Abstract:
A method of heat treating a nickel base superalloy comprising solution treatment at 2050° to 2150° F. (1121° to 1177° C.) for about 2 hours and cooling at a rate at least as rapid as still air; stabilization at 1750° to 1850° F. (954° to 1010° C.) for 1/4 to 4 hours and cooling at a rate at least as rapid as still air; and precipitation hardening at 1350° F. (732° C.) for at least about 8 hours and air cooling. The heat treated product contains a low level of precipitated grain boundary carbides, and exhibits an optimum balance of tensile strength, stress rupture life and creep strength, along with reduced residual stress in the product.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 449,482 filed Dec. 13, 1982, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a heat treatment of a nickel base alloy to produce an article exhibiting an acceptable level of grain boundary precipitates, reduced residual stress, with an optimum balance of tensile, stress rupture and creep properties. The invention has particular utility in the production of components for gas turbine and jet engines, such as turbine discs. 
     For the compositions hereinafter defined, heat treatment steps are maintained within relatively narrow, critical limits which have been found to be necessary to achieve the novel combination of reduced residual stress and optimum mechanical properties, while at the same time effecting a reduction of about 50% in processing time and cost, as compared to a conventional prior art treatment of a nickel base alloy. 
     So-called &#34;superalloys&#34; which are widely used for components in gas turbine and jet engines include nickel base alloys sold under the trademarks &#34;IN-100&#34; by International Nickel Co., Inc. and &#34;Rene 100&#34; by General Electric Company. The International Nickel Co., Inc. alloy is disclosed in U.S. Pat. No. 3,061,426. According to &#34;Aerospace Structural Metals Handbook Chapter IN-100&#34;, by S. S. Manson, Code 4212, 1978 revision, page 6, the composition of IN-100 is as follows: 
     cobalt 13-17% 
     chromium 8-11% 
     aluminum 5-6% 
     titanium 4.5-5.0% 
     aluminum plus titanium 10-11% 
     molybdenum 2-4% 
     iron 0-1% 
     vanadium 0.7-1.2% 
     boron 0.01-0.02% 
     carbon 0.15-0.20% 
     manganese 0.10% maximum 
     sulfur 0.015% maximum 
     silicon 0.15% maximum 
     nickel balance 
     The same literature source indicates the composition of Rene 100 to be as follows: 
     cobalt 14-16% 
     chromium 9-10% 
     aluminum 5.3-5.7% 
     titanium 4.0-4.4% 
     molybdenum 2.7-3.3% 
     iron 0-1% 
     vanadium 0.9-1.1% 
     boron 0.01-0.02% 
     carbon 0.15-0.20% 
     nickel balance 
     In this same literature source, introductory comments at page 1 include the following: 
     &#34;Because of the large quantities of strengthening elements included in the composition, the alloy is not hot worked, and is therefore used in the as-cast condition. Recently, however, there has been considerable development of a powder metallurgy product which permits working of the alloy. At high temperatures the powder consolidated product becomes superplastic, thus opening many possibilities in fabrication-to-shape of wrought complex components. 
     &#34;Also, because of the high content of gamma prime precipitate that constitutes one of the strengthening components of the alloy, the equilibrium solution temperature approaches the solidus, so the material is usually used in the as-cast condition, without heat treatment. However, it is subjected to heat treatment during the deposition of protective coatings. The powder metallurgy product is heat treated to achieve desirable properties.&#34; 
     It is next pointed out that protective coatings may be needed for high temperature applications due to the relatively low oxidation and corrosion resistance of the alloy. A number of types of coatings such as aluminizing or chromizing have been found to provide sufficient protection. Additionally, precipitation of sigma phase with resulting embrittlement has been found to occur after exposure to high temperature and stress for long periods of time. Restriction of the aluminum plus titanium contents has been found to be effective in minimizing sigma phase formation, and the limitation on the aluminum plus titanium levels is based on electron vacancy density calculations. 
     Page 1 of this literature source further states: 
     &#34;For the powder metallurgy product, Pratt and Whitney Aircraft recommends solutioning at 2050° F., stabilization at 1600° and 1800° F., and precipitation hardening at 1200° and 1400° F. Typical heat treatment used . . . 2215° F., 4 hrs+2000° F., 4 hrs+1550° F., 16 hrs.&#34; 
     Data relating to IN-100 are also contained in &#34;Alloy Digest&#34;, filing code: Ni-151, March 1970; &#34;Properties of Superalloys/243&#34; and &#34;Guide to Selection of Superalloys&#34;, pages 14 and 15, W. F. Simmons et al. 
     United States Patents relating to nickel base alloys and treatment thereof include U.S. Pat. Nos. 3,653,987; 3,667,938; 4,083,734; 4,093,476; 4,121,950 and 4,253,884. 
     U.S. Pat. No. 3,653,987, issued Apr. 4, 1972 to W. J. Boesch, discloses an alloy consisting essentially of up to 0.18% carbon, 14.2 to 20% cobalt, 13.7 to 16% chromium, 3.8 to 5.5% molybdenum, 2.75 to 3.75% titanium, 3.75 to 4.75% aluminum, up to 4% iron, 0.005 to 0.035% boron, up to 0.5% zirconium, up to 0.5% hafnium, up to 0.75% columbium, up to 0.5% rhenium, up to 0.75% tantalum, up to 1.0% manganese, up to 3% tungsten, up to 0.5% rare earth metals, and balance essentially nickel with incidental impurities. This alloy is heat treated to develop gamma prime particles consisting essentially of randomly dispersed irregularly shaped particles less than 0.35 micron in diameter. The treatment involves heating at a temperature of at least 2000° F., cooling, and heating at a temperature of about 1500° to about 1850° F. An optional third stage of heat treatment for precipitation hardening may be conducted at 1350° to 1450° F. This patent points out that a prior art heat treatment for nickel base alloys comprised the steps of heating at a temperature of 2135° F. for 4 hours and cooling; heating at a temperature of 1975° F. for 4 hours and cooling; heating at a temperature of 1550° F. for 4 hours and cooling; and heating at a temperature of 1400° F. for 16 hours and cooling. 
     U.S. Pat. No. 4,083,734, issued Apr. 11, 1978 to W. J. Boesch, discloses a nickel base alloy consisting essentially of from 12.0 to 20.0% chromium, 4.75 to 7.0% titanium, 1.3 to 3.0% aluminum, 13.0 to 19.0% cobalt, 2.0 to 3.5% molybdenum, 0.5 to 2.5% tungsten, 0.005 to 0.03% boron, 0.005 to 0.045% carbon, up to 0.75% manganese, 0.01 to 0.08% zirconium, up to 0.5% iron, up to 0.2% rare earth elements, up to 0.02% of magnesium, calcium, strontium, barium, and mixtures thereof, and balance essentially nickel, with titanium plus aluminum from 6.5 to 9.0%. A maximum carbon level of 0.045% is alleged to increase the hot impact strength of the alloy without adversely affecting stress rupture properties. An exemplary treatment for a wrought alloy of this patent was heating at 2150° F. for 4 hours and air cooling; heating at 1975° F. for 4 hours and air cooling; heating at 1550° F. for 24 hours and air cooling; and heating at 1400° F. for 16 hours and air cooling. 
     U.S. Pat. No. 4,093,476, issued June 6, 1978 to W. J. Boesch, differs from U.S. Pat. No. 4,083,734 principally in permitting from 0.05 to 0.15% carbon and requiring from 0.031% to 0.048% boron. Carbon within the range of 0.02% to 0.04% and boron within the range of 0.032% to 0.045% are alleged to provide the best combination of stress rupture life and impact strength. An exemplary heat treatment of this patent differed from that of U.S. Pat. No. 4,083 734 only by specifying a first heating step of 2135° F. for 4 hours. 
     U.S. Pat. No. 4,121,950, issued Oct. 24, 1978 to A. R. Guimier et al, discloses a nickel base alloy consisting essentially of 13 to 20% cobalt, 13 to 19% chromium, 3% to 6% molybdenum, tungsten or mixtures thereof, 0.01 to 0.20% carbon, 2 to 4% aluminum, 0.10 to 3% titanium, 0.30 to 1.50% hafnium and remainder nickel. The heat treatment process is described and claimed functionally as &#34;(a) placing at least a portion of the gamma prime phase back into solution, (b) effecting the coalescence of carbides and the initiation of the reprecipitation of the gamma prime phase, and (c) completing the reprecipitation of the gamma prime phase.&#34;The actual steps involve heating at about 1050° to 1200° C. for at least one hour and cooling; heating at about 850° C. for 10 to 30 hours and cooling; and heating at about 760° C. from 10 to  30 hours. Preferably aluminum plus titanium ranges between about 4% and 7% with the ratio of titanium to aluminum about 0.20 to 1.5. 
     U.S. Pat. No. 4,253,884, issued Mar. 3, 1981 to G. E. Maurer et al, discloses a method of heat treating and incorporating a coating operation therewith for a nickel base alloy consisting essentially of from 12.0 to 20.0% chromium, 4.0 to 7.0% titanium, 1.2 to 3.5% aluminum, 12.0 to 20.0% cobalt, 2.0 to 4.0% molybdenum, 0.5 to 2.5% tungsten, 0.005 to 0.048% boron, 0.005 to 0.15% carbon, up to 0.75% manganese, up to 0.5% silicon, up to 1.5% hafnium, up to 0.1% zirconium, up to 1.0% iron, up to 0.2% rare earth elements, up to 0.1% magnesium, calcium, strontium, barium and mixtures thereof, up to 6.0% rhenium and/or ruthenium, and balance essentially nickel, with titanium plus aluminum being from 6.0 to 9.0% and a titanium to aluminum ratio of 1.75 to 3.5. The heat treatment to which this alloy is subjected comprises heating at a temperature of at least 2050° F., cooling; heating between 1800° and 2000° F., cooling; heating between 1500° and 1800° F.; coating the alloy with a cobalt, nickel or iron base alloy; heating the coated alloy to a temperature of at least 1600° F., cooling; and heating the alloy within the range of 1300° and 1500° F. 
     It is therefore evident that there are numerous specific compositions within the general class of nickel base superalloys and a variety of heat treatments therefor. All heat treatments of which applicants are aware appear to have in common the objective of placing in solution the gamma prime particles or phase which is composed of M 3  (Al, Ti) wherein M is primarily nickel with relatively minor amounts of chromium and molybdenum. Thereafter the next stage of heat treatment is for the purpose of reprecipitating the gamma prime phase and to form a grain boundary precipitate of metal carbides. The third stage (if practiced) is a precipitation hardening or aging treatment wherein nickel, aluminum and titanium compounds are precipitated. In substantially all the prior art patents discussed above it is pointed out that MC carbides are precipitated in the grain boundaries, with M being principally titanium, molybdenum and/or chromium. Even in U.S. Pat. No. 4,083,734, which limits carbon to a maximum of 0.045%, it is emphasized that carbides are formed and precipitate in the grain boundaries, but it is alleged that the carbon level specified in this patent inhibits transformation in service of MC carbides to M 23  C 6  carbides (wherein M is predominantly chromium), the latter being alleged to be responsible for a loss of hot impact strength. 
     SUMMARY OF THE INVENTION 
     The present invention constitutes a discovery that control of the formation of carbide precipitates in the grain boundaries results in improvement in mechanical properties, particularly stress rupture life. At the same time the composition responds to a simplified heat treatment process of relatively short duration which reduces residual stresses in articles and obtains optimum tensile and creep strength properties. 
     The method of the invention is applicable inter alia, to isothermal forgings produced from hot isostatically pressed powdered alloys, to forgings produced from forward extrusion consolidated billets, to components used in the direct hot isostatically pressed condition, and to components forged from material produced by advanced vacuum melting methods. 
     According to the invention there is provided a method of heat treating an article fabricated from a nickel base alloy consisting essentially of, in weight percent, from 0.015% to 0.09% carbon, up to 0.020% manganese, up to 0.10% silicon, up to 0.010% phosphorus, up to 0.010% sulfur, 10.90% to 13.90% chromium, 18.00% to 19.00% cobalt, 2.80% to 3.60% molybdenum, 4.15% to 4.50% titanium, 4.80% to 5.15% aluminum, 0.016% to 0.024% boron, up to 0.50% hafnium, up to 1.60% columbium, 0.04% to 0.08% zirconium, up to 0.05% tungsten, up to 0.98% vanadium, up to 0.30% iron, up to 0.07% copper, up to 0.0002% (2 ppm) lead, up to 0.00005% (0.5 ppm) bismuth, and balance essentially nickel, said method comprising the steps of: 
     (1) solution treating at 2050° to 2150° F. (1121° to 1177° C.), for about 2 hours and cooling at a rate at least as rapid as still air: 
     (2) stabilizing at 1750° to 1850° F. (954° to 1010° C.) for 1/4 to 4 hours and cooling at a rate at least as rapid as still air; 
     (3) precipitation hardening at about 1350 °F. (732° C.) for about 8 hours and cooling at a rate at least as rapid as still air; 
     whereby to precipitate grain boundary carbides to an acceptable low level, to obtain an optimum balance of tensile strength, stress rupture life, creep strength and reduced residual stress in the article. 
     The invention further provides a heat treated article fabricated from the nickel base alloy defined above, said article having a yield strength of at least 140 ksi (98.43 kg/mm 2 ), a tensile strength of at least 215 ksi (136.4 kg/mm 2 ) and a percent elongation of at least 15% at room temperature, a combination bar stress rupture life of at least 23 hours at 1350° F. (732° C.) and at least 92.5 ksi stress, and substantial freedom from deleterious grain boundary carbide precipitates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a photomicrograph at 500× of a forged sample solution treated at 2090° F. for 2 hours, oil quenched; stabilized at 1600° F. for 4 hours Furnace Time, air cooled; and aged at 1350° F. for 8 hours, air cooled; 
     FIG. 2 is a photomicrograph at 500× of a forged sample solution treated at 2090° F. for 2 hours, oil quenched; stabilized at 1700° F. for 1 hour, air cooled; no aging; 
     FIG. 3 is a photomicrograph at 500× of a forged sample solution treated at 2090° F. for 2 hours, oil quenched; stabilized at 1750° F. for 1 hour, air cooled; no aging. 
     FIG. 4 is a photomicrograph at 500× of a forged sample solution treated at 2090° F. for 2 hours, oil quenched; stabilized at 1800° Fo for 1 hour, air cooled; and aged at 1350° F. for 8 hours, air cooled; and 
     FIG. 5 is a photomicrograph at 500× of a forged sample solution treated at 2090° F., oil quenched; stabilized at 1800° F. for 4 hours, air cooled; and aged at 1350° F. for 8 hours, air cooled. 
    
