AU624463B2

AU624463B2 - Tantalum-containing superalloys

Info

Publication number: AU624463B2
Application number: AU47652/90A
Authority: AU
Inventors: Thomas Joseph Kelly
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1989-04-10
Filing date: 1990-01-04
Publication date: 1992-06-11
Anticipated expiration: 2010-01-04
Also published as: DE4011129A1; AU4765290A; CA2010147A1; IT9019977A1; IT1240749B; FR2646171A1; GB2230274A; GB2230274B; GB9001431D0; IL93341A0; JPH0317243A; IT9019977A0; CN1046944A

Description

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AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: Related Art: Applicant(s): Address for Service: General Electric Company One River Road SCHENECTADY New York 12345 UNITED STATES OF AMERICA ARTHUR S. CAVE CO.

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Complete specification for the invention entitled "Tantalum-containing superalloys".

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la 5160L 13DV-9629 TANTALUM-CONTAINING SUPERALLOYS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to improved nickel base alloys which have a particularly high combination of strength properties and ductility over the temperature range extending from about room temperature to approximately 1500°F. The improvement is provided by incorporating a substantial amount of OS tantalum in the alloy, generally as an atom-for-atom S* replacement for niobium, and then heat treating the alloy at very high temperatures for extended periods.

DESCRIPTION OF THE PRIOR ART c*S Prior art nickel-base superalloys while steadily being improved, have disadvantages either from a standpoint of strength or ductility, particularly at elevated temperatures, above about 1200 0

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These alloys are generally based upon nickel in combination with one or more of chromium, iron, and cobalt. In addition, they may contain a variety of L _ee 5160L 13DV-9629 -2elements in a large number of combinations to produce desired effects. Some of the elements which have been utilized in nickel-base superalloys to provide or improve one or more of the following properties are: strength (Mo, Ta, W, Re), oxidation resistance (Cr, Al), phase stability (Ni) or increased volume fractions of favorable secondary precipitates (Co).

Other elements are added to form hardening precipitates such as gamma prime (Al, Ti) and gamma double prime Minor elements B) are added to form carbides and borides and others (Ce, Mg) are added for purposes of tramp element control. Some elements Zr, Hf) also are added to promote favorable grain boundary effects. Many elements Co, Mo, W, Cr), although added for their favorable alloying qualities, can participate, in some circumstances, in the formation of undesirable phases sigma, mu, Laves).

Gamma double prime is generally'considered to be a body centered tetragonal ordered Ni 3 Nb strengthening precipitate which is formed when niobium is present in nickel-base superalloys. A superalloy in which gamma S* double prime strengthening occurs is Inconel 718 which is within the scope of U.S. Patent No. 3,046,108 (Eiselstein). Eiselstein teaches that the alloy must e"s "'contain about 4 to about 8 weight percent columbium and that the columbium in the alloy may be replaced in part with tantalum in an amount of up to 4% of the alloy. In partially replacing the columbium content .vo. *of the alloy with tantalum, Eiselstein teaches that double the weight of tantalum should be used to obtain the same effect on properties. He also teaches that only tantalum-free alloys and/or alloys wherein not 1 VT °^m -0635k/lfg 3 more than 50% of the columbium is replaced by tantalum are notch-ductile at elevated temperatures.

Eiselstein thus teaches that tantalum and niobium act the same in nickel-based alloys provided that only a limited amount of tantalum is present.

The gamma double prime phase is not normally a stable phase since it can convert to gamma prime or to delta on extended exposure to elevated temperatures. Alloys hardened with gamma double prime achieve high tensile strength and very good creep rupture properties at lower temperatures, but the conversion of gamma double prime to gamma prime or delta above about 1250°F causes a sharp reduction of strength. (Donachie, "Relationship of Properties to Microstructure in Superalloys" in Superalloys Source Book, S: American Society for Metals, 1984).

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SUMMARY OF THE INVENTION It has now been discovered that tantalum does not act the same as niobium in nickel base superalloys. Rather, tantalum has been found to produce an alloy which has greater phase stability and different phase relationships than -the corresponding niobium containing alloy. This difference in phase stability makes the Ta containing alloys much stronger to much higher temperatures than Nb containing alloys. In addition, the gamma double prime in the alloys of the invention does not readily convert to delta phase as occurs in niobium-bearing counterpart alloys.

