Iron-nickel alloy having improved weldability
The present invention relates to an Fe-Ni alloy with a low thermal expansion
coefficient intended to be used for making welded assemblies for applications in which
high dimensional stability under the effect of temperature variations is required.
5 The alloy according to the invention is more particularly intended to be used in
cryogenic applications, and notably for making assemblies intended to contain liquefied
gases, and notably tubes for transporting or tanks for transporting or storing liquefied
gases.
Presently, such welded assemblies are made by using an iron-nickel alloy of the
10 Invar8 type as a base metal. Indeed, the lnvar8 are known for their low thermal
expansion coefficient, and are therefore particularly suitable for the applications
mentioned above.
However, presently used Fe-Ni alloys do not give entire satisfaction. Indeed, the
inventors noticed that the welded assemblies made from these alloys had weld defects.
15 In particular, they observed that the use of these alloys led to irregular welding seams and
having islets of oxides at their surface.
An object of the present invention is to find a remedy to these drawbacks and to
propose an Fe-Ni alloy with which welded assemblies may be made with great
dimensional stability and having improved weldability.
20 For this purpose, the invention relates to an alloy based on iron comprising, by
weight: .
35% 5 Ni 5 37%
trace amounts 5 Mn 5 0.6%
trace amounts 5 C 5 0.07%
25 trace amounts 5 Si r 0.35%
trace amounts 5 Cr 5 0.5%
trace amounts 5 Co 5 0.5%
trace amounts 5 P 5 0.01%
trace amounts 5 Mo < 0.5%
30 trace amounts 5 S 5 0.0035%
trace amounts S 0 5 0.0025%
0.011% r [(3.138 x Al + 6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S)] 50.038%
0.0003% < Ca < 0.0015%
0.0005% < Mg 5 0.0035%
35 0.0020% < A1 5 0.0085%
the remainder being iron and residual elements resulting from the elaboration.
2
According to particular embodiments, .the alloy according to the invention
comprises one or several of the following features, taken individually or according to all
the technically possible combination(s):
-the silicon content is greater than or equal to 0.1% by weight;
5 - the manganese content is greater than or equal to 0.15% by weight, the carbon
content is greater than or equal to 0.02% by weight and the silicon content is greater than
or equal to 0.1% by weight;
-the carbon content is less than or equal to 0.05% by weight;
-the calcium content is less than or equal to 0.0010% by weight;
10 -the magnesium content is less than or equal to 0.0020% by weight; and
-the aluminium content is comprised between 0.0030% and 0.0070% by weight.
The invention also relates to a method for manufacturing a strip made in an alloy
as defined earlier, the method comprising the following successive steps:
- an alloy is elaborated as defined earlier;
15 - a semi-finished product of said alloy is formed;
-this semi-finished product is hot-rolled in order to obtain a hot strip;
-the hot strip is cold-rolled in one or several passes in order to obtain a cold strip.
The invention also relates to a strip made in an alloy as defined earlier.
The invention also relates to a method for manufacturing a welding wire
20 comprising the following successive steps:
- an alloy is elaborated as defined earlier;
- a semi-finished product of said alloy is formed;
-this semi-finished product is hot-rolled in order to manufacture an initial wire;
-the initial wire is cold-drawn in order to obtain the welding wire.
25 The invention also relates to a welding wire made in an alloy as defined earlier.
The invention also relates to the use of an alloy based on iron comprising, by
weight:
35% r Ni r 37%
0.15% 5 Mn 2 0.6%
30 0.02% 5 C r 0.07%
0.1% 5 Si 5 0.35%
trace amounts 5 Cr 2 0.5%
trace amounts r Co 5 0.5%
trace amounts 5 P r 0.01%
35 trace amounts r Mo < 0.5%
trace amounts 5 S 5 0.0035%
3
trace amounts 5 0 5 0.0025%
0.011% r [(3.138 x Al + 6 x Mg + 13.418 x Ca) -(3.509 x 0 + 1.770 x S)] 50.038%
0.0003% < Ca r 0.0015%
0.0005% < Mg s 0.0035%
5 0.0020% < AI r 0.0085%
the remainder being iron and residual elements resulting from the elaboration,
in order to manufacture tanks or tubes intended to receive a liquefied gas.
