,821
issued June 26, 1973 to Athey et al., but requires complex
equipment, extremely close temperature control, and two
separate heat treatments. A mechanical process for pro-
viding fine surface grains and coarse internal grains is
described in U.S. Patent 3,S05,130, issued to Paul on
April 7, 1970. It requires special surface cold working
and a recrystallization heat treatment. Such cold working
is impractical on some alloys because of the susceptibil-
ity to cold work cracking and, further, the results of
such mechanical working can be lost to relaxation at
turbine operating temperatures. Thornburg's U.S. Patent
3,597,286 issued August 3, 1971 uses annealed cold rolled
iron-cobalt-vanadium alloy and produces a partially re-
crystallized grain structure with a balance of room temper-
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ature mechanical properties commensurate with magnetic
characteristics. Such a recrystallization process is
generally difficult with age hardenable nickel base comp-
ositions and can also result in relaxa-tion at high operat-
ing temperatures.
Boron is known to (up to certain amounts, where
incipient melting occurs at operating temperature) improve
stress rupture properties (see 4,093,476 issued to Boesch)
It also acts as a grain boundary ductilizer (see the
0 aforementioned Re28,681).
U.S. Patent 2,763,584, issued to Badger on Sep-
tember 18, 1956 employs decarbonization and deboronization
of exterior surfaces of articles for the purpose improving
thermal shock resistance. Heating in a hydrogen atmos-
phere presents problems and is expensive, and the removal
of carbon results in loss of control on grain size.
SUMMARY OF THE INVENTION
This is a method for producing a composite grain
structure in nickel-based superalloy articles. The size
of the grains is varied in a controlled manner (both the
size of enlarged grains and location of the enlarged
grains is controlled).
This method comprises preparing a nickel-based
superalloy containing, by weight percent, 0.01-.10 carbon
and 0.02-.08 boron, forming this alloy into a configura-
tion approximating the final configuration, and heating
the formed alloy in a non-reducing atmosphere to a temper-
ature of 1800-2500C (but less than the homogenization
temperature) for 1-10 hours. Diffusion of boron from
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portions of the article which are exposed (without mask-
ing) to the atmosphere allows grain growth selectively in
these exposed portions of the article. The non-reducing
atmosphere generally maintains the carbon content constant
Thus, the diffusion of boron of the exposed surfaces into
the atmosphere allows grain growth in those regions of the
alloy, but the carbon content is maintained to provide an
effective upper limit to grain growth.
Preferably, unmasked passages are provided into
0 the interior of the article and the outside surfaces of
the article are masked. This provides for grain growth
only in the interior of the article adjacent to the pas-
sages and thus a composite structure with larger grains in
the interior and smaller grains on the exterior of the
article.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
.... .. . _
If the carbon content of a nickel-based super-
alloy is reduced, without replacing the carbon with boron,
grain coarsening during solution heat treatment will
~0 result. If boron is substituted for the carbon, however,
a fine grain structure can be maintained. Apparently, the
grain boundary pinning by carbon provides grain size
control during solution heat treatment but this control
can also be provided by boron. The boron, however, can
diffuse during solution heat treatment, resulting in
deboronization of exposed metal surfaces. By maintaining
the carbon level, and controlling which regions are ex-
posed, controlled grain growth in selective regions can be
achieved.
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The usefulness of such a system is exemplified
by rotating turbine blade hardware. Extensive efforts are
being made to increase rupture strength capability of such
hardware.
For rotating turbine blades, increased grain
size would normally increase creep rupture strength, but,
with all the grains increased in size, this would result
in decreased fatigue resistance. Thus, with such effec-
tively homogeneous grain sizes, properties are currently
0 compromised. The controlled deboronization of this inven-
tion, however, can provide for turbine blades which util-
ize large internal grains for creep strength and fine
surface grain structure for maximum resistance to high
cycle fatigue initiation.
A special alloy was prepared using a version of
the Udimet-710 composition generally described in U.S.
Patent 3,667,938 issued to Boesch, modified to have low
carbon (.024% versus the normal .08%) and higher boron
(.05% versus the normal .01%). After forging, the grain
structure was extremely fine and uniform. After solution
heat treatment (2135~F for 4 hours in air) the grains
exposed to the furnace atmosphere grew substantially in
comparison to the unexposed grain structure, providing a
composite structure having an extremely uniform band of
enlarged grains, both of uniform band thickness and uni-
form grain size.
As noted previously, the optimum turbine blade
design should have large internal grains and fine external
grains. This can be achieved by machining cooling pas-
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blades) prior to solution heat treatment and coating the
external surfaces with oxide or glass coatings to gener-
ally prevent diffusion of boron out of the external sur-
faces. Thus, during so:Lution heat treatment, the cooling
passages are exposed to the furnace environment and the
external surfaces are masked. This process provides a
structure having fine, fatigue-resistant grains on the
exterior surfaces and other regions remote from the cool-
ing passages together with enlarged, creep-resistant
grains in regions adjacent to the cooling passages.
This same structure with large internal grains
and fine external grains can alternately be achieved
without the oxide or glass coatings by fabricating an
oversized forging with cooling passages, heat treating,
and then machining the surface. Thus, large grains are
formed both in regions adjacent to the cooling passages
and on the external surface during solution heat treatment,
but the large grains on the external surfaces are removed
during machining to provide a fine grain external surface.
This composite grain structure technology is not
limited to forged structures, but can also be expanded to
other structures such as cast or powder metallurgy and hot
pressing structures. Cast or powder products may need
additional hot working to increase stored strain energy
for recrystallization and growth, and thus it may be
convenient to use powder and cast structures as forging
preforms. Heat treatment in a non-reducing atmosphere
provides deboronization without decarburization and thus
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provides control of the amount of grain growth and the
location of grain growth in these types of structures as
well.
It should be noted that no significant further
deboronization (after solution heat treatment) will occur
during service (e.g. during the operation of the turbine)
as maximum service temperatures are not sufficiently high
for significant diffusion of boron. While a variety of
compositions can be used, this invention can be conven-
o iently practiced by modifying a known alloy by substitut-
ing (approximately an equal atom percentage) boron for a
portion of the carbon in the commercially available com-
position. Preferably between 25 and 75 atom percent of
the carbon in the known composition is replaced with
boron.
Generally the solution heat treatment is per-
formed at a temperatwre between 2100F and 2500F and the
alloy is held at that temperature for 2-4 hours. The
atmosphere during the treatment can be either inert (e.g.
argon) or oxidizing (e.g. air).
Thus, it can be seen that grain size control by
a deboronization process of carbon, nickel-base super-
alloys can be utilized to produce a composite grain struc-
ture with enlarged grain structures of controlled maximum
size in specific regions to attain significantly improved
hardware creep rupture life and simultaneously fine grain,
high cycle fatigue-resistant regions. This is attained by
a single heat treatment procedure and this procedure may
be employed with forged, cast, or powder metallurgical
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thermal treatment cycles.
As variations additional to the above can be
made without varying the inventive concept described
herein, the invention should not be construed as limited
to the particular forms described, as these described
forms are to be regarded as illustrative rather than
restrictive. The invention is intended to cover all forms
which should not depart from the spirit and scope of the
invention.
