CA1260292A - Cobalt-base superalloy and cast and welded industrial gas turbine components thereof - Google Patents

Abstract

NOVEL COBALT-BASE SUPERALLOY AND
CAST AND WELDED INDUSTRIAL GAS
TURBINE COMPONENTS THEREOF

Abstract of the Disclosure Cobalt-base superalloys having special utility in the production of industrial gas turbine hot gas path components because of their unique combination of properties including excellent hot corrosion resistance, stress rupture strength at high temperature, metallurgical stability, tensile ductility and weldability, consist essentially of 0.3 to 0.6% carbon, 27-35% chromium, 9-16% nickel, 6-9% tungsten, 0.45 to 2.0% tantalum, up to 3.0%
hafnium, up to 0.7% zirconium, not more than 2.0%
iron, 1.5% manganese and silicon and 0.05% boron, balance cobalt, the carbide formers being selected to satisfy the following equation:

Description

]
~OVEL COBALT-BASE SUPERAL,LOY AND CAST
AND W~LDED I~DUSTRIAL GAS TURBINE
_ .
COMPO~E~TS THEREOF.

Field of the _nvention This invention re]ates generally to the superalloy branch of the metallurgical art, and is more specifically concerned with new cobalt-base superalloys having a unique combination of properties and consequent special utility in the production of both cast articles and ~elded structures, and with novel industrial gas turbine hot gas path components of those new alloys.
Background Cobalt-base superalloys disclosed and claimed in U.S. Patent No. 3,383,205 - Sims et al, issued May 14, 1968, have superior oxidation and hot corrosion resistance and as a consequence have long been used extensively in commercial production of industrial gas turbine nozzles. In fact, one of those superalloys is the current first stage nozzle alloy of the Gas Turbine Division of Genera]. Electric Company, the assignee hereof. The creep rupture and fatigue strength of that al].oy, however, are marginal for new industria]. gas turbine nozzle applications and in 5lDV28]9 recognition of that ~act, a program was launched to improve those properties without significan-tly diminishing the resistance of the supera]loy either to oxidation or to hot corrosion. While the resu]ting superalloys met those objectives as a consequence of their relatively high carbon contents (0.40 to 0.50~), they were still not the answer to the problem because of their inferior weldability and low tensile ductility.
Summary of the Invention Through our discoveries and new concepts detailed below, we have created new cobalt-base super-alloys haviny a previously unobtainable combination of desirable properties. Thus we have found the way to avoid having to make the trade-offs of desirable properties exemplified by the problem mentioned above. This invention in providing the answers to that problem embodies those discoveries and new concepts of ours and they are epitomized in the appended claims directed both to alloy compositions and to articl.es of manufacture of those compositions.
One of our concepts upon which this invention is based is that we].dability and tensile ductility of cobalt-base superalloys need not be significantly compromised in order to increase creep strength and fatigue strength very substantially. In particular, beneficial effects of increased carbon content can be obtained without the normally attending detrimental effects thereof by addition of one or more of the following strong monocarbide MC-formers:
~ hafnium, tantalum, .G~b}Jm, zirconium and titanium~
~ We have discovered that these additive elements are effective for this purpose in relatively small amounts and that within certain J.imits they can be used singly or together in any desired combination to secure consistently the new results and advantages of this invention.
In making this invention, we have established that the beneficial effects of carbon on creep strength and fatigue strength are not forfeited to any appreciable degree as a result of isolating the carbon in the form of monocarbide throughout the grain and in the grain boundaries of the superalloy. Further, we have established that such segregation and isolation of carbon results in good weldabillty, metallurgical stability and tensile ductility, all of which are normally adversely affected by carbon in proportions preferred in accordance with this invention.
We have further discovered that the new results and advantages of this invention can consistently be obtained only through the use of at least 0.45% tantalum, and that while selection of other elements of the monocarbide MC-carbide former group is a matter of choice for the operator as to kind, the total amounts used are critically important. Thus the balance between the carbon content of the alloy and the total of those elements expressed as the ratio of the sum of the atomic percent of those elements to the atomic percent of carbon must be within the range of 0.4 to 0.8. In the superalloy of our present preference that ratio is 0.62.
Briefly described in its composition of matter aspect, the present invention is a cobalt-base superalloy having a unique combination of properties at high temperature ~ .
~ `

