Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention is directed to an improved method for
treating superalloy castings subse~uent to the casting opera
tion. The inventioniis particularly concerned with the treat-
ment of castings which are formed by means of casting, in
shell or permanent molds~ or under similar conditions.
In the production of superalloy castings, it is well-
- known that internal or externally connected material voids are
relatively common. Specifically, defects occasioned by micro-
shrinkage, gas entrapment, cavity shrinkage, as well as other
10 defects resulting in internal and externally connected material ~`
voids are commonly encountered. All such voids adversely af-
fect properties, and the internal voids naturally create par-
ticular problems. Since precision casting involves close dimen-
` sional tolerances, even the externally connected defects are
problematical since they cannot be cured by simply grinding
away the objectionable areas. ^--
Where castings are formed of highly expensive alloys,
the presence of casting defects represents a particularly se-
vere problem. Such castings cannot be used where the parts
are employed for critical purposes, for example as jet engine
turbine blades. Thus, it is known that the presence of inter-
nal and/or externally connected defects will cause part failure
under the operating conditions of the parts and, accordingly,
such parts must be rejected. The parts are then only useful
as scrap, and this represents a great loss to producers in view
of the high production costs involved in precision casting.
various procedures have been developed for treating
castings to eliminate the internal and externally connected
defects. One such process known as hot isostatic pressing in-
3 volves the simultaneous application of high temperature and~
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pressure in a gaseous atmosphere, the combination of the tem-
perature and pressure resulting in creep, yielding, or similar
movement of material whereby voids can be eliminated or at least
greatly reduced.
In the case of internal voids, the pressure differen-
tial is possible since the material surrounding the voids
effectively seals the voids with respect to the high pressure -
gaseous atmosphere surrounding the article being treated. In
the case of externally connected voids, however, equilibrium
develops as the high pressure gas enters the voids in which
case it becomes impossible to achieve the desired metal move-
ment for closlng up the voids.
Procedures have been developed for healing the
external defects as described for example in U.S. Patent No.
3,758,347 issued September 11, 1973. This procedure involves
coating of external surfaces to seal off the material voids,
and to thereby prevent the penetration of the high pressure
gas into the voids. Under such conditions, material movement
can develop at high pressure and temperature whereby healing
of the external voids is accomplished.
This invention relates to a method for treating super-
alloy castings wherein the cast:Lngs are formed in a mold with
the solidified surface thereof comprising a substantially ~ ;
continuous skin, and wherein defects in the form of material
voids are located immediately ad~acent the skin, the improvement
comprising the steps of heating the casting without any surface
finishing operation to a temperature sufficient to permit metal
movement in the casting, locating the casting in a gaseous
atmosphere, and applying pressure between about 10,000 psi and
50,000 psi through the gas to the casting while the casting is
at said temperature, the combination of heat and pressure
serving to provide said metal movement and to substantially
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eliminate said voids by collapsing and consolidating the casting
material in the area of the voids.
This invention provides an improved method for treating
superalloy castings.
Moreover, this invention provides a method for treating
superalloy castings which effectively eliminates the presence
of detrimental externally connected voids in the castings and
which, at the same time, provides for elimination of internal
voids.
These and other purposes of this invention will appear
hereinafter and for purposes of illustration but not of limi-
tation, illustrations relating to the invention are shown in the
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; accompanying drawings in which:
Figure 1 is a photomicrograph illustrating the struc-
ture of a superalloy casting; and,
Figure 2 is an enlarged photomicrograph of a section
5 of the casting shown in Figu re 1. ^;~
The sub~ect matter of this invention generally com-
prises a method for treating superalloy castings which are sub-
ject to the formation of externally connected defects and which
may be considered unsuitable for use without the elimination of
10 the defects. The invention particularly relates to superalloy ' ~
; castings of the type formed in shell molds and the like, that ;
is, castings which are ccnsidered precision castings and which
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are, therefore, relatively expensive.
