US 3008855 A
Description (OCR text may contain errors)
Nov. 14, 1961 E. SWENSON TURBINE BLADE AND METHOD OF MAKING SAME Filed Jan. 26, 1959 and ATTORNEY United States Patent 3,008,855 TURBINE BLADE AND METHOD OF MAKING SANIE Ernest L. Swenson, Bedford, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Jan. 26, 1959, Ser. No. 788,944 3 Claims. (Cl. 14832) This invention relates to turbine blades, compressor blades or the like having improved thermal shock resistance and to a method for manufacturing the same.
In turbojet engines a turbine operated by burning gases drives a blower which furnishes air to the burner. Such turbines operate at a relatively high temperature and the capacity of such engines is limited by the ability of the turbine blades to withstand the high temperatures involved. An important factor is the ability of the blades to withstand the thermal shock involved in the heating and cooling of the blades in the operation of such engines. It has been found that blade failures due to thermal shock are associated with the crystal or grain boundary of the metal of which the blades are formed.
It is an object of this invention to provide metal articles having improved thermal shock resistance and more particularly to provide turbine blades, compressor blades and like articles having improved thermal shock resistance. A further object of the invention is to provide turbine blades or the like formed of high temperature nickel or cobalt base alloys having airfoils of which at least the leading edge thereof consists of substantially a single crystal. 'It is another object of this invention to provide a method for forming a turbine blade or the like having improved thermal shock resistance, and preferably a blade or the like of which at least the leading edge thereof consists of a single crystal.
These and other objects are accomplished by forming a blade of a high temperature nickel or cobalt base alloy containing both chromium and carbon in a process wherein the alloy is melted and cast under vacuum, the alloy melt being held under vacuum for a prolonged period before the casting thereof, and preferably for a time sufficient to provide a cast blade having at least the leading edge thereof formed substantially of a single crystal. The resulting blade has markedly superior thermal shock resistance as compared to a blade cast of the same alloy utilizing conventional casting procedures Other objects and advantages will more fully appear from the following detailed description of the invention in conjunction with the accompanying drawing in which:
FIGURE 1 is a schematic illustration of apparatus suitable for carrying out the method of the present invention.
FIGURE 2 is an enlarged photolithic view of a macroetched blade which has been vacuum cast by conventional procedures.
FIGURE 3 is an enlarged photolithic view of a macroetched blade cast in accordance with the process of the present invention.
As above-indicated, the present invention is concerned with the manufacture of turbine blades or like articles having markedly improved resistance to thermal shock and with a process of manufacture wherein the article or blade is cast from a nickel or cobalt base alloy containing both chromium and carbon and wherein the melting of the alloy is performed under a high vacuum and the melt is maintained under such vacuum for a prolonged period of time, and preferably for a time sufficient so that when the article or blade is cast while still under vacuum, a blade is formed which has at least the leading edge thereof consisting substantially of a single grain or crystal.
Suitable high temperature alloys for use in the present invention include an alloy of the type set forth in the Callaway et al. Patent 2,688,536, assigned to the assignee of the present invention and consisting essentially as follows:
0.10% to 0.20% carbon 0.25% max. manganese 0.75% max. silicon 13% to 17% chromium 4% to 6% molybdenum 1.50% to 3.00% titanium 2.00% to 4.00% aluminum 0.01% to 0.10% boron 8% to 12% iron Balance nickel Another alloy suitable for use in the present invention consists of the above alloy in which the iron content is reduced from about 0.5% to 8.00% and preferably from about 3.5% to 6.0%.
Other suitable high temperature nickel base alloys and cobalt base alloys containing both chromium and carbon, well known in the art, are as follows:
A-286 Hastelloy R Hastelloy C 25.00% nickel. 65.0% nickel. 55.00% nickel. 141.75% chromium. 15.5% chromium. 16.00% chromium. 0.08% carbon. 1.50% cobalt. 0.15% carbon.
