Note: Descriptions are shown in the official language in which they were submitted.
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AUSTENITIC STAINLESS STEEL
FIELD OF TECHNOLOGY
The present disclosure relates to austenitic stainless steels. More
specifically,
the present disclosure relates to austenitic stainless steels having improved
creep
resistance and/or improved corrosion resistance when subjected to high
temperature
environments.
DESCRIPTION OF THE BACKGROUND OF THE TECHNOLOGY
High temperature air presents a particularly corrosive environment. Even
more aggressive corrosion conditions can occur if significant water vapor is
present.
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The combination of high temperature air and significant water vapor is
common in energy generation devices such as, for example, gas turbines, steam
turbines, and fuel cells, and in heat exchangers and recuperators handling the
gas
streams used or generated by such energy generation devices, as well as in
equipment for treating, processing, or extracting chemicals or minerals at
high
temperatures. Accordingly, parts of such devices subjected to these conditions
have
been fabricated from a variety of austenitic stainless steels.
To enhance corrosion resistance, austenitic stainless steels include various
combinations of chromium, nickel, manganese, and other alloying additions.
Nevertheless, stainless steels and certain other chromium-bearing heat-
resistant
alloys are susceptible to attack in high temperature air and in high
temperature air
containing water vapor. This attack takes two distinct forms. Low-alloy
content
stainless steels such as, for example, AISI Type 304 (nominally 18 weight
percent
chromium and 8 weight percent nickel, balance iron), suffer from accelerated
oxidation in the presence of water vapor. The slow-growing chromium oxide film
is
displaced by a thick scale comprised of rapidly growing mixed iron and
chromium
oxides. The result is rapid metal wastage by conversion to oxide. High-alloy
content
materials such as, for example, superferritic iron-chromium stainless steels
and
nickel-chromium superalloys, appear to be immune to this form of attack, but
have
been observed to suffer from weight loss during exposure to water vapor. The
oxide
that forms on certain of the high-alloy content materials is very pure
chromium oxide
and is susceptible to evaporation through the formation of volatile chromium
oxyhydroxides. The result of this evaporative loss of chromium to the
atmosphere is
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an abnormally high level of chromium depletion in the metal substrate, and
this can
lead to a loss of high temperature oxidation resistance. The transition
between the
foregoing corrosion states is relatively complex, with aspects of both states
noted in
some alloys.
In addition to corrosion, articles and parts in high temperature environments
may suffer from creep. Creep is the undesirable plastic deformation of alloys
held
for long periods of time at stresses lower than the normal yield strength.
Thus, creep
may affect certain structural parts and other parts subject to high stresses
and high
temperatures in, for example, energy generation devices and related devices,
and in
equipment and parts for high temperature processing, treating, or extracting
chemicals or minerals, or for high temperature treating or processing alloys.
In such
applications, it is often desirable that parts are formed from a material that
has
substantial resistance to corrosion in high temperature environments, and that
also
has substantial creep resistance.
The alloying element manganese has been shown play a role in mitigating the
effects of chromium oxide vaporization. Many stainless steel specifications
include
manganese at levels limited to 2 weight percent or less, with no required
minimum
level. The manganese in these steels is not an intentional alloying addition
but,
instead, is included in the steel as an incidental ingredient derived from the
scrap
starting materials. One austenitic stainless steel adapted for use in high
temperature, high water vapor content environments that includes an
appreciable
allowance for incidental manganese is NF709 alloy. NF709 alloy has been
available
from Nippon Steel Corporation in forms including of seamless tubing for boiler
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applications. The composition of NF709 alloy, which is provided in the Nippon
Steel
publication "Quality and Properties of NF709 Austenitic Stainless Steel for
Boiler
Tubing Applications," is shown in Table 1. The published composition specifies
a
manganese limit of 1.5 weight percent, with no specified minimum. According to
various published accounts of research on this alloy, the typical commercial
manganese content is approximately 1 weight percent. Certain other austenitic
stainless steels are also shown in Table 1. Elemental concentrations
throughout the
present description are weight percentages based on total alloy weight unless
otherwise indicated. "NS" in Table 1 indicates that the particular UNS
specification
does not specify a concentration for the element.