In accordance with the procedure of the invention, '-
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15 superalloy castings are formed in molds at which time the por- -
tions of the castings which solidify directly against the mold
wall constitute a substantially continuous skin. In discuss- `
ing the continuous skin which comprises a critical considera-
tion of this invention, it will be understood that a skin which
substantially eliminates the penetration of gas ~rom a surround-
ing atmosphere is intended.
` Figures 1 and 2 comprise illustrations by way of pho-
tomicrographs of the condition of castings adapted to be uti-
lized in the practice of the invention. Figure 1 i5 a 50
times magnification illustrating a portion of the cross section
of a superalloy casting. The casting was sectioned and photo-
graphed after separation from a shell mold but without any sur-
face cleaning or other surface treatment taking placeO ~s
shown in Figure 1, a plurality of voids (black areas) are -
3 formed in the casting and some voids are quite close to the
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surface of the casting. It is noted, however, that in all in-
stances, a barrier of metal (white areas~ is present so that ~;
none of the voids are exposed to the exterior of the casting.
Figure 2 is a 5Q0 times magnification of a portion
of the microstructure shown in Figure 1. In particular, the
illustration of Figure 2 illustrates the vold 10 and the metal
barrier 12 showing that the void is completely closed off from
the atmosphere, and a skin which substantially eliminates the
penetration of gas is, therefore, provided.
This skin comprising the particular barrier 12 has a
thickness in the order of .0015 inches. As noted, and as will
be more fully discussed, this barrier must be preserved to
achieve the advantayes of the invention. It has been found
that conventional finishing operations will remove as much as
15 .002 inches of metal in which case the void 10 would be exposed
whereby the hot isostatic pressing will not eliminate this
void.
The material voids, including voids immediately be-
neath the surface skin, may be the result of gases which are
present in the course of the casting operation or due to shrink-
age. Unless these voids or defects are removed, serious limi-
tations on the properties of articles produced from the cast-
ings are imposed. ;~
~his invention provides for the simultaneous applica-
tion of heat and pressure to castings which are unfinished,
that is castings characterized by the continuous skin. The
heating is preferably in the range from 50 F. below the gamma
prime solvus temperature up to the solidus temperature for the
superalloy being treated. The pressure application should be
30 at least about 10,000 psi with maximum pressures being dependent
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primarily upon equipment limitations and being upwards of 50,000
psi. Pressures between 15,000 and 30,000 psi provide satisfac- -
tory conditions in virtually all instances. `~
The process of the invention is more particularly ;`
5 characterized by the utilization of the sound skin bf material ` ~ -
which solidifies directly against the mold wall. This skin is
maintained after casting by eliminating conventional finishiny
operations such as blasting or belting. The shell mold or
other mold structure may be subjected to a gentle knock-out
operation, but areas of the mold may remain on the parts during
the subsequent processingO It is particularly contemplated
that the entire part cluster be subjected to the hot isostatic
treatment with the mold in place where this represents a con-
venient manuacturing procedure. Thus, the mold p~rosity per-
mits gas penetration for purposes of applying the desired pres-
sure to the castings. Equipment space limitations represent `
the greatest drawback to this procedure.
Gate and runner portions, if removed,may expose sur-
face connected defects, and the exposed area should then be
sealed before hot isostatic pressing. For this reason, the
presence of at least a portion of the gate and runner portions
may be particularly desirable, and in any event is not detri-
mental to the hot isostatic pressing. These cast portions
will, along with the aforementioned skin, provide sound re-
gions of material for sealing the casting surfaces. Underthese circumstances, the application of the gas pressure will
result in closing of porosity beneath these surfaces since the
gas will not penetrate to equalize the pressure in the area of
the material voids.
In addition to the presence of mold material and
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superfluous cast portions, any cores present in the castings
may be maintained during the hot isostatic pressing. It has
been found that the core material is not in any way detrimen-
tal to the processing and that the properties of the superal-
loys are not adversely affected by the presence of the corematerial during the processing. The core removal can be con-
veniently carried out during any subsequent finishing opera-
tions, and the core material may itself assist in sealing dur-
ing the processing and will assist in preventing oxidation of
the casting surfaces defining the passages, and will aid in
preventing any collapse of small passagesO It is also contem-
plated that a plating or other sealing material be employed
~or closing o~f the core passages in which case the core mate-
rial therein acts as a,pressure applying media relative to the
passage surfaces for insuring healing of surface connected de-
fects.
