U.S. patent application number 11/666903 was filed with the patent office on 2008-06-12 for duplex stainless steel.
Invention is credited to Karin Jakobsson, Pasi Kangas.
Application Number | 20080138232 11/666903 |
Document ID | / |
Family ID | 33488170 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080138232 |
Kind Code |
A1 |
Kangas; Pasi ; et
al. |
June 12, 2008 |
Duplex Stainless Steel
Abstract
A duplex stainless steel alloy which contains in weight %: Cr
25-35%, Ni 4-10%, Mo 1-6%, N 0.3-0.6%, Mn greater than 0-3%, Si max
1.0%, C max 0.06%, Cu and/or W and/or Co 0.1-10%, W 0.1-5%, balance
Fe and normally occurring impurities wherein the ferrite content is
30-70%. The alloy has a yield point in tension being min 760
MPa.
Inventors: |
Kangas; Pasi; (Sandviken,
SE) ; Jakobsson; Karin; (Sandviken, SE) |
Correspondence
Address: |
Carter, DeLuca, Farrell & Schmidt
445 Broad Hollow Road, Suite 225
Melville
NY
11747
US
|
Family ID: |
33488170 |
Appl. No.: |
11/666903 |
Filed: |
November 4, 2005 |
PCT Filed: |
November 4, 2005 |
PCT NO: |
PCT/SE05/01661 |
371 Date: |
May 2, 2007 |
Current U.S.
Class: |
420/38 |
Current CPC
Class: |
B23K 35/3053 20130101;
C22C 38/04 20130101; C21D 2211/005 20130101; C22C 38/001 20130101;
C22C 38/02 20130101; C22C 38/44 20130101; C21D 2211/001 20130101;
C22C 38/42 20130101 |
Class at
Publication: |
420/38 |
International
Class: |
C22C 38/52 20060101
C22C038/52; C22C 38/44 20060101 C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 2004 |
SE |
0402698-5 |
Claims
1. A duplex stainless steel alloy comprising: from about 0.1 weight
% to about 5 weight % W; optionally from about 0.1 weight % to
about 5 weight % Cu; optionally from about 0 weight % to about 3.5
weight % Co; and the balance Fe and normally occurring impurities,
wherein the ferrite content is 30 weight % to about 70 weight %,
and the combination of W, optional Cu, and optional Co is from
about 0.1 weight % to about 10 weight %, and the alloy has a yield
point in tension being at least about 760 MPa.
2. The alloy according to claim 1, wherein the alloy contains 0.1
weight % to about 5 weight % Cu.
3. The alloy according to claim 1, wherein the alloy contains 0.1
weight % to about 3 weight % Cu.
4. The alloy according to claim 2, wherein the alloy contains at
least about 0.8% Cu.
5. The alloy according to claim 1, wherein the alloy contains less
than 0.15% Si and less than 0.05% C.
6. The alloy according to claim 1, wherein the alloy contains less
than 0.1% Si and less than 0.05% C.
7. The alloy according to claim 1, wherein the alloy contains 0.40
weight % to about 0.55 weight % N.
8. The alloy according to claim 1, wherein the alloy contains 1
weight % to about 3 weight % W.
9. The alloy according to claim 1, wherein the following
relationship is satisfied: 0.5(% W)+1 (% Mo)=2-10%.
10. The alloy according to claim 1, wherein the following
relationship is satisfied: 0.5(% W)+1(% Mo)=3-7%.
11. The alloy according to claim 1, wherein the alloy contains less
than 3.5 weight % Co.
12. The alloy according to claim 1, wherein the alloy contains 28
weight % to about 33 weight % Cr.
13. The alloy according to claim 1, wherein the alloy contains 5
weight % to about 1.5 weight % Mn.
14. The alloy according to claim 13, wherein the alloy contains 5
weight % to about 9 weight % Ni.
15. The alloy according to claim 1, wherein the alloy contains 35
weight % to about 55 weight % ferrite.
16. The alloy according to claim 1, wherein the alloy is
manufactured using a conventional metallurgical method.
17. The alloy according to claim 1, wherein the alloy comprises a
maximum of 1 weight % of further alloying additions in total.
18. An article in the form of a tube, wire, strip, rod, sheet or
bar, comprising the alloy according to claim 1.
19. An article according to claim 18, wherein the article is made
of the alloy according to claim 1.
20. An article according to claim 18, wherein the alloy comprises a
coating or a cladding of the article.
21. An article comprising the alloy of claim 1, the article
selected from the group consisting of an umbilical, a downhole, an
integrated production unit, and a welding wire.
Description
TECHNICAL FIELD OF THE INVENTION AND STATE OF THE ART
[0001] The present invention concerns to a duplex stainless steel
alloy having a high Cr-, Mo- and N- content and a ferrite content
of 30-70%.
[0002] Duplex stainless steels are characterised by an
austenite-ferrite structure where both phases have different
chemical compositions. They are attractive as structural materials
where both high mechanical strength and excellent resistance to
corrosion are required. Duplex stainless steels are often used as
alternatives to austenitic stainless steels and nickel-based alloys
due to their lower cost, which is a consequence of the lower nickel
content in duplex stainless steels.
