US3329535A - Pressure treatment of superalloys and method of making turbine blade therefrom - Google Patents

Description

y 1967 P. H. LANGER ETAL 3,

PRESSURE TREATMENT OF SUPERALLOYS AND METHOD OF MAKING TURBINE BLADE THEREFROM 4 Sheets-Sheet 1 Filed May 11, 1965 July 4. 1967 P. H. LANGER ETAL 3,329,535

PRESSURE TREATMENT OF SUPERALLOYS AND METHOD OF MAKING TURBINE BLADE THEREFROM Filed May 11, 1965 4 Sheets-Sheet 4;

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2 400 I]! m \f m v 5 arm uNPRE55uR|zED flfl/ I w K --r" :1 1 z a 4 a 10 1mm TIME-HOURS uglfi INVENTORS ARNEILD R. MARDER PAUL H. LANEIER EEUREIE. \J.FIEEIE.HE.R

3,329,535 PRESSURE TREATMENT OF SUPERALLOYS AND RETHOD OF MAKING TURBINE BLADE THERE- M Paul H. Langer, Allentown, Pa., and George J. Fischer, Hillside, and Arnold R. Marder, Palisades Park, N.J., assignors to Curtiss-Wright Corporation, a corporation of Delaware Filed May 11, 1965, Ser. No. 454,857 17 Claims. (Cl. 1484) This invention relates to a method of improving the properties of various metals, and more particularly to a method of improving the tensile strength, hardness, resistance to creep, corrosion resistance, and temperature resistance of superalloys, and for reducing their brittleness.

The term superalloys as defined in volume 1 of Metals Handbook is applied to alloys developed for very high temperature service where relatively high stresses (tensile, thermal, vibratory, and shock) are encountered and where oxidation resistance is frequently required. There are many such superalloys, falling into various composition classes, such as for example, high-nickel austenitic alloys, precipitation-hardening stainless steels, ironnickel-chrornium-molybdenum alloys, nickel-base alloys, and cobalt-chromium-nickel-base alloys, and sold under various trade names. The compositions of a large number of such alloys are given on page 467 of volume 1 of Metals Handbook, 8th edition, 1961, published by the American Society for Metals.

The superalloys are particularly useful in jet engines, especially as turbine blades, which are subjected to high temperature service and are exposed to highly corrosive atmospheres. Turbine blades are also subject to high ten sile stresses resulting from centrifugal forces, and to bending and torsional forces from gas pressure.

The phenomenon of aging is characteristic of the austenitic structure found in superalloys, that is, the tendency of the carbon component originally in solid solution in the metal to gradually precipitate out as a metal carbide. Such carbide precipitation commonly takes place largely at the grain boundaries of the alloy, embrittling the metal and reducing its tensile strength and its resistance to torsional and bending stresses. Such grain boundary precipitation of carbides also provides paths along which intergranular corrosion can occur, resulting in further deterioration of the metal.

At room temperatures aging and carbide precipitation takes place so slowly that it may generally be ignored during the ordinary service life of parts in which it may occur. However, it is accelerated by high temperatures (but below the solution temperature of carbon in the metal), and with the increasing demand for higher powered jet engines and consequent higher operating temperatures the likelihoodof failure of the metal becomes correspondingly greater. This results in expensive and timeconsuming inspection, maintenance, and replacement programs. I

No means has heretofore been known of obviating carbide deposition at the grain boundaries. However, this invention provides a method of altering the mode of carbide precipitation by applying high pressure to metal stock or fabricated parts, so that carbon atoms or molecules of metal carbide are expelled from the matrix irrespective of grain boundaries, and thus form discrete nuclei for subsequent carbide precipitation randomly and substantially homogeneously throughout the metal.

It is therefore an object of this invention to provide a methodvof altering the mode of carbide precipitation in superalloys. I

United States Patent It is another object of the invention to provide preaged superalloys in which the precipitated carbides are substantially homogeneously distributed.

A further object is to provide superalloys having discrete carbon nuclei distributed irrespective of grain boundaries.

Yet another object is the provision of a method of treatment of superalloys to improve their physical and mechanical properties.

A still further object of the invention is to provide a pressure and heat treatment of superalloys to reduce brittleness and increase resistance to intergranular corrosion.

Other objects and advantages will become apparent on reading the following specification in connection with the accompanying drawings, in which FIG. 1 is a representation of typical heavy deposition of carbides at the grain boundaries of'superalloys of the prior art;

FIG. 2 is a similar representation of a superalloy treated according to the present invention;

FIG. 3 is a representation of another superalloy treated according to the present invention;

FIGS. 4-8 are graphs of hardness measurements of superalloys treated according to the invention for various times at various temperatures; and

FIG. 9 is a turbine blade formed of superalloy and treated according to the invention.

