U.S. patent number RE41,504 [Application Number 12/230,179] was granted by the patent office on 2010-08-17 for heat and corrosion resistant cast cf8c stainless steel with improved high temperature strength and ductility.
This patent grant is currently assigned to Caterpillar Inc.. Invention is credited to Philip J. Maziasz, Tim McGreevy, Michael James Pollard, Chad W. Siebenaler, Robert W. Swindeman.
United States Patent |
RE41,504 |
Maziasz , et al. |
August 17, 2010 |
Heat and corrosion resistant cast CF8C stainless steel with
improved high temperature strength and ductility
Abstract
A CF8C type stainless steel alloy and articles formed therefrom
containing about 18.0 weight percent to about 22.0 weight percent
chromium and 11.0 weight percent to about 14.0 weight percent
nickel; from about 0.05 weight percent to about 0.15 weight percent
carbon; from about 2.0 weight percent to about 10.0 weight percent
manganese; and from about 0.3 weight percent to about 1.5 weight
percent niobium. The present alloys further include less than 0.15
weight percent sulfur which provides high temperature strength both
in the matrix and at the grain boundaries without reducing
ductility due to cracking along boundaries with continuous or
nearly-continuous carbides. The disclosed alloys also have
increased nitrogen solubility thereby enhancing strength at all
temperatures because nitride precipitates or nitrogen porosity
during casting are not observed. The solubility of nitrogen is
dramatically enhanced by the presence of manganese, which also
retains or improves the solubility of carbon thereby providing
additional solid solution strengthening due to the presence of
manganese and nitrogen, and combined carbon.
Inventors: |
Maziasz; Philip J. (Oak Ridge,
TN), McGreevy; Tim (Washington, IL), Pollard; Michael
James (Peoria, IL), Siebenaler; Chad W. (Dunlap, IL),
Swindeman; Robert W. (Oak Ridge, TN) |
Assignee: |
Caterpillar Inc. (Peoria,
IL)
|
Family
ID: |
24961116 |
Appl.
No.: |
12/230,179 |
Filed: |
August 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09736741 |
Dec 14, 2000 |
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Reissue of: |
10195724 |
Jul 15, 2002 |
07153373 |
Dec 26, 2006 |
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Current U.S.
Class: |
148/327; 420/45;
420/44; 420/46 |
Current CPC
Class: |
C22C
38/48 (20130101); C22C 38/001 (20130101); C22C
38/58 (20130101); C22C 38/04 (20130101); C22C
38/42 (20130101); C22C 38/44 (20130101); C21D
6/005 (20130101); C22C 38/02 (20130101); C22C
38/52 (20130101) |
Current International
Class: |
C22C
38/58 (20060101) |
Field of
Search: |
;148/327 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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313006 |
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Mar 1956 |
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CH |
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0296439 |
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Dec 1988 |
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EP |
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0340631 |
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Nov 1989 |
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EP |
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0467756 |
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Jan 1992 |
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EP |
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0668367 |
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Aug 1995 |
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EP |
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1061511 |
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Mar 1967 |
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GB |
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1413935 |
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Nov 1975 |
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GB |
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Other References
Davis, J.R., "High-Alloy Cast Steels," ASM Specialty Handbook
(Heat-Resistant Materials) (1997), pp. 200-202. cited by other
.
Davis, J.R. "Metallurgy and Properties of Cast Stainless Steels,"
ASM Specialty Handbook (Stainless Steels) 1994, pp. 66-88. cited by
other .
Chen et al., "Development of the 6.8L V10 Heat Resisting Cast-Steel
Exhaust Manifold," SAW Technical Paper Series (Oct. 14-16, 1996),
pp. 57-64. cited by other .
Search report from EPO for corresponding Application No. EP01124942
dated Feb. 27, 2003 (3 pages). cited by other.
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Primary Examiner: King; Roy
Assistant Examiner: Yang; Jie
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner
Government Interests
This invention was made with U.S. Government support under U.S.
Department of Energy Contract No.: DE-AC05-960R2264 awarded by the
U.S. Department of Energy. The U.S. Government has certain rights
in this invention.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 09/736,741 filed Dec. 14, 2000 now abandoned, the disclosure of
which is incorporated by reference herein.