The invention will be better understood upon reading the description which follows,
only given as an example, and made with reference to the appended drawings, wherein:
10 - fig. 1 is an image, taken with an optical microscope, of a part having
filamentary corrosion; and
- fig. 2 is a graph showing the results of experiments conducted by the inventors.
In the whole description, the contents are given as weight percentages. Moreover,
the Al, Mg, Ca, S and 0 contents correspond to the total contents of these elements in the
15 alloy.
The alloy according to the invention is an iron-based alloy comprising, by weight:
35% 5 Ni 5 37%
trace amounts r Mn r 0.6%
trace amounts r C 5 0.07%
20 trace amounts r Si 5 0.35%
trace amounts r Mo < 0.5%
trace amounts 5 Co S 0.5%
trace amounts 5 Cr 2 0.5%
trace amounts 5 P 5 0.01%
25 trace amounts r S 2 0.0035%
trace amounts r 0 r 0.0025%
0.01 1% r [(3.138 x Al + 6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S)] 50.038%
0.0003% < Ca 5 0.0015%
0.0005% < Mg r 0.0035%
30 0.0020% < A1 5 0.0085%
the remainder being iron and residual elements resulting from the elaboration.
The alloy according to the invention is an alloy of the lnvar8 type.
By residual elements resulting from the elaboration, are meant elements which are
present in the raw materials used for elaborating the alloy or which stem from the
35 apparatuses used for its elaboration, and for example refractory materials of ovens.
These residual elements do not have any metallurgical effect on the alloy.
4
The residual elements in particular comprise elements from the family of lead (Pb),
which are reduced to a minimum in order to limit the sensitivity of the alloy to solidification
cracks and to avoid degradation of the weldability.
Phosphorus (P), molybdenum (Mo), sulfur (S) and oxygen (0) are impurities
5 resulting from the elaboration for which the total amounts present in the alloy should be
limited to contents below the specified contents.
In the alloy according to the invention, the carbon content is limited in order to
avoid precipitation of carbides of the MC type wherein M is a residual element which may
be associated with carbon for forming carbides, such as titanium (Ti), niobium (Nb),
10 vanadium (V), zirconium (Zr). Indeed, some carbides degrade the resistance of the alloy
to hot cracking. The carbon content is also limited in order to limit the formation of
porosities during welding by effervescence.
The alloy according to the invention further has a low average thermal expansion
coefficient, in particular of less than or equal to 2.10.~ K-' between -180°C and O0C, and
15 advantageously less than or equal to 1.5.10-"K-' between -180°C and O0C, and less than
or equal to 2.5.10.~ K-' between 20°C and 100°C.
Further, it is stable towards martensitic transformation as far as below the
liquefaction temperature of nitrogen (-196°C). In particular, its contents of gammagenic
elements, i.e. nickel (Ni), manganese (Mn) and carbon (C), are adjusted so that its
20 metallurgical structure is stable at 4.2 Kelvins (liquefaction temperature of helium) in the
absence of any plastic deformation or that its volume fraction of martensite remains less
than or equal to 5% when it is subject to a 25% deformation by planar traction interrupted
at -1 96°C.
The contents of cobalt (Co), manganese (Mn) and silicon (Si) in the alloy are
25 limited in order to avoid degradation of the stability of the alloy towards martensitic
transformation, as well as the average expansion coefficient between -180°C and 0°C.
The alloy according to the invention has a low elastic modulus, in particular less
than 150,000 MPa.
It does not have any "ductile-fragile" resilience transition. More particularly, it has
30 a resilience at -196°C greater than 150 jouleslcmz, and in particular greater than 200
joules/cm2.
These properties make it particularly suitable for applications wherein dimensional
stability under the effect of temperature variations is required.
In the alloy according to the invention, the sulfur (S) and oxygen (0) contents are
35 reduced as much as possible for improving the capability of hot transformation of the
5
alloy. In particular, it is sought to reduce as much as possible the sulfur (S) and oxygen
(0) contents in solid solution in the alloy.
