U.S. patent application number 10/767841 was filed with the patent office on 2004-12-23 for high-chromium nitrogen containing castable alloy.
Invention is credited to Radon, Roman.
Application Number | 20040258554 10/767841 |
Document ID | / |
Family ID | 34826538 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040258554 |
Kind Code |
A1 |
Radon, Roman |
December 23, 2004 |
High-chromium nitrogen containing castable alloy
Abstract
A corrosion and erosion resistant alloy comprising as mandatory
elements besides iron, in % by weight, about 31 to about 48
chromium, about 0.01 to about 0.7 nitrogen, about 0.5 to about 30
manganese and about 0.3 to about 2.5 carbon. This abstract is
neither intended to define the invention disclosed in this
specification nor intended to limit the scope of the invention in
any way.
Inventors: |
Radon, Roman; (Belleview,
FL) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1950 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Family ID: |
34826538 |
Appl. No.: |
10/767841 |
Filed: |
January 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10767841 |
Jan 30, 2004 |
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10040357 |
Jan 9, 2002 |
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6761777 |
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Current U.S.
Class: |
420/12 ; 420/38;
420/57; 420/582 |
Current CPC
Class: |
C22C 38/42 20130101;
C22C 30/00 20130101; C22C 38/58 20130101; C22C 38/001 20130101;
C22C 38/52 20130101; C22C 38/02 20130101; C22C 38/54 20130101; C22C
38/44 20130101 |
Class at
Publication: |
420/012 ;
420/038; 420/057; 420/582 |
International
Class: |
C22C 030/02; C22C
038/58 |
Claims
What is claimed is:
1. A corrosion and erosion resistant alloy comprising, in % by
weight: from about 31 to about 48 chromium from about 0.01 to about
0.7 nitrogen from about 0.5 to about 30 manganese from about 0.3 to
about 2.5 carbon from 0 to about 5 boron from 0 to about 6
molybdenum from 0 to about 5 silicon from 0 to about 8 copper from
0 to about 4 cobalt from 0 to about 25 nickel plus cobalt, said
alloy further comprising from 0 to about 2% of each of zirconium,
vanadium, cerium, titanium, tantalum, tungsten, niobium, aluminum,
calcium and rare earth elements, the balance comprising iron and
inevitable impurities, said alloy having a microstructure
comprising chromium carbides, nitrides and optionally borides in an
austenitic matrix, said matrix having a face centered cubic crystal
structure and being supersaturated with nitrogen in solid solution
form, the composition of the alloy satisfying the relationship: 3 %
Ni + % Co + 0.5 ( % Mn + % Cu ) + 30 ( % N + % C ) + 5 x % B % Cr +
% Mo + % Si + 1.5 ( % Ti + % Ta + % V + % Nb + % Ce + % Al ) 1.5
.
2. The alloy of claim 1, wherein the alloy comprises of at least
one of molybdenum, silicon, boron, copper and (nickel plus cobalt),
each in an amount of at least about 0.01% by weight.
3. The alloy of claim 1, wherein the alloy comprises at least about
32% by weight of chromium.
4. The alloy of claim 3, wherein the alloy comprises of at least
one of molybdenum, silicon, boron, copper and (nickel plus cobalt),
each in an amount of at least about 0.01% by weight.
5. A corrosion and erosion resistant alloy comprising, in % by
weight: from about 32 to about 34 chromium from about 0.35 to about
0.45 nitrogen from about 6 to about 9 manganese from about 0.5 to
about 2.5 carbon from 0 to about 4.5 boron from 0 to about 5
molybdenum from 0 to about 3 silicon from 0 to about 4 copper from
0 to about 4 cobalt from 0 to about 4 nickel plus cobalt, said
alloy further comprising 0 to about 2% of each of zirconium,
vanadium, cerium, titanium, tantalum, tungsten, niobium, aluminum,
calcium and rare earth elements, the balance comprising iron and
inevitable impurities, said alloy having a microstructure
comprising chromium carbides, nitrides and optionally borides in an
austenitic matrix, said matrix having a face centered cubic crystal
structure and being supersaturated with nitrogen in solid solution
form, the composition of the alloy satisfying the relationship: 4 %
Ni + % Co + 0.5 ( % Mn + % Cu ) + 30 ( % N + % C ) + 5 x % B % Cr +
% Mo + % Si + 1.5 ( % Ti + % Ta + % V + % Nb + % Ce + % Al ) 1.5
.
6. The alloy of claim 5, wherein the alloy comprises, in % by
weight, one or more of the following: from about 2 to about 5
molybdenum from about 0.5 to about 3 silicon from about 0.7 to
about 4 copper from about 1.5 to about 4 nickel plus cobalt.
7. The alloy of claim 6, wherein the alloy comprises, in % by
weight: from about 2 to about 4 molybdenum from about 0.5 to about
2 silicon from about 0.7 to about 3 copper from about 1.5 to about
3 nickel plus cobalt.
8. The alloy of claim 6, wherein the alloy comprises at least about
0.01% by weight of boron.
9. A corrosion and erosion resistant alloy comprising, in % by
weight: from about 35 to about 40 chromium from about 0.4 to about
0.6 nitrogen from about 4.5 to about 15 manganese from about 0.8 to
about 1.6 carbon from 0 to about 5 boron from 0 to about 5
molybdenum from 0 to about 3 silicon from 0 to about 6 copper from
0 to about 4 cobalt from 0 to about 13 nickel plus cobalt, said
alloy further comprising from 0 to about 2% of each of zirconium,
vanadium, cerium, titanium, tantalum, tungsten, niobium, aluminum,
calcium and rare earth elements, the balance comprising iron and
inevitable impurities, said alloy having a microstructure
comprising chromium carbides, nitrides and optionally borides in an
austenitic matrix, said matrix having a face centered cubic crystal
structure and being supersaturated with nitrogen in solid solution
form, the composition of the alloy satisfying the relationship: 5 %
Ni + % Co + 0.5 ( % Mn + % Cu ) + 30 ( % N + % C ) + 5 x % B % Cr +
% Mo + % Si + 1.5 ( % Ti + % Ta + % V + % Nb + % Ce + % Al ) 1.5
.