Claims
What is claimed is:
1. A heat resistant and .[.cast,.]. corrosion resistant austenitic
stainless steel alloy comprising: from about 0.07 weight percent to
about 0.15 weight percent carbon; from about 18.0 weight percent to
about 22.0 weight percent chromium and 11.0 weight percent to about
14.0 weight percent nickel; from about 0.3 weight percent to about
1.5 weight percent niobium; from 0.2 weight percent to about 0.5
weight percent nitrogen; from about 2.0 weight percent to about 10
weight percent manganese; less than about 0.03 weight percent
sulfur; 0.45 weight percent molybdenum or less; and 0.75 weight
percent silicon or less.
2. The stainless steel alloy of claim 1 wherein niobium and carbon
are present in a weight ratio of niobium to carbon ranging from
about 8 to about 11.
3. The stainless steel alloy of claim 1 further including less than
about 0.04 weight percent phosphorous.
4. The stainless steel alloy of claim 1 further including about 3.0
weight percent copper or less.
5. The stainless steel alloy of claim 1 further including from
about 0.2 weight percent titanium or less.
6. The stainless steel alloy of claim 1 further including from
about 5.0 weight percent cobalt or less.
7. The stainless steel alloy of claim 1 further including from
about 3.0 weight percent aluminum or less.
8. The stainless steel alloy of claim 1 further including from
about 0.01 weight percent boron or less.
9. The stainless steel alloy of claim 1 further including from
about 3.0 weight percent tungsten or less.
10. The stainless steel alloy of claim 1 further including about
3.0 weight percent vanadium or less.
11. The stainless steel alloy of claim 1 wherein nitrogen and
carbon are present in a cumulative amount ranging from 0.1 weight
percent to 0.65 weight percent.
12. An article formed from the heat resistant and .[.cast,.].
corrosion resistant austenitic stainless steel alloy of claim
1.
13. A heat resistant and .[.cast,.]. corrosion resistant austenitic
stainless steel alloy comprising: from about 18.0 weight percent to
about 22.0 weight percent chromium and 11.0 weight percent to about
14.0 weight percent nickel; from about 0.07 weight percent to about
0.15 weight percent carbon; from 0.2 weight percent to about 0.5
weight percent nitrogen; from about 2.0 weight percent to about
10.0 weight percent manganese; from 0.65 weight percent to about
1.5 weight percent niobium and about 0.75 weight percent silicon or
less.
14. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the carbon
content is from about 0.08 weight percent to about 0.12 weight
percent carbon.
15. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the manganese
content is from about 2.0 weight percent to about 6.0 weight
percent manganese.
16. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the manganese
content is from about 4.0 weight percent to about 6.0 weight
percent manganese.
17. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the niobium
content is from about 0.65 weight percent to about 1.0 weight
percent.
18. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein niobium and
carbon are present in a weight ratio of niobium to carbon ranging
from about 8 to about 11.
19. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 further including
sulfur in an amount of less than 0.1 weight percent.
20. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the alloy is
fully austenitic with any carbide formation being substantially
niobium carbide.
21. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the alloy is
characterized as a CF8C steel alloy substantially free of manganese
sulfides.
22. The heat resistant and .[.cast,.]. corrosion resistant
austenitic stainless steel alloy of claim 13 wherein the alloy is
characterized as a CF8C steel alloy substantially free of chrome
carbides along grain and substructure boundaries.
Description
TECHNICAL FIELD
This invention relates generally to cast steel alloys of the CF8C
type with improved strength and ductility at high temperatures.
More particularly, this invention relates to CF8C type stainless
steel alloys and articles made therefrom having excellent high
temperature strength, creep resistance and aging resistance, with
reduced niobium carbides, manganese sulfides, and chrome carbides
along grain and substructure boundaries.
BACKGROUND
There is a need for high strength, oxidation resistant and crack
resistant cast alloys for use in internal combustion engine
components such as exhaust manifolds and turbocharger housings and
gas-turbine engine components such as combustor housings as well as
other components that must function in extreme environments for
prolonged periods of time. The need for improved high strength,
oxidation resistant, crack resistant cast alloys arises from the
desire to increase operating temperatures of diesel engines,
gasoline engines, and gas-turbine engines in effort of increasing
fuel efficiency and the desire to increase the warranted operating
hours or miles for diesel engines, gasoline engines and gas-turbine
engines.
