Note: Descriptions are shown in the official language in which they were submitted.
-5781/CAN ~ 736
This invention relates to nickel-base superalloys
which are particularly suitable for the production of cast
parts for use at elevated temperatures in corrosive
atmospheres, such as, for example, in gas turbines.
State of the Art
The continual demand by gas turbine manufacturers
for alloys with improved high temperature properties has
lead to extensive development work. One proposal disclosed
in U.K. specification No. 1,395,125 to improve the high
temperature properties of a wide range of nickel-base
superalloys was to control the carbon and boron contents
such that the carbon content was maintained at a relatively
low level whereas the boron content was between 0.05 and
0.3% which is considerably above that normally employed;
preferably the boron content did not exceed 0.25~, the
most preferred range being from 0.05 to 0.15%.
Discovery
We have now surprisingly found that with certain
nickel-base alloys in which the alloying constituents are
carefully controlled and closely correlated with each
other, improvements can be obtained with boron contents
greater than 0.3% and up to as high as 1.2~.
Objects
It is an object of the present invention to
provide improved alloys and castings.
Other objects and advantages will become
apparent from the following description.
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Description
Accordingly the present invention provides an a]loy
containing, by weight, from 14 to 22% chromium, from 5 to
25% cobalt, from 1 to 5% tungsten, from 0.5 to 3% tantalum,
from 2 to 5% titanium, from 1 to 4.5% aluminum, the
sum of the titanium plus aluminum being about 4.5~ to 9%,
from 0 to 2% niobium, 0.31 to 1.2% boron, from 0 to 3.5
molybdenum, from 0 to 0.5% zirconium, from 0 to 0.2%
in total of yttrium or lanthanum or both, and from 0 to
0.1% carbon, the balance, apart from impurities, being nickel.
The alloys must contain at least 14% chromium for
good corrosion resistance but no more than 22% chromium in
order to minimize the risk of detrimental sigma phase
formation during extensive high temperature service.
Preferably the chromium content is from 15 to 21~, for
example from 15 to 17% or from 19 to 21~. The presence in
the alloys of from 5 to 25% cobalt has a strengthening
effect but more than 25% cobalt could lead to sigma phase
formation. Preferably the cobalt content is from 5 to 22~,
for example 7 to 20%.
The presence of tantalum, titanium, aluminum and
niobium also has a strengthening effect on the alloys.
At least 0.5% tantalum must be present, preferably 0.8 to
2.5%, for example 1.0 to 2.0% but more than 3% leads to
embrittlement. Niobium can be optionally present in an
amount up to 2% and preferably is present in an amount of
at least 0.2 or 0.5%. However more than 2~ can cause
embrittlement and the niobium content preferably does not
exceed 1.5%.
~9~7;~i
The titanium and aluminum contents must be in
the ranges 2 to 5% and 1 to 4.5%, respectively, with the sum
of the percentages of titanium and aluminum being about
4.5 to 9 and preferably no more than 8.5~. More than
the maximum of either of these elements leads to embrittlement
and preferred titanium contents are from 2.5 to 4.5%, for
example 3 to 4%, with preferred aluminum contents being
from 1.5 to 4%, for example 1.8 to 3.8~.
For optimum stress rupture properties the titanium,
aluminum, noibium, tantalum and chromium contents are
preferably correlated such that:
%Ti + %Al + %Nb + 0.5 (~Ta) + 0.2 (%Cr) = 11.2 to 12.4.
The boron content is critical for achieving the
alloys' excellent properties and must be present in amounts
at least 0.31%, e.g., at least 0.35% but not exceeding 1.2%.
Contents outside this range lead to a reduction in stress
rupture life properties. Preferably the boron content
is at least 0.4% preferably from 0.4 to 1%, for example
0.5 to 1~. Coupled with this boron content, the carbon
should be kept as low as possible and must not exceed 0.1~,
preferably not more than 0.05% and most advantageously
not more than 0.03~, as this also leads to a reduction in
stress rupture life properties.
Tungsten and molybdenum, when present, contribute
to strength. Tungsten must be present in an amount of from
1 to 5%, preferably from 1.5 to 4%, for example 1.8 to 3~,
and molybdenum must not be present in amounts greater than
3.5%. Preferably the molybdenum content is at least 0.2%
but no more than 2%. Zirconium improves strength and
ductility of the alloy and can optionally be present in an
amount not exceeding 0.5%. A suitable zirconium range is
from 0.01 to 0.3%, preferably 0.02 to 0.2%.
