Note: Descriptions are shown in the official language in which they were submitted.
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CA 02580933 2007-03-05
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Description
HEAT AND CORROSION RESISTANT CAST AUSTENITIC STAINLESS
STEEL ALLOY WITH IMPROVED HIGH TEMPERATURE STRENGTH
Technical Field
This disclosure relates generally to cast austenitic stainless steel
alloys of the CF8C type with improved high temperature strength. More
particularly, this disclosure relates to CF8C type stainless steel alloys and
articles
cast therefrom having excellent high temperature creep strength, and aging
resistance, and which exhibit a stable austenitic microstructure with
substantially
no delta ferrite after casting and high temperature aging.
Back rg ound
There is a need for high strength, oxidation resistant and crack
resistant cast alloys for use in components subject to extreme temperature
environments. Advanced diesel engines must continue to have high fuel
efficiency as well as reduced exhaust emissions, without sacrificing
durability
and reliability. More demanding duty cycles require exhaust manifolds and
turbocharger housing materials to withstand temperatures above 750 C. Such
materials must withstand both prolonged, steady high-temperature exposure as
well as more rapid and severe thermal cycling. New emissions reduction
technology and transient power excursions can push temperatures in these
critical
components even higher. The current material of choice for diesel engine
components is Silicon-Molybdenum (SiMo) cast iron. However, it is being
pushed beyond its high temperature strength and corrosion limitations. Nickel
based super alloys are candidate materials for other high temperature
applications
like gas turbines, due to its excellent high temperature properties. However,
the
cost of nickel makes nickel based super alloys expensive, and turbine
manufacturers are considering lower-cost alternatives for casings and large
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structural components. These material issues are not unique to diesel engines
and
combustion turbines. Distributed power applications that utilize advanced
natural
gas reciprocating engines need low-cost high-temperature capable materials as
expectations for efficiency and service temperature increase. Any new material
for these applications should have low cost and good high temperature creep
and
fatigue resistance.
Because these components are made by casting processes, any
new material should have good casting characteristics like melt fluidity, hot
tear
resistance, and weldability. A significant factor in the cost of producing a
casting
is the post casting stress relief or solution heat treatment typically
required for
stainless steel castings. Eliminating the need for post casting heat
treatments can
result in substantial time and money savings for casting manufacturers. These
cost savings can be even higher for large components like steam turbine
casings
where large furnaces have to be used. Therefore, any new material should have
the desired properties in the as-cast state, that is, without the need for
post casting
heat treatment.
CF8C is a commercially available cast austenitic stainless steel
that is relatively inexpensive. However, standard practice is to solution
treat
CF8C castings at 1050 C, which like discussed above, may increase its cost for
some applications. Currently-available cast austenitic stainless CF8C steels
may
include from 18 wt. % to 21 wt. % chromium, 9 wt. % to 12 wt. % nickel and
smaller amounts of carbon, silicon, manganese, phosphorous, sulfur and
niobium.
CF8C typically includes about 2 wt. % silicon, about 1.5 wt. % manganese and
about 0.04 wt. % sulfur. CF8C is a niobium stabilized grade of austenitic
stainless steel most suitable for applications at temperatures below 500 C. In
the
standard form CF8C has poor strength at temperatures above 600 C. It also does
not provide adequate cyclic oxidation resistance at temperatures exceeding
700 C., does not provide sufficient ductility, does not have the requisite
long-
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term stability of the original microstructure after high temperature aging,
and
lacks long-term resistance to cracking during severe thermal cycling.
In austenitic stainless steel castings, such as CF8C, delta ferrite is
present in the as-cast microstructure. This delta-ferrite in the
microstructure
transforms to sigma (6) phase during prolonged high-temperature exposure,
decreasing the ductility of the material, particularly at lower or ambient
temperatures. The absence of delta ferrite and sigma phase in the
microstructure
in the as-cast state and after prolonged exposure to high temperatures (high
temperature aging) is an important advantage to preserve the as-cast
properties of
the material during the lifetime of a component created with the material.
