U.S. patent application number 11/567944 was filed with the patent office on 2009-02-26 for cast heat-resistant austenitic steel with improved temperature creep properties and balanced alloying element additions and methodology for development of the same.
Invention is credited to Philip J. Maziasz, Govindarajan Muralidharan, Roman I. Pankiw, Vinod Kumar Sikka.
Application Number | 20090053100 11/567944 |
Document ID | / |
Family ID | 40418865 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090053100 |
Kind Code |
A1 |
Pankiw; Roman I. ; et
al. |
February 26, 2009 |
CAST HEAT-RESISTANT AUSTENITIC STEEL WITH IMPROVED TEMPERATURE
CREEP PROPERTIES AND BALANCED ALLOYING ELEMENT ADDITIONS AND
METHODOLOGY FOR DEVELOPMENT OF THE SAME
Abstract
The present invention addresses the need for new austenitic
steel compositions with higher creep strength and higher upper
temperatures. The present invention also discloses a methodology
for the development of new austenitic steel compositions with
higher creep strength and higher upper temperatures.
Inventors: |
Pankiw; Roman I.;
(Greensburg, PA) ; Muralidharan; Govindarajan;
(Knoxville, TN) ; Sikka; Vinod Kumar; (Oak Ridge,
TN) ; Maziasz; Philip J.; (Oak Ridge, TN) |
Correspondence
Address: |
Meyer, Unkovic & Scott LLP
1300 Oliver Building, 535 Smithfield Street
Pittsburgh
PA
15222
US
|
Family ID: |
40418865 |
Appl. No.: |
11/567944 |
Filed: |
December 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60748239 |
Dec 7, 2005 |
|
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|
60789905 |
Apr 6, 2006 |
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Current U.S.
Class: |
420/585 ;
148/419; 420/586.1; 420/590 |
Current CPC
Class: |
C22C 30/00 20130101;
C22C 38/02 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/585 ;
420/586.1; 148/419; 420/590 |
International
Class: |
C22C 30/00 20060101
C22C030/00; C22C 33/00 20060101 C22C033/00 |
Goverment Interests
STATEMENT OF FEDERAL FUNDING
[0002] The United States government has rights to this invention
pursuant to contract no. DE-AC05-00OR22725 between the United
States Department of Energy and UT-Battelle, LLC. This invention
was made under Cooperative Research and Development Agreement
("CRADA") ORNL 02-0632 between Duraloy Technologies, Inc. and
UT-Battelle, LLC.
Claims
1. A cast heat-resistant austenitic steel alloy with improved
temperature creep properties and balanced alloyed element
additions, said alloy comprising: about 0.4 to about 0.7 wt. %
carbon; about 20 to about 35 wt. % chromium; about 30 to about 45
wt. % nickel; about 0.5 to about 1.5 wt. % manganese; about 0.6 to
about 2.0 wt. % silicon; up to about 1.5 wt. % niobium; up to about
1.5 wt. % tungsten; up to about 1.5 wt. % molybdenum; and, the
remainder being iron.
2. The cast heat-resistant austenitic steel alloy of claim 1,
further comprising about 0.01 to about 1.0 wt. % titanium.
3. The cast heat-resistant austenitic steel alloy of claim 1,
further comprising about 0.01 to about 1.0 wt. % zirconium.
4. The cast heat-resistant austenitic steel alloy of claim 2,
further comprising about 0.01 to about 1.0 wt. % zirconium.
5. The cast heat-resistant austenitic steel alloy of claim 1,
further comprising about 0.02 wt. % cobalt.
6. The cast heat-resistant austenitic steel alloy of claim 1,
wherein the creep life of said alloy at about 1200.degree. C. and
about 500 psi is in excess of about 600 hours.
7. The cast heat-resistant austenitic steel alloy of claim 1,
wherein the total wt. % of carbides in said alloy at about
1200.degree. C. is between about 3.18 and about 7.84.
8. The cast heat-resistant austenitic steel alloy of claim 1,
wherein M.sub.23C.sub.6 and MC carbides are present in the
microstructure of said alloy at high temperatures up to about
1204.degree. C.
9. The cast heat-resistant austenitic steel alloy of claim 1,
wherein said alloy has the following composition: about 0.41 wt. %
carbon; about 23.6 wt. % chromium; about 34.6 wt. % nickel; about
1.0 wt. % manganese: about 0.7 wt. % silicon; about 0.05 wt. %
molybdenum; about 0.08 wt. % tungsten; about 0.33 wt. % niobium;
about 0.02 wt. % cobalt; about 0.1 wt. % titanium; and the
remainder being iron; said alloy has a maximum temperature of
stability of M.sub.23C.sub.6 carbide of about 1173.degree. C.; a
maximum wt. % of M.sub.23C.sub.6 between about 600.degree. C. and
about 1500.degree. C. of about 6.4; a maximum wt. % of MC between
about 600.degree. C. and about 1500.degree. C. of about 0.43; and a
creep life at about 1204.degree. C. and about 500 psi of about 675
hours.
10. The cast heat-resistant austenitic steel alloy of claim 1, said
alloy having the following composition: about 0.42 wt. % carbon;
about 23.7 wt. % chromium; about 35.1 wt. % nickel; about 1.1 wt. %
manganese; about 0.7 wt. % silicon; about 0.28 wt. % molybdenum;
about 0.07 wt. % tungsten; about 0.34 wt. % niobium; about 0.02 wt.
% cobalt; about 0.4 wt. % titanium; about 0.1 wt. % zirconium; and
the remainder being iron; said alloy having a maximum temperature
of stability of M.sub.23C.sub.6 carbide of about 1227.degree. C.; a
maximum wt. % of M.sub.23C.sub.6 between about 600.degree. C. and
austenitic 1500.degree. C. of about 6.7; a maximum wt. % of MC
between about 600.degree. C. and austenitic 1500.degree. C. of
about 0.74; and a creep life at about 1204.degree. C. and about 500
psi of about 1251 hours.
11. The cast heat-resistant austenitic steel alloy of claim 1, said
alloy having the following composition: about 0.41 wt. % carbon;
about 23.4 wt. % chromium; about 34.34 wt. % nickel; about 1.0 wt.
% manganese; about 0.7 wt. % silicon; about 0.5 wt. % molybdenum;
about 0.34 wt. % niobium; about 0.1 wt. % tungsten; about 0.3 wt. %
titanium; and the remainder being iron; said alloy having a maximum
temperature of stability of M.sub.23C.sub.6 carbide of about
1253.degree. C.; a maximum wt. % of M.sub.23C.sub.6 between about
600.degree. C. and about 1500.degree. C. of about 6.6; a maximum
wt. % of MC between about 600.degree. C. and about 1500.degree. C.
of about 0.63; and a creep life at about 1204.degree. C. and about
500 psi of about 1293 hours.