~6~2 and consequent special utility in the production of industrial gas turbine hot yas path components, which alloy consists essentially of 0.3-0.6% carbon, 27-35% chromium, 9-16% nickel, 6-9% tungsten, up to 3% hafnium, .45-2.0%
tantalum, up to .7% zirconium, up to .5%
titanium, up to 1% manganese and silicon, up to .05% boron, up to 2.0% iron, remainder essentially cobalt. An additional important requirement is that the carbide-forming elements be so selected as to satisfy the relationship stated above and represented by the following equation:

Atomic Percent (Ta+Hf+Ti+Zr) 0 4 to 0 8 Atomic Percent C

Similarly described in its article-of-manufacture aspect, the present invention is a cast cobalt-base superalloy industrial gas turbine nozzle consisting of the new alloy set forth immediately above. Also, in this aspect the invention takes the form of transition pieces and shrouds, and of a fabricated cobalt-base superalloy gas turbine combustion chamber comprising a plurality of sheets of the said new alloy rolled and formed in predetermined shape and assembled and welded together.
Brief Description of the Drawings In the drawings accompanying and forming a part of this specification., Figure 1 is a view in perspective of an 5].DV2~1 industrial g~s turbine nozz].e of this invention;
Figure 2 is a Larson-Mi].ler p].ot of the stress-rupture properties of an a].loy of U.S. Patent 3,383,205 and one of this invention;
Figure 3 is a chart beariny curves illustrating varestraint welding test results of tests on five alloys of this invention and two prior art alloys including that of U.S. Patent 3,3~3,205 treated in Figure 2, total crack length in mils being plotted against percent augmented strain; and Figure 4 is a view in perspective of an industrial gas turbine transition piece of this invention.
Detailed Description of the Preferred Embodiments ~Yhile our present preference is to prepare these new alloys by the vacuum melting and vacuum casting procedure, we alternatively contemplate usiny the air melting, air casting approach. Additions of hafnium, titanium, zirconium and tantalum are made in ; 20 the former while e~_=bi=h=~ tantalum and optionally hafnium are employed in the air melting case. In any event the amounts of these additives used in producing the alloys of this invention are carefully controlled to insure that the cast or fabricated products of these alloys have all the desirable characteristics described above. Likewise, the best practice along each of these two lines involves controlling the amounts of the elements other than these severa].
monocarbide MC-carbide formers as to both the ranges of the major constituents and the maximum amounts o~
the minor or impurity e].ements such as iron, manganese, silicon and boron.
As stated above and shown be].ow, the consequence of failure to exert such control is the loss of one or more of the important advantages of gz this invention. The excellent weldability of these new alloys are forfeited, for example, when the amounts of monocarbide MC-carbide formers used are not in balance with the alloy carbon content as described above and set forth in the appended claims. Further in this regard the chromium content of these alloys is preferably targeted at 28-30% in recognition that departures in each direction can penalize alloy properties, specifically amounts less than about 27% result in loss of oxidation and hot corrosion resistance and amounts greater than about 35% result in loss of ductility without offsetting gain in either oxidation resistance or hot corrosion resistance.
The cast and fabricated bodies of this invention being components of industrial gas turbines are quite different from aircraft jet engine components especially in respect to size and mass. Because of this, they represent problems unlike those of the relatively lighter weight counterparts such as marked cracking tendency associated with welding operations.
This has significant implication for cast as well as fabricated industrial gas turbine components as it would obviously be highly desirable to be able to weld repair industrial gas turbine nozzles to avoid the time and expense of replacement. Gaining this advantage without forfeiting any other constitutes an important advance in the art. Likewise, the opportunity to build industrial gas 51~V2~9 turbine combustion chamber structures by weldiny preformed sheets or p]ates together which is enabJed as a resu].t of this invention, its al].oys having exceJ.lent we]dability, is an important new advance in the production of industrial gas turbines. In our practice of such welding operations as these we prefer to use the gas tungsten arc technique and equipment in general use in industry in the fabrication of both ferrous and nonferrous metal structures, inc]uding those of cobalt-base superal].oys.
The first stage nozz]e 10 of an industrial gas turbine shown in Figure ]. is a casting of our preferred alloy composition produced by the injection molding and investment castiny technique in general use in the art~ Also, the shape and size and the design details of nozzle 10 essentially duplicate those features of the present standard first stage nozzle. Transition piece 20 similarly resembles that which has long been in general use in industrial gas turbines differing importantly, however, in that it is constructed of parts of an alloy of this invention welded together to provide a strong crack-free assembly of integra].ly bonded elements. Thus, bracket 22 is fitted in place on body 23 and welded securely and fixed tightly thereto.
q'hose skilled in the art will gain a further and better understanding of this invention and its important new advantages and results from the following illustrative, but not limiting, examples.
Example I
Investment castings for test purposes were made of a commercial cobalt-base aJ.loy of the following ana].ysis:

