U.S. patent application number 15/210107 was filed with the patent office on 2016-11-03 for high strength precipitation hardenable stainless steel.
The applicant listed for this patent is CRS HOLDINGS, INC.. Invention is credited to Michael L. Schmidt, David E. Wert.
Application Number | 20160319406 15/210107 |
Document ID | / |
Family ID | 49883242 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160319406 |
Kind Code |
A1 |
Wert; David E. ; et
al. |
November 3, 2016 |
High Strength Precipitation Hardenable Stainless Steel
Abstract
A precipitation hardenable, martensitic stainless steel alloy is
disclosed. The alloy has the following composition in weight
percent, about TABLE-US-00001 C 0.03 max Mn 1.0 max Si 0.75 max P
0.040 max S 0.020 max Cr 10-13 Ni 10.5-11.6 Mo 0.25-1.5 Co 0.5-1.5
Cu 0.75 max Ti 1.5-1.8 Al 0.3-0.8 Nb 0.3-0.8 B 0.010 max N 0.030
max The balance is iron and usual impurities. The disclosed alloy
provides a unique combination of corrosion resistance, strength,
and toughness.
Inventors: |
Wert; David E.; (Wyomissing,
PA) ; Schmidt; Michael L.; (Sinking Spring,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CRS HOLDINGS, INC. |
Wilmington |
DE |
US |
|
|
Family ID: |
49883242 |
Appl. No.: |
15/210107 |
Filed: |
July 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13706800 |
Dec 6, 2012 |
|
|
|
15210107 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
C21D 6/04 20130101; C22C 38/44 20130101; C21D 9/0068 20130101; C22C
38/52 20130101; C22C 38/004 20130101; C22C 38/54 20130101; C22C
38/001 20130101; C22C 38/002 20130101; C22C 38/50 20130101; C22C
38/04 20130101; C22C 38/06 20130101; C21D 2211/008 20130101; C21D
1/06 20130101; C21D 6/02 20130101; C21D 6/004 20130101; C22C 38/48
20130101 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C22C 38/52 20060101 C22C038/52; C22C 38/50 20060101
C22C038/50; C22C 38/00 20060101 C22C038/00; C22C 38/44 20060101
C22C038/44; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 1/06 20060101
C21D001/06; C22C 38/48 20060101 C22C038/48 |
Claims
1. A precipitation hardenable, martensitic stainless steel alloy
consisting essentially of, in weight percent, TABLE-US-00010 Mn up
to about 1.0 Si up to about 0.75 Cr about 10 to about 13 Ni about
10.75 to about 11.6 Mo about 0.25 to about 1.5 Cu not more than
about 0.25 Co about 0.9 to about1.5 Ti about 1.5 to about 1.8 Al
about 0.4 to about 0.8 Cb about 0.3 to about 0.6 B up to about
0.010
and the balance is iron and usual impurities, said impurities
including about 0.040% max. phosphorus, about 0.005% max. sulfur,
about 0.03% max. carbon, and about 0.030% max. nitrogen.
2. The alloy recited in claim 1 which contains at least about 0.4%
columbium.
3. The alloy recited in claim 1 which contains at least about 10.5%
chromium.
4. The alloy recited in claim 1 which contains not more than about
12.5% chromium.
5. The alloy recited in claim 1 which contains not more than about
1.7% titanium.
6. The alloy recited in claim 1 which contains not more than about
1.25% molybdenum.
7. The alloy recited in claim 1 which contains at least about 0.75
weight percent molybdenum.
8. The alloy recited in claim 1 which further contains up to about
0.005% calcium.
9. The alloy recited in claim 1 which further contains up to about
0.025% cerium.
10. A precipitation hardenable, martensitic stainless steel alloy
consisting essentially of, in weight percent, TABLE-US-00011 Mn up
to about 0.25 Si up to about 0.25 Cr about 10.5 to about 12.5 Ni
about 10.75 to about 11.25 Mo about 0.75 to about 1.25 Cu not more
than about 0.25 Co about 0.9 to about 1.25 Ti about 1.5 to about
1.7 Al about 0.45 to about 0.8 Cb about 0.4 to about 0.6 B about
0.001 to about 0.005
and the balance is iron and usual impurities, said impurities
including about 0.015% max. phosphorus, about 0.005% max. sulfur,
about 0.02% max. carbon, and about 0.015% max. nitrogen.
11. The alloy recited in claim 10 which contains not more than
about 1.1% cobalt.
12. The alloy recited in claim 10 which contains at least about
10.85% nickel.