1.25% molybdenum. 0.10% carbon. 17.0% molybdenum. 1.90% titanium. 5.00% molybdenum. 5.00% tungsten. 0.35% aluminum. 2.50% titanium. 1.00% aluminum. 0.25% vanadium. 2.25% aluminum. 5.50% iron. Balance iron. 7.00% iron.
Waspalloy Hastelloy 06 8-497 13.5% cobalt.
19.5% chromium. 0.10% carbon.
4.25% molybdenum. 2.50% titanium. 1.50% aluminum. 2.00% :iron.
20.0% chromium. 20.0% nickel.
0.10% carbon. 2.00% tungsten. 8.00% molybdenum. Balance iron.
19.0% cobalt. 14.0% chromium. 19.5% nickel.
4.00% tungsten. 1.00% molybdenum.
; 4.00% columbinm.
Balance nickel. Balance iron.
Suitable apparatus for carrying out the invention is shown in FIGURE l1 which includes an air-tight chamber 10 which may be evacuated through the conduit 12 and vented to the atmosphere through the conduit 14. A closure 16 is provided on the chamber 10 to permit access thereto. Tiltahly mounted within the chamber 10 is an induction furnace 18 containing therein a crucible or suitable receptacle .20 for holding molten metal.
Below the vacuum chamber 10 is a second vacuum chamber 22 in communication with the chamber 10 through the opening 24 when the closure 26, shown in a partially open position, is in an open or vertical position. The second chamber 2 2 is also provided with a vacuum conduit 28, a venting conduit 30 and an access door 32 shown in a closed position. Within the chamber 22 is a vertically movable platform 34 mounted on a vertical shaft 36. Means is provided (not shown) for moving the shaft 36 vertically and thereby moving the platform 34 from its lowered position as shown in the drawing to a raised position within the chamber 10.. The shaft 36 is provided with a suitable sealing arrangement whereby the chamber 22 may be maintained in a highly evacuated condition.
In general the operation of the above-described apparatus is as follows. The access door 16 'is opened and the crucible 20 is charged with the constituents of the alfloy to be used in the process of the invention. Thereafter the closures 16 and 26 are closed, and the chamber 10 is evacuated to a high vacuum of about 20 microns of mercury or less. Next, the induction furnace 18 is placed into operation to melt the metal within the crucible 20. The alloy melt is then maintained under vacuum for a predetermined time as will be hereinafter more fully described.
Meanwhile a suitable mold 38 in which a turbine blade or the like is to be cast is placed on the platform 34 through the access door 32. The chamber 22 is then evacuated to approximately the same degree as chamber 10. The interconnecting door 26 is then opened, the mold 38 is raised into chamber and the molten metal within the crucible 20 is poured into the mold. Thereafter the mold 38 is lowered into the chamber 32, the interconnecting door 26 is closed, the chamber 22 is vented and the mold 338 is removed. The apparatus is, of course, provided with suitable remote control means and suitable Windows to enable the above operations to be performed without breaking vacuum within the chambers in the course of operating the apparatus.
As indicated above, the method of the present invention involves melting and casting a nickel or cobalt alloy containing both chromium and carbon under vacuum.. It has been found that holding the alloy melt under high vac uum for a prolonged period of time before casting provides the alloy with an improved high temperature duetility and further results in casting having remarkably larger crystals. By holding the melt under vacuum for a suflicient time after the melting thereof, a time in the order of one hour, a turbine blade or the like may be cast wherein the leading and trailing edges thereof consist of substantially a single crystal. The thermal shock resistance of such a blade has been found to be remarkably improved.
The above-described phenomenon is not clearly understood, however, experimental evidence indicates that the increased grain size is a function of the time that the melt is held under vacuum and the chemical composition of the alloy. Grain size has been found to increase with the increased time that the melt is held under vacuum before casting. This increase in grain size with increased time under vacuum is accompanied by the decrease in the percentage of both the chromium and carbon in the alloy. It is believed that nucleating centers for crystal formation contained in the carbon and chromium become more dispersed or decreased by the vacuum application and that these centers are contained essentially in the chromium and carbon. It has further been found that chromium and carbon added to the vacuum melt will again result in relatively fine grain structure on casting, and after such additions of chromium and carbon, it is necessary to again hold the melt under vacuum for a prolonged period of time to achieve a coarsening of grain structure.
