Note: Descriptions are shown in the official language in which they were submitted.
6 8 7
1 sackground o~ the ~nvent:ion
This invention relates to an improvement in low
thermal expansion coefficient high-nickel, iron-nickel alloys,
commonly known as "Invar" alloys.
Invar alloy, a typical low thermal expansion co-
e~icient alloy~ according to the ASTM, comprises 35 ~ 37% Ni
and the balance Fe, and may contain up to 0.5~ of Mn, up to
0.5% of Co, up to 0.5% of Cr, up to 0.5~ of Mo, up to 0.1% of
C, up to 0.3~ of Si, up to 0.025% of S and up to 0.025% of P
as allowable additive elements and/or impurities. And a
small amount of Al, which is used for deoY.idation, may exist
as the residue.
Generally speaking, in the Fe-Ni alloys containing
high-Ni activities of C, N and 0 are remarkably high because
of the high Ni conkent, and in the course of solidification
CO bubbles formed from C and O, and ~2 bubbles originated
from dissolved N are generated, which result in formation
of blisters in the formed ingots. These formed.blisters
make the hot working of the alloy impossible, and therefore,
vacuum melting is usually employed in ~he production of
Invar alloys.
As the high-Ni Fe-Ni alloys solidify in the form of
the homogeneous austenite phase, impurities are liable to
segregate. Such segregatea impurities are not homogenized
by heat.ing in a soaking pit, and therefore they cause cracking
in the course of slabbing (intergranular fracture), which
constitutes another cause of difficulty in the hot working.
It was already reported that the intergranular fracture
could be substantially prevented by limiting the S content
30. -2-
*Trade Mark
to O.O~o~ or less, the Al conten-t to 0.020~ or less and -the
O content to 0~0~5~r or less (Japanese Laying-Open Paten^t
Publication No. 52922/76~.
In the meanwhile, demand for the liquefied natural gas
(hereinafter called LNG) has been remarkably expanded, and
thus need for tankers, storage tanks, -tank trailers, etc.
and related large scale constructions for transporta-tion
and storage thereof as wcll as the equipments therefor
(hereinafter referred to as "the containers and equipments")
is increasing.
Characteris-tic prop~rties required for the material for
constructing these containers and equipments are, (a) That
the metalographic structure of the material is s-table over
the temperature range down to -162C, to which the material
is exposed, and thus does not lose its toughness at this low
-temperature. (b) That dimensional change in the temperature
range from room temperature to -162C, to which the contain-
ers and equipments are exposed, is minimum, namely, the
thermal expansion coefficient is sufficiently small in this
temperature range. (c) That the welding work, which is in-
dispensable for constructing the containers and equipments,
can be easily carried out, and welding defect, -that is, high
temperature cracking, which will result in gas leak or break-
down of the containers and equipments, does not develop. (d)
That the containers and equipments built of the material are
not susceptible to delayed cracking such as stress corrosion
cracking, etc.
There is no material -that satis:~actorily mee-ts all these
requirements. Among the materials which mee-t some of -these
requirements, there are 9~o-Ni steel, aluminum alloys, aus-tenite
~ 1 6~B~7
stainless steels, 36~-Ni Invar alloy, e-tc., whlch are now
used as the materials for the containers and equipmen-ts for
LNG ~
The 3~o-Ni Invar allo~ satisfies (a) and (b) among -the
above-mentioned requirements, That is, this alloy main-tains
-the metal structure of face~centered cubic lattice down to
-196~', which is the -temperature of lique:~ied nitrogen and
thus easily effected in a laboratory, and maintains its
toughness without exhibi-ting the ductility-brittleness tran-
sition phenomenon, and thus retains sufficient toughness at
this temperature. Also this alloy is characterized in that
it retains low thermal expansion coefficient over a wide
range from room temperature to -196C.
