Note: Descriptions are shown in the official language in which they were submitted.
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"Nickel-Chromium-lron-Molybdenum Alloy"
The present invention relates to novel nickel-chromium-iron-molybdenum alloys,
corresponding products and articles and their uses. The alloys according to
the
invention allow achieving good mechanical properties combined with a good
corrosion
resistance in water with high salinity, especially at elevated temperatures.
They are
thus particularly suited for use in geothermal power plants, e.g. as down-hole-
headers.
Down-hole-headers in geothermal power plants are required to withstand hot
geothermal fluids (e.g. above 100 C) containing high concentrations of
chloride ions
(e.g. above 100 g/1). These conditions are particularly demanding and often
lead to
pitting and crevice corrosion. Alloys used for these applications are required
to have a
sufficient corrosion resistance under these conditions.
Up to now titanium based alloys and nickel based alloys are regularly used for
1 5 constructing such down-hole-headers. These alloys are generally thought
to be the
only practical and reliable alternative when it comes to this application.
However, the
use of these alloys is uneconomical. For example, nickel is one of the most
costly
constituents in corrosion resistant alloys. A59 is an example of an alloy
often used. It
contains high amounts not only of nickel, but also of molybdenum, which also
strongly
2 0 contributes to the overall cost of the alloy. Moreover, A59 performs
considerably worse
under reducing conditions than under oxidizing conditions.
CONFIRMATION COPY
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Austenitic stainless steels, such as 316L, 254SM0 or A31 have been proposed as
alternatives. Unfortunately these materials are well-known to be prone to
crevice
corrosion. Even A31, which is the most highly alloyed steel in this group of
materials, is
not sufficiently resistant to corrosion in some situations. Hence, it is
obvious that =
neither the traditional austenitic steels nor the nickel based alloys are
fully satisfactory.
Corrosion resistant nickel based alloys with lower amounts of nickel than in
A59 are
proposed in US 5,424,029. However, the alloys disclosed therein still require
a
relatively high proportion of nickel, rendering them quite costly. In most
applications,
the costs are a decisive criterion.
The object of the present invention is to provide alloys combining low
material costs
and a high corrosion resistance. In particular, the alloys should have a high
wet
corrosion resistance in water with high salinity, especially at temperatures
above
100 C. The alloys should have a good resistance against pitting and crevice
corrosion
attack. Preferred alloys have a good resistance to reducing conditions (as
measured
e.g. by ASTM G 28 A) and at the same time to pitting corrosion and chloride
ion attack
(as measured e.g. by ASTM G 28 B). Advantageous alloys combine a high
corrosion
resistance with good mechanical properties, e.g. a high strength. Alloys with
such
properties are particularly suitable for, but not limited to, down-hole-
headers in
geothermal power plants operating using hot geothermal fluids containing high
chloride
concentrations.
This object is solved by a nickel-chromium-iron-molybdenum alloy, comprising
40 to 48
wt% (percent-by-weight) nickel, 30 to 38 wt% chromium, 4 to 12 wt% molybdenum,
and
iron, wherein the alloy optionally further comprises up to 5 wt% manganese, up
to 2
wt% copper up to 0.6 wt% nitrogen, up to 0.5 wt% aluminium and up to 0.5 wt%
vanadium.
According to a second aspect of the present invention, the object of the
invention is
solved by a nickel-chromium-iron-molybdenum alloy, consisting of 40 to 48 wt%
nickel
(Ni), 30 to 38 wt% chromium (Cr), 4 to 12 wt% molybdenum (Mo), optionally
manganese (Mn), optionally copper (Cu), optionally nitrogen (N), optionally
tungsten
(W), optionally niobium (Nb), optionally cobalt (Co), optionally carbon (C),
optionally
tantalum (Ta), optionally titanium (Ti), optionally silicon (Si), optionally
aluminium (Al)
and optionally vanadium (V) and balance iron (Fe) plus impurities.