The invention is illustrated specifically by melting an alloy under vacuum, holding the alloy under vacuum for successively greater periods of time, and then casting a turbine blade after each period while the molten metal is still under vacuum. The following table gives the chemical analysis of each casting and the time for which the It will be noted that the percentage composition of chromium and carbon in each casting decreases with time under vacuum. FIGURE ZilIustrates the macro-etched crystalline structure of casting No. 1, cast under vacuum after holding the melt under vacuum of about 20 microns of mercury for about 30 minutes. In conducting the above experiment, approximately 25 minutes were required to melt the alloy and to bring it to a pouring temperature of about 2650" F. and an additional 5 minutes were required to cast each mold. Accordingly, casting N0. 1, cast after 30 minutes under vacuum, represents a casting formed as soon as feasible, after the vacuum melting step has been accomplished. It will be noted that the leading edge 40 of the blade shown in FIGURE 2 consists of a relatively large number of crystals. This blade failed after approximately 800 thermal shock cycles. In each cycle the leading edge of the blade was flameheated to about 800 F. and air-cooled to about 500 F.
FIGURE 3 illustrates the macro-etched crystalline structure of the casting No. 10, cast under vacuum while holding the melt under vacuum'for a period of about 96 minutes. As shown in FIGURE 3, the leading edge 42 of this blade consists of a single crystal. This blade did not fail after being subjected to 1500 thermal shock cycles of the type above-indicated. The macro-etched castings 2 to 9 (not shown) indicated a progressively coarser crystalline structure, but the leading edge of each of these blades consisted of a plurality of crystals. These castings also evinced a progressively greater resistance to thermal shock. Markedly improved thermal shock resistance resulted in blades which were cast of metal held under Vacuum for at least 15 minutes after the melting of the alloy, and markedly superior results were obtained when holding the melt under vacuum about 30 minutes before casting. Thus, casting No. 2 also failed after about 800 thermal shock cycles whereas casting No. 3 failed after 900 thermal shock cycles and castings Nos. 4, 6 and 8 withstood 1000 thermal shock cycles. It is readily apparent from the above data that a prolonged vacuum treatment after melting in the order of one hour produces castings having greatly superior thermal shock resistance and that an optimum blade results when at least the leading edge of the blade consists of a single crystal.
Since the above-described process involves the removal of chromium and carbon from the melt, it is necessary to periodically add chromium and carbon to the alloy melt to maintain the chromium and carbon content in the casting within the desired specifications.
Apart from the vacuum aspects of the above-described process, more or less conventional investment molding procedures are preferably employed. In forming the mold 38 a pattern of wax or other suitable low temperature, heat-destructible material such as polystyrene or resinous, polymerized derivatives of acrylic or methacrylic acid is formed in suitable injection mold apparatus or by other suitable means.
The surfaces of the pattern are next coated with a ceramic wash or coating material which is to provide the smooth casting surface on the refractory mold to be melt was was held under vacuum. formed. This coating material comprises an aqueous Time 7 Heat Ostg. No. Gr Fe O Si Mn Ti Al Mo B Held Under Vacuum OriginalAnalysis. 15.13 4.90 15 .110 0.10 2. 55 3. 53 5.85 .054 1 15. 50 5. 50 15 .015 005 2. 45 3. 87 4.90 035 30 5. 45 15 005 2. 50 3. 93 4. 90 039 35 5.50 15 005 2.45 3.85 4.90 034 43 5. 50 14 004 2. 50 3. s5 4. 90 037 53 5. 40 14 003 2. 5s 3. 95 4. 37 034 58 5. 45 13 124 003 2. 47 3. 90 4. 55 075 55 5. 40 13 152 003 2. 50 3. 00 4. 35 039 70 5. 40 12 003 2. 55 3. 97 4. 37 090 35 5. 40 12 150 003 2. 68 4. 10 4. 55 037 90 5. 55 10 154 003 2. 52 4. 00 4. 37 085 95 dispersion of conventional finely comminuted refractory materials, a binder, such as an air-setting silicate cement, and defoaming and Wetting agents.