This limitation of the oxygen and sulfur contents in solid solution is notably
obtained by adding silicon, which acts as a deoxidizer and indirectly as a desulfurizer via
5 chemical reactions between the liquid metal and the slag during the elaboration of the
alloy. Indeed it is known that the sulfur content %S of the liquid metal of an alloyed steel
-
verifies, during the refining operation in a liquid phase, the following relationship:
wherein
10 (%S) is the sulfur content of the slag
C', is the sulfur capacity of the slag
a,, is the activity of the oxygen of the liquid metal
Manganese participates in the desulfurization in solid phase.
Moreover, the inventors noticed that too high calcium, aluminium and magnesium
15 contents were detrimental to the weldability of the alloy. Therefore, the contents of these
elements should be limited. More particularly, the inventors of the present invention
discover that when:
(al) the calcium content is less than or equal to 0.0015%,
(bl) the magnesium content is less than or equal to 0.0035%
20 (cl) the aluminium content is less than or equal to 0.0085%,
and when, moreover the aluminium, magnesium, calcium, oxygen and sulfur con.tents in
the alloy observe the following relationship:
((3.138 x Al + 6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S)] 2 0.038% (dl).
the weld beads made on parts made in the alloy according to the invention are regular.
25 On the contrary, when the relationships (al), (bl), (cl) and (dl) above are not
observed, the weld beads are irregular.
The inventors believe that the regularity of the beads in the alloy according to the
invention results from the fact that, for the specified contents, the electric arc of the
welding tool is stable on the one hand, and the surface of the beads is without any oxide
30 aggregates on the other hand. On the contrary, when the alloy contains contents of these
elements above specified limits, the electric arc of the welding tool is unstable, but also,
oxide islets pin the base of the beads, which results in variable bead widths, and therefore
in irregular beads. The inventors believe that this pinning notably stems from variations of
6
surface energy of the melted area when the calcium, aluminium and magnesium do not
observe the relationships above. By pinning of the base of the beads, is meant that the
base of the beads cannot migrate, it remains immobile, blocked out of an equilibrium
condition. If the pinning force disappears, the base of the beads is able to move in order
to converge towards its equilibrium condition.
Preferably,
- the calcium content is less than or equal to 0.0010% by weight; andlor
- the magnesium content is less than or equal to 0.0020% by weight; andlor
- the aluminium content is less than or equal to 0.0070% by weight.
However, the inventors of the present invention noticed that when, according to the
invention:
(a2) the calcium content (Ca) is strictly greater than 0.0003% by weight,
(b2) the magnesium content (Mg) is strictly greater than 0.0005% by weight,
(c2) the aluminium content (Al) is strictly greater than 0.0020% by weight,
and when, moreover, the total contents of aluminium, magnesium, calcium, sulfur and
oxygen in the alloy observe the following relationship:
[(3.138 x Al + 6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S)] 2 0.011% (d2), the
obtained alloy has good resistance to hot cracking.
In particular, the alloy according to the invention develops a total length of cracks
of less than or equal to 10 mm (+I- 0.5mm) during a Varestraint test conducted according
to the European standard FD CEN ISOITR 17641-3 under a plastic deformation of 3.2%.
On the contrary, the inventors observe that when the relationships (a2), (b2), (c2)
and (d2) above are not observed, the alloy has a resistance to hot cracking which is not
satisfactory. In particular, the alloy then develops a total length of cracks greater than 10
mm (+I- 0.5mm) during a Varestraint test as mentioned above.
Preferably, the aluminium content is greater than or equal to 0.0030%.
The inventors believe that, in the alloy according to the invention, this improvement
in the resistance to hot cracking stems from the presence, in limited amounts, of calcium,
magnesium and aluminium in the alloy at contents allowing these elements to trap
residual sulfur and oxygen as sulfides andlor oxides in liquid phase.
