U.S. patent application number 14/632159 was filed with the patent office on 2016-09-01 for corrosion pitting resistant martensitic stainless steel.
The applicant listed for this patent is General Electric Company. Invention is credited to Theodore Francis Majka.
Application Number | 20160251737 14/632159 |
Document ID | / |
Family ID | 55405257 |
Filed Date | 2016-09-01 |
United States Patent
Application |
20160251737 |
Kind Code |
A1 |
Majka; Theodore Francis |
September 1, 2016 |
CORROSION PITTING RESISTANT MARTENSITIC STAINLESS STEEL
Abstract
A forged, martensitic, stainless steel alloy is disclosed. The
alloy comprises, by weight: about 12.0 to about 16.0 percent
chromium; greater than 16.0 to about 20.0 percent cobalt, about 6.0
to about 8.0 percent molybdenum, about 1.0 to about 3.0 percent
nickel, about 0.020 to about 0.040 percent carbon; and the balance
iron and incidental impurities. The forged, martensitic, stainless
steel alloys are highly resistant to pitting corrosion and provide
a combination of tensile strength, ductility, and fracture
toughness suitable for use as turbine compressor airfoils.
Inventors: |
Majka; Theodore Francis;
(Simpsonville, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Family ID: |
55405257 |
Appl. No.: |
14/632159 |
Filed: |
February 26, 2015 |
Current U.S.
Class: |
148/605 |
Current CPC
Class: |
C22C 38/44 20130101;
C21D 8/005 20130101; C21D 2211/008 20130101; C21D 2211/001
20130101; C22C 38/52 20130101; C21D 6/007 20130101; C21D 2211/004
20130101; C21D 6/004 20130101 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/52 20060101 C22C038/52; C22C 38/44 20060101
C22C038/44; C21D 6/00 20060101 C21D006/00 |
Claims
1. A forged, martensitic, stainless steel alloy comprising, by
weight: about 12.0 to about 16.0 percent chromium; greater than
16.0 to about 20.0 percent cobalt, about 6.0 to about 8.0 percent
molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to
about 0.040 percent carbon; and the balance iron and incidental
impurities.
2. The alloy of claim 1, wherein the alloy comprises about 16.5 to
about 20.0 percent cobalt.
3. The alloy of claim 1, wherein the alloy has a microstructure
that contains substantially no laves phase.
4. The alloy of claim 1, wherein the alloy has a microstructure
that comprises substantially no chi phase.
5. The alloy of claim 1, wherein the alloy has a microstructure
that comprises substantially no delta ferrite phase.
6. The alloy of claim 1, wherein the alloy has a microstructure
that comprises substantially no laves phase, chi phase and delta
ferrite phase.
7. The alloy of claim 1, wherein the alloy has a microstructure
that comprises a retained austenite phase.
8. The alloy of claim 7, wherein the retained austenite phase
comprises at least about 15 percent by volume of the
microstructure.
9. The alloy of claim 8, wherein the retained austenite phase
comprises about 15 percent to about 25 percent by volume of the
microstructure.
10. The alloy of claim 1, wherein the alloy is configured to
provide a tensile elongation of at least about 14 percent.
11. The alloy of claim 10, wherein the elongation is about 14 to
about 24 percent.
12. The alloy of claim 1, wherein the alloy is configured to
provide a tensile reduction in area of at least about 41
percent.
13. The alloy of claim 12, wherein the reduction in area is about
41 to about 49 percent.
14. The alloy of claim 1, wherein the alloy has a pitting
resistance equivalence number of about 31.8 or more.
15. The alloy of claim 1, wherein the alloy has an ultimate tensile
strength of about 150 ksi or more.
16. The alloy of claim 1, wherein the alloy comprises a turbine
compressor airfoil.
17. A method of making a forged, martensitic, stainless steel
alloy, comprising: providing a forged preform of martensitic,
pitting corrosion resistant stainless steel alloy comprising, by
weight: about 12.0 to about 16.0 percent chromium; greater than
16.0 to about 20.0 percent cobalt, about 6.0 to about 8.0 percent
molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to
about 0.040 percent carbon; and the balance iron and incidental
impurities; heating the forged preform to a solutionizing
temperature for a time sufficient to form a solutionized
microstructure; cooling the forged preform and solutionized
microstructure to room temperature to form a martensitic
microstructure; heating the forged preform to a tempering
temperature of about 600.degree. F. for a tempering time sufficient
to form a tempered forged preform comprising a tempered martensitic
microstructure; and cooling the tempered forged preform to room
temperature.