The present invention provides a nickel base superalloy which consists of 8 to 16% tantalum, 17 to 22% chromium, up to 25% iron, up to 16% cobalt, 2 to 6% molybdenum, 0.75 to titanium, 0.1 to 5% alufinium, 30 to 150 ppm boron, 0.01 to 0.1% carbon, the balance nickel except for incidental impurities, and wherein the total amount of iron plus cobalt is not less than about 8%.

The invention also provides a nickel base superalloy which consists of 30 to 40% nickel, 30 to 40% iron, 15 to 23% cobalt, 8 to 16% tantalum, and 30 to 150 ppm boron.

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:0635k/lfg 4 In addition, other elements such as manganese, silicon, phosphorus, sulfur, lead, bismuth, tellurim, selenium, niobium and silver may also be present as incidental impurities. However as used above, throughout the description and in the claims the term "incidental impurities" is not to be restricted to the elements listed above.

The invention also provides a method of improving the high temperature strength properties of a nickel base superalloy which consists of about 8 to 16% tantalum, 17 to 22% chromium, up to 25% iron, up to 16% cobalt, 2 to 6% molybdenum, 0.75 to 5% titanium, 0.1 to 5% aluminium, 30 to 150 ppm boron, 0.01 to 0.1% carbon, the balance nickel except for incidental impurities, and wherein the total amount of iron plus cobalt is not less than about 8% 00 comprising the steps of: a) heat treating at about 2000°F for about 1 hour, b) hot isostatic pressing at about 2050°F at a pressure of 12 to 15 ksi for about 5 hours, c) heating to about 1925°F and holding for about 4 hours, and o 0 d) heating to about 1600 F and holding for about 2 S* hours.

The invention also provides a method of improving the 0 high temperature strength properties of a niobium-free 0 nickel-base superalloy consisting of 8.5 to 10% tantalum, 18 to 20% chromium, 17 to 19% iron, 2.5 to 4% molybdenum, 0.75 to 2.5% titanium, 0.25 to 0.75% aluminium, 30 to 60 ppm boron if the alloy is to be cast or 80 to 100ppm boron if Sthe alloy is to be wrought, 0.03 to 0.05% carbon, the balance nickel, except for incidental impurities, comprising the steps of: a) heat treating at about 2000'F for about 1 hour, b) hot isostatic pressing at about 2050"F at a pressure of 12 to 15 ksi for about 5 hours, c) heating to about 1925"F and holding for about 4 hours, and d) heating to about 1600'P and holding for about 2 I $,WA4hours.

1( 2^ U Ji\7 '0635k/lfg 4a DESCRIPTION OF THE PREFERRED EMBODIMENT The alloys of the present invention contain at least about 30% nickel (all percents expressed herein and in the claims are by weight unless otherwise specified) and about 8 to about 16% tantalum. The balance of the alloy will consist of other elements which are conventionally alloyed with nickel to form superalloys such as elements selected from the group consisting of chromium, iron, cobalt, molybdenum, titanium, zirconium, tungsten, hafnium, aluminium, I S *•l 1 i i r i i i:t is 5160L 13DV-9629 boron, carbon and combinations thereof. Further, other elements such as manganese, silicon, phosphorus, sulfur, lead, bismuth, tellurium, selenium, and silver may also be found in the alloy as incidental impurities. These alloys will be substantially niobium-free, they will contain less than about preferably less than and most preferably less than about 0.1% Nb.

Generally, the alloy will contain, in addition to nickel and tantalum, up to about 25% chromium, up to about 40% iron, up to about 25% cobalt, up to about 8% molybdenum, up to about 3% titanium, up to about 2% aluminum, up to about 7% tungsten about 30 to about 150 ppm boron, and up to about 0.1% carbon. Other elements, such as those other alloying elements specified above, may be present in amounts up to about 1% each with a total maximum of up to about One preferred alloy consists essentially of about 8 to about 16% tantalum, about 17 to about 22% chromium, up to about 25% iron, up to about 16% cobalt, but not less than 12% total Fe plus Co, about 2 to about 6% molybdenum, about 1 to about titanium, about 0.1 to about 5% aluminum, about 30 to about 150 ppm boron, about 0.01 to about 0.1% carbon, the balance nickel (including incidental impurities), wherein the total amount of iron plus cobalt is about 12to about A second preferred alloy consists essentially of about 8.5 to about 10% tantalum, about 18 to about chromium, about 17 to about 19% iron, about 2.5 to about 4% molybdenum, about 0.75 to about titanium, about 0.25 to about 0.75% aluminum, about \to about 60 ppm boron, if the alloy is to be cast, or 0 0 0 0