In this alloy, if the ~acuum meling is employed, the
above-mentioned problem of residual bubbles can be solved,
and hot work cracking caused by intergranular fracture can
be prevented in accordance with the teaching of Japanese
1aying-Open Patent Publication No. 52922/76. Although these
difficulties are solved, the alloy has defects that it easily
develop high -temperature cracking when welded, and its corro-
sion resistance is rather poor and it cannot be used when
stress corrosion cracking must be considered, because its
nickel content is high and the nickel forms low melting com-
pounds with sulfur, which is inevi-tably incidental to the
raw materials. That is to say, the alloy does not satisfy
-the conditions (c) and (d).
We have for many years studied prevention of high tem-
: perature cracking in welding and s-tress corrosion crackingof the Fe-Ni alloys, and we have now invented a new Invar
alloy whi.ch is provided with the abcve-mentioned characteristic
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8 7
properties (c) and (d) wi.-thout sacrificing -the properties
(a) and (b).
~hat is to say, there is provided a new improved Invar
alloy, of which the stress corrosion cracking sensi-tivity
is remarkably reduced by res-tricting the Co content -to 0.05%
or less by means of careful selection of Ni source; the sen-
sitivity to the high temperature cracking in welding is re-
duced by modifying the content of Mn and that controlling
the Mn conten-t depending upon the content level of S and hl;
and the low thermal expansion coefficient and s-tability in
the structure are well maintained by defining the Ni content
as 34.5 - 37.~% in relation with the amount of the Mn con-
tained.
Summary of the Invention
According to this invention, in -the Invar alloy which
comprises 36% Ni and the balance Fe and may contain C up to
0.1%, Si up to 1%, Mn up to 0.5%, P up to 0.025%, S up to
0.025%, Co up to 0.5%, Cr up to 0.5%, Mo up to 0.5%, and Al
up to 0.02% as allowable additives and/or impurities; an
improved alloy, whereof the Ni content is 34.5 _ 37,5%, the
Co content is not more than 0.05%, and the Mn content is up
to 1,2% when both the S content and the Al content are not
more than 0.005~0, and the Mn content is at least 0.5% and up
to 1.2% when either of the S content or the Al con-tent is
not more -than 0.005~0, is provided.
In the alloy of this invention, the Ni content range is
a little more expanded than that specified in the ASTM, This
is based on the finding tha-t, in -this alloy, in order to
maintain the structural stability in the low tempera-ture down
to _162Oc~ at least about 34.5% of Ni is required, and there
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1 J B~687
1 is no increase in the thermal expans:ion coefficient in the
a~orementioned temperature ranye with the Fe-Ni alloys
containiny high Ni content up to 37.5% in the relation with the
amount o~ Mn which is added to the alloy in accordance with
this invention.
Carbon may be contained in this alloy up to 0.1~ as
specified in the ASTM when the corrosion resistance of the alloy
is not a critical problem. However, its content should pre-
ferably be as low as possible for the above-mentioned reason
--generation of CO gas. The preferred con~ent is less than 0.01%.
Silicon has undesirable effect in the high Ni Fa-Ni
alloys leading to the high temperature cracking in welding. In
this invention, however, the problem of the high temperature
cracking has been sol~ed by the increase in the Mn content
within the Si content range according to the ASTM. The Si
content is preferably less than 0.25~.
Manganese is added in the alloy of this invention
in excess of the content specified in the ASTM in order to over-
come the deleterious effect of S and Al. However, the upper
limit of the Mn content is restricted to 1.26 because of the
adverse effect of Mn on the low thermal expansion co-efficient
of the all~y.
Phosphorus has little influence upon the properties
required in the alloy o~ this invention. Therefore, P is
allowed to be contained up to 0.025% as specified in ASTM. The
preferred P content is less than 0.01%.
Cobalt has been revealed to conduce to stress cor-
rosion cracking. Therefore the content thereo~ is limited to
not more than 0.05%. The preferable Co content is less than
0.03%.
Chromium is an impurit~ more or less coming ~rom the raw
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-~t~
~ .
~46~7
materials. Eut this elemen-t has li-t-tle influence upon the
properties required in the alloy of this invention, and
therefore, it is allowed to be contained up to 0.5%. How-
ever, the preferable alloy of this inven-tion con-tains sub-
stantially no chromium.