    
     DETAILED DESCRIPTION 
     The heat treatment process of the present invention results in formation of randomly dispersed, irregularly shaped gamma prime particles and carbides throughout the grains of the alloy, rather than substantial concentrations of carbides along grain boundaries. 
     The above-mentioned U.S. Pat. No. 3,653,987 states at column 3, lines 12-16: 
     &#34;The second stage of the heat treatment is designed to initiate the formation of and form the randomly dispersed irregularly shaped fine gamma prime particles and to form a grain boundary precipitate, M 23  C 6  (M is generally chromium which improves grain boundary ductility.&#34; 
     Contrary to the teaching of this patent, applicants have discovered that extensive carbide grain boundary precipitates adversely affect stress rupture life. This problem is avoided in the present invention by conducting a stabilizing heating step at a relatively high temperature (1750° to 1850° F.). In the exemplary disclosure of U.S. Pat. No. 3,653,987 a carbon content of 0.08% was used, and the &#34;second stage&#34; heat treatments were conducted at 1975° F., 1700° F., and 1750° F., respectively. Similarly, it is clear from FIGS. 1 and 2 of U.S. Pat. No. 4,083,734 and column 2, lines 39-42 and column 3, lines 1-3 of U.S. Pat. No. 4,253,884 that carbide particles are precipitated at the grain boundaries, and this is considered desirable. 
     Within the above broad composition ranges, the following narrower compositions represent alloys which have recently become commercially available, and which respond to the improved heat treatment of the present invention: 
     