It will be noted that the expression: [(3.138 Al + 6 Mg + 13.41 8 Ca) - (3.509 0 +
1.770 S)], developed by the inventors of the present invention, compares the contents of
calcium, magnesium and aluminium with those of oxygen and sulfur. It expresses the
idea according to which the amount of calcium, magnesium and aluminium which
degrades the weldability corresponds to the fraction of the total contents of Ca, Mg and Al
7
which corresponds to the amount of these elements present in solid solution in the alloy,
i.e., not precipitated as oxides or sulfides.
The weighted coefficients of calcium, magnesium and of aluminium in this
relationship express the relative affinity of each of these elements with sulfur and oxygen
5 as ascertained by the inventors, i.e. the capability of each of these elements of trapping
sulfur and oxygen in order to form sulfides or oxides.
In this expression, Al, Mg, Ca, 0 and S correspond to the total contents of these
elements in the alloy, expressed as weight percentages.
It will be noted that calcium, magnesium and aluminium are usually considered as
10 simple impurities in the alloys falling within the field of the invention. However, as
explained above, the inventors of the present invention noticed that these elements may
have the beneficial effects indicated above when they are present in small amounts in the
alloy, i.e. in the specified ranges.
Taking into account the foregoing, the alloy according to the invention allows
15 producing welded assemblies made of Invat@ which do not have the welding defects
observed in the case of the alloys customarily used.
Preferably, the alloy according to the invention comprises:
0.15% 5 Mn 5 0.6%
0.02% 5 C 5 0.07%
20 0.1% 5 Si 5 0.35%.
This alloy is particularly suitable for cryogenic applications, i.e. notably the
transport and the storage of liquefied gases, such as liquid hydrogen, liquid nitrogen,
liquid methane or liquid propane.
In particular, the manganese (Mn) and carbon (C) contents respectively greater
25 than or equal to 0.15% and 0.02% improve the stability of the alloy towards martensitic
transformation at -196°C.
Moreover, the inventors discovered that the silicon present in the alloy at contents
greater than 0.10% improves the resistance of the alloy to filamentary corrosion by
formation of a cortical silicon oxide layer developed by means of a suitable final heat
30 treatment.
The filamentary corrosion results from the extended contact of the alloy with the
atmosphere. It in particular occurs under the effect of oxygen and of the pollutants of the
air, as well as of the water vapor. In English, the filamentary corrosion is also designated
by the term of "filiform corrosion". Fig. 1 illustrates an example of filamentary corrosion.
35 The alloy according to the invention may be elaborated by any suitable method
known to one skilled in the art. As an example, it is elaborated in an electric arc furnace,
8
and then is refined in a ladle by usual methods, which may in particular comprise a step of
placing under reduced pressure. Alternatively, the alloy according to the invention is
elaborated in a vacuum furnace from starting materials with a low content of residual
elements.
5 For example cold strips are then manufactured from the thereby elaborated alloy.
As an example, the following method is used for manufacturing such cold strips.
The alloy is cast as semi-finished products such as ingots, remelted electrodes,
slabs, notably thin slabs with a thickness of less than 180 mm, or billets.
When the alloy is casted as a remelted electrode, the latter is advantageously re-
10 melted in vacuo or under an electro-conductive slag in order to obtain better purity and
more homogeneous semi-finished products.
The thereby obtained semi-finished product is then hot-rolled. at a temperature
comprised between 950°C and 1300°C in order to obtain a hot strip. The thickness of the
hot strip is notably comprised between 2 mm and 6 mm.
15 According to an embodiment, the hot-rolling is preceded with a chemical
homogeneization heat treatment at a temperature comprised between 950°C and 1300°C
for a period comprised between 30 minutes to 24 hours.
The hot strip is then cooled to room temperature in order to form a cooled strip,
and then wound into coils.
20 The cooled strip is then cold-rolled in order to obtain a cold strip having a final
thickness advantageously comprised between 0.5 mm and 2 mm. The cold-rolling is
carried out in one pass or in several successive passes.