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Table 1
Type Esshete NitroniCTM Type Type
NF709 201L 1250 60 309S 310S
UNS none S20100 S21500 S21800 S30908 S31008
Number
Carbon 0.10 0.03 max 0.06-0.15 0.10 max
0.08 0.08 max
max max
Molybdenum 1.0-2.0 NS 0.8-1.2 NS NS NS
Chromium 19.0- 16-18 14-16 16-18 22-24 24-26
23.0
Nickel 22.0- 3.5-5.5 9-11 8-9 12-15 19-22
28.0
Niobium 0.10- 0.75-1.25 0.75-1.25 NS NS
NS
0.40
Manganese 1.50 5.5-7.5 5.5-7.5 7-9 2.0 max
2.0 max
max
Silicon NS NS NS 3.5-4.5 0.75 0.75
max
max
Titanium 0.02- NS NS NS NS NS
0.20
Nitrogen 0.10- 0.25 max NS 0.08-0.18 NS NS
0.25
With reference to Table 1, basic AISI Type 201 stainless steel is similar to
standard 18 chromium-8 nickel stainless steels, but with a fraction of nickel
replaced
with manganese to lower alloy cost. In general, Type 201 alloy does not
possess
sufficient creep and oxidation resistance for use at elevated temperatures.
Higher-
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alloyed materials such as the NITRONIC family of alloys, Esshete 1250 alloy,
and
21--6-9 alloy (UNS S21900), include low nickel levels (about 11 weight percent
maximum) and significant manganese levels (5-10 weight percent), and are
typically
designed for high creep strength and moderate environmental resistance.
Commercially available heat-resistant stainless steels such as AISI Types 309S
and
310S generally include manganese at levels up to about 2 weight percent. These
alloys are somewhat deficient in terms of metallurgical stability, which may
be tied to
their basic compositions inasmuch as the nickel-to-chromium ratio for these
two
grades results in the formation of significant amounts of brittle phases at
typical use
temperatures.
It would be advantageous to provide austenitic stainless steels having
improved high temperature creep resistance and/or resistance to corrosive
attack in
high temperature air and/or in high temperature air containing appreciable
levels of
water vapor. For example, stainless steels exhibiting substantial corrosion
resistance in high temperature air including water vapor could be
advantageously
employed in, for example, parts of energy generation devices including, for
example,
gas turbines, steam turbines, and fuel cells, which are subjected to highly
corrosive
high temperature-high water vapor content environments. Such parts include
heat
exchangers, recuperators, tubing, pipe, and certain structural parts. Alloys
exhibiting
substantial corrosion resistance in high temperature air also may be
advantageously
applied in certain devices for high temperature processing, treatment, or
extraction of
chemicals or minerals, or for high temperature processing or treatment of
alloys.
Stainless steels exhibiting both substantial high temperature creep resistance
as well
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as significant corrosion resistance could be advantageously adapted for use in
parts
of the foregoing devices that are subjected to high stresses.
SUMMARY
According to the present disclosure, austenitic stainless steels are provided
having improved high temperature creep resistance and/or improved resistance
to
corrosion when exposed to a high temperature air environment. As used herein,
"high temperature" refers to temperatures in excess of about 100 F (about 37.8
C).
According to one aspect of the present disclosure, an austenitic stainless
steel is
provided including: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23
chromium; 25
to 27 nickel; Ito 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to
0.75
niobium; up to 0.1 titanium; iron; and incidental impurities. In certain non-
limiting
embodiments, the manganese content of the steel is at least 1.6 up to 4.0
weight
percent. Also, in certain non-limiting embodiments, the austenitic stainless
steel
further includes one or more of the following elements: greater than 0 to 0.50
silicon;
greater than 0 to 0.30 aluminum; greater than 0 to 0.02 sulfur; greater than 0
to 0.05
phosphorus; greater than 0 to 0.1 zirconium; and greater than 0 to 0.1
vanadium.
According to certain non-limiting embodiments, the titanium and/or aluminum
content
of the steel is no greater than 0.1 weight percent.
As used herein, the use of "up to" without reference to a lower limit includes
the absence of the referenced element. Also, as used herein, "no greater than"
with
reference to titanium and aluminum content includes the absence of these
elements.
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According to another aspect of the present disclosure, an austenitic stainless
steel is provided that includes: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20
to 23
chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0
manganese;
0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to 0.30
aluminum; up
to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1
vanadium; iron;
and incidental impurities. In certain non-limiting embodiments, the manganese
content of the steel is at least 1.6 up to 4.0 weight percent. Also, according
to
certain non-limiting embodiments, the titanium and/or aluminum content of the
steel
is no greater than 0.1 weight percent.