         ______________________________________Weight Percent   Powder        Vacuum   Metallurgy    Remelted______________________________________Carbon    0.015-0.035     0.015-0.035Manganese 0.020 max.      0.020 max.Silicon   0.10 max.       0.10 max.Phosphorus     0.010 max.      0.010 max.Sulfur    0.010 max.      0.010 max.Chromium  11.90-12.90     10.90-13.90Cobalt    18.00-19.00     18.00-19.00Molybdenum     2.80-3.60       2.80-3.60Titanium  4.15-4.50       4.15-4.50Aluminum  4.80-5.15       4.80-5.15Boron     0.016-0.024     0.016-0.024Hafnium   0.30-0.50       0.30-0.50Columbium 1.20-1.60       1.20-1.60Zirconium 0.04-0.08       0.04-0.08Tungsten  0.05 max.       0.05 max.Iron      0.30 max.       0.30 max.Copper    0.07 max.       0.07 max.Vanadium  0.10 max.       --Lead      0.0002 (2 ppm) max.                     0.0002 (2 ppm) max.Bismuth   0.00005 (0.5 ppm) max.                     0.00005 (0.5 ppm) max.Oxygen    0.020 (200 ppm) max.                     --Nitrogen  0.005 (50 ppm) max.                     --Nickel    Remainder       Remainder______________________________________ 
    