12. The cast heat-resistant austenitic steel alloy of claim 1, said
alloy having the following composition: about 0.6 wt. % carbon;
about 23.6 wt. % chromium; about 35.5 wt. % nickel; about 1.0 wt. %
manganese; about 0.7 wt. % silicon; about 0.85 wt. % molybdenum;
about 0.34 wt. % niobium; about 0.07 wt. % tungsten; about 0.4 wt.
% titanium; about 0.1 wt. % zirconium; the remainder being iron;
and said alloy having a maximum temperature of stability of
M.sub.23C.sub.6 carbide of about 1267.degree. C.; a maximum wt. %
of M.sub.23C.sub.6 between about 600.degree. C. and about
1500.degree. C. of about 10.1; a maximum wt. % of MC between about
600.degree. C. and about 1500.degree. C. of about 0.75; and a creep
life at about 1204.degree. C. and about 500 psi of about 1558
hours.
13. A cast heat-resistant austenitic steel alloy with improved
temperature creep properties and balanced alloyed elements
additions, said alloy comprising: about 0.4 wt. % carbon; about 24
wt. % chromium; about 35 wt. % nickel; about 1.0 wt. % manganese;
about 0.7 wt. % silicon; about 0.3 wt. % niobium; about 0.08 wt. %
tungsten; about 0.5 to about 1.5 wt. % molybdenum; about 0.1 to
about 0.4 wt. % titanium; and, the rest being iron.
14. The cast heat resistant austenitic steel alloy of claim 13,
further comprising less than about 0.1 wt. % zirconium.
15. The cast heat-resistant austenitic steel alloy of claim 13,
said alloy having a total calculated wt. % of carbides in the range
of about 3.18 to about 5.06.
16. A method for evaluating compositions of HP and HK alloys
comprising the steps of: calculating the effect of an addition of
alloying elements on the stability of M.sub.23C.sub.6 and MC
utilizing thermodynamic models; using said HP and HK alloys as a
base reference for said calculations; and, wherein said addition of
alloying elements are used to stabilize strengthening phases at a
required temperature as indicated by the results of said
thermodynamic calculations to provide improved creep-resistance.
Description
PRIORITY TO PROVISIONAL APPLICATIONS
[0001] This application hereby claims priority to provisional
application Ser. No. 60/748,239, filed on Dec. 7, 2005, and
provisional application Ser. No. 60/789,905, filed on Apr. 6, 2006.
Both applications are incorporated herein by reference.
INVENTORS
[0003] Govindarajan Muralidharan, Vinod Kumar Sikka, Philip J.
Maziasz, and Roman I. Pankiw
BACKGROUND OF THE INVENTION
[0004] The present invention addresses the need for new austenitic
steel compositions with higher creep strength and higher upper
temperatures as compared to the presently used H-Series of cast
stainless steels. Heat-resistant cast austenitic stainless steels
and alloys are the backbone of the chemical, petrochemical,
heat-treating and metals processing industries today, with
applications continuing to drive performance, durability and
use-temperatures higher, while economics tries to lower the cost of
such alloys. There is a significant and continued need for low-cost
austenitic stainless steel alloys that can be used in the as-cast
condition at high temperatures up to 1200.degree. C. Alloys
currently used for this purpose have a significant Nickel (Ni)
content added (.about.35-45 wt. %), a large Cobalt (Co) content (up
to 15 wt. %), or a large Tungsten (W) or Molybdenum (Mo) content.
For these alloys, creep properties at 1200.degree. C. can vary
widely within the composition ranges specified in these inventions
and a better definition of alloy compositions is needed for optimum
creep properties for the temperature range of operation.
[0005] The alloy compositions of the present invention are designed
specifically for improved creep properties at high temperatures of
up to and including about 1200.degree. C. Microstructure is a
unique characteristic of the alloys of the present invention and
forms the basis of their high temperature strength. The key
problems solved by the present invention are the reduction of
Cobalt (Co) content and the need for only small quantities of other
alloying elements. The microstructure design creates a stable
austenite resistant and an optimum combination of MC and
M.sub.23C.sub.6 carbides, which is promoted through the addition of
alloying elements.
[0006] For service temperatures above 850.degree. C. to 900.degree.
C., the dominant alloy for use in steam methane reforming or
ethylene cracking applications was initially HK-40 stainless steel.
More recently modified or micro-alloyed HP stainless alloys have
been used for these applications. In 1941, the Alloy Casting
Institute introduced the classifications used today, designating
the heat-resistant grades as H-grades and the corrosion-resistant
stainless steels and alloys as C-grades. Fairly complete
descriptions of properties, compositions, and standard industrial
practice for the various grades of cast austenitic stainless steels
and alloys can be found in handbooks or data available from the
Steel Founders Society of America, ASM International, The Nickel
Development Institute, The Specialty Steel Industry of North
America, and/or data sheets compiled by the various leading alloy
casters. The HK-40 stainless steel is essentially a
Fe-25Cr-20Ni-0.4C alloy, whereas HP-40 stainless alloy is
Fe-25Cr-35-Ni-0.4C, with more creep-resistant modifications
occurring in the HP modified (+Nb) or the HP micro-alloyed (+Nb+Ti
or +Nb+Zr) materials. In the 1960s and 1970s, efforts to improve
the carburization-resistance of the HK-40 steels led to additions
of up to 2% Silicon (Si) and increases in Nickel (Ni) (IN-519,
25Cr-25Ni-1.5Nb and HP alloys), while efforts to increase the
strength and creep-resistance added Niobium (Nb). Costly upgrades
of the modified HP alloys include additions of Tungsten (W) and
Cobalt (Co) to further increase the high-temperature strength.
