U.S. patent application number 10/087642 was filed with the patent office on 2002-11-07 for cast shaped article made from high strength, precipitation-hardenable stainless steel and a process for making same.
Invention is credited to Martin, James W..
Application Number | 20020164261 10/087642 |
Document ID | / |
Family ID | 23042030 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020164261 |
Kind Code |
A1 |
Martin, James W. |
November 7, 2002 |
Cast shaped article made from high strength,
precipitation-hardenable stainless steel and a process for making
same
Abstract
A shaped article which is manufactured by casting in air or
under inert-gas shrouding at atmospheric pressure is disclosed. The
shaped article provides a superior combination of high strength,
hardness, ductility, and corrosion resistance compared to the known
air-castable stainless steels. A cast article in accordance with
this invention is made from a high strength, castable, stainless
steel alloy having the following weight percent composition. 1 C
0.1 max. Mn 2 max. Si 1 max. P 0.05 max. S 0.05 max. Cr 9-13 Ni 4-8
Mo 4-8 Co 8-16 N 0.1 max. The balance of the alloy is essentially
iron and the usual impurities. A process for making such an article
is also described.
Inventors: |
Martin, James W.; (Sinking
Spring, PA) |
Correspondence
Address: |
DANN DORFMAN HERRELL & SKILLMAN
SUITE 720
1601 MARKET STREET
PHILADELPHIA
PA
19103-2307
US
|
Family ID: |
23042030 |
Appl. No.: |
10/087642 |
Filed: |
February 28, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60272976 |
Mar 2, 2001 |
|
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Current U.S.
Class: |
420/67 ; 420/34;
420/69 |
Current CPC
Class: |
C21D 6/004 20130101;
C21D 6/02 20130101; C22C 38/44 20130101; A63B 2209/00 20130101;
C22C 38/004 20130101; A63B 53/04 20130101; C21D 6/007 20130101;
C22C 38/54 20130101; C22C 38/52 20130101 |
Class at
Publication: |
420/67 ; 420/69;
420/34 |
International
Class: |
C22C 038/18; C22C
038/20; C22C 038/26; C22C 038/34 |
Claims
What is claimed is:
1. A cast, shaped article that is made from a high strength,
precipitation-hardenable stainless steel alloy consisting
essentially of, in weight percent, about
8 C 0.1 max. Mn 2 max. Si 1 max. P 0.05 max. S 0.05 max. Cr 9-13 Ni
4-8 Mo 4-8 Co 8-16 N 0.1 max.
the balance being essentially iron and the usual impurities.
2. A shaped article as set forth in claim 1 which provides a room
temperature yield strength of at least about 220 ksi in the cast
and heat treated condition.
3. A shaped article as set forth in claim 1 in which the
precipitation hardenable stainless steel alloy contains up to about
1% niobium.
4. A shaped article as set forth in claim 1 in which the
precipitation hardenable stainless steel alloy contains up to about
0.02% boron.
5. A shaped article as set forth in claim 1 in which the
precipitation hardenable stainless steel alloy consists essentially
of, in weight percent, about
9 C 0.025 max. Mn 1 max. Si 1 max. P 0.04 max. S 0.03 max. Cr
9.5-12 Ni 5-8 Mo 4-8 Co 8-16 N 0.025 max.
the balance being essentially iron and the usual impurities.
6. A shaped article as set forth in claim 5 in which the
precipitation hardenable stainless steel alloy contains up to about
1% niobium.
7. A shaped article as set forth in claim 5 in which the
precipitation hardenable stainless steel alloy contains up to about
0.02% boron.
8. A shaped article as set forth in claim 1 in which the
precipitation hardenable stainless steel alloy consists essentially
of, in weight percent, about
10 C 0.015 max. Mn 0.5 max. Si 0.5 max. P 0.03 max. S 0.01 max. Cr
10.0-11.5 Ni 5.5-7.5 Mo 5-6 Co 9.5-13.5 N 0.015 max.
the balance being essentially iron and the usual impurities.
9. A shaped article as set forth in claim 8 in which the
precipitation hardenable stainless steel alloy contains up to about
1% niobium.
10. A shaped article as set forth in claim 8 in which the
precipitation hardenable stainless steel alloy contains up to about
0.02% boron.