     
         ______________________________________Weight Percent   Powder        Vacuum   Metallurgy    Remelted______________________________________Carbon     0.05-0.09       0.05-0.09Manganese 0.020 max.      0.020 max.Silicon   0.10 max.       0.10 max.Phosphorus     0.010 max.      0.010 max.Sulfur    0.010 max.      0.010 max.Chromium  11.90-12.90     10.90-13.90Cobalt    18.00-19.00     18.00-19.00Molybdenum     2.80-3.60       2.80-3.60Titanium  4.15-4.50       4.15-4.50Aluminum  4.80-5.15       4.80-5.15Boron     0.016-0.024     0.016-0.024Vanadium  0.58-0.98       0.58-0.98Zirconium 0.04-0.08       0.04-0.08Tungsten  0.05 max.       0.05 max.Columbium 0.04 max.       0.04 max.&amp; TantalumIron      0.30 max.       0.30 max.Copper    0.07 max.       0.07 max.Lead      0.0002 (2 ppm) max.                     0.0002 (2 ppm) max.Bismuth   0.00005 (0.5 ppm) max.                     0.00005 (0.5 ppm) max.Oxygen    0.010 (100 ppm) max.                     --Nickel    Remainder       Remainder______________________________________ 
    
     
         ______________________________________Weight Percent   Powder        Vacuum   Metallurgy    Remelted______________________________________Carbon    0.015-0.035     0.015-0.035Manganese 0.020 max.      0.020 max.Silicon   0.10 max.       0.10 max.Phosphorus     0.010 max.      0.010 max.Sulfur    0.010 max.      0.010 max.Chromium  11.90-12.90     10.90-13.90Cobalt    18.00-19.00     18.00-19.00Molybdenum     2.80-3.60       2.80-3.60Titanum   4.15-4.50       4.15-4.50Aluminum  4.80-5.15       4.80-5.15Boron     0.016-0.024     0.016-0.024Hafnium   0.30 max.       0.03 maxColumbium 1.20-1.60       1.20-1.60Zirconium 0.04-0.08       0.04-0.08Tungsten  0.05 max.       0.05 max.Iron      0.30 max.        0.3 max.Copper    0.07 max.       0.07 max.Vanadium  0.10 max.       --Lead      0.0002 (2 ppm) max.                     0.0002 (2 ppm) max.Bismuth   0.00005 (0.5 ppm) max.                     0.00005 (0.5 ppm) max.Oxygen    0.020 (200 ppm) max.                     --Nitrogen  0.005 (50 ppm) max.                     --Nickel    Remainder       Remainder______________________________________ 
    
     A series of billets was prepared by hot isostatic compression of nickel base alloy powders within the ranges of alloy 1 above. The billets were 61/4 inch diameter and were prepared in accordance with existing specifications by heating to a temperature of 2110° to 2140° F. (1154° to 1171° C.) for 2.5 to 3.5 hours at 15 ksi pressure (10.55 kg/mm 2 ). Half the billet material comprised -325 mesh powder (U.S. Standard), i.e. passing sieve openings of 0.044 mm, and the other half comprised -100 mesh powder, i.e. passing 0.149 mm sieve openings. The compositions of the experimental billets are set forth in Table I. The first two compositions set forth in Table I were prepared from -325 mesh powder while the remaining compositions were prepared from -100 mesh powder. 
     For identification purposes the samples from the various billets were designated as follows: 
     
         ______________________________________Powder Size     Example  Serial No.______________________________________-325 mesh       A        A1-325 mesh       B        B1-100 mesh       C        C1-100 mesh       D        D1______________________________________ 
    
     The initial heat treatment conditions were modifications of existing prescribed requirements for components of this type which were as follows: 
     Solution treat at 2125° F. for 2 hours, 60 second delay and oil quench. 
     Stabilize by preheating furnace to 1600° F., hold 40 minutes after furnace has recovered to 1600° F. and air cool. Preheat furnace to 1800° F., hold 45 minutes after furnace has recovered to 1800° F. and air cool. 
     Age at 1200° F. for 24 hours and air cool followed by heating at 1400° F. for 16 hours and air cool. 
     The selected heat treatment sequence was derived for test purposes as a modification of the above standard treatment utilizing time at temperature as a basis for the stabilizing cycle, and applied to Serial Nos. A1, B1, C1 and D1 as follows: 
     