10. The alloy of claim 9, wherein the alloy comprises, in % by
weight, one or more of the following: from about 2 to about 4
molybdenum from about 0.5 to about 2 silicon from about 1 to about
4 copper from about 4 to about 13 nickel plus cobalt.
11. The alloy of claim 9, wherein the alloy comprises, in % by
weight: from about 0.9 to about 1.6 carbon from about 5 to about 13
manganese from about 2 to about 4 molybdenum from 0 to about 4.5
boron from about 0.5 to about 1.5 silicon from about 1 to about 3
copper from about 0.01 to about 4 cobalt from about 4 to about 12.5
nickel plus cobalt.
12. The alloy of claim 10, wherein the alloy comprises, in % by
weight: from about 1 to about 1.55 carbon from about 5 to about 12
manganese from about 2 to about 3.5 molybdenum from 0 to about 4
boron from about 0.6 to about 1.2 silicon from about 1 to about 2.5
copper from about 0.02 to about 4 cobalt from about 4 to about 12
nickel plus cobalt.
13. The alloy of claim 1, which exhibits a PREN of from 58 to
66.
14. The alloy of claim 11, which exhibits a PREN of from 58 to
66.
15. The alloy of claim 13, wherein the matrix comprises from about
0.25% to about 0.45% by weight of nitrogen in solid solution
form.
16. The alloy of claim 1, wherein the alloy comprises, in % by
weight: from about 41 to about 48 chromium from about 0.45 to about
0.7 nitrogen from about 6 to about 30 manganese from about 0.9 to
about 1.5 carbon from 0 to about 3.5 boron from 0 to about 4
molybdenum from 0 to about 3 silicon from 0 to about 8 copper from
0 to about 25 nickel plus cobalt, the balance comprising iron and
inevitable impurities.
17. The alloy of claim 16, wherein the alloy comprises of at least
one of molybdenum, silicon, boron, copper and (nickel plus cobalt),
each in an amount of at least about 0.01% by weight.
18. The alloy of claim 17, wherein the alloy comprises, in % by
weight, one or more of the following: from about 1 to about 4
molybdenum from about 0.5 to about 3 silicon from about 1 to about
8 copper from about 10 to about 25 nickel plus cobalt.
19. The alloy of claim 18, wherein a PREN is from 51 to 72.
20. The alloy of claim 16, wherein the matrix comprises from about
0.25% to about 0.45% by weight of nitrogen in solid solution
form.
21. A casting which comprises the alloy of claim 1.
22. A casting which comprises the alloy of claim 12.
23. A part of a slurry pump which comprises the alloy of claim
1.
24. The part of claim 23, wherein the part comprises one of a
casing, impeller, suction liner, pipe, nozzle, agitator and a valve
blade.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 10/040,357 filed Jan. 9, 2002, the entire
disclosure whereof is expressly incorporated by reference
herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to the art of alloys and
more particularly to a high-chromium, nitrogen containing alloy
having high corrosion resistance. The instant invention also
relates to a high-chromium, nitrogen containing castable alloy, a
high-chromium nitrogen content alloy, and a process for producing
the high-chromium, nitrogen containing alloy, and articles prepared
from the same.
[0004] 2. Discussion of Background Information
[0005] Equipment used in highly corrosive environments typically is
made of alloys such as stainless steel and other highly alloyed
materials. These alloys must be able to withstand the extremely
corrosive environments created by chemicals such as concentrated
sulfuric acid or concentrated phosphoric acid. A particularly
difficult environment is encountered in the production of phosphate
fertilizer. In the digestion of phosphate rock with hot,
concentrated sulfuric acid, the equipment is required to be
resistant to the environment at temperatures up to about
100.degree. C. The crude phosphoric acid produced can be extremely
corrosive and contains some residual sulfuric acid. The corrosive
effect is often increased by other impurities in the phosphoric
acid, particularly by halogen ions such as chloride and fluoride,
which are normally present in the phosphate rock feedstock that is
used in the process. A particularly corrosive environment is
encountered during the concentration of the crude phosphoric
acid.
[0006] Phosphate rock deposits at various locations in the world
vary greatly in chemical composition. The most severe corrosive
environments are typically encountered in the processing of
deposits of phosphate rock which contains a high content of
halides, such as chloride or fluoride.
[0007] It is known that increasing the Cr content improves the
corrosion resistance of steel. Hi-chromium alloys containing 23-40%
Cr, 0.8-2% C, 2.5% Si, and up to 5% Mo, have been known since the
1930's. See, for example, German Patent No. 701,807. U.S. Pat. No.
5,252,149 represents an improvement of this alloy, followed by
German Patent Application Nos. 195 12 044 and 44 17 261. According
to both patents the alloys exhibit a high resistance to abrasion
and good resistance to corrosion. However, both alloys exhibit poor
mechanical properties, especially low toughness, brittleness,
sensitivity to heat, and low notched impact resistance, thereby
limiting their usefulness. It is evident that their structures
comprise ferrite (Fe-.alpha.).
[0008] The ferritic structure comprised in these alloys is
inherently very brittle, and the carbide phase embedded in such a
brittle phase results in a very low toughness, high notch
sensitivity, as well as sensitivity to heat. Additionally, the
ferritic structure is supersaturated with chromium, resulting in a
formation of the sigma phase, which drastically lowers toughness
and corrosion resistance.