[0003] Duplex stainless steels are extensively used in the onshore
and offshore sectors of the oil and gas industry due to their
corrosive resistance to the various corrosive media, such as
CO.sub.2, H.sub.2S and chlorides, found in such onshore/offshore
environments. Umbilical pipes, or "umbilicals", that interconnect
units on the land or sea surface with units at the bottom of the
sea to transport substances therebetween, such as to crude oil and
gas from a source to an oil rig, are often made of duplex stainless
steel pipes that are welded together. Downhole tubes, which are
grooved tubes that are generally installed within a drill-hole, and
integrated production tubes (IPUs), which are composite tubes
comprising umbilicals and downhole tubes, are also often made of
duplex steel.
[0004] A downhole tube has to be resistant both to corrosion in the
sea water that surrounds it and to corrosion in the substances that
it transports. Downhole tubes are supplied in threaded finish and
joined to the necessary lengths by means of couplings. Since oil
and gas wells are situated at a considerable depth below sea level
the length of a downhole tube can be quite considerable. The
demands on the material that is used for downhole tubes can be
summarised as follows: [0005] Yield point in tension; minimum 110
ksi (kilos per square inch) (760 MPa) [0006] Resistance to
corrosion caused by CO.sub.2 or H.sub.2S. [0007] Good impact
strength down to -46.degree. C., at least 50 J. [0008] The material
has to be capable of being manufactured in the shape of seamless
tubes and in forms in which threads and fitting couplings for tubes
can be produced.
[0009] U.S. Pat. No. 6,749,697 discloses a duplex stainless steel
alloy with austenite-ferrite structure having a high Cr-, Mo- and
N- content. This alloy fulfils the above-mentioned requirements
since when in hot extruded and annealed finish the alloy shows high
strength, good corrosion resistance in several acids and bases and
has especially good pitting resistance in chloride environments, as
well as good weldability. The pitting resistance of an alloy is
often described as a Pitting Resistance Equivalent number, PRE
number=% Cr+3.3% Mo+16% N. The alloy is therefore optimised
according to the property. The PRE number of this alloy is over 40.
The alloy contains in weight-% max 0.05% C, 0-2.0% Si 0-3.0% Mn,
25-35% Cr, 4-10% Ni, 2-6% Mo, 0.3-0.6% N, balance Fe and normally
occurring impurities whereby the content of ferrite is 30-70%.
[0010] WO 03/020994 describes an alloy characterised by Mn 0-3%, Cr
24-30%, Ni 4.9-10%, Mo 3-5, Cu 0-2%, W 0-3%, N 0.28-0.5% and Co
0-3.5%. This alloy has a high Cr-, Mo and N content, which
increases the alloy's pitting resistance but on the other hand
increases the risk of poor structural stability. By alloying with
Co the alloy is considered to be more structurally stable so at
least 0.5% Co, preferably at least 1.5%, max 3.5% Co can be added
to increase the corrosion resistance and this is also reported to
increase structural stability. Since the alloy may contain W, the
PRE number is modified to include the element W having a weight
corresponding to half of the weight for Mo, namely PREW=% Cr+3.3%
(% Mo+0.5% W)+16N. This alloy has a PRE/PREW number over 40.
[0011] U.S. Pat. No. 6,312,532 discloses a duplex stainless steel
alloy containing Mn 0.3-4%, Cr 27-35%, Ni 3-10%, Mo 0-3%, N
0.3-0.55%, Cu 0.5-3% and W 2-5%. The alloy exhibits a relatively
high resistance to corrosion in chloride environments due to
alloying with W. Alloying with Cu in combination with high W or Mo
contents is stated to decrease the precipitation of intermetallic
phases on slow cooling. This property is of great importance when
manufacturing stainless steel products of large dimensions where
the rate of cooling is relatively slow, which in general increases
the risk of intermetallic phases precipitating in the temperature
range of about 700-1000.degree. C. This alloy has a PREW number
over 40. The patent states that at least 2% W should be added for
optimal effect and combinations of Mo+0.5 W should not exceed 3.52.
When using high contents of Mo and W the Cu content should exceed
1.5% to maximise the structural stability. If large amounts of Cu
are used the Mo content should be low to ensure good protection
against inter-crystalline corrosion.
[0012] A disadvantage with duplex stainless steels is that their
high alloy content makes them susceptible to the formation of
intermetallic phases, such as the sigma and chi phases, from
extended exposure to high temperatures. The sigma phase is a hard,
brittle and highly corrodible intermetallic compound that is rich
in Cr and Mo. The chi phase is an intermetallic compound with a
manganese sulphide structure.
[0013] Significant intermetallic precipitation may lead to a loss
of corrosion resistance and sometimes to a loss of toughness.
Furthermore the production of thick and/or long pipes with large
diameters is adversely affected because of the precipitation of
intermetallic phases inside the products where the cooling rate is
relatively slow after annealing.