[0007] The native microstructure established in these
fully-austenitic alloys consists of dendritic structures of
austenite matrix with finer dispersions of carbides (Cr-rich
M.sub.23C.sub.6 or Nb-rich MC, depending on the alloy), and heavier
clusters of NbC in the interdendritic regions (the last liquid to
solidify) and dispersions of M.sub.23C.sub.6 along the seams
between colonies of dendrites. Aging effects can vary, with little
deleterious effects above 950.degree. C. to 1000.degree. C.,
particularly in the modified HP alloys, but with potential
embrittlement (severe ductility loss at ambient temperatures) due
to M.sub.23C.sub.6 films and/or sigma phase formation during
prolonged exposure below 900.degree. C., mainly in the HK-40 type
alloy. Additions of Cobalt (Co) are made mainly to strengthen and
stabilize the austenite matrix phase, while additions of Tungsten
(W) promote solid solution strengthening as well as Tungsten
Carbide (WC) formation. Ethylene cracking and radiant furnace tubes
generally involve prolonged exposure at relatively steady
temperatures, where creep-resistance and oxidation/corrosion
resistance are the life-defining properties. However, materials
processing applications of such alloys, including steel mill
furnace rollers and retorts for calcining, involve more than just
creep-strength, and must include thermal fatigue resistance to
prevent surface cracking (critical for coiling drums) or
catastrophic through-section fracture (retorts). While one can
possibly use the more expensive chemical/petrochemical premium
alloys for such materials processing equipment applications, the
materials processing industries probably would be better served by
improving the strength and aging resistance of the standard HK-40
grade steel to provide a more cost-effective solution.
[0008] Alloy development of complex engineering alloys based on
single or multiple alloying element additions or changes over wide
ranges can often be very labor intensive, time consuming and
costly. Usually such traditional brute-force efforts produce only
modest incremental improvements, and then such improvements must be
further verified by testing relevant to real-time component
service. Therefore, most applications engineers attempt to redesign
components or to solve their materials problems by selecting
alternate materials, and they only turn to traditional alloy
development as a last resort.
[0009] A far more scientific and yet practical method of precise
microstructural analysis and identification of the
degradation/failure mechanisms was devised at Oak Ridge National
Laboratory to improve the creep-resistance of 300-series austenitic
stainless steels at about 700.degree. C. to about 800.degree. C.
This method provided a framework for translating various single or
combined alloying element effects directly into their effects on
precipitation behavior or stability of the parent matrix phase
(austenite). When this scientific knowledge of how to stabilize
desirable phases and reduce or eliminate undesirable phases was
coupled with a thorough knowledge of microstructure/properties
relationships and failure mechanisms, precise microstructures were
designed that produced outstanding long-term creep-resistance the
first time those modified stainless steels and alloys were made.
The alloying addition effects were classified as (a) direct
reactant effects (i.e., Nb+C=NbC), (b) catalytic effects (i.e.,
solutes that enhance the formation of a phase even though they are
not direct reactants, like Silicon (Si) enhancing the formation of
Fe.sub.2Mo Laves phase or Boron (B) enhancing the formation of TiC
or M.sub.23C.sub.6 carbides), (c) inhibitor effects (i.e., solutes
that retard or prevent the formation of particular precipitate
phases, like Carbon (C), Boron (B), and Phosphorus (P) retarding or
preventing the formation of intermetallic phases like sigma, chi or
Laves), and (d) interference effects (i.e., two or more phases
competing for the same element to decouple or simplify phase
behavior and control in complex alloys). Some of the
microstructural effects that have been controlled to create
extremely long-term creep-rupture resistance include: (a) the
elimination of creep voids; (b) the promotion of fine dispersions
of MC carbides; (c) the prevention of dissolution or coarsening of
fine MC carbides; (d) the delay or prevention of the formation of
embrittling grain-boundary intermetallic phases; and (e) the
prevention of dislocation recovery or recrystallization (mainly for
wrought alloys).
[0010] As discussed more fully below, the related art recognizes
and discusses previous efforts to obtain the improved alloys of the
present invention. However, these efforts suffered from various
shortcomings, including the need for costly element additions and
low creep lives at high temperatures.
[0011] For example, U.S. Pat. No. 3,627,516 describes Fe--Ni--Cr
alloys with a preferred composition of 26% Chrominium, 32% Nickel,
0.4% Carbon, 1.1% Manganese, 1.2% Silicon, 0.08% Nickel, 1.2%
Niobium and the rest Iron for use at temperatures ranging from
800.degree. C. to 1200.degree. C. This patent also notes that 0.5%
to 1.5% Molybdenum was beneficial in some cases. The present
invention, on the other hand, identifies alloys with higher Nickel
content. Niobium (Nb) contents in the present invention are also
higher than in the alloys identified in U.S. Pat. No. 3,627,516
(thus lower overall costs). Molybdenum (Mo) contents are also lower
in the present invention, which, again, is different from the
related art.
[0012] Similarly, U.S. Pat. No. 3,758,294 discloses improved alloys
with additions of 0.18% Nitrogen, 1.7% Tungsten, and 1.3% Niobium.
The preferred embodiment including resistance to carburization
included 0.4 wt. % Carbon, 25 to 28 wt. % Chromium, 32 to 36%
Nickel, 0.5 to 1.0% Manganese, 1.2 to 1.6% Silicon, 1.4 to 2.0%
Tungsten, 1.0 to 1.8% Niobium and 0.15% Nitrogen. Comparison of the
creep life of the alloys identified in U.S. Pat. No. 3,627,516 and
U.S. Pat. No. 3,785,294 with those in the present invention reveals
that the present invention's new alloys possess improved creep
properties at about 1200.degree. C. Thus, improved properties were
obtained with much lower alloying element additions, particularly
Tungsten (W) and Niobium (Nb), and without the addition of Nitrogen
(N).
[0013] U.S. Pat. No. 4,853,185 and U.S. Pat. No. 4,981,674 identify
Fe--Ni--Cr alloys with 25-45% Nickel, 12-32% Chromium, 0.1 to 2.0%
Niobium (minimum of nine times Carbon content), 2.0% Titanium max,
3% Silicon max, 0.05-0.5% Nitrogen, 0.02 to 0.2% Carbon, 2.0%
Manganese max, 1.0% Aluminum max, 5.0% Tungsten/Molybdenum max,
0.02% Boron max, 0.2% Zirconium max, 5% Cobalt max, Yttrium,
Lanthanum, Copper, REM up to 0.1% max. At least one Niobium,
Tantalum, or Vanadium has to be present in the alloy along with
Carbon and Nitrogen. Niobium (Nb) has to be added to a level of at
least nine times the Carbon content. In contrast, the alloys of the
present invention need no additions of Tantalum (Ta) or Vanadium
(V) with very small simultaneous additions of Niobium (Nb),
Tungsten (W), and Molybdenum (Mo). In addition, no Nitrogen (N) is
intentionally added in the alloys of the present invention and thus
the concentration can be much lower than 0.05%. Furthermore, the
Carbon (C) levels in the present invention's alloys are much larger
than those identified by Rothman et al.
