CA1040515A - Method for treating superalloy castings - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for treating superalloy castings wherein the portions of the castings which solidify directly against the mold walls comprise a substantially continuous skin. In such castings, defects in the form of material voids, including defects present immediately beneath the skin, are eliminated by the application of heat and pressure. The castings are heated to temperatures sufficient to achieve metal movement, and a pressure of at least about 10,000 psi is applied by means of a surrounding gaseous atmosphere. The heat and pressure applica-tion collapses the material in the area of the voids and con-solidates this material to substantially remove the voids and to thereby improve the performance characteristics of the castings.

Description

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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 .. '1 :"
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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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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 ,. ~ . .
are, therefore, relatively expensive.

In accordance with the procedure of the invention, '-,~ .
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 , S~5 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 ~L04f~515 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-!
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 - 1~ - ~`

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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 -,. .. ..
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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{)5~L5 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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Claims (10)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In a method for treating superalloy castings wherein the castings 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 adjacent 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 eliminate said voids by collapsing and consolidating the casting mate-rial in the area of the voids.

2. A method in accordance with Claim 1 wherein said pressure application takes place at least partly when said castings are at a temperature in a range from 50° below the gamma prime solvus temperature of the castings up to the solidus temperature of the castings.

3. A method in accordance with Claim 1 wherein said pressure is about 15,000 psi.

4. A method in accordance with claim 1 wherein said casting is separated from the mold prior to the heat and pres-sure application.

5. A method in accordance with claim 1 wherein said castings are formed in gas permeable molds, said skin solidify-ing directly against the mold surface, and wherein the heat and pressure application is accomplished with the mold in place around the casting.

6. A method in accordance with Claim 5 wherein a cluster of castings are solidified within the mold.

7. A method in accordance with Claim 1 wherein said castings solidify with gate and runner portions, said heat and pressure application occurring prior to removal of said portions.

8. A method in accordance with Claim 1 wherein said castings define internal passages with cores located within the passages, and wherein said heat and pressure application occurs prior to removal of said cores.

9. A method in accordance with Claim 1 wherein said castings are located in a container with said pressure being applied to said container and being transferred to said castings.

10. In a method for the production of a superalloy part which is characterized by specific nominal dimensions and which must be produced with close tolerances, the improvement com-prising the steps of preparing an investment casting mold defining a cavity dimensioned to minimize the need for machining to final dimensions, including nucleating agents in the molten metal engaging surfaces of the mold to promote rapid nucleation and freezing of said metal, pouring molten metal of said super-alloy composition into said mold, and solidifying the molten metal in said mold, the solidified casting portions adjacent said molten metal engaging surfaces comprising a sound, substantially continuous skin, and wherein defects in the form of material voids are located immediately beneath the skin, locating the casting in a chamber subsequent to complete solidification thereof and without any surface finishing opera-tions whereby said skin remains intact, introducing a sur-rounding gaseous atmosphere into the chamber, applying pressure to the casting through the atmosphere between about 10,000 psi and 50,000 psi, heating the casting to an elevated temperature, said skin being of sufficient thickness to substantially prevent the penetration of said gas into said voids, said casting being at said temperature for a period of time during application of said pressure sufficient to permit metal movement in the casting when the casting is subjected to said pressure, the combination of heat, pressure and time serving to achieve said metal move-ment and to substantially eliminate said voids by deforming and consolidating the material in the area of said voids, and thereafter subjecting said casting to a finishing operation which removes only the non-metallic components on the part surfaces including mold material, any superfluous gate and runner portions, and only that additional amount of metallic material necessary to assure complete removal of said non-metallic components.