U.S. patent application number 10/360961 was filed with the patent office on 2004-08-12 for austenitic stainless steels including molybdenum.
Invention is credited to Rakowski, James M..
Application Number | 20040156737 10/360961 |
Document ID | / |
Family ID | 32824097 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040156737 |
Kind Code |
A1 |
Rakowski, James M. |
August 12, 2004 |
Austenitic stainless steels including molybdenum
Abstract
An austenitic stainless steel comprises, by weight, 9 to 23%
chromium, 30 to 35% nickel, 1 to 6% molybdenum, 0 to 0.03%
titanium, 0.15% to 0.6% aluminum, up to 0.1% carbon, 1 to 1.5%
manganese, 0 to less than 0.8% silicon, 0.25 to 0.6% niobium and
iron. Embodiments of austenitic stainless steels according to the
present invention exhibit enhanced resistance to corrosion. Thus,
the stainless steels of the present invention may find broad
application as, for example, automotive components and, more
particularly, as automotive exhaust system flexible connectors and
other components, as well as in other applications in which
corrosion resistance is desired.
Inventors: |
Rakowski, James M.;
(Pittsburgh, PA) |
Correspondence
Address: |
Allegheny Technologies Incorporated
1000 Six PPG Place
Pittsburgh
PA
15222
US
|
Family ID: |
32824097 |
Appl. No.: |
10/360961 |
Filed: |
February 6, 2003 |
Current U.S.
Class: |
420/53 ;
420/586.1 |
Current CPC
Class: |
C22C 38/58 20130101;
F01N 13/1816 20130101; C22C 38/002 20130101; C22C 38/04 20130101;
C22C 38/44 20130101; B21D 26/033 20130101; H05B 3/40 20130101; C22C
38/02 20130101; F01N 2530/04 20130101; H05B 3/16 20130101; C22C
38/50 20130101; B23K 31/02 20130101; C22C 38/001 20130101; C22C
38/06 20130101; C22C 38/42 20130101; C22C 38/48 20130101; H05B
3/141 20130101; H05B 3/0014 20130101 |
Class at
Publication: |
420/053 ;
420/586.1 |
International
Class: |
C22C 038/50 |
Claims
I claim:
1. An austenitic stainless steel comprising, by weight: 19% to 23%
chromium; 30% to 35% nickel; 1% to 6% molybdenum; 0 to 0.03%
titanium; 0.15% to 0.6% aluminum; up to 0.1% carbon 1% to 1.5%
manganese; 0 to less than 0.8% silicon; 0.25% to 0.6% niobium; and
iron.
2. The austenitic stainless steel of claim 1, comprising 19% to
21.5% chromium.
3. The austenitic stainless steel of claim 1, comprising 2% to 4%
molybdenum.
4. The austenitic stainless steel of claim 1, comprising 1% to 2.7%
molybdenum.
5. The austenitic stainless steel of claim 1, comprising 0 to 0.01%
titanium.
6. The austenitic stainless steel of claim 1, comprising 0 to
0.005% titanium.
7. The austenitic stainless steel of claim 1, comprising 0.15% to
0.4% aluminum.
8. The austenitic stainless steel of claim 1, comprising up to
0.025% carbon.
9. The austenitic stainless steel of claim 1, comprising 0 to 0.4%
silicon.
10. The austenitic stainless steel of claim 1, comprising 0.3% to
0.5% niobium.
11. The austenitic stainless steel of claim 1, further comprising 0
to 0.75% copper.
12. The austenitic stainless steel of claim 1, further comprising 0
to 0.4% copper.
13. The austenitic stainless steel of claim 1, further comprising:
no greater than 0.05% phosphorus; no greater than 0.02% sulfur; and
no greater than 0.1% nitrogen.
14. The austenitic stainless steel of claim 1 comprising, by
weight: 19% to 21.5% chromium; 30% to 35% nickel; 1% to 2.7%
molybdenum; 0 to 0.03% titanium; 0.15% to 0.4% aluminum; up to
0.025% carbon 1% to 1.5% manganese; 0 to less than 0.8% silicon;
0.25% to 0.6% niobium; and iron.
15. The austenitic stainless steel of claim 1, consisting
essentially of, by weight: 19% to 23% chromium; 30% to 35% nickel;
1% to 6% molybdenum; 0 to 0.03% titanium; 0.15% to 0.6% aluminum;
up to 0.1% carbon 1% to 1.5% manganese; 0 to less than 0.8%
silicon; 0.25% to 0.6% niobium; 0 to 0.75% copper; up to 0.05%
phosphorus; no up to 0.02% sulfur; up to 0.1% nitrogen; iron; and
incidental impurities.
16. An article of manufacture including an austenitic stainless
steel comprising, by weight: 19% to 23% chromium; 30% to 35%
nickel; 1% to 6% molybdenum; 0 to 0.03% titanium; 0.15% to 0.6%
aluminum; up to 0.1% carbon 1% to 1.5% manganese; 0 to less than
0.8% silicon; 0.25% to 0.6% niobium; and iron.
17. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 19% to 21.5% chromium.
18. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 2% to 4% molybdenum.
19. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 1% to 2.7% molybdenum.
20. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 0 to 0.01% titanium.
21. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 0 to 0.005% titanium
22. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 0.15% to 0.4% aluminum.
23. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises up to 0.025% carbon.
24. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 0 to 0.4% silicon.
25. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises 0.3% to 0.5% niobium.
26. The article of manufacture of claim 16, wherein the austenitic
stainless steel further comprises 0 to 0.75% copper.
27. The article of manufacture of claim 16, wherein the austenitic
stainless steel further comprises 0 to 0.4% copper.
28. The article of manufacture of claim 16, wherein the austenitic
stainless steel further comprises: no greater than 0.05%
phosphorus; no greater than 0.02% sulfur; and no greater than 0.1%
nitrogen.
29. The article of manufacture of claim 16, wherein the austenitic
stainless steel comprises, by weight: 19% to 21.5% chromium; 30% to
35% nickel; 1% to 2.7% molybdenum; 0 to 0.03% titanium; 0.15% to
0.4% aluminum; up to 0.025% carbon 1% to 1.5% manganese; 0 to less
than 0.8% silicon; 0.25% to 0.6% niobium; and iron.
30. The article of manufacture of claim 16, wherein the austenitic
stainless steel consists essentially of, by weight: 19% to 23%
chromium; 30% to 35% nickel; 1% to 6% molybdenum; 0 to 0.03%
titanium; 0.15% to 0.6% aluminum; up to 0.1% carbon 1% to 1.5%
manganese; 0 to less than 0.8% silicon; 0.25% to 0.6% niobium; 0 to
0.75% copper; up to 0.05% phosphorus; up to 0.02% sulfur; up to
0.1% nitrogen; and iron; and incidental impurities.
31. The article of manufacture of any of claims 16, 29 and 30,
wherein the article of manufacture is selected from the group
consisting of an automobile, an automotive exhaust system
component, an automotive exhaust system flexible connector, a
heating element sheath, and a gasket.