11. A process for making a shaped article, comprising the steps of:
melting a stainless steel alloy having the following composition in
weight percent
11 C 0.1 max. Mn 2 max. Si 1 max. P 0.05 max. S 0.05 max. Cr 9-13
Ni 4-8 Mo 4-8 Co 8-16 N 0.1 max.
the balance being essentially iron and the usual impurities;
casting the molten alloy into a mold to form a cast article;
allowing the cast article to solidify; and then heat treating the
cast article to age harden the cast article.
12. A process as set forth in claim 11 wherein the molten alloy is
cast in air.
13. A process as set forth in claim 11 wherein the molten alloy is
cast under an inert gas atmosphere.
14. A process as set forth in claim 11 comprising the step of heat
treating the cast article to substantially homogenize the
composition and microstructure of the cast article, said
homogenization heat treating being performed after the cast article
has solidified, but before the cast article is age hardened.
15. A process as set forth in claim 14 wherein the homogenization
heat treating step comprises the step of heating the cast article
at a homogenization temperature of about 2000 to 2300.degree. F.
and then cooling the cast article.
16. A process as set forth in claim 15 wherein the homogenization
heat treating step comprises the step of heating the cast article
for about 1 to 4 hours.
17. A process as set forth in claim 15 comprising the step of
cooling the cast article from the homogenization temperature to
about room temperature.
18. A process as set forth in claim 11 wherein the age hardening
heat treating step comprises the steps of: heating the cast article
at an elevated temperature sufficient to substantially completely
austenitize the stainless steel alloy; rapidly cooling the cast
article to about room temperature; and then heating the cast
article at a hardening temperature of about 900 to 1100.degree.
F.
19. A process as set forth in claim 18 wherein the step of
austenitizing the stainless steel alloy comprises heating the cast
article at a temperature of about 1400 to 2000.degree. F. for at
least 30 minutes.
20. A process as set forth in claim 18 wherein the step of heating
the austenitized alloy comprises the step of heating the
austenitized alloy at a temperature of about 950 to 1025.degree. F.
to provide a yield strength of at least about 220 ksi.
21. A process as set forth in claim 18 or 20 wherein the
austenitized alloy is heated for about 1 to 4 hours.
22. A golf club head made from a high strength,
precipitation-hardenable stainless steel alloy consisting
essentially of, in weight percent, about
12 C 0.1 max. Mn 2 max. Si 1 max. P 0.05 max. S 0.05 max. Cr 9-13
Ni 4-8 Mo 4-8 Co 8-16 N 0.1 max.
the balance being essentially iron and the usual impurities, said
golf club head having been formed by casting said alloy into a
mold, solidifying the alloy, and then heat treating the alloy under
conditions of temperature and time sufficient to provide a yield
strength of at least about 220 ksi for said golf club head.
Description
[0001] This application claims the benefit of priority from
copending U.S. Provisional Application No. 60/272,976, filed Mar.
2, 2001.
FIELD OF THE INVENTION
[0002] This invention relates to shaped articles that are cast from
a high strength precipitation-hardenable stainless steel and a
process for making such articles. More particularly, the invention
relates to a cast golf club head made from such an alloy.
BACKGROUND OF THE INVENTION
[0003] In order to gain a competitive edge, golf club manufacturers
demand progressively higher-strength, corrosion-resistant alloys
which are air castable. Hitherto, precipitation-hardenable
stainless steels have been used for such applications because they
provide both high strength and hardness together with good
corrosion resistance. For example, international application
publication number WO 01/79576 describes a high-strength,
martensitic precipitation-hardenable stainless steel that is
suitable for air casting of shapes. In the context of that patent
application, high-strength is defined as a room-temperature yield
strength of at least about 190 ksi when the alloy is in the
solution-treated-and-age-hardened condition. The alloy described in
the published international application typically provides a yield
strength of up to about 200 ksi and an ultimate tensile strength of
up to about 210 ksi.
[0004] While varying combinations of higher strength, toughness,
and corrosion resistance can be achieved by the known
precipitation-hardenabl- e stainless steel alloys, those alloys
utilize highly reactive elements, such as titanium or aluminum
which react with nickel to form a strengthening precipitate during
an age-hardening heat treatment. However, titanium and aluminum
have a strong affinity for oxygen and nitrogen. Therefore, alloys
that employ those elements are unsuitable for air casting of shaped
articles because of undesirable reactions with oxygen and nitrogen
when the molten metal is exposed to the air. Efforts to isolate the
molten metal from air such as by inert gas shrouding have not
provided adequate protection. Consequently, the economic benefits
of air or non-vacuum melting and casting are not readily obtainable
with such steels.