In -the alloy of this invention, little or no Mo will
be contained, if it is not posi-tively added. And it iæ no-t
a necessary element and thus is not added. However, i-t has
no deleterious effect if it is contained up to 0.~ as
specified in the ASTM.
Aluminum has adverse effect on the low thermal expansion
coefficient and the intergranular fracture. However, this
element is effective for deoxidation and denitrid;ng and a
small amount of the Al used in the melting step remains. The
prior art publication, e.g., the above-mentioned Japanese
Laying-Open Patent Publication ~2922/76, teaches that this
element should be restricted to 0,02% or less. A1 has also
undesirable influence on the high -temperature cracking in
welding. However, in this invention, the deleterious effect
thereof is overcome by addition of -the specified amount of Mn.
A slight amount of O and N is inevitably involved. In
consideration of the intergranular fracture, the O content
should preferably be not more than 0. 025~o, Nitrogen is usu-
ally contained in the alloys of this kind to the exten-t of
2~ 0.04% or so. But the N content should be as low as possible,
since it is a cause of blistering. The preferred N content
is less than 0.01~.
The alloy of this invention is usually prepared by vacuum
melting.
3 The correlation of -the contents of S, A1 and Mn will be
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made clear in, the following descrip-tion of specific embodi,-
ments of the inven-tion.
Now the invention is illustrated by way of the working
examples with reference to the a-ttached drawings.
Brief Explanation of the Attached Drawings
Fig. 1 is a schematic presentation of the appara~tus
used for the stress corrosion cracking tex-t in this inven-
tion.
Fig. 2 is a diagram showing the results of the stress
corrosiGn cracking.
Fig, 3 is a graph showing the relation between the Mn
conten-t and the average thermal expansion coefficient of
the Invar alloys at lower temperatures.
Detailed Description of_the Invention
The chemical anal~vses of the samples used in -these
examples are listed in Table 1. Samples 1 - 13 are of alloys
of this invention, while samples A - N are comparative alloys.
1 1 6 ~ 7
Table 1
Chemical Analyses o:E Sample A]loys
-
, . . . _
Sam- _ _ ~
ple C N 0 P Si Ni Co Mn S A l
No .
_ . _ _
1 <0 . 010 <0 . C10 ~0 . o25 o ~ oo7 o .23 35 ~ 85 o ~ OlC 0 . 22 o ~ oo3 o ~ oo3
2 ~l l~ .l co~ 005 o ~ 17 35~ 96 0 ~ o37 0 ~ 29 0 ~ oo4 0 ~ oo4
- ---- - -
3 ~l ll llo ~ oo6 o ~ 21 33 ~ 7g o ~ oo7 o ~ 47 o ~ oo3 o ~ oo4
- - -
4 ll --wll -- ll<o~ oo5 o~ 28 35.80 o ~ oo9 o ~ 55 o ~ oo3 0.013
ll l~ llll o ~ 16 36 ~ Ol o ~ OlO o ~ 65 o ~ Oll o ~ oo3
~ - ~ - - ~ - ~
d 6 .l n l~.l 0,14 35.82 O,Oll 0-72 o,ol3 o,oo4
- ~ --~ ----
7 ~l ll llo,oo6 0,20 35.90 O,Oll o,98 o,oo4 0,002
~1 _ ~ _
8 ll ll l~~o ~ oo5 o ~ 23 35 ~ 64 o ~ 02s l ~ 02 ll o ~ 019
~ - ~ -
9 ll ll l~ ll 0,17 35.71 O,OlO 1,17 0,014 o,oo4
H .... _ . ~ _
,21 37.34 O,Oll 0,27 0,002 o,oo5