9~
51~V2819 carbon 0.25 chromium 29.0 nickel 10.0 tungsten 7.0 manganese 0.7 silicon 0.7 phosphorus 0.02 sulphur 0.02 iron 1.0 boron 0.015 cobalt remainder This superalloy is disclosed and claimed in U.S. Patent 3,383,205 assigned to the assignee hereof and has long been in general use in the production of industrial gas turbine hot stage components, particularly cast non-rotating parts such as first stage nozzles.
~he cast test specimens were subjected to . standard tensile, creep rupture and varestraint i 20 weldability tests, the tensile and 5 ~ rupture data being set out in Table I and the varestraint data illustrated in Figure 2. Curve A of Figure 2 illustrates the Larson-Miller data and curve AA of Figure 3 represents the varestraint data.
Example II
A cobalt-base superalloy of this invention was tested in a duplication of the test conditions and procedures of Example I the superalloy having the ~ollowing analyses:
carbon 0.357 chromium 28.56 nickel 10.88 tungsten 7.33 tantalum 0.53 hafnium 1.00 , 5 lDV2 8 ]. g zirconium 0.496 titanium 0.1~4 iron 0.270 silicon 0.024 sulphur 0.004 phosphorus ~ 0.005 manganese ~ 0.005 cobalt remainder The resulting test data are set forth in Tables I, 2 and 3 for a ready comparison with those of Example I and those detai].ed below. Curve B of Figure 2 illustrates the Larson-Miller data and curve BB of Figure 3 represents the varestraint data.
Further, this superalloy was found on the performance of standard tests to have the superior oxidation and hot corrosion resistance of the cobalt-base alloy of ~xample I.
Examp].e III
The same experimental tests were carried out on four additional superalloys of this invention of the following compositions:

A].loy A Al].oy B A].lo~ C A].lo~ D

Carbon0.25 0.25 0.35 0.35 Manganese 0.70 0.70 0.70 0.70 Silicon0.75 0.75 0.75 0.75 Phosphorus < 0O04 ~ 0.04 ~ 0.04 < 0-04 Sulphur< 0.04~ 0.04~ 0.04 ~ 0.04 Chromium28.0 28.0 29.0 29.0 ~ickel10.0 10.0 10.0 10.0 10 Tungsten7.0 7.0 7.0 5.0 Iron~ 0.5 ~ 0.5 < 0-5 < 0 5 ~irconium Hafnium Titanium lS Columbium 0.5 1.0 1.0 1.25 Tantalum0.5 0.5 0.5 Boron 0.01 CobaltREM REM REM REM
Again the test data developed in measuring the properties of these alloys as described above are stated in Tables 1, 2 and 3.
Example IV
Another superalloy of the prior art of the cobalt-base type was likewise tested as to the foregoing properties with the results stated in the three tables below, this particular alloy (Alloy E) being of the following compositions:

51DV28].9 -- ].1 --Carbon 0.35 Manganese 0.70 Silicon 0.75 Phosphorus < 0.04 5ulphur< 0.04 Chromium29.0 Nicke].10.0 Tungsten7.0 Iron~ 0.5 Zirconium 0.20 ~afnium Titanium0.]5 Columbium 0.25 Tanta].um Boron0.01 CobaltRE~
In regard to the tests carried out in the course of this experimental work to measure the properties of these various alloy compositions, as indicated above, standard test procedures were followed in every instance and the same procedures were applied for each respective alloy in the sev~ral tests so that comparisons could be made directly and conclusions could be drawn from the resulting data which were reliable. The ASTM procedures were used, , ~ therefore, in the tensile andS~ rupture tests and ..~ .
`~ in the case of the vare~traint test the procedure followed was that described in Welding Research Council Bulletin 280 in the artic].e entitled "The Varestraint Test", C. D. Ludlum, et al, August ].982.

51DV2~19 ~1 .
~i co ~

~ ~ O
U~ O o ~ t ~D CO O ~7 --t N ~ ~ N_t N~1 N
r _ ~r ~ _ t ~ ~ r~ ~`t ~ Cr~ ~ O
U~
~ q~ ,~ er ~o _t~ ~ ~`t a~
.a t:l ~ ~ C~ t o~
~ _ C~ N _t _t N N t~ _t tl~ ~`t U~ ~rt ~ t t ~ t _t r t O O O C~ O O O O O ~ O
~ o In 1~ t~~ ~ ~ ~ t~

~O ~ O
H ' ~ t ~ ~t ~ ` -at ~` '`t , C~O o o o ~:~ er a,t tn ut ~i_ ~i ~ o o t ~ o u~t ~t -: ~:7 o o I t~ r t~
3~
~ H t, H H H~H, ~;
~ .. , ~ " .' -1 3- 51~V2819 .~ _ 5 ~ ooo ooo ooo ~ ~ ~ o ~ ~ ~io o ~D O

5-1 ~ o oo ~ o o o ~ o ~ ~ In o c~ o ~ ~ cO o ~ ~D ~ O ~ ~ ~
~ ~ l~ o a~ r o~

1~1 ~ o co ~1~ o ~ 1~ ~ t~ U~ l 01`
G O O ~ O u7 ,1 ~r o a~ a~ Ln ~ In ~ o o ~ D O 00 r~
.~ N N
i~
O I

~3~P
O ~ D O O ~ o O O ~ O ~ ~a o o eP U~ O O ~ O
iIrl O ~ 7 O Irl L"l Irl O L'~ L') It~ O Lr~ O L~ 7 L~l O L~
~ ~ ri _i ~ O ~ i 0 r~ ~i ~ O ~ J O _i _i ~ O ~

H H H H ~;

As evident from Table I, the superalloys of this invention (Examples II and Examples IIIA-D) have ultimate tensile strenyths equal to or better than the commercial superalloy of Example I and have stress rupture strength substantially greater than that commercial superalloy. Further it is apparent from Table I that these new superalloys have good room temperature tensile elongation characteristics and as Table II shows and Figure 3 graphically illustrates, the weldability of the superalloys of this invention is superior to commercial superalloys A and E and even spectacularly so in the case of the superalloy of Example II which as indicated above is our present preferred embodiment of the invention. It will also be noted that as indicated in parentheses on that chart, the superalloys of thls invention set forth in Examples II and III have carbideformer-carbon atomic percent ratios within the above prescribed critical range of 0.4 to 0.8, while the prior art alloys of Examples I and IV do not come close to meeting that important requirement.
It should be further understood that the stress-rupture properties of these new superalloys can be substantially increased by special and critical heat treatment. Moreover this can be accomplished without penalizing the superior weldability or the hot corrosion resistance of these new superalloys. Thus by solutioning substantially the M23C6 eutectic carbide phase of a casting of one of these new superalloys and thereafter aging the body to precipitate the carbide phase in fine particulate form distributed substantially uniformly throughout the casting microstructure, the stress-rupture strength and the tensile strength are critically increased as the benefits of the carbide constituent of the

2~

superalloy are maximized and the usual detriments of carbon are minimized or totally eliminated.
Briefly described in its composition of matter aspect, the present invention is a cobalt-base superalloy having an unique combination of properties at high temperature and consequent special utility in the production of industrial gas turbine hot gas path components, consisting essentially of 0.3-0.6% carbon, 27-35% chromium, 9-16% nickel, 6-9% tungsten, up to 3%
hafnium, 0.45-2.0% tantalum, up to .7% zirconium, up to 0.5% titanium, up to 1.0% manganes2 and silicon, up to .05% boron, up to 2.0% iron, remainder essentially cobalt. An additional important requirement is that the monocarbide MC-carbide former elements be so selected as to satisfy the relationship stated above and represented by the following equation:

Atomic Percent (Ta+Hf+Ti+Zr) 0 4 to 0 8 Atomlc Percent C
and Atomic Percent Ta _ >o 1 Atomic Percent C
Similarly described in its article-of-manufacture aspect, the present invention is a cast cobalt-base superalloy industrial gas turbine nozzle consisting of the new alloy set forth immediately above. Also, in this aspect the invention takes the form of transition pieces and shrouds, and of a fabricated cobalt-base superalloy gas turbine combustion chamber comprising a plurality of sheets of the said new alloy rolled and formed in predetermined shape and assembled and welded together. Still further in both cast and fabricated form these new bodies have an unique combination of properties attributable to special critical heat treatment. In particular, they have excellent hot corrosion resistance, stress-rupture strength at hiyh temperature, metallurgical stability, tensile ductility and weldability.
The method aspect of the present invention, likewise described in broad general terms, comprises the steps of casting a superalloy of this invention and subjecting the resulting cast body to elevated temperatures and thereby substantially solutioning the eutectic carbide phase (M23C6), and thereafter subjecting the cast body to a substantially lower temperature well above room temperature to precipitate the carbide phase in fine particulate form distributed substantially uniformly throughout the microstructure of the cast body.
It should also be understood that columbium, another strong monocarbide MC-carbide former, is not a preferred addition to these new superalloys because of its detrimental effect on superalloy hot corrosion resistance and because it is not necessary for the purposes of this invention.

Claims (17)

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

1. A cobalt-base superalloy having a unique combination of desirable properties at high temperature and consequent special utility in the production of industrial gas turbine hot gas path components including nozzles and combustors, said superalloy consisting essentially of, by weight:
0.3 to 0.6 percent carbon, 27 to 35 percent chromium, 9 to 16 percent nickel, 6 to 9 percent tungsten, 0.45 to 2.0 percent tantalum, up to 0.5 percent titanium, up to 3.0 percent hafnium, up to 0.7 percent zirconium, up to 1.0 percent manganese, up to 1.0 percent silicon, up to 0.05 percent boron, up to 2.0 percent iron, Balance cobalt, the carbon (C), tantalum (Ta), hafnium (Hf), titanium (Ti) and zirconium (Zr) being so selected as to satisfy the following equation:

= 0.4 to 0.8.

2. A cobalt-base superalloy of claim 1 in which the atomic percent ratio of carbide-forming element to carbon is about 0.65.

3. A cobalt-base superalloy of claim 1 which contains about 0.35% carbon, about 29% chromium, about 10% nickel, about 7% tungsten, about 0.5%
zirconium, about 0.2% titanium, less than 0.01%
manganese, less than 0.07% silicon, about 1.0%
tantalum, less than about 0.4% iron, about 0.5%
hafnium, remainder essentially cobalt.

4. An industrial gas turbine nozzle of cobalt-base superalloy having excellent hot corrosion resistance, creep strength and stress rupture strength at high temperature, metallurgical stability, tensile ductility and weldability, said superalloy consisting essentially of, by weight:
0.3 to 0.6 percent carbon, 27 to 35 percent chromium, 9 to 16 percent nickel, 6 to 9 percent tungsten, 0.45 to 2.0 percent tantalum, up to .05 percent titanium, up to 3.0 percent hafnium, up to 0.7 percent zirconium, up to 1.0 percent manganese, up to 1.0 percent silicon, up to 0.05 percent boron, up to 2.0 percent iron, Balance cobalt, the carbon (C), tantalum (Ta), hafnium (Hf), titanium (Ti), and zirconium (Zr) being so selected as to satisfy the following equation:

= 0.4 to 0.8.

5. A cobalt base superalloy consisting essentially of:
0.357 percent carbon, 28.56 percent chromium, 10.88 percent nickel, 7.33 percent tungsten, 0.53 percent tantalum, 1.00 percent hafnium, 0.496 percent zirconium, 0.184 percent titanium, 0.270 percent iron, 0.024 percent silicon, 0.0004 percent sulfur, 0.005 percent phosphorus, 0.005 percent manganese, cobalt remainder.

