Note: Descriptions are shown in the official language in which they were submitted.
CA 02718576 2010-09-15
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TITLE OF THE INVENTION
METHOD OF MAKING A HIGH STRENGTH, HIGH TOUGHNESS,
FATIGUE RESISTANT, PRECIPITATION HARDENABLE STAINLESS STEEL
AND PRODUCT MADE THEREFROM
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to precipitation-hardenable stainless steel alloys and
in
particular to a method of making such alloys to reduce the size and
distribution of inclusions
that adversely affect the fatigue resistance and fracture toughness provided
by such alloys.
Description of the Related Art
US Patent No. 5,681,528 and US Patent No. 5,855,844 describe high-strength,
notch-
ductile, precipitation-hardening stainless steels. Those alloys are used for
structural
applications in the aerospace industry and in many additional non-aerospace
uses. Testing of
the known alloys by the aerospace industry has indicated that the fatigue life
provided by the
alloys, while considered to be acceptable, leaves something to be desired.
Fatigue life is a
very important parameter for the design of aerospace structural members.
Improved fatigue
life would allow for either product weight savings or longer design service
life for structural
components. It is desired to provide improved fatigue-strength relative to the
known alloys,
while still maintaining the excellent combination of strength, toughness, and
corrosion
resistance that the known alloys provide.
The abovementioned fatigue testing has demonstrated that the majority of
fatigue
failures initiate at large second phase inclusions, which are present in the
material as a result
of the alloy composition and processing. The alloy according to the present
invention is
designed to provide strength and toughness that are equivalent to the known
alloy, but
without the resultant large second phase inclusions that adversely affect the
fatigue resistance
of the known alloy.
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SUMMARY OF THE INVENTION
The improvement in fatigue life desired for the known precipitation
hardenable, stainless steel alloys is achieved to a large degree by the alloy
in
accordance with the present invention. The alloy according to this invention
is a
precipitation hardening Cr-Ni-Ti-- Mo martensitic stainless steel alloy that
provides a
unique combination of corrosion resistance, fatigue resistance, strength, and
toughness.
The present invention provides a method of making a precipitation hardenable,
high strength, high toughness stainless steel, having broad, intermediate, and
preferred
compositional ranges, in weight percent:
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
Ti 1.5-1.8 1.5-1.7 1.5-1.7
Mo 0.25-1.5 0.75-1.25 0.9-1.1
Cu 0.95 max 0.50 max 0.25 max
Al 0.25 max 0.050 max 0.025 max
Nb 0.3 max 0.050 max 0.025 max
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
objectionably detract from the desired combination of properties provided by
this
alloy. The alloy according to this invention is further characterized by a
plurality of
non-strengthening, calcium-based inclusions that are sparsely dispersed in the
matrix
steel.
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The method includes the step of melting a precipitation-hardenable stainless
steel alloy having the weight percent composition set forth above. The method
further
includes the step of adding calcium to the molten alloy in an amount
sufficient to
combine with available sulfur and oxygen in the molten alloy to form calcium-
based
inclusions selected from the group consisting of calcium sulphides, calcium
oxides,
calcium oxysulfides, and combinations thereof. The method also includes the
steps of
processing the alloy to remove at least a portion of the inclusions from the
alloy and
then solidifying the refined alloy, whereby rare earth metal additions are not
used in
the alloy, and after said processing and solidifying steps said alloy contains
substantially no rare-earth based inclusions and any residual calcium-based
inclusions
are sparsely dispersed in the alloy.
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. Throughout this application percent (%) means percent by weight
unless
otherwise indicated. The term "inclusion" encompasses secondary particles and
phases such as sulfides, oxides, oxysulfides, carbides, nitrides, and
carbonitrides.
DETAILED DESCRIPTION
In the alloy according to the present invention, the unique combination of
strength, notch toughness, and stress-corrosion cracking resistance is
achieved by
balancing the elements chromium, nickel, titanium, and molybdenum. 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 commensurate with that of
a
conventional stainless steel under oxidizing conditions. 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 it benefits the notch toughness of
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the alloy. At least about 1.5% titanium is present in the alloy to benefit the
strength of the
alloy through the precipitation of a nickel-titanium-rich phase during aging.
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 it 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.