Surprisingly, it was found that it is possible to achieve a high corrosion
resistance in
spite of reducing the amounts of molybdenum and nickel, which in practice due
to their
4 0 high price are the two primary constituents that determine the overall
final cost of a
corrosion resistant alloy. In order to obtain a single phase alloy, the
inventors have
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found that it is possible to increase the chromium content at the expense of
the
molybdenum and nickel instead of following the conventional way of increasing
the iron
content. It has been found that it is possible to dissolve very high amounts
of chromium
in an (austenitic) single phase matrix together with an optimal amount of
molybdenum
and very low amounts of nickel and iron. The alloys according to the invention
have the
desired good corrosion resistance and allow for achieving favourable
mechanical
properties. At the same time the amounts of costly materials may be reduced.
This way
the alloys according to the invention provide an economically viable and
robust
alternative for demanding applications such as contact with hot fluids having
a high
salinity (e.g. above 100 C, above 100 g/I chloride ions).
The indicated ranges of nickel, chromium and molybdenum allow balancing these
three
main alloying elements in order to achieve the desired favourable properties.
Outside
the ranges of 40 to 48 wt% nickel, 30 to 38 wt% chromium and 4 to 12 wt%
molybdenum, at least one of the following properties cannot be expected to be
favourable: corrosion resistance, structural properties (e.g. number of
phases) and
mechanical properties. Additionally, higher amounts of nickel and/or
molybdenum
would render the alloy uneconomical.
Other elements may be additionally added to the alloy according to the
invention.
Manganese and nitrogen may be useful for stabilizing a desired austenite
phase.
Tungsten, niobium, tantalum and titanium may be used for optimizing the
mechanical
properties. Silicon and manganese may improve melting and casting of the
alloy.
Copper and nitrogen may further improve the corrosion resistance of the alloy.
Carbon
2 5 may be present either as a side effect or as a deliberate addition. On
the one hand
carbon affects the corrosion resistance adversely; on the other hand when
added in the
right amounts carbon improves the mechanical properties. Aluminium may improve
forgeability and if used for castings can be used as deoxidation. Vanadium may
produce a fine grain structure while forging. Accordingly it is possible to
fine tune the
3 0 required properties of the alloy according the invention and thus to
either add a certain
amount of carbon e.g. to improve mechanical strength or to limit it to the
minimum
amount possible.
The alloy according to the invention is suitable for use in water with high
salinity (also
3 5 at high temperatures) and in the geothermal, off-shore, chemical, oil
and gas industry.
A preferred alloy according to the inventions consists of 40 to 48 wt% nickel,
30 to 38
wt% chromium, 4 to 12 wt% molybdenum, up to 5 wt% manganese, up to 2 wt%
copper, up to 0.6 wt% nitrogen, up to 5 wt% tungsten up to 3 wt% niobium, up
to 2 wt%
4 0 cobalt, up to 0.2 wt% carbon, up to 1 wt% tantalum, up to 1 wt%
titanium, up to 1 wt%
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silicon, up to 0.5 wt% aluminium, up to 0.5 wt% vanadium and balance iron plus
impurities.
In the context of the alloy according to the invention the term "impurities"
preferably
refers to unavoidable impurities, i.e. impurities that result automatically
when the alloy
is made up of the other components. Preferably the term "impurities" refers to
unwanted components. It goes without saying that the term "impurities" does
not
include any of the following elements: nickel, chromium, molybdenum,
manganese,
copper, nitrogen, tungsten, niobium, cobalt, carbon, tantalum, titanium,
silicon,
aluminium, vanadium and iron.
In a preferred alloy according to the invention, the sum of impurities is at
most 0.1 wt%,
preferably at most 0.05 wt% and more preferably at most 0.02 wt%. This allows
controlling the effect the impurities may have on the properties of the alloy.
A preferred alloy according to the invention as described above contains at
least 2
wt%, preferably at least 4 wt% iron.