SUMMARY OF THE INVENTION
[0014] The object of the present invention is to provide a duplex
stainless steel that shows high strength, good corrosion
resistance, good workability and which is weldable.
[0015] This object is fulfilled by optimising the alloy described
in U.S. Pat. No. 6,749,697 by utilising knowledge of the influence
of the elements Cu, W and Co on the structural stability of the
alloy and its corrosion properties while retaining or improving the
alloy's tensile properties. The object is fulfilled by a duplex
stainless steel alloy having the composition disclosed herein
namely an alloy that contains (in weight %): Cr 25-35%, Ni 4-10%,
Mo 1-6%, N 0.3-0.6%, Mn greater than 0 to 3%, Si max 1.0% and C max
0.06%, Cu and/or W and/or Co 0.1-10%, W 0.1-5%, balance Fe and
normally occurring impurities wherein the ferrite content is
30-70%, and which alloy has a yield point in tension being minimum
760 MPa.
[0016] Such an alloy having high contents of Cr, Mo and N and
containing W or W and Cu and/or Co has surprisingly good mechanical
and corrosion properties, particularly as regards pitting in a
chloride environment. The high contents of Cr, Mo and N give the
alloy a very high strength and simultaneously a good workability,
especially for hot extrusion into articles such as seamless tubes.
The addition of W or W and Cu and/or Co enhances the alloy's
corrosion resistance in acid environments, improves its structural
stability and its weldability and confers greater resistance to
some types of corrosion attack by seawater.
[0017] Besides exhibiting excellent mechanical properties the
inventive alloy has a high resistance to stress corrosion cracking
caused by hydrogen sulphide. The alloy has good hot workability, is
easier to roll and is well suited for applications that require
welding, such as the manufacture of butt-welded seamless tubes and
seam-welded tubes for various coiled tubing applications.
Consequently, the alloy is especially suited for hydraulic tubes,
such as umbilicals, downhole tubes and IPUs. However, the most
remarkable characteristic of the alloy according to the invention
is the extraordinary combination of a high yield point in tension
and a high impact toughness.
[0018] The present inventors has found the following relationship
between yield point in tension and composition for a duplex
stainless steel alloy:
R.sub.p0.2=31.6% Cr+34(% Mo+% W)+153% N+10.2% Cu-426.
Tungsten, which is similar to molybdenum in function and effect in
terms of corrosion chemistry, is used to partly replace the
molybdenum in the alloy since tungsten is not as active as
molybdenum in promoting the precipitation of intermetallic phases
such as the sigma phase. Partly substituting molybdenum with
tungsten also increases the alloy's low temperature impact
toughness. The utilization of both molybdenum and tungsten improves
duplex stainless steel alloy's corrosion resistance. Furthermore
since molybdenum is much more expensive than tungsten the
substitution of molybdenum with tungsten provides a more
cost-effective alloy.
[0019] An addition of W or W and Cu and/or Co is also essential for
suppressing the precipitation of intermetallic phases. The alloy's
pitting corrosion properties and its resistance to intergranular
corrosion are furthermore enhanced by a simultaneous addition of W
and Cu, where W at least partly substitutes Mo. However high
contents of W in combination with high contents of Cr and Mo
increase the risk of intergranular precipitations so the content of
W should therefore be limited to max 5 weight %.
[0020] According to an embodiment of the invention the alloy
contains 0.40-0.55% N. It has been found that this high content of
nitrogen results in a particularly favourable combination of a high
yield point in tension and a high impact toughness.
[0021] According to another embodiment of the invention, where the
inventive duplex stainless steel alloy contains tungsten, the
following relationship is satisfied:
0.5(% W)+1(% Mo)=2-10%, or preferably 3-7%.
where (% W) and (% Mo) refer to the content of tungsten and
molybdenum respectively in weight %.
[0022] According to another embodiment of the invention the alloy
is manufactured using a conventional metallurgical method, such as
melting in an arc furnace. The inventive alloy may therefore be
readily melted and cast using conventional techniques and
equipment. Alternatively the alloy is manufactured by a powder
metallurgy method.
[0023] According to a further embodiment of the invention the alloy
comprises a maximum of 1 weight % alloying additions that are added
for process metallurgical or hot workability reasons.
[0024] The present invention also concerns an article in the form
of a tube, wire, strip, rod, sheet or bar or any other article
having high strength and/or good corrosion resistance, which
comprises an alloy according to any of the embodiments disclosed
above. Such an article may be a seamless tube, a welding wire, a
seam-welded tube, a flange, a coupling, a rotor blade, a fan, a
cargo tank, weld material or high strength highly resistant wiring.
Said article is either made of the inventive alloy or it comprises
a coating of the inventive alloy. Alternatively the article
comprises the inventive alloy metallurgically or mechanically
bonded (or clad) to a base material such as carbon steel.
[0025] Due to the good structural stability and weldability of the
inventive alloy its field of application is much larger than the
fields of application for the alloys constituting the state of the
art.