When chromium, nickel, titanium, and/or molybdenum are not properly balanced,
the
alloy's ability to transform fully to a martensitic structure using
conventional processing
techniques is inhibited. Furthermore, improper balancing of chromium, nickel,
titanium, and
molybdenum in this alloy impairs the alloy's ability to remain substantially
fully martensitic
when solution treated and age-hardened. Under such conditions the strength
provided by the
alloy is significantly reduced. Therefore, chromium, nickel, titanium, and
molybdenum
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% and nickel is limited to not more than about 11.6% and preferably to not
more than
about 11.25%. Titanium is restricted to not more than about 1.8% and
preferably to not more
than about 1.7% and 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%.
Sulfur in this alloy tends to combine with manganese and/or titanium to form
manganese sulfides (MnS) and/or titanium sulfides (TiS) which adversely affect
the fracture
toughness, notch toughness, and notch tensile strength of the alloy. A product
form of this
alloy having a large cross-section, i.e., >0.7 in2 (>4 cm), does not undergo
sufficient
thermomechanical processing to homogenize the alloy and neutralize the adverse
effect of
the sulfide inclusions. A small addition of calcium is preferably made to the
alloy to benefit
the fatigue strength of the alloy by combining with sulfur to facilitate the
removal of sulfur
from the alloy. In the known alloy, small additions of cerium, lanthanum,
and/or other rare
earth metals are used to benefit the toughness and fracture toughness
properties, especially in
large section sizes. However, although the use of such rare earth treatment
benefits the
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toughness of the alloy, it has now been found that remnants of such rare earth
inclusions may
also serve as crack initiation sites that adversely affect the fatigue
strength of the alloy.
Therefore, rare earth additions are not used in the present alloy so as to
avoid the presence of
the rare earth inclusions. Rare earth metals including cerium, lanthanum,
yttrium, etc. are
restricted such that the combined amounts of such elements are not more than
about 0.001%.
Preferably, the alloy contains not more than about 0.0008%, and better yet not
more than
0.0007% of such elements.
The elimination of the rare earth treatment would have been expected to
adversely
affect the fracture toughness of the alloy, especially in larger section
sizes. However, it has
been found that the use of the calcium treatment instead of the rare earth
treatment not only
benefits the fatigue strength of the alloy, but does not adversely affect the
combination of
toughness and fracture toughness provided by this alloy. Therefore, it is
believed that the
alloy according to the present invention provides strength and toughness
equivalent to the
known alloys.
Additional elements such as boron, aluminum, niobium, manganese, and silicon
may
be present in controlled amounts to benefit other desirable properties
provided by this alloy.
More specifically, up to about 0.0 10% 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.
Aluminum and/or niobium can be present in the alloy to benefit the yield and
ultimate
tensile strengths. More particularly, up to about 0.25%, better yet up to
about 0.10%, still
better up to about 0.050%, and preferably up to about 0.025% aluminum can be
present in
the alloy. Also, up to about 0.3%, better yet up to about 0.10%, still better
up to about
0.050%, and preferably up to about 0.025% niobium can be present in the alloy.
Although
higher yield and ultimate tensile strengths are obtainable when aluminum
and/or niobium are
present in this alloy, the increased strength is developed at the expense of
notch toughness.
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Therefore, when optimum notch toughness is desired, aluminum and niobium are
restricted
to the usual residual levels.
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 deleterious effects on toughness, corrosion
resistance, and the
austenite-martensite phase balance in the matrix material.
The balance of the alloy is essentially iron apart from the usual impurities
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.
In particular, too much carbon and/or nitrogen impair the corrosion resistance
and
deleteriously affect the toughness and fatigue strength 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%, and preferably not more than about 0.0 10%
nitrogen is
present in the alloy. When carbon and/or nitrogen are present in larger
amounts, the carbon
and/or nitrogen combines with titanium to form titanium-rich non-metallic
inclusions, such
as titanium carbonitrides. That reaction inhibits the formation of the nickel-
titanium-rich
phase which is a primary factor in the high strength provided by this alloy.
Moreover, such
carbonitrides serve as crack-initiation sites and adversely affect the
fracture toughness and
fatigue resistance provided by the alloy.