Current materials used for applications such as exhaust manifolds,
turbo-charger housings and combustor housings are limited by
oxidation and corrosion resistance as well as by strength at high
temperatures and detrimental effects of aging. Specifically,
current exhaust manifold materials, such as high silicon and
molybdenum cast ductile iron (Hi--Si--Mo) and austenitic ductile
iron (Ni-resist) must be replaced by cast stainless steels when
used for more severe applications such as higher operating
temperatures or when longer operating lifetimes are demanded due to
increased warranty coverage. The currently commercially available
cast stainless steels include ferritic stainless steels such as
NHSR-F5N or austenitic stainless steels such as NHSR-A3N, CF8C and
CN-12. However, these currently-available cast stainless steels are
deficient in terms of tensile and creep strength at temperatures
exceeding 600.degree. C., do not provide adequate cyclic oxidation
resistance for temperatures exceeding 700.degree. C., do not
provide sufficient room temperature ductility either as-cast or
after service exposure and aging, do not have the requisite
long-term stability of the original microstructure and lack
long-term resistance to cracking during severe thermal cycling.
Currently-available cast austenitic stainless CF8C steels 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
aqueous corrosion resistance at temperatures below 500.degree. C.
In the standard form CF8C has inferior strength compared to CN12 at
temperatures about 600.degree. C.
It is therefore desirable to have a CF8C type steel alloy and
articles made from a steel alloy that have improved strength at
high temperatures and improved ductility for engine component
applications requiring severe thermal cycling, high operation
temperatures and extended warranty coverage.
SUMMARY OF THE INVENTION
The present invention may be characterized as a heat resistant and
cast, corrosion resistant austenitic stainless steel alloy. In
particular, the heat resistant and cast, corrosion resistant
austenitic stainless steel alloy comprises from about 0.05 weight
percent to about 0.15 weight percent carbon, from about 2.0 weight
percent to about 10 weight percent manganese; and less than about
0.03 weight percent sulfur.
In another aspect, the invention also be characterized as a heat
resistant and cast, corrosion resistant austenitic stainless steel
alloy comprising from about 18.0 weight percent to about 22.0
weight percent chromium and 11.0 weight percent to about 14.0
weight percent nickel, from about 0.05 weight percent to about 0.15
weight percent carbon, from about 2.0 weight percent to about 10.0
weight percent manganese, and from about 0.3 weight percent to
about 1.5 weight percent niobium.
Various advantages of the present invention will become apparent
upon reading the following detailed description and appended
claims.
DETAILED DESCRIPTION
The present invention is directed toward steel alloys of the CF8C
type. Table 1 presents the optimal and permissible minimum and
maximum ranges for the compositional elements of CF8C stainless
steel alloys made in accordance with the present invention. Boron,
aluminum and copper also may be added. However, it will be noted
that allowable ranges for cobalt, vanadium, tungsten and titanium
may not significantly alter the performance of the resulting
material. Specifically, based on current information, that cobalt
may range from 0 to 5 wt. %, vanadium may range from 0 to 3 wt. %,
tungsten may range from 0 to 3 wt. % and titanium may range from 0
to 0.2 wt. % without significantly altering the performances of the
alloys. Accordingly, it is anticipated that the inclusion of these
elements in amounts that fall outside of the ranges of Table 1
would still provide advantageous alloys and would fall within the
spirit and scope of the present invention.
TABLE-US-00001 TABLE 1 Composition by Weight Percent Modified CF8C
OPTIMAL PERMISSIBLE Element MIN MAX MIN MAX Chromium 18.0 21.0 18.0
25.0 Nickel 12.0 15.0 8.0 20.0 Carbon 0.07 0.1 0.05 0.15 Silicon
0.5 0.75 0.20 3.0 Manganese 2.0 5.0 0.5 10.0 Phosphorous 0 0.04 0
0.04 Sulfur 0 0.03 0 0.1 Molybdenum 0 0.5 0 1.0 Copper 0 0.3 0 3.0
Niobium 0.3 1.0 0 1.5 Nitrogen 0.1 0.3 0.02 0.5 Titanium 0 0.03 0
0.2 Cobalt 0 0.5 0 5.0 Aluminum 0 0.05 0 3.0 Boron 0 0.01 0 0.01
Vanadium 0 0.01 0 3.0 Tungsten 0 0.1 0 3.0 Niobium:Carbon 9 11 8 11
Carbon + Nitrogen 0.15 0.4 0.1 0.5
Unexpectedly, the inventors have found that substantially reducing
the sulfur content of austenitic stainless steels increases the
creep properties. The inventors believe machineability is not
significantly altered as they believe the carbide morphology
controls machining characteristics in this alloys system. While
sulfur may be an important component of cast stainless steels for
other applications because it contributes significantly to the
machineability of such steels, it severely limits the high
temperature creep-life and ductility and low temperature ductility
after service at elevated temperatures.