[0014] In related art, U.S. Pat. No. 4,615,658 teaches a material
for gas turbine shrouds that contains 0.35 to 0.5 wt. % Carbon, 22
to 24 wt. % Chromium, 24 to 26 wt. % Nickel, 0.15 to 0.35 wt. %
Titanium, 0.2 to 0.5 wt. % Niobium, 0.1 to 1.2 wt. % Manganese,
less than 0.8 wt. % Silicon and the balance Iron. Also suggested
were additions of 0.05 to 5 wt. % rare-earth elements, 5 to 20 wt.
% Cobalt, less than 7 wt. % Tungsten, and less than 7 wt. %
Molybdenum. The present invention does not require Cobalt (Co) and
rare-earth elements. Further, U.S. Pat. No. 4,615,658 does not show
the creep properties of the alloys at high temperatures, and the
amount of Tungsten (W) required by the invention of U.S. Pat. No.
4,615,658 is significant, which influences the cost and other alloy
properties.
[0015] U.S. Pat. No. 6,485,674 discloses a heat resistant
austenitic stainless steel with 0.04 to 0.1% Carbon, not more than
0.4% Silicon no more than 0.6% Manganese, 20 to 27% Chromium, 22.5
to 32% Nickel, not more than 0.5% Molybdenum, 0.2 to 0.6% Niobium,
0.4 to 4.0% Tungsten, 0.1 to 0.3% Nitrogen, 0.002 to 0.008% Boron,
less than 0.05% Aluminum, and at least one of Calcium or Manganese.
The alloys of the present invention contain a larger amount of
Carbon (C), are able to accept more Silicon (Si) and Manganese (Mn)
do not need any Nitrogen (N) or Calcium (Ca) additions, and achieve
excellent heat resistance at temperatures up to 1200.degree. C.
[0016] U.S. Pat. No. 6,685,881 discloses an austenitic stainless
steel for use up to 950.degree. C. with good heat resistance and
machinability. The compositions of those alloys are in the range of
0.2 to 0.4% Carbon, 0.5 to 2.0% Silicon, 0.5 to 2.0% Manganese, 8
to 42% Nickel, 15 to 28% Chromium, 0.5 to 7.0% Tungsten, 0.5 to
2.0% Niobium, up to 0.05% Titanium, up to 0.15% Nitrogen, 0.001 to
0.5% Selenium, up to 0.1% Phosphorus, and 0.04 to 0.2% Sulfur. The
present invention alloys have good heat resistance up to about
1200.degree. C. In addition, the Tungsten (W) and Niobium (Nb)
contents of the present invention alloys are much lower with no
intentional additions of Nitrogen (N). The creep lives of the
alloys in the U.S. Pat. No. 6,685,881 are not specified.
[0017] In summary, the present invention increases the
high-temperature strength and upper use temperature of H-Series of
cast austenitic stainless steels without the requirement of certain
costly element additions. The present invention may be used in,
among other things, steam methane reformer tubes, ethylene cracking
furnace tubes, radiant furnace tubes, steel mill furnace transfer
rollers, and retorts for calcining. The result is a significant
energy and cost savings each year.
SUMMARY OF THE INVENTION
[0018] The present invention deals with the development of new cast
heat-resistant austenitic alloys whose compositions are
scientifically designed based on an understanding of the effect of
alloying elements on the thermodynamic stabilities and compositions
of various phases in the Fe--Cr--Ni alloy system. One of the unique
aspects of this invention is the addition of balanced quantities of
alloying elements achieved through thermodynamic calculations.
These thermodynamic calculations are used to calculate the
stabilities and compositions of various phases as a function of
alloying element content. Using such calculations, desirable phases
such as austenite, M.sub.23C.sub.6, and MC have been retained in
the microstructure to higher temperatures. Moreover, the new alloys
contain an optimum combination of complex carbides with alloying
element additions enhancing their stability. For example, Tungsten
(W) and Molybdenum (Mo) improve the stability of M.sub.23C.sub.6,
while Molybdenum (Mo) and Niobium (Nb) improve the stability of MC
carbides. The new compositions, when tested at about 2200.degree.
F. (1204.degree. C.), showed improved properties when compared with
standard HP and HP modified alloys.
[0019] Although there is a need for alloys with improved
high-temperature strength properties, very little work has been
performed in recent times on improving the properties of H-series
alloys. One of the reasons for the lack of significant efforts is
the cost and time associated with development of new alloys through
traditional methods. Alloy development using traditional Edisonian
methods involves repeated cycles of trial-and-error additions of
alloying elements, followed by extensive experimental work to
fabricate and test the alloys. This work entails extensive labor,
time and thus cost, with no assurance of success. Furthermore, with
multiple alloying element additions, possible permutations and
combinations can be extensively large and prohibitively expensive.
In recent times, using the vast amount of experimental data
available, significant advances have been made in computational
thermodynamic and kinetic models to describe alloy phase equilibria
and microstructural evolution. The present invention uses these
computer models to test the effect of adding a certain element on
the phase equilibria without actually preparing the alloy, thus
replacing the traditional trial-and-error methods. This
computer-aided alloy development work significantly reduces the
cost and time of the development effort and is thus one of the
important features of the present invention. The alloys of the
present invention were developed by searching alloy composition
phases for alloying element additions that would result in a
desirable microstructure, with the following experimental work
limited to a selected number of potentially promising alloys.
[0020] The present invention teaches a novel methodology for the
development of new cast heat-resistant austenitic alloys. The alloy
compositions of the present invention were developed to primarily
increase the high-temperature creep strength of existing HP- and
HK-type alloys. It was determined that the presence of both types
of carbides, M.sub.23C.sub.6 and MC, had a positive influence of
the high-temperature creep properties of alloys. To understand the
stabilities of these carbides in existing alloys, thermodynamic
calculations were carried out using computational tools. The
results of these calculations showed that M.sub.23C.sub.6 phase
dissolution at temperatures between about 2100-2200.degree. F.
(depending on the composition of the alloy) was one of the most
important factors responsible for the loss in strength. Using
computational thermodynamics, the effects of adding various other
elements on the stabilities of the carbide phases were determined,
and additional alloying element additions were selected to increase
the stability of those phases at high temperatures. In addition,
alloy composition was optimized to remove or minimize expensive
alloying elements such as Tungsten and Cobalt used for
strengthening the matrix through solid solution strengthening in
alloys such as Supertherm.RTM.. Since computational thermodynamics
can predict the stability of deleterious phases such as sigma,
alloy composition was also optimized to decrease the stability of
that phase over the temperature range of interest.