[0005] An age-hardenable stainless iron-base alloy has been sold
under the tradename PYROMET X-23 by Carpenter Technology
Corporation in wrought product forms. That alloy provides good
notch tensile strength (i.e., NTS/UTS.gtoreq.1) in combination with
good tensile ductility at an ultimate tensile strength of up to
about 260 ksi. The PYROMET X-23 alloy was developed primarily for
gun barrel applications because of its combination of high
strength, toughness, corrosion resistance and thermal stability. In
that context, thermal stability refers to the alloy's resistance to
embrittlement at temperatures within the range of 700 to
1000.degree. F. The PYROMET X-23 alloy has been manufactured
typically as a single vacuum-melted (i.e., VIM) or double
vacuum-melted (i.e., VIM/VAR) product.
SUMMARY OF THE INVENTION
[0006] In accordance with the present invention, there is provided
a shaped article which is manufactured by casting in air or under
inert-gas shrouding at atmospheric pressure. The article according
to this invention provides a superior combination of high strength,
hardness, ductility, and corrosion resistance compared to the known
air-castable stainless steels. A cast article in accordance with
this invention is made from a high strength, castable, stainless
steel alloy having the following broad, intermediate, and preferred
weight percent ranges.
2 Broad Intermediate Preferred C 0.1 max. 0.025 max. 0.015 max. Mn
2 max. 1 max. 0.5 max. Si 1 max. 1 max. 0.5 max. P 0.05 max. 0.04
max. 0.03 max. S 0.05 max. 0.03 max. 0.01 max. Cr 9-13 9.5-12
10.0-11.5 Ni 4-8 5-8 5.5-7.5 Mo 4-8 4-8 5-6 Co 8-16 8-16 9.5-13.5 N
0.1 max. 0.025 max. 0.015 max.
[0007] The balance of the alloy is essentially iron and the usual
impurities found in commercial grades of age-hardenable stainless
steels intended for similar use or service. In addition, the alloy
may optionally contain up to about 1% niobium or niobium and/or
tantalum. The alloy may also contain up to about 0.02% boron. A
cast article, such as a golf club head, made from this alloy
provides a yield strength of at least about 220 ksi together with
good toughness and ductility.
[0008] 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 intermediate or preferred
compositional ranges. In addition, a minimum or maximum for an
element of the broad, intermediate, or preferred range can be used
with the maximum or minimum for that element from one of the other
ranges.
[0009] Here and throughout this application, the term "percent" or
the symbol "%" means percent by weight, unless otherwise indicated.
Also, the term "yield strength" means the offset yield strength
determined by the stress corresponding to the intersection of the
stress-strain curve and a line parallel to the elastic part of the
curve offset by a strain of 0.2%.
[0010] In accordance with another aspect of this invention, there
is provided a process for making a shaped article. The process
includes the step of melting a stainless steel alloy having the
composition in weight percent of any of the alloys set forth above.
The molten alloy is cast into a mold to form a shaped article which
is allowed to solidify. The shaped article is then heated at an
elevated temperature for a time sufficient to substantially fully
homogenize the composition and microstructure of the shaped
article. The shaped article is then heat treated to develop the
combination of strength and toughness desired for the use to which
the shaped article will be put.
DETAILED DESCRIPTION
[0011] Alloy compositions falling within the weight percent
compositions set forth in the table above are particularly suited
for air-cast golf club components. The alloy can economically
achieve room-temperature ultimate tensile strengths of at least 220
ksi while retaining sufficient ductility and corrosion resistance
for the golf-club application. Wrought forms of the subject alloy
composition, including forgings, strip and tubular products, are
also useful for golf club applications and are compatible with
air-cast components. Articles of the invention also may find
application for small parts unrelated to the golf industry such as
firearm components.
[0012] An important aspect of this invention is the ability of the
subject alloy compositions to respond to age hardening without the
need for highly reactive elements such as titanium or aluminum.