[0009] U.S. Pat. No. 5,320,801 is directed to alloys having the
following composition in % by weight: Cr--27 to 34, Ni+Co--13 to
31, Si--3.2 to 4.5, Cu--2.5 to 4, C--0.7 to 1.6, Mn --0.5 to 1.5,
Mo --1 to 4, and Fe--essentially the balance. The alloy of the '801
patent possesses good toughness, but exhibits very poor hardness
and very poor wear resistance and low tensile strength. Its
hardness of 208 to 354 HB is similar to that of CD4MCU stainless
steel (260-350 HB), which has excellent corrosion resistance, but
poor wear resistance. The alloy disclosed in U.S. Pat. No.
5,320,801 is similar to austenitic, high-nickel stainless steels in
that it has good toughness, but very low tensile strength and
hardness, as well as poor wear resistance. The nickel present in
corrosion resistant alloys mainly assists in structural
stabilization but contributes very little to an improvement in
corrosion resistance. Representative examples thereof are the
stainless austenitic steels which contain 12-35% Ni and have a
corrosion resistance which approaches that of duplex stainless
steels which contain a low percentage of nickel (4-8%), or that of
high-chromium stainless steels with a Ni content of not more than
4%. The primary elements of stainless alloys are Cr, Mo and
nitrogen, as shown in the Examples below which illustrate how
various alloying elements influence the corrosion resistance of
stainless steel. For example, the Pitting Resistance Equivalent
Number (PREN)=% Cr+3.3.times.% Mo+16.times.% N illustrates that
nitrogen is an important, very powerful alloying element of
corrosion resistant alloys.
[0010] One of the main shortcomings of the high-chromium alloys of
the prior art is the difficulty in dissolving Cr, Mo and N in the
matrix without adversely affecting the mechanical properties of the
alloy, such as toughness, tensile strength, brittleness, heat
sensitivity and weldability. This difficulty is due to the
precipitation of the sigma phase from alloys which are saturated
with chromium and molybdenum. Premature wearing out of pump parts
made from the above-mentioned high-chromium alloys is a common
occurrence. The main contributing factors in this respect are very
low toughness, brittleness and low endurance. Very often a failure
occurs due to a casting which is worn thin in an isolated area
where, due to the poor mechanical properties of the alloy, a crack
developed. Eventually, this leads to the destruction of an
otherwise still viable component.
[0011] The mechanisms of corrosion and erosion in acidic
environments of the alloys of the prior art involve accelerated
corrosion due to the continuous removal of the passive corrosion
resistant layer by particles contained in corrosive fluids. This is
most evident in alloys which contain a higher content of Cr and Mo,
where a significant amount of sigma phase is unavoidable and the
metal matrix possesses very poor toughness properties. In order to
restore the passive layer, it is necessary to use as high as
possible concentrations of Cr and Mo.
[0012] An increase in the Cr/C, or (Cr+Mo)/C ratio increases the
corrosion resistance up to the critical point, after which the
formation of the sigma phase begins, which drastically reduces the
toughness and lowers the corrosion resistance of the alloy by
depleting the Cr in the vicinity of the sigma phase
precipitates.
[0013] The present invention provides an increase in the ratio
(Cr+N)/(C-N), or (Cr+Mo+N)/C and (Cr+Mo+N+B)/(C-N) by reducing the
carbon content in the matrix and introducing nitrogen as a powerful
additional alloying element to the high-chromium alloys, where it
is present in a high concentration in solid solution.
[0014] Nitrogen, like carbon, forms interstitial solids with
body-centered-cubic (bcc) .alpha.-iron, and face-centered-cubic
(fcc) .gamma.-iron. The size of the nitrogen atom is smaller than
that of the carbon atom, wherefor the nitrogen atom can occupy the
interstitial sites in the .alpha.-as well as in the .gamma.-phases
more easily than the carbon atom.
[0015] The maximum solubility of nitrogen in Fe-.alpha. and
Fe-.gamma. is several times higher than that of carbon at the same
temperature, which leads to a substantial expansion and distortion
of elementary lattices. Nitrogen has a solid solution hardening and
strengthening effect that is much greater than that of carbon,
while at the same time maintaining a higher level of toughness.
[0016] The solubility limits of nitrogen in the prior art
high-chromium alloys are a very low, 0.15% of N at the most. This
limit is dictated by the inherently low physico-chemical solubility
of nitrogen and carbon (0.02 to 0.08% max. for C+N) in the
Fe-.alpha. structure, which constitutes up to a maximum of 40% of
the alloys of German Patent Application Nos. 44 17 261 or 195 12
044 as well as by a low Mn content of .ltoreq.1.5
[0017] The addition of nitrogen is the most effective means for
improving the mechanical properties of austenitic high-chromium
alloys without deleteriously affecting the ductility and corrosion
resistance thereof. It has now been found that if Mn and/or Mo are
present in considerable amounts in high-chromium alloys, nitrogen
can be fully effective as an anti-corrosive agent, and may have a
wide range of positive effects on the mechanical properties of a
casting such as, e.g., increased tensile strength, hardness and
toughness, without giving rise to a loss in ductility. In
particular, under these conditions nitrogen dissolves in the solid
state two to four times better than in any other high-chromium
alloy of in the prior art. Similarly, in high-manganese stainless
steels, which dissolve up to 0.8% nitrogen, and even up to 1% under
high nitrogen partial pressure, the tensile strength and the
hardness may be increased by a factor of two to four, with as good
a ductility as for the same steel without nitrogen.