At the final thickness, the cold strip is subject to a recrystallization heat treatment
in a static oven for a period ranging from 10 minutes to several hours and at a
25 temperature above 700°C. Alternatively, it is subject to a recrystallization heat treatment
in a continuous annealing oven for a period ranging from a few seconds to about 1
minute, at a temperature above 800aC, in the holding area of the oven, and under a
protected atmosphere of the N2/H2 type (30%/70%) with a frost temperature comprised
between -50°C and -15°C.
30 A recrystallization heat treatment may be carried out, under the same conditions,
during cold-rolling, at an intermediate thickness between the initial thickness
(corresponding to the thickness of the hot strip) and the final thickness. The intermediate
thickness is for example selected to be equal to 1.5 mm when the final thickness of the
cold strip is 0.7 mm.
35 The methods for elaborating the alloy and for manufacturing cold strips made of
this alloy are only given as an example.
9
Any other methods for elaborating the alloy according to the invention and for
manufacturing finished products made of this alloy known to one skilled in the art may be
used for this purpose.
Tests
The inventors carried out laboratory castings of alloys having Ni, Mn, C, Si, Co, Cr,
Mo, S, 0 and P contents in the specified ranges, and Ca, Mg and Al contents varying
between a few ppm and approximately 0.001%. The thereby obtained ingots were hot
formed by rolling in order to produce plates a few millimeters thick. These plates were
then machined in order to obtain a surface without any hot oxidation.
The alloy compositions of each of the tested plates are described in the table
hereafter.
The inventors made, on the thereby obtained plates, fusion lines with the TIG
(Tungsten Inert Gas) method in order to show the incidence of calcium, magnesium and
aluminium on the regularity of the weld beads. The results of these tests are described in
the column entitled (c TIG fusion line )> in the table hereafter.
The width of the weld beads was measured by optical microscopy and the
regularity of the beads was defined as follows:
Regularity = 100 x (Lmax - Lmin)lLmax (I),
wherein Lmin corresponds to the minimum measured width of the weld bead and Lmax
corresponds to the maximum measured width of the weld bead.
It was considered that the regularity of the weld bead was good (Index 1 in the
table hereafter) when the regularity calculated by applying formula (1) is less than or equal
to 2.5%.
It was considered that the regularity of the weld bead was acceptable (Index 2 in
the table hereafter) when the regularity calculated by applying formula (1) is comprised
between 2.5% and 5%.
It was considered that the regularity of the weld bead was poor (Index 3 in the
table hereafter) when the regularity calculated by applying formula (1) is strictly greater
than 5%.
Moreover, the inventors conducted on the obtained plates Varestraint tests
according to the FD CEN ISOITR 17641-3 European standard under 3.2% of plastic
deformation in order to evaluate their resistance to hot cracking. They measured the total
length of cracks developed during the test, and classified the plates in two categories:
10
- the plates having, at the end of the test, a total length of cracks of less than or
equal to 10 mm +I- 0.5 mm were considered as having good resistance to hot cracking,
while
the plates having a total length of the cracks strictly greater than 10 mm +/- 0.5
5 mm were considered as having insufficient resistance to hot cracking.
The results of these tests are described in the column entitled "Varestraint Tests
with 3.2% deformation" of the table hereafter. In this column, the plates having good
resistance to hot cracking are those which have a total length of cracks noted as (( 1 to
10 D, while the plates having insufficient resistance to hot cracking are those which have a
10 total length of cracks noted as (c 10 to 15 s.
In the table hereafter, the (( behavior law s column shows the value taken by the
expression: [(3.138 x Al + 6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S)] for the
relevant alloy, wherein Al, Mg, Ca, 0 and S respectively designate the total contents of Al,
Mg, Ca, 0 and S in weight percentages in the alloy.