A class of stainless steel alloys is described in U.S. Patent
5,340,534 issued to Magee (hereinafter, "the '534 patent".) The 1534 patent
seeks to improve the galling resistance and corrosion resistance of stainless
steel
alloys. A concentration of silicon above 2.25% is an important contributor to
the
improved galling resistance of the alloy. Silicon is also important for metal
fluidity of casting steels. However, silicon promotes the formation of
ferrite,
sigma phases and niobium rich lanes or other silicide phases in the steel, and
ferrite volume measurements indicate ferrite volumes between 2.3 and 7 percent
in different heats of alloys described in the '534 patent. As described
earlier,
presence of ferrite and sigma phases deteriorates the properties of steels
exposed
to high temperatures. Another class of stainless steel alloys is described in
U.S.
Patent 4,341,555 issued to Douthett et al. (hereinafter, "the '555 patent".)
In the
alloys described in the '555 patent, the concentration of carbon is restricted
to
0.06%, and a concentration of molybdenum is kept between 2 and 4.5% for good
pitting and acid corrosion resistance. The alloys described in the '555 patent
rely
on post casting stress relief heat treatments to improve their mechanical
properties.
It is, therefore, desirable to have a modified CF8C type steel alloy
which has good casting characteristics, improved strength and creep properties
at
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temperatures above 600 C in the as-cast state, and which exhibits a stable and
completely austenitic microstructure after casting and high temperature aging,
so
that the improved strength and ductility of the material is maintained over
the
lifetime of the alloy. Completely austenitic microstructure refers to a nearly
100% austenitic microstructure which is substantially free of delta ferrite
and
sigma phases of steel.
The disclosed system is directed to overcoming one or more of the
problems set forth above.
Summary of the Invention
In one aspect, the present disclosure is directed to a heat and
corrosion resistant cast austenitic stainless steel alloy which contains less
than
aboutl5% nickel. The alloy has a completely austenitic microstructure in an as-
cast state, and a creep rupture life exceeding 20,000 hrs at a stress of 35
MPa and
a temperature of 850 C, when creep tested in the as-cast state under ASTM E139
test conditions.
In another aspect, the present disclosure is directed to a heat and
corrosion resistant cast austenitic stainless steel alloy containing less than
15%
nickel. The alloy has a creep rupture life exceeding 3,000 hrs and a minimum
creep rate of less than 1 x 10"3 at a stress of 100 MPa and a temperature of
750 C,
when creep tested in the as-cast state under ASTM E139 test conditions. The
alloy also has a 0.2% yield strength exceeding 130 MPa at 750 C in the as-cast
state, and a decrease in 0.2% yield strength from 750 to 900 C of less than
20%;
and, a completely austenitic microstructure after casting.
In another aspect, the present disclosure is directed to an article
made of a heat and corrosion resistant cast austenitic stainless steel alloy
which
contains less than about 15% nickel and has a completely austenitic
microstructure. The article also shows no detectable ferromagnetic phases like
ferrite or martensite when measured with a measurement device after casting
and
after high temperature aging for 3000 hrs at 750 C. The article has a creep
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rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of
850 C, when creep tested in the as-cast state under ASTM E139 test conditions
and a creep rupture life exceeding 2000 hours and a minimum creep rate less
than
x 10"3 at a stress of 100 MPa and a temperature of 750 C, when creep tested in
5 the as-cast state under ASTM E 139 test conditions.
The present disclosure also discloses a heat and corrosion resistant
cast austenitic stainless steel alloy which has a completely austenitic
microstructure in the as-cast state. The alloy includes about 0.05 weight
percent
to about 0.15 weight percent of carbon, about 1.5 weight percent to about 3.5
weight percent copper, about 0.25 weight percent to about 1.0 weight percent
tungsten, and about 0.6 weight percent to about 1.5 weight percent of niobium.
Brief Description of the Drawings
FIG. 1 a is an SEM micrograph of the microstructure of an
exemplary polished and etched as-cast CF8C alloy.
FIG. 1 b is an SEM micrograph of the microstructure of an
exemplary polished and etched as-cast CFBC-Plus alloy.
FIG. 2a shows the microstructure of an exemplary CF8C alloy
before high temperature aging.
FIG. 2b shows the microstructure of an exemplary CF8C alloy
after high temperature aging.
