Note: Descriptions are shown in the official language in which they were submitted.
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Nickel-Chromium-Molvbdenum Allo
The present invention relates to an austenitic nickel-
chromium-molybdenum alloy having a high corrosion resistance in
relation to oxidizing and reducing media.
As a rule, austenitic nickel alloys, which display
outstanding resistance to reducing media such as hydrochloric acid,
are alloyed with 26 to 30% molybdenum. Examples of known nickel-
1o molybdenum alloys are Alloy B-2 (2.4617) and Alloy B-4 (2.4600).
These alloys are resistant to hydrochloric acid up to the boiling
point providing that there are no traces of oxidizing components
such as Fe3+ in the acid. Such traces can, however, be introduced
very easily as impurities as the acid is being transported. In a
similar way, the presence of traces of HN03 causes a sharp increase
in the corrosion rate of nickel-molybdenum alloys. For this
reason, there has been no lack of attempts to render quasi-binary
nickel-molybdenum alloys immune to oxidizing impurities by
modification of their composition.
2o As an example, EP 0 723 029 A1 describes a nickel-
molybdenum-chromium-iron alloy that possesses outstanding
resistance to corrosion in relation to acid, lower-oxygen media.
This has been achieved in that on the average, the molybdenum
content was cut back to 23% and in that, in contrast to this, the
material was alloyed, on the average, with about 8% chromium.
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On the other hand, corrosion-resistant nickel alloys with
chromium and molybdenum as the principal alloying components are
also known, these containing 14 to 26% chromium and 3 to 18%
molybdenum or 20 to 24% chromium, 12 to 17% molybdenum, 2 to 4%
tungsten, and 2 to 8% iron.
According to EP 0 334 410 B1, an alloy with 22 to 24%
chromium and 15 to 16.5% molybdenum can be used advantageously for
the working conditions of chemical process technology and
environmental protection technology. According to their topical
to composition, these materials possess good resistance to eroding
corrosion and against pitting and fissure corrosion. Because of
their high chromium content, however, their resistance under
reducing conditions is not as good as in oxidizing media. Attempts
have been made to correct this deficiency in that some 2 to 4%
tungsten is added to an alloy with about 19 to 23% chromium and
about 14 to 17% molybdenum.
One disadvantage to alloys of such composition is that
they require a special homogenizing-annealing treatment, as is
described in detail in European Patent Application EP 0 392 484 A1.
2o In addition, this alloy possesses low thermal stability, which is
a disadvantage when it has to be processed, for example by welding.
This is made clear by the results tabulated below, which show
corrosion loss in mm/year in a standard test of inter-crystalline
corrosion according to ASTM G 28 A, whereby a typical alloy
composition according to EP 0 392 484 A1 is checked, once in the
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. solution-annealed state and once in a thermally stressed state for
1 hour at 870~C. In one case, corrosion loss was 2.63 mm/year and
in the other case it was 22.14 mm/year. One can recognize the low
stability in relation to thermal stress in that the 1-hour change
at 870~C causes corrosion loss in the standard test according to
ASTM G-28 A to increase by a factor of more than 8 in the case of
the alloy according to EP 0 392 484 Al. Alternatively, European
Patent Application EP 0 693 565 A2 proposes a nickel-chromium-
molybdenum alloy in which--amongst others--1.0 to 3.5% copper is
1o alloyed in with 22.0 to 24.50 chromium and 14.0 to 18.0%
molybdenum. Nitrogen in amounts of up to 0.150 can be added to
this combination as a strength-enhancing element, although it is
preferred that no more than 0.06% be added, because according to
the teachings of EP 0 693 565 A2 nitrogen in this alloying
combination with copper is detrimental to corrosion resistance in
hydrochloric acid, the typical reducing medium. Besides, the
addition of copper diminishes thermal stability, which is a
disadvantage from the standpoint of processing, for example by
welding. This is shown clearly by the results set out below, which
2o were obtained once as corrosion loss in mm/year in the standard
test for intercrystalline corrosion according to ASTM G-28 A,
whereby a typical alloying composition according to EP 0693 565 A 2
was tested once in the solution-annealed state and once in an
altered state after 1 hour at 870~C. In one case, corrosion loss
was 0.68 mm/year and in the other case it was 2.90 mm/year.
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One can recognize the low stability in relation to
thermal stress in that the 1-hour change at 870~C causes corrosion
loss to increase by a factor of more than 4.