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I S 0 0 i 0 ***ooo 00 8 0 j pr 5160L 13DV-9629 -6about 80 to"abou.t, 150 ppm boron if the alloy is to be wrought, about-'0..03 to about 0.05% carbon, the balance nickel. A mbst preferred version of this alloy consists essentially of about 9% tantalum, about 19% chromium, about 18% iron, about 3% molybdenum, about 1% titanium, about 0.5% aluminum, about 30 to about ppm boron, if the alloy is to be cast, or about 80 to about 100 ppm boron if the alloy is to be wrought, about 0.05% carbon, the balance nickel.

A third preferred alloy consists essentially of about 30 to about 40% nickel, about 30 to about i iron, about 15 to about 23% cobalt, about 8 to about i 16% tantalum, and about 30 to abput 150 ppm boron. A more preferred version of this alloy consists essentially of about 35 to about 38% nickel, about to about 38% iron, about 17 to about 20% cobalt, about 8 to about 10% tantalum, and about 30 to about 60 ppm boron, if the alloy is to be cast, or about 80 to about 100ppni bo'roh if the alloy is to be wrought. A most preferred version of this alloy consists 1 essentially of about 36 to about 37% nickel, about 36 i to about 37% iron, about 17 to about 19% cobalt, about 8.5 to about 9.5% tantalum, and about 30 to about ppm boron, if the alloy is to be cast, or about 80 to S about 100 ppm boron if the alloy is to be wrought.

SThe alloys of this invention may be cast or wrought and may be produced by conventional methods.

t 00OO For the alloys of the invention to develop their 0 0 improved high temperature properties, they need to be heat treated. The heat treatment is conducted at a higher temperature for a substantially longer period than is conventionally used for similar niobium containing alloys.

40O4 _Cl__i 5160L 13DV-9629 -7- The presently preferred heat treatment cycle for the second preferred alloy entails heating at about 2000 0 F for about 1 hour, followed by hot isostatic pressing at about 2050 0 F, at a pressure of about 12 to about 15 ksi, for about 3 to about 5 hours, followed by heating at about 1925 0 F for about 4 hours, and followed by heating at about 1600 0 F for about 2 hours. An additional heating (aging) at about 1350°F for about 8 hours may be helpful to produce optimal properties with some alloys. The conventional heat treatment for this alloy in its niobium containing version would not include the 1600°F step and would include a lower temperature aging step at about 1150°F for about 4 to 8 hours.

By the use of tantalum in the substantial absence of niobium in combination with the higher heat treatment conditions, alloys are produced which make greater use of gamma double prime strengthening than in conventional niobium-containing alloys. The alloys of the invention are age-hardenable, malleable, and are characterized by a high combination of strength and ductility, particularly at elevated temperatures.

In addition, it is believed that the amount of aluminum plus titanium, if included in the alloy, can be increased above that conventionally found in niobium containing alloys without inducing strain age cracking of weldments. Another benefit of using tantalum instead of niobium in the alloys is improved weldability. This is due to an increased resistance to heat affected zone microcracking due to the higher Ta-Ni eutectic temperature compared to that of the Nb-Ni eutectic.

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5160L 13DV-9629 -8- The following non-limiting examples are provided to demonstrate the preparation of alloys of the present invention and their improved properties, especially at high temperatures.

EXAMPLE I A tantalum-containing alloy like 718 was produced by melting a composition of 48.6% nickel, 19.2% chromium, 18.0% iron, 0.02% niobium, 9.1% tantalum, molybdenum, 1.04% titanium, 0.47% aluminum, 0.0043% boron, 0.044% carbon, and 0.02% silicon, in a vacuum induction furnace. The melted alloy was cast into ceramic molds to form 2" x 4" x 1/4" slabs.

Specimens from the slabs were subjected to Keat treatment as follows: 2000 0 F for 1 hour, 2050°F hot isostatic pressing at 14.7 KSI for 3 hours, 1925 0 F for 4 hours, 1600"F for 2 hours, and then 1350 0 F for 8 hours.

A conventional 718 alloy of the same composition containing essen.ially no tantalum, but about 4.6% niobium, was produced in the same manner as above and heat treated to conventional 718 practice (as noted in footnote 1 to Table I below).