According to yet another aspect of the present disclosure, an austenitic
stainless steel is provided that consists essentially of the following: 0.05
to 0.2
carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2
molybdenum;
greater than 1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium;
up to
0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus;
up to 0.1
zirconium; up to 0.1 vanadium; iron; and incidental impurities. According to
certain
non-limiting embodiments, the manganese content of the steel is at least 1.6
up to
4.0 weight percent.
According to yet a further aspect of the present disclosure, an austenitic
stainless steel is provided that consists of: 0.05 to 0.2 carbon; 0.08 to 0.2
nitrogen;
20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0
manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50 silicon; up to
0.30
aluminum; up to 0.02 sulfur, up to 0.05 phosphorus; up to 0.1 zirconium; up to
0.1
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vanadium; iron; and incidental impurities. In certain non-limiting embodiments
of the
steel, the manganese content of the steel is at least 1.6 up to 4.0 weight
percent.
Another aspect of the present disclosure is directed to an austenitic
stainless
steel including, in weight percentages based on total weight of the steel:
0.05 to 0.2
carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2
molybdenum;
up 4.0 manganese; 0.20 to 0.75 niobium; at least one of no greater than 0.1
titanium
and no greater than 0.1 aluminum; iron; and incidental impurities.
A further aspect of the present disclosure is directed to an austenitic
stainless
steel consisting essentially of, in weight percentages based on total weight
of the
steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27
nickel; 1
to 2 molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no
greater than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50
silicon; up to
0.02 sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium;
iron;
and incidental impurities. In certain non-limiting embodiments, the steel
includes at
least 1.5 up to 4.0 weight percent manganese, while in other embodiments the
steel
includes 1.6 up to 4.0 weight percent manganese.
Yet a further aspect of the present disclosure is directed to an austenitic
stainless consisting of, in weight percentages based on total weight of the
steel:
0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel;
1 to 2
molybdenum; up to 4.0 manganese; 0.20 to 0.75 niobium; at least one of no
greater
than 0.1 titanium and no greater than 0.1 aluminum; up to 0.50 silicon; up to
0.02
sulfur; up to 0.05 phosphorus; up to 0.1 zirconium; up to 0.1 vanadium; iron;
and
incidental impurities. In certain non-limiting embodiments, the steel includes
at least
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. .
1.5 up to 4.0 weight percent manganese, while in other embodiments the steel
includes 1.6 up to 4.0 weight percent manganese.
In a more preferred aspect, in the aforementioned austenitic stainless steel
the
ratio of niobium to carbon in the steel satisfies the formula 0.7 <
(niobium/carbon)
1.0, wherein the niobium and carbon contents in the formula are expressed in
atom
percentages.
According to yet another aspect of the present invention, an article of
manufacture is provided including an austenitic stainless steel having a
composition
according to the present disclosure. Non-limiting embodiments of the article
of
manufacture include, for example, energy generation devices and parts of such
devices. For example, the article of manufacture may be selected from a gas
turbine,
a steam turbine, a fuel cell, a heat exchanger, a recuperator, a tube, a pipe,
a
structural part, and other parts for any of those devices. Other examples of
the article
of manufacture include equipment or piping, tubing, and other parts for
equipment for
high temperature processing, treatment, or extraction of chemicals and
minerals, or
for high temperature processing or treatment of alloys.
According to yet another aspect of the present invention, there is provided an
austenitic stainless steel consisting of, in weight percentages based on total
weight
of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23 chromium; 25
to 27
nickel; 1 to 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to 0.75
niobium;
up to 0.1 titanium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02
sulfur; up to
0.05 phosphorus; up to 0.1 vanadium; residual iron; and incidental impurities.
According to yet another aspect of the present invention, there is provided an
article of manufacture including an austenitic stainless steel consisting of,
in weight
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percentages based on total weight of the steel: 0.05 to 0.2 carbon; 0.08 to
0.2
nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater than
1.5
to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.1 titanium; up to 0.50
silicon; up
to 0.30 aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1
vanadium;
residual iron; and incidental impurities.