         ______________________________________Serial No. A1ASerial No. A1:Solution Treat   2090 F./2 hrs./OQStabilize        HoldAge              HoldSerial No. A1BSerial No. A1:Solution Treat   2090 F./2 hrs./OQStabilize        1600 F./1 hr./ACAge              1350 F./8 hrs./ACSerial No. B1ASerial No. B1:Solution Treat   2090 F./2 hrs./90 sec.DOQStabilize        1500 F./1 hr./ACAge              1350 F./8 hrs./ACSerial No. B1BSerial No. B1:Solution Treat   2090 F./2 hrs./90 sec.DOQStabilize        1600 F./1 hr./ACAge              1350 F./8 hrs./ACSerial No. C1ASerial No. C1:Solution Treat   2065 F./2 hrs./OQStabilize        1600 F./1 hr./ACAge              1350 F./8 hrs./ACSerial No. C1BSerial No. C1:Solution Treat   2065 F./2 hrs./OQStabilize        HoldAge              HoldSerial No. D1ASerial No. D1:Solution Treat   2090 F./2 hrs./OQStabilize        1600 F./1 hr./ACAge              1350 F./8 hrs./ACSerial No. D1BSerial No. D1:Solution Treat   2065 F./2 hrs./OQStabilize        1600 F./1 hr./ACAge              1350 F./8 hrs./AC______________________________________ 
    
     Serial Nos. A1, B1 and C1 were sectioned in half after solution treatment. 
     Serial Nos. A1A and C1B were held after solution treatment, while the remainder of the samples were subjected to stabilizing and aging heat treatment and cross-sectional testing. 
     The mechanical properties of the cross-sectioned specimens are set forth in Table II. 
     Serial No. B1A exhibited acceptable tensile strength and ductility while Serial No. D1A exhibited optimum stress rupture life. However, this first iteration heat treatment did not produce the combination of tensile ductility and stress rupture life required for gas turbine and jet engine components. 
     Additional heat treatment sequences were performed on the remaining material from the forging half sections Serial Nos. A1B, B1A, B1B and D1A. In this second heat treatment iteration the samples were identified as A1BT, B1AT, B1BT and D1AT, respectively. The heat treat cycles were as follows: 
     
         ______________________________________Serial No. A1BTSerial No. A1B:Solution Treat         2090 F./2 hrs./Direct Oil QuenchStabilize     1600 F./40 min/AC         1800 F./45 min/ACAge           1350 F./8 hrs./ACSerial No. B1ATSerial No. B1A:Solution Treat         2090 F./2 hrs./Direct Oil QuenchStabilize     1750 F./4 hrs. total furnace time         with 2 hrs. min. at temp./ACAge           1350 F./8 hrs./ACSerial No. B1BTSerial No. B1B:Solution Treat         2090 F./2 hrs./Direct Oil QuenchStabilize     NoneAge           1350 F./8 hrs./ACSerial No. D1ATSerial No. D1A:Solution Treat         2090 F./2 hrs./Direct Oil QuenchStabilize     1600 F./30 min. total furnace time         with max. metal temp. of 1400 F./ACAge           1350 F./8 hrs./AC______________________________________ 
    
     Mechanical properties of the second heat treat iteration are summarized in Table III. The higher stabilizing heat treatments Serial No. A1BT and Serial No. B1AT reduced residual stress from the oil quench after solution treatment while at the same time produced acceptable tensile and stress rupture properties. 
     Microstructural samples from the heat treatments were polished and etched with Murakami&#39;s etchant, and a grain boundary precipitate was evident on the samples from each heat treat section. However, a reduced amount of precipitate was present in samples which had a minimum exposure in the 1600° to 1750° F. temperature range. A microspecimen from Serial No. B1BT (which was not previously stabilized) was stabilized at 1800° F. for one hour and air cooled, and this exhibited virtual freedom from grain boundary precipitate. 
     Additional bars were obtained from Serial No. A1A and Serial A1B material and were used to develop a microstructural phase diagram for the grain boundary precipitate. The gradient bar study was conducted with stabilizing temperature ranges between 1500° and 1800° F. for time periods ranging from 1/2 to 4 hours. FIGS. 1 through 5 are photomicrographs of representative polished and etched samples. It is evident from FIGS. 1 and 2 that relatively massive precipitation occurs along grain boundaries by stabilizing at 1600° and 1700° F., respectively. In FIG. 3, wherein stabilization was at 1750° F. for 1 hour, less grain boundary carbide precipitates were evident. In FIGS. 4 and 5, wherein stabilization was conducted at 1800° F., for 1 hour and 4 hours, respectively, it is apparent that the precipitates were randomly dispersed and irregularly shaped with no concentration of precipitates along grain boundaries. Since a temperature of 1750° F. appears to be the upper limit at which grain boundary precipitation occurs, the range of 1750°  to 1850° F. for a time period of 1/4 to 4 hours, is considered to be the operative conditions for the stabilizing step of the method of the present invention. A maximum of 1850° F., should be observed in order to avoid tensile yield and ultimate strength degradation. 
     Since the samples of FIGS. 2 and 3 were not subjected to the standard aging or precipitation hardening treatment, it is evident that this treatment does not affect concentrations of precipitates along grain boundaries. Rather, this is a function of the stabilizing heat treatment conducted between 1750° and 1850° F. in accordance with the present invention. 
     Remaining half sections of Serial No. A1A and C1B were sectioned and identified as Serial Nos. A1AA, A1AB, C1BA and C1BB, respectively. These quarter sections were heat treated as follows: 
     