Coating of the pattern with the ceramic Wash is preferably accomplished by dipping the pattern in the coating solution. Although in some instances the ceramic coating may also be applied by spraying or painting it on the pattern or in any other suitable manner, dipping is preferred because it assures more uniform coating of all the pattern surfaces and is the simplest method of application.
The clip coat slurry is preferably kept in constant motion by stirring means except during the actual dipping operation. However, the mixing action should not be such as to unnecessarily introduce air into the slurry, and care should be exercised in immersing the pattern in the slurry to prevent air entrapment on the pattern. Normally the dip coat solution is retained at room temperature during the dipping operation because excessive heat can result in distortion of the plastic or wax pattern. The excess coating material is permitted to drain off prior to subsequent treatment and investment.
After the pattern has been completely coated with the dip coat slurry, it may be sanded or stuccoed to provide a rough surface on the coating, thus insuring greater adhesion between the principal refractory portion of the mold and the dip coat on the pattern. This sanding may be accomplished by merely screening or otherwise applying silica sand or other suitable refractory materials in known manner to the outer coated surface of the destructible pattern. When all the molding surfaces of the pattern have been effectively covered with sand, the pattern should be air dried.
Thereafter the pattern is invested in a mold containing a relatively coarse refractory material such as silica. Among the procedures of investing the wax pattern which may be used is that of providing the wax pattern with a base portion of wax or suitable heat-destructible material and then placing the base on a removable base of a metal flask. The refractory mold material which may consist of a mixture of conventional silica having an ethyl silicate binder is then poured about the pattern. An example of an investement dry mix or grog which may be used is one comprising major proportions of a finely ground, dead burned fire clay and silica flour and minor proportions of magnesium oxide and borax glass. The binder for this grog may include an aqueous solution of condensed ethyl silicate, ethyl alcohol and hydrochloric acid. Preferably to 2% of borax is added to the mix to provide the refractory mix with greater strength enabling it to withstand the vacuum process. When the mold body has solidified or set to a suflicient extent to be self-supporting, the base of the flask is removed and heat is applied to the mold whereby the pattern material is melted and permitted to run out of the mold at the point where the base of the pattern rested on the base of the flask, and to convert the refractory material of the mold into a relatively strong, self-sustaining mass. Preferably the mold is first heated to about 1300 F. to melt and remove the wax pattern material and then heated rapidly to about a 1900 F firing temperature. The opening formed in the base of the mold serves as a sprue and gate of the mold.
While the present invention has been described by means of specific examples, it will be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.
1. A cast turbine blade comprising an alloy taken from the class consisting of nickel and cobalt base alloys containing both chromium and carbon having the leading and trailing edges thereof each consisting of substantially a single crystal.
2. A cast turbine blade comprising a nickel base alloy containing both chromium and carbon and having the leading and trailing edges thereof each consisting of substantially a single crystal.
3. A cast turbine blade comprising an alloy consisting of 0.10% to 0.20% carbon, up to 0.25% manganese, up to 0.75% silicon, 13% to 17% chromium, 4% to 6% molybdenum, 1.5% to 3% titanium, 2% to 4% aluminum, .01% to 0.1% boron, 3.5% to 6% iron, and the balance nickel, having at least the leading and trailing edges thereof each consisting of substantially a single crystal.
Vacuum Metallurgy, pages to 105. Papers presented at the Vacuum Metallurgy Symposium of the Electrothermics and Metallurgy Division of the Electrochemical Society, October 6 and 7, 1954, at Boston, Massachusetts.