The low thermal stability of this alloy is even more
clearly seen if it is tested once in the solution-annealed state
and once in the altered state after 1 hour at 870°C in the
alternative standard test for intercrystalline corrosion according
to ASTM G-28 B. Corrosion losses of 0.11 and 57.8 mm/year are then
seen, which is to say it is increased by more than 500-fold by one-
to hour aging, when the corrosive attack was so powerful along the
grain borders that complete grains fell out of the material during
the test.
In addition, the use of a nickel-chromium-molybdenum
alloy with 20% chromium, 20% molybdenum, and 2o tantalum for heat-
exchanger tubes to recover heat from the flue gasses of power
stations fired by fossil fuels is also known. According to
CORROSION 96, Paper No. 413, this alloy is reputed to be
particularly suitable for withstanding the dew-point corrosion of
the sulphuric acid that occurs in this application. Because of the
2o composition of the alloy, its thermal stability is estimated to be
no better than that of the two alloys according to EP 0 392 484 Al
and EP 0 693 565 A2 discussed heretofore.
This results in the task of describing a nickel-chromium-
molybdenum alloy that has a balanced corrosion resistance in a
number of both oxidizing and reducing media, whereby it is superior
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to the prior art, but without the disadvantage that, on the one
hand, it requires a special
homo-agenizing-annealing
treatment and,
on the other, without he disadvantage of low thermal stability.
t
According to the present invention, this has been
achieved with a nickel-chromium-molybdenum
alloy of the following
composition (o by mass)
Chromium 20.0 to 23.0%
Molybdenum 18.5 to 2l.Oo
Iron max. 1.50
1o Manganese max. 0.50
Silicon max. O.lOo
Cobalt max. 0.3%
Tungsten max. 0.3%
Copper max. 0.30
Aluminum 0.1 to 0.3%
Magnesium 0.001 to 0.015%
Calcium 0.001 to 0.010%
Carbon max. 0.01%
Nitrogen 0.05 to 0.15%
2o Vanadium 0.1 to 0.3%
the rest consisting of nickel and impurities resulting from the
melting process.
A preferred nickel-chromium-molybdenum
alloy contains the
following composition (% by mass):
Chromium 20.5 to 21.50
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Molybdenum 19.5 to 20.5%
Iron max. 1.5%
Manganese max. 0.5%
Silicon max. 0.10%
s Cobalt max. 0.3%
Tungsten max. 0.3%
Copper max. 0.3%
Aluminum 0.1 to 0.3%
Magnesium 0.001 to 0.015%
1o Calcium 0.001 to 0.010%
Carbon max. 0.01%
Nitrogen 0.08 to 0.12%
the rest consisting of nickel and impurities resulting from the
melting process.
15 According to another concept of the present invention,
the following additives (% by mass) can be alloyed with the one or
the other nickel-chromium-molybdenum alloy:
Boron max. 0.0030%
Phosphorous max. 0.020%
2o Sulphur max. 0.010%
Niobium max. 0.20%
Titanium max. 0.02%
A preferred area of application for the object of the
present invention can be seen in the fact that the nickel-chromium-
25 molybdenum alloy is used in chemical plants for structural elements
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that must be highly resistant to oxidizing and to reducing media at
one and the same time.
It is also possible to use the alloy according to the
present invention as a welding filler for super-alloyed welding of
highly alloyed NiCrMo materials and as a welding filler of the same
type, with little segregation, with itself.
An alloy that is of especially universal use has a
chromium:molybdenum ratio between 1.9 and 2.3, as calculated in
substance content based on % by atoms, with a ratio of
to approximately 2.0 being considered as the optimal.
Most surprisingly, it has been shown that in the case of
this alloy composition of, on average for example, approximately
21o chromium and 20o molybdenum it is possible to so broaden the
nickel-chromium-molybdenum three-substance system such that is
approximately present here, that--on the one hand-- the substance
requires no homogenizing-annealing treatment but--on the other
hand--the corrosion resistance remains unimpaired in reducing media
such as hydrochloric acid, which would have to be anticipated here,
too, according to the prior art described heretofore.
2o The present invention will now be described in comparison
to the known nickel alloys according to the prior art.
At the designations 1, 2, and 3, Table 1 shows
embodiments of the alloy according to the present invention. These
three embodiments were smelted in the usual manner in an induction
oven and processed into 5 and 12-mm thick plates, in conformity
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. with the standard production methods for nickel-based materials.