The microstructure of the tantalum-bearing alloy is found to have equal or less stable Laves phase on solidification as the conventional 718 alloy. In addition, the tantalum-bearing alloy does not produce 0.1o the delta phase after exposure in the 1600°F to 1800°F o' range; a heat treatment which is used to determine element segregation in 718 alloys (delta dump). The microstructure of the tantalum-bearing alloy has an excellent distribution of gamma prime and gamma double 9**

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9.9e i 1 5160L 13DV-5629 -9prime of a size which produces a reasonable strengthening effect. The gamma prime and gamma double prime precipitate in the tantalum-bearing alloy is much more uniformly distributed throughout the dendrite cores and interstices than in conventional cast 718.

Specimens of the two alloys were evaluated to determine their mechanical properties at both room temperature (RT) and at elevated temperature. The results are: TABLE I Cast Ta 7181,3 RT 1200 130O Cast Nb 7181 RT 1200 Cast Ta 7182,4 .T 1200 1400 UTS 155.3 0.2% 118.1 %El 19 %RA 29.1 130 114 11.5 22.5 122 106.5 9 21.5 151 133 15 29 117 104 11 25 178.2 147.7 142.5 117.8 12 11 18 8 hr; 1350/8 hr; hr; 1600/2 hr; 133.3 112.6 6 6 1 2000 0 F/1 hr; 2050 hip/14.7ksi/3 hr; 1925/1 !1150/8 hr.

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4 one specimen one specimen As may be noted from Table I, the tantalum-bearing 718 type superalloy showed improved elevated temperature strength properties over its niobium-bearing counterpart and these properties were even further improved by the use of the Spreferred heat treatment.

0** 00.0 p I I 5160L 13DV-9629 EXAMPLE II The procedure of Example I was repeated with an alloy whose composition was 36.6% nickel, 36.6% 'iron, 17.7% cobalt, 9.1% tantalum, and 45 ppm boron. The corresponding conventional alloy in which the tantalum is replaced with niobium on an atom for atom basis, i.e. the niobium content is was also prepared for comparison purposes. The alloys are evaluated for mechanical properties as in Example I. The results are: TABLE II Cast Ta Alloy Cast Nb Alloy 1200°F R.T. 1200°F Ultimate tensile 182,5 141.8 135 108 strength (KSI) 0.2% Yield strength (KSI) 159.4 128.6 120 89 Elongation 4.5 3.0 4.0 Reduction in area 6.5 6.5 7.0 13.0 As is evident, ihe tantalum-bearing alloy of the I S oo present invention exhibits substantially increased ultimate tensile and yield strengths, reduced reduction in area, and similar elongation as compared to the same alloy containing niobium.

o'oo Evaluations of the various alloys again demonstrate the superiority of the tantalum-bearing alloy of this invention as compared to the comparable niobium-bearing alloy.

ee 0 i i! 5160L 13DV-9629 -11- EXAMPLE III Although the conventional 718 alloy of Example I is highly resistant to strain-age cracking during weld stress relief, the alloy can be susceptible to both liquation cracking in the weld heat-affected-zone (HAZ) and, under conditions of high restraint, solidification cracking in the weld fusion zone. To evaluate the effect of the tantalum for niobium substitution of the present invention, the alloy formation steps of Example I are repeated to produce cast-to-size weldability test specimens 5 mm in thickness. Prior to weldability testing all specimens were heat treated in vacuum at 2000°F for one hour and cooled to 1200 0 F in twenty minutes. Spot Varestraint and Mini Varestraint weldability tests were utilized to evaluate HAZ liquidation and fusion zone solidification cracking susceptibilities. In the Spot Varestraint test, strain is applied to a gas-tungsten-arc spot weld immediately after extinguishing the arc, thereby restricting cracking to the weld HAZ. During Mini Varestraint testing, straining occurs during the generation of a continuous gas tungsten-arc weld, with cracks forming primarily in the previously solidified fusion zone. Total crack length is utilized as the quantitative measure of cracking susceptibility.

As shown in Table III, the tantalum-bearing alloy exhibits the lowest susceptibility to weld HAZ cracking over the entire range of strain levels tested, i.e. 0.2S to 3% augmented strain, by the Spot Varestraint test.