13. The alloy recited in claim 10 which contains not more than
about 0.6% columbium.
14. The alloy recited in claim 10 which contains at least about
0.45% aluminum.
15. The alloy recited in claim 10 which contains not more than
about 0.65% aluminum.
16. The alloy recited in claim 10 which contains at least about
0.9% molybdenum.
17. The alloy recited in claim 10 which further contains up to
about 0.005% calcium.
18. The alloy recited in claim 10 which further contains up to
about 0.025% cerium.
19. A precipitation hardenable, martensitic stainless steel alloy
consisting essentially of, in weight percent, TABLE-US-00012 Mn up
to about 0.10 Si up to about 0.10 Cr about 11.0 to about 12.0 Ni
about 10.85 to about 11.25 Mo about 0.9 to about 1.1 Co about 0.9
to about 1.1 Cu not more than about 0.25 Ti about 1.5 to about 1.7
Al about 0.45 to about 0.65 Cb about 0.4 to about 0.6 B about
0.0015 to about 0.0035 Ca up to about 0.005 Ce up to about
0.025
and the balance is iron and usual impurities, said impurities
including about 0.010% max. phosphorus, about 0.005% max. sulfur,
about 0.015% max. carbon, and about 0.010% max. nitrogen.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 13/706,800, filed on Dec. 6, 2012, the entirety of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to precipitation hardenable,
martensitic stainless steel alloys and in particular to a
martensitic stainless steel alloy and an article made therefrom,
having a novel combination of strength and corrosion
resistance.
[0004] 2. Description of the Related Art
[0005] The aerospace industry has been looking for a stainless
steel alloy for landing gear for many years. The main alloy
currently used for the commercial landing gear application is 300M
alloy. 300M alloy can be quenched and tempered to provide an
ultimate tensile strength of at least 280 ksi and fracture
toughness (K.sub.Ic) of at least 50 ksi in. However, 300M alloy
does not provide effective corrosion resistance. Therefore, it has
been necessary to plate the landing gear components with a
corrosion resistant metal such as cadmium. Cadmium is a highly
toxic, carcinogenic material and its use has presented significant
environmental risks in the manufacture and maintenance of aircraft
landing gear and other components made from 300M alloy.
[0006] Precipitation hardenable stainless steel alloys having
commercially acceptable combinations of strength and toughness are
known and used for various aerospace applications. However, some of
those alloys do not provide strength equivalent to 300M, so they
cannot be considered as "drop-in" replacements for that alloy. The
other known precipitation hardenable stainless steels may provide
adequate strength for the landing gear application, but leave
something to be desired in the resistance to corrosion they
provide. The corrosion resistance desired for the aircraft landing
gear application includes general corrosion resistance, pitting
corrosion resistance, and resistance to stress corrosion
cracking.
[0007] In view of the foregoing discussion, there is a need for a
steel alloy with mechanical properties comparable to those of 300M,
so the new alloy can be used as a drop-in replacement, combined
with effective corrosion resistance in the variety of environments
in which commercial aircraft are used.
SUMMARY OF THE INVENTION
[0008] The disadvantages associated with the known precipitation
hardenable, martensitic stainless steel alloys are solved to a
large degree by the alloy in accordance with the present invention.
The alloy according to the present invention is a precipitation
hardening Cr--Ni--Ti--Mo martensitic stainless steel alloy that
provides a unique combination of strength, toughness, and corrosion
resistance.
[0009] The broad, intermediate, and preferred compositional ranges
of the alloy according to the present invention are set forth below
in weight percent.
TABLE-US-00002 Broad Intermediate Preferred C 0.03 max 0.02 max
0.015 max Mn 1.0 max 0.25 max 0.10 max Si 0.75 max 0.25 max 0.10
max P 0.040 max 0.015 max 0.010 max S 0.020 max 0.010 max 0.005 max
Cr 10-13 10.5-12.5 11.0-12.0 Ni 10.5-11.6 10.75-11.25 10.85-11.25
Mo 0.25-1.5 0.75-1.25 0.9-1.1 Cu 0.75 max 0.50 max 0.25 max Co
0.5-1.5 0.75-1.25 0.9-1.1 Ti 1.5-1.8 1.5-1.7 1.5-1.7 Al 0.3-0.8
0.4-0.7 0.45-0.65 Cb 0.3-0.8 0.4-0.7 0.4-0.6 B 0.010 max
0.001-0.005 0.0015-0.0035 N 0.030 max 0.015 max 0.010 max
The balance of the alloy is essentially iron except for the usual
impurities found in commercial grades of such steels and minor
amounts of additional elements which may vary from a few
thousandths of a percent up to larger amounts that do not adversely
affect the desired combination of properties provided by this
alloy.