FIG. 3a shows the microstructure of an exemplary CFBC-Plus
alloy before high temperature aging.
FIG. 3b shows the microstructure of an exemplary CF8C-Plus
alloy after high temperature aging.
FIG. 4a is a TEM image of the microstructure of an exemplary
CF8C alloy after creep testing at 850 C and 35MPa.
FIG. 4b is a TEM image of the microstructure of an exemplary
CFBC-Plus alloy after creep testing at 850 C and 35MPa.
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Detailed Description
CF8C is the traditional cast equivalent of type 347 stainless steel.
The chemistry of CF8C-Plus is based on the composition of CF8C with precise
additions of nickel (Ni), manganese (Mn), and nitrogen (N) combined with a
reduction in silicon (Si) and adjustments of other minor alloying elements.
These
alloy modifications were made to improve the high-temperature mechanical
properties and the casting characteristics of the CF8C steel using inexpensive
alloying elements without the need for post casting heat treatments.
Table I
Weight percentage (%)
Compositional range Example
CF8C alloy
Min Max CF8C+
Carbon (C) 0.05 0.15 0.07 0.1
Chromium (Cr) 18 25 19 19
Nickel (Ni) 10 15 10 12.5
Niobium (Nb) 0.1 1.5 0.8 0.8
Nitrogen (N) 0.05 0.5 0.25
Manganese (Mn) 0.5 10 1 4
Sulphur (S) 0 0.05 trace trace
Molybdenum (Mo) 0 1 0.3 0.3
Phosphorous (P) 0 0.04 trace trace
Co er Cu 0 3.5
Silicon (Si) 0.2 1 1 0.5
Titanium (Ti) 0 0.2
Cobalt Co 0 5
Aluminum AI 0 3
Boron (B) 0 0.01
Tungsten 0 3
Vanadium 0 3
Iron (Fe) Balance Balance Balance
Table I is directed towards the maximum and minimum ranges of
the compositional elements made in accordance with the present disclosure.
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Table I also includes (in column labeled "Example alloy") an example of an
embodiment of an alloy made in accordance with the present disclosure.
Embodiments covered by the present disclosure include alloys with any subset
of
compositional ranges falling within the minimum and maximum ranges shown in
Table I. It should be noted that allowable ranges of cobalt (Co), vanadium
(V),
and titanium (Ti) may not significantly alter the performance of the resulting
material. Specifically, based upon current information, Co may range from 0 to
about 5 weight percent, V may range from 0 to about 3 weight percent, and Ti
may range from 0 to about 0.2 weight percent without significantly altering
the
performance of the alloys.
To study the effect of these modifications on the mechanical
properties and creep behavior of the materials, mechanical testing was carried
out, and the test results of the modified alloy (named CFBC-Plus) samples are
compared with those of the traditional CF8C steel alloy. Samples of the
traditional CF8C and the CFBC-Plus material were cast for experiments using
centrifugal casting. Table I also shows the composition of CF8C and CF8C-Plus
steel alloys that were used for these studies.
FIG. 1 a shows the microstructure of an exemplary polished and
etched as-cast CF8C alloy, and FIG. lb shows the microstructure of an
exemplary polished and etched as-cast CFBC-Plus alloy. The microstructure of
the as-cast CF8C alloy includes an austenite matrix with delta ferrite 10
pools in
the interdentrite core regions, and niobium carbide (NbC) 12 in the
interdentritic
regions. In contrast, the microstructure of the as-cast CF8C-Plus alloy does
not
show any delta ferrite 10. The microstructure of CFBC-Plus alloy is fully
austenitic with a mixture of chromium carbide (Cr23C6) and NbC 12 in the
interdentritic regions. A digital Fisher Feritscope was used to measure the
ferrite content of both the CF8C and CF8C-Plus steel castings. The CF8C had a
ferrite number of about 16.8 +/-1.1, which is equivalent to about 14% delta
ferrite, and the CFBC-Plus did not register any detectable ferromagnetic
behavior,
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CA 02580933 2007-03-05
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meaning it has less than about 0.1% delta ferrite. Both these macroscopic
measurements and microscopic studies indicate that the CF8C-Plus material in
the as-cast state is substantially free of delta ferrite 10 in the as-cast
state.