18. The method of claim 17, wherein the solutionizing temperature
comprises about 2,000 to about 2,100.degree. F. and the time
comprises about 1 to about 3 hours.
19. The method of claim 17, wherein the tempering time is about 3
to about 6 hours.
20. The method of claim 17, wherein the tempered forged preform
comprises a turbine airfoil preform.
Description
BACKGROUND OF THE INVENTION
[0001] The subject matter disclosed herein generally relates to
corrosion resistant stainless steels. More particularly, it relates
to corrosion pitting resistant, martensitic, stainless steels,
including those suitable for turbine rotating components.
[0002] The metal alloys used for rotating components of a gas
turbine, particularly the front stage compressor airfoils,
including rotating and stationary blades, must have a combination
of high strength, toughness, fatigue resistance and other physical
and mechanical properties in order to provide the required
operational properties of these machines. In addition, the alloys
used must also have sufficient resistance to various forms of
corrosion and corrosion mechanisms, particularly pitting corrosion,
due to the extreme environments in which turbines are operated,
including exposure to various ionic reactant species, such as
various species that include chlorides, sulfates, nitrides and
other corrosive species. Corrosion can also diminish the other
necessary physical and mechanical properties, such as the high
cycle fatigue strength, by initiation of surface cracks that
propagate under the cyclic thermal and stresses associated with
operation of the turbine.
[0003] At present, there are no high-strength steels available that
sufficiently resist corrosion pitting to survive harsh
marine/industrial environments, such as coastal industrial power
plants, for more than 2-3 years. Even alloys that are known to have
many advantageous corrosion resistance properties, including
resistance to intergranular attack, such as 450 and 450+ stainless
steel, are still susceptible to corrosion pitting mechanisms. While
these martensitic stainless steels have provided a combination of
corrosion resistance, mechanical strength and fracture toughness
properties sufficient to make them suitable for use in rotating
steam and gas turbine components, these alloys are still known to
be susceptible to corrosion pitting phenomena. For example,
stainless steel airfoils, such as those used in the front stage
compressors of industrial gas turbines, have shown susceptibility
to corrosion pitting on the surfaces, particularly the leading edge
surface, of the airfoil. Without being limited by theory, corrosion
pitting is believed to be associated with various electrochemical
reaction processes enabled by airborne deposits, especially
corrosive species present in the deposits, and moisture from intake
air on the airfoil surfaces. Electrochemically-induced corrosion
pitting phenomena occurring at the airfoil surfaces can in turn
result in cracking of the airfoils due to the cyclic thermal and
operating stresses experienced by these components. High levels of
moisture can result from various sources, including use in high
moisture environments, such as facilities located near oceans or
other bodies of water, as well as on-line water washing, fogging,
evaporative cooling, or various combinations thereof, to enhance
compressor efficiency. Corrosive contaminants usually result from
the environments in which the turbines are operating because they
are frequently placed in highly corrosive environments, such as
those near chemical or petrochemical plants, where various chemical
species may be found in the intake air, or those at or near ocean
coastlines or other saltwater environments where various sea salts
may be present in the intake air, or combinations of the above, or
in other applications where the inlet air contains corrosive
chemical species.
[0004] In view of the above, stainless steel alloys suitable for
use as turbine airfoils, particularly industrial gas turbine
airfoils, in the operating environments described and having
improved resistance to corrosion pitting are very desirable.
BRIEF DESCRIPTION OF THE INVENTION
[0005] According to one aspect of the invention, a forged,
corrosion pitting resistant, martensitic, stainless steel alloy is
disclosed. The alloy comprises, by weight: about 12.0 to about 16.0
percent chromium; greater than 16.0 to about 20.0 percent cobalt,
about 6.0 to about 8.0 percent molybdenum, about 1.0 to about 3.0
percent nickel, about 0.020 to about 0.040 percent carbon; and the
balance iron and incidental impurities.