[0010] The foregoing tabulation is provided as a convenient summary
and is not intended thereby to restrict the lower and upper values
of the ranges of the individual elements of the alloy of this
invention for use in combination with each other, or to restrict
the ranges of the elements for use solely in combination with each
other. Thus, one or more of the element ranges of the broad
composition can be used with one or more of the other ranges for
the remaining elements in the preferred composition. In addition, a
minimum or maximum for an element of one preferred embodiment can
be used with the maximum or minimum for that element from another
preferred embodiment. Moreover, the alloy according to this
invention may comprise, consist essentially of, or consist of the
constituent elements described above and throughout this
specification. Here and throughout this application, unless
otherwise indicated, the term percent or the symbol "%" means
percent by weight or mass percent.
DETAILED DESCRIPTION
[0011] The alloy according to the present invention provides a
unique combination of strength, toughness, and corrosion resistance
which results from a novel balancing of the elements chromium,
nickel, cobalt, molybdenum and also the elements titanium,
aluminum, and columbium. At least about 10%, better yet at least
about 10.5%, and preferably at least about 11.0% chromium is
present in the alloy to provide corrosion resistance similar to
that of a conventional stainless steel. At least about 10.5%,
better yet at least about 10.75%, and preferably at least about
10.85% nickel is present in the alloy because nickel benefits the
toughness and notch toughness of the alloy. Nickel also contributes
to the corrosion resistance by enhancing the ability of the alloy
to repassivate. This alloy contains at least about 0.5%, better yet
at least about 0.75%, and preferably at least about 0.9% cobalt
because cobalt contributes to the high strength and corrosion
resistance provided by the alloy. At least about 0.25%, better yet
at least about 0.75%, and preferably at least about 0.9% molybdenum
is also present in the alloy because molybdenum contributes to the
alloy's notch toughness. Molybdenum also benefits the alloy's
corrosion resistance in reducing media and in environments which
promote pitting attack and stress-corrosion cracking.
[0012] The alloy of this invention also contains at least about
1.5% titanium to benefit the strength of the alloy through the
precipitation of a nickel-titanium-rich phase during aging.
Columbium and aluminum also contribute to the strength provided by
this alloy. Therefore, the alloy contains at least about 0.3% and
better yet at least about 0.4% of each of columbium and aluminum.
Preferably the alloy contains at least about 0.45% aluminum.
[0013] When chromium, nickel, cobalt, molybdenum, titanium,
columbium, and aluminum are not properly balanced, the alloy's
ability to transform fully to a martensitic structure using
conventional processing techniques is inhibited. Furthermore, the
alloy's ability to remain substantially fully martensitic when
solution treated and age-hardened is impaired. Under such
conditions the strength provided by the alloy is significantly
reduced. Therefore, the amounts of chromium, nickel, cobalt,
molybdenum, titanium, columbium, and aluminum present in this alloy
are restricted. More particularly, chromium is limited to not more
than about 13%, better yet to not more than about 12.5%, and
preferably to not more than about 12.0%. Nickel is limited to not
more than about 11.6% and preferably to not more than about 11.25%.
Too much cobalt adversely affects the strength and toughness
provided by this alloy. Therefore, cobalt is restricted to not more
than about 1.5%, better yet to not more than about 1.25%, and
preferably to not more than about 1.1%. Molybdenum is restricted to
not more than about 1.5%, better yet to not more than about 1.25%,
and preferably to not more than about 1.1%.
[0014] Too much titanium adversely affects the toughness and notch
toughness of the alloy. Therefore, titanium is restricted to not
more than about 1.8% and preferably to not more than about 1.7% in
this alloy. Too much aluminum can adversely affect the toughness
and corrosion resistance provided by the alloy. Therefore, aluminum
is restricted to not more than about 0.8%, better yet to not more
than about 0.7%, and preferably to not more than about 0.65%. Too
much columbium is likely to result in undesirable alloy segregation
and the precipitation of unwanted secondary phases such as Laves
phase. Therefore, columbium is restricted to not more than about
0.8%, better yet to not more than about 0.7%, and preferably to not
more than about 0.6% in this alloy.