According to the invention, an alloy is preferred that contains one or more of
the
following: (i) 42 to 48 wt% nickel, (ii) 32 to 38 wt% chromium, (iii) 4 to
11.5 wt%
molybdenum, (iv) 0.01 to 5 wt% manganese, (v) 0.1 to 2 wt% copper, (vi) 0.01
to 0.6
wt% nitrogen, (vii) up to 2 wt% tungsten, (viii) up to 1 wt% niobium, (ix) up
to 1.8 wt%
cobalt, (x) 0.002 to 0.2 wt% carbon, (xi) up to 0.5 wt% tantalum, (xii) up to
0.5 wt%
titanium, (xiii) 0.01 to 1 wt% silicon, (xiv) 0.01 to 0.5 wt% aluminium, (xv)
0.01 to 0.5
wt% vanadium.
The alloy according the invention accordingly has at least one, several or all
of the
features described above with reference to points (i) to (xv), i.e. the amount
of one,
several or all of the mentioned components is in corresponding ranges.
More preferred is an alloy to the invention that contains one or more of the
following: (i)
43 to 47 wt% nickel, (ii) 33 to 37 wt% chromium, (iii) 4 to 11 wt% molybdenum,
(iv) 0.02
to 2 wt% manganese, (v) 1 to 2 wt% copper, (vi) 0.05 to 0.4 wt% nitrogen,
(vii) up to 1
wt% tungsten, (viii) up to 0.2 wt% niobium, (ix) up to 1.5 wt% cobalt, (x)
0.005 to 0.1
wt% carbon, (xi) up to 0.2 wt% tantalum, (xii) up to 0.2 wt% titanium, (xiii)
0.02 to 0.7
wt% silicon, (xiv) 0.01 to 0.5 wt% aluminium, (xv) 0.01 to 0.5 wt% vanadium.
Even more preferred is an alloy according to the invention, wherein the alloy
contains
one or more of the following: (i) 43 to 46.5 wt% nickel, (ii) 33.5 to 37 wt%
chromium, (iii)
4.5 to 10.5 wt% molybdenum, (iv) 0.05 to 0.5 wt% manganese, (v) 1.5 to 1.8 wt%
copper, (vi) 0.1 to 0.3 wt% nitrogen, (vii) up to 0.5 wt% tungsten, (viii) up
to 0.05 wt%
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niobium, (ix) up to 1 wt% cobalt, (x) 0.01 to 0.02 wt% carbon, (xi) up to 0.05
wt%
tantalum, (xii) up to 0.05 wt% titanium, (xiii) 0.05 to 0.4 wt% silicon, (xiv)
0.01 to 0.5
wt% aluminium, (xv) 0.01 to 0.5 wt% vanadium.
5
Preferably the alloy according to the invention has a PREN value, calculated
as wt%Cr
+ 3.3*wt%Mo + 16*wt%N, of at least 40.
The PREN (pitting resistance equivalent number) value is a measure for the
corrosion
resistance of an alloy. Generally, the higher the PREN value, the more
resistant the
alloy is against corrosion. The PREN value of the alloy according to the
invention is
preferably at least 50, 55, 60, 65 or even 70.
Preferably the alloy according to the invention is austenitic. Preferably the
alloy
consists of a single phase. Alternatively the matrix of the alloy may
preferably consist of
a single phase harbouring precipitates that have a positive influence on the
desired
properties of the steel.
According to the invention, an alloy is preferred that is characterized by one
or more of
the following features (a) to (d) combined with one or more of the following
features (e)
to (f):
(a) an Rp0.2 proof strength measured according to DIN EN 10 002-
1:2001-12 of at
least 300 MPa at 25 C,
(b) an Rp0.2 proof strength measured according to DIN EN 10 002-1:2001-12
of at
least 250 MPa at 150 C,
(c) an Rm ultimate tensile strength measured according to DIN EN 10 002-
1:2001-
12 of at least 450 MPa at 25 C,
(d) an Rm ultimate tensile strength measured according to DIN EN 10 002-
1:2001-
12 of at least 400 MPa at 150 C,
(e) a material loss measured according to ASTM G 28 A of at most 0.5
mm/year,
(f) a material loss measured according to ASTM G 28 B of at most 2.5
mm/year.