         ______________________________________Serial No. A1AASerial No. A1A:Solution Treat         2090 F./2 hrs./90 sec.         Oil Quench DelayStabilize     1800 F./2 hrs./ACAge           1350 F./8 hrs./ACSerial No. A1ABSerial. No. A1A:Solution Treat         2090 F./2 hrs./90 sec.         Oil Quench DelayStabilize     1800 F./4 hrs./ACAge           1350 F./8 hrs./ACSerial No. C1BASerial No. C1B:Solution Treat         2090 F./2 hrs./90 sec.         Oil Quench DelayStabilize     1600 F./1 hr./ACAge           1350 F./8 hrs./ACRe-Stabilize  1800 F./Time to reach temp./ACRe-Age        1350 F./8 hrs./ACSerial No. C1BBSerial No. C1A:Solution Treat         2090 F./2 hrs./90 sec.         Oil Quench DelayStabilize     1600 F./30 min. total furnace time         with max. metal temp. of 1400 F./ACAge           1350 F./8 hrs./AC______________________________________ 
    
     Mechanical properties of these samples are summarized in Table IV. Although the data for the four different heat treat conditions met the component property goals, the results indicate grain boundary carbide precipitation is affecting the stress rupture--creep property response. The best balance of creep and stress rupture values was obtained with a minimum exposure at 1800° F. (Serial No. C1BA) but this cycle would not be practical from a production control viewpoint. The 1600° F. furnace exposure (Serial No. C1BB) would not provide an adequate stress relief. Therefore, a stabilizing cycle of 1800° F. for 1 hour at temperature would provide the best property balance, an effective stress relief and heat treat control in a production situation. 
     A full-scale component test program was next performed. The stabilizing cycle was modified to include a fan air cool in order to accommodate the larger cross section of components and furnace loads. Mechanical properties of a cross-section component, which was a first stage turbine disc, are set forth in Table V, while mechanical properties of another cross section component, which was a second stage turbine disc, are summarized in Table VI. As will be apparent from these tables the mechanical properties substantially exceeded the goal of the manufacturer of the components in all instances. 
     The grain sizes reported in Tables II, V and VI indicate a uniform microstructure of desirably small average grain size after heat treatment, with an average of ASTM 11 to 12, with occasional grains as large as ASTM 8 or 9. 
     An alloy within the ranges of commercial alloy 2 above was fabricated into engine components which were subjected to the heat treatment method of the present invention, viz.: 
     
         ______________________________________Solution Treat    2050° F./2 hrs./OQStabilize         1815° F./45 min./ACAge               1200° F./24 hours/AC             1400° F./4 hrs./AC______________________________________ 
    
     The properties of these components after heat treatment are summarized in Table VII. It is evident that the properties were substantially superior to the minimum goals established for these components. 
     
                       TABLE I______________________________________CHEMICAL ANALYSIS  Percent by WeightELEMENT  Example A Example B Example C                                Example D______________________________________Carbon   0.031     0.031     0.027   0.032Manganese    &lt;0.01     &lt;0.01     &lt;0.01   &lt;0.01Silicon  0.08      0.06      0.06    0.06Phosphorus    0.002     0.002     0.001   0.002Sulfur   0.0012    0.0014    0.0012  0.0012Chromium 12.26     12.26     12.26   12.25Cobalt   18.05     18.03     18.10   18.06Molybdenum    3.27      3.29      3.29    3.26Titanium 4.23      4.24      4.24    4.24Aluminum 5.15      5.10      5.15    5.14Boron    0.018     0.018     0.017   0.018Hafnium  0.39      0.49      0.50    0.44Columbium    1.38      1.39      1.39    1.38Zirconium    0.07      0.07      0.08    0.08Tungsten 0.05      0.05      &lt;0.05   &lt;0.05Iron     0.08      0.09      0.09    0.09Copper   &lt;0.05     &lt;0.05     &lt;0.05   &lt;0.05Lead     0.00006   0.00004   0.00007 0.00004Bismuth  0.00001   0.00000   0.00001 0.00000Oxygen   0.015     0.014     0.010   0.008Nitrogen 0.002     0.002     0.002   0.002Nickel   54.98     54.91     54.78   54.94______________________________________GAS ANALYSISHYDROGEN         OXYGEN      NITROGENExample 0°           180°                    0°                          180°                                0°                                      180°______________________________________Ex. A   0.00085 0.00058  0.0146                          0.0129                                0.0022                                      0.0018Ex. B   0.00046 0.00036  0.0141                          0.0134                                0.0016                                      0.0016Ex. C   0.00055 0.00043  0.0102                          0.0094                                0.0025                                      0.0018Ex. D   0.00044 0.00041  0.0085                          0.0084                                0.0016                                      0.0018______________________________________ 
    