[0006] According to another aspect of the invention, a method of
making a forged, martensitic, pitting corrosion resistant,
stainless steel alloy is disclosed. The method includes providing a
forged preform of martensitic, pitting corrosion resistant
stainless steel alloy comprising, by weight: about 12.0 to about
16.0 percent chromium; greater than 16.0 to about 20.0 percent
cobalt, about 6.0 to about 8.0 percent molybdenum, about 1.0 to
about 3.0 percent nickel, about 0.020 to about 0.040 percent
carbon; and the balance iron and incidental impurities. The method
also includes heating the forged preform to a solutionizing
temperature for a time sufficient to form a solutionized
microstructure. The method further includes cooling the forged
preform and solutionized microstructure to room temperature to form
a martensitic microstructure. Yet further, the method includes
heating the forged preform to a tempering temperature of about
600.degree. F. for a tempering time sufficient to form a tempered
forged preform comprising a tempered martensitic microstructure.
Still further, the method includes cooling the tempered forged
preform to room temperature.
[0007] These and other advantages and features will become more
apparent from the following description taken in conjunction with
the drawings.
BRIEF DESCRIPTION OF THE DRAWING
[0008] The subject matter, which is regarded as the invention, is
particularly pointed out and distinctly claimed in the claims at
the conclusion of the specification. The foregoing and other
features, and advantages of the invention are apparent from the
following detailed description taken in conjunction with the
accompanying drawings in which:
[0009] The FIGURE is a flow chart of an embodiment of a method of
making the martensitic stainless alloys disclosed herein.
[0010] The detailed description explains embodiments of the
invention, together with advantages and features, by way of example
with reference to the drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0011] Corrosion pitting as described above is presently observed
in service on front stage compressor airfoils. The corrosion
pitting resistant, martensitic, stainless steel alloys described
herein provide an iron-based, pitting corrosion resistant material
that is a significant enhancement for many heavy marine and
industrial applications that are susceptible to corrosion pitting
phenomena as described above, including front stage turbine
compressor airfoils, in regards to service reliability, reduction
of maintenance concerns and costs, and avoidance of unplanned
downtime due to airfoil failures. The stainless steel alloys
described herein specifically have greater resistance to corrosion
pitting than GTD-450 and GTD-450+ stainless steels. Due to the
significant operational costs associated with downtime of an
industrial gas turbine, including the cost of purchased power to
replace the output of the turbine, as well as the maintenance cost
of dismantling the turbine to effect repair or replacement of the
airfoils and the repair or replacement costs of the airfoils
themselves, the enhancements in pitting corrosion resistance of the
alloys and methods of making them have significant commercial
value. An additional benefit of the corrosion pitting resistant
iron-base alloys and methods of making them is that they do not
require the addition of separate coatings for pitting corrosion
protection. The stainless steel alloys described herein are
particularly configured and well suited for forging, particularly
the forging of turbine airfoil articles
[0012] In an exemplary embodiment, a forged, martensitic, stainless
steel alloy includes, by weight: about 12.0 to about 16.0 percent
chromium; greater than 16.0 to about 20.0 percent cobalt, about 6.0
to about 8.0 percent molybdenum, about 1.0 to about 3.0 percent
nickel, about 0.020 to about 0.040 percent carbon; and the balance
iron and incidental impurities. More particularly, the forged,
martensitic, stainless steel alloy includes, by weight: about 13.5
to about 14.5 percent chromium; greater than 16.0 to about 20.0
percent cobalt, about 6.0 to about 6.5 percent molybdenum, about
1.0 to about 3.0 percent nickel, about 0.020 to about 0.30 percent
carbon; and the balance iron and incidental impurities. Even more
particularly, the forged, martensitic, stainless steel alloy
includes, by weight: about 14 percent chromium; greater than 16.0
to about 20.0 percent cobalt, about 6.0 molybdenum, about 1.0 to
about 3.0 percent nickel, about 0.025 carbon; and the balance iron
and incidental impurities. The stainless steel alloy composition is
selected and configured to provide a martensitic microstructure by
heat treatment as described herein. The stainless steel alloy
composition is selected and configured to provide a martensitic
stainless steel alloy with a minimum tensile strength of about 150
ksi, a molybdenum content of greater than 6%, and a pitting
resistance equivalent number, or PREN, of greater than about 31.8.
The stainless steel alloys disclosed herein achieve these corrosion
and strength properties by a combination of compositional chemistry
and heat treatment. For example, the stainless steel alloys
disclosed herein exhibit exceptional resistance to corrosion
pitting and may be heat treated to provide high strength and
fracture toughness suitable for application as early stage turbine
compressor airfoils (e.g. stages 1 through stage 5), including both
blades and vanes, for industrial gas turbines. In another aspect,
the stainless steel alloys described herein obtain strength
primarily from the development of a martensitic microstructure and
solid solution strengthening in conjunction with the martensitic
reaction, while also developing a predetermined amount of retained
austenite and substantially no delta ferrite, which in an
embodiment also includes no delta ferrite.