[0015] Additional elements such as manganese, silicon, and boron
may be present in controlled amounts to benefit other desirable
properties provided by this alloy. More specifically, up to about
1.0%, better yet up to about 0.5%, still better up to about 0.25%,
and preferably up to about 0.10% manganese and/or up to about
0.75%, better yet up to about 0.5%, still better up to about 0.25%,
and preferably up to about 0.10% silicon can be present in the
alloy as residuals from scrap sources or deoxidizing additions.
Such additions are beneficial when the alloy is not vacuum melted.
Manganese and/or silicon are preferably kept at low levels because
of their adverse effect on toughness, corrosion resistance, and the
austenite-martensite phase balance in the matrix material.
[0016] Up to about 0.010% boron, better yet up to about 0.005%
boron, and preferably up to about 0.0035% boron can be present in
the alloy to benefit the hot workability of the alloy. In order to
provide the desired effect, at least about 0.001% and preferably at
least about 0.0015% boron is present in the alloy.
[0017] The balance of the alloy is essentially iron apart from the
usual impurities inevitably found in commercial grades of alloys
intended for similar service or use. The levels of such elements
are controlled so as not to adversely affect the desired
properties.
[0018] In particular, too much carbon and/or nitrogen impair the
corrosion resistance and adversely affect the toughness provided by
this alloy. Accordingly, not more than about 0.03%, better yet not
more than about 0.02%, and preferably not more than about 0.015%
carbon is present in the alloy. Also, not more than about 0.030%,
better yet not more than about 0.015%, not more than about 0.010%
nitrogen is present in the alloy. When carbon and/or nitrogen are
present in larger amounts, the carbon and/or nitrogen bond with
titanium, aluminum, and/or columbium to form undesirable
non-metallic inclusions such as carbides or nitrides and/or
carbonitrides. Those reactions inhibit the formation of the
nickel-titanium/aluminum/columbium intermetallic phases which are a
primary factor in the development of the high strength provided by
this alloy.
[0019] Phosphorus is maintained at a low level because of its
adverse effect on toughness and corrosion resistance. Accordingly,
not more than about 0.040%, better yet not more than about 0.015%,
and preferably not more than about 0.010% phosphorus is present in
the alloy.
[0020] Not more than about 0.020%, better yet not more than about
0.010%, and preferably not more than about 0.005% sulfur is present
in the alloy. Larger amounts of sulfur promote the formation of
non-metallic sulfide inclusions which, like carbon and nitrogen,
inhibit the desired strengthening effect provided by titanium,
aluminum, and columbium. These sulfide inclusions impair the
toughness of the alloy, especially in the transverse direction.
Also, a greater amount of sulfur adversely affects the hot
workability and corrosion resistance of this alloy.
[0021] Although sulfur and phosphorus can be reduced to very low
levels through the selection of high purity charge materials or by
employing alloy refining techniques, their presence in the alloy
cannot be entirely avoided under large scale production conditions.
Therefore, a small amount of calcium may be added in controlled
amounts to combine with phosphorus and/or sulfur to facilitate the
removal and stabilization of those two elements in the alloy.
Calcium is also used to deoxidize the alloy. When used, the
retained amount of calcium is not more than about 0.010% and
preferably to not more than about 0.005% in this alloy. As an
alternative to the calcium treatment, one or more rare earth metals
(REM), particularly cerium and lanthanum, can be added to the
alloy. In this regard, the alloy may contain at least about 0.001%
REM and better yet, at least about 0.002% REM. Too much REM
recovery adversely affects the hot workability and the toughness of
this alloy. Excessive REM content also results in the formation of
undesirable oxide inclusions in the alloy. Therefore, the amount of
REM present in this alloy is limited to not more than about 0.025%,
better yet to not more than about 0.015%, and preferably to not
more than about 0.010%, in this alloy. It is further contemplated
that magnesium can be added as an alternative to calcium or REM for
desulfurization and deoxidation.
[0022] Too much copper adversely affects the notch toughness,
ductility, and strength of this alloy. Therefore, the alloy
contains not more than about 0.75%, better yet not more than about
0.50%, and preferably not more than about 0.25% copper.
[0023] No special techniques are required for melting, casting, or
working the alloy of the present invention. Vacuum induction
melting (VIM) and vacuum induction melting followed by vacuum arc
remelting (VAR) are the preferred methods of melting and refining
this alloy, but other practices can be used. In addition, this
alloy can be made using powder metallurgy techniques, if desired.
Further, although the alloy of the present invention can be hot or
cold worked, cold working enhances the mechanical strength of the
alloy.