32. An automotive exhaust system flexible connector including an
austenitic stainless steel comprising, in weight percent: 19% to
21.5% chromium; 30% to 35% nickel; 1% to 6% molybdenum; 0 to 0.03%
titanium; 0.15% to 0.6% aluminum; up to 0.1% carbon 1% to 1.5%
manganese; 0 to less than 0.8% silicon; 0.25% to 0.6% niobium; and
iron.
33. A method of making an article of manufacture, the method
comprising: forming at least a portion of the article of
manufacture from an austenitic stainless steel comprising, by
weight: 19% to 23% chromium; 30% to 35% nickel; 1% to 6%
molybdenum; 0 to 0.03% titanium; 0.15% to 0.6% aluminum; up to 0.1%
carbon 1% to 1.5% manganese; 0 to less than 0.8% silicon; 0.25% to
0.6% niobium; 0 to 0.75% copper; and iron.
34. The method of claim 33, wherein the article of manufacture is
selected from the group consisting of an automobile, an automotive
exhaust system component, an automotive exhaust system flexible
connector, a heating element sheath, and a gasket.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn. 120 from co-pending U.S. patent application Ser. No.
09/641,317, filed Aug. 18, 2000.
FEDERALLY SPONSORED RESEARCH
[0002] Not Applicable
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
[0003] The present invention relates to oxidation and corrosion
resistant austenitic stainless steels. More particularly, the
present invention relates to austenitic stainless steels adapted
for use in high temperature and corrosive environments, such as,
for example, use in automotive exhaust system components. The
austenitic stainless steels of the invention find particular
application in components exposed to temperatures up to
1800.degree. F. (982.degree. C.) and to corrosive environments,
such as, for example, chloride-rich waters.
DESCRIPTION OF THE INVENTION BACKGROUND
[0004] In the manufacture of automotive exhaust system components,
concurrent goals are to minimize both cost and weight, while also
maintaining the integrity of the system. Typically, automobile
components for these applications are fabricated from thin
stainless steel stock in order to minimize the weight of the
components and, therefore, the components' resistance to corrosive
attack must be high to prevent failure by perforation or other
means. Corrosion resistance is complicated by the fact that
components used for certain automotive exhaust system applications
are exposed to severely corrosive chemical environments at elevated
temperatures. In particular, automotive exhaust system components
and other automotive engine components are exposed to contamination
from road deicing salts under conditions of elevated temperature
due to the hot exhaust gases. The stainless steel and other metal
components subjected to these conditions are susceptible to a
complex mode of corrosive attack known as hot salt corrosion.
[0005] Typically, at higher temperatures, stainless steel
components undergo oxidation on surfaces exposed to air to form a
protective metal oxide layer. The oxide layer protects the
underlying metal and reduces further oxidation and other forms of
corrosion. However, road deicing salt deposits may attack and
degrade this protective oxide layer. As the protective oxide layer
is degraded, the underlying metal may be exposed and become
susceptible to severe corrosion.
[0006] Thus, metal alloys selected for automotive exhaust system
components are exposed to a range of demanding conditions.
Durability of automotive exhaust system components is critical
because extended lifetimes are demanded by consumers, by federal
regulations, and also under manufacturers' warranty requirements.
To further complicate alloy selection for automotive exhaust system
components, a recent development in these applications is the use
of metallic flexible connectors, which act as compliant joints
between two fixed exhaust system components. Flexible connectors
may be used to mitigate problems associated with the use of welded,
slip, and other joints. A material chosen for use in a flexible
connector is subjected to a high temperature corrosive environment
and must be both formable and have resistance to hot salt corrosion
and various other corrosion types, such as, for example,
intermediate temperature oxidation, general corrosion, and chloride
stress corrosion cracking.
[0007] Alloys for use in automotive exhaust system flexible
connectors often experience conditions in which elevated
temperature exposure occurs after the alloy has been exposed to
contaminants such as road deicing salts. Halide salts can act as
fluxing agents, removing the protective oxide scales which normally
form on the connectors at elevated temperatures. Degradation of the
connectors may be quite rapid under such conditions. Therefore,
simple air oxidation testing may be inadequate to reveal true
resistance to corrosive degradation in service.
[0008] The automotive industry uses several alloys for
manufacturing automotive exhaust system components. These alloys
range from low cost materials with moderate corrosion resistance to
high cost, highly alloyed materials with much greater corrosion
resistance. A relatively low cost alloy with moderate corrosion
resistance is AISI Type 316Ti (UNS Designation S31635). Type 316Ti
stainless steel corrodes more rapidly when exposed to elevated
temperatures and, therefore, is not generally used in automotive
exhaust system flexible connectors when temperatures are greater
than approximately 1200.degree. F. (649.degree. C.). Type 316Ti is
typically only used for automotive exhaust system components which
do not develop high exhaust temperatures.
[0009] Higher cost, more highly alloyed materials are commonly used
to fabricate flexible connectors for automotive exhaust systems
exposed to higher temperatures. A typical alloy used in the
manufacture of flexible connectors that are subjected to elevated
temperature corrosive environments is the austenitic nickel-base
superalloy of UNS Designation N06625, which is sold commercially
as, for example, Allegheny Ludlum ALTEMP.RTM. 625 (hereinafter "AL
625") alloy. AL 625 is an austenitic nickel-based superalloy
possessing excellent resistance to oxidation and corrosion over a
broad range of corrosive conditions and displaying excellent
formability and strength. Alloys of UNS Designation N06625
generally comprise, by weight, approximately 20-25% chromium,
approximately 8-12% molybdenum, approximately 3.5% niobium, and 4%
iron. Although alloys of this type are excellent choices for
automotive exhaust system flexible connectors, they are quite
expensive compared to Type 316Ti alloys.
[0010] Automotive exhaust system component manufacturers may use
other alloys for constructing exhaust system flexible connectors.
However, none of those alloys provide high corrosion resistance,
especially when exposed to elevated temperatures and corrosive
contaminants such as road deicing salts.
[0011] Thus, there exists a need for a corrosion resistant material
for use in high temperature corrosive environments that is not as
highly alloyed as, for example, alloys of UNS Designation N06625
and which, therefore, is less costly to produce than such
superalloys. More particularly, there exist a need for an iron-base
alloy which may be formed into, for example, light-weight flexible
connectors and other components for automotive exhaust systems and
which will resist corrosion from corrosive substances such as salt
deposits and other road deicing products at elevated
temperatures.
SUMMARY OF THE INVENTION
[0012] The present invention addresses the above described needs by
providing an austenitic stainless steel comprising, by weight, 19
to 23% chromium, 30 to 35% nickel, 1 to 6% molybdenum and less than
0.8% silicon. The addition of molybdenum to the iron-base alloys
increases their resistance to corrosion at high temperatures.
[0013] Unless stated otherwise, all composition percentages herein
are weight percentages based on total weight of the alloy.