15
Alloy
S.OuWl fur
Sulfur
Chemical composition
(weight %)
remainder
TIG fusion line
Regularity
of the bead
I = Good
2 =Acceptable
3 = Poor
1
I
Varestraint tests
with 3,2%
deformation
Total length of
cracks
(mm)
Behavior
law
Sulfur
v
W
remainder
-K
-L
2
-N
0,
remainder
0.0003
0.0003
0.0005
0.0003
0.0005
Low
Sulfur
0.0009
0.0012
0.0013
0.0009
0.0010
1
1
2
2
3
36
36
36
36
36
1 to10
I to10
1 to10
1 to10
1 to10
0.35
0.35
0.35
0.35
0.35
0.020
0.020
0.030
0.038
0.054
0.03
0.03
0.03
0.03
0.03
0.25
0.25
0.25
0.25
0.25
0.0005
0.0004
0.0005
0.0005
0.0005
0.0015
0,0010
0.0015
0.0015
0.0030
0.0025
0.0045
0.0065
0.0085
0.0110
In the table above, the examples which are not according to the invention are
noted in bold characters.
In the group of examples referenced from A to E, the calcium content was varied
5 between 0.0005% and 0.0050% while retaining substantially constant silicon, magnesium,
aluminium, sulfur and oxygen contents, in order to evaluate the effect of calcium on the
regularity of the weld bead and on the hot cracking of the alloy.
In the group of examples referenced from F to J, the magnesium content was
varied between 0.0010% and 0.0056% while retaining substantially constant silicon,
10 calcium, aluminium, sulfur and oxygen contents, in order to evaluate the effect of
magnesium on the regularity of the weld bead and on the hot cracking of the alloy.
In the group of examples referenced from K to 0, the aluminium content was
varied between 0.0025% and 0.0110% while retaining substantially constant silicon,
calcium, magnesium, sulfur and oxygen contents in order to evaluate the effect of
15 aluminium on the regularity of the weld bead and on the hot cracking of the alloy.
In the group of examples referenced from P to W, alloys having a higher sulfur
contents than in the groups of preceding examples were evaluated in order to determine
the lower limits of the contents of each of the Ca, Al and Mg elements which allow
avoiding hot cracking.
20 In the examples referenced as D, E, I, J, 0, P and W, the relationship (3.138 x Al +
6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S) assumes values greater than the upper
limiting value of 0.038% defined in the composition of the alloy. Now, it is observed that in
these examples, the weld bead has regularity considered as poor (index 3), while the
resistance to cracking of the alloy resulting from the Varestraint test is good (length of the
25 cracks comprised between 1 and 10 mm).
In the examples R, U and V, the relationship (3.138 x Al + 6 x Mg + 13.418 xCa) -
(3.509 x 0 + 1.770 x S) assumes values less than the lower limit boundary of 0.01 1 as
specified. Now, it is observed that, in these examples, the weld bead obtained has a good
regularity (index I), but the resistance to cracking of the alloy is poor.
30 In all the other examples, the relationship (3.138 x Al + 6 x Mg + 13.418 x Ca) -
(3.509 x 0 + 1.770 x S) assumes values comprised between the lower limit of 0.01 1% and
the upper limit of 0.038%, as specified. It is observed that the alloy has a resistance to
cracking considered as good (total length of the cracks comprised between 0 and 10 mm)
and that the weld beads obtained are regular.
35 Thus, by a very specific control of the calcium, aluminium and magnesium
contents in a range of very low contents and by observing the relationships (dl) and (d2)
13
between these elements, sulfur and oxygen, an Fe-Ni alloy is obtained having a low
thermal expansion coefficient, and which further has excellent metallurgical weldability.
Thus, the alloy according to the invention may advantageously be used as a base metal
for producing welded assemblies with great dimensional stability.
5 In order to verify the effect of the silicon content on the sensitivity to filamentary
corrosion, the inventors also conducted experiments on sheets made of alloys (a), (b) and
(c) having Ni, Mn, C, Co, Cr, Mo, S, 0, P, Ca, Mg and Al contents in the specified ranges,
but variable silicon contents.
Thus, the alloy (a) has a silicon content strictly less than 0.01% by weight, the alloy
10 (b) has a silicon content equal to 0.1% by weight and the alloy (c) has a silicon content
equal to 0.25% by weight.
These sheets were subject to an industrial recrystallization heat tr.eatment under H,
with a frost temperature comprised between -50°C and -1 5"C, and then were left for 4000
h in a weathering chamber at 55°C under 95% of relative humidity.