According to yet a further aspect of the present invention, there is provided
an austenitic stainless steel consisting of, in weight percentages based on
total
weight of the steel: 0.05 to 0.2 carbon; 0.08 to 0.2 nitrogen; 20 to 23
chromium; 25
to 27 nickel; Ito 2 molybdenum; greater than 1.5 to 4.0 manganese; 0.20 to
0.75
niobium; up to 0.50 silicon; up to 0.30 aluminum; up to 0.02 sulfur; up to
0.05
phosphorus; up to 0.1 vanadium; residual iron; and incidental impurities;
wherein
the austenitic stainless steel comprises an oxide scale characterized by a
MnO/Cr203 molar ratio of at least 1.0, determined using X-ray energy-
dispersive
spectroscopy.
According to yet a further aspect of the present invention, there is provided
an article of manufacture including an austenitic stainless steel consisting
of, in
weight percentages based on total weight of the steel: 0.05 to 0.2 carbon;
0.08 to
0.2 nitrogen; 20 to 23 chromium; 25 to 27 nickel; 1 to 2 molybdenum; greater
than
1.5 to 4.0 manganese; 0.20 to 0.75 niobium; up to 0.50 silicon; up to 0.30
aluminum; up to 0.02 sulfur; up to 0.05 phosphorus; up to 0.1 vanadium;
residual
iron; and incidental impurities; wherein the austenitic stainless steel
comprises an
oxide scale characterized by a MnO/Cr203 molar ratio of at least 1.0,
determined
by using X-ray energy-dispersive spectroscopy.
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. .
The reader will appreciate the foregoing details, as well as others, upon
considering the following detailed description of certain non-limiting
embodiments
within the present disclosure. The reader also may comprehend additional
advantages and details upon evaluating or using alloys and articles of
manufacture within the present disclosure
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the alloys and articles described herein
may be better understood by reference to the accompanying drawing in which:
Figure 1 is a plot of weight change over time for alloy samples exposed at
1300 F (704 C) in air containing 10% water vapor;
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Figure 2 is a plot of weight change over time for alloy samples exposed at
1400 F (760 C) in air containing 7% water vapor;
Figure 3 is a plot of weight change over time for alloy samples exposed at
1500 F (815 C) in air containing 7% water vapor;
Figure 4(a) and 4(b) are micrographs of oxide scale formed on alloy samples
exposed to high temperature environments including water vapor;
Figure 5 is a graph of oxide composition, measured as a molar ratio of MnO to
Cr203, for several alloys subjected to high temperature environments including
water
vapor;
Figure 6 is a plot of chromium content of two alloy samples as a function of
depth into the sample;
Figure 7 is a plot of chromium content of two alloy samples as a function of
depth into the sample;
Figure 8 is a graph of oxide composition, measured as a molar ratio of MnO to
Cr203, for high manganese and low manganese samples subjected to high
temperature environments including 7% water vapor; and
Figure 9 is a plot of weight change over time for alloy samples exposed at
1400 F (760 C) in air containing 10% water vapor.
DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS
Other than in the operating examples, or where otherwise indicated, all
numbers expressing quantities of ingredients, processing conditions and the
like
used in the present description and claims are to be understood as being
modified in
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. .
all instances by the term "about". Accordingly, unless indicated to the
contrary, any
numerical parameters set forth in the following description and the attached
claims
are approximations that may vary depending upon the desired properties one
seeks
to obtain in the alloys and articles according to the present disclosure.
Notwithstanding that the numerical ranges and parameters setting forth the
broad scope of the present disclosure are approximations, the numerical values
set
forth in any specific examples herein are reported as precisely as possible.
Any
numerical values, however, inherently contain certain errors, such as, for
example,
equipment and/or operator errors, necessarily resulting from the standard
deviation
found in their respective testing measurements. Also, it should be understood
that
any numerical range recited herein is intended to include the range boundaries
and
all sub-ranges subsumed therein. For example, a range of "1 to 10" is intended
to
include all sub-ranges between (and including) the recited minimum value of 1
and
the recited maximum value of 10, that is, having a minimum value equal to or
greater
than 1 and a maximum value of equal to or less than 10.
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. .
As described above, certain austenitic stainless steels have been used in
articles
and parts exposed to high temperature air or to high temperature air
containing
significant water vapor. Parts subjected to such conditions include, for
example,
affected parts of energy generation devices, such as gas turbines, steam
turbines,
and fuel cells, and heat exchangers and recuperators, and in equipment and
parts for
high temperature processing, treatment, or extraction of chemicals or
minerals, or
high temperature processing or treatment of alloys. These steels, however,
still
suffer from a level of corrosive attack when subjected over time to these
conditions.