Age-hardenable stainless steels containing aluminum and/or titanium
achieve high strength through the precipitation of Ni-Al or Ni-Ti
compounds within a low-carbon martensitic matrix. In contrast, the
alloy described in this application is age hardened through the
precipitation of a Co/Mo/Cr-rich intermetallic compound known as
"R" phase. Because of the absence of highly reactive elements, the
alloy used in this invention exhibits a reduced tendency to form
oxide and/or nitride compounds in the alloy matrix. Further, the
alloy used in this invention is less susceptible to other
compositional changes such as decarburization, when the molten
metal is exposed to air during remelting and casting.
[0013] The alloy used in the cast article according to this
invention contains at least about 9%, better yet at least about
9.5%, and preferably at least about 10.0% chromium to provide
adequate resistance to corrosion under oxidizing conditions,
including atmospheric exposure. While increased chromium levels may
provide additional corrosion resistance, too much chromium
adversely affects the toughness and phase stability of the alloy.
Chromium adversely affects phase stability because it promotes the
formation of ferrite and an excessive amount of retained austenite.
Therefore, chromium is limited to not more than about 13%, better
yet to not more than about 12%, and preferably to not more than
about 11.5%.
[0014] Cobalt serves multiple purposes in the alloy used in this
invention. For example, cobalt promotes the formation of austenite
and benefits the toughness of the alloy. Cobalt also participates
in age-hardening through the precipitation of "R" phase. To achieve
those benefits the alloy contains at least about 8%, and preferably
at least about 9.5% cobalt. Too much cobalt stabilizes the
austenite in this alloy, such that a full martensitic
transformation during quenching is inhibited, thereby preventing
the alloy from achieving the very high strength and hardness of
which it is capable. Increasing cobalt content also significantly
adds to the cost of the alloy without any further significant
benefit. For these reasons, cobalt is limited to not more than
about 16% and preferably to not more than about 13.5% in the
alloy.
[0015] Nickel, like cobalt, promotes austenite formation and
benefits the toughness provided by the alloy. Therefore, the alloy
used in a cast article according to this invention contains at
least about 4%, better yet at least about 5%, and preferably at
least about 5.5% nickel to achieve good toughness and ductility.
However, nickel also has a strong effect on suppressing the
austenite-to-martensite transformation on quenching. Therefore,
nickel is limited to not more than about 8%, and preferably to not
more than about 7.5%.
[0016] At least about 4%, and preferably at least about 5%,
molybdenum is present in the steel alloy used in this invention
because molybdenum contributes not only to strength through its
role in the formation of the "R" phase strengthening precipitate,
but also because it benefits the toughness, ductility, and
corrosion resistance of the alloy. On the other hand, the
molybdenum content is limited to not more than about 8% and
preferably to not more than about 6% because too much molybdenum
leads to excessive retained austenite and promotes undesirable
formation of ferrite in the alloy. All or part of the molybdenum
can be replaced by an equivalent amount of tungsten. As is known to
those skilled in the art, the amount of tungsten required to
replace a given amount of molybdenum and provide an equivalent
effect is in the proportion of approximately 2% tungsten for each
1% of molybdenum.
[0017] A small amount of silicon, for example, about 0.01 to 0.02%,
may be present in the alloy used in this invention because it
benefits the fluidity of the alloy during casting. Silicon is also
beneficial as a deoxidizing agent. Because silicon is a ferrite
forming element, its concentration, when present in the alloy, is
limited to not more than about 1% , and preferably to not more than
about 0.5%.
[0018] The alloy used in a cast article according to the present
invention may optionally include up to about 1% niobium. Because
niobium is far less reactive with oxygen and nitrogen than aluminum
or titanium, the presence of niobium in the alloy does not
compromise the castability of the alloy in air. Moreover, niobium
benefits the strength of the alloy because it reacts with some of
the nickel to form a nickel-niobium rich intermetallic compound
that strengthens the martensitic matrix of the alloy. The amount of
niobium that is used in the alloy is limited to the aforesaid
amount because more than about 1% niobium in the alloy adversely
affects the toughness and ductility of the alloy. Tantalum may be
substituted for all or part of the niobium on a 2-for-1 weight
percent basis.
[0019] The alloy used in the cast articles of this invention may
also contain a small but effective amount of boron up to about
0.02% because boron benefits the hot workability and toughness of
the alloy. Boron is also useful as a deoxidizing agent. Although
hot-workability is not typically a concern with regard to cast
articles, hot working operations may be employed to manufacture
product forms that can be remelted for casting articles according
to this invention.