[0024] The preferred method of providing calcium in this alloy is
through the addition of a nickel-calcium compound during VIM. The
nickel-calcium compound, such as the Ni-Cal.RTM. alloy sold by
Chemalloy Co. Inc., is added in an amount effective to combine with
available phosphorus, sulfur, and oxygen. Other techniques for
adding calcium may also be used. For example, capsules of elemental
calcium or calcium master alloys can be added to the melt. It is
believed that a slag containing calcium or a calcium compound may
also be used. The chemical reactions result in the formation of
secondary phase inclusions such as calcium sulfides, calcium
oxides, and calcium oxysulfides that are readily removed during
primary or secondary melting. When used, REM are added to the
molten alloy in the form of mischmetal which is a mixture of rare
earth elements, an example of which contains about 50% cerium,
about 30% lanthanum, about 15% neodymium, and about 5%
praseodymium.
[0025] The precipitation hardenable alloy of the present invention
is processed in multiple steps to develop the desired combination
of properties. In a first step, the alloy is solution annealed. The
solution annealing temperature is selected to be high enough to
dissolve essentially all of the undesired precipitates into the
alloy matrix material and to ensure that the grain structure is
fully recrystallized. Unrecrystallized grains can lead to increased
anisotropy of the mechanical properties, particularly the ductility
and toughness, of the alloy. However, if the solution annealing
temperature is too high, it will impair the fracture toughness of
the alloy by promoting excessive grain growth. Preferably, the
alloy of the present invention is solution annealed at
1850EF-1950EF (1010EC-1066EC) for a time sufficient to
substantially completely dissolve any precipitates in the alloy
matrix and to fully recrystallize the grain structure. The time at
the solution temperature depends on the thickness of the part. The
alloy is then quenched, preferably in oil.
[0026] To further develop the high strength of the alloy, it is
subjected to a refrigeration treatment after it is quenched. The
refrigeration treatment cools the alloy to a temperature
sufficiently below the martensite finish temperature to ensure the
completion of the martensite transformation. Preferably, the
refrigeration treatment comprises cooling the alloy to about -100EF
(-73EC) or lower for a time sufficient to ensure that the alloy has
substantially completely transformed to martensite. The need for a
refrigeration treatment will be affected, at least in part, by the
martensite finish temperature of the alloy. If the martensite
finish temperature is sufficiently high, the transformation to a
martensitic structure can proceed without the need for a
refrigeration treatment. In addition, the need for a refrigeration
treatment may also depend on the section size of the piece being
manufactured. As the section size of the piece increases,
segregation in the alloy becomes more significant and the use of a
refrigeration treatment becomes more beneficial. Further, the
length of time that the piece is chilled may need to be increased
for large pieces in order to complete the transformation to
martensite. For example, it has been found that a refrigeration
treatment lasting a minimum of about 8 hours is preferred for
developing the high strength that is characteristic of this
alloy.
[0027] The alloy of the present invention is age hardened in
accordance with techniques used for the known precipitation
hardening, stainless steel alloys, which treatments are known to
those skilled in the art. For example, the alloys are preferably
aged at about 950-975EF (510-524EC) for a time sufficient to ensure
that the alloy is substantially uniformly heated to the aging
temperature depending on the thickness of the part and typically
for an additional 4 to 8 hours to complete the aging reaction and
to reach the desired combination of strength and toughness. The
specific aging temperature used is selected by considering that:
(1) the ultimate tensile strength of the alloy decreases as the
aging temperature increases; and (2) the time required to age
harden the alloy to a desired strength level increases as the aging
temperature decreases.
[0028] The alloy of the present invention can be formed into a
variety of product shapes for a wide variety of uses and lends
itself to the formation of billets, bars, rod, wire, strip, plate,
or sheet using conventional practices. The alloy of the present
invention is useful in a wide range of practical applications which
require an alloy having a good combination of corrosion resistance,
strength, and toughness. In particular, the alloy of the present
invention can be used to produce structural members for aircraft,
including but not limited to landing gear components and fasteners.
The alloy is also well suited for use in medical and dental
applications such as dental tools and medical scrapers, cutters,
and suture needles.
WORKING EXAMPLES
[0029] In order to demonstrate the novel combination of strength,
toughness, and corrosion resistance provided by the alloy according
to this invention, a comparative testing program was carried out.
Seven 35 lb. heats having the weight percent compositions set forth
in Table I below were produced by VIM.
TABLE-US-00003 TABLE I Elmt. Ex. 1 Ex. 2 Ex. A Ex. B. Ex. C Ex. D.