[0021] A preferred embodiment of the cast heat-resistant austenitic
steel alloy with improved temperature creep properties and balanced
alloyed element additions of the present invention includes about
0.4 to about 0.7 wt. % Carbon; about 20 to about 35 wt. % Chromium;
about 30 to about 45 wt. % Nickel; about 0.5 to about 1.5 wt. %
Manganese; about 0.6 to about 2.0 wt. % Silicon; up to about 1.5
wt. % Niobium; up to about 1.5 wt. % Tungsten; up to about 1.5 wt.
% Molybdenum; and, the remainder being Iron. The alloy further
optionally comprises about 0.01 to about 1.0 wt. % Titanium, about
0.01 to about 1.0 wt. % Zirconium and about 0.02 wt. % Cobalt. The
creep life of the alloy at about 1200.degree. C. and about 500 psi
is in excess of about 600 hours and the total wt. % of carbides in
the alloy at about 1200.degree. C. is between about 3.18 and about
7.84. M.sub.23C.sub.6 and MC carbides are present in the
microstructure of the alloy at high temperatures up to about
1204.degree. C.
[0022] The method of the present invention specifically includes
the steps of evaluating compositions of HP and HK alloys;
calculating the effect of an addition of alloying elements on the
stability of M.sub.23C.sub.6 and MC utilizing thermodynamic models,
using the HP and HK alloys as a base reference for the
calculations, and wherein the alloying element additions are used
to stabilize strengthening phases at a required temperature as
indicated by the results of the thermodynamic calculations to
provide improved creep-resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For the present invention to be easily understood and
readily practiced, the invention will now be described, for the
purposes of illustration and not limitation, in conjunction with
the following figures, wherein:
[0024] FIG. 1 illustrates equilibrium thermodynamic calculations
showing phases present at various temperatures in the HP-7 alloy,
having a composition of: about 0.41 wt. % C, about 23.6 wt. % Cr,
about 34.6 wt. % Ni, about 1.0 wt. % Mn, about 0.7 wt. % Si, about
0.05 wt. % Mo, about 0.33 wt. % Nb, about 0.08 wt. % W, about 0.1
wt. % Ti, and the rest Fe.
[0025] FIG. 2 illustrates equilibrium thermodynamic calculations
showing phases present at various temperatures in the HP-15 alloy,
having a composition of: about 0.42 wt. % C, about 23.7 wt. % Cr,
about 35.1 wt. % Ni, about 1.1 wt. % Mn, about 0.7 wt. % Si, about
0.28 wt. % Mo, about 0.34 wt. % Nb, about 0.07 wt. % W, about 0.4
wt. % Ti, about 0.1 wt. % Zr, and the rest Fe.
[0026] FIG. 3 illustrates equilibrium thermodynamic calculations
showing phases present at various temperatures in HP-16 alloy,
having a composition of: about 0.41 wt. % C, about 23.4 wt. % Cr,
about 34.34 wt. % Ni, about 1.0 wt. % Mn, about 0.7 wt. % Si, about
0.5 wt. % Mo, about 0.34 wt. % Nb, about 0.1 wt. % W, about 0.3 wt.
% Ti, and the rest Fe.
[0027] FIG. 4 illustrates equilibrium thermodynamic calculations
showing phases present at various temperatures in HP-14R1 alloy,
having a composition of: about 0.6 wt. % C, about 23.6 wt. % Cr,
about 35.5 wt. % Ni, about 1.0 wt. % Mn, about 0.7 wt. % Si, about
0.85 wt. % Mo, about 0.34 wt. % Nb, about 0.07 wt. % W, about 0.4
wt. % Ti, about 0.1 wt. % Zr, and the rest Fe.
[0028] FIG. 5 illustrates equilibrium thermodynamic calculations
showing phases present at various temperatures in HP-40 alloy,
having a composition of: about 0.4 wt. % C, about 23.5 wt. % Cr,
about 0.7 wt. % Mn, about 33.2 wt. % Ni, about 1.75 wt. % Si, and
the rest Fe.
[0029] FIG. 6 illustrates a comparison in creep rupture life
between new alloys: HP-7, HP-11, HP-13, HP-14R1, HP-15, HP-16 and
the HP-40 baseline alloy.
[0030] FIG. 7(a) illustrates the thermodynamic calculations for
HP-11.
[0031] FIG. 7(b) illustrates the creep strain as a function of time
obtained during testing at about 2200.degree. F. (1204.degree. C.)
and about 500 psi, showing that failure occurred after about 14
hours.
[0032] FIGS. 8(a) and 8(b) illustrate thermodynamic calculations
showing the effect of Titanium additions on the stabilities of
M.sub.23C.sub.6 and MC.
[0033] FIGS. 9(a) and 9(b) illustrate thermodynamic calculations
showing the effect of Tungsten additions on the stabilities of
M.sub.23C.sub.6 and MC carbides.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The invention will now be described in detail in relation to
the preferred embodiments and implementation thereof which is
exemplary in nature and descriptively specific as disclosed. As is
customary, it will be understood that no limitation of the scope of
the invention is thereby intended. The invention encompasses such
alterations and further modifications in the illustrated
compositions and methods, and such further applications of the
principles of the invention illustrated herein, as would normally
occur to persons skilled in the art to which the invention
relates.
[0035] As used herein in the specification and claims, including as
used in the examples and unless otherwise expressly specified, all
numbers may be read as if prefaced by the word "about", even if the
term does not expressly appear. Also, any numerical range recited
herein is intended to include all sub-ranges subsumed therein.
[0036] And now with reference to FIGS. 1-9b, the present invention
is described below. There is a need for low-cost, heat resistant
austenitic stainless steels for operation at temperatures up to
about 1200.degree. C. and higher. For these alloys, a significant
property of interest is creep-resistance, with oxidation resistance
being the second most important property. Many of these alloys
typically contain significant quantities of Nickel (Ni) (up to
about 45 wt. %) along with significant quantities of Cobalt (Co)
and Tungsten (W) making the alloys very expensive. A primary
motivation of the present invention is the development of alloy
compositions that are lower in cost than existing alloys but which
have comparable or improved creep properties when tested in a
typical environment, such as but not limited to air. The alloys
identified in a preferred embodiment of the present invention are
Iron-Nickel-Chromium (Fe--Ni--Cr) alloys with the composition of
the alloys in the typical range of:
[0037] Carbon: about 0.2 to about 0.7 wt. %;
[0038] Chromium: about 20 to about 40 wt. %;
[0039] Nickel: about 25 to about 60 wt. %;
[0040] Manganese: about 0.1 to about 2.5 wt. %;
[0041] Silicon: about 0.1 to about 2.5 wt. %;
[0042] Niobium: up to about 2.0 wt. %;
[0043] Tungsten: up to about 2.0 wt. %;
[0044] Molybdenum: up to about 2.0 wt. %;
[0045] Titanium: up to about 1.0 wt. %; (optional)
[0046] Zirconium: up to about 1.0 wt. %; (optional) and,
[0047] Iron: Rest/remainder
[0048] The alloying element additions in the preferred embodiments
of the present invention are kept to a minimum along with decreased
Nickel (Ni) contents, which lowers the cost of the alloy. The
preferred alloy compositions have been derived through a novel
methodology that emphasizes the role of microstructure over that of
each specific alloying element.