37 ~ 40 o ~ OlO o ~ 61 o ~ oo9 o ~ oo4
- - ~-- - -
12 ll ll .- ~0.005 O,l9 37.21 0,012 l,lO O,Oll o,oo4
~ - ~ - - ~- -
13 ~ ll l~ ~ 0,21 34,72 O,Oll o,24 o,oo4 o,oo5
- - ~ - -- ~--
A .. .. .. .. 0,15 36,02 O,OlO 0,2L~ o,oo4 o,oo9
_ _ _ . _ A _ _ _ _ . --
B . __ 0,22 35.56 o ~ 2 a 0,27 O, Ol O 0,008
C -- ~ o, oo6 o ~ 1 9 35 ~ 94 o ~ 012 o ~ 28 o, ol 5 0 ~ o o3
D ll ll ll0 . oo7 o ~ 23 35 ~ 61 o ~ 65 o ~ 30 o ~ oo9 o ~ 002
- - - - - -
E _ o, oo c o . 1 8 36 ~ 02 0 . o74 o ~ 32 o ~ 012 o ~ 032
F .l .l .. .. 0 . 18 35 ~ 9l 0 ~ Ol O 0 ~ 41 0 ~ 002 0.016
¢
G _ _ _ .- 0, 21 35.6 a 0 ~ o o9 o ~ 47 0 ~ o o5 o ~ 01 0
H __ 0 . 16 35 ~ 70 ~ Ol l l ~ 32 o . oo4 0 ~ oo3
I .. .. .. .. 0 . 20 36 ~ 94 o,4g 0 ~ 84
o - - ~ - ~ -
J ll " ._ " 0 . 00 ~ 0, 19 37 ~ 21 o ~ Ol O l ~ 29 o ~ 01
K _=_ ~0.00~ 0,27 3a,L~6 o,oo9 o,L~7 0,012 o,oo4
L ll ll ............... " o~2L~ 38,30 o~oo9 o,96 ~ 0,012
_ . _ _ ~ ~_ ~ __ ~ _ . .
Mll ll ll ll o . 18 34 ~ 36 ll o ~ 20 llo ~ o34
~ --- - - - - - -~l - - ~
. N L_ __ _ _ o ~ 1 4 34 ~ 01 o, o l 5 1 ~ o4 .l o ~ 008
- ~
~ 3 6~7
Each sample was mel-ted in a vacuum high frequency e]ec-
-tric furnace of lOkg capacity, and cast. The cast specimen
was forged at about 1150C, and by repeti-tion of thermal
treatment and cold working, i-t was formed into plates o~
predetermined thickness (2.Omm and l.Omm). Thereafter the
specimen was finally subjected to a thermal trea-tment at
80ooc for 10 minutes.
The control of the Co content was effected by combined
use of ferronickel and electroly-tic nickel. Manganese was
added in the form of me-talic manganese. In order to prepare
low S level specimens the desulfuration was carried ou-t by
using lime and fluorspar.
Using -these specimens and the apparatus schematlcally
shown in ~ig. 1, a stress corrosion cracking test was carried
out and the results are shown in Fig. 2, The tes-t solution
was a 20% aqueoussolution of NaCl containing o.46N Cr6~.
The te~pera-ture was 450C and the applied stress was 30kg/mm2.
The test resul-ts show that the s-tress corrosion cracking
has nothing to do wi-th N1 and Mn but 1argely depends on the
amount of Co contained. Therefore, the Co content should pref-
erably be as low as possible. Although the Co conten-t can be
lowered by strict selection of the Ni source, there is a limit
as a matter of course and the allowable upper limit is 0.05%.
Cobalt has the same effect as Ni for the structural s-ta-
bility and ferronickel is far more inexpensive than electroly-tic
nickel. Thus economically i-t is advantageous to set the allow-
able limit of the Co con-ten-t high. Bu-t ;-t is essen-tlal -to re-
strict the Co content in order to con-trol -the s-tress corrosion
cracking sensitivity of the alloy.
3 We studied the high tempera-ture cracking in welding by
-- 10 --
1 3 64B~37
way of the arc strike -tes-t, and -the resul-ts are summarized
in Table 2. The text condi-tions were as follows. Curren-t:
llOA, arc length: 2rnm and arcking time: 4 seconds.