[0013] The pitting resistance equivalent number provides a
guideline for comparing the pitting corrosion resistance of
stainless steel alloys based on alloy chemistry. The higher the
PREN the more resistance to pitting corrosion, but there are
practical limits to how much the value can be increased before the
ability to successfully heat treat the alloy is compromised. The
PREN may be calculated using equation 1 below.
PREN=(% Cr)+3.3(% Mo)+16(% N) (1)
The martensitic stainless steel alloys described herein have a PREN
greater than about 31.8, and more particularly greater than about
33.3. In one embodiment, the PREN ranged from greater than about
31.8 to about 42.4, and more particularly about 33.3 to about
36.0.
[0014] The stainless steel alloys disclosed herein may be described
as iron-based alloys comprising five alloy constituents, including
Cr, Mo, Co, Ni, and C. All other elements are impurities incidental
to the manufacture of stainless steel, and may include, in weight
percent, Mn (0.25 max.), Al (0.03 max.), V (0.10 max.), Si (0.25
max.), S (0.005 max.), or P (0.02 max.), for example, and are kept
below the maximum prescribed levels described herein to ensure the
consistency of properties and microstructure from lot to lot. When
balanced within the stated ranges the disclosed stainless steel
alloys provide a martensitic microstructure with the desired
strength and fracture toughness levels along with corrosion pitting
resistance.
[0015] As noted, Cr is a required constituent and will be present
in an amount sufficient to form a passive film of chromium oxide on
the alloy surface. In one embodiment, Cr is present in an amount of
at least about 11.5 weight percent. In another embodiment, Cr is
present in an amount of about 12 to about 16 weight percent, and
more particularly about 13.5 to about 14.5 weight percent, and even
more particularly about 14 weight percent.
[0016] As indicated by equation 1, Mo has a larger effect than Cr
on the corrosion pitting resistance of stainless steel. In one
embodiment, Mo is present in an amount of about 6.0 to about 8.0
weight percent, and more particularly about 6.0 to about 6.5 weight
percent, and even more particularly about 6 weight percent. At
least about 6 weight percent is required to ensure sufficient
resistance to pitting in marine, chloride environments. Studies
have shown that Mo enhances the repassivation capability of
stainless steel. Conventional high Mo content stainless steels are
typically either ferritic grades or austenitic grades with high Ni
levels. Martensitic high Mo content stainless steels grades that
have been investigated have generally focused on exploiting the
ultra-high strength capabilities present in high-temperature
tempered materials and have been designed and heat treated at high
tempering temperatures, such as 1,100.degree. F., for use at
elevated operating temperatures. However, in these materials
corrosion resistance and toughness is sacrificed at the high
tempering temperatures due to the precipitation and formation of
Mo-rich and Cr-rich intermetallic phases, which deplete the matrix
of the corrosion resisting elements Mo and Cr. At high tempering
temperatures a secondary hardening effect also occurs due to
formation of these intermetallic compounds. The intermetallic
phases include the laves phase (Fe.sub.2Mo), Fe.sub.7Mo.sub.6,
FeMo, the sigma phase (Fe--Cr--Mo), and a complex BCC chi phase
(Fe--Cr--Mo). Cobalt does not participate in the phases associated
with these precipitation reactions. These intermetallic phases also
drastically decrease the toughness of the alloy. Thus, martensitic
stainless alloys of this invention are tempered at low tempering
temperatures as described herein to avoid the precipitation of
these intermetallic phases. The tempered alloys are suitable for
use in relatively lower temperature applications where corrosion
resistance with moderate strength and good toughness are important.
The martensitic stainless alloys of this invention balance high Mo
additions with the low-tempering temperature region of the hardness
vs tempering temperature curve to avoid the formation of
intermetallic phases and keep Mo and Cr in solution to maintain a
high level as toughness. In one embodiment, the microstructure of
the martensitic stainless steel alloys of this invention contains
substantially no laves phase, which in an embodiment also includes
no laves phase. In another embodiment, the microstructure of the
martensitic stainless steel alloys of this invention contains
substantially no chi phase, which in an embodiment also includes no
chi phase. In yet another embodiment, the microstructure of the
martensitic stainless steel alloys of this invention contains
substantially no delta ferrite phase, which in an embodiment also
includes no delta ferrite phase. In still another embodiment, the
microstructure of the martensitic stainless steel alloys of this
invention contains substantially no laves phase, chi phase and
delta ferrite phase, which in an embodiment also includes no laves
phase, chi phase and delta ferrite phase.