15 The surface filamentary corrosion fraction was then measured by automatic analysis
of images captured by means of an optical microscope at a magnification of 200.
Fig. 2 is a graph illustrating the results of the experiments conducted by the
inventors. These results show that, in the case of examples (a) and (b), in which the
silicon content is greater than or equal to 0.1%, the surface filamentary corrosion fraction
20 remains less than 5% under the conditions mentioned above. On the contrary, in the case
of example (a), wherein the silicon content is strictly less than 0.1%, the surface
filamentary corrosion fraction becomes greater than 5% under the conditions mentioned
above.
Thus, the alloys having a silicon content greater than or equal to 0.1% have better
25 resistance to filamentary corrosion than alloys having silicon contents strictly less than
0.1%.
The alloy according to the invention may also be used for manufacturing a welding
wire. Such a welding wire has all the advantages mentioned above in terms of resistance
to cracking and of regularity of the weld beads obtained when the wire is used as a
30 welding. Moreover, the weld bead obtained will have a low thermal expansion coefficient
low.
As an example, such a welding wire is made by the following method. The alloy is
elaborated by for example using the elaboration methods described herein before. Next,
this alloy is cast into semi-finished products and notably into billets. These semi-finished
35 products are then hot-rolled in order to obtain an initial wire, also called a machine wire.
Such a machine wire generally has a diameter comprised between 4 mm and 6 mm.
14
Next, the initial wire is cold-drawn in order to reduce the diameter and obtain the welding
wire. The diameter of the welding wire is preferably comprised between 0.5 mm and 1.5
mm.

CLAIMS
1 .-An iron-based alloy comprising, by weight:
35% r Ni 5 37%
trace amounts r Mn 5 0.6%
5 trace amounts r C r 0.07%
trace amounts 5 Si r 0.35%
trace amounts 5 Cr 5 0.5%
trace amounts 5 Co 2 0.5%
trace amounts 5 P 5 0.01%
10 trace amounts 5 Mo < 0.5%
trace amounts 5 S 2 0.0035%
trace amounts 5 0 5 0.0025%
0.01 1% 5 [(3.138 x Al + 6 x Mg + 13.418 x Ca) - (3.509 x 0 + 1.770 x S)] 50.038%
0.0003% < Ca 5 0.0015%
15 0.0005% < Mg 5 0.0035%
0.0020% < A1 5 0.0085%
the remainder being iron and residual elements resulting from the elaboration.
2.- The alloy according to claim 1, wherein:
Mn 2 0.15% by weight
20 C 2 0.02% by weight
Si 2 0.1% by weight.
3.- The alloy according to claim 1 or 2, wherein the calcium content is less than or
equal to 0.0010% by weight.
4.- The alloy according to any one of the preceding claims, wherein the
25 magnesium content is less than or equal to 0,0020% by weight.
5.- The alloy according to any one of the preceding claims, wherein the aluminium
content is comprised between 0.0030% and 0.0070% by weight.
6.- A method for manufacturing a strip in an alloy according to any one of claims 1
to 5, comprising the following successive steps:
30 - elaboration of an allo{according to any one of claims 1 to 5;
- forming a semi-finished product of said alloy ;
- hot-rolling this semi-finished product in order to obtain a hot strip;
- cold-rolling the hot strip in one or several passes in order to obtain a cold strip.
7.- A strip made of an alloy according to any one of claims 1 to 5
35 8.- A method for manufacturing a welding wire comprising the following successive
steps:
- elaborat~nga n alloy accordtng to any one of cla~rns1 to 5,
-forming a semi-finished product of said alloy;
- hot-rolling this semi-finished product in order to manufacture an initial wire;
- cold-drawing the initial wire in ordel- to obtain the welding wire. , ,
5 9.- A welding wire made of an alloy according to any one of claims 1 to 5.
10.- The use of the alloy according to any one of claims 1 to 5, taken in
comb~nat~own~ thc la~m2 , in order to manufacture tanks or tubes Intended for recelvlng a
liquefied gas.