To investigate the microstructural evolution of CF8C and CF8C-
Plus steel during aging, sand cast keel bars were encapsulated in quartz
tubes,
evacuated, and backfilled with argon. These were aged in an air box furnace at
750 C for 3,000 hours. These specimens were polished and etched for optical
microscopy using an etchant composed of glycerin, hydrochloric acid, nitric
acid,
and acetic acid having a volumetric ratio of 3:3:1:1. Scanning Electron
Microscopy (SEM) analysis was performed on polished and unetched specimens
using backscatter electron (BSE) imaging, and x-ray energy dispersive
spectroscopy (XEDS) was performed on areas of interest.
FIG. 2a shows a BSE image of the microstructure of an exemplary
CF8C alloy before high temperature aging, and FIG. 2b shows a BSE image of
the microstructure of an exemplary CF8C alloy after high temperature aging at
750 C for 3,000 hours. Comparison of FIGS. 2a and 2b indicate a change in BSE
image contrast in delta ferrite 10 of the aged material. XEDS analysis of
these
regions in the aged material indicates that they are enriched in silicon (Si)
and
chromium (Cr) compared to the delta-ferrite 10 found in the as-cast structure.
Comparison of FIGS. 2a and 2b indicates that delta-ferrite 10 has transformed
into sigma phase 14 after high temperature aging. Based on the chemical
composition of the phase and the knowledge that delta-ferrite 10 can transform
rapidly to sigma phase 14 in stainless steels, it is concluded that aging for
3,000
hours at 750 C transforms a majority of the delta ferrite 10 in CF8C steel
into
sigma phase 14. Electron diffraction patterns from these regions were studied
using transmission electron microscopy (TEM) and confirmed the presence of
body centered tetragonal (bct) sigma phase.
FIG. 3a shows a BSE image of the microstructure of an exemplary
CF8C-Plus alloy before high temperature aging, and FIG. 3b shows a BSE image
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of the microstructure of an exemplary CF8C-Plus alloy after high temperature
aging at 750 C for 3,000 hours. In contrast to the CF8C alloys shown in FIG 2a
and FIG 2b, the CF8C-Plus steel alloys in FIG 3a and FIG 3b do not show the
formation of any delta ferrite 10 or sigma phases 14 after high temperature
aging.
The structure of CFBC-Plus alloy before and after high temperature aging
samples is austenitic with interdendritic carbides 16. No obvious change was
observed in carbide size or morphology after aging. These studies indicate
that in
contrast to CF8C alloy, the CFBC-Plus alloy is substantially free of delta
ferrite
or sigma phases 14 of steel after high temperature aging at 750 C for 3,000
10 hours.
From the centrifugal castings, tensile, creep, and fatigue
specimens were machined in both the hoop and longitudinal orientations. Room
temperature and elevated temperature tensile tests were performed in
accordance
with ASTM E8 and E21. Air creep testing was performed in accordance with
ASTM E139 at constant load in lever-arm type creep machines with
extensometers attached to the shoulders of the specimens to measure creep
deformation. Low cycle fatigue (LCF) and creep-fatigue (C-F) testing was
performed in servo-hydraulic test systems in strain control using induction
heating in accordance with ASTM E606. For the creep-fatigue tests, a strain
hold
was imposed at maximum tensile strain during the cycle.
Table II compares the average tensile properties, namely 0.2%
offset yield strength (YS), ultimate tensile strength (UTS), and ductility as
measured as the percentage elongation at fracture (Elong.), and percentage
reduction of cross sectional area at fracture (RA), for CFBC, and CF8C-Plus
(CF8C+) steels as a function of temperature. The average yield strength of
CF8C-Plus changes very little above 700 C, while that of CF8C steel shows
significant weakening. The average ultimate tensile strength of the CFBC-Plus
steel is higher for the entire temperature range compared to CFBC. This
increase
is significantly higher (> 70 Mpa) at temperatures above 700 C. The ductility,
as
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CA 02580933 2007-03-05
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measured by elongation and reduction of area, of CFBC-Plus steel are both
higher
than that of CF8C steel above 700 C.
Table II
Tensile tests ASTM E8 &E12)
Temp YS (Mpa) UTS (Mpa) Ductili
(C) Elong. (%) RA (%)
CF8C CF8C+ CF8C CF8C+ CF8C CF8C+ CF8C CF8C+
25 252 273 555 587 41.0 42.5 41.0 34.1
200 184 194 425 473 37.5 40.0 54.0 36.0
400 162 169 415 476 32.0 41.4 55.0 42.5
600 138 143 330 398 34.4 40.0 40.5 45.5
700 136 135 237 324 20.5 32.0 24.0 38.5
750 129 333 29.8 42.5
800 121 136 151 255 11.0 27.4 16.5 43.2
850 132 243 33.4 56.0
900 66 120 73 170 25.5 49.5 43.5 65.5
Table III compares the average creep rupture life of CF8C and
CF8C-Plus (CF8C+) alloys at different stresses and temperatures. As seen in
the
table, the creep rupture life of CFBC-Plus steel is over an order of magnitude
higher than that of CF8C steel in all cases. The creep ductility of CF8C-Plus
steel, both as measured as a percentage change in elongation and percentage
change in area, also shows a significant improvement over that of CF8C steel.
In
most cases, this improvement in ductility over CF8C steel is over 100%. The
minimum creep rate of CF8C-Plus steel also shows a significant decrease over
that of CFBC. In most cases, this decrease in minimum creep rate is over an
order of magnitude lower than that of CFBC.
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CA 02580933 2007-03-05
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Table III
Creep strength (ASTM E139)
Creep ductility (%) Creep Resistance
Stress Temp Creep rupture life Elongo) RA o Minimum Creep
(Mpa) (C) . (/o ( /o) Rate (%/hr)
CF8C CF8C+ CF8C CF8C+ CF8C CF8C+ CF8C CF8C+
35 850 1159 24100 8.4 7.8 8.3 13.5
75 850 104 28.6 60.9 1.8E-02
100 750 87.4 2443.5 5.9 25.6 4.9 44.6 1.0E-02 2.2E-03
140 750 3.7 120.5 8.2 31.6 12.5 51.1 9.6E-01 1.1 E-02
180 650 88.5 3913.2 8.1 20.5 12.4 43.3 2.2E-02 1.OE-04
Fig. 4a is a TEM image of the microstructure of CF8C after 493
hours of creep testing at 850 C and 35MPa. FIG. 4b is the TEM image of the
microstructure of CF8C-Plus after over 20,000 hours of creep testing at 850 C
and 35MPa. Comparison of FIGS. 4a and 4b shows that the NbC 12 precipitates
in the CF8C-Plus steel were less than about 50 nanometers in average diameter
(as shown in FIG. 4b,) while the average diameter of these precipitates was
over
about 250 nanometers, with much larger spacing after only 493 hours of
testing,
in CF8C alloys (as shown in FIG. 4a.)
To study the effect on low cycle fatigue, fully reversed (R-ratio =-
1) strain controlled low cycle fatigue tests were run at 650 C and 800 C at
constant frequency. Table IV compares the low cycle fatigue life of CF8C and
CF8C-Plus (CF8C+) steels at different strain ranges at the two different
temperatures. At 650 C both materials show similar behavior at high strains,
but
CF8C-Plus alloys show significant improvement in cycles to failure for the
lowest strain ranges. A similar result is found at 800 C.
Table IV
Strain range 650C 850
CF8C CF8C+ CF8C CF8C+
0.3 30161 >255567 8535 30471
0.5 7333 22545 3392 3826
0.7 7647 4099 2624 2203
1 1794 2396 1394 717
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Additionally, low cycle fatigue tests were run at 750 C with an R-
ratio of 0 to 0.45% strain (0 to 0.45% total strain) at a strain rate of
0.001/sec.
For these creep-fatigue experiments, a 180 second hold time at the maximum
strain (0.45%) was utilized. Table V shows the results for these tests. For
the
low cycle fatigue tests, the cycles to failure for the CF8C was 50% that of
the
CFBC-Plus. Both materials had a reduction in cycle life when the 180 second
peak strain hold was added, but the CF8C showed a more dramatic reduction
(75%) compared to the CF8C (60%). The creep-fatigue cycle life of the CF8C-
Plus steel was three times that of the CF8C steel.
Table V
Low cycle fatigue Approximate cycles to
life failure
CF8C CF8C+
Continuous cycle 5000 9750
180 Sec peak 1100 4000
strain hold
The effects of further alloying elements in CFBC-Plus material
was also studied. Four separate alloying additions, B, W, Cu, and Al, were
chosen for evaluation on CF8C-Plus steel. Fifteen pound lab-scale heats of
CFBC-Plus with minor alloy additions were produced by induction melting with
an argon cover gas and cast into graphite blocks (152mm 102mm X 25.4mm).
One heat was cast to the CFBC-Plus composition, and four other heats contained
a single alloy addition each. The approximate measured compositions of these
five castings (wt%) are given in Table VI. The colunm titled 'CF8C+' lists the
approximate composition of an embodiment of an alloy made in accordance with
the present disclosure. This alloy is used as the baseline to compare the
effect of
additional alloying elements in the alloy. The columns titled 'CF8C+B',
'CF8C+W', 'CF8C+Cu', and 'CF8C+Al' list the composition of the alloys
obtained by adding about 0.005 weight percent of boron, about 0.45 weight
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CA 02580933 2007-03-05
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percent of tungsten, about 2.5 weight percent of copper, and about 1.3 weight
percent of aluminum, respectively, to the composition of the CF8C-Plus alloy.
Table VI
App roxima e wei ercen
CF8C+ CF8C+B CF8C+W CF8C+Cu CF8C+AI
C 0.1 0.1 0.1 0.09 0.01
Mn 4.25 4.25 4.16 4.26 4.44
Si 0.5 0.5 0.5 0.5 0.6
Ni~V 12.7 12.7 12.6 12.9 12.3
Cr 19.3 19.4 18.9 19.1 19.2 Mo 0.25 0.25 0.25 0.25 0.26
V 0.008 0.01 0.008 0.008 0.008
Nb 0.78 0.79 0.75 0.77 0.8
N 0.28 0.22 0.26 0.25
Fe Bal Bal Bal Bal Bal
Addition .005B .45W 2.5Cu 1.3AI
No post-casting stress-relief or solution annealing treatment was
given to these castings. Tensile bars were machined from the casting blocks,
and
tensile tests and creep tests were performed on these materials. The test
condition chosen to screen all the specimens was 850 C and 75MPa. Alloy
samples that had creep rupture lives comparable to the CFBC-Plus material were
then tested at 750 C and 140MPa.
Table VII compares the tensile test and creep test results of the
four alloy additions to the CF8C material. As the results indicate, samples
with
Al and B additions exhibited worse creep life than CF8C-Plus material, and
were
not, therefore, chosen for creep testing at 750 C and 140MPa. The results
indicate that the alloys with the Cu and W additions performed better than the
base CF8C-Plus material in high temperature creep.
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Table VII
Tensile tests ASTM E8 &E72 Creep strength (ASTM E139)
Alloy YS M a UTS M a Creep Ru ture life (hrs)
25C 850C 25C 850C (850C, 75MPa) (750C, 140MPa
CF8C-Plus 291 151 655 231 170 320
CF8C-Plus + B 267 145 608 220 85 -
CF8C-Plus + W 298 153 604 233 180 340
CF8C-Plus + Cu 283 153 667 231 190 450
CF8C-Plus + Al 227 123 555 176 30 -
Based upon these results, both Cu and W together were added to
the CF8C-Plus material to obtain and alloy having the approximate composition
(in weight percent) of 0.09C, 3.9Mn, 0.46Si, 13.lNi, 20.1Cr, 0.28Mo, 0.008V,
0.77Nb, 0.28N, 2.94Cu, 1 W, and the balance Fe. Tensile bars were machined
from the casting blocks, and tensile tests and creep tests were repeated on
this
new alloy.
Table VIII lists the results of these tensile and creep tests.