[0017] As will also be understood from equation 1, N has a large
effect on the PREN, and may optionally be included in the claimed
stainless steel materials. However, N is difficult to add in
significant amounts in vacuum melted materials. In addition, N can
also combine with Cr in the alloy microstructure to form chromium
nitrides, which can embrittle and sensitize the stainless steel
materials by local depletion of chromium within the alloy
microstructure, particularly at the alloy surface, where contact
with corrosive species is possible, as described herein. Thus,
where present, N will generally be present in amount of 0.02 weight
percent or less, and more particularly about 0.001 to about 0.02
weight percent.
[0018] The development of a martensitic microstructure from the
martensite transformation requires a high temperature austenitic
microstructure. Thus, the composition of the claimed stainless
steel alloys will have a high temperature microstructure that
includes austenite. Since both Cr and Mo are ferrite stabilizers,
consequently, an austenite former is required to balance the phase
diagram and develop a high temperature austenite phase to
facilitate a martensitic heat treatment and provide the martensitic
microstructure, while also developing a predetermined amount of
retained austenite and substantially no delta ferrite, which in an
embodiment also includes no delta ferrite. Co was selected to
stabilize austenite. In one embodiment, Co is present in an amount
of about 16.0 to about 20.0 weight percent, and more particularly
about 16.5 to about 20.0 weight percent, and even more particularly
about 16.5 to about 18.0 weight percent. As an austenite
stabilizer, cobalt provides a sufficiently large austenite phase
field for temperature and/or time latitude in the heat treatment
process. In addition, the effect of Co on the martensite start,
M.sub.s, temperature is not as pronounced as that of Ni, providing
for the use of standard quench and temper heat treatment
protocols.
[0019] Ni is a required constituent and will be present in an
amount sufficient to stabilize austenite. Ni is an austenite
stabilizer and increases the amount of retained austenite in these
alloys. Thus, the amount of Ni should be controlled to provide a
predetermined amount of a retained austenite phase in the alloy
microstructure. In one embodiment the predetermined amount of the
retained austenite phase comprises at least about 15 percent by
volume of the alloy microstructure. In another embodiment, the
predetermined amount of retained austenite phase comprises about 15
percent to about 25 percent by volume of the alloy microstructure.
In one embodiment, the amount of Ni comprises about 1.0 to about
3.0 weight percent, and more particularly, about 1.0 to about 2.0,
and yet more particularly about 1.0 to about 1.5 weight percent.
The predetermined amount of retained austenite improves the
fracture toughness of the claimed alloys. Ni significantly
depresses the M.sub.s temperature and the quantities disclosed
herein provide a M.sub.s temperature that is compatible with the
heat treatment temperatures and times disclosed herein to provide
the desired martensitic structure while also promoting an increased
amount of retained austenite. Ni in the amounts described herein
also increases the Charpy V-notch toughness of the martensitic
stainless steel alloys described herein.
[0020] As noted, C is a required constituent and will be present in
an amount sufficient to provide a predetermine hardness and/or a
predetermined tensile strength. The amount of C is also selected to
avoid the formation of coarse M.sub.23C.sub.6 carbides. These
carbides preferentially nucleate at grain boundaries and cause
reduced toughness. Chromium carbides also deplete the matrix
surrounding the carbide of chromium, leading to a reduction of
corrosion resistance. In one embodiment, C is present in an amount
less than about 0.05 weight percent. In another embodiment, C is
present in an amount of about 0.020 to about 0.40 weight percent,
and more particularly about 0.20 to about 0.30 weight percent, and
even more particularly about 0.025 weight percent. In one
embodiment, the predetermined hardness is about 30 to about 42 HRC,
and the predetermined ultimate tensile strength (UTS) is about 150
to about 200 ksi. The amount of C may be used together with a low
temperature tempering heat treatment, as described herein, to
provide a predetermined strength and a predetermined fracture
toughness that are sufficient for use as turbine airfoil
components, including turbine compressor vanes and blades, and more
particularly turbine compressor vanes and blades suitable for use
in the first through fifth stages of an industrial gas turbine
compressor.