     
                       TABLE II______________________________________MECHANICAL PROPERTIES - FIRST HEATTREAT ITERATION______________________________________ROOM TEMPERATURE   1150° F. ELEVATED TEM-TENSILE            PERATURE TENSILEY.S.     U.S.    %      %    Y.S.  U.S.  %    %(KSI)    (KSI)   EL     RA   (KSI) (KSI) EL   RA______________________________________A1B Example A solution 2090° F./2 Hrs./Direct Oil QuenchStabilize 1600° F./1 Hour/AC Age 1350° F./8 Hrs./AC165     240     17   16   162   220   16   19161     230     15    --14                          157   213   24   29157     230     16    --14                          148   209   28   36163     227      --14                     15   153   207   25   34157     225      --14                      --13                          159   212   16   19Goal 140     215     15   15   140   194   12   12B1A Example B solution 2090° F./2 Hrs./90 Sec. Oil QuenchDelay Stabilize 1500° F./1 Hour/AC Age 1350° F./8 Hrs./AC161     241     24   21   159   216   27   31161     239     21   20   159   213   22   27160     235     19   17   158   209   27   33165     239     20   19   158   209   24   29158     235     19   19   157   215   24   28Goal 140     215     15   15   140   194   12   12B1B Example B Solution 2090° F./2 Hrs./90 Sec. Oil QuenchDelay Stabilize 1600° F./1 Hour/AC Age 1350° F./8 Hrs./AC159     227     15    --14                          158   213   22   26158      221      --13   --12                        Invalid Test159     233     17   16   156   206   28   34159     229     15   15   155   210   27   33156     223      --13                      --13                          164   215   12   15Goal 140     215     15   15   140   194   12   12C1A Example C solution 2065° F./2 Hrs./15 Sec. Oil QuenchDelay Stabilize 1600° F./1 Hour/AC Age 1350° F./8 Hrs./AC162     223      --13                      --13                           165  220   15   17159     231     17   15   158   211   17   20158     215      --13                      --11                          155   208   20   21164     235     16   16   155   209   25   30158     195       -9   -7 156   206    --9.5                                           13Goal 140     215     15   15   140   194   12   12D1A Example D Solution 2090° F./2 Hrs./Direct Oil QuenchStabilize 1600° F./1 Hour/AC Age 1350° F./8 Hrs./AC164     232     15   15   165   218   14   17161     235     17   16   158   213   22   25157     231     17   16   155   213   24   25160     231     15    --13                          155   213   25   28165     222      --11                      --12                          158   209    --10                                           12Goal 140     215     15   15   140   194   12   12D1B Example D Solution 2065° F./2 Hrs./Direct Oil QuenchStabilize 1600° F./1 Hour/AC Age 1350° F./8 Hrs./AC163     230      --14                     15   161   215   15   16159     231     16   15   159   213   20   22157     233     17   15   155   209   23   24164     232     15    --12                          161   218   20   21156         ##STR1##                 --10                      --12                          155   212   12   16Goal 140     215     15   15   140   194   12   12______________________________________COMBINATIONSTRESS          MICROSTRUCTURALRUPTURE         EVALUATIONKt = 3.6 Temper-           ASTM GRAIN SIZEature 1350° F.       FORGED &amp;Stress 95 KSI               HEATSERIAL STRESS           AS-HIP    TREATED*NO.    HRS.     % EL    AVG.  ALA   AVG.  ALA______________________________________A1B    27.2     Notch   10    8     12    8  24.9     NotchB1A   ##STR2##           Notch   10    9     12    8  24.5     NotchB1B    29.7     Notch   10    9     12    8  25.9     NotchC1A    25.4     Notch   10    9     12    9  27.6     NotchD1A    40.1     14       9    8     12    8  37.4     NotchD1B    30.8     Notch     9   8     12    9  31.8     11Goal   23        5______________________________________ *MICROSTRUCTURAL REVIEW INDICATED MICROSTRUCTUAL UNIFORMITY FROM RIM TO BORE 
    
     
                                           TABLE III__________________________________________________________________________MECHANICAL PROPERTIES - SECOND HEAT TREAT ITERATION                        TENSILE PROPERTIES  COMBINATION                        TEST                STRESS RUPTURE                                            1350°NUMBER SOLUTION*        STABILIZE*                AGE*    TEMP*                             YS  UTS                                    % EL                                        % RA                                            LOAD HRS.                                                     %__________________________________________________________________________                                                     ELA1BT  2090°/2 H/        1600° F./40                1350°/8 H/AC                        R.T. 162 235                                    26  30  95   41.8                                                     Notch Oil Quench        min/AC        1800°/45 1150 160 213                                    20  22        min/ACB1AT  2090°/4 H/        1750°/4 H                1350°/8 H/AC                        R.T. 164 237                                    21  21  95   36.1+                                                     Notch Oil Quench        Total           1150 162 216                                    18  18        Furnace        Time/ACB1BT  2090°/2 H/        None    1350°/8 H/AC                        R.T. 164 240                                    25  27  95   65.5                                                     Notch                        1150 161 219                                    23  23D1AT  2090°/2 H/        1600°/30                1350°/8 H/AC                        R.T. 164 241                                    24  24  95   116.8                                                     10 Oil Quench        Min. Total           1150                                 159                                    217 22  20        Furnace Time                Goals   RT   140 215                                    15  15  95   23   5                        1150 140 194                                    12  12__________________________________________________________________________ *Temperature in °F. 
    
     
                       TABLE IV______________________________________MECHANICAL PROPERTIES - THIRD HEATTREAT ITERATION______________________________________ROOM TEMPERATURE   1150° F. ELEVATED TEM-TENSILE            PERATURE TENSILEY.S.    UTS    % EL    % RA  Y.S. UTS  % EL  % RA______________________________________A1AA Quarter Section Solution 2090°/2 H/90 Sec Oil QuenchDelay Stabilize 1800°/2 H/AC Age 1350°/8 H/AC153     230    28      26    Void - Testing Problem153    232    28    28    152  200  29    31152    230    26    24    152  207  26    29153    232    28    28    152  204  29    33153    230    26    25    152  204  24    27Goal 140    215    15    15    140  194  12    12A1AB Quarter Section Solution 2090°/2 H/90 Sec Oil QuenchDelay Stabilize 1800°/4 H/AC Age 1350°/8 H/AC152    231    28    27    153  204  26    21153    230    27    26    152  201  25    27150    229    28    26    151  204  26    29151    229    28    27    153  201  26    32152    230    26    24    152  202  22    26Goal 140    215    15    15    140  194  12    12C1BA Quarter Section Solution 2090°/2 H/90 Sec Oil QuenchDelay Stabilize 1600°/1 H/AC Age 1350°/8 H/ACReStabilize 1800°/Time to Reach Temperature/AC Re-Age1350°/8 H/AC153    232    26    27    152  206  25    29154    232    26    27    154  202  26    29154    230    25    25    151  212  26    34151    229    22    22    154  211  26    32151    214    15    15    153  207  18    19Goal 140    215    15    15    140  194  12    12C1BB -100 Mesh Quarter Section Solution 2090°/2 H/90 SecOil QuenchDelay Stabilize 1600°/30 min Total F.T./AC (1400° F.Max. Temp.)160    239    27    27    158  216  24    20158    238    24    23    158  212  25    27158    240    27    26    Void165    243    26    25    Void155    232    20    15    155  214  20    17Goal 140    215    15    15    140  194  12    12______________________________________                          CREEP  COMBINATION STRESS      1300°  F.SERIAL RUPTURE                 AT 80 KSINUM-   STRESS           FAIL   HOURS   HOURSBER    HOURS    % EL    LOC.   TO 0.1% TO 0.2%______________________________________A1AA   40.3     --      Notch  146     181A1AB   48.3     5.5     Smooth 109     152C1BA   81.8     --      Notch  227     Test Dis-                                  continuedC1BB   40.9     6       Notch  125     155Goal   23       5              --      100______________________________________ 
    