6. An industrial gas turbine nozzle made of cobalt-base superalloy having excellent hot corrosion resistance, and stress-rupture strength at high temperature, metallurgical stability, tensile ductility, and weldability, said superalloy consisting essentially of:
0.357 percent carbon, 28.56 percent chromium, 10.88 percent nickel, 7.33 percent tungsten, 0.53 percent tantalum, 0.184 percent titanium, 1.00 percent hafnium, 0.496 percent zirconium, 0.005 percent manganese, 0.024 percent silicon, 0.005 percent phosphorus, 0.270 percent iron, cobalt remainder.

7. A fabricated industrial gas turbine transition piece made of cobalt-base superalloy comprising a plurality of sheets rolled and formed in predetermined shape and assembled and welded together to define the piece, said superalloy consisting essentially of:
0.357 percent carbon, 28.56 percent chromium, 10.88 percent nickel, 7.33 percent tungsten, 0.53 percent tantalum, 0.184 percent titanium, 1.00 percent hafnium, 0.496 percent zirconium, 0.005 percent manganese, 0.024 percent silicon, 0.005 percent phosphorus, 0.270 percent iron, cobalt remainder.

8. The method of producing a cobalt-base superalloy body having an unique combination of superior stress rupture strength and weldability properties and consequent special utility in application to industrial gas turbine hot gas path components which comprises the steps of casting in desired size and shape a superalloy consisting essentially of, by weight:
0.3 to 0.6 percent carbon, 27 to 35 percent chromium, 9 to 16 percent nickel, 6 to 9 percent tungsten, 0.45 to 2.0 percent tantalum, up to 0.5 percent titanium, up to 3.0 percent hafnium, up to 0.7 percent zirconium, up to 1.0 percent manganese, up to 1.0 percent silicon, up to 0.05 percent boron, up to 2.0 percent iron, Balance cobalt, the carbon, tantalum, hafnium, titanium and zirconium being so selected as to satisfy the following equation:

= 0.4 to 0.8.

9. The method of claim 8 wherein the following equation is satisfied:

= >0.1 subjecting the resulting cast body containing M23C6 eutectic phase to elevated temperature and thereby solutioning substantially all M23C6 eutectic phase, thereafter cooling the said body and thereby precipitating substantially all the M23C6 carbide phase in the form of fine particulate distributed substantially uniformly throughout the body microstructure.

10. The method of claim 9 in which the cast body is subjected to temperature about 2250°F until solutioning of the eutectic phase is substantially complete, thereafter subjecting the body to temperature of approximately 1475°F until precipitation of the M23C6 particulate phase is substantially complete and finally cooling the body to room temperature .

11. The method of claim 10 in which the body is air cooled from solutioning temperature to about room temperature and thereafter is heated to precipitation temperature and upon completion of the precipitation of the particulate carbide phase the body is finally air cooled to room temperature.

12. The method of claim 11 in which the solutioning temperature is 2250°F and the body is maintained at that temperature for approximately 4 hours, and in which the precipitation temperature is about 1475°F and the body is maintained at that temperature for about 8 hours.

13. The method of claim 12 in which the superalloy consisting essentially of:
0.357 percent carbon, 28.56 percent chromium, 10.88 percent nickel, 7.33 percent tungsten, 0.53 percent tantalum, 1.00 percent hafnium, 0.496 percent zirconium, 0.184 percent titanium, 0.270 percent iron, 0.024 percent silicon, 0.0004 percent sulfur, 0.005 percent phosphorus, 0.005 percent manganese, cobalt remainder.

14. The superalloy of claim 5 wherein said superalloy has microstructure characterized by substantially all M23C6 eutectic carbide phase being in the form of fine particulate distributed substantially uniformly throughout the superalloy transition piece microstructure.

15. The nozzle of claim 6 wherein said nozzle has microstructure characterized by substantially all M23C6 eutectic carbide phase being in the form of fine particulate distributed substantially uniformly throughout the superalloy transition piece microstructure.

16. The transition piece of claim 7 wherein said transition pieces has microstructure characterized by substantially all M23C6 eutectic carbide phase being in the form of fine particulate distributed substantially uniformly throughout the superalloy transition piece microstructure.

17. The cobalt-base superalloy as claimed in claim 1, 3 or 4 wherein the following equation is satisfied:

= >0.1.