The alloy accordingly preferably has one, two, three or all of the features
listed under
points (a) to (d) above and at the same time either or both of the features
listed under
(e) and (f). These preferred alloys have a combination of a high strength and
a high
corrosion resistance.
However according to another embodiment, preferred alloys according to the
invention
may simply have one or more of the features (a) to (f) as described above.
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Features (a) to (d) relates to the mechanical properties and specifically to
mechanical
strength. These values are determined according to standard DIN EN 10 002-
1:2001-
12. Points (e) to (f) refer to corrosion resistance as measured by ASTM G 28 A
(reducing conditions: 50% H2SO4 + 2.7% Fe2(S0.4)3) and ASTM G 28 B (pitting
corrosion and resistance against chloride attack: 23% H2SO4 + 1.2% HCI 4-1 %
FeCI3 +
1% CuC12). Preferably the Rp0.2 proof strength is at least 325 or 350 MPa at
25 C
and/or at least 275 or 300 MPa at 150 C, in each case measured according to
DIN 10
002-1:2001-12. Preferably the Rm ultimate tensile strength is at least 475 or
500 MPa
at 25 C and/or at least 425 or 450 MPa at 150 C, in each case measured
according to
DIN 10 002-1:2001-12. Preferably the material loss is at most 0.4, 0.3 or 0.2
mm/year
(measured according to ASTM G 28 A) and/or at most 2, 1.5 or 1 mm/year
(measured
according to ASTM G 28 B).
According to another aspect the present invention relates to a product
comprising an
alloy according to the invention. Preferred products are selected from the
group
consisting of powders, granules, sheets, plates, bars, wires, pipes, cast
products,
wrought products, rolled products, forgings and welding materials (e.g. filler
materials).
The present invention according to another aspect also relates to an article
for
applications in water with high salinity, comprising an alloy according to the
invention.
The water preferably has a chloride concentration above 100 g/I. Since the
alloys
according to the invention may withstand water with high salinity, the
articles according
to the invention are suitable for such applications. Preferred articles are
selected from
the group consisting of down-hole-headers, pipelines, tubes, valves, pumps and
housings.
According to another aspect the present invention relates to the use of an
alloy
according to the invention, a product according to the invention or an article
according
to the invention for applications in water with high salinity, preferably with
a chloride
concentration above 100 g/I. The present invention also relates to the use of
an alloy
according to the invention, a product according to the invention or an article
according
to the invention for applications in the geothermal, off-shore, chemical, oil
and gas
industry. Applications at high temperature are preferred.
The following examples further illustrate the invention.
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Examples
Examples 1 to 15: Experimental alloy castings
A number of experimental melts were cast using either a vacuum furnace and a
ceramic mould (examples 1-4), exposed to air using an induction furnace and a
Cu-
mould (examples 5-7), in production vertical spun-cast tubes with a diameter
of 330
mm, a height of 300 mm and a wall thickness of 70 mm (examples 8-10) and
horizontally-spun cast tubes with a diameter of 115 mm and a wall thickness of
12 mm
(examples 11-13). Example 1 is a comparative example in which the alloy has a
very
low molybdenum content.
Example 14 (alloy A31, comparative example) and example 15 (alloy A59,
comparative
example) were produced horizontally cast in order to allow comparison. The
chemical
compositions of the castings are listed in Table I, together with the
calculated PREN
values.