The fabricated parts of superalloy, or the metal stock, are first solution-treated at suitable temperature for an appropriate time to put the carbon component of the alloy into solid solution in the matrix. For most superalloys, solution of carbon atoms ordinarily will not take place below a temperature of about 1800 F., and solutiontreatment should not be carried out at a temperature above the softening point of the metal. A suitable temperature range is therefore from about 1800 F. to about 2500 F. At the lower temperatures a somewhat longer solutioning time is required than at higher values, and for the compositions of some superalloys a temperature rather toward the higher end of the range is preferred, owing to the tendency of the carbides of some metals to require higher temperature for solution. A rule of thumb for heattreating in general is considered to be one hour per inch of cross-section of the metal, at the recommended temperature. In the present procedure it was found that a temperature from 2000 F. to 2300 F. was satisfactory, and that the time of treatment should be between ten minutes and two hours, with an optimum of about one hour at about 2250 F. for parts of moderate size, such as turbine blades. The parts were then cooled in air, although in a production process the procedure may be shortened by quenching them.

After cooling, the parts were subjected to high hydrostatic pressure, which markedly reduced the solubility of carbon in austenite. Under high pressure, carbon atoms or molecules of metal carbides were squeezed out of the matrix in a coherent precipitate to form nuclei for subsequent carbide precipitation. Such a coherent precipitate forming the nuclei, however, is not squeezed out at the grain boundaries, nor do the particles making up the coherent precipitate migrate to the grain boundaries. They are squeezed out at rand-om locations in a homogeneous dispersion throughout the metal, and hence when carbides are precipitated upon them during the aging process there is no preferential location for carbide deposition and no embrittlement of the metal, as occurs when carbides are deposited principally at the grain boundaries. Such particles of a coherent precipitate whether carbon atoms or carbide molecules or clusters of either or both, are herein referred to as carbon nuclei. The hydrostatic pressure applied may range from '10 to 50 kilobars, and need only be maint-ained for suflicient time for the pressure effect to equalize throughout the metal, substantially instantaneously for small parts and not over ten minutes for the largest parts on which it is practicable to practice the method. It has been found that a pressure of approximately 25 kilobars for about five minutes is satisfactory-for parts of moderate size, such as turbine blades.

The parts, or the metal stock, were then artificially aged at an accelerated rate at high temperature. Various aging temperatures may be used, from 1200 F. to 1800 F., the latter value being employed only on those compositions having a somewhat higher solution temperature. During the aging process carbides precipitate on the nuclei formed by the previous pressurization of the metal, the rate of precipitation being more rapid and sooner completed at the higher temperatures. Metallographic examination of specimens shows that no marked precipitation along grain boundaries occurs, in contrast to the previously known morphology.

FIG. 1 is a rnicrographic drawing of the grain structure of a typical metal of the superalloy group of the prior art. An untreated specimen 11 of superalloy was prepared for .metallographic examination by polishing and etching in the customary manner. Under high magnification the typical grain structure appears, with a heavy precipitate of carbide particles 12 along the grain boundaries, which produces the typical embrittlernent known in the prior art.

Two metals of the superalloy group treated according to the present invention were selected for extensive investigation. N-155 is the trade name for a cobalt-chromiumnickel-base alloy having the following nominal composition.

Percent Carbon 0.15 Manganese 1.5 Silicon 0.5 Chromium 21 Nickel 20 Cobalt 20 Molybdenum 3 Tungsten 3 Columbium 1 Iron Balance Rene 41 is a trade name for a nickel-base alloy of the superalloy group, having the following nominal composition.

Percent Carbon 0.1 Chromium 19 Cobalt 11 Molybdenum Titanium 3 Aluminum 1.5 Iron 3 Nickel Balance It will be understood that the foregoing formulations are nominal only, and that minor variations in composition may 'be made without departing from the general class of the alloy. In any given sample slight deviations from stated nominal percents are likely to occur.

FIG. 2 is a micrographic representation of the appearance under high magnification of a specimen 13 of N-155 after pressurization and aging according to the invention. The grain boundaries 12a are less distinct and pronounced than those found in superalloys of the prior art. In the specimen 13 treated by the process of the invention there is no selective deposition of carbides along the grain boundaries, owing to their precipitation at random locations homogeneously throughout the matrix on the car bon nuclei produced by pressure treatment.

In FIG. 3 there is shown a similar micrographic representation of the grain structure of a specimen 14 of Rene 41. Here again, the grain boundaries 12b are not 4 heavily outlined by a deposit of carbide particles, which have precipitated randomly instead, on the carbon nuclei generated by treatment according to the method of invention.