Accordingly, the present inventors undertook to determine whether certain
modified
austenitic stainless steel chemistries further improved corrosion resistance
in high
temperature environments. As further described below, the inventors determined
that alloys containing 1.5 weight percent or less manganese are subject to
oxide
scale evaporation and subsequent degradation in air containing water vapor.
The
inventors' work, in part, focused on certain novel austenitic stainless steel
chemistries including more than 1.5 weight percent manganese, along with
appreciable levels of chromium and nickel. As a result of their work, the
present
inventors concluded that an austenitic stainless steel having the broad
composition
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and, more preferably, the nominal composition listed in Table 2 would have
substantial resistance to chromium oxide scale evaporation in high temperature
air
environments and in high temperature air environments including water vapor.
The
proposed alloy's manganese content is controlled at a minimum level, which was
found to significantly improve resistance to high temperature corrosive
attack.
Table 2
Minimum Maximum Nominal
Carbon 0.05 0.2 0.10
Nitrogen 0.08 0.2 0.15
Chromium 20 23 20.5
Nickel 25 27 25.5
Molybdenum 1 2 1.5
Manganese greater than1.5 4.0 1.6
Silicon 0 0.50 0.30
Aluminum 0 0.30 0.25
Sulfur 0 0.02 0.005
Phosphorus 0 0.05 0.03
Niobium 0.20 0.75 0.6
Titanium 0 0.1
Zirconium 0 0.1
Vanadium 0 0.1
Table 3 provides information on several alloys evaluated during the testing.
All heats were melted and subsequently rolled to foil gauge. Heats 1 and 3
were lab
heats, heat 2 was prepared as a pilot coil, and heat 4 was a plant heat
prepared as a
production coil. Heats 1, 3, and 4 were prepared with an aim of 1.0 weight
percent
manganese, and heat 2 was prepared with an aim of 1.6 weight percent
manganese.
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Table 3
Heat 1 Heat 2
Heat 3
Heat 4
Carbon 0.10 0.087 0.076 0.078
Molybdenum 1.54 1.53 1.54
1.50
Chromium 20.01 21.0 20.19
20.4
Nickel 25.42 26.0 25.57 26.0
Niobium 0.65 0.30 0.30 0.34
Manganese 0.99 1.61 1.03
0.99
Titanium 0.077 0.01 0.02
Nitrogen 0.143 0.10 0.13 0.1
A comparison of the 1.6 and 1.0 weight percent manganese (nominal)
variants listed in Table 3 as heats 2 and 4, respectively, showed that the
lower-
manganese version is significantly more susceptible to oxide scale evaporation
in
humidified air, particularly at higher temperatures. This could result in
significant
environmental attack over time. Testing was conducted as follows.
Samples were exposed in the temperature range 1300-1500 F (704-815 C) in
wet air. As shown in Figure 1, both the high manganese sample (approximately
1.6
weight percent manganese, heat 2) and the low manganese sample (approximately
1.0 weight percent manganese, heat 3) exhibited similar oxidation kinetics in
terms
of weight change (mg/cm2) over time when exposed at 1300 F (704 C) in air
containing 10% water vapor. The low manganese sample generally exhibited a
slightly lower weight gain, with somewhat irregular behavior.
Figure 2 shows weight change over time for samples of high manganese
(heat 2) and the low manganese (heat 4) alloys when the samples were exposed
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CA 02603526 2013-07-22
1400 F (760 C) in air containing 7% water vapor. The samples exhibited
significantly different oxidation kinetics under these conditions. The low
manganese
sample gained weight rapidly during the initial portion of the test, but then
the weight
gain slowed significantly. After completion of the 5,000 hour test, the two
samples
exhibited essentially identical weight gain.
Figure 3 shows weight change over time for samples of high manganese
(heat 2) and the low manganese (heat 4) alloys when the samples were exposed
at
1500 F (815 C) in air containing 7% water vapor. The curve shows that the
lower
manganese sample exhibited significant oxide scale evaporation during the test
period. The higher manganese alloy did not exhibit the same weight change over
the limited test exposure.
Samples subjected to 5,000 hours of total exposure under the 1300 F (704 )
and 1400 F (760 C) conditions above were mounted, polished, and examined. The
oxide scale that formed on the high manganese samples appeared thin, compact,
and essentially featureless. The low manganese variant exhibited subscale void
formation after exposure at 1300 F (704 ) in humid air. The oxide scale over
these
voids, shown in Figure 4a, was slightly thicker than the scale elsewhere.