[0026] The alloy and the article according to any of embodiments
described above are intended for use particularly but not
exclusively as a construction material or a mechanical or
structural component, such as an umbilical, a downhole tube or an
integrated production unit (IPU), in sea-water environments, in
chloride environments, in corrosive environments, in chemical
plants, in the paper industry or as welding wire.
[0027] Further advantages as well as advantageous features of the
invention appear from the following description and the other
dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] In the appended drawings:
[0029] FIG. 1 is a diagram in the form of a plot of the impact
toughness versus the yield point in tension for test charges of
alloys according to embodiments of the invention, and
[0030] FIG. 2 is a diagram showing the relation for test charges of
alloys according to embodiments of the invention at measured values
of yield point in tension and a prediction according to a formula
drawn up by the present inventors.
DESCRIPTION OF THE INVENTION
[0031] The principles and advantages of the alloy of the present
invention and selection of the desired ranges of the constituent
elements of the alloy which render the unexpected superiority of
the alloy can be stated as follows:
[0032] Chromium (Cr) is a very active element that improves the
resistance to a plurality of corrosion types. Moreover chromium
increases the strength of the alloy. High chromium content
additionally implies a very good solubility of N in the material.
Consequently it is desirable to keep the Cr-content as high as
possible in order to improve the strength and resistance to
corrosion. For very good strength properties and resistance to
corrosion the content of chromium should be at least 25 weight %,
preferably at least 28 weight %. However the content should not
exceed 33%. However high contents of Cr increase the risk of
forming intermetallic precipitations. For this reason the chromium
content preferably not exceed 35 weight %.
[0033] Nickel (Ni) is used as an austenite-stabilising element and
is added to the alloy at a suitable level in order to attain the
desirable content of austenite and ferrite, respectively. In order
to attain ferrite contents of between 30-70%, the content of nickel
should be at least 4 weight %, preferably at least 5 weight % and
should not exceed 10 weight %, preferably not exceed 9 weight
%.
[0034] Molybdenum (Mo) is an active element which improves the
resistance to corrosion in chloride environments as well as in
reducing acids. An excessive Mo-content in combination with a high
Cr-content means that the risk of forming intermetallic
precipitations increases. Since Mo increases the strength of the
alloy, the content of Mo should be in the range of at least 1
weight %, preferably at least 3%, it should not exceed 6 weight %,
preferably not exceed 5 weight %.
[0035] Nitrogen (N) is a very active element which partly increases
the resistance to corrosion and partly increases the structural
stability as well as the strength of the material. Furthermore, a
high N-content improves the reformation of austenite after welding,
which ensures good properties for welded joints. In order to attain
a good effect at least 0.3 weight % N should be added. High
contents of N increase the risk of precipitation of chromium
nitrides, especially when the content of chromium is also high.
Furthermore, a high N-content implies that the risk of porosity
increases because the solubility of N in the steel melt or weld
pool will be exceeded. The N-content should therefore be limited to
max 0.60 weight %, it should preferably be at least 0.40 weight %,
and should not exceed 0.55 weight % N.
[0036] Manganese (Mn) is added in order to increase the solubility
of N in the material, among other things. There are however other
elements that have a higher influence on the solubility. Mn in
combination with high contents of sulphur can also give rise to the
formation of manganese sulphides, which act as initiation points
for pitting corrosion. The content of Mn should therefore be
limited to being greater than 0 weight %, preferably at least 0.5
weight %, it should not exceed 3 weight %, preferably not exceed
1.5 weight %.
[0037] Silicon (Si) is utilized as a deoxidiser during steel
production and it also increases the floatability under production
and welding. It is known that high silicon contents support the
precipitation of an intermetallic phase. It has been surprisingly
shown that an increased content of silicon favourably reduces the
precipitation of sigma phase. For this reason a certain content of
silicon should be optionally permitted. The content of silicon
should however be limited to max 1.0 weight %. Silicon would for
example be added up to 0.15% or 0.10%.
[0038] Carbon (C) strengthens stainless steel but promotes the
formation of precipitates harmful to corrosion resistance and
therefore has to be considered to be a contaminant in this
invention. Carbon has a limited solubility in both ferrite and
austenite and this implies a risk of precipitation of chromium
carbides. The carbon content should therefore be limited to max
0.05 weight %, preferably to max 0.03 weight % and most preferably
to max 0.02 weight %.
[0039] Copper (Cu) is added in order to improve the duplex
stainless steel's resistance to certain corrosive environments such
as in acid environments, such as sulphuric acid, and it also
decreases the alloy's susceptibility to stress corrosion cracking
and provides age-hardening effects. It has been found that Cu
decreases the precipitation rate of intermetallic phase on slow
cooling in materials with relatively high contents of Mo and/or W.