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Yttrium or lanthanum or both may be present up
to 0.2% in total for improved ductility. However, more
than 0.2~ leads to inadequate ductility.
Overall, for optimum properties it is preferred
that the boron content is more than 0.6% and a preferred
alloy contains either from 15 to 17% chromium with 7 to 10
cobalt, or from l9 to 21% chromium with 13 to 17% cobalt,
from 2.1 to 2.8% tungsten, from 1.4 to 2.0% tantalum, from
3.2 to 4.0~ titanium, from 2.2 to 3.8~ aluminum, from 0.5 to
1.5~ niobium, from 0.6 to 1.0~ boron, from 0.2 to 2.0~
molybdenum, from 0.03 to 0.08% zirconium, from 0 to 0.2% in
total of yttrium or lanthanum or both, and from 0 to 0.03%
carbon, balance nickel.
In addition, in most preferred alloys the titanium,
aluminum, niobium, tantalum and chromium correlation stated
above should be applied.
Of the elements that may be present as impurities,
silicon has a deleterious effect on corrosion resistance
and should be kept below 1% and preferably below 0.5~.
Other impurities may include up to l~ mannanese and up to
3% iron together with additional elements which are commonly
associated with alloys of this type and which do not have
a detrimental affect on their properties.
To develop the full stress rupture properties of
the alloys of the invention, they should be subjected to
a heat treatment comprising solution-heating and subse~uent
ageing. The solution treatment advantageously comprises
heating for from 1 to 12 hours at a temperature in the
range of from 1100 to 1180C and the alloys may then be
aged by heating for from 8 to 48 hours at a temperature
736
in the range of from 800 to 900C. The single final
ageing treatment may advantageously be replaced by a two
stage ageing treatment comprisina heating for from 4 to 24
hours at a temperature in the range of from 900 to 1000C
followed by heating for from 8 to 48 hours at a temperature
in the range of from 700 to 800C. Cooling after each
heat treatment stage may be carried out at any convenient
rate, and air cooling is generallv suitable.
In thé heat treated state alloys according to the
invention have minimum stress rupture lives which to some
extent decrease with increasing chromium content. Thus for
a chromium content of 15 to 17% the alloys would have a
stress rupture life of at least 260 hours at 550 M/mm~
and 760C and for a chromium content of 19 to 21~ would
have a stress rupture life of at least 200 hours at 550 N/mm2
and 760C. However, it should be noted that a surprisina
feature of the invention is that with the most preferred
alloys the best results appear to be obtained with the
higher chromium contents.
The fact that the alloys of this invention possess
an excellent combination of properties including stress
rupture properties coupled with corrosion resistance in
particular is illustrated by the following examples.
Examples
A number of alloys with compositions shown in
Table I were vacuum melted and cast in vacuum to tapered
test bar blanks from which test pieces were machined.
Prior to the machining of the test pieces, the blanks were
heat treated by solution heating at 1121C for 2 hours,
air cooling, and ageing at 843C for 24 hours, and air
cooling in respect of Alloys A and 1 to 4, and by solution
heating at 1160C for 4 hours, air cooling, and ageing at
850C for 16 hours and air cooling in respect of Alloys B,
73~
5 and 6. The heat treated test pieces were then subjected
to various stress rupture tests with the results shown
in Table II. In Tables I and II Alloys 1 to 6 are
according to the present invention and Alloys A and B
are comparative alloys outside the scope of the present
invention.
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736
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It can be seen from a comparison of the results
in Table II that the lower chromium content Alloys l to 4
had better stress rupture life and elongation properties
over the entire range of test conditions employed than
Alloy A. Similarly the higher chromium content Alloys
5 and 6 also had better stress rupture life and elonga-
tion properties, at the test conditions employed, than
Alloy B.
Considering the lower chromium content Alloys
1 to 4 containing nominally 16% chromium, it can be seen
from the results of Table II, that the stress rupture
properties increase with increasing boron content at
550 N/mm2/760C, peak at about 0.60~ boron at
330 N/mm2/816C and are generally good over the whole
boron range at 228 N/mm2/927C. The higher chromium
content Alloys 5 and 6 containing nominally 20% chromium
showed improving stress rupture properties with increas-
ing boron content up to 0.80%. Thus for optimum stress
rupture properties it is preferred that alloys according
to the invention should contain between 0.4, preferably
0.5, and 1.0% boron.
Further tests were conducted to compare the
properties of Alloy No. 7 (being a preferred alloy of the
invention) with a known commercial alloy (Alloy C, avail-
able under the designation IN-792). The compositions of
both Alloys are shown in Table III. Again test pieces
were prepared by vacuum melting and casting in vacuum
to produce tapered test bar blanks from which test pieces
could be machined. The heat treatment used in these
further tests, prior to machining was a simple two stage
~09~736
treatment comprising solution heating for four hours at
1150C and air cooling followed by a~eing at 850C for
16 hours and air cooling.
TABLE III
Alloy Cr C Co Mo W Nb Ta Ti A1 zr s Ni
7 20.5 0.021 15.0 0.53 2.31 0.98 1.63 3.70 2.64 0.065 0.79 Bal
7 20.0 0.01 15.0 0.50 2.2 1.0 1.5 3.6 2.5 0.05 0.80 Bal
(modified)
C 12.6 0.125 9.0 1.98 3.91 - 3.95 4.30 3.62 0.08 0.018 Bal
The heat treated test pieces were then subjected to
various standard stress-rupture tests, the results of
which are shown in Table IV.
-- 10 --
3 Cli9~736
TABLE IV
StressTest Alloy No. 7 Alloy C
(N/mm2)(C)
Life Elong.Life Elong.
620 760 498 2.7 161 5.2
550 " 1797 2.5 499 5.2
500 ">2089 1668 2.6
545 816l33x 2 5X
414 816 581 3.1 543 6.0
400 " 873 5.2 917 4.4
345 " 2461 3.6 2085 3.7
330 ~3404x 1 7x
300 ">2785 >1439
269 927 97 8.2 133 8.2
228 " 199 4.7
200 " 516 6.2 692 8.2
154 ">1336 ~985
152 980185X 6.8x
x modified Alloy No. 7
~0~736
It should be noted that two different heats of Alloy No. 7
were used in these tests and it is shown in Table IV which
heat was employed for each particular test.
These latter test results demonstrate that in
general Alloy No. 7 of the invention has a strength which is
at least equivalent to and, in some cases, significantly
superior to that of Alloy C (IN-792), particularly at lower
tempertures, for example 760C, which has hitherto always
been considered to be an extremely strong alloy. In addi-
tion, the ductility of Alloy No. 7 (based on a comparison of
the elongation figures) is in general equivalent to that of
Alloy C with the exception of that at 760C where the
strength of Alloy No. 7 is superior.
Comparison of these stress rupture test properties
of Alloy No. 7 with published data of another commercially
available alloy sold under the designation IN-100 also shows
the superiority of Alloy No. 7 at 760C and at least equality
at 816C, 927C and 980C.
In addition to the high strength of the alloys of
the invention, they are also characterized by high corrosion
resistance. This fact is demonstrated by crucible tests in
which standard size cylindrical samples of Alloy No. 7 were
immersed in a 25% sodium chloride, 75% sodium sulphate solu-
tion.
In a first test of 900C for 300 hours with the
salts being replenished after 150 hours, the weight loss of
the sample after descaling was found to be as low as 2 mg/cm2.
In a more aggressive test at the same temperature in which
the salt was replenished every 24 hours, the weight loss was
also very low at 16 mg/cm2.
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'6~
~0~736
B~ comparison, a similar sample of comparative
Alloy C (IN-792) was found to have corroded extremely
badly after only 48 hours in a test at 850C, with a
weight loss of 562 mg/cm~.
The alloys of the invention may be used in cast
or wrought form for high temperature uses such as for gas
turbine en~ine parts, for example rotor or stator blades
and integrally bladed discs.
The heat treatments described above to develop
1~ the properties of the allovs may be supplemented by other
more complex treatments which are known to be appropriate
to alloys of this type. In addition to normal casting
techniques, other techniques such as unidirectional
solidification may be employed if desired.
While the present invention has been described
with reference to the foregoing embodiments, these
embodiments are not to be taken as limiting since persons
skilled in the art will appreciate that modifications
and variations can be resorted to without departing from
the spirit and scope of the invention.
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