The inventors have found that removing or substantially reducing
the presence of sulfur alone provides a four-fold improvement in
creep life at 850.degree. C. at a stress load of 110 MPa.
Further, the inventors have found that reducing the maximum carbon
content in the alloys of the present invention reduces the coarse
NbC and possibly some of the coarse Cr23C6 constituents from the
total carbide content. Table 2 includes the compositions of two
experimental modified CF8C type alloys I and J in comparison with a
standard CF8C alloy.
TABLE-US-00002 TABLE 2 Composition by Weight Percent Element
STANDARD CF8C I J Chromium 19.16 19.14 19.08 Nickel 12.19 12.24
12.36 Carbon 0.08 0.09 0.08 Silicon 0.66 0.62 0.67 Manganese 1.89
1.80 4.55 Phosphorous 0.004 0.004 0.005 Sulfur 0.002 0.002 0.004
Molybdenum 0.31 0.31 0.31 Copper 0.01 0.01 0.01 Niobium 0.68 0.68
0.68 Nitrogen 0.02 0.11 0.23 Titanium 0.008 0.006 0.006 Cobalt 0.01
0.01 0.01 Aluminum 0.01 0.01 0.01 Boron 0.001 0.001 0.001 Vanadium
0.004 0.007 0.001 Niobium:Carbon 8.40 7.82 8.52 Carbon + Nitrogen
0.10 0.20 0.31
The elevated tensile properties for alloys I, J, and CF8C were
measured at 850.degree. C. and are displayed in Table 3. Creep
properties of alloys I, J, and CF8C were measured at 850.degree. C.
and are displayed in Table 4.
TABLE-US-00003 TABLE 3 Strain Temp Rate YS UTS Elong Alloy
Condition (.degree. C.) (l/sec) (ksi) (ksi) (%) CF8C As-Cast 850
IE-05 11.7 12.6 31.2 I As-Cast 850 IE-05 17.1 18.1 45.9 J As-Cast
850 IE-05 21.5 22.1 35
TABLE-US-00004 TABLE 4 Temp Stress Life Elong Heat Condition
(.degree. C.) (ksi) (Hours) (%) CF8C As-Cast 850 35 1824 7.2 I
As-Cast 850 35 5252* 2 J As-Cast 850 35 6045* 0.4
The critical test conditions for the alloys in Table 4 (CF8C type
alloys) of 850.degree. C. and 35MPa were again chosen because of
expected operating temperatures and the harmful precipitates, which
form readily. The stress of 35MPa was chosen for accelerated test
conditions that would again equate to much longer durability at
lower stress levels during engine service. The increase in nitrogen
results in a dramatic increase in room and elevated temperature
strength and ductility with at least a three-fold improvements in
creep life at 850.degree. C.
A solution annealing treatment (SA) was applied to each alloy to
analyze the effect of a more uniform distribution of carbon. The
alloys were held at 1200.degree. C. for one hour. They were then
air cooled rather than quenched to allow the small niobium carbide
and chromium carbide precipitates to nucleate in the matrix during
cooling. The resulting microstructure was found to be very similar
to the as-cast (AS) structure except for the formation of small
precipitates.
Unfortunately, the solution annealing treatment lowered creep life
significantly while increasing creep ductility, therefore proving
that the strategy to optimize the as-cast microstructures was best
as well as most cost effective.
Alloys I and J aged at 850.degree. C. for 1000 hours showed
improved strength compared to the commercially available CF8C.
TABLE-US-00005 TABLE 5 Strain Temp Rate YS UTS Elong Alloy
Condition (.degree. C.) (l/sec) (ksi) (ksi) (%) CF8C Aged 1000 hr
at 850.degree. C. 22 IE-05 28.3 67.5 27 I Aged 1000 hr at
850.degree. C. 22 IE-05 34.4 82 25 J Aged 1000 hr at 850.degree. C.
22 IE-05 42.3 79.4 11.3
Manganese is an effective austenite stabilizer, like nickel, but is
about one tenth the cost of nickel. The positive austenite
stabilizing potential of manganese must be balanced with its
possible affects on oxidation resistance at a given chromium level
relative to nickel, which nears maximum effectiveness around 5 wt.