As can be seen from Table 1, the comparison materials that
correspond to the prior art all have nitrogen contents that are
clearly below 0.040.
Table 2 shows the test results for the three embodiments
of the alloy according to the present invention, as well as for
comparison materials for two oxidizing test solutions of different
strengths (ASTM G 28 A and B), as well as two reducing test media
(SEP 1877 III and DuPont SW 800 M). As expected, the high mol
content variants Alloy B-2/B-4 and B-10 are not resistant to
corrosion under these oxidizing conditions. Even in the weaker
oxidizing solution of the ASTM G 28 B test, these materials display
even more pronounced intercrystalline corrosion and decompose
actively. In contrast to this, the embodiments of the alloy
according to the present invention are distinguished both in the
oxidizing and in the reducing media by resistance to
intercrystalline corrosion and exhibit the lowest overall mass loss
rates for both media ranges. Embodiment 2 of the alloy according
to the present invention, which has a Cr:Mo ratio that is slightly
less than 2, is distinguished by particularly low rates of
corrosion when examined in this way.
This is made clear in Table 3, that presents the results
of testing the embodiments of the alloy according to the present
invention and the comparison materials as total values of the
measured corrosion loss in tests conducted under oxidizing and
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. under reducing conditions, as set out in Table 2. Alloy 1, with
18.6% molybdenum is just above the value according to the present
invention, although it has properties that are better than the
known prior art according to C-2000. This indicates that such an
alloy can be used to better effect for certain applications.
As can be seen from Table 4, in another part of the test,
the behaviour of the materials was determined in 2-% boiling
hydrochloric acid. Under these conditions, the corrosion rate of
the three embodiments of the alloy according to the present
to invention is at the level found for the nickel-molybdenum alloys
groups of materials--the so-called B alloys--and is clearly below
the other comparison materials that correspond to the prior art. A
higher value is also cited for the alloy NiCr20-Mo20Ta (Corrosion
96, Paper No. 413).
A test medium that occupies a position mid-way between
oxidizing and reducing conditions is represented by the test in 90%
sulphuric acid. As is shown by Table 5, the embodiments 1 to 3 of
the alloy according to the present invention are extremely
resistant to corrosion under these conditions. In particular, the
2o alloy variation No. 2, that has a Cr:Mo ratio of just under 2
exhibits resistance to corrosion that is at the level of the B-
allot's.
The excellent resistance to local corrosion of
embodiments 1 to 3 of the alloy according to the present invention
when in an acid medium that contains chloride could be proven by
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testing in "Gruner Tod" [Green Death] medium. According to Table
6, the material Alloy 59 according to the prior art exhibits
excellent resistance to corrosion. In "Gruner Tod" test medium,
the critical pitting temperature is 125~C. In contrast to this,
under these conditions, no pitting could be identified in the
samples according to the present invention, even at 135°C. The
alloys from the group of B-materials are not resistant in this test
medium. The three embodiments of the new alloys according to the
present invention exhibit the best behaviour.
1o Despite the high alloy shares, the test alloys exhibit
structural stability that is superior to that of the comparison
materials. As is made clear by Table 7, samples were sensibi-lized
for 1 hour and 3 hours at 870~C and then subjected to testing
according to ASTM G 28 B. After thermal stressing, the comparison
materials C-276, C-22, 686, and C-2000 exhibit not only a sharp
increase of corrosion rate, but also exhibit inter-crystalline
corrosion to such a degree that individual grains fall out. In
contrast to this, the materials according to the present invention
were free of any manifestations of local corrosion, and were
2o distinguished by a particularly low rate of mass loss. It is true
that in this test, only alloy 59 exhibited better behaviour,
although under the "Gruner Tod" test conditions (Table 6) and in
90-o sulphuric acid (Table 5) and in 2-o hydro-chloric acid, as
well as in the overview according to Table 3 it is less resistant
to corrosion.
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Table 8 combines the results of testing of the mechanical
properties of the three embodiments of the alloy according to the
present invention, and compares them to typical values for the
comparison materials for the identical range of dimensions. It can
be seen that in comparison to the alloys forming the prior art, the
mechanical properties are not affected detrimentally.
Table 9 shows a comparison of the upper and the lower
limiting values on the one hand of chromium and, on the other hand,
of molybdenum. In each instance, the data are cited in by mass and
1o in o by atoms. Based on the lower and upper limiting values cited
in Claim 1, the Cr:Mo ratio is between 1.99 and 2.02 (calculated as
o by atoms)
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