Unexpectedly, the inventors found that adding both the Cu and W together
introduced synergistic effects that decreased the creep rate and increased the
creep rupture life of the material significantly. Microscopic analyses of this
alloy
indicated that its microstructure was substantially free of delta ferrite 10
or sigma
phases 14 of steel in the as-cast and the post high temperature aged
microstructure.
Table VIII
Tensile tests ASTM E8 &E12) Creep strength (ASTM E139)
Creep Creep ductility Creep
Temp YS UTS Ductili Stress Temp rupture resistance
(C) (Mpa) (Mpa) (Mpa) (C) life (hrs) Elong. (%) RA (%) Min. creep
Elon. % RA (%) rate (%/hr)
252 585 40.0 39.0
200 165 475 38.0 41.5 75 850 80.4 22.4 43.2 2.80E-03
400 145 440 40.5 44.5
600 135 375 37.0 41.0
700 140 310 20.0 18.5 100 750 3904.4 19.1 32.4 5.10E-04
750 150 275 12.0 13.5
800 140 235 13.0 19.5
850 150 225 14.0 17.0 140 750 289.6 21.1 39.2 2.50E-03
900 120 155 36.0 39.0
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CA 02580933 2007-03-05
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Industrial Applicability
The disclosed heat and corrosion resistant cast austenitic stainless
steel alloy can be used for the production of any articles exposed to extreme
temperatures and/or extreme thermal cycling conditions. The disclosed alloy
can
be used for components in engines and power systems. However, the present
disclosure is not limited to these applications, as other applications will
become
apparent to those skilled in the art.
By employing the stainless steel alloys of the present disclosure,
manufacturers can provide a more reliable and durable high temperature
component. The absence of delta-ferrite 10 in the microstructure after casting
produces a stable austenitic microstructure in CFBC-Plus. Delta-ferrite 10
transforms to sigma phase 14 during prolonged high-temperature exposure
causing embrittlement. CF8C-Plus has a nearly 100% austenite microstructure,
substantially free of delta ferrite and sigma phase.
The improved creep ductility combined with a lower creep rate for
CF8C-Plus steel results in increased low cycle fatigue life and creep rupture
strength. Increased low cycle fatigue life and creep rupture strength allows
components made of CF8C+ to be long lasting. The increased creep strength and
fatigue life of the disclosed CF8C-Plus steel alloys over traditional CF8C
material is unexpected because both these materials are castings, and
therefore,
deformation processes are not involved in creating a dislocation structure
upon
cooling. Potential reasons for the significant improvement of low cycle
fatigue
life and creep rupture life of CF8C-Plus over traditional CF8C alloys are that
the
presence of Mn alters the stacking fault energy of the CF8C-Plus alloy giving
rise
to higher energy stacking faults, and the presence of manganese and nitrogen
in
the alloy composition helps nucleation of NbC. The size and density of the NbC
12 precipitates in the matrix may also contribute to the observed improvement
in
fatigue life and creep rupture life. The presence of these fine particulates
of NbC
12 could likely pin dislocations, improving the creep rupture life of CF8C-
Plus
. , ..,,...w._ .~...r,..~. . , ,.~....~.w..,.,,.,.,, .. ..._
CA 02580933 2007-03-05
-16-
alloys. The increased fatigue and creep rupture life, the decreased creep
strain
rate, and the lower decrease in 0.2% yield strength at high temperatures may
allow engine and turbine manufacturers to increase power density by allowing
engines and turbines to run at higher temperatures, thereby providing possible
increase in fuel efficiency.
Engine and turbine manufacturers may also reduce the weight of
components as a result of the increased power density by thinner section
designs
allowed by increased high temperature strength and corrosion resistance
compared to conventional high-silicon molybdenum ductile irons. Further, the
stainless steel alloys of the present disclosure provide superior performance
over
other cast stainless steels for a comparable or lower cost. Finally, stainless
steel
alloys disclosed herein will assist manufacturers in meeting emission
regulations
for diesel, turbine and gasoline engine applications.
While only certain embodiments have been set forth, alternative
embodiments and various modifications will be apparent from the above
description to those skilled in the art. These and other alternatives are
considered
equivalents and within the spirit and scope of the present disclosure.