[0014] The present invention also provides an austenitic stainless
steel consisting essentially of, by weight, 19 to 23% chromium, 30
to 35% nickel, 1 to 6% molybdenum, 0 to 0.1% carbon, 0 to 1.5%
manganese, 0 to 0.05% phosphorus, 0 to 0.02% sulfur, less than 0.8%
silicon, 0.15 to 0.6% titanium, 0.15 to 0.6% aluminum, 0 to 0.75%
copper, iron, and incidental impurities.
[0015] The present invention further provides an austenitic
stainless steel comprising, by weight, 9 to 23% chromium, 30 to 35%
nickel, 1 to 6% molybdenum, 0 to 0.03% titanium, 0.15% to 0.6%
aluminum, up to 0.1% carbon, 1 to 1.5% manganese, 0 to less than
0.8% silicon, 0.25 to 0.6% niobium, iron, and incidental
impurities.
[0016] The present invention additionally provides an austenitic
stainless steel consisting essentially of, by weight, 19 to 23%
chromium, 30 to 35% nickel, 1 to 6% molybdenum, 0 to 0.03%
titanium, 0.15 to 0.6% aluminum, up to 0.1% carbon, 1 to 1.5%
manganese, 0 to less than 0.8% silicon, 0.25 to 0.6% niobium, 0 to
0.75% copper, up to 0.05% phosphorus, up to 0.02% sulfur, up to 0.1
% nitrogen, iron, and incidental impurities.
[0017] Certain embodiments of austenitic stainless steels according
to the present invention exhibit enhanced resistance corrosion by
salt at a broad temperature range up to at least 1500.degree. F.
(816.degree. C.). Articles of manufacture of the austenitic
stainless steels as described above are also provided by the
present invention. Thus, the stainless steels of the present
invention would find broad application as, for example, automotive
components and, more particularly, as automotive exhaust system
components and flexible connectors, as well as in other
applications in which corrosion resistance is desired. The alloys
of the present invention exhibits excellent oxidation resistance at
elevated temperatures and therefore, finds broad application in
high temperature applications, such as heating element sheaths.
[0018] The present invention also provides methods of fabricating
an article of manufacture from austenitic stainless steels
comprising, by weight, 19 to 23% chromium, 30 to 35% nickel, 1 to
6% molybdenum, and less than 0.8% silicon.
[0019] The present invention additionally provides methods of
fabricating an article of manufacture wherein the method comprises
forming at least a portion of the article of manufacture from an
austenitic stainless steel comprising, by weight, 19 to 23%
chromium, 30 to 35% nickel, 1 to 6% molybdenum, 0 to 0.03%
titanium, 0.15 to 0.6% aluminum, up to 0.1% carbon, 1 to 1.5%
manganese, 0 to less than 0.8% silicon, 0.25 to 0.6% niobium, iron,
and incidental impurities. Non-limiting examples of articles of
manufacture that may be made using such method of the present
invention include an automobile, an automotive exhaust system
component, an automotive exhaust system flexible connector, a
heating element sheath, and a gasket.
[0020] The reader will appreciate the foregoing details and
advantages of the present invention, as well as others, upon
consideration of the following detailed description of embodiments
of the invention. The reader also may comprehend such additional
details and advantages of the present invention upon making and/or
using the stainless steels of the present invention.
BRIEF DESCRIPTION OF THE FIGURES
[0021] The features and advantages of the present invention may be
better understood by reference to the accompanying figures in
which:
[0022] FIG. 1 is a graph of weight change data comparing the
results of hot salt corrosion testing of flat coupon samples of an
alloy of the present invention (Sample 2) and prior art alloys
coated with 0.0, 0.05 and 0.10 mg/cm.sup.2 salt layers and exposed
for 72 hours to 1200.degree. F. (649.degree. C.);
[0023] FIG. 2 is a graph of weight change data comparing the
results of hot salt corrosion testing of flat coupon samples of an
alloy of the present invention (Sample 2) and prior art alloys
coated with 0.0, 0.05 and 0.10 mg/cm.sup.2 salt layers and exposed
for 72 hours to 1500.degree. F. (816.degree. C.);
[0024] FIG. 3 is a graph of weight change data comparing the
results of hot salt corrosion testing of welded teardrop samples of
an alloy of the present invention (Sample 2) and prior art alloys
coated with a nominal 0.10 mg/cm.sup.2 salt layer and exposed to
1200.degree. F. (649.degree. C.);
[0025] FIG. 4 is a graph of weight change data comparing the
results of hot salt corrosion testing of welded teardrop samples of
an alloy of the present invention (Sample 2) and prior art alloys
coated with a nominal 0.10 mg/cm.sup.2 salt layer and exposed to
1500.degree. F. (816.degree. C.);
[0026] FIG. 5 is a graphical illustration of a typical corroded
metal sample illustrating terms results of analysis procedure of
ASTM G54--Standard Practice for Simple Static Oxidation
Testing;
[0027] FIG. 6 is a depth of penetration graph comparing the results
of measurements taken according to ASTM G54 for welded teardrop
samples with a nominal 0.10 mg/cm.sup.2 salt coating exposed to
1200.degree. F. (649.degree. C.) for a sample of the alloy of the
present invention (Sample 2) and prior art alloys;
[0028] FIG. 7 is a depth of penetration graph comparing the results
of measurements taken according to ASTM G54 for welded teardrop
samples with a nominal 0.10 mg/cm.sup.2 salt coating exposed to
1500.degree. F. (816.degree. C.) for a sample of the alloy of the
present invention (Sample 2) and prior art alloys; and
[0029] FIGS. 8-12 are micrographs of alloy specimens including
varying levels of titanium and niobium, and which were prepared as
described in Example 2.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0030] The present invention provides austenitic stainless steels
resistant to corrosion at elevated temperatures. The corrosion
resistant austenitic stainless steels of the present invention find
particular application in the automotive industry and, more
particularly, in automotive exhaust system components. Austenitic
stainless steels are alloys including iron, chromium and nickel.
Typically, austenitic stainless steels are used in applications
requiring corrosion resistance and are characterized by a chromium
content above 16% and nickel content above 7%.
[0031] In general, the process of corrosion is the reaction of a
metal or metal alloy with their environment. The corrosion
resistance of a metal or alloy in a particular environment is
generally determined at least partly by its composition, among
other factors. The byproducts of corrosion are generally metal
oxides such as iron oxides, aluminum oxides, chromium oxide, etc.
The formation of certain oxides, particularly chromium oxide, on
stainless steel is beneficial and effectively prevents further
degradation of the underlying metal. Corrosion may be accelerated
by the presence of heat or corrosive agents.
[0032] Corrosion resistance of stainless steels used in automotive
applications is complicated by exposure to contamination from road
deicing salts under conditions of elevated temperature. This
exposure results in a complex form of corrosion due to the
interaction between the oxides which form at elevated temperatures
and the contaminating salts. Elevated temperature oxidation is
typified by the formation of protective oxides by reaction of the
metal directly with the oxygen in the air. The road deicing salts
which deposit on the automotive components may attack and degrade
the protective oxide layer. As the protective layer degrades, the
underlying metal is exposed to further corrosion. Halide salts,
particularly chloride salts, tend to promote localized forms of
attack such as pitting or grain boundary oxidation.