The reason for this is possibly that the precipitation of a
copper-rich austenite or epsilon phase prevents the precipitation
of other intermetallic phases such as the sigma phase. Since
precipitation of the epsilon phase should not have the same
negative influence on the corrosion properties as the sigma phase,
the appearance of small amounts of copper-rich epsilon phase is a
positive factor in the inventive alloy. However, high contents of
copper mean that the solubility limit is exceeded so the Cu-content
should be limited to max 5 weight %. When present, the Cu-content
should be at least 0.1 weight %, preferably at least 0.8 weight %,
and should not exceed 5 weight %, preferably not exceed 3.5 weight
%.
[0040] Tungsten (W) improves the resistance to corrosion in
chloride environments as well as in reducing acids and the alloy's
resistance to pitting and crevice corrosion. It has been found that
alloying with W as a replacement for Mo increases the alloy's low
temperature impact strength. At the same time alloying with W and
Cu, where W replaces the element Mo in the alloy with the aim of
improving pitting resistance properties, can take place with the
aim of reducing the risk of worsening the inter-crystalline
corrosion resistance. However a too high W-content in combination
with a high Cr-content increases the risk of precipitation of
intermetallic phases, such as the sigma phase. When present, the
W-content should therefore be limited to at least 0.1 weight %, it
should not exceed 5 weight %, preferably not exceed 3 weight %, and
it may be min weight 1%.
[0041] Cobalt (Co) is added to reduce the precipitation of sigma
phase. It increases the alloy's corrosion resistance and structural
stability. Cobalt dissolves in the ferrite matrix, like nickel and
silicon, and strengthens the ferrite. Cobalt also tends to
stabilise austenite. When present, the content of cobalt should be
greater than 0%, preferably greater than 0.5% and should not exceed
3.5%, preferably not exceed 2% Co.
[0042] Ferrite: The content of ferrite is important in order to
obtain good mechanical properties and corrosion properties as well
as good weldability and workability. From a corrosion and welding
point of view it is desirable to obtain good properties with a
ferrite content between 30-70%. High ferrite contents cause
deterioration in low temperature impact toughness and resistance to
hydrogen embrittlement. The ferrite content is therefore at least
30%, max 70%, preferably at least 35%, and should not exceed 55%,
the remainder being austenite.
[0043] Alloying additions: Elements added for process metallurgical
reasons, in order to obtain melt purification from S or O, for
example, or added in order to improve the workability of the
material. Examples of such elements are Al, B, Ca, Ce and Mg. In
order for such elements not to have a harmful effect on the
properties of the alloy, the levels of each individual element
should be less than 0.1%. The total level of alloying additions
should be less than 1%, preferably max 0.1%.
MODELLING EXAMPLES
[0044] Modelling of 21 different compositions was carried out using
the thermodynamic calculation program ThermoCalc Version Q. The
compositions of the experimental charges are given in Table 1.
[0045] Table 2 gives the compositions in the ferrite and the
austenite phases respectively. Table 3 contains parameters taken
from the calculated phase diagrams; such as the amount of sigma
phase at 900.degree. C., the maximum temperature for sigma phase
(SIGMA) i.e. the temperature at which the sigma phase starts to
precipitate at thermodynamic equilibrium, which means that this
parameter is a dimension for the structural stability of the alloy,
the maximum temperature for chromium nitrides Cr.sub.2N and the
maximum temperature for the precipitation of chromium-rich
austenite phase.
Observations
[0046] An increase of the W content in alloys 1-4 increases the
balance in the PREW number (PRENW) between austenite and ferrite.
The Cr content in the austenite phase also decreases. A high Cr
content implies the risk of poor impact strength at low
temperatures (-46.degree. C.) so an increasing W content therefore
improves the alloy's impact strength (see Table 2, alloys 1-4).
[0047] Cu decreases the maximum temperature for sigma phase in
alloys with W (see Table 3, compare alloys 3 and 4 with alloys 7
and 8). For each weight % Cu T.sub.maxsigma decreases by
20-30.degree. C.
[0048] W as a replacement for Mo should give an increased tensile
yield limit because W is a bigger atom, which should have a greater
effect on solution hardening. By replacing Mo with W in the ratio
1:2 the structural stability will be largely unchanged but a better
strength will be achieved.
[0049] Co decreases the risk of sigma phase precipitation by
lowering the maximum temperature for sigma phase precipitation.
(See Table 3, compare alloy 10 with alloy 11 and alloy 1 with alloy
9.)
TEST EXAMPLES
[0050] Sixteen test charges were produced by casting 170 kg blooms.
The blooms were hot-forged to round bars, from which test materials
for investigations with respect to corrosion, strength and
structural stability were taken.
[0051] The composition of the sixteen test charges successfully
hot-forged to round bars with a diameter of 40 mm are given in
Table 4.
[0052] For investigating the structural stability of the test
charges test plates from the rods were subjected to solution heat
treatment at 7 temperatures between 900-1200.degree. C. (in steps
of 50.degree. C.). The best possible heat treatment temperature
with the lowest degree of intermetallic phase was determined by
studies in a light optical microscope. The material was then
subjected to solution heat treatment at this temperature during 5
minutes before the test material was taken out. The ferrite content
was determined by means of point counting in a light optical
microscope (LOM). The results are presented in Table 5.