     
                       TABLE V______________________________________FIRST STAGE TURBINE DISC - HEAT NO. 022081 -HEAT CODE SERIAL NO. 2001______________________________________        Yield    Ultimate % ElTest Identity        KSI      KSI      4D      % RA______________________________________ROOM TEMPERATURE TENSILEO.D. - Tangential        147      225      27      26Web - Radial 148      225      28      29Bore - Tangential        156      230      25      26Spacer - Tangential        153      230      26      24Integral - Tangential        159      234      25      26Goal         140      215      15      15ELEVATED TEMPERATURE TENSILE - 1150° F.O.D. - Tangential        151      202      26      31Web - Radial 148      206      24      24Bore - Tangential        152      208      28      34Spacer - Tangential        149      201      27      29Integral - Tangential        155      213      26      31Goal         140      194      12      12______________________________________COMBINATION BAR STRESS RUPTURE @ 1350° F., 95 KSI        Total               FailureTest Identity        Hours       % EL    Loc.______________________________________O.D. - Tangential        49.2        13      SmoothBore - Tangential        45.2        8.5     SmoothIntegral - Tangential        53.8        9.0     SmoothSpecification (Min.)        23.0        5.0______________________________________CREEP RUPTURE TEST @  1300° F., 80 KSI         Creep      CreepTest Identity Hrs. @ 0.1%                    Hrs. @ 0.2%______________________________________O.D. - Tangential         120        166O.D. - Tangential          88        152______________________________________ASTM GRAIN SIZETest Identity        Average  As-Large-As______________________________________O.D.         11       9Web          11       9Bore         12       9Spacer       12       9Integral     11       9______________________________________ 
    
     
                       TABLE VI______________________________________FIRST STAGE TURBINE DISC - HEAT NO. M0029C, HEATCODE CNDN SERIAL NO. 2001 - CROSS-SECTIONALPROPERTY ANALYSIS______________________________________      YIELD      ULTIMATE      STRENGTH   STRENGTH   % ELTEST IDENTITY      (KSI)      (KSI)      4D    % RA______________________________________ROOM TEMPERATURE TENSILEO.D.       151        228        22    28TANGENTIALWEB RADIAL 151        228        21    26BORE       152        230        20    25TANGENTIALSPACER     152        229        21    24TANGENTIALINTEGRAL   154        230        21    27TANGENTIALGOAL       140        215        15    15ELEVATED TEMPERATURE TENSILE 1150° F.O.D.       150        203        27    31TANGENTIALWEB RADIAL 150        203        27    35BORE       150        204        28    33TANGENTIALSPACER     147        203        26    33TANGENTIALINTEGRAL   148        203        26    33TANGENTIALGOAL       140        194        12    12______________________________________COMBINATION BAR STRESS RUPTURE 1350° F. AT 95 KSI         TOTAL    % ELON-   FAILURETEST IDENTITY HOURS    GATION    LOCATION______________________________________O.D. TANGENTIAL         47.1     11        SmoothBORE TANGENTIAL         27.4     13        SmoothINTEGRAL      35.3     11        NotchTANGENTIALSMOOTH SECTION         36.2     11        SmoothCONT.GOAL          23.0     5.0______________________________________ASTM GRAIN SIZETEST IDENTITY    AVERAGE______________________________________O.D. TANGENTIALWEB RADIAL       11BORE TANGENTIAL  11SPACER TANGENTIAL            11INTEGRAL TANGENTIAL            11GOAL             8 or Finer______________________________________ 
    
     
                       TABLE VII______________________________________ROOM TEMPERATURE TENSILE  YIELD  STRENGTH      TENSILE    %      %  0.2% OFFSET   STRENGTH   ELONG. R.A.  MIN. KSI      MIN. KSI   MIN.   MIN.______________________________________3rd Stage  160           230        28     25DiscGoal   150           215        15     15______________________________________COMBINATION STRESS RUPTURE  TEMPER-    STRESS    TIME TO  %  ATURE      KSI       RUPTURE  ELONG.______________________________________3rd Stage  1350° F.             92.5      38 Hrs.  7Disc4th Stage  1350° F.             92.5      52.8     15DiscGoal   1350° F.             92.5      23.0     5______________________________________CREEP                    STRESS   TIME TO     TEMPERATURE    KSI      0.2%______________________________________3rd Stage Disc     1300° F.                    80       1774th Stage Disc     1300° F.                    80       237Goal      1300° F.                    80       100______________________________________