[0049] This methodology of the present invention involves the
consideration of certain reference compositions of HP and HK
alloys. Using these compositions as a starting point, the effect of
the addition of alloying elements on the stabilities of
M.sub.23C.sub.6 and MC are calculated using thermodynamic models.
Sufficient alloying element additions are used to stabilize the
strengthening phases at the required temperature (for example about
2200.degree. F.), as indicated by results of the thermodynamic
calculations, while minimizing the formation of deleterious
topologically closed packed phases at low temperatures. The focus
of the methodology is on high temperatures since an aim of the
invention is to increase the operating temperature of alloys. The
following describes some examples of the methodology.
[0050] FIG. 7(a) shows thermodynamic predictions for phase
equilibria in an alloy HP-11 with poor creep performance at about
2200.degree. F. (creep rupture life of about 14 hours at a stress
of about 500 psi) (see FIG. 7(b)). Specifically FIGS. 7(a) and 7(b)
show that the M.sub.23C.sub.6 carbides are predicted to be stable
only up to about 2100.degree. F. and are replaced by M.sub.7C.sub.3
carbides above this temperature. The approach adopted in this
methodology is to add alloying elements to extend the range of
stability of M.sub.23C.sub.6 carbides to higher temperatures.
[0051] FIGS. 8(a) and 8(b) illustrate the effects of the addition
of varying levels of Titanium, one of the commonly used alloying
elements, on the stabilities of the M.sub.23C.sub.6 and MC carbides
in HP-11. As increasing levels of Titanium are added, the
M.sub.23C.sub.6 content in the alloy decreases while
correspondingly, the MC content increases. The highest temperature
at which the M.sub.23C.sub.6 carbide remains stable increases
slightly with the addition of Titanium.
[0052] FIGS. 9(a) and 9(b) disclose the effect of the addition of
varying levels of Tungsten on the stabilities of the
M.sub.23C.sub.6 and MC carbides. With increasing levels of
Tungsten, the highest temperature of stability of the
M.sub.23C.sub.6 carbide increases significantly along with a small
increase in the weight % of the carbide phase. Tungsten has very
little effect on the MC carbide phase as shown in FIG. 8(b). Thus,
if a goal is to increase the amount of MC carbide, the addition of
Titanium is also desirable as an alloying element. The addition of
Tungsten would also be important to increase the amount of
M.sub.23C.sub.6 carbide and to increase its temperature range of
stability.
[0053] The preferred embodiments of these alloys have been designed
based upon certain observed properties for potential alloying
elements: [0054] (1) Silicon (Si) contents (about 0.6 to about 2.5
wt. %) are used for ease of casting, carburization resistance, and
oxidation resistance; [0055] (2) Nickel (Ni) is restricted to the
range of about 25 to about 60 wt. % to reduce the cost of the
alloy, although a minimum amount of Nickel content is essential to
maintain the austenitic structure; [0056] (3) Chromium (Cr) is
essential for oxidation resistance but is a ferrite stabilizer (the
selected range of about 20 to about 40 wt. % will provide
sufficient corrosion resistance but enables retention of the
austenitic structure); and [0057] (4) the intentional addition of
Nitrogen (N) is not required to achieve good properties.
[0058] In addition to these considerations on the alloying
elements, it is well recognized that the phases present in the
alloy are related in a complex manner to the nature and extent of
alloying element additions. Related inventions have used
trial-and-error methods to arrive at a few compositions. In the
present invention, thermodynamic calculations were performed to
study, systematically, the nature and amount of various phases
present in the newly designed alloys at equilibrium at various
temperatures. FIGS. 1-5 show summary reports of the phases present
as a function of temperature of various alloy compositions of the
present invention.
[0059] Phases present at temperatures in the range of about
1000.degree. C.-1200.degree. C. include austenite, M.sub.7C.sub.3,
M(C,N), and M.sub.23C.sub.6. In particular, differences are
observable in the calculated values of the various types of
carbides present at about 1200.degree. C. Table 1 shows the two
examples of the preferred composition of the alloys. Creep testing
was performed in air at about 1204.degree. C. (2200.degree. F.) and
about 500 psi.
TABLE-US-00001 TABLE 1 Examples of Preferred Embodiments of
Compositions, Wt. % and their Creep Life. Creep Life (Hrs)
1204.degree. C. (2200.degree. F.), Alloy C Cr Ni Mn Si Mo W Nb Co
Ti Zr Fe 500 psi HP-7 0.41 23.6 34.6 1.0 0.7 0.05 0.08 0.33 0.02
0.1 0.0 38.1 675 HP-15 0.42 23.7 35.1 1.1 0.7 0.28 0.07 0.34 0.02
0.4 0.1 37.8 1251 HP-16 0.41 23.4 34.3 1.0 0.7 0.5 0.1 0.34 0.02
0.3 0.0 38.9 1293 HP- 0.6 23.6 35.5 1.0 0.7 0.85 0.07 0.34 0.02 0.4
0.1 36.8 1558 14R1 Super- 0.5 25.9 34.3 0.7 1.5 0.02 4-6 0.0 14-16
0.0 0.0 Rest 487 Therm
[0060] Table 2 compares the predicted equilibrium wt. % of the
M.sub.7C.sub.3, M(C,N), and M.sub.23C.sub.6 in these alloys at
about 1200.degree. C. The carbides/carbonitrides are the
strengthening phases in these alloys. The increased wt. % carbides
correlate well with improved creep properties.