[0053] For determining the structural stability for the test
charges the test material was rapidly heated to the dissolving
temperature, were annealed 3 minutes and cooled with a cooling rate
of -17.5.degree. C./minute and -100.degree. C./minute down to room
temperature. The amount of sigma phase in the test charges was then
determined by picture analysis of pictures from the BSE-detector in
a Scanning Electron Microscope (SEM). The results are presented in
Table 6.
[0054] It has been found that for a good structural stability it is
necessary to restrict the amount of the alloying elements as Cr, Mo
and W, while an increased content of N results in an improved
structural stability. Two important relations have been observed,
namely when there is a requirement of a good structural stability
it is advantageous to replace Mo by W. Furthermore, high con-tents
of N are favourable for the structural stability. It is shown in
the example that 5542 has a considerably better structural
stability than 5543, where an essential difference is that W
replaces Mo in a relation 2:1 (2% W for each % Mo).
[0055] The mechanical strength of the test charges was determined
at room temperature and the impact toughness was determined at room
temperature and -50.degree. C. The results are presented in Table
7. However, a number of the test bars exhibited cracks. The results
are also shown in diagram form in FIG. 1, which is a plot of the
impact toughness versus the yield point in tension.
[0056] The yield point in tension R.sub.p0.2 is strongly dependant
upon solution hardening elements. The relation between yield point
in tension and the composition satisfies with a comparatively good
correlation the formula:
R.sub.p0.2=31.6% Cr+34(% Mo+% W)+153% N+10.2% Cu-426.
The appended FIG. 2 shows the relation for the test charges at the
measured values of R.sub.p0.2 and the prediction according to this
formula. It appears from the formula that for a high yield point in
tension N has the strongest influence, while Cr, Mo and W have the
same influence. Since W is an element which does not influence the
structural stability as negatively as Mo, it is favourable to alloy
with W while lowering the content of Mo for avoiding problems with
the structural stability. However, Mo has a greater influence upon
the corrosion properties. For a maintained structural stability it
is possible to alloy with W that replaces Mo by a factor 2, which
means that the content of W may be increased with 2% if the content
of Mo is lowered by 1%, for optimizing the yield point in
tension.
[0057] It appears clearly that for the test charges 5536 in
comparison with 5542 and 5548 it is possible to increase the yield
point in tension for the materials by lowering the content of Mo
and N and at the same time increase the content of W and Cu.
[0058] A problem for high tensile materials in general is that it
is very difficult to obtain a combination of a good impact
toughness and a high yield point in tension. It has for the present
invention been demonstrated that for charges having a very high
yield point in tension, where R.sub.p0.2 exceeds 800 MPa, it is
possible to obtain an acceptable impact toughness at -50.degree. C.
for charges where the content of W and Cu is high at the same time
as the content of N has been reduced. It was by that possible to
obtain a combination of two important properties for construction
materials, which so far has been difficult to obtain for duplex
steels.
[0059] A comparison of these charges 5536 with 5542 and 5548 shows
clearly this relationship, where an increase of the content of W
and Cu in combination with a lowering of the content of N results
in an attractive combination of an acceptable low temperature
impact toughness and a high yield point in tension. An optimization
of the properties may be obtained by further increasing the content
of W and Cu while considering the requirement of a good structural
stability.
[0060] The resistance of the test materials to pitting and crevice
corrosion were measured according to ASTM G48C and MTI-2. The
critical pitting corrosion temperature (CPT) and the critical
crevice corrosion temperature (CCT) were determined and are shown
in Table 8. However, several of the test bars had cracks. The
composition in the ferrite and austenite phase, respectively, has
been determined by means of microprobe analysis (EPMA), and the
results are shown in Table 9. The PRE number may be calculated
according to PRE=% Cr+3.3 (% Mo+0.5% W)+16% N for the respective
phase and the total composition. The PRE number should be as
balanced as possible between the austenite and the ferrite
phases.
[0061] The properties (positive/negative+0-) of the test material
are compared in Table 10, where also a judgement of the
forgeability of the material has been made on a scale from 0 (the
worst) to 5 (the best).
[0062] It appears that the charge 5548 is the best one with respect
to the combination of corrosion resistance, yield point in tension
and impact toughness. It appears from Table 4 that this charge has
a content of Cu of about 2%, W about 4% and Co about 0.1% in
weight. Thus, it is favourable to have all these three elements
present in the alloy.
[0063] An optimum composition of a duplex stainless steel alloy
according to the invention where all the properties are considered
may be as follows:
[0064] Alloy with high contents of Cr, Cu and W and with a content
of N which does not negatively influence the low temperature impact
toughness. Restrict the content of Mo so that the requirement of a
good structural stability may be met. A high yield point in tension
is obtained when the content of N is high. It is possible to lower
the content of N without lowering the yield point in tension if the
content of W or Cu is increased. An acceptable low temperature
impact toughness in combination with a high yield point in tension
is obtained when the content of N is comparatively low and the
content of W and Cu is high.