[0033] The present austenitic stainless steels include 1 to 6%
molybdenum by weight. Molybdenum is added as an alloying agent to
provide corrosion resistance, toughness, strength, and resistance
to creep at elevated temperatures. The austenitic stainless steels
of the present invention also include 19 to 23 weight percent
chromium, 30 to 35 weight percent nickel and less than 0.8 weight
percent silicon. The present austenitic stainless steels provides
better elevated temperature corrosion resistance than the prior art
type 316Ti alloys and, therefore, would enjoy more generalized
application as an automotive exhaust component. However, certain
alloys within the present invention provide this corrosion
resistance at a lower cost than the UNS Designation N06625 alloys
because, for example, the present invention is an iron-base alloy,
while the N06625 alloys are more expensive nickel-base
superalloys.
[0034] The austenitic stainless steels of the present invention
preferably contain greater than 2 weight percent of molybdenum.
Another preferred embodiment of the present invention includes less
than 4 weight percent molybdenum. This concentration of molybdenum
provides improved corrosion resistance at a reasonable cost.
Certain embodiments of alloys within the present invention may
optionally contain additional alloying components, such as, for
example, manganese, phosphorous, sulfur, and copper. Certain
embodiments of the stainless steel of the present invention also
may contain, for example, from 0.15 to 0.6 weight percent titanium,
0.15 to 0.6 weight percent aluminum, and other incidental
impurities.
[0035] Electric heat element sheaths typically comprise a
resistance conductor enclosed in a metal sheath. The resistance
conductor may be supported within and electrically insulated from
the sheathing by a densely packed layer of refractory,
heat-conducting material. The resistance conductor may generally be
a helically wound wire member while the refractory heat-conducting
material may be granular magnesium oxide.
[0036] Examples of alloys within the present invention follow.
EXAMPLE 1
[0037] Certain embodiments of stainless steels of the present
invention were prepared and evaluated for resistance to corrosion
in high temperature, corrosive environments. Two heats were melted
with a target composition including, by weight, 19 to 23% chromium
and 30 to 35% nickel. The first alloy had a target molybdenum
concentration of 2%, and the second alloy had a target molybdenum
concentration of 4%. The actual compositions of the heats of the
invention are shown in Table 1 as Sample 1 and Sample 2. Sample 1
contained 1.81% molybdenum and Sample 2 contained 3.54% molybdenum.
The alloy Samples 1 and 2 were prepared by a conventional method,
specifically, by vacuum melting the alloy components in
concentrations to approximate the target specification. The formed
ingots were then ground and hot rolled at approximately
2000.degree. F. (1093.degree. C.) to about 0.1 inches thick by 7
inches wide. The resulting plate was grit blasted and descaled in
an acid. The plate was then cold rolled to a thickness of 0.008
inches and annealed in inert gas. The resulting plate was formed
into both flat coupon and welded teardrop samples.
[0038] For comparison, additional commercially available alloys
were obtained and formed into flat coupon and welded teardrop
samples. Sample 3 was melted to specifications of a commercially
available AISI Type 332 (UNS Designation N08800) alloy. Type 332 is
an austenitic stainless steel characterized by a composition
similar to that of Samples 1 and 2, but includes no deliberately
added molybdenum. Type 332 is, generally, a nickel and chromium
stainless steel designed to resist oxidation and carburization at
elevated temperatures. The analysis of the Type 332 sample tested
is shown in Table 1. Type 332 typically is characterized as an
alloy comprising approximately 32 weight percent nickel and
approximately 20 weight percent chromium. Type 332 was chosen for
comparison purposes to determine the improvement offered by the
addition of molybdenum in Samples 1 and 2 to the corrosion
resistance in hot salt corrosion testing.
[0039] Also tested for comparison purposes were samples of AISI
Type 316Ti (UNS Designation S31635) (Sample 4) and AL 625 (UNS
Designation N06625) (Sample 5). These two alloys are currently
employed in flexible connectors for automotive exhaust systems
because they are formable and resist intermediate temperature
oxidation, general corrosion, and chloride stress corrosion
cracking, particularly in the presence of high levels of road
contaminants such as deicing salts. The composition of Samples 4
and 5 are shown in Table 1. AISI Type 316Ti is a low cost alloy
presently used in low temperature automotive exhaust system
flexible connector applications. AL 625, on the other hand, is a
higher cost material which presently finds broad application,
including use as automotive exhaust system flexible connectors
subjected to temperatures in excess of 1500.degree. F. (816.degree.
C.).
1TABLE 1 Chemical composition of test samples Sample 1 Sample 2
Sample 3 Sample 4 Sample 5 T332 + 2Mo T332 + 4Mo T332 T316Ti AL625
Alloy C 0.020 0.019 0.013 0.08 max. 0.05 N -- 0.0045 -- 0.10 max.
-- Al 0.34 0.30 0.55 0.30 Si 0.37 0.40 0.41 0.75 max. 0.25 Ti --
0.35 0.37 0.70 0.30 Cr 20.72 20.70 20.55 16-18 22.0 Mn 0.95 0.91
0.97 2 max. 0.30 Fe Bal Bal Bal Bal 4.0 Ni 31.07 30.74 31.19 10-14
Balance Nb + -- -- -- -- 3.5 Ta Mo 1.81 3.54 0.19 2-3 9.0
[0040] A test was devised to examine the elevated temperature
corrosion and oxidation resistance of the above samples in the
presence of deposited corrosive solids. Special corrosion tests
have been developed to simulate these high temperature corrosive
environments. Currently, most testing of alloy resistance to
corrosion from salt at elevated temperatures can be categorized as
a "cup" test or a "dip" test.
[0041] In the cup test a sample of alloy is placed in a cup,
generally of Swift or Erichsen geometry. The cup is then filled
with a known volume of aqueous test solution having known salt
concentration. The water in the cup is evaporated in an oven,
leaving a salt coating on the sample. The sample is then exposed to
elevated temperature under either cyclic or isothermal conditions
and the sample's resistance to salt corrosion is assessed. In the
dip test a sample, either flat or in a U-bend configuration, is
dipped in an aqueous solution having known salt concentration. The
water is evaporated in an oven, leaving a coating of salt on the
sample. The sample may then be assessed for resistance to salt
corrosion.
[0042] There are, however, problems with both of the above tests to
determine resistance to corrosion from salt. The results of the
test may be inconsistent and not easily compared from test to test
because the salt coating is not evenly distributed across the
extent of the surface to be tested or consistent between samples.