Hydrostatic pressurization of superalloys and subsequent aging at elevated temperature also produces a marked increase in hardness of the metals, with a concomitant increase in tensile strength and other desirable metallurgical properties. FIGS. 4-8 show a series of semilogarithmic graphs of hardness measurements of samples pressurized and aged at various temperatures, plotted against hardness measurements of samples which were not pressurized but which were subjected to the same aging treatment. Hardness was measured by the Knoop procedure of diamond point indentation, with a 300-gram load. The pressurized samples were given a pressure treatment in which the entire sample was surrounded by a pressure-transmitting medium and subjected to 25 kilo-bars for five minutes. Any suitable pressure apparatus may be used. All curves shown in the graphs, both for pressurized and unpressurized samples, are averages for a large number of samples at each value.

As will be seen in FIG. 4, the unpressurized samples before aging had an initial Knoop Hardness Number (KHN) of about 240, whereas the pressurized samples had an initial KHN of about 370. Both sets of samples were aged at a temperature of 1200 F. In the unpressurized samples there is a general slight rise in KHN over the entire aging period of hours, but never going above a value of 300. In the pressurized samples there is a small early peak in hardness during the first half hour of aging, followed by a slight decline, and subsequent rise to a peak of about 450.

Similar curves are shown in FIGS. 5-8 for aging periods of 100 hours at 0, 1400", l500, 1600. At

each of these temperatures the curves for unpressurized samples are much the same. Over the entire aging period there is a general slight rise in Knoop Hardness Number from the starting value of about 240, and never going above 300. The unpressurized samples were least affected by aging at 1600 F., as shown in FIG. 8, where the curve is very nearly a horizontal straight line and the highest KHN value reached is about 260.

With the pressurized samples, however, the situation is very different. The maximum peak in KHN comes generally earlier with increase in temperature of aging, indicating that increase in temperature accelerates the aging process. Higher KHN peaks, approximately 450, are also achieved with the four lower temperatures of aging, and only about 390 at 1600 F. Further, as aging temperature is increased the eventual decline in KHN among the pressurized samples becomes somewhat greater, so that the final KHN after 100 hours is progressively lower. In all cases, nevertheless, the pressurized samples maintain throughout a significantly greater hardness and consequent superiority than the unpressurized samples. Similar results were achieved with aging at 1700 and 1800".

This greater hardness of the pressurized materials is not the equivalent of cold working of the metal. Comparisons were made with unpressurized samples which were cold-rolled to the same initial hardness as the pressurized pieces, about 37 0 KHN. The cold-rolled samples exhibited the characteristic surface hardness known as work hardening, whereas the hardness of pressurized superalloys extends throughout the material. The coldrolled samples also did not maintain their initial equality with the pressurized samples during the aging procedure, but declined at an increasing rate to a hardness value considerably below the final value of pressurized samples at all aging temperatures.

In FIG. 9 there is shown a view of a turbine blade 15 formed of a superalloy, preferably Rene 41 or N-155. The blade may be either a stator blade or a rotor blade, and although shown with the conventional fir-tree rootit may be provided with any other convenient mounting means. The fabricated blade is first solution treated at a temperature between 1800" F. and 2500 F., preferably about 2250 F., for a period between ten minutes and two hours, with an optimum being about one hour. The blade is then subjected to hydrostatic pressure between and 50 kilobars, preferably at about 25 kilobars, for a period of one to ten minutes. For turbine blades of usual size range, about five minutes is sufiicient for pressure to equalize throughout the blade.

The blade is next aged at a temperature which will depend somewhat on the environment in which the blade is intended to be used. In a high-rated turbine engine a small portion of the blade may operate at a temperature as high as 1600 F., generally along a short portion of the leading edge in the mid-span region. In such an environment the remainder of the blade runs at temperatures from about 1100 to 1550 F. in various portions, the coolest regions being usually at the root and tip.

A blade intended for use in such an environment should be aged at about 1500 F., since only small parts of the blade will operate at any higher temperature. The time of aging at the selected temperature may also vary according to the expected operating temperature of the blade, and should ordinarily not be carried much beyond the point at which the highest KHN peak is reached, whereupon the hardness remains substantially stable during operation. For a blade aged at 1500 one hour is sufiicient, the hardness peak being reached at about that time, and aging need not be carried out longer than two hours.

Aging times for other temperatures will be selected on a similar basis. For instance, if a part is aged at 1600 an aging period of thirty minutes to one hour is satisfactory. For the 1400 aging temperature a time of one to five hours will suffice; for 1-300, five to ten hours; and for 1200, five to fifty hours.

When a turbine blade or other superalloy part, or the superalloy stock, is treated according to the invention by solution-treatment, pressurization, and aging at a suitable temperature for an appropriate time, carbide precipitation takes place randomly throughout the structure as discussed above, instead of selectively along the grain boundaries as previously known. Such treatment consequently results in a less brittle structure and a longer service life, and harder, stronger material better able to resist stresses and therefore suitable for more severe duty over longer periods.

Although the invention has been described above in a preferred form, it will be understood that various modifications may be made by those skilled in the art without departing from the concept of the invention. It is intended to cover all such modifications by the appended claims.

What is claimed is:

1. A method of modifying the structure of a superalloy metal, comprising solution-treating said metal at a temperature between 1800 F. and 2500 F. for a period from ten minutes to two hours, cooling said metal, subjecting said metal to hydrostatic pressure between 10 and 50 kilobars for a period of one to ten minutes, and aging said metal for a period of /2 to 100 hours at a temperature of 1200 F. to 1800 F.

2. The method of claim 1, in which said superalloy metal is Nl55.

3. The method of claim 1, in which said superalloy metal is Rene 41.

4. A method of modifying the structure of a superalloy metal, comprising solution-treating said metal at a temperature of about 2000 F. to 2300 F. for a period of about one hour, cooling said metal, subjecting said metal to hydrostatic pressure of about 25 kilobars for a period of about five minutes, and aging said metal for a period of /2 to hours at a temperature of 1200 F. to 1800 F.

5. The method of claim 4, in which said superalloy metal is N-l55.

6. The method of claim 4, in which said superalloy metal is Rene 41.

7. A method of modifying the structure of a superalloy metal of austenitic structure containing carbon, comprising solution-treating said metal at a temperature between 1800 F. and 2500 F. for a period from ten minutes to two hours to cause said carbon to go into solid solu tion in said metal, cooling said metal, subjecting said metal to hydrostatic pressure between 10 and 50 kilobars for a period of one to ten minutes to form carbon nuclei randomly dispersed in the structure of said metal, and aging said metal for a period of /2 to 100 hours at a temperature of 1200 F. to 1600" F. to cause metallic carbide to precipitate on said randomly dispersed nuclei.

8. The method of claim 7, in which said superalloy metal is N-l55.

9. The method of claim 7, in which said superalloy metal is Rene 41.

10. A method of modifying the structure of a superalloy metal of austenitic structure conaining carbon, comprising solution-treating said metal at a temperature of about 2250 F. for a period of about one hour to cause said carbon to go into solid solution in said metal, cooling said metal, subjecting said metal to hydrostatic pressure of about 25 kilobars for a period of about five minutes to form carbon nuclei randomly dispersed in said metal, and aging said metal for a period of /2 to 100 hours at a temperature of 1200 F. to 1600" F. to cause metallic carbide to precipitate on said randomly dispersed nuclei.

11. The method of claim 10, in which said superalloy metal is N45.

12. The method of claim 10, in which said superalloy metal is Rene 41.

13. The method of making a turbine blade, comprising forming said blade of a superalloy met-a1, solution-treating said blade at a temperature between 1800" F. and 2500 F. for a period from ten minutes to two hours to cause carbon to go into solid solution in said metal, cooling said blade, subjecting said blade to hydrostatic pressure between 10 and 50 kilobars for a period of one to ten minutes to form carbon nuclei randomly dispersed in the metal of said blade, and aging said blade for a period of /2 to 100 hours at 1200 F. to 1600" F. to cause metallic carbide to precipitate on said randomly dispersed nuclei.

14. The method of claim 13, in which said blade is solution-treated at a temperature of about 2250" F. for a period of about one hour.

15. The method of claim 14, pressurized at about 25 kilobars 16. The method of claim 15, formed of N-155.

17. The method of claim 15, formed of Rene 41.

in which said blade is for about five minutes. in which said blade is in which said blade is References Cited UNITED STATES PATENTS 3,156,974 11/1964 Bobrowsky l48-4 X 3,157,540 11/1964 Bobrowsky 148-4 X DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

Claims (1)

1. A METHOD OF MODIFYING THE STRUCTURE OF A SUPERALLOY METAL, COMPRISING SOLUTION-TREATING SAID METAL AT A TEMPERATURE BETWEEN 1800*F. AND 2500*F. FOR A PERIOD FROM TEN MINUTES TO TWO HOURS, COOLING SAID METAL, SUBJECTING SAID METAL TO HYDROSTATIC PRESSURE BETWEEN 10 AND 50 KILOBARS FOR A PERIOD OF ONE TO TEN MINUTES, AND AGING SAID METAL FOR A PERIOD OF 1/2 TO 100 HOURS AT A TEMPERATURE OF 1200*F. TO 1800*F.

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