TABLE-US-00001 TABLE 1 Alloy C Si Mn Cr Ni Mo Cu W N Co 1 0.015
0.15 1.0 31.0 7.81 3.5 0 0 0.5 0 2 0.015 0.15 1.0 31.0 7.98 3.0 0
1.0 0.5 0 3 0.015 0.15 1.0 31.0 8.15 2.5 0 2.0 0.5 0 4 0.015 0.15
1.0 31.0 8.46 1.5 0 4.0 0.5 0 5 0.015 0.15 1.0 31.0 7.57 2.5 1.0
2.0 0.5 0 6 0.015 0.15 1.0 31.0 6.95 2.5 2.0 2.0 0.5 0 7 0.015 0.15
1.0 31.0 7.21 1.5 2.0 4.0 0.5 0 8 0.015 0.15 1.0 31.0 6.59 1.5 3.0
4.0 0.5 0 9 0.015 0.15 1.0 31.0 8.54 3.5 0 0 0.5 1 10 0.015 0.15
1.0 31.0 7.24 1.5 2.0 4.0 0.5 1 11 0.015 0.15 1.0 31.0 6.85 1.5 2.0
4.0 0.5 3 12 0.015 0.15 1.0 31.0 7.27 3.5 1.0 0 0.5 0 13 0.015 0.15
1.0 31.0 6.66 3.5 2.0 0 0.5 0 14 0.015 0.15 1.0 31.0 6.02 3.5 3.0 0
0.5 0 15 0.015 0.15 1.0 31.0 7.70 3.5 1.0 0 0.5 1 16 0.015 0.15 1.0
31.0 6.86 3.5 2.0 0 0.5 1 17 0.015 0.15 1.0 31.0 6.05 3.5 3.0 0 0.5
1 18 0.015 0.15 1.0 28.0 6.27 2.5 1.5 2.0 0.3 0 19 0.015 0.15 3.0
28.0 6.23 2.5 1.5 2.0 0.4 0 20 0.015 0.15 1.0 33.0 9.27 2.5 1.5 2.0
0.4 0 21 0.015 0.15 3.0 33.0 9.12 1.5 1.5 4.0 0.6 0
TABLE-US-00002 TABLE 2 Cr Mo W N Cu Co PRENW Alloy Ferr. Aust.
Ferr. Aust. Ferr. Aust. Ferr. Aust. Ferr. Aust. Ferr. Aust. Ferr.
Aust. 1 32.2 29.2 4.19 2.72 0 0 0.063 0.83 0 0 0 0 47.0 51.4 2 32.3
28.9 3.59 2.32 1.18 0.76 0.065 0.80 0 0 0 0 47.1 50.6 3 32.4 28.6
3.00 1.93 2.35 1.50 0.067 0.76 0 0 0 0 47.2 49.6 4 32.7 28.1 1.80
1.14 4.69 2.94 0.071 0.70 0 0 0 0 47.5 47.9 5 33.1 26.4 2.95 1.90
2.30 1.40 0.069 0.49 0.53 1.57 0 0 47.8 42.9 6 33.9 24.7 2.92 1.91
2.28 1.36 0.072 0.32 0.90 3.36 0 0 48.4 38.4 7 34.1 24.4 1.75 1.14
4.56 2.70 0.077 0.30 0.86 3.42 0 0 48.6 37.4 8 34.8 23.2 1.73 1.17
4.16 2.71 0.081 0.20 1.12 5.35 0 0 49.2 34.8 9 32.5 27.4 4.15 2.65
0 0 0.056 0.57 0 0 1.07 0.97 47.1 45.2 10 34.3 23.9 1.75 1.13 4.46
2.72 0.071 0.24 0.81 3.50 0.91 1.19 48.4 35.7 11 34.4 23.2 1.76
1.11 4.3 2.76 0.062 0.14 0.77 3.58 2.66 3.68 48.3 33.7 12 32.9 26.8
4.13 2.67 0 0 0.065 0.53 0.55 1.53 0 0 47.6 44.1 13 33.6 25.0 4.08
2.68 0 0 0.067 0.35 0.94 3.30 0 0 48.1 39.4 14 34.2 23.6 4.02 2.73
0 0 0.071 0.24 1.22 5.19 0 0 48.6 36.4 15 33.1 25.6 4.11 2.64 0 0
0.058 0.38 0.50 1.61 1.00 1.06 47.6 40.4 16 33.8 24.3 4.07 2.66 0 0
0.062 0.26 0.88 3.39 0.95 1.13 48.2 37.2 17 34.4 23.2 4.02 2.70 0 0
0.066 0.18 1.18 5.28 0.91 1.18 48.7 35.0 18 30.8 24.4 3.01 1.94
2.44 1.47 0.068 0.42 0.79 2.26 0 0 45.8 39.9 19 30.4 23.6 2.99 1.90
2.37 1.40 0.070 0.39 0.80 2.31 0 0 45.3 38.4 20 37.1 26.9 2.95 1.94
2.37 1.40 0.071 0.37 0.61 2.51 0 0 51.9 41.5 21 36.3 25.3 1.75 1.13
4.49 2.57 0.076 0.32 0.61 2.67 0 0 50.7 38.4
TABLE-US-00003 TABLE 3 Tmax PRENW PRENW Sigma at Tmax Tmax fcc-
Alloy Mo Cu W Co ferrite austenite 900.degree. C. % sigma Cr2N Cu