TABLE-US-00002 TABLE 2 Examples of Preferred Embodiments of
Compositions and the Calculated Wt. % Carbides Present at
Equilibrium at about 1200.degree. C. Creep Life (Hrs) Total
1204.degree. C. Wt. % Wt. % Wt. % Wt. % (2200 F), Alloy C Cr Ni Mn
Si Mo W Nb Co Ti Zr Fe M.sub.7C.sub.3 MC M.sub.23C.sub.6 Carbides
500 psi HP-7 0.41 23.6 34.6 1.0 0.7 0.05 0.08 0.33 0.02 0.1 0.0
38.1 2.9 .037 0.0 3.27 675 HP-15 0.42 23.7 35.1 1.1 0.7 0.28 0.07
0.34 0.02 0.4 0.1 37.8 0.0 0.73 3.7 4.43 1251 HP-16 0.41 23.4 34.3
1.0 0.7 0.5 0.1 0.34 0.02 0.3 0.0 38.9 0.0 0.6 4.0 4.6 1293 HP-14R1
0.6 23.6 35.5 1.0 0.7 0.85 0.07 0.34 0.02 0.4 0.1 36.8 0.0 0.74 7.1
7.84 1558 Super- 0.53 25.9 34.3 0.7 1.5 0.02 4-6 0.0 14-16 0.0 0.0
Rest 0.0 0.0 9.6 9.6 487 Therm HP-40 0.4 23.5 33.2 0.7 1.75 -- --
-- -- -- -- 40.45 3.5 0 0 3.5 389
[0061] Table 3 shows the highest temperature stabilities of the
M.sub.23C.sub.6 phase and the maximum carbide contents in the three
alloys. Notably, the best properties are obtained when both
M.sub.23C.sub.6 and MC are present in the microstructure and in
certain amounts.
TABLE-US-00003 TABLE 3 Maximum Temperatures of Stability of
M.sub.23C.sub.6 and Maximum wt. % of M.sub.23C.sub.6 and MC.
Maximum wt. % Maximum wt. % Creep Life Maximum of M.sub.23C.sub.6
of MC Between (Hrs) about Temperature Between about about
600.degree. C. 1204.degree. C. of Stability of 600.degree. C. and
and about (2200 F.), about 500 Alloy M.sub.23C.sub.6 (.degree. C.)
about 1204.degree. C. 1204.degree. C. psi HP-7 1173 6.4 0.43 675
HP-15 1227 6.7 0.74 1251 HP-16 1253 6.6 0.63 1293 HP-14R1 1267 10.1
0.75 1558
[0062] The various embodiments of the present invention include all
variations on the above-identified compositions, which such
variations and analogous compositions will be apparent to those
skilled in the art. By way of example, but not limitation, the
following preferred element ranges, in combination, have been
identified as having improved creep properties: [0063] Carbon:
about 0.4 to about 0.7 wt. %; [0064] Chromium: about 20 to about 35
wt. %; [0065] Nickel: about 30 to about 45 wt. %; [0066] Manganese:
about 0.5 to about 1.5 wt. %; [0067] Silicon: about 0.6 to about
2.0 wt. %; [0068] Niobium: up to about 1.5 wt. %; [0069] Tungsten:
up to about 1.5 wt. %; [0070] Zirconium: about 0.01 to about 1.0
wt. %; (optional) [0071] Molybdenum: all compositions in the range
of up to about 1.5 wt. %; [0072] Titanium: all compositions in the
range of about 0.01 to about 1.0 wt. %; [0073] Cobalt: about 0.02
wt. %; and [0074] Iron: Rest/remainder
[0075] Table 4 shows further examples of preferred embodiments of
the alloy of the present invention within the above range, together
the calculated wt. % of carbides present at equilibrium at about
1200.degree. C.
TABLE-US-00004 TABLE 4 Further Examples of Preferred Embodiments of
Compositions and the Calculated Wt. % Carbides Present at
Equilibrium at about 1200.degree. C. Calc. Calc. Calc. Total Creep
Wt. % Wt. % Wt. % Wt. % Life Alloy C Cr Ni Mn Si Nb W Zr Mo Ti Fe
M.sub.7C.sub.3 M.sub.23C MC Carbides (Hrs) 1 0.4 24 35 1.0 0.7 0.3
0.1 0.0 0.1 0.1 Rest 2.86 0.0 0.32 3.18 714.7 2 0.4 24 35 1.0 0.7
0.3 0.1 0.0 0.2 0.1 Rest 1.64 1.99 0.32 3.95 793.6 3 0.4 24 35 1.0
0.7 0.3 0.1 0.0 0.3 0.1 Rest 0.31 4.16 0.32 4.79 872.4 4 0.4 24 35
1.0 0.7 0.3 0.1 0.0 0.4 0.1 Rest 0.0 4.69 0.32 5.01 951.3 5 0.4 24
35 1.0 0.7 0.3 0.1 0.0 0.5 0.1 Rest 0.0 4.74 0.32 5.06 1030.2 6 0.4
24 35 1.0 0.7 0.3 0.1 0.0 0.1 0.2 Rest 2.51 0.22 0.43 3.16 846.2 7
0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.2 0.2 Rest 0.96 2.73 0.43 4.12
925.1 8 0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.3 0.2 Rest 0.0 4.31 0.43
4.74 1003.9 9 0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.4 0.2 Rest 0.0 4.36
0.43 4.79 1082.8 10 0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.5 0.2 Rest 0.0
4.41 0.43 4.84 1161.7 11 0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.1 0.3 Rest
1.77 1.04 0.55 3.36 977.7 12 0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.2 0.3
Rest 0.23 3.53 0.55 4.31 1056.6 13 0.4 24 35 1.0 0.7 0.3 0.1 0.0
0.3 0.3 Rest 0.0 3.95 0.55 4.5 1135.4 14 0.4 24 35 1.0 0.7 0.3 0.1
0.0 0.4 0.3 Rest 0.0 3.99 0.55 4.54 1214.3 15 0.4 24 35 1.0 0.7 0.3
0.1 0.0 0.5 0.3 Rest 0.0 4.04 0.55 4.59 1293.2 16 0.4 24 35 1.0 0.7
0.3 0.1 0.0 0.1 0.4 Rest 0.98 1.91 0.67 3.57 1109.2 17 0.4 24 35
1.0 0.7 0.3 0.1 0.0 0.2 0.4 Rest 0.0 3.51 0.67 4.19 1188.1 18 0.4
24 35 1.0 0.7 0.3 0.1 0.0 0.3 0.4 Rest 0.0 3.56 0.67 4.23 1266.9 19
0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.4 0.4 Rest 0.0 3.61 0.67 4.28
1345.8 20 0.4 24 35 1.0 0.7 0.3 0.1 0.0 0.5 0.4 Rest 0.0 3.66 0.67
4.33 1424.7
[0076] The total calculated wt. % of the carbides in these
preferred embodiments of the alloys of the present invention is in
the range about 3.18 wt. % to about 5.06 wt. % and hence lies
between the previously identified preferred embodiments. As such,
the creep properties of these alloys are also improved over that of
related alloys.