1 3 ~8~7
-~-rc~
..1 , , .,
C~3 1
O ~ O
. . ... ~.. ..,.... .
. . O
. C~l O O ~0 C~
.. .... . ,..~..., ~0,
C~ O ~ O C)
.. .. ~ O .. .
. O O ~ O C)_
O ~. O ~
E~ _ _ _ _ -- ----_ _ _ . . .. _ _ ~
~ O ~ O ~C
c~ ~ . , .,. ..
~ C~ ¢ O C~l O ~ .
h o o o
td ¢ _ __ _ _. _ _ _ _ _ _ _. _ _ _ C)
4 1 O ~\1 O r h
.; O ~ O ~1 O ~) ~
,- ~1 . -t ..
CD CO O O O O
. ~. . ~ G
. o_ .
: . . ., O
o~ ~ ~oO G ~,
. o o ..
o ~o o ~ (^J
_ . __ _~
Z ~ ~ ~ *
~1 U~ ~ ¢ ~ *
~ _ ,. _ ~
-- 12 --
B 8 7
From this table, -the following -three fac-ts are learned.
(1~ When the S content is 0.~05% or less and the content of
the residual Al is also 0.005% or less, high -temperature
cracking does not occur regardless of the Mn content.
(2) When the S content is 0.005% or less and -the Mn conten-t
is not less than 0.5%, high temperature cracking does not
occur even if the amount of the residual A1 is large.
(3) When the S content is more than 0.005% bu-t the residual
Al content is not more than 0,00~%, high temperature cracking
is prevented, if the Mn content is not less than 0.5%.
Therefore it is concluded that the high temperature
cracking which frequently occur in the high-Ni Fe-Ni alloy
can be controlled by regula-ting the Mn content depending on
the S level and the content of the residual Al. From this
viewpoint, the higher the Mn content, the better. ~ut it is
restricted from the aspect of thermal expansion coefficient
as explained below.
Fig. 3 shows the change of the average thermal expansion
coefficient over the temperature range 0C - -180C of the
Fe~Ni alloys respec-tively containing 3~.8(~0.10)% and 37.3
(~0,10)% Ni when Mn content is varied. Also average therrnal
expansion coefficient of 38.4~ Ni level alloys is shown.
From Fig. 3, it is learned that average thermal expansion
coefficient is largely influenced by the contents of Ni and Mn
and it becomes remarkably large when Ni content exceeds 37.~%
and Mn content exceeds 1.2%,
We also studied the structural stabili-ty of the alloy a-t
low temperature (-162C). Specimens were kept at -162C for
10 hours and thereafter formation of rnartensi-te was checked.
The results are shown in Table 3. The amout of mar-tensi-te was
1~6~7
measured by point counting under an op-tical microscope.
'rable 3
Amount of martensite aftcr kept at -162C for 10 hours
- Sample No. 1 13 M N
Amount of martensite 0 0 5.2% ~.9%
From Table 3, it is learned that both Mn and Ni have in-
fluence on the formation of martensite, and at least 3L~,5% Ni
is required in consideration of the case where the Mn con-tent
is low, in order to obtain Fe-Ni alloy with good s-tructural
stability that does not form martensite at -162C.
As has been described in the above examples, the Fe-Ni
alloy used for the containers and equipments for LNG is largely
restricted in composition. That is, in order to maintain the
- structural stability at low temperature (-162C), it must con-tain at least 34.5% Ni. Cobalt which is incidental to Ni must
be restricted to 0.05% or less for the prevention of stress
- corrosion cracking.
For the purpose of keeping the thermal expansion coeffi-
cient low, the Ni content cannot exceeds 37.5%. Manganese is
; 20 effective for the prevention of high temperature cracking in
welding. But the content thereof must be not more than 1.2%.
The high temperature cracking in welding largely depends on
the S level and the amountof the residual Al, but i-t is com-
pletely preven-ted by addition of Mn in an amount determined
by the content of S and Al.
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