Using either the cup or dip tests, salt will generally be deposited
most heavily in the areas which are last to dry. In order to impose
a more uniform deposition of salt on the samples, a simple salt
application method was utilized by the present inventor. The method
comprised spraying an aqueous salt solution on a flat sample. An
even layer of salt may be deposited from an aerosol spray
consisting essentially of sodium chloride dissolved in deionized
water using this method. During deposition of the aerosol spray,
the samples are heated to approximately 300.degree. F. (149.degree.
C.) to ensure rapid, uniform evaporation of the water from the
aqueous solution. The amount of salt deposited is monitored by
weighing between sprays, and is reported as a surface concentration
(mg salt/cm.sup.2 surface area of sample). Calculations indicate
that the salt deposition may be controlled by careful use of this
method to approximately .+-.0.01 mg/cm.sup.2. After spraying, the
samples may be exposed to at least one 72-hour thermal cycle at an
elevated temperature in a muffle furnace in still lab air or any
other environmental conditions as desired. Preferably, a dedicated
test furnace and labware should be used for this test in order to
avoid cross-contamination from other test materials. After
exposure, the samples and any collected non-adherent corrosion
products are independently weighed. The results are reported as a
specific weight change relative to the original (uncoated) specimen
weight as previously described.
[0043] Flat coupons were initially tested since this is the
simplest method to screen alloys for susceptibility to hot salt
corrosion. The weight of each sample was determined before testing.
An even layer of salt was applied to 1 inch by 2 inch test samples
of each test alloy. A dilute aqueous solution of chloride salts
dissolved in deionized water was sprayed on each such sample. The
samples were preheated to approximately 300.degree. F. (149.degree.
C.) on a hot plate to ensure rapid, uniform evaporation of the
water from the solution. The amount of salt deposited on each
sample was monitored by weighing after each spraying. After
spraying, the samples were placed in high form alumina crucibles
and exposed in a muffle furnace to elevated temperatures to
1500.degree. F. (816.degree. C.). The typical exposure cycle was 72
hours at the elevated temperature in still lab air. After exposure
the specimens were weighed. Any non-adherent corrosion products
were collected and weighed separately. Any calculated weight gains
or losses of the samples are due to the reaction of metal species
with the atmosphere and any remaining salt from the coating. The
amount of applied salt is generally much less than the weight
change due to interaction with the environment, and as such can
generally be discounted.
[0044] The effects of residual stresses resulting from forming or
welding were also investigated. For this test, samples were formed
into welded "teardrop" samples. The "teardrop" samples were
fabricated by bending 0.062" thick flat samples into a teardrop
shape on a jig and then autogenously welding the mating edges.
Prior to exposure to the elevated temperatures, the samples were
coated with chloride salts using a method similar to that described
for coating the flat samples. The coatings on the teardrops were
not applied in a quantitative manner. However, the result of
coating was an even, uniform deposition of salt. It is estimated
that the amount of salt deposited on the outer surface of the
teardrop samples was approximately 0.05 to 0.10 mg/cm.sup.2. The
coated specimens were exposed in the automated thermogravimetric
cyclic oxidation laboratory setup. Every 24 hours the salt coating
on each sample was removed by evaporation and the samples were then
weighed so as to determine weight loss or gain caused by exposure
to the environment. After weighing, the salt coatings were
reapplied and the test was continued.
[0045] Table 2 summarizes the tests carried out on each of Samples
1 through 5.
2TABLE 2 Test specimen stock identification matrix Grade Coupon
testing Teardrop testing Sample 1 Present Invention -- -- Sample 2
Present Invention 0.008" thick 0.061" thick Sample 3 T-332 0.008"
thick 0.058" thick Sample 4 T-316Ti 0.008" thick 0.062" thick
Sample 5 AL625 0.008" thick 0.059" thick
Results from Corrosion Testing (Example 1)
[0046] Flat coupon testing was used to provide an initial measure
of performance and then welded teardrop tests were tested to
confirm flat coupon testing and expand the test results.
Flat Coupon Testing Results
[0047] Testing was conducted of flat coupon samples of four test
materials, samples 2 through 5 listed in Table 1, to determine the
affect of increased salt concentrations and increased temperatures
on the corrosion resistance of the alloy. Coupons of each
composition for samples 2 through 5 listed in Table 1 were tested
with no added salt coating and salt coatings of 0.05 mg/cm.sup.2
and 0.10 mg/cm.sup.2. The coupons were tested at two temperatures,
1200.degree. F. (649.degree. C.) and 1500.degree. F. (816.degree.
C.). The samples were weighed prior to being coated with salt to
determine their initial weight and then coated with the appropriate
amount of salt for each test and placed in a 1200.degree. F.
(649.degree. C.) environment to determine the resistance of each
alloy to hot salt oxidation corrosion. After 72 hours of exposure
to the elevated temperature, the samples were removed from the oven
and allowed to cool to room temperature. The salt remaining on the
sample was removed and the sample was weighed to determine the
final weight of the sample.
[0048] The results of the flat coupon sample hot oxidation
corrosion test are shown in FIG. 1. FIG. 1 is a graph of weight
change data comparing the results of hot salt corrosion testing of
flat coupon samples of an alloy of the present invention (Sample 2)
and prior art alloys coated with a 0.0, 0.5 and 0.10 mg/cm.sup.2
salt layer and exposed for 72 hours to 1200.degree. F. (649.degree.
C.). The change in weight was determined by subtracting the initial
weight of the sample by the final weight of the sample and, then,
dividing this result by the initial surface area of the flat coupon
sample.
[0049] All alloys performed well in this test at 1200.degree. F.
(649.degree. C.). Each sample of each alloy showed a slight weight
gain indicating the formation of an adherent oxidation layer. The
formation of this metal oxide layer protects the body of the
material if it remains adherent to the surface of the metal.
Generally, the samples showed a greater weight gain with an
increase in level of salt coating. This result indicates increased
levels of oxidation on the surface of the sample with increased
salt concentrations. T316Ti, Sample 4, showed the greatest weight
gain of over 1 mg/cm.sup.2 while the tested alloy of the present
invention, Sample 2, and the T332, Sample 3, showed the least
weight gain of less than 0.5 mg/cm.sup.2.
[0050] A similar test was conducted on the same samples at
1500.degree. F. (816.degree. C.) and the results are shown in FIG.
2. The low temperature application alloy T-316Ti performed poorly,
as expected. Heavy spalling was noted and the coupons coated with
0.05 and 0.10 mg/cm.sup.2 lost over 10 mg per square centimeter of
initial surface area. This test confirmed that T-316Ti is
unsuitable for use in elevated temperature applications, above
1200.degree. F. (649.degree. C.), and confirmed the reliability of
the test method developed for comparing resistance of the alloys to
hot salt oxidation. All other tested alloys performed well. T-332,
Sample 3, showed weight loss of about 1.3 mg/cm.sup.2 under the
test conditions. The higher cost AL625 superalloy, Sample 5,
exhibited a weight gain of approximately 1.7 mg/cm.sup.2 under
these test conditions. This weight gain is consistent with the
formation of the protective layer of metal oxides on the surface of
the alloy and minimal spalling of this protective layer. The alloy
of the present invention, Sample 2, exhibited almost no weight
change under the test conditions. The presence of about 4 weight
percent molybdenum in Sample 2 increased the hot salt corrosion
resistance of the alloy of the invention relative to the prior art
T-332 alloy, Sample 3. Sample 3 showed almost no weight change for
the sample without a salt coating or with a coating of 0.05
mg/cm.sup.2. However, when exposed to a salt concentration of 0.10
mg/cm.sup.2, Sample 3 showed a degradation of the protective
oxidation layer and a weight loss of greater than 1.5
mg/cm.sup.2.
[0051] The alloy of the present invention displayed a strong
resistance to hot salt oxidation corrosion in this testing. The
molybdenum concentration in Sample 2 increased the corrosion
resistance of the alloy over the corrosion resistance of the T332
alloy, Sample 3.
Welded Tear Drop Testing Results
[0052] Welded tear drop testing was consistent with the flat coupon
testing. The results of the welded teardrop testing are reported in
percentage of weight change. The coupons were weighed initially and
periodically throughout the extended period of testing, over 200
hours. FIGS. 3 and 4 are graphs of the weight change data comparing
the results of hot salt corrosion testing of welded teardrop
samples of an alloy of the present invention (Sample 2) and prior
art alloys coated with a nominal 0.10 mg/cm.sup.2 salt layer and
exposed to 1200.degree. F. (649.degree. C.) and 1500.degree. F.
(816.degree. C.), respectively. On both figures, it can be easily
recognized that T316Ti again performed very poorly and proved to be
an unacceptable alloy for elevated temperature corrosive
environments. All other tested samples were substantially
equivalent in performance as shown in both FIGS. 3 and 4. The
tested alloy of the present invention, Sample 2, displayed the
greatest resistance to corrosion under these conditions with less
than 1% weight loss and no additional weight change after
approximately the first 30 hours of the test. This compares
favorably with the performance of the higher performance prior art
alloy AL625, Sample 5, which lost approximately 3% of its initial
weight over the length of testing at 1500.degree. F. (816.degree.
C.). The tested alloy of the present invention better resisted hot
salt oxidation compared with the other tested alloys.
[0053] Weight change information alone is generally an incomplete
parameter for measuring the total effect of degradation in a highly
aggressive environment. Attack in highly aggressive environments,
such as in hot salt oxidation corrosion, is often irregular in
nature and can compromise a significantly larger portion of the
cross-section of an alloy component than would appear to be
affected from analysis of weight change data alone. Therefore,
metal loss (in terms of percentage of remaining cross-section) was
measured in accordance with ASTM-G54 Standard Practice for Simple
Static Oxidation Testing. FIG. 5 illustrates the definitions of the
parameters derived from this analysis. Test Sample 30 has an
initial thickness, T.sub.o, shown as distance 32 in FIG. 5. The
percentage of metal remaining is determined by dividing the
thickness of the test sample after exposure to the corrosion
testing, T.sub.ml, shown as distance 34, by the initial thickness,
32. The percentage of unaffected metal is determined by dividing
the thickness of the test sample showing no signs of corrosion,
T.sub.m, shown as distance 36 in FIG. 4, by the initial thickness,
32. These results give a better indication than simple weight loss
measurements as to when corrosion will totally degrade the metal
coupon.
[0054] The results of the metallographic investigation are shown in
FIGS. 6 and 7. Analysis of the low temperature alloy, T-316Ti
(Sample 4), displayed significant corrosion under the both test
conditions, 1200.degree. F. (649.degree. C.) and 1500.degree. F.
(816.degree. C.). Only 25% of the initial cross-section remained in
the T316Ti coupon after testing at 1500.degree. F. (816.degree.
C).
[0055] The other tested alloys performed well at 1200.degree. F.
(649.degree. C.), greater than 90% of the initial material remained
unaffected for Samples 2, 3 and 5. The results of analysis of the
coupons after exposure to 1500.degree. F. (816.degree. C.)
indicated that the higher cost nickel-base AL625 superalloy Sample
5, experienced low percentage loss of initial thickness but began
to exhibit the formation of pitting, as indicated by the difference
between the percentage of remaining cross-sectional area,
approximately 93%, and the percentage of unaffected metal,
approximately 82%. Localized pitting of the material as indicated
by the results of analysis according to ASTM-G54 procedures
provides data indicating the potential for localized failure of the
material. The coupon comprised of T332 alloy also showed slight
pitting after exposure to 1500.degree. F. (816.degree. C.) with
less than 75% of the initial material remained unaffected.
[0056] The alloy of the present invention, Sample 2, showed the
greatest percentage of unaffected area remaining after testing at
both temperatures. This result indicates that the molybdenum
retards the degradation and separation of the protective oxidation
layer. The remaining cross-section and the percentage of unaffected
area remaining after testing are approximately equal, about 90%.
This indicates that hot salt corrosion of the alloys of the present
invention is uniform across the surface of the test coupon and that
premature failure should not occur due to localized failure.
Conversely, this type of localized corrosion was exhibited by the
prior art T-332 alloy, Sample 3. The analysis of Sample 3 indicated
slight pitting, a potential for localized failure.
EXAMPLE 2
[0057] Austenitic stainless steels can be subject to sensitization
when exposed to high temperatures. As is known in the art,
sensitization is the intergranular precipitation of chromium
carbides in austenitic stainless steel when the steel is exposed to
temperatures in the approximate range of 800-1500.degree. F.
(427-816.degree. C.). A result of sensitization is that regions of
the affected grains are depleted in chromium content, promoting
susceptibility to intergranular corrosion in the presence of
aqueous chlorides. In order to investigate the susceptibility to
sensitization of alloys within the present invention, the present
inventor prepared and tested five 50 lb. VIM heats having the
chemical compositions shown in Table 3. Table 3 identifies the
heats as Heats 6-10 so as to distinguish them from Samples 1-5 in
above Example 1. The heats included varying additions of the
carbide-forming elements titanium and niobium. Heat 6 was
formulated with an aim of zero titanium and zero niobium, and was
found to include residuals levels of 0.002% titanium and 0.003%
niobium. Heat 7 was formulated as a titanium-stabilized heat with
an aim of 0.3% titanium and zero niobium, and was found to include
0.320% titanium and 0.003% niobium. Thus, Heat 7 represented an
alloy similar in composition to Sample 2 in Example 1 above. Heats
8-10 were formulated to include varying levels of addition of
niobium and an aim of zero titanium, and were found to include
0.24-0.46% niobium and a residuals level of 0.002% titanium.
Accordingly, the susceptibility to sensitization of Heat 6, which
was substantially free of both titanium and niobium, and Heat 7,
which was titanium-stabilized and substantially free of niobium,
were compared with that of Heats 8-10, which included significant
niobium and were substantially free of titanium.