Scattered
oxide nodules were present on the low manganese samples exposed in humid air
at
1400 F (760 C). Examples of the nodules are shown in Figure 4b. Numerous small
"emergent" nodules appeared to be in the process of disrupting the oxide
scale.
Samples also were examined under magnification after being exposed to 1500 F
(815 C) air containing water vapor. It was observed that small nodules of
mixed
oxides formed in the oxide scale on the low manganese (approximately 1.0
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weight percent manganese, heat 4) sample after about 3,000 hours. The low
manganese samples was again examined under magnification after about 8,000
hours exposure, and the oxide nodules were found to have grown significantly
in
size. The high manganese (approximately 1.6 weight percent manganese, heat 2)
sample was examined at about 3,500 hours, and no nodules were observed in the
oxide scale.
Microanalysis in the scanning electron microscope (SEM) was used to study
the general compositional makeup of the oxide scales. The scales were
relatively
thin (2-3 microns), which made it difficult to extract a detailed
compositional profile.
Measurements were generally limited to sites near the scale/alloy interface
and near
the scale/gas interface. It was observed that the high manganese alloy (heat
2)
exhibited significantly greater manganese segregation from the alloy to the
scale.
See Figure 5, which plots oxide composition, measured as a molar ratio of MnO
to
Cr203, as determined using X-ray energy-dispersive spectroscopy (XEDS) in the
SEM (semi-quantitative) for several samples at the scale/alloy interface and
the
scale/gas interface. The low manganese material did not exhibit manganese
saturation (i.e., a MnO/Cr203 ratio of 1.0) at the scale/gas interface at 1300
F
(704 C) and was borderline saturated at 1400 F (760 C). Achieving manganese
saturation in the spinet is believed to be important in providing resistance
to
evaporation.
The same technique (XEDS in the SEM, quantified using standardless and
standards-based methods) was used to determine the level and extent of
chromium
depletion in the underlying metal after exposure to high temperature air
including
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water vapor. Figure 6 plots the chromium concentration as a function of depth
into
the sample surface for high manganese and low manganese samples, from heats 2
and 4, respectively, exposed for 5,000 hours at 1300 F (704 C) in air
containing 10%
water vapor. The depletion observed for the low manganese sample is
significantly
greater in terms of chromium concentration directly adjacent the scale/metal
interface Depth of depletion between the samples does not appear to be
noticeably
different. The chromium profiles derived from each sample appear extremely
sharp,
indicating that chromium cannot diffuse rapidly from the interior of the
sample to the
scale/alloy interface.
Figure 7 is a plot of chromium concentration as a function of depth into the
sample surface for high manganese and low manganese samples, heats 2 and 4,
respectively, exposed for 5,000 hours at 1400 F (760 C) in air containing 7%
water
vapor. As with Figure 6, chromium depletion for the low manganese sample was
significantly greater than for the high manganese sample at the scale/metal
interface. It was observed that the effect of chromium depletion at 1400 F
(760 C)
is not substantially greater in terms of terminal chromium content at the
scale/alloy
interface relative to what is shown in Figure 6, but the gradient shown in
Figure 7
runs much deeper into the substrate. This may have resulted because the
diffusion
of chromium in the metal is rapid enough at 1400 F (760 C) to delocalize the
effects
of chromium depletion due to oxidation.
Figure 8 is a graph showing oxide composition, measured as a molar ratio of
MnO to Cr203, using XEDS in the SEM (semi-quantitative) for high manganese and
low manganese samples, derived from heats 2 and 4, respectively, subjected to
high
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Attorney Docket No. 050053/RL-2066
temperature air containing 7% water vapor. The measurements were taken at the
scale/alloy interface and the scale/gas interface. The evaluations conducted
after
exposure to 1300 F (704 C) and to 1400 F (760 C) air were conducted after
about
5,000 hours of exposure time. Those conducted after exposure at 1500 F (815 C)
were performed after about 3,000 hours of exposure time. The low manganese
material did not exhibit manganese saturation (i.e., a MnO/Cr2O3 ratio of 1.0)
at the
scale/gas interface at 1300 F (704 C) and at 1500 F (815 C), and was
borderline
saturated at 1400 F (760 C).