[0021] Referring to the FIGURE, according to another aspect, a
method 100 of making a forged, martensitic, pitting corrosion
resistant, stainless steel alloy is disclosed. The method 100
includes providing 110 a forged preform of a martensitic, pitting
corrosion resistant stainless steel alloy comprising, by weight:
about 12.0 to about 16.0 percent chromium; greater than 16.0 to
about 20.0 percent cobalt, about 6.0 to about 8.0 percent
molybdenum, about 1.0 to about 3.0 percent nickel, about 0.020 to
about 0.040 percent carbon; and the balance iron and incidental
impurities. The stainless steel alloys can be provided in any
suitable manner, including being processed by substantially
conventional methods. For example, the alloy may be produced by
electric furnace melting with argon oxygen decarburization (AOD)
ladle refinement, followed by electro-slag remelting (ESR) of the
ingots. Other similar melting practices may also be used. A
suitable forming operation, such as various forging methods, may
then be employed to produce bar stocks and forging preforms that
have a precursor shape of the desired article, including the
various articles described herein, such as, for example, turbine
compressor airfoils.
[0022] The method 100 also includes heating 120 the forged preform
to a solutionizing temperature for a time sufficient to form a
solutionized microstructure. In one embodiment, the solutionizing
temperature comprises about 2,000 to about 2,100.degree. F. and the
solutionizing time comprises about 1 to about 3 hours.
[0023] The method further includes cooling 130 the forged preform
and solutionized microstructure to room temperature to form a
martensitic microstructure. Any suitable method of cooling may be
employed that provides a cooling rate sufficient to promote a
martensitic transformation of the alloy microstructure. In one
embodiment, cooling comprises water, polymer, oil, gas, or air
quenching.
[0024] The method also includes heating 140 the forged preform to a
tempering temperature of about 600.degree. F. or less for a
predetermined tempering time sufficient to form a tempered forged
preform comprising a tempered martensitic microstructure. Any
suitable heating method and tempering time may be employed. In one
embodiment, the predetermined tempering time is about 3 to about 6
hours. In one embodiment, the wherein the tempered forged preform
comprises a turbine compressor airfoil preform. Still further, the
method includes cooling 150 the tempered forged preform to room
temperature. Low tempering temperatures, 600.degree. F. or less,
are utilized to avoid the formation of the precipitates described
herein, particularly the embrittling chi and laves phases. It has
been shown that when more than 3.5% Mo is present in a 12% Cr steel
there is a high-temperature aging reaction based on the
precipitation of the laves phase. High Mo contents can also result
in the high temperature formation of the intermetallic chi phase
which gives rise to brittleness and low tensile ductility. The
formation of these compounds result in a dramatic loss in impact
resistance. Consequently, the focus will be on solid solution
strengthening (from both substitutional elements and interstitial
carbon) and low-temperature tempering at a temperature of
600.degree. F. or lower. The low temperature tempering also
establishes a predetermined maximum operating temperature of these
alloys that is less than the tempering temperature, preferably at
least about 50 to about 100.degree. F. lower than the tempering
temperature to avoid subsequent tempering of the martensite and
changes to the alloy microstructure. It is desirable to keep as
much Cr and Mo as possible in solution to provide corrosion
resistance and not have the elements bound in intermetallic
compounds or carbides.