Table l - Chemical analysis
Ex. C Cr Ni Mo Fe Cu N
Mn Si PREN Melting Casting
1 0.014 34.7 44.3 0.03
Bal. 1.79 0.19 0.43 0.24 38 V CM
2 0.012 35.9 44.9 4.78
Bal. 1.50 0.16 0.46 0.29 54 V CM
3 0.020 35.6 43.4 7.12
Bal. 1.68 0.23 0.35 0.36 63 V CM
4 0.012 35.8 46.3 9.58
Bal. 1.51 0.27 0.40 0.23 72 V CM
5 0.012 34.6 44.8 5.50
Bal. 1.79 0.20 0.38 0.08 56 A Cu
6 0.020 34.8 45.0 7.43
Bal. 1.61 0.19 0.43 0.10 62 A Cu
7 0.020 34.0 , 43.3
9.44 Bal. 1.70 0.22 0.47 0.11 69 A Cu
8 0.017 34.5 44.9 5.64
Bal. 1.77 0.19 0.37 0.08 57 A VC
9 0.018 34.6 45.2 7.51
Bal. 1.63 0.18 0.41 0.11 63 A VC
10 0.017 33.9 43.6 9.43 Bal. 1.67 0.20 0.46 0.13 69 A
VC
11 0.015 33.7 46.4 4.84 Bal. 1.59 0.27 0.07 0.07 54 A
HC
12 0.015 33.8 46.0 7.23 Bal. 1.48 0.19 0.10 0.07 61 A
HC
13 0.010 36.7 46.0 9.59 Bal. 1.53 0.26 0.08 0.08 72 A
HC
14 0.020 26.4 30.1 6.6 Bal. 1.12 0.26 0.40 0.24 52 A
HC
15 0.010 22.5 Bal. 15.2 1.2 0.01 0.07 0.25 0.10 76 A
HC
The chemical compositions are given in wt%. The PREN value is calculated as
wt%Cr
+ 3.3*wt%Mo + 16*wt%N. Melting is indicated as V = Vacuum and A = Air. The
casting
method is indicated as CM = Ceramic mould, Cu = Cu-mould, VC = Vertical
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Centrifugally cast (70 mm wall thickness) and HC = Horizontal Centrifugally
cast (12
mm wall thickness). Further abbreviations are Ex. = Example, Bal. = Balance.
Example 16: Mechanical properties
In Table II the mechanical properties for the alloy castings from Examples 1
to 15 are
presented. All alloys according to the invention (alloys of Examples 2 to 13)
show very
good values. The alloys of Example 4, 7, 10 and 13 show particularly good
values, the
proof strength is 30-40 MPa above that of A59 (alloy of Example 15) at room
temperature. At 150 C, the alloys of Examples 4 and 7 still have a proof
strength well
above 300 MPa, and the alloy of Example 10 slightly below. The alloys of
Examples 2,
3, 5, 6, 8 and 9 have proof strengths roughly equal to A59. The alloy of
Example 1,
substantially without Mo, exhibits good, but in comparison to the other
castings the
poorest mechanical values. The impact toughness values for all alloys
according to the
1 5 invention are well above 200, i.e. excellent. The ductility values
are as well high, all well
above 14%, which is the common limit for pressure vessels.
The mechanical properties of the alloy according to the invention are
significantly better
than those of A31 (Example 14) and with a Mo content in the lower range,
comparable
to those of A59 (Example 15). With a Mo content in the higher range, the alloy
becomes superior to both A31 and A59, especially at elevated temperatures
(T>100 C).
Table II ¨ Mechanical properties
Alloy of Room Temperature 150 C Room
Temperature
Example Rp0.2 Rm A5 R0.2 Rm
A5 Impact Toughness
MPa MPa % MPa MPa % J/cm2
1 287 523 51 194 444 56 273
2 325 547 61 271 487 62 344
3 338 559 46 297 518 60 250
4 372 497 37 318 444 52 352
5 347 592 64 298 526 53 273
6 338 584 61 272 519 59 264
7 379 577 40 319 502 44 259
8 .314 580 59 272 513 59 445
9 335 578 55 290 439 49 256
10 368 595 64 293 463 50 256
11 345 588 60 267 511 62 317
12 349 611 64 273 516 64 303
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13 399 630 64 307 532 61 338
14 317 614 66 243 516 64 324
15 326 655 74 272 581 71 359
14* 276 650 40 185
15* ?.340 690 40 --- ?. 225
* = Literature values, rolled sheet material.
= Proof strength
Rm = Ultimate tensile strength
A5 = Ductility
= not available
Example 17: Corrosion resistance properties
In Table III the results from the corrosion tests on the alloys according to
the invention
of Examples 2 to 4 and 8 to 10 according to ASTM G 28 A (reducing conditions),
ASTM
G 28 B (pitting corrosion and resistance against Cl- attack) and ASTM A 262 C
(oxidizing conditions) are shown. The alloys A31 (Example 14), A59 (Example
15) and
the alloy of Example 1, none of which according to the invention, were also
tested for
comparison.
The theoretical values for the Critical Pitting Temperature (CPT) and Critical
Crevice
Temperature are also included as well as the calculated PREN value. The
results show
that A59 can withstand pitting corrosion and Cl- induced attack (G 28 B) very
well,
2 0 while it performs significantly worse when exposed to reducing
conditions (G 28 A). For
A31 it is the other way around, and the material loss in the presence of Cl-
ions
reaches several mm per year. All investigated alloys according to the
invention show
that these alloy compositions, regardless of their exact Mo content, have
excellent
corrosion resistance when it comes to both reducing and oxidizing acids. Mo
has an
2 5 effect on the resistance against Cl- induced general corrosion and
pitting corrosion.
The alloy of Example 1 (comparative example) substantially without Mo suffers
from
severe material loss as well as from pitting corrosion. As soon Mo in the
range
according to the invention is added, the corrosion rate is retarded by about a
hundredfold. Additional Mo reduces the rate even further and with 9-10 %wt Mo,
values
30 close to A59 are reached, but without losing the excellent properties of
the alloys
according to the invention in reducing acid. In the end the alloy according to
the
invention allow combining the favourable properties of A31 (corrosion
resistance under
reducing conditions) and A59 (resistance against pitting corrosion and Cl-
induced
attack) without suffering from their unfavourable properties.
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The theoretical values have been added in order to allow for a qualitative
comparison
between the castings and A31 and A59. The CPT and CCT values listed are the
calculated starting values for the CPT and CCT test according to the ASTM G 48
standard, which standard however only allows testing up to 85 C.
5
The alloys according to the invention have several benefits compared to the
two
commercial alloys used as benchmark, A31 (Example 14) and A59 (Example 15).
They
have a good corrosion resistance against both reducing and oxidizing acids and
induced pitting corrosion. Alloy 31 (A31) can resist reducing conditions but
has rather
10 poor pitting resistance, while Alloy 59 (A59) has good pitting
corrosion resistance but
cannot compete with A31 or the alloys according to the invention in reducing
solutions.
Table III ¨ Corrosion resistance
Alloy of Experimental Theoretical
Example G 28 A G 28 B A 262 C *CPT *CCT PREN
mm/year, mm/year mm/year C C
1 0.11 364 0.08 52 32 38
2 0.10 2.51 90 72 54
3 0.20 1.95 0.14 109 89 63
4 0.20 0.36 0.18
130 109 72
5 --- --- 94 70 56
6 108 89 62
7 123 102 69
8 0.16 4.2 0.04 94 74 57
9 0.23 1.2 0.05 108 89 63
10 0.15 0.08 0.07
125 105 69
11 89 66 54
12 105 84 61
13 132 112 72
14 0.15 2.8 82 57 52
1.25 0.25 138 114 76
G 28 A: 50% H2SO4 + 2.7% Fe2(SO4)3
G 28 B: 23% H2SO4 + 1.2% HCI +1 % FeCI3 + 1% CuCl2
A 262: 65% HNO3
CPT = Critical Pitting Temperature, CCT = Critical Crevice Temperature
* = Values calculated according to ASTM G48
PREN = wt%Cr + 3.3*wt%Mo + 16*wt%N
= not available