[0077] A further preferred embodiment of the cast heat-resistant
austenitic steel alloy with improved temperature creep properties
and balanced alloyed elements additions, is comprised of about 0.4
to about 0.7 wt. % Carbon; about 20 to about 35 wt. % Chromium;
about 30 to about 45 wt. % Nickel; about 0.5 to about 1.5 wt. %
Manganese; about 0.6 to about 2.0 wt. % Silicon; up to about 1.5
wt. % Niobium; up to about 1.5 wt. % Tungsten; up to about 1.5 wt.
% Molybdenum; and the remainder being Iron. In one preferred
embodiment, the alloy further comprises about 0.01 to about 1.0 wt.
% Titanium and/or about 0.01 to about 1.0 wt. % Zirconium. In a
preferred embodiment, the alloy is also comprised of about 0.02 wt.
% Cobalt. The creep life of the alloy at about 1200.degree. C. and
about 500 psi is in excess of about 600 hours. The alloy has a
total wt. % of carbides at about 1200.degree. C. between about 3.18
and about 7.84. The M.sub.23C.sub.6 and MC carbides are present in
the microstructure of the alloy at temperatures up to about
1204.degree. C.
[0078] The alloy of the present invention has the following
composition in another preferred embodiment: about 0.41 wt. %
Carbon; about 23.6 wt. % Chromium; about 34.6 wt. % Nickel; about
1.0 wt. % Manganese; about 0.7 wt. % Silicon; about 0.05 wt. %
Molybdenum; about 0.08 wt. % Tungsten; about 0.33 wt. % Niobium;
about 0.02 wt. % Cobalt; about 0.1 wt. % Titanium; and the
remainder being Iron. This preferred composition has a maximum
temperature of stability of M.sub.23C.sub.6 carbide of about
1173.degree. C.; a maximum wt. % of M.sub.23C.sub.6 between about
600.degree. C. and about 1500.degree. C. of about 6.4; a maximum
wt. % of MC between about 600.degree. C. and about 1500.degree. C.
of about 0.43; and a creep life at about 1204.degree. C.
(2200.degree. F.) and about 500 psi of about 675 hours.
[0079] The alloy of the present invention has the following
composition in yet another preferred embodiment: about 0.42 wt. %
Carbon; about 23.7 wt. % Chromium; about 35.1 wt. % Nickel; about
1.1 wt. % Manganese; about 0.7 wt. % Silicon; about 0.28 wt. %
Molybdenum; about 0.07 wt. % Tungsten; about 0.34 wt. % Niobium;
about 0.02 wt. % Cobalt; about 0.4 wt. % Titanium; about 0.1 wt. %
Zirconium; and the remainder being Iron. This preferred composition
has a maximum temperature of stability of M.sub.23C.sub.6 carbide
of about 1227.degree. C.; a maximum wt. % of M.sub.23C.sub.6
between about 600.degree. C. and about 1500.degree. C. of about
6.7; a maximum wt. % of MC between about 600.degree. C. and about
1500.degree. C. of about 0.74; and a creep life at about
1204.degree. C. (2200.degree. F.) and about 500 psi of about 1251
hours.
[0080] The alloy of the present invention has the following
composition in yet another preferred embodiment: about 0.41 wt. %
Carbon; about 23.4 wt. % Chromium; about 34.34 wt. % Nickel; about
1.0 wt. % Manganese; about 0.7 wt. % Silicon; about 0.5 wt. %
Molybdenum; about 0.34 wt. % Niobium; about 0.1 wt. % Tungsten;
about 0.3 wt. % Titanium; and the remainder being Iron. This
preferred composition has a maximum temperature of stability of
M.sub.23C.sub.6 carbide of about 1253.degree. C.; a maximum wt. %
of M.sub.23C.sub.6 between about 600.degree. C. and about
1500.degree. C. of about 6.6; a maximum wt. % of MC between about
600.degree. C. and about 1500.degree. C. of about 0.63; and a creep
life at about 1204.degree. C. (2200.degree. F.) and about 500 psi
of about 293 hours.
[0081] The alloy of the present invention has the following
composition in another preferred embodiment: about 0.6 wt. %
Carbon; about 23.6 wt. % Chromium; about 35.5 wt. % Nickel; about
1.0 wt. % Manganese; about 0.7 wt. % Silicon; about 0.85 wt. %
Molybdenum; about 0.34 wt. % Niobium; about 0.07 wt. % Tungsten;
about 0.4 wt. % Titanium; about 0.1 wt. % Zirconium; and the
remainder being Iron. This composition has a maximum temperature of
stability of M.sub.23C.sub.6 carbide of about 1267.degree. C.; a
maximum wt. % of M.sub.23C.sub.6 between about 600.degree. C. and
about 1500.degree. C. of about 10.1; a maximum wt. % of MC between
about 600.degree. C. and about 1500.degree. C. of about 0.75; and a
creep life at about 1204.degree. C. (2200.degree. F.) and about 500
psi of about 1558 hours.
[0082] Another preferred embodiment of the cast heat-resistant
austenitic steel alloy of the present invention with improved
temperature creep properties and balanced alloyed element additions
is comprised of: about 0.4 wt. % Carbon; about 24 wt. % Chromium;
about 35 wt. % Nickel; about 1.0 wt. % Manganese; about 0.7 wt. %
Silicon; about 0.3 wt. % Niobium; about 0.08 wt. % Tungsten; less
than about 0.1 wt. % Zirconium; about 0.05 to about 0.5 wt. %
Molybdenum; about 0.1 to about 0.4 wt. % Titanium; and the rest
being Iron. This alloy has a total calculated wt. % of carbides in
the range of about 3.18 to about 5.06.
[0083] The present invention further includes a preferred method of
making the cast heat-resistant austenitic steel alloy with improved
temperature creep properties and balanced alloyed elements
additions of the present invention, which comprises the steps of:
evaluating compositions of HP and HK alloys; calculating the effect
of an addition of alloying elements on the stability of
M.sub.23C.sub.6 and MC utilizing thermodynamic models; using the HP
and HK alloys as a base reference for said calculations; and
whereby said alloying element additions are used to stabilize
strengthening phases at a required temperature as indicated by the
results of the thermodynamic calculations to provide improved
creep-resistance.
[0084] It is to be understood that the present invention is not
limited to the preferred embodiments described above, but
encompasses any and all embodiments within the scope of the
description and any claims subsequently added hereto.
* * * * *