[0025] In addition to resistance to pitting corrosion, the
martensitic stainless steels alloys disclosed herein have a
combination of strength, ductility, and fracture toughness that
makes them suitable for use to form various turbine airfoil and
other components. In one embodiment, the martensitic stainless
steel alloys exhibited better pitting corrosion resistance than
GTD-450 and GTD-450+ after salt fog exposure for 500 hours in
accordance with ASTM G85, and in another embodiment exhibited
substantially no pitting corrosion after 500 hours of exposure in
accordance with ASTM G85, which may also be described in an
embodiment as no pitting corrosion in conjunction with this salt
fog exposure. In another embodiment, the martensitic stainless
steels alloys disclosed herein exhibited substantially no pitting
corrosion after 1,000 hours of salt fog exposure in accordance with
ASTM B117, which may also be described in an embodiment as no
pitting corrosion in conjunction with this salt fog exposure. In
one embodiment, the martensitic stainless steels alloys have an
ultimate tensile strength of about 150 ksi or more, and more
particularly about 150 to about 200 ksi. In another embodiment, the
martensitic stainless steels alloys have an elongation of at least
about 14 percent, and more particularly an elongation of about 14
to about 24 percent. In yet another embodiment, the martensitic
stainless steels alloys have a tensile reduction in area of at
least about 41 percent, and more particularly about 41 to about 49
percent. In yet another embodiment, the martensitic stainless
steels alloys have a Charpy V-notch toughness of about 85 to about
95 J.
[0026] The alloys disclosed herein may be used to form turbine
airfoil components, including those used for compressor airfoil
components of industrial gas turbines. A typical compressor airfoil
in the form of a turbine compressor blade is well known. A
compressor blade has a leading edge, a trailing edge, a tip edge
and a blade root, such as a dovetailed root that is adapted for
detachable attachment to a compressor disk. The span of a blade
extends from the tip edge to the blade root. The surface of the
blade comprehended within the span constitutes the airfoil surface
of the turbine airfoil. The airfoil surface is that portion of the
turbine compressor airfoil that is exposed to the flow path of air
from the turbine inlet through the compressor section of the
turbine into the combustion chamber and other portions of the
turbine. While the alloys disclosed herein are particularly useful
for use in turbine compressor airfoils in the form of turbine
compressor blades and vanes, they are broadly applicable to all
manner of turbine compressor airfoils used in a wide variety of
components. These include turbine airfoils associated with turbine
compressor vanes and nozzles, shrouds, liners and other turbine
compressor airfoils, i.e., turbine components having airfoil
surfaces such as diaphragm components, seal components, valve
stems, nozzle boxes, nozzle plates, or the like. Also, while these
alloys are useful for gas turbine compressor blades and vanes, they
can potentially also be used for the turbine components of
industrial steam turbines, including compressor blades and vanes,
steam turbine buckets and other steam turbine airfoil components,
oil and gas machinery components, as well as other applications
requiring high tensile strength, fracture toughness and resistance
to pitting corrosion so long as the operating temperature range of
the components is compatible with the predetermined maximum
operating temperature of the alloys as described herein.
[0027] The terms "a" and "an" herein do not denote a limitation of
quantity, but rather denote the presence of at least one of the
referenced items. The modifier "about" used in connection with a
quantity is inclusive of the stated value and has the meaning
dictated by the context (e.g., includes the degree of error
associated with measurement of the particular quantity).
Furthermore, unless otherwise limited all ranges disclosed herein
are inclusive and combinable (e.g., ranges of "up to about 25
weight percent (wt. %), more particularly about 5 wt. % to about 20
wt. % and even more particularly about 10 wt. % to about 15 wt. %"
are inclusive of the endpoints and all intermediate values of the
ranges, e.g., "about 5 wt. % to about 25 wt. %, about 5 wt. % to
about 15 wt. %", etc.). The use of "about" in conjunction with a
listing of constituents of an alloy composition is applied to all
of the listed constituents, and in conjunction with a range to both
endpoints of the range. Finally, unless defined otherwise,
technical and scientific terms used herein have the same meaning as
is commonly understood by one of skill in the art to which this
invention belongs. The suffix "(s)" as used herein is intended to
include both the singular and the plural of the term that it
modifies, thereby including one or more of that term (e.g., the
metal(s) includes one or more metals). Reference throughout the
specification to "one embodiment", "another embodiment", "an
embodiment", and so forth, means that a particular element (e.g.,
feature, structure, and/or characteristic) described in connection
with the embodiment is included in at least one embodiment
described herein, and may or may not be present in other
embodiments.
[0028] It is to be understood that the use of "comprising" in
conjunction with the alloy compositions described herein
specifically discloses and includes the embodiments wherein the
alloy compositions "consist essentially of the named components
(i.e., contain the named components and no other components that
significantly adversely affect the basic and novel features
disclosed), and embodiments wherein the alloy compositions "consist
of the named components (i.e., contain only the named components
except for contaminants which are naturally and inevitably present
in each of the named components).
[0029] While the invention has been described in detail in
connection with only a limited number of embodiments, it should be
readily understood that the invention is not limited to such
disclosed embodiments. Rather, the invention can be modified to
incorporate any number of variations, alterations, substitutions or
equivalent arrangements not heretofore described, but which are
commensurate with the spirit and scope of the invention.
Additionally, while various embodiments of the invention have been
described, it is to be understood that aspects of the invention may
include only some of the described embodiments. Accordingly, the
invention is not to be seen as limited by the foregoing
description, but is only limited by the scope of the appended
claims.
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