Ex. E C 0.002 0.003 0.005 0.002 0.002 0.002 0.003 Mn <0.01
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Si <0.01
<0.01 0.04 <0.01 <0.01 <0.01 <0.01 P <0.005
<0.005 0.005 <0.005 <0.005 <0.005 <0.005 S 0.0006
0.0005 0.0009 0.0005 0.0005 0.0005 0.0005 Cr 11.42 11.49 11.35
11.48 11.47 11.47 11.47 Ni 11.04 10.66 10.86 11.01 11.04 10.41
10.00 Mo 0.95 0.95 0.94 0.95 0.95 0.95 0.95 Co 0.98 0.98 (Note)
1.96 2.94 1.96 2.94 Ti 1.63 1.62 1.54 1.63 1.65 1.63 1.63 Al 0.55
0.57 0.07 0.53 0.50 0.54 0.54 Cb 0.50 0.51 <0.01 0.51 0.54 0.50
0.51 B 0.0021 0.0019 0.0023 0.0019 0.0021 0.0021 0.0022 N 0.0015
0.0014 0.0013 0.0011 0.0014 0.0015 0.0015 Ca 0.0019 0.0021 0.0006
0.0021 0.0019 0.0019 0.0021 Note: No positive addition.
The balance of each heat is iron and usual impurities. Examples 1
and 2 are representative of the alloy according to the present
invention. Examples A to E are comparative alloys. In particular,
Example A is within the scope of the alloy described in U.S. Pat.
No. 5,681,526.
[0030] The VIM heats were melted and cast into 4'' square ingots.
The ingots were charged into a furnace operating at 1500.degree. F.
and the furnace temperature was ramped up to 2300.degree. F. Ingots
were held at 2300.degree. F. for 16 hours after which the furnace
temperature was lowered to 2000.degree. F. The ingots were held at
2000.degree. F. until they were substantially fully equalized in
temperature. The ingots were then double-end forged to 23/4''
square billet from starting temperature of 2000.degree. F. and then
hot cut into 3 pieces each. Pieces were re-heated at 2000.degree.
F., and double-end forged to 11/4'' square. The bars were again hot
cut into 3 pieces and reheated at 2000.degree. F. The bars were
then single-end forged to 11/16'' square with no reheats. The bars
were cooled in air, overage annealed at 1250.degree. F. for 8
hours, and then air cooled.
[0031] Longitudinal smooth and notched (K.sub.t=3) tensile samples,
longitudinal Charpy V-notch (CVN) samples, and longitudinal rising
step load (RSL) fracture toughness samples were machined from the
bars of each heat. The samples from Examples 1, 2, B, C, D, and E
were solution treated at 1900.degree. F. for 1 hour and oil
quenched. The samples from Example A were solution treated at
1800.degree. F. in accordance with the usual practice for that
alloy. After solution treatment, all samples were refrigerated at
-100.degree. F. for 24 hours then warmed in air to room
temperature. The samples were then age-hardened at various
temperatures ranging from 900.degree. F. to 1000.degree. F. Aging
was conducted by holding the samples at temperature for 4 hours in
air and then quenching the samples in water.
[0032] The results of room temperature tensile testing on the
samples of each heat are shown in Tables IIA and IIB below
including the 0.2% offset yield strength (Y.S.) and the ultimate
tensile strength (U.T.S) in ksi, the percent elongation (% El.),
the percent reduction in area (% R.A.), and the notch tensile
strength (N.T.S.) in ksi.
TABLE-US-00004 TABLE IIA Heat Solution Age Y.S. U.T.S. % El. % R.A.
N.T.S. Ex. 1 1900.degree. F. 900.degree. F. -- -- -- -- 257 280 7.1
29.5 257 283 6.6 28.8 925.degree. F. 255 280 9.1 36.6 263 286 8.0
31.8 263 286 8.2 35.1 950.degree. F. 268 286 9.8 45.8 282 261 284
10.0 44.0 320 258 283 8.9 40.9 282 975.degree. F. 260 280 10.1 43.8
263 280 10.8 49.8 258 280 9.7 47.0 Ex. 2 1900.degree. F.
900.degree. F. --* --* --* --* 259 285 --* --* 252 284 --* --*
925.degree. F. 270 292 7.6 34.4 271 294 7.6 35.4 267 289 9.0 41.0
950.degree. F. 272 292 8.9 37.8 274 290 11.0 47.0 262 283 9.5 46.6
975.degree. F. 264 283 10.2 46.6 227 259 279 11.5 50.3 239 267 285
10.6 47.6 233 Ex. A 1800.degree. F. 925.degree. F. 250 265 11.3
56.7 -- 248 262 11.4 58.2 -- 250 265 12.5 58.9 -- 950.degree. F.
245 258 10.9 56.1 384 247 261 13.5 60.4 396 247 261 11.6 55.8 402
975.degree. F. 237 249 12.6 63.4 -- 230 241 11.7 55.3 -- 231 241
11.9 60.7 -- Ex. B 1900.degree. F. 900.degree. F. 241 273 7.6 24.6
248 274 7.7 29.9 246 274 7.8 30.7 925.degree. F. 251 275 9.2 38.8
254 277 10.4 39.5 247 273 9.2 41.1 950.degree. F. 252 277 9.6 42.6
259 281 8.5 35.2 244 277 9.1 39.6 975.degree. F. 241 270 9.0 42.7
244 266 11.3 53.3 249 272 10.8 50.4 *Samples fractured in a manner
such that valid results could not be obtained.
TABLE-US-00005 TABLE IIB Heat Solution Age Y.S. U.T.S. % El. % R.A.
N.T.S. Ex. C 1900.degree. F. 900.degree. F. 241 272 8.4 30.2 237
272 8.0 30.4 243 272 8.1 29.6 925.degree. F. 244 273 9.5 36.8 239
274 9.8 37.4 244 276 8.4 36.2 950.degree. F. 253 275 10.4 43.3 250
274 9.9 38.7 247 271 --* 39.3 975.degree. F. 243 264 11.9 52.6 243
267 10.8 50.9 241 264 11.3 49.8 Ex. D 1900.degree. F. 925.degree.
F. 269 275 --* --* 274 293 6.0 27.5 --* --* --* --* 950.degree. F.
264 291 9.9 43.7 260 291 9.9 36.4 268 295 9.4 42.3 975.degree. F.
263 281 9.3 48.5 271 276 289 8.9 47.4 283 273 290 9.4 44.2 240
1000.degree. F. 251 269 11.6 59.0 252 270 11.5 54.6 250 275 11.4
54.5 Ex. E 1900.degree. F. 925.degree. F. 270 295 3.6 9.4 274 295
4.5 10.6 271 293 8.7 34.6 950.degree. F. 276 296 8.0 42.1 270 290
8.7 40.8 280 295 7.4 34.4 975.degree. F. --* --* --* --* 268 291
8.5 43.5 269 287 8.7 43.5 1000.degree. F. 257 272 10.6 49.9 263 277
10.4 49.3 259 278 9.1 45.1
[0033] The results of Charpy V-notch (CVN) impact testing of
Examples 1, 2, and D are shown in Table III below including the
aging temperature, the Rockwell C-scale hardness (HRC), and the
impact toughness (CVN) in foot-pounds. CVN testing was performed in
accordance with ASTM Standard Test Procedure E23.
TABLE-US-00006 TABLE III Example Age HRC CVN Avg. Ex. 1 950.degree.
F. 54.0 4.4, 4.3, 3.8 4 Ex. 2 975.degree. F. 53.5 4.3, 4.4, 4.0 4
Ex. D 975.degree. F. 54.0 4.1, 4.6, 3.7 4
[0034] Rising Step Load (RSL) samples for plane-strain fracture
toughness testing and stress corrosion cracking resistance (SCC)
were finish machined from the age-hardened bars of Examples 1, 2,
A, and D. Two samples from each heat were tested in air to provide
a fracture toughness value (K.sub.Ic). Additional samples were
tested in 3.5% NaCl solution, natural pH, at room temperature, to
provide a threshold stress intensity value (K.sub.ISCC). Testing
was performed on a test machine that meets the requirements of ASTM
Standard Test Procedure E1290. The results of room temperature
fracture toughness testing (K.sub.Ic) and stress corrosion cracking
testing for Examples 1, 2, A, and D are presented in Table IV below
including the plane-strain fracture toughness (K.sub.Ic) in ksi in
and the threshold stress intensity to produce stress corrosion
cracking (K.sub.ISCC) in ksi in. K.sub.ISCC is reported for each
step interval and as a final value. The lowest of the measured
values for each example is designated as the final value of
K.sub.ISCC in accordance with the standard test procedure. The
tensile strength values for each example are also reported in Table
IV to show that the fracture toughness and stress corrosion
cracking resistance were measured on alloys having similar levels
of strength.