[0020] The balance of the alloy is essentially iron and the usual
impurities found in commercial grades of precipitation-hardenable
stainless steels intended for similar use or service. In this
regard, carbon, nitrogen, manganese, phosphorus, and sulfur are
inevitably present in the alloy used in this invention. However,
the amounts of those elements are controlled because the presence
of too much of them, either individually or in combination,
adversely affects the strength and toughness provided by the alloy.
Carbon and nitrogen are strong austenite stabilizing elements when
present in the solid solution. Their presence in too great a
concentration adversely affects the phase stability of the alloy.
Also, carbon and nitrogen are likely to combine with chromium to
form undesirable carbide, nitride, and carbonitride compounds.
Therefore, carbon and nitrogen are each restricted to not more than
about 0.1%, better yet to not more than about 0.025%, and
preferably to not more than about 0.015% in the alloy.
[0021] Manganese is limited to not more than about 1% and
preferably to not more than about 0.5%. Phosphorus is restricted to
not more than about 0.050%, better yet to not more than about
0.040%, and preferably to not more than about 0.030%. Sulfur is
limited to not more than about 0.05%, better yet to not more than
about 0.030%, and preferably to not more than about 0.01% because
it adversely affects the mechanical properties and corrosion
resistance of the alloy. Other elements including copper, vanadium,
zirconium, calcium, titanium, aluminum, and rare-earth metals can
be present in the article at tramp levels or as residual amounts
retained from alloying additions.
[0022] The alloy is readily prepared and cast into a mold to form a
component such as a golf club head. It can be melted in air in the
known ways or under an inert gas atmosphere. Although better
results are obtained when the alloy is vacuum melted, as by vacuum
induction melting (VIM), the added cost of VIM may not be warranted
for golf club components. A relatively simple heat treatment of the
cast component is used to bring out the unique properties of the
alloy. Preferably, the cast article is heated at a temperature of
about 2000-2300.degree. F. for about 1 to 4 hours to homogenize the
alloy material. The cast article is then cooled in air from the
homogenizing temperature. After the homogenizing heat treatment,
the cast article is solution annealed from about 1400.degree. F. to
about 2000.degree. F. for a time sufficient to ensure substantially
complete austenitizing of the alloy. At least about 30 minutes at
temperature is sufficient for cast, shaped articles made in
accordance with this invention. The cast article is rapidly cooled
from the solution annealing temperature to room temperature,
preferably by quenching in water, oil, or a polymer solution, to
ensure optimum response to the age-hardening heat treatment that
follows. Forced gas cooling has also been used successfully.
[0023] After the solution treatment, the cast article is
age-hardened by heating at about 900.degree. F. to about
1100.degree. F., preferably at about 950.degree. F. to about
1025.degree. F. for about 1 to 4 hours, and then cooled in air.
EXAMPLE
[0024] As an example of an article according to the present
invention, a small heat having the weight percent composition shown
in Table 1 below.
3TABLE 1 C Mn Si P S Cr Ni Mo Co Cu B N 0.006 <0.01 <0.01
<0.005 <0.001 10.12 7.01 5.48 10.05 <0.01 0.003 0.002
[0025] The balance of the alloy is iron and the usual impurities.
The example heat was melted under an argon shroud at atmospheric
pressure. An evaluation of the mechanical properties of the example
heat in the cast+heat-treated condition was performed. The results
of the evaluation are set forth in Table 2 below.
4TABLE 2 Heat 0.2% YS % Elong. Hardness Treatment* (ksi) UTS (ksi)
(in 4D) % R.A. NTS (ksi) NTS/UTS (HRC) A 230.0 245.7 13.4 51.6
351.7 1.43 49.5 B 233.6 253.1 13.2 48.5 346.7 1.37 50.0 C 222.8
253.4 14.1 54.2 324.3 1.28 49.0 D 214.2 249.7 14.7 52.4 321.6 1.29
48.0 E 201.6 242.8 16.4 54.5 297.6 1.23 47.5 F 192.0 236.0 15.5
51.1 284.5 1.21 47.0 *For the above testing, all specimens were
homogenized + solution-annealed after casting as follows: Heat at
2100.degree. F. for 4 hours and then air cool; heat at 1700.degree.