A set of heats of higher-manganese alloys was prepared to assess how
oxidation resistance responds to further increased manganese levels. Table 4
shows the chemical composition of the additional heats, referenced as heats 5
and 6.
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CA 02603526 2007-09-28
,
,
Attorney Docket No. 050053/RL-2066
Table 4
Heat 5 Heat 6
Carbon 0.04 0.03
________________________________ ___ _________________________________
Manganese 2.04 3.82
Phosphorus 0.006 0.006
Sulfur 0.0069 0.003
Silicon 0.26 0.17
Chromium 19.4 19.81
_ _____________________________________________________________________
Nickel 23.19 23.22
Aluminum 0.07 0.17
Molybdenum 1.2 1.25
Copper 0.010 0.010
Titanium 0.004 0.004
Nitrogen 0.051 0.058
Niobium 0.39 0.39
Figure 9 is a plot of sample weight change over time for samples of the alloys
of heat 2 (1.61 weight percent manganese), heat 5 (2.04 weight percent
manganese), and heat 6 (3.82 weight percent manganese) exposed at 1400 F
(760 C) in air containing 7% water vapor. The results indicate that higher
manganese levels produce higher initial weight gain through oxide scale
formation.
While the weight gains shown in Figure 9 did not appear to be problematic, it
is
believed that higher manganese levels, above about 4 weight percent, would
result
in further scale formation and weight gains, and the consequent undesirable
result of
spallation of the material.
CA 02603526 2007-09-28
Attorney Docket No. 050053/RL-2066
Additional heats 7 through 11 in Table 5 were prepared. The heats included
less than 0.1 weight percent titanium. Heats 7, 8 and 11 also included less
than 0.1
weight percent aluminum.
Table 5
'1/4----------------,..s.s.õ Heat 7 Heat 8 Heat 9
Heat 10 Heat 11
Carbon 0.086 0.088 0.078 0.091
0.080
Molybdenum 1.54 1.52 1.50 1.52
1.54
Chromium 20.99 20.95 20.4 ' 20.35
25.83
Nickel 25.92 ' 26.02 26.0 25.7
20.42
_
Niobium 0.30 0.30 0.34 0.38
0.36
Manganese 1.61 1.79 0.99 . 1.03
1.52
Titanium 0.010 <0.01 0.02 0.001
0.06
Nitrogen 0.0955 0.1130 0.10 0.104
0.12
Silicon 0.41 0.40 0.47 0.33
0.36
Sulfur <0.01 <0.01 0.0001 ' 0.0001
0.0005
Aluminum <0.01 <0.01 0.16 0.34
0.02
Boron 0.0033 0.0029 0.0047 0.0047
0.0052
As discussed above, austenitic stainless steels subjected to stress at high
temperature for prolonged periods can be subject to creep. Most austenitic
stainless
steels include relatively minor levels of titanium and aluminum to facilitate
deoxidation of the molten metal during melting and casting. These elements
also
are precipitated as nitrides and, possibly, intermetallic phases in the solid
state.
These precipitated phases are very difficult or impractical to dissolve during
processing. Excessive nitride formation will have the effect of reducing the
level of
nitrogen in solid solution, which will reduce the creel; strength of the
alloy. Nitrides
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Attorney Docket No. 050053/RL-2066
and intermetallic phases also can make processing more difficult, particularly
when
the steel is formed by being folded or stamped into part shapes.
Accordingly, in order to improve creep strength and the formability of the
alloy
during folding, stamping, and similar mechanical processing steps, a preferred
chemistry for the austenitic stainless steels of the present disclosure
includes at least
one of no greater than 0.1 weight percent titanium and no greater than 0.1
weight
percent aluminum. More preferably, to better enhance creep resistance and
formability, the austenitic stainless steels of the present disclosure
includes no
greater than 0.1 weight percent titanium and no greater than 0.1 weight
percent
aluminum.
Based on the above, an austenitic stainless having the investigated
chemistries and including manganese at levels greater than 1.5 weight percent
and
up to about 4 weight percent should exhibit advantageous resistance to high
temperature attack in air, which may include significant water vapor, and
without
suffering from excessive scale formation and spallation. More specifically,
the broad
and nominal alloy compositions shown in Table Z are proposed as austenitic
stainless steels with substantial resistance to corrosive attack in high
temperature air
and in high temperature air including water vapor. A preferred manganese level
is at
least 1.6 up to about 4 weight percent, and a more preferred manganese level
is at
least 1.6 up to about 2.0 weight percent manganese.
An additional proposed alloy chemistry having improved creep resistance and
improved formability has the general chemistry shown in Table 2, but includes
no
greater than 0.1 weight percent titanium and/or no greater than 0.1 weight
percent
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Attorney Docket No. 050053/RL-2066
aluminum. The expected improvement in creep resistance resulting from the
limits
on titanium and/or aluminum content is not necessarily tied to the improved
high
temperature corrosion resistance provided by controlling the manganese content
to
the range of greater than 1.5 weight percent up to about 4 weight percent.
Instead,
the manganese content of the alloy proposed herein having improved creep
resistance and formability may be any level up to about 4.0 weight percent.
Accordingly, the alloy in the following Table 6 should exhibit advantageous
creep
resistance and formability properties, and a preferred chemistry includes no
greater
than 0.1 weight percent titanium and no greater than 0.1 weight percent
aluminum.
Table 6
Minimum Maximum
Carbon 0.05 0.2
Nitrogen 0.08 0.2
Chromium 20 23
Nickel 25 27
Molybdenum 1 2
Manganese 0 4.0
Silicon 0 0.50
Aluminum* 0 0.30
Sulfur 0 0.02
Phosphorus 0 0.05
Niobium 0.20 0.75
Titanium* 0 0.1
Zirconium 0 0.1
Vanadium 0 0.1
* At least one of Ti and Al is no greater than 0.1.
An alloy exhibiting advantageous high temperature creep resistance,
improved formability, and advantageous resistance to corrosive attack in high
temperature air including water vapor would have the composition shown in
Table 6
and wherein the composition is further controlled such that the manganese
content is
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CA 02603526 2011-06-09
greater then 1.5 up to about 4.0 weight percent, preferably is at least 1.6 up
to about
4.0 weight percent, and more preferably is at least 1.6 up to about 2.0 weight
percent. Such an alloy could be advantageously applied in making, for example,
structural parts and other parts of the previously mentioned energy generation
devices and processing, treatment, or extraction devices that are both
subjected to
stress and exposed to high temperature air including water vapor.
An optional limitation, on the austenitic stainless steels chemistries
proposed
herein, established to better ensure substantial resistance to creep, is that
the
niobium to carbon ratio in the alloys satisfies the following formula:
0.7< (niobium/carbon) 1.0
wherein the niobium and carbon contents in the formula are expressed in atom
percentages.
Heats of the novel corrosion resistant austenitic stainless steels disclosed
herein may be made by conventional means, such as by the conventional
technique
of vacuum melting scrap and other feed materials. The resulting heats may be
processed by conventional techniques into billets, slabs, plates, coils,
sheets, and
other intermediate articles, and then further processed into final articles of
manufacture. The enhanced formability of embodiments of alloys within the
present
disclosure including no greater than 0.1 weight percent of titanium and/or no
greater
than 0.1 weight percent aluminum allows flat mill products (such as strip,
sheet,
plate, coil, and the like) formed from the alloys to be further processed into
articles
having relatively complicated shapes. This characteristic of the alloys is an
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CA 02603526 2013-07-22
advantage relative to NF709 alloy, which has more limited formability and has
commonly only been processed by extrusion into seamless pipe.
The novel austenitic stainless steels according to the present disclosure may
be used in any suitable application and environment, but the alloys are
particularly
suited for use in equipment and parts subjected for extended periods to high
temperature, or to both high temperature and significant water vapor. For
example,
the creep resistance and/or high temperature corrosion resistance of the
alloys
disclosed herein makes them particularly suitable for use in: tubing, piping,
structural parts, and other parts of equipment adapted for high temperature
processing, treatment, or extraction of chemicals or minerals, or high
temperature
processing or treatment of alloys; tubing, piping, structural parts, and other
parts of
energy generation devices such as, for example, gas turbines, steam turbines,
and
fuel cells; and parts of heat exchangers, recuperators, and other equipment
handling gas streams used or generated by energy generation devices. Other
applications for the alloys disclosed herein will be apparent to those of
ordinary skill
upon considering the present description of the alloys.
Although the present invention has been described in connection with certain
preferred embodiments, it is to be understood that the scope of the claims
should not
be limited by the preferred embodiments set forth in the examples, but should
be
given the broadest interpretation consistent with the description as a whole.