PRENW 1 3.5 0 0 0 47.0 51.4 10 920 1120 -- 50.5 2 3.0 0 1.0 0 47.1
50.6 15 950 1150 -- 50.5 3 2.5 0 2.0 0 47.2 49.6 20 950 1175 --
50.5 4 1.5 0 4.0 0 47.5 47.9 25 1000 1220 -- 50.5 5 2.5 1.0 2.0 0
47.8 42.9 20 950 1250 -- 50.5 6 2.5 2.0 2.0 0 48.4 38.4 15 940 1250
850 50.5 7 1.5 2.0 4.0 0 48.6 37.4 20 970 1250 850 50.5 8 1.5 3.0
4.0 0 49.2 34.8 20 950 1250 1050 50.5 9 3.5 0 0 1 47.1 45.2 0 900
1250 -- 50.5 10 1.5 2.0 4.0 1 48.4 35.7 15 950 1250 850 50.5 11 1.5
2.0 4.0 3 48.3 33.7 0 880 1250 850 50.5 12 3.5 1.0 0 0 47.6 44.1 5
910 1250 -- 50.5 13 3.5 2.0 0 0 48.1 39.4 0 900 1250 850 50.5 14
3.5 3.0 0 0 48.6 36.4 0 890 1220 1050 50.5 15 3.5 1.0 0 1 47.6 40.4
0 880 1250 -- 50.5 16 3.5 2.0 0 1 48.2 37.2 0 880 1250 850 50.5 17
3.5 3.0 0 1 48.7 35.0 0 860 1250 1000 50.5 18 2.5 1.5 2.0 0 45.8
39.9 5 920 1120 -- 44.3 19 2.5 1.5 2.0 0 45.3 38.4 0 850 1250 --
44.3 20 2.5 1.5 2.0 0 51.9 41.5 30 990 1250 800 49.3 21 1.5 1.5 4.0
0 50.7 38.4 10 940 1250 -- 54.1
Composition of Test Charges
TABLE-US-00004 [0065] TABLE 4 Chargenr C Si Cr Ni Mo Cu W Co N 5536
0.008 0.12 28.4 7.3 3.51 0.00 0.00 0.00 0.47 5537 0.037 0.42 30.6
7.3 2.27 0.95 2.72 0.98 0.45 5539 0.037 0.51 30.2 7.3 2.23 1.05
2.94 1.06 0.46 5542 0.056 0.89 29.2 7.5 1.54 1.95 3.77 0.01 0.42
5543 0.052 0.11 31.7 7.8 3.50 1.96 0.05 1.96 0.44 5544 0.037 0.48
30.5 7.5 2.26 0.98 2.93 0.99 0.45 5546 0.052 0.13 28.8 6.9 3.46
2.00 0.06 0.01 0.54 5548 0.007 0.11 31.5 9.2 1.47 1.92 3.94 0.10
0.41 5549 0.008 0.71 32.0 8.6 3.44 1.95 0.45 0.02 0.50 5550 0.007
0.12 29.2 6.1 1.57 1.96 4.03 1.98 0.54 5552 0.061 1.00 29.4 5.1
3.43 0.00 0.00 2.01 0.54 5553 0.011 0.12 32.2 6.9 3.46 0.00 0.00
2.01 0.53 5554 0.044 0.07 29.0 5.7 1.53 0.00 3.86 1.95 0.43 5556
0.007 1.02 30.0 6.9 1.46 0.00 4.10 0.00 0.55 5557 0.061 0.10 31.8
6.2 1.47 0.00 3.90 0.00 0.55 5558 0.007 0.94 29.1 6.9 3.36 1.96
0.23 1.93 0.44
TABLE-US-00005 TABLE 5 Annealing temperature Ferrite Chargenr
(.degree. C.) (%) 5536 1100 56 5537 1100 47 5539 1100 54 5542 1150
50 5543 1100 44 5544 1100 47 5546 1025 35 5548 1100 49 5549 1150 47
5550 1100 43 5552 1050 50 5553 1050 18 5554 1100 48 5556 1075 46
5557 1050 61 5558 1050 44
TABLE-US-00006 TABLE 6 Sigma phase content (%) in dilatomer test
and test charges subjected to solution heat treatment Chargenr
-17.5.degree. C./min -100.degree. C./min 5536 4 0 5537 39 na 5539
32 5 5542 35 2 5543 39 12 5544 42 8 5546 2 0 5548 47 3 5549 na 23
5550 32 1 5552 15 0 5553 na 9 5554 5 0 5556 46 1 5557 5 0 5558 43 2
na = not analyzed
TABLE-US-00007 TABLE 7 Impact Yield point in tension Ultimate
toughness Elong Rