Note: Descriptions are shown in the official language in which they were submitted.
PC-5740
103~Z05
BACKGROUND~OF THE INVENTION
This invention relates to low-expansion nickel-iron
alloys and is concerned with the provision of such an alloy which
in the cast form has high strength and low thermal expansion char-
acteristics at service temperatures.
It is known that certain nickel-iron alloys have a
remarkably low coefficient of thermal expansion such as, for example,
an alloy of 36% nickel and 64% iron known under the trade name "Invar"
which has a coefficient of thermal expansion approaching zero over
the temperature range O to around 200 C. A major problem with low-
expansion nickel-iron alloys is their low strength. One method by
which the strength of such alloys can be increased is by the addition
of elements such as aluminum, titanium or niobium, and a subsequent
ageing treatment.
Titanium has generally been added in amount of between
0.75% and 2.5% by weight to increase the strength of wrought alloys,
and we have found that to achieve an increase in strength to comparable
levels in cast alloys requires the addition of rather more titanium,
that is, between 1.5 and 5% titanium by weight. However, as is well
known, the increase in strength resulting from titanium additions is
achieved at the expense of the low coefficient of thermal expansion
which is increased in proportion to the increasing titanium content
I of the alloy.
! Surprisingly we have now found that an optimum balance
between high strength and a low coefficient of thermal expansion at
temperatures in the range of 20 to 300 C can be achieved in a cast
and aged nickel-iron alloy strengthened with titanium, by correlating
the nickel and titanium contents and optional cobalt and niobium contents
according to a specific relationship.
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It ls an ob~ect of the present invention to provide im-
proved age-hardened iron-nickel-titanium alloys with predetermined
low expansion characteristics whicll have high mechanical strength.
It is another object of this invention to provide low
expansion iron-nickel-titanium alloys which may contain cobalt,
and/or niobium and in which concentrations of nickel, cobalt,
titanium and niobium are controlled and correlated.
It is a further obJect to provide low expansion iron-nickel-
titanium alloys having in the as-cast and aged condition a thermal
expansion coefficient between 20 and 300C of less than about
6 x 10 6/oC and preferably less than about 5 x 10 6/oC, and a 0.2%
proof stress at 20C higher than about 350N/n~ .
A still further object is to provide an alloy having low
expansivity and high strength at working temperatures - which can be
cast directly into intricately-shaped castings with good surface
properties.
The invention also contemplates providing stnlctural com-
ponents of machinery, for example turbine shafts and blades, in which
close dimensional tolerances must be maintained at temperatures up
to about 500C, made of cast, age-hardenable alloys.
Other objects and advantages will become apparent from the
following description and examples.
THE INVENIION
According to one aspect of the invention there is provided
an alloy which, when in the as-cast and aged condition, has a thermal
expansion coefficient between 20 and 300C of less than 6 x 10 6/oC and
a 0.2% proof stress at 20C higher than 350 N/mm (Newtons per square
millimeter), comprising by weight up to about 47%, e.g. from about 27
to about 47% nickel, from above 5% to about 16% cobalt, from about 1 to
about 4% titanium, up to about 1.5~ niobium, the contents of nickel,
cobalt, titanium and niobium being such that:
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~03~Z05
%Ni + 0.7(%Co) - 1.25 [%Ti + 0.35(%Nb)] - 2 (%Ti) / (%Ti + ~Nb)
= 37 to ~0,
and the balance, apart from impurities and residual elements,
being essentially iron.
According to another aspect of the invention there is
provided an alloy which, when in the as-cast and aged condition, has
a thermal expansion coefficient between 20 and 30QC of less than
5 x 10 /C and a 0.2% proof stress at 20C higher than 350N/mm ,
comprising by weight up to about 47%, e.g. from about 27 to about 47%
nickel, from above 5% to about 16% cobalt, from about 1 to about 4
titanium, up to about 1.5% niobium, the contents of nickel, cobalt,
titanium and niobium being such that:
%Ni + 0.7 (%Co) - 1.25 [%Ti + 0.35(%Nb)~ - 2 (%Ti)/~%Tî + ~Nb?
= 37 to 39,
and the balance, apart from impurities and residual elements,
being essentially iron.
Alloys according to the invention may also contain by
weight, up to about 1%, e.g. up to about 0.3% silicon, up to about
a. 4% manganese and up to about 0.2% aluminum and not more than about
0.1% carbon. The presence of silicon, manganese and/or aluminum is
particularly beneficial when the alloys are to be produced by melting
in air.
The attainment of high strength in alloys according to the
invention depends upon precipitation hardening by the formation of a
precipitate, Ni3(Ti), when ageing the alloy at elevated temperature.
The ageing treatment preferably carried out in the temperature range
550 to 700C, with the optimum temperature being dependent upon the
titanium content of the alloy. For lower levels of titanium content,
optimum properties may be achieved after heat treating at the lower
end of the temperature range, e.g. 575 to 625C for about 24 hours,
whereas for higher levels of titanium content, a heat treatment of
about 24 hours at the higher end of the temperature range e.g. 625 to
675C, may give optimum properties.
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The tensile strength of alloys according to the invention
is thought to be a function of the titanium content. It will be
noted that Canadian Patent No. 991,889 discloses nickel-cobalt-iron-
titanium alloys having low expansion characteristics and high strength
in the wrought condition. Approximately twice the level of titanium
is required in the cast and aged alloy to give comparable strengths
to similar alloys in the wrought and aged state. For example, an
iron-base alloy containing nominally 34% nickel and 13% cobalt would
require approximately 3% titanium to achieve in the cast condition
the strength achieved in the wrought condition by the same iron-base
alloy containing approximately 1.5% titanium. The alloys of the
invention con~ain titanium in an amount of between 1 and 4% by weight,
preferably between 1.5 and 3.5% and most preferably between 1.7 and
2.7%. Although niobium i6 not essential for obtaining the required
properties, it can be added in an amount of up to 1.5% to help in the
achievement of good mechanical properties.
The nickel content of alloys according to the invention is
from about 27 to about 47%. The correlation between the nickel and
titanium contents in alloys of the invention is critical if the desired
balance of strength and low expansion properties are to be achieued
between 20 and 300C. To this end the nickel, cobalt, titanium and
niobium contents of the alloy should satisfy the following relation-
ships:
%NI + 0.7 ~%Co) - 1.25 [%Ti + 0.35(%Nb)] - 2 t%Ti~/(%Ti + %Nb)
= 37 to 40 - - - _ _ (1)
or
%Ni + 0.7 (%Co) - 1.25 [%Ti +`0.35~%Nb)] - 2 (%Ti)/(%Ti + %Nb)
= 37 to 39 - - - - - (2)
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Cast and aged alloys which do not satisfy the foregoing
relationships, whilst possibly having a 0.2~ proof stress at
20 C higher than 350N/mm2 depending upon their titanium content
will not also have the desired thermal expansion coefficient
between 20 and 300 C of less than 6 x 10 / C for relationship
(1) and less than 5 x 10 / C for relationship (2).
Thus experiments have shown that for iron-base alloys
containing nominally 2.5% titanium and 13.5% cobslt it is necessary
to have a nickel content of between approximately 32.5% and
approximately 34.5% in order to maintain in the cast and aged
~ondition a mean coefficient of thermal expansion between 20
and 300 C of le~s than 5 x 10 / C. If the nickel content
falls below approximately 32.5% in the nomlnally 2.5% titanium,
13.5% cobalt alloy there is a possibility of martensite formation,
which has a high coefficient of thermal expansion, by refrigerating
or cold working. Nickel contents higher than approximately 34.5%
in the nominally 2.5% titanium, 13.5% cobalt alloy result in thermal
expansion coefficients greater than the tesired 5 x 10 / C.
. _ .
The inclusion of c ~ ~ ~ nZ ~ ~oys of the lnvention is
not essential to achieve the desired 1QW expansion coefficient between
20 and 300 C or the desired strength, but it is desirable for alloy
parts which ~ust withstand temperatures greater than 300 C. This
is because the effect of cobalt is to reduce thermal expansivity
at temperatures above 300 C. Preferably the cobalt content i9 between
above 5 and about 16 wt.%, and most preferflbly between abou~ 10 and
above 15 wt.%. The effect of the inclusion of cobalt can be seen
from Example I of the following examples.
EXAMPLE I
Samples of an alloy containing, by weight 13~5~/o cobalt,
33. 0% nickel, 2. 5% titanium, balance iron and samples of a cobalt-
free alloy containing 43% nickel, 2~5Z titanium, balance iron, were
tested and the thermal expansion coefficient measured for various
temperature3 for the cobalt-free alloy and the cobal~ containing alloy
with the results shown in Table 1.
TABLE 1
.
Mean Coefficient of Thermfll Expansion
Wt% Cobalt ~ X 10 /C
20-300 C2D-400C 20-500 C20-600 C
:
0 5 7~1 8~8 10~0
13~5 5 5~7 7~6 9~2
The reduction in thermal expanslon coefficient at temperature~ in
excess of 300 C due to the addition of cobalt i~ clearly apparent
from Table 1.
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1038205
EXAMPLE II
An alloy of composition 33.0% nickel, 13.4~ cobalt, 2.5%
titanium, less than 0.002~ carbon balance apart from impurities being
iron, was inventment cast in vacuum, was given an ageing heat treatment
at 650C for 24 hours and when tested had the properties shown in
Tables 2 and 3:
TABLE 2
Test Tensile Properties, N/mm
Temperature (1) (2)
U.T.S. 0.2~ P.S.
20C 8~0 750
500C 610 500
(1) ~ltimate Tensile Strength
; (2) Proof Stress
TABLE 3
Test Temperature Coefficient of Thermal Expansion /C
RangeC c~ X 10
20 - 100 4.2
20 - 200 3.8
2020 - 300 3.9
20 - 400 5.5
20 - 500 7-4
20 - 600 9.1
EXAMPLE III
An alloy of nominal composition 37% nickel, 8% cobalt, 2.1%
titanium, 0.002% carbon balance, apart from impurities, being iron, was
invest~ent cast in vacuum, WhS given an ageing heat treatment at 650C
for 24 hours and when tested had the properties shown in Tables 4 and 5:
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TABLE 4
~ .
Test TemperatureTensile Properties, N/mm
U. . 0.2% P.S.
Room temp. (20 C) 820 680
500C 700 490
TABLE 5
Test TemperatureCoefficient of Thermal Expansion / C
Range C ~c X 10
20 - 100 4.6
20 - 200 4.3
10 20 - 300 4.3
20 - 350 4.6
20 - 400 5.6
20 - 500 7.7
20 - 600 9.4
Alloys of the invention can, if required, be cast directly
into intricately-shaped inve~tment castings with good surface properties
requiring little or no surface machining prior to use. Nickel-iron(-cobalt)
castings wbich are not strengthened are prone to surface cracking due
in part to "hot shortness" and in part to poor oxidation resistance.
The presence of æuch cracks cansevere~y limit or reduce mechanicfll
properties, such as fatigue life, and their presence, particularly in
investment castings, i8 undesirable. The presence of titanium in nickel-
iron(-cobalt) alloys of the invention remarkably limits the incidence of
cracking due to hot shortness and poor oxidation resistance, and such
castings have good surface finishes over castlng~ lacking in titanium.
Tkese good surface propertles are p~rticularly noticeable in cHstlngs
made from alloys of the invention containing hlgh titanium levels (e.g.
2% and above). Alloys according to the invention can be produced by air
melting and casting, but it is preferred to melt in vacuo or under an
inert atmosphere.
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Alloys according to the invention are particularly
useful for structural components which reach high temperatures
in use and must have such a combination of low expansivity and
high strength at working temperatures. Such structural components
include parts of rotating and reciprocating machinery, for example
turbine shafts and blades, in which close dimensional tolerances
have to be maintained under varying temperatures from ambient
temperature up to 300 C or even higherS for example up to 500 C.
These requirements arise in a p rticularly acute form in high-
efficiency propulsion machinery for land, ~ea and air uses.
Although the present invention has been described in
conjunction with preferred embodiments, it is to be understood
that modificaeions and variations may be resorted to without
departin~ from the spirit and scope of the invention as those skilled
in the art will readily understand. Such modifications and variations
are considered to be within the purview and scope of the invention
and appended claims.
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103~0S
SUPPLEMENTAX~ DISCLOSURE
_ . .
As noted previously, the present invention is concerned
with cast alloys. Heretofore, many low expansion alloys were
wrought alloys for which the desired properties have been
developed after processing which consisted of casting, hot or
cold working, solution treatment and final ageing. After such
treatment the alloys consist of an essentially homogeneous matrix
containing Ni3Ti precipitate. According to the present invention
the alloy is a cast material which has t~e desired properties
after simply ageing the casting, or even (as explained previously)
in the as-cast condition. In the alloys of this invention the - ~-
microstructure in the as-cast condition is inhomogeneous, there
being a significant segregation o the major alloying elements,
viz. Ni, Co, Fe and Ti. Because of the sensitivitv of expansivity
to compositional effects it is surprising that such inhomogeneous
products have low expansivities.
It is a significant feature of the present invention
that the cast alloys can be age-hardened directly to achieve
dimensionally stable castings having the indicated properties,
viz. a linear thermal expansivity over a temperature range of 20
to 300C of less than 6 x 10 6/oC, and preEerably less than
5 x 10 6/oC .and a 0.2 proof stress at 20C greater than 350 N/mm~.
Casting temperatures of about 1500C have been found
particularly suitable. However, casting temperatures for the
alloys of this invention may range from about 1475C to about
1600C.
With respect to the alloy composition, it was noted
that the present low expansion alloy contains about 27~ to
about 47% nickel. The minimum and maximum nickel must be cor-
related to the Co, Ti and Nb contents and the composition factor,
according to the given equation. ~or example, if the cobalt
range is 0 to 16% and the composition factor is 37 to 40, to
satisfy the equation, the minimum nickel content in the alloy
o
103820S
can be calculated, and it is about 29%. Similarly, if the compo-
sition factor is 37 to 39 and the cobalt 5 to 16%, the nickel
content is about 29% to about 44%. Thus, preferred embodiments
of this invention contain about 29% to about 44% nickel. ~ore
preferred embodiments contain ahout 32% to 33% nickel. Where the
cohalt content is a minimum oE 5%, the maximum nickel present is
between about 42% and 43% for alloys having an expansivity at
20-300C of less than 5 x 10 6/oC. As indicated previously, the
cobalt content is preferably between above 5% and about 16%.
More preferably, it is between about 7% and about 14%, e.g. about
7.5 to about 8.5%. The attainment of high strength in alloys
according to the invention depends upon precipitation hardening
by the formation of a precipitate, ~i3(Ti), when ageing the alloy
at elevated temperature, and it is known that titanium combined
with carbon will not enter this precipitate. For this reason it
is the content of titanium that is not combined with carbon which
is important. The total amount of titanium preferably exceeds the
uncombined amount by four times the weight of the carbon content,
which itself must not exceed 0.1%. Preferably, carbon should not
exceed 0.04%, e.g. it is lower than 0.02% or even lower than
0.002%. When the carbon content is so low, the total titanium
content is effectively the same as the uncombined titanium. As
indicated above, the alloys of the present invention contain
titanium (uncombined) in an amount of between 1 and 4% by weight,
preferably between 1.5 and 3.5~ more preferably between 1.7 and
2.7%. Most preferably the uncombined titanium level is above
1.75%, e.g. between 1.9% and 2.2%. Although niobium is not es-
sential for obtaining the re~uired properties, it can be added in
an amount of up to about 1.5% to help in the achievement of good
mechanical properties.
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103~9Z05
To obtain good castings according to the invention it
is preferable to control the silicon, manganese and aluminum con-
tent of the alloy from which the casting is made. Less than 0.3%
and preferably less than 0.1~ silicon in alloys used for castings
according to the invention decreases the expansion coefficient.
More than 0.3% silicon can increase the proof stress but undesir-
ably increases the expansion coefficient. ~langanese facilities
deoxidation, castability and improved proof stress but at the
expense of increased expansion and for this reason the manganese
content must not exceed 0.4% and for optimum proof stress and
expansion properties preferably should not exceed 0.3%. Aluminum
assists the production of castings by the air melting and air
casting route. For this purpose it is advarltageous for the alloy
to contain at least 0.05% aluminum but it must not be present in
quantities greater than 0.3% otherwise it increases the expansion
coefficient. Preferably, for optimum proof stress and expansion
properties the aluminum content should not exceed 0.2%. Magnesium
may be present, e.g. preferahly in an amount not more than 0.1%.
Briefly, in preferred embodiments of this invention
alloys used for castings, the nickel content preferably is from
about 32% to about 38%, the cobalt content is from 7% to about
14%, the uncombined titanium content is from above 1.75% up to
about 2.7%, the carbon content does not exceed 0.04%, with the
nickel, cobalt, titanium and niobium correlated as indicated
previously. Up to 1.5% niobium may be ~resent. The alloys may
contain up to about 0.3% Si, up to 0.3% aluminum and up to 0.3~
manganese. More preferably, alloys used for castings according
to the invention contain from about 36.5% to about 38% nickel,
from about 7.5% to about 8.5% cobalt, from about 1.9% to about 2.2%
uncombined titanium and from about 0.3% to about 0.6% niobium.
103~ 5
A particularly pre~erred alloy composition range
from which castings according to the invention can be made is,
by weight, 36.5% to 38% nickel, 7.5~ to 8.5~ cobalt, 1.9~ to 2.2%
uncombined titanium, 0.3~ to 0.6~ niobium, not more than 0.002%
carbon, not more than 0.3% silicon, not more than 0.2% aluminum,
not more than 0.3% manganese, balance, apart from impurities,
being iron. Test results of a casting made from such an allov
are described in the following Example IV.
EXAMPLE IV
An alloy of composition, by weight, 37.3~ nickel, 7.9%
cobalt, 2.02% uncombined titanium, 0.54~ niobium, 0.002% carbon,
0.05% aluminum, %Ni + 0.7(%Co~ - 1.25[%Ti + 0.35(%Nb)] -
2 (%Ti)/(%Ti + %Nb) = 38.49, balance, apart from impurities,
being iron was vacuum melted and investment cast in vacuum at a
temperature in the range of 1500 to 1550C to a casting according
to the invention. The casting was given an ageing heat treatment ~` -
in air at 650C -for 24 hours and when tested had the properties
shown in Tables 6 and 7.
TABLE 6
Test Tensile Properties (N~mm ? Elongation
Temperature ~C) U.T.S. 0.2% P.S. (%)
820 710 5
500 650 510 9
TABLE 7
Test Coefficient of Thermal Ex~ansion
Range C ~-~ C)
20 - 100 4.3 x 10 6
20 - 200 4.5 x 10 6
20 - 300 4.6 x 10 6
20 - 350 4.9 x 10 6
20 - 400 6.0 x 10 6
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