3TABLE 3 Chemical composition of test samples Heat 6 Heat 7 Heat 8
Heat 9 Heat 10 C 0.020 0.04 0.02 0.02 0.03 Mn 1.01 1.00 1.01 1.01
1.00 P 0.002 0.002 0.002 0.002 0.002 Si 0.41 0.41 0.40 0.40 0.53 Cr
21.74 21.62 21.66 21.64 21.43 Ni 33.43 33.83 33.43 33.4 33.42 Al
0.28 0.32 0.32 0.32 0.24 Mo 2.40 2.39 2.39 2.39 2.39 Cu 0.010 0.010
0.010 0.010 0.010 Ti 0.002 0.320 0.002 0.002 0.002 N 0.016 0.020
0.020 0.021 0.026 Cb 0.003 0.003 0.24 0.36 0.46
[0058] Each of the five heats was rolled to 0.075 inch thickness
and solution annealed at 2050.degree. F. (1121.degree. C.) for 2
minutes time-at-temperature. Samples were sheared from each of the
annealed finished panels and tested for sensitization according to
the ASTM A262 (Practice A) test procedure, as revised in 2002. As
is known in the art, the ASTM A262 (Practice A) test procedure
involves deliberately exposing samples to a sensitizing heat
treatment (1 hour at a 1200.degree. F. (649.degree. C.) furnace
temperature), and then mounting, polishing and etching the samples
to reveal the microstructure of each sample. The samples are then
compared to reference micrographs, and each sample's revealed
microstructure is classified as being in one of the following three
categories:
[0059] "Step"--grain boundaries are revealed, normal structure
[0060] "Ditch"--grain boundaries are heavily etched, revealing
sensitization
[0061] "Mixed--some amount of both step and ditch structures
present
[0062] The results of the sensitization tests are provided in Table
4. Representative micrographs of the observed microstructures of
the samples from Heat 6 and Heat 7 are shown in FIGS. 8 and 9,
respectively. FIGS. 11-12, respectively, are representative
micrographs of the observed structures of the samples from Heats 8
(lowest level of intentional niobium addition), 9, and 10 (highest
level of intentional niobium addition. The micrographs of FIGS.
10-12 appear essentially the same despite significant variation in
the niobium content.
4TABLE 4 Sensitization test results Heat 6 Heat 7 Heat 8 Heat 9
Heat 10 Observed Ditch Mixed, biased Step Step Step Structure to
ditch
[0063] It is apparent from the results in Table 4 that the addition
of niobium in Heats 8-10 substantially inhibited sensitization as
measured by ASTM A262, Practice A, even though those heats included
only very low levels of titanium. Moreover, all niobium levels in
Heats 8-10 exhibited a step structure, indicating no significant
level of sensitization. In contrast, sensitization occurred in the
material of Heat 6, which substantially lacked both titanium and
niobium. Although Heat 7 included titanium in an amount similar to
the 0.34% level in Sample 2 of Example 1 above, Heat 7 exhibited a
microstructure biased to a ditch structure and, thus, had an
observable level of sensitization. The Heat 7 sample exhibited more
ditching of grain boundaries than not, indicating severe but not
total sensitization. Thus, an unexpected and surprising result of
the tests is that by modifying the composition of Heat 7 to
substitute an addition of niobium for all or substantially all of
the titanium in Heat 7, the resulting alloys, embodied in Heats
8-10, were not subject to sensitization at a level observable in
the tests.
[0064] Accordingly, it was determined that niobium more effectively
prevents sensitization than titanium in austenitic stainless steels
of the type tested. The addition of too high a level of niobium may
result in over-stabilized material, wherein the excess stabilizing
element produces inclusions that may detrimentally affect, for
example, corrosion, mechanical properties, fatigue life, surface
finish, and formability. On the other hand, the addition of too
little niobium may produce an under-stabilized material. It is
believed that providing at least 0.25% and up to 0.6% niobium in,
for example, an alloy having the general composition of Sample 2 in
Example 1, will significantly reduce sensitization without
significantly impairing other important properties of the alloy.
Although it does not appear necessary to include titanium in the
alloys, it is believed that alloys of the present invention
including 0.25-0.6% niobium can tolerate the presence of titanium
up to 0.03% and exhibit improved sensitization properties. It also
appears from the sensitization test results that a
carbon-to-niobium ratio of about 1:10 provides sufficient
stabilization to significantly inhibit sensitization.
[0065] The improved sensitization performance of Heats 8-10 should
manifest itself in the form of improved corrosion resistance at
high temperatures in the presence of aqueous chlorides. An
additional advantage of substituting niobium for some or all
titanium is that there may be no need for a stabilizing anneal (an
intermediate temperature heat treatment designed to pre-form
stabilizing carbides), thereby allowing standard solution or
mill-annealed material to be used without the danger of
sensitization during service.
[0066] Considering the above observations in Example 2, one aspect
of the present invention is directed to an austenitic stainless
steel comprising, by weight, 19% to 23% chromium, 30% to 35%
nickel, 1% to 6% molybdenum, 0 to 0.03% titanium, 0.15% to 0.6%
aluminum, up to 0.1% carbon, 1% to 1.5% manganese, 0 to less than
0.8% silicon, 0.25% to 0.6% niobium, and iron. For ease of
reference only, such alloy is referred to hereinafter as the
"niobium-containing stainless steel of the present invention" or,
more simply, as the "niobium-containing stainless steel".
[0067] In certain embodiments, the niobium-containing stainless
steel of the present invention includes 0.3% to 0.5% niobium. It is
believed that a niobium content within this range provides a
further cushion against the possibility of under-and
over-stabilization, while still providing improved sensitization
properties.
[0068] At least 19% chromium is present in the niobium-containing
stainless steel to provide a basic level of corrosion and high
temperature oxidation resistance. If chromium is present at too
high a level, then it can be difficult to adjust carbon to desired
levels, the tendency for second phase formation increases, and the
cost and difficulty of making the alloy increases. Accordingly, in
certain forms, the niobium-containing stainless steel of the
present invention includes 19% to 21.5% chromium, and may include
about 21% chromium.
[0069] Increasing molybdenum content enhances resistance to
corrosion and, in particular, localized corrosion such as pitting
and crevice corrosion. The addition of molybdenum is generally more
effective at improving pitting/crevice corrosion than the addition
of chromium. Adding too high a level of molybdenum, however,
results in sigma phase formation at temperatures greater than about
1000.degree. F. (538.degree. C.). Sigma phase reduces corrosion
resistance and can make the alloy brittle at room temperature. In
addition, molybdenum is relatively expensive. Thus, in general, the
level of molybdenum should be minimized while still providing the
desired level of corrosion resistance. Accordingly, certain
embodiments of the niobium-containing stainless steel of the
present invention may include 2% to 4% molybdenum, while other
embodiments include 1% to 2.7% molybdenum. In one form, the
stainless steel includes about 2.5% molybdenum.
[0070] When present at high levels, titanium causes surface
defects. Titanium also forms inclusions in the presence of carbon
and nitrogen, which adversely affects formability and fatigue
resistance. Accordingly, in certain embodiments, the titanium
content of the niobium-containing stainless steel of the present
invention is restricted to the range of 0 to 0.01%, while in other
embodiments is restricted to 0 to 0.005%.