F. for 1 hour followed by water quench. Specimens were then age
hardened utilizing different temperatures as follows: Treatment A -
950 F., 4 hours, air cool Treatment B - 975 F., 4 hours, air cool
Treatment C - 1000 F., 4 hours, air cool Treatment D - 1025 F., 4
hours, air cool Treatment E - 1050 F., 4 hours, air cool Treatment
F - 1075 F., 4 hours, air cool
[0026] As the data in Table 2 show, a peak hardness of about HRC 50
is achieved upon aging the cast article at about 975.degree. F. The
results also reveal that non-vacuum cast specimens of the subject
invention can achieve a yield strength well in excess of 220 ksi
ultimate tensile strength with useful levels of ductility and notch
tensile strength over a wide range of aging temperatures.
[0027] Castings made in accordance with this invention also respond
to age hardening temperatures below about 950.degree. F. However,
such treatments are considered to be "underaging" heat treatments,
that is, they result in the alloy developing less than the peak
strength of which it is capable. Useful strength levels are still
provided when this alloy is age-hardened at such "underaging" heat
treatments. That feature makes the alloy of this invention "design
compatible" with other precipitation-hardenable stainless steels
that reach peak strength when aged at lower temperatures, such as
about 900-950.degree. F. This feature is advantageous in golf club
head designs in which a face formed from wrought strip of one grade
of precipitation-hardenable stainless steel is joined to a body of
another precipitation-hardenable stainless steel alloy. When used
for golf club head designs that do not employ multiple materials,
the alloy according to this invention is preferably age-hardened at
the highest temperature capable of providing the prescribed or
specified strength requirement.
[0028] The presence of a controlled amount of reverted austenite
benefits the toughness and ductility provided by the alloy of this
invention. Set forth in Table 3 below is the amount of austenite
present in the samples of the alloy of Table 1 above. Heat
treatment identifiers correspond to those specified in Table 2
above.
5 TABLE 3 Heat Treatment Vol. % Austenite Homogenized + Soln Annld
Trace (<1) A Trace (<1) B 4 C 5 D 8 E 13 F 18
[0029] The data set forth in Table 3 shows that as the
age-hardening temperature increases, the amount of austenite
present in the age-hardened steel increases.
[0030] It is noteworthy that the mechanical properties described
herein are achieved in spite of a somewhat coarse grain size (i.e.,
ASTM Grain Size No. 0-1), a condition which has been frequently
encountered during the casting of golf club heads. Furthermore, it
is an additional feature of this invention that the use of a
homogenizing heat treatment as described above, prior to the
solution annealing treatment, provides several benefits to the cast
component including improved strength capability, tensile ductility
and uniformity of properties. To demonstrate the benefit of the
homogenization heat treatment, a second example heat having the
weight percent composition shown in Table 4 was prepared.
6TABLE 4 C Mn Si P S Cr Ni Mo Co Cu B N 0.002 <0.01 <0.01
<0.005 <0.001 10.09 6.98 5.48 10.96 <0.01 0.0020 0.005
[0031] The balance of the alloy was iron and the usual
impurities.
7TABLE 5 Heat 0.2% YS UTS % Elong. Treatment (ksi) (ksi) (in 4D) %
RA A.sup.1 192.0 230.8 9.3 15.4 B.sup.2 206.8 245.3 13.4 42.0
.sup.1Treatment A: Solution anneal at 1700.degree. F. for 1 hour,
then water quenched + Aging at 1025.degree. F. for 4 hours, then
air cooled. .sup.2Treatment B: Homogenize at 2100.degree. F. for 4
hours, then air cool + Solution anneal at 1700.degree. F. for 1
hour, then water quenched + Aging at 1025.degree. F. for 4 hours,
then air cooled.
[0032] The data in Table 5 show that the sample which received the
homogenizing heat treatment prior to solution annealing achieved a
significantly better combination of strength and ductility than the
sample that was not homogenized.
[0033] The terms and expressions which have been employed herein
are used as terms of description, not of limitation. There is no
intention in the use of such terms and expressions of excluding any
equivalents of the elements, features, or steps shown and described
or portions thereof. However, it is recognized that various
modifications are possible within the scope of the invention
claimed.
* * * * *