SUMMARY OF THE INVENTION
[0018] The present invention provides a corrosion and erosion
resistant alloy which comprises, in % by weight, about 31 to about
48 chromium, about 0.01 to about 0.7 nitrogen, about 0.5 to about
30 manganese, about 0.3 to about 2.5 carbon, 0 to about 5 boron, 0
to about 6 molybdenum, 0 to about 5 silicon, 0 to about 8 copper, 0
to about 4 cobalt and 0 to about 25 nickel plus cobalt. This alloy
further comprises 0 to about 2% of each of zirconium, vanadium,
cerium, titanium, tantalum, tungsten, niobium, aluminum, calcium
and rare earth elements, the balance comprising iron and inevitable
impurities. It has a microstructure which comprises chromium
carbides, nitrides and optionally borides in an austenitic matrix,
which matrix has a face centered cubic structure and is
supersaturated with nitrogen in solid solution form. The
composition of the alloy satisfies the relationship: 1 % Ni + % Co
+ 0.5 ( % Mn + % Cu ) + 30 ( % N + % C ) + 5 x % B % Cr + % Mo + %
Si + 1.5 ( % Ti + % Ta + % V + % Nb + % Ce + % Al ) 1.5 .
[0019] In one aspect, the alloy may comprise molybdenum, silicon,
boron, copper and/or (nickel plus cobalt), each in an amount of at
least about 0.01% by weight.
[0020] In another aspect, the alloy may have a PREN of from 58 to
66 and/or the matrix may comprise from about 0.25% to about. 0.45%
of nitrogen in solid solution form.
[0021] In yet another aspect, the alloy may comprise at least about
32% by weight of chromium.
[0022] In a still further aspect, the alloy may comprise about 32
to about 34 chromium, about 0.35 to about 0.45 nitrogen, about 6 to
about 9 manganese, about 0.5 to about 2.5 carbon, 0 to about 4.5
boron, 0 to about 5 molybdenum, 0 to about 3 silicon, 0 to about 4
copper, 0 to about 4 cobalt and 0 to about 4 nickel plus cobalt.
This alloy may comprise about 2 to about 5 molybdenum, about 0.5 to
about 3 silicon, about 0.7 to about 4 copper and/or about 1.5 to
about 4 nickel plus cobalt. For example, about 2 to about 4
molybdenum, about 0.5 to about 2 silicon, about 0.7 to about 3
copper and about 1.5 to about 3 nickel plus cobalt may be present
in this alloy. Moreover, the alloy may comprise at least about
0.01% by weight of boron.
[0023] In a still further aspect, the alloy of the present
invention may comprise, in % by weight, about 35 to about 40
chromium, about 0.4 to about 0.6 nitrogen, about 4.5 to about 15
manganese, about 0.8 to about 1.6 carbon, 0 to about 5 boron, 0 to
about 5 molybdenum, 0 to about 3 silicon, 0 to about 6 copper, 0 to
about 4 cobalt and 0 to about 13 nickel plus cobalt. This alloy may
comprise, for example, about 2 to about 4 molybdenum, about 0.5 to
about 2 silicon, about 1 to about 4 copper and/or about 4 to about
13 nickel plus cobalt, e.g., it may comprise about 0.9 to about 1.6
carbon, about 5 to about 13 manganese, about 2 to about 4
molybdenum, 0 to about 4.5 boron, about 0.5 to about 1.5 silicon,
about 1 to about 3 copper, about 0.01 to about 4 cobalt, and about
4 to about 12.5 nickel plus cobalt, or it may comprise about 1 to
about 1.55 carbon, about 5 to about 12 manganese, about 2 to about
3.5 molybdenum, 0 to about 4 boron, about 0.6 to about 1.2 silicon,
about 1 to about 2.5 copper, about 0.02 to about 4 cobalt and about
4 to about 12 nickel plus cobalt. This alloy may have a PREN is
from 58 to 66 and/or the matrix thereof may comprise from about
0.25% to about 0.45% by weight of nitrogen in solid solution
form.
[0024] In yet another aspect of the alloy of the present invention,
the alloy may comprise, in % by weight, about 41 to about 48
chromium, about 0.45 to about 0.7 nitrogen, about 6 to about 30
manganese, about 0.9 to about 1.5 carbon, 0 to about 3.5 boron, 0
to about 4 molybdenum, 0 to about 3 silicon, 0 to about 8 copper
and 0 to about 25 nickel plus cobalt. This alloy may comprise, for
example, molybdenum, silicon, boron, copper and/or (nickel plus
cobalt), each in an amount of at least about 0.01% by weight.
Particularly, this alloy may comprise about 1 to about 4
molybdenum, about 0.5 to about 3 silicon, about 1 to about 8 copper
and about 10 to about 25 nickel plus cobalt. Also, it may have a
PREN of from 51 to 72 and/or the matrix thereof may comprise from
about 0.25% to about 0.45% by weight of nitrogen in solid solution
form.
[0025] The present invention also provides a casting of the alloy
of the present invention, including the various aspects thereof.
For example, the casting may be a casing, impeller, suction liner,
pipe, nozzle, agitator or a valve blade.
[0026] The present invention provides a high-chromium alloy and,
more specifically, a corrosion and erosion resistant high-chromium,
nitrogen containing castable alloy. The alloy of the present
invention may be used, for example, for the manufacture by casting
of slurry pump parts, such as casings, impellers, suction liners,
pipes, nozzles, agitators, valve blades, in particular, for casting
parts will be exposed to highly corrosive fluids and abrasive
slurries. A typical application for such parts is in the wet
processing of phosphoric acid. Industrial phosphoric acid solutions
are chemically complex, containing sulfuric acid, hydrofluoric
acid, hydrochloric acid, chlorides, fluorides and gypsum, all
highly depassivating species which are highly detrimental to the
parts exposed thereto. Another application for such parts is in
power plant scrubbers i.e., flue gas desulfurization processes
where the parts are exposed to sulfuric components and gypsum.
[0027] One of the objects of the present invention is to provide a
material with high resistance to chloride environments, which at
the same time exhibits extraordinary properties in acidic and basic
environments, combined with good mechanical properties and high
structural stability. This combination can be very useful in
applications in, for example, the chemical industry, where problems
exist with respect to corrosion caused by acids, and a
contamination of the acids with chlorides amplifies the corrosive
effect. These properties of the alloy in combination with a high
strength lead to advantageous design solutions from an economic
point of view. Currently available materials with good properties
in acidic environments include steels with high contents of Ni,
which makes these materials very expensive. Another disadvantage of
austenitic steels is that they usually exhibit a very low
strength.
[0028] It has been found that the maximum solubility of nitrogen in
a solid solution of the FeCr--Mn alloys of the present invention is
about 0.013 to about 0.0155% N with 1% of Cr and a minimum of 6% of
Mn and a minimum of 2% of Mo as the best enhancement.
[0029] Nitrogen has a much lower affinity toward Cr than carbon.
The above-mentioned properties of nitrogen in
high-chromium-manganese alloys cause the carbon in these alloys to
be transformed into the carbide phase, forming hard eutectic
chromium carbides, with the surplus carbon being dissolved in the
matrix together with nitrogen.
[0030] Nitrogen introduced in a high concentration in solid
solution has a much stronger effect than carbon on the retardation
of the formation the sigma phase, thereby allowing larger
quantities of Cr and Mo to be dissolved in the Fe--Cr--Mn alloys to
enhance passivation.
[0031] Nitrogen generally improves corrosion resistance,
particularly in chloride containing media. In stainless steels its
effectiveness has been tested and expressed by the PREN value
(Pitting Resistance Equivalent Number)=% Cr+3.3.times.%
Mo+16.times.% N. The higher the level of the passivating elements
(Cr, Mo, N), the higher the resistance to corrosion/erosion.
[0032] Additionally, boron reacts with many elements in the
Periodic Table to form a wide variety of compounds. The strong
covalent bonds of most borides are responsible for their high
melting points, corrosion resistance and hardness values. The
chemical resistance of borides is superior to that of most of their
nitride and carbide counterparts. Because of the larger atomic size
of B.apprxeq.0.91 .ANG., compared to C.apprxeq.0.77 .ANG. and
N.apprxeq.0.71 .ANG., interstitial substitution of boron in an
undistorted octahedral site is rare, resulting primarily in
boron-boron bonding, for borides M.sub.nB.sub.m (NiB, CoB, MnB,
FeB, CrB)
[0033] In addition, nickel, manganese and iron react strongly with
boron and form very hard compounds, much harder than the
corresponding nitrides or carbides. For extremely abrasive and
corrosive applications boron is preferably employed in
concentrations of up to about 5% B, with a carbon content of from
about 0.3% to about 1.2% and a nitrogen content of form about 0.4%
to about 0.6%.
[0034] Overall superior results are realized according to the
present invention by the novel microstructure, with a highly
corrosion-resistant matrix, preferably austenitic, of face centered
cubic crystal structure, and supersaturated by nitrogen in solid
solution form. This matrix is very hard, tough, non-brittle and has
carbides and nitrides (and optionally borides) embedded therein,
which additionally imparts high wear resistance to the matrix.
[0035] In practicing the instant invention, it is preferred for the
matrix to contain high levels of Cr, Mo and N in a solid solution,
without Cr, or Mo, being trapped by sigma phase precipitates. It is
desired that the constituting elements of the alloys of the present
invention satisfy the following relationship, which is a measure of
the austeniticity of the present alloys: 2 % Ni + % Co + 0.5 ( % Mn
+ % Cu ) + 30 ( % N + % C ) + 5 x % B ) % Cr + % Mo + % Si + 1.5 (
Ti + Ta + V + Nb + Ce + Al ) 1.5
[0036] Due to the addition of the austenite-formers nickel and
cobalt in a concentration range of about 0.01% to about 25 wt.-%,
it is possible to control the ratio of the ferrite and austenite
phases in the matrix in a defined manner. The normally extremely
high brittleness of chilled casting types with high carbon contents
and a carbide lattice in a ferritic matrix is avoided by the
predominant deposition of the chromium carbides in the only phase
present, i.e., the austenitic phase. Since the austenitic phase,
unlike the ferrite phase, is not embrittled by segregation of
intermetallic phases or by segregation processes, the danger of
fractures due to stresses between the carbides and the matrix is
not as great as it is in the case of a purely ferritic or
ferritic-austenitic matrix.
[0037] A molybdenum content within the range of from about 0.01% to
about 6 weight %, preferably from about 2% to about 4 weight %, and
especially about 2% to about 3 weight %, increases the corrosion
resistance, especially in chloride-containing, acidic media.
[0038] Also, by varying the alloy components carbon and chromium
within the range of from about 0.3% to about 2.5 weight % for
carbon and from about 31% to about 48 weight % for chromium, the
corrosion resistance and wear resistance of the material of the
invention can be adjusted to correspond to a prescribed
specification profile.
[0039] The corrosion resistant high-chromium, nitrogen containing
austenitic alloy of the present invention has excellent high
temperature strength and is suitable as construction material for
boilers, chemical plant reactors and other equipment which is
regularly exposed to high temperatures and/or corrosive
environments. The present invention also provides a metal casting
material the wear resistance of which corresponds approximately to
that of materials of common commercial types of white iron, and
which additionally exhibits high corrosion resistance in aggressive
media. In addition to high corrosion and wear resistance, the alloy
according to the invention has good casting characteristics.
Accordingly, it can be produced in conventional high-grade steel
foundries. The casting material also has good working
characteristics. These advantageous characteristics of the alloy of
the present invention may be attributed primarily to a chromium
content of about 31 to about 48 wt. %, a carbon content of about
0.3 to about 2.5 wt. %, and a nitrogen content of about 0.01 to
about 0.7 wt. %, which affords a sufficiently high volume
proportion of carbides and nitrides. The high-chromium content
decreases the chromium depletion of the matrix. Compared to the
known types of castings previously utilized in applications
involving hydroabrasive wear, the material according to the present
invention shows a much better combination of corrosion resistance
and wear resistance. The present invention is also directed to an
air-meltable, castable, workable alloy which exhibits resistance to
the corrosive action of acids such as sulfuric acid and phosphoric
acid over a wide range of acid strengths.
[0040] The high-chromium, nitrogen containing alloy composition of
the present invention is also highly responsive to a cryogenic
hardening process, thereby becoming super-hard. When hardened by
the cryogenic treatment, the composition possesses higher abrasion
resistance, greater hardness, and a durable matrix without the
usual precipitation of secondary carbides.
[0041] The alloys of the invention may be prepared by conventional
methods of melting, and no special conditions, such as, e.g.,
controlled atmosphere, special furnace linings, protective slags or
special molding materials are required.
[0042] In the treatment process of the present invention, the
high-chromium, nitrogen containing castable alloy has many of the
alloying elements entirely distributed in the austenitic phase or
its transformation products, when subjected to sub-zero treatment
of at least -100.degree. F., preferably -100.degree. F. to
-300.degree. F., attain much greater hardening than that achieved
through conventional high temperature treatments.
[0043] Generally, the high-chromium, nitrogen containing alloys of
this invention are made by preparing a molten metal mass of all the
required elements in the presence of air or additional nitrogen,
pouring castings therefrom, cooling of the castings, and subjecting
the castings to a cryogenic cooling treatment to produce the
desired hardness. The surface of the casting may be cleaned and
finished, either before or after cryogenic cooling. In more detail,
the preferred process involves the following steps:
[0044] (1) mixing the necessary components to be fed to the
furnace;
[0045] (2) melting the mixture in the furnace to form a pourable
molten metal;
[0046] (3) pouring the molten metal composition into an appropriate
mold;
[0047] (4) allowing the mold and the casting therein to cool slowly
to room temperature under ambient conditions;
[0048] (5) cleaning and finishing the surface of the casting, for
example, by grinding or the like to smooth the surface; and,
[0049] (6) immersing the finished casting in a cryogenic cooling
medium at a temperature of -100.degree. F. to -300.degree. F. for a
time sufficient to reach the desired hardness.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0050] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to show structural
details of the present invention in more detail than is necessary
for the fundamental understanding of the present invention, the
description making apparent to those skilled in the art how the
several forms of the present invention may be embodied in
practice.
[0051] Several mechanical tests as described below were conducted
which included the following measurements:
[0052] Tensile Strength--(Ksi)
[0053] Deflection--(mm), 30.5 mm diameter cast bar, 300 mm
span.
[0054] Impact Resistance--(J), IZOD test, unnotched 30.5 mm
diameter bar, struck 76 mm above support.
[0055] Hardness--(BHN): Brinell test, 3000 kg load on 10 mm
tungsten carbide ball.
[0056] For the testing, alloys according to the present invention
were compared to alloys of the prior art alloys, and stainless
steel as a reference.
[0057] The specific alloys tested were as follows:
[0058] Preferred alloys (composition in wt. %) of U.S. Pat. No.
5,252,149:
1 1 2 3 Cr 36.6 Cr 38.2 Cr 39.3 C 1.9 C 2.06 C 2.02 Mn 1.2 Mn 1.5
Mn 1.1 Si 1.5 Si 1.4 Si 1.5 Ni 2 Mo 1.2 Mo 1.8 Cu 1 Ni 1.2 Ni 1.6
Balance - Fe Cu 1.2 Cu 1.6 plus inevitable impurities Balance - Fe
Balance - Fe plus plus inevitable inevitable impurities
impurities
[0059] Preferred alloys (composition in wt. %) of U.S. Pat. No.