A demonstration of the advantages of the invention
was obtained upon casting turbine blades of the following
superalloy compositions:
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For testing purposes, blades formed from Alloy A
were prepared, and these blades were of a relatively difficult
to cast design including five cooling passages requiring the
use of cores. The blades have been found to be particularly
subject to the presence of porosity in the as-cast condition,
and are subject to a high reject rateO
- In one instance, 150 blades were cast, and these -~
were of a design known to have substantial porosity in the as-
cast condition. 32 of the blades were then subjected to sur-
10 face treatment by grit blasting. The ceramic cores were re- -
moved, and the castings were then subjected to hot isostatic
pressing comprising exposure of the castings to a temperature
of 2200 F. (+ 25 F.). The specimens were heated in a pres-
sure chamber for two hours while a pressure 14,500 psi t~ 500
psi) was maintained in the chamber. Argon gas was utilized
for all tests referred to in this application.
The hot isostatic pressing resulted in improved in-
tegrity; however, all 32 blades exhibited varying amounts of
porosity in critical regions. ;
118 blades were subjected to the same hot isostatic
pressing without any prior surface treatment. After casting,
the majority of the ceramic mold portions were gently removed
from the castings; however, all castings retained some por~
tions of ceramic, there was extensive gate evidence in each ~`
instance, and the cores utilized or forming the internal pas-
; sages remained in place. ` ;
Examination of these castings revealed that 116 of
the 118 parts had no detectable porosity, and could be termed
completely healed. The remaining two castings exhibited some
porosity in pedestal regionsO The presence during processing
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of the ceramic cores, gate portions, and mold portions in no
way affected the characteristics of the blade structure when
subsequent finishing operations were performed to remove these
materials.
In addition to the examination of the microstructure
of the castings, tests were performed for purposes of evaluat-
ing the properties of the blade. In a first series of tests
comprising 1400F/94 KSI rupture life tests, the castings sub-
jected to the hot isostatic pressing in the as-cast condition
showed a substantially improved rupture life in the order of
a two to three fold increase. Improvements in rupture elonga-
tion in the order of 30 percent were also noted.
Rupture life tests at 1800F/29 KSI showed a 50 per-
cent improvement in some instances while rupture elongation ~
15 was improved by about 20 percent. ~ -
The improvements in rupture life and elongation are
accompanied by improved thermal and mechanical fatigue prop-
erties. It is concluded that the performance characteristics ~-
of the difficult to cast cored blades as well as the rejection
20 rate of the blades can be materially improved on a production -;
basis.
Although the foregoing discussion has primarily re-
ferred to turbine blades~ it will be appreciated that various
investment castings, superalloy parts, including turbine vanes,
wheels and other structural components can be readily subject-
ed to the described processes with highly beneficial results.
In the production of turbine wheels formed of Alloy B, the cast
parts were obtained in the unfinished condition. When subject-
ed to hot isostatic pressing, the parts exhibited significant-
3 ly improved low cycle fatigue propexties when tested at 900F.
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This represents an extremely important improvement in view of
the fact that the useful life of such parts operating under
such conditions has been a significant pro~lem. In testing ;~
the same and similar parts, si~nificant room temperature prop-
erty increases were also observed whereby the applicability
of the invention to superalloy parts employed at room temper-
- ature or at relatively low temperatures is recognized. In
this connection, the subjecting of the parts to hot isostatic
pressing has been found to improve the machinability of the -
parts which is a feature having value irrespective of the oper-
ating conditions of the parts. ~ `
The particular conditions ~or treatment o superal- '`
loy parts will vary with material composition and are also de-
pendent upon processing equipment limitations. It has been
found, for example, that with commercially available, large
diameter hot isostatic pressing units, a maximum of about
15,000 psi operating pressure is now available. Under such `
conditions, maximum improvements in properties for the blades
tested were available when heating within the aforementioned ~;~
20 preferred range, or example at 2200 F. + 25 F. Other ex-
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periments indicate that where pressures in the order of 30,000 `
psi could be obtained, processing in the order o 2150 F. +
25 F. provided improved results. Higher pressure will permit
the use of correspondingly lower temperatures. In all instan-
ces, a treating time of about 2 hours was employed; however,
the treatment time may vary from about 10 minutes to about 10
hours and preferably is maintained for from 2 to ~ hours.