% and therefore addition of manganese in excess of 10 wt. % is not
recommended. Manganese in an amount of less than 2 wt. % may not
provide the desired stabilizing effect. Manganese also dramatically
increases the solubility of carbon and nitrogen in austenite. This
effect is especially beneficial because dissolved nitrogen is an
austenite stabilizer and also improves strength of the alloy when
in solid solution without decreasing ductility or toughness.
Manganese also improves strength ductility and toughness, and
manganese and nitrogen have synergistic effects.
The dynamic reduction in the sulfur content to 0.1 wt. % or less
proposed by the present alloys substantially eliminates the
segregation of free sulfur to grain boundaries and further
eliminates MnS particles found in conventional CF8C alloys, both of
which are believed to be detrimental at high temperatures.
An appropriate niobium:carbon ratio reduces excessive and
continuous networks of coarse niobium carbides (NbC) or finer
chrome carbides (M23C6) along the grain or substructure boundaries
(interdentritic boundaries and cast material) that are detrimental
to the mechanical performance of the material at high temperatures.
Accordingly, by providing an optimum level of the niobium and
carbon ratio ranging from about 9 to about 11 for the modified CF8C
alloys disclosed herein, niobium and carbon are present in amounts
necessary to provide high-temperature strength (both in the matrix
and at the grain boundaries), but without reducing ductility due to
cracking along boundaries with continuous or nearly-continuous
carbides.
Strength at all temperatures is also enhanced by the improved
solubility of nitrogen, which is a function of manganese. For
alloys of the modified CF8C type disclosed herein, the nitrogen
content can range from 0.02 wt. % to about 0.5 wt. %. The presence
of nitride precipitates is reduced by adjusting the levels and
enhancing the solubility of nitrogen while lowering the
chromium:nickel ratio.
In addition to the nitrogen levels disclosed above, the silicon
content can be limited to about 3.0 wt. % or less, the molybdenum
content can be limited to about 1.0 wt. % or less, the niobium
content can range from 0.0 wt. % to about 1.5 wt. %, the carbon
content can range from 0.05 wt. % to about 0.15 wt. %, the chromium
content can range from about 18 wt. % to about 25 wt. %, the nickel
content can range from about 8.0 wt. % to about 20.0 wt. %, the
manganese content can range from about 0.5 wt. % to about 1.0 wt.
%, the sulfur content can range from about 0 wt. % to about 0.1 wt.
%, the niobium carbon ratio can range from about 8 to about 11, and
the sum of the niobium and carbon contents can range from about 0.1
wt. % to about 0.5 wt. %.
Also, for the modified CF8C alloys disclosed herein, the
phosphorous content can be limited to about 0.04 wt. % or less, the
copper content can be limited to about 3.0 wt. % or less, the
tungsten content can be limited to about 3.0 wt. % or less, the
vanadium content can be limited to about 3.0 wt. % or less, the
titanium content can be limited to about 0.20 wt. % or less, the
cobalt content can be limited to about 5.0 wt. % or less, the
aluminum content can be limited to about 3.0 wt. % or less and the
boron content can be limited to about 0.01 wt. % or less.
Because nickel is an expensive component, stainless steel alloys
made in accordance with the present invention are more economical
if the nickel content is reduced.
INDUSTRIAL APPLICABILITY
The present invention is specifically directed toward a cast
stainless steel alloy for the production of articles exposed to
high temperatures and extreme thermal cycling such as
air/exhaust-handling equipment for diesel and gasoline engines and
gas-turbine engine components. However, the present invention is
not limited to these applications as other applications will become
apparent to those skilled in the art that require an austenitic
stainless steel alloy for manufacturing reliable and durable high
temperature cast components with any one or more of the following
qualities: sufficient tensile and creep strength at temperatures in
excess of 600.degree. C.; adequate cyclic oxidation resistance at
temperatures at or above 700.degree. C.; sufficient room
temperature ductility either as-cast or after exposure; sufficient
long term stability of the original microstructure and sufficient
longterm resistance to cracking during severe thermal cycling.
By employing the stainless steel alloys of the present invention,
manufacturers can provide a more reliable and durable high
temperature component. Engine and turbine manufacturers can
increase power density by allowing engines and turbines to run at
higher temperatures thereby providing possible increased fuel
efficiency. Engine manufacturers may also reduce the weight of
engines as a result of the increased power density by thinner
section designs allowed by increased high temperature strength and
oxidation and corrosion resistance compared to conventional
high-silicon molybdenum ductile irons. Further, the stainless steel
alloys of the present invention provide superior performance over
other cast stainless steels for a comparable 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 invention.
* * * * *