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5160L 13DV-9629 -12- TABLE III Cast Alloy 718 Cracks TCL MCL Cast Ta 718 Cracks TCL MCL 0.29% 0.29% 1.16% 1.16% 2.9% 2.9% .422 .493 .671 .775 1.008 1.108 .032 .033 .040 .040 .055 .053 .214 .240 .391 .462 .664 .669 .025 .028 .034 .034 .039 .045 Cracks: number of cracks per weld TCL: Tota' Crack Length MCL: Maximum Crack Length Although the present invention has been described in connection with specific examples and embodiments, it will be understood by those skilled in the arts involved that the present invention is capable of modification without departing from its spirit and scope as represented by the appended claims.

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Claims

1. A nickel base superalloy which consists of 8 to 16% tantalum, 17 to 22% chromium, up to 25% iron, up to 16% cobalt, 2 to 6% molybdenum, 0.75 to 5% titanium, 0.1 to aluminium, 30 to 150 ppm boron, 0.01 to 0.1% carbon, the balance nickel except for incidental impurities, and wherein the total amount of iron plus cobalt is not less than about 8%.

2. A nickel base superalloy which consists of 8.5 to tantalum, 18 to 20% chromium, 17 to 19% iron, 2.5 to 4% molybdenum, 0.75 to 2.5% titanium, 0.25 to 0.75% aluminium, to 60 ppm boron if the alloy is to be cast or 80 to 100 ppm boron if the alloy is to be wrought, and 0.03 to 0.05% carbon, and the balance nickel except for incidental impurities.

3. The alloy of Claim 2 which in the alloy consists of about 9% tantalum, about 19% chromium, about 18% iron, about 3% molybdenum, about 1% titanium, about 0.5% aluminium, to 60 ppm boron if the alloy is to be cast or 80 to 100 ppm boron if the alloy is to be wrought, and about 0.05% carbon, and the balance nickel except for incidental impurities.

4. A nickel base superalloy which consists of 30 to nickel, 30 to 40% iron, 15 to 23% cobalt, 8 to 16% tantalum, and 30 to 150 ppm boron.

5. The alloy of Claim 4 wherein the alloy consists of 38% nickel, 35 to 38% iron, 17 to 20% cobalt, 8 to 10% tantalum, and 30 to 60 ppm boron if the alloy is to be cast or 80 to 100 ppm boron if the alloy is to be wrought.

6. The alloy of Claim 5 wherein the alloy consists of 36 to 37% nickel, 36 to 37% iron, 17 to 19% cobalt, 8.5 to f tantalum, and 30 to 60 ppm boron if the alloy is to be cast or 80 to 100ppm boron if the alloy is to be wrought.

7. A method of improving the high temperature strength properties of a nickel base superalloy which consists of about 8 to 16% tantalum, 17 to 22% chromium, up to 25% iron, up to 16% cobalt, 2 to 6% molybdenum, 0.75 to 5% titanium, 0.1 to 5% aluminium, 30 to 150 ppm boron, 0.01 to 0.1% carbon, the balance nickel except for incidental impurities, 7 and wherein the total amount of iron plus cobalt is not less I"^riiii-*^ J J ,0635k/lfg 1iu than about comprising the steps of: a) heat treating at about 2000°F for about 1 hour, b) hot isostatic pressing at about 2050°F at a pressure of 12 to 15 ksi for about 5 hours, c) heating to about 1925°F and holding for about 4 hours, and d) heating to about 1600°F and holding for about 2 hours. i 8. A method of improving the high temperature strength properties of a niobium-free nickel-base superalloy consisting of 8.5 to 10% tantalum, 18 to 20% chromium, 17 to 19% iron, 2.5 to 4% molybdenum, 0.75 to 2.5% titanium, 0.25 to 0.75% aluminium, 30 to 60 ppm boron if the alloy is to be S cast or 80 to 100ppm boron if the alloy is to e wrought, i 0.03 to 0.05% carbon, the balance nickel, except for o: incidental impurities, comprising the steps of: a) heat treating at about 2000°F for about 1 hour, b) hot isostatic pressing at about 2050°F at a pressure of 12 to 15 ksi for about 5 hours, c) heating to about 1925°F and holding for about 4 hours, and d) heating to about 1600 F and holding for about 2 hours.

9. The method of Claim 7 or Claim 8 wherein the alloy is further aged at about 1350°F for about 8 hours following the step of heating to 1600 F. A nickel base niobium-free alloy, substantially as herein described with reference to the examples.

11. A method of treating a nickel base niobium-free alloy, substantially as herein described with reference to the examples. DATED this 11th day of March, 1992. GENERAL ELECTRIC By Its Patent Attorneys DAVIES COLLISON CAVE f,11