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

SUPERALLIAGE ET PIECE MOULEE, ET COMPOSANTS SOUDES POUR TURBINE A GAZ INDUSTRIELLE FAITS DUDIT SUPERALLIAGE

English 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:

Image = 0.4 to 0.8.

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:

Image = 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.

- 18 -

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:

Image = 0.4 to 0.8.


- 19 -

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.

- 20 -

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,

- 21 -

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:

Image = 0.4 to 0.8.

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

Image = >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

- 22 -
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

- 23 -
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:

Image = >0.1.

Description

51DV2819
]
~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
-- 2 --
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


- 3 - 51DV2819

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
- 4 - 51DV2819

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
-- 5 --
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
- 6 - 51DV2819

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
-- 7
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
-- 8 --
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
- 9 -
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:

51DV2819
-- 10 --
A].loy A Al].oy B A].lo~ C A].lo~ D
481 482 483 485
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
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H H H H ~;

- 14 - 51DV2819

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~
- 15 - 51DV2819

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

- 16 - 51DV2819

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.