Phosphorus is maintained at a low level because of its deleterious 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 0.0 10% phosphorus is present
in the alloy.
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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 titanium-rich non-metallic inclusions which, like carbon and
nitrogen,
inhibit the desired strengthening effect of the titanium and serve as crack-
initiation sites that
adversely affect the fracture toughness and fatigue resistance provided by the
alloy. Also,
greater amounts of sulfur deleteriously affect the hot workability and
corrosion resistance of
this alloy and impair its toughness, particularly in a transverse direction.
Oxygen is limited
to not more than about 25 parts per million (ppm). Tramp elements such as
lead, bismuth,
antimony, arsenic, tellurium, selenium, tin, germanium, and gallium are
limited to about
0.003% max. each, better yet to not more than about 0.002% each, and
preferably to not
more than about 0.001% each.
Too much copper deleteriously affects the notch toughness, ductility, and
strength of
this alloy. Therefore, the alloy contains not more than about 0.95%, better
yet not more than
about 0.75%, still better, not more than about 0.50%, and preferably not more
than about
0.25% copper.
The method according to the present invention is preferably carried out by
vacuum
induction melting (VIM) the constituent elements as described above.
Preferably, VIM is
followed by vacuum arc remelting (VAR), but other practices can be used. 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 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 can be readily removed
during primary
or secondary melting. It is believed that any residual calcium-based
inclusions are sparsely
dispersed in the alloy matrix material upon solidification. It is expected
that after VAR the
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alloy contains less than about 0.001% calcium and not more than about 0.001%
sulfur. The
inclusions are generally smaller in major cross-sectional size than the rare-
earth-based
inclusions and Ti-rich non-metallic inclusions that are present in the known
alloys. It is also
believed that the size distribution of the calcium-based inclusions is about
0.5 m to about
3.00 gm in major cross-sectional dimension, when such inclusions are present.
The very
small size and sparse dispersion of Ca-based inclusions benefits the strength,
toughness, and
fatigue resistance provided by the alloy.
This alloy can be made using powder metallurgy techniques, if desired.
Although the
alloy of the present invention can be hot or cold worked, cold working
enhances the
mechanical strength of the alloy.
The precipitation hardening alloy of the present invention is solution
annealed and
then age hardened to develop the desired high strength and hardness. The
solution annealing
temperature should be high enough to dissolve essentially all of the undesired
precipitates
into the alloy matrix material. However, if the solution annealing temperature
is too high, it
will impair the fracture toughness of the alloy by promoting excessive grain
growth.
Typically, the alloy of the present invention is solution annealed at about
1700 F-1900 F
(927 C-1038 C) for about 1 hour and then quenched.
When desired, this alloy can also be subjected to a deep chill treatment after
it is
quenched, to further develop the high strength of the alloy. The deep chill
treatment cools the
alloy to a temperature sufficiently below the martensite finish temperature to
ensure the
completion of the martensite transformation. Typically, a deep chill treatment
consists of
cooling the alloy to below about -100 F (-73 C) for about 1 to 8 hours. The
need for a deep
chill treatment will be affected, at least in part, by the martensite finish
NO temperature of
the alloy. If the MF temperature is sufficiently high, the transformation to a
martensitic
structure will proceed without the need for a deep chill treatment. In
addition, the need for a
deep chill treatment may also depend on the cross-sectional size of the piece
being
manufactured. As the size of the piece increases, segregation in the alloy
becomes more
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significant and the use of a deep chill 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 ensure
that the transformation to martensite is completed. For example, it has been
found that in a
piece having a large cross-sectional area as described above, a deep chill
treatment lasting
about 8 hours is preferred for developing the high strength that is
characteristic of this alloy.
The alloy of the present invention is age hardened in accordance with
techniques used
for the known precipitation hardening, stainless steel alloys, as are known to
those skilled in
the art. For example, the alloys are aged at a temperature between about 900 F
(482 C) and
about 1150 F (621 C) for about 4 to 8 hours. The specific aging conditions
used are 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.
The terms and expressions that have been employed herein are used as terms of
description and not of limitation. There is no intention in the use of such
terms and
expressions to exclude any equivalents of the features described or any
portions thereof. It is
recognized, however, that various modifications are possible within the scope
of the
invention claimed.
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