5,320,801:
2 4 5 6 Cr 29.8 Cr 32.7 Cr 34.8 Ni + Co 17.2 Ni + Co 26.5 Ni + Co
34.5 Si 3.4 Si 3.2 Si 3.5 Cu 1.9 Cu 3.1 Cu 3.8 C 1.65 C 1.28 C 1.26
Mn 1.1 Mn 1.5 Mn 1.6 Mo 0.9 Mo 1.8 Mo 2.2 Balance - Fe Balance - Fe
Balance - Fe plus plus plus inevitable inevitable inevitable
impurities impurities impurities
[0060] Alloys according to the present invention (composition in
wt. %):
3 7 8 8B 9 9a* 9b* Cr 35.8 Cr 37.3 Cr 37.9 Cr 38.3 Cr 39.1 Cr 33.4
N 0.42 N 0.48 N 0.4 N 0.52 N 0.56 N 0.41 Mn 6.1 Mn 9.8 Mn 5.2 Mn
11.1 Mn 8.2 Mn 5.1 C 1.26 C 1.33 C 1.33 C 1.41 C 1.55 C 2.2 B 0.2 B
0.15 B 3.8 B 0.1 B 0 B 0 Mo 3 Mo 2.6 Mo 2.6 Mo 2.2 Mo 2.1 Mo 2.7 Si
0.9 Si 0.8 Si 1 Si 0.7 Si 0.7 Si 0.94 Cu 1.5 Cu 1.7 Cu 1 Cu 1.9 Cu
1.5 Cu 0.8 Co 2.1 Co 0.6 Co 0.5 Co 4 Co 0.02 Co 0.004 Ni 3.25 Ni
3.6 Ni 8.2 Ni 0.2 Ni 11.0 Ni 1.8 Balance - Fe Balance - Fe Balance
- Fe Balance - Fe Balance - Fe Balance - Fe plus plus plus plus
plus plus inevitable inevitable inevitable inevitable inevitable
inevitable impurities impurities impurities impurities impurities
impurities *= not tested
[0061] Alloys of German Patent Application Nos. 19512044 and
4417261 (composition in wt. %):
4 10 11 12 Cr 38.8 Cr 43 Cr 44 Ni 5 Ni 8 Ni 10 Mo 2 Mo 3 Mo 3.5 Cu
2 Cu 2.5 Cu 2.1 N 0.19 N 0.09 N 0.15 Si 1 Si 1.5 Si 1.5 Mn 1 Mn 1.2
Mn 1.1 C 1.6 C 1.7 C 1.6 V 1.2 Balance - Fe Balance - Fe Balance -
Fe plus inevitable plus inevitable plus inevitable impurities
impurities impurities
[0062] Stainless Steel Alloy (composition in wt. %):
5 20Cb3 Cd-4MCu+N 317L Cr 20 Cr 26.5 Cr 18 Ni 37.5 Ni 5.5 Ni 11 Mo
3 Mo 2.5 Mo 3.1 Cu 3 Cu 2.9 C Min. Nb 0.4 N 0.23 C Min C Min
Balance - Fe plus Balance - Fe plus Balance - Fe plus inevitable
inevitable inevitable impurities impurities impurities
[0063]
6TABLE 1 Sample No. Tensile U.S. Pat. Strength Deflection Impact
Hardness No. (Ksi) Elongation % (mm) (J) (BHN) Comments 5,252,149 1
61 0 2/3 12 19 450 as cast 2 64 0 1.3/1.9 11 18 460 3 58 0 0.9-1.9
10 16 490 Heat treatment at 1450.degree. F. for 3 hrs 5,320,801 4
53 0 8-11 22-26 360 Sample: 5 54 0.3-0.6 9-13 26-34 330 Hardened at
1400.degree. F. for 4 hrs 6 48 0.3-0.5 8-13 22 31 320 Hardened at
1400.degree. F. for 4 hrs Present invention 7 95 0.5-1.1 14-18
48-59 512 Cryogenic C hardened at - 300.degree. F. 8 111 0.4-1.0
10-16 41-49 450 Heat Treated 8B 109 0 8-12 30-36 530 as cast 9 95
0.3-0.6 9-12 36-47 490 as cast German Patent Appls. 4417261,
19512044 10 68 0 1.5-2.2 11-16 500 Heat treatment at 1800.degree.
F. for 2 hrs 11 65 0 1 2.0 10-15 450 12 64 0 0.6 1.6 8-14 490
[0064] The alloys 1, 2, 3, 10, 11 and 12 of the prior art have an
eutectic microstructure where the matrices are essentially ferritic
(Fe-.alpha.).
[0065] The alloys according to German Patent Application Nos.
4417261 or 19512044, identified as 10, 11 and 12, can have up to
40% or Fe-.alpha. phase in the matrix. The Fe-.alpha. phase in the
high-chromium alloys inherently posses very low toughness because
of the very low solubility of carbon and nitrogen in the
Fe-.alpha.. Even a small, limited addition of nitrogen has a
detrimental effect on the toughness, deflection and heat
sensitivity, making the alloy more brittle.
[0066] Alloys 4, 5 and 6 of U.S. Pat. No. 5,320,801 are chromium
high-nickel alloys with an austenitic microstructure. These
high-nickel alloys inherently possess a very low tensile strength,
a very low hardness, as cast above 200 HB, and after hardening from
the range of 300 HB, they lose their toughness and corrosion
resistance.
[0067] As can be appreciated from Table 1 above, alloys 7, 8 and 9
of the present invention possess the following properties in
comparison to prior art alloys:
[0068] 2 to 3 times greater toughness
[0069] 1.6 to 2.3 times higher tensile strength
[0070] Very high as cast hardness after cryogenic hardening
[0071] Measurable elongation or malleability
[0072] Excellent deflection
[0073] 1.5 to 2.5 higher max. hydraulic pressure vessel test.
[0074] Low heat sensitivity
[0075] Good machinability, especially threadability, which is very
poor in prior art alloys
[0076] Best castability with melting and pouring temp.-150.degree.
F. lower
[0077] The alloys of the prior art as well as the alloys of the
present invention were subjected to corrosion test to show the
superiority of the alloys of the instant invention:
[0078] The corrosion tests were conducted in synthetic
P.sub.2O.sub.5 acid at 80.degree. C., with a chloride content of
from 1000 to 3000 ppm. Agitated, 96 hr test. (mmy). The results of
the corrosion tests are summarized in Table 2.
7TABLE 2 PREN = Sample No. Hardness Chloride Corrosion % Cr + 3.3
.times. % Patent No. (BHN) Content (PPM) Rate (mmy) Mo + 16 .times.
% N U.S. Pat. No. 5,320,801 260 1000 17 PREN.sub.5 = 38 5 2000 28
As cast 3000 56 5 330 1000 23 Hardened 2000 36 At 1400.degree. F./4
hr 3000 65 U.S. Pat. No. 5,252,149 460 1000 15 PREN.sub.2 = 42 2
2000 23 as cast 3000 49 Present 450 1000 8 PREN.sub.8 = 53
Invention 2000 11 8 3000 16 As Cast Stainless Steel 180 1000 13
PREN = 30 20Cb-3 2000 14 (20Cb-3) 3000 32 Stainless Steel 280 1000
11 PREN = 38 CD-4MCuN 2000 15 3000 19 CD-4MCuN 330 1000 17 Hardened
2000 28 3000 45 Stainless Steel 185 1000 0.68 PREN = 38 317L 2000
1.1 (317L) 3000
[0079] The following conclusions can be drawn from Table 2:
[0080] The high-chromium alloy No. 5 of U.S. Pat. No. 5,320,801
containing 26% Nickel has a lower corrosion resistance than alloy
No. 2 of U.S. Pat. No. 5,252,149, which has a nickel content of
only 1%.
[0081] The same conclusion applies to the stainless steel alloy
20Cb3, in which the Ni content is 37%. The alloy CD4MCuN contains
only 5% Ni. The main function of Ni in corrosion resistant alloys
is as a structural component.
[0082] The high-chromium, nitrogen containing alloy No. 8 of the
present invention contains only 3.6% Ni, but 0.48% nitrogen, which
is a very powerful corrosion inhibitor. Nitrogen interacts with the
chlorides and somehow buffers their detrimental effect on the
alloy. Alloy No. 8 according to the present invention with the
higher PREN=53, has a 2 to 3 times better corrosion resistance than
the prior art alloys No. 5 and No. 2. Alloy No. 8 of the present
invention which contains high levels of Cr, Mo and a high
concentration of nitrogen, possesses the best corrosion resistance
in acidic environments which contain high levels of chlorides.
[0083] Prior art alloys and the alloys of the present invention
were also subjected to corrosion/erosion tests as described
below.
Corrosion/Erosion Test
[0084] The corrosion erosion tests were done using 30% by weight of
alumina (80 microns) suspended in 28% P.sub.2O.sub.5 synthetic
acid, 1.5% H.sub.2SO.sub.4, 0.05% hydrofluoric acid plus 1000 ppm
Cl, temperature 800.degree. C., rotation 650 RPM, duration 12 hr.
Mass loss (mg). The results of the erosion/corrosion testing are
shown in Table 3 below.
8TABLE 3 PREN = Hardness Weight Loss CR % + 3.3 .times. Sample No.
BMN (mg) Mo % + 16 .times. N U.S. Pat. No. 5,320,801 260 306.6 PREN
(5) = 38 5 as cast 5 330 282.6 age hardened at 1400.degree. F./4
hr. Present invention 530 96.3 PREN (8B) = 53 8 - B 450 123.3 8 as
cast 8 anneal/S solution 450 125.1 PREN (8) = 53 at 2000.degree.
F./4 hr. Stainless Steel 280 426 PREN = 38 CD4McuN (CD-4mcUn)
solution annealed CD-4MCuN 330 328.2 age hardened 20cb - 3 180
660.3 PREN = 30 solution annealed (20Cb-3)
[0085] The slurry corrosion/erosion tests indicate that alloy
20Cb-3 which has the lowest hardness shows the highest mass loss.
Prior art alloy No. 5 has a low hardness, comparable to the
hardness of the reference stainless steel CD-4MCuN.
[0086] The loss of mass of alloy No. 5 of U.S. Pat. No. 5,320,801
is 50% less than that of the stainless steel alloy Cd4MCuN. With
alloy sample No. 8 according to the present invention the loss of
mass is 245% less than that of the reference alloy Cd4MCuN. Alloy
No. 8 with the highest PREN factor=53, possesses the highest
corrosion/erosion resistance, i.e., about 3.5 times better than
that of the reference alloy CD4MCuN and 2.3 times better than that
of alloy No. 5 according to U.S. Pat. No. 5,320,801.
[0087] Alloy No. 8B with boron according to the present invention
with the highest hardness and PREN=53 possesses the highest
corrosion/erosion resistance, i.e., about 4.4 times better than
that of the reference alloy CD-4MCuN and 2.9 times better than that
of alloy No. 5 according to U.S. Pat. No. 5,320,801.
[0088] Any conventional or under nitrogen partial pressure casting
technology may be used to produce the alloys of the present
invention.
[0089] It is preferred that the alloys are formed by a conventional
casting technology and then are heat-treated at a temperature in
the range of 1800.degree. to 2000.degree. F., followed by air
cooling.
[0090] The most preferred hardening method for the alloy of the
present invention is by cryogenic treatment: cooling to at least
from -100.degree. F. to -300.degree. F., and maintaining at these
temperatures for a time of one hour per one inch of casting wall
thickness.
[0091] The cryogenic tempering process may be performed with
equipment and machinery which is conventional in the thermal
cycling treatment field. First, the articles-under-treatment are
placed in a treatment chamber which is connected to a supply of
cryogenic fluid, such as liquid nitrogen or a similar low
temperature fluid. Exposure of the chamber to the influence of the
cryogenic fluid lowers the temperature until the desired level is
reached. In the case of liquid nitrogen, this is about -300.degree.
F. (i.e., 300.degree. F. below zero).
[0092] It is noted that the foregoing examples have been provided
merely for the purpose of explanation and are in no way to be
construed as limiting of the present invention. While the present
invention has been described with reference to exemplary
embodiments, it is understood that the words which have been used
herein are words of description and illustration, rather than words
of limitation. Changes may be made, within the purview of the
appended claims, as presently stated and as amended, without
departing from the scope and spirit of the present invention in its
aspects. Although the present invention has been described herein
with reference to particular means, materials and embodiments, the
present invention is not intended to be limited to the particulars
disclosed herein; rather, the present invention extends to all
functionally equivalent structures, methods and uses, such as are
within the scope of the appended claims.
* * * * *