In the course of treating parts in accordance with `~
this invention, an inert atmosphere may be employed to avoid
3 surface contamination. On the other hand, it is contemplated
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that the parts may be enclosed within a container and the pres-
sure of the atmosphere will then be applied to the container
in which event the parts themselves will not be subjected to
any contaminants in the atmosphere. A suitable filler mate-
rial may be placed within the container whereby the pressureof the atmosphere will be applied to the parts after being
transmitted through the container and the filler material.
It will be apparent that the basic feature of this
invention involves the treatment of the castings in an unfin-
ished state whereby the continuous skin necessary for curingde~ects close to the sur~ace will be preserved. It is contem-
plated that various steps may be taken for purposes of accen-
tuating the formation of the continuous skin to thereby com-
pletely insure the improved results of the invention. This
; 15 may be accomplished by utilizing nucleants in the surface por-
, tion of the mold which is first contacted by the molten metal.
The presence of cobalt oxide, cobalt aluminate and/or cobalt -
silicate will tend to promote rapid freezing of the casting
surface whereby subsequent freezing will take place behind a
continuous skin which is initially formed.
It is also contemplated that mold preheat can be uti-
lized as a means for accentuating the formation of the contin-
uous sXin. Specifically, by maintaining a lower than normal
mold temperature, more rapid freezing will occur in the sur-
face areas of the casting whereby a continuous barrier can beformed without shrinkage voids or the like developing near the
surface.
i The procedures of this invention will provide savings
in terms of a reduction in the number of reject castings since
3 the internal porosity problems can be effectively minimized in
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accordance with the practice of the invention. It is also
contemplated that utilization of extra gates which are often
employed for purposes of reducing porosity in c~eas which are
otherwise difficult to cast can be avoided. Thus, even if
such porosity occurs, the presence of the continuous skin and
the application of the hot isostatic pressing will provide
for substantial elimination of the porosity.
Although the hot isostatic pressing is recognized
as contributing to the improved physical properties of the
castings, it is believed that the presence of the aforemention~
ed sound skin regions also contributes to improved properties
so that these improvements result from a combination of
the hot isostatic treatment and the fact that this treatment -
is carried out in the as-cast condition. The improved pro-
perties are believed to result from the uniquely sound
character of the skin which develops in the casting of the -
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superalloys. It is preferred that the as-cast dimensions be
as close as possible to final dimensions so that there is a
minimum requirement for machining or other surface finishing.
The system of the invention also, of course, provides ;~
advantages to the extent that the system permits the elimination
of any coating such as described in United States Patent No. `i~
3,758,347. Such coatings inherently develop problem areas since
the presence of any material which is foreign to the casting it-
self can create problems from the standpoint of part performance.
For example, even minor alloying or the presence of minor a-
mounts of foreign material could create an area making the
castings more susceptible to fatigue failure. It is also clear
that the necessity for coating requires several addition-
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al handling steps including cleaning operations over and above
the coating operations whereby the possibility of damage and `
contamination is multiplied.
The recognition of the sound continuous coating in
superallpy castings has, as indicated above, provided substan-
tial and highly significant improvements in the casting prop-
erties whereby the performance of the castings, particularly
at elevated temperatures, is materially improved. The pres-
ence of the continuous skin which enables the hot isostatic
10 processing is apparently uni~ue to superalloys insoar as cur- -
rent investigations are concerned. The ability to cure the
internal defects which is allowed by the presence of the skin
also is of most importance in the case of superalloys in view
of the high-performance demands placed on such alloys.
It will be understood that variOus changes and modi-
fications may be made in the above described structure which
provide the characteristics of the invention without departing
from the spirit thereof particularly as defined in the follow-
ing claims.
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