[0071] Carbon content dictates the amount of carbides that will
form when carbon solubility is exceeded. The addition of carbon
beyond the solubility limit is generally accompanied by increasing
levels of stabilizing elements, such as titanium and niobium, so as
to form an excess of carbides, which enhance high temperature creep
strength. Such higher carbon additions, however, can adversely
affect the ability to roll to thin gauge, harm formability, and
reduce fatigue strength. Accordingly, certain embodiments of the
niobium-containing stainless steel include no more than 0.03%
carbon. Other embodiments include no more than 0.025% carbon.
Certain embodiments of the niobium-containing stainless steel also
may include one or both of 0.15% to 0.4% aluminum, and up to 0.4%
silicon. In certain forms, the niobium-containing stainless steel
includes one or more of about 0.30% aluminum, about 0.020% carbon,
and about 0.30% silicon.
[0072] Copper increases resistance to certain types of corrosion,
such as corrosion in reducing environments like dilute sulfuric
acid. High copper levels, however, can result in formation of
undesirable second phases. Accordingly, the niobium-containing
stainless steel may include up to 0.75% copper, while certain
embodiments of the steel may include up to 0.4% copper. In one
form, the niobium-containing stainless steel includes about 0.3%
copper.
[0073] Sulfur content preferably is minimized to avoid adversely
affecting hot workability. Phosphorus is an impurity that can
adversely affect properties at too high a level. Accordingly, in
certain forms, the niobium-containing stainless steel is limited to
no greater than 0.05% phosphorus and/or no greater than 0.02%
sulfur.
[0074] Nitrogen generally increases strength, austenite stability
(for example, resistance to sigma formation) and corrosion
resistance. Too high a level of nitrogen, however, may tie up
niobium and reduce resistance to sensitization, and also may form
inclusions. Accordingly, in certain embodiments, the
niobium-containing stainless steel includes no more than 0.1%
nitrogen, in certain other embodiments includes no more than 0.025%
nitrogen, and in one form includes about 0.020% nitrogen.
[0075] Considering advantages that may be derived from certain of
the foregoing modifications to the broad composition for the
niobium-containing stainless steel, an additional aspect of the
present invention is directed to an austenitic stainless steel
comprising, by weight, 19% to 21.5% chromium, 30% to 35% nickel, 1%
to 2.7% molybdenum, 0 to 0.03% titanium, 0.15% to 0.4% aluminum, up
to 0.025% carbon, 1% to 1.5% manganese, 0 to less than 0.8%
silicon, 0 to 0.75% copper, 0.25% to 0.6% niobium, and iron. In one
form the niobium-containing stainless steel includes, by weight,
21.5% chromium, 34.5% nickel, 2.5% molybdenum, 0.02% carbon, 1.2%
manganese, no greater than 0.03% titanium, 0.5% niobium, up to
0.05% phosphorus, up to 0.02% sulfur, 0.30% silicon, 0.30%
aluminum, 0.30% copper, 0.020% nitrogen, iron and incidental
impurities.
[0076] Again taking into consideration the results of Example 2, a
further aspect of the present invention is directed to an
austenitic stainless steel including molybdenum and niobium and
consisting essentially of, by weight, 19% to 23% chromium, 30% to
35% nickel, 1% to 6% molybdenum, 0 to 0.03% titanium, 0.15% to 0.6%
aluminum, up to 0.1% carbon, 1% to 1.5% manganese, 0 to less than
0.8% silicon, 0.25% to 0.6% niobium, 0 to 0.75% copper, up to 0.05%
phosphorus, up to 0.02% sulfur; up to 0.1% nitrogen, iron and
incidental impurities. Incidental impurities may include, for
example, residual levels of impurities derived from scrap and other
materials from which the alloys are produced. Given the above
possible modifications to the composition of the niobium-containing
stainless steel, another form of the present invention is directed
to an austenitic stainless steel including molybdenum and niobium
and consisting essentially of, by weight, 19% to 21.5% chromium, 30
to 35% nickel, 1% to 2.7% molybdenum, 0 up to 0.03% titanium, 0.15%
to 0.4% aluminum, up to 0.025% carbon, 1 % to 1.5% manganese, 0 to
less than 0.8% silicon, 0.25% to 0.6% niobium, up to 0.05%
phosphorus, up to 0.02% sulfur, up to 0.1% nitrogen, iron and
incidental impurities.
[0077] It will be understood that the present invention also
encompasses articles of manufacture made wholly or partially from
austenitic stainless steels as set forth in the present disclosure,
and further encompasses methods of making such articles. Without
intending to limit the possible embodiments of such articles of
manufacture, examples of articles that may include the austenitic
stainless steel described herein and that may be made by such
methods include automobiles, automotive exhaust system components
(such as , for example, automotive exhaust system flexible
connectors), heating element sheaths, and gaskets. Those having
ordinary skill may readily design a suitable process for producing
such articles of manufacture using the stainless steels of the
present invention.
[0078] Given the corrosion resistance properties of the austenitic
stainless steels described in this Example 2, it is believed that
the steels would be particularly well suited for application as
automotive exhaust system flexible connectors. Material with a
relatively fine grain size is required when fabricating automotive
exhaust flexible connectors and other light gauge articles.
Material having to coarse a grain size would not form well in the
hydroforming process typically used to fabricate automotive exhaust
system flexible connectors. Accordingly, niobium-containing
stainless steel of the present invention having an ASTM grain size
number of 7 or higher (for example, 8-10) would be used to form
such flexible connectors.
[0079] When producing automotive exhaust system flexible connectors
from the niobium-containing stainless steel, the steel may be made
by electric furnace/AOD melting, casting, hot rolling, and then
multi-stage rolling on a cluster mill to light gauge. The light
gauge material may be bright annealed and slit to a relatively
narrow strip having a thickness of, for example, 0.006-0.010 inch.
The continuous coil of material is welded into a tube on an
automated tube mill, and then hydroformed into a corrugated
flexible connector bellows. This requires that the material have a
consistent edge, a relatively clean and stabilized microstructure
free of gross defects, a surface free of scale, and high intrinsic
ductility and fracture toughness. Those having ordinary skill will
be familiar with suitable methods of processing material for use as
automotive exhaust system flexible connectors. Accordingly, further
description of such methods is considered unnecessary.
[0080] It is to be understood that the present description
illustrates those aspects of the invention relevant to a clear
understanding of the invention. Certain aspects of the invention
that would be apparent to those of ordinary skill in the art and
that, therefore, would not facilitate a better understanding of the
invention have not been presented in order to simplify the present
description. Although the present invention has been described in
connection with certain embodiments, those of ordinary skill in the
art will, upon considering the foregoing description, recognize
that many modifications and variations of the invention may be
employed. All such variations and modifications of the invention
are intended to be covered by the foregoing description and the
following claims.
* * * * *