TABLE-US-00007 TABLE IV Avg. K.sub.ISCC Final Example Solution Age
U.T.S. 1 hour steps 2 hour steps 4 hour steps K.sub.ISCC K.sub.IC
Ex. 1 1900.degree. F. 950.degree. F. 284 26.3 26.0 28.8 26 47.3,
46.0 Ex. 2 1900.degree. F. 975.degree. F. 282 29.0 22.0 34.8 22
45.5, 49.0 Ex. A 1800.degree. F. 950.degree. F. 260 71.6 32.3 36.0
32 90.5 Ex. D 1900.degree. F. 975.degree. F. 287 31.4 23.6 27.3 24
43.5, 42.1
[0035] Duplicate salt spray corrosion test cones were finish
machined from the bars of Examples 1, 2, A, D after age-hardening.
The cone samples were prepared by turning and hand polishing to a
600 grit finish. Prior to testing, all salt spray cones were
passivated using 20% Nitric acid+3 oz./gallon Sodium Dichromate at
120/140.degree. F. for 30 minutes. Samples were tested in
accordance with ASTM B 117, using a 5% NaCl concentration, natural
pH, at 95.degree. F. for 200 hour test duration. Time to first rust
was noted for all samples, as well as a final rating after the
completion of 200 hours test duration. The results of the
salt-spray testing are shown in Table V below including the time to
first appearance of rust and a final rating after the completion of
the test duration. The ratings are defined as follows: 1=no rust,
2=1-3 rust spots, 3=<5% rust, 4=5-10%, 5=10-20%, 6=20-40%,
7=40-60%, 8=60-80%, 9=>80%.
TABLE-US-00008 TABLE V Example Solution Age First Rust Final Rating
Ex. 1 1900.degree. F. 950.degree. F. None, None 1, 1 Ex. 2
1900.degree. F. 975.degree. F. None, None 1, 1 Ex. A 1800.degree.
F. 950.degree. F. None, None 1, 1 Ex. D 1900.degree. F. 975.degree.
F. None, None 1, 1
[0036] Cyclic polarization (pitting) test samples were finish
machined from the aged bars of Examples 1, 2, A, and D. Scans to
measure pitting resistance were run on duplicate samples from each
of those examples. The samples were tested in 3.5% NaCl solution,
natural pH, at room temperature and were cleaned but not passivated
prior to testing. Testing was performed with a modified ASTM
Standard Test procedure G61 as described below. Voltage values at
the knee of the curve and protection potentials were measured for
all samples. The results of the potentiodynamic pitting tests are
shown in Table VI below including the pitting potential and the
protection potential in millivolts (mV).
TABLE-US-00009 TABLE VI Example Solution Age mV @ knee Protection
Potential Ex. 1 1900.degree. F. 950.degree. F. 62.7, 66.7 11.1,
34.9 Ex. 2 1900.degree. F. 975.degree. F. 76.2, 126.2 -12.7, -60.3
Ex. A 1800.degree. F. 950.degree. F. 76.2, 118.0 19.5, -8.7 Ex. D
1900.degree. F. 975.degree. F. 110.0, 126.2 -52.4, none.sup.
[0037] A steel article made from the alloy described above and
processed in accordance with the foregoing processing steps
provides a combination of properties that make it particularly
useful for aircraft landing gear and other aircraft structural
components, including but not limited to flap tracks and slat
tracks, and for other applications where both high strength and
corrosion resistance are required. In particular, a steel article
fabricated from the alloy that is solution heat treated and age
hardened as described above provides a tensile strength of at least
280 ksi and a fracture toughness (K.sub.Ic) of at least 45 ksi in
when tested with a test machine that meets the requirements of ASTM
Standard Test Procedure E1290. A steel article in accordance with
this invention is also characterized by a Charpy V-notch impact
energy of at least about 4 ft-lbs when tested in accordance with
ASTM Standard Test Procedure E23. Further, a steel article in
accordance with this invention is characterized by general
corrosion resistance such that the article does not rust when
tested in accordance with ASTM Standard Test procedure B 117 and by
sufficient pitting corrosion resistance such that the article has a
pitting potential of at least 62 mV when tested in accordance with
a modified ASTM Standard Test procedure G61. The ASTM G61 test
procedure was modified by using round bar rather than flat samples.
The use of round bar samples exposes the end grains and can be
considered to be a more severe test than the standard G61
procedure.
[0038] The terms and expressions which are employed in this
specification are used as terms of description and not of
limitation. There is no intention in the use of such terms and
expressions of excluding any equivalents of the features shown and
described or portions thereof. It is recognized that various
modifications are possible within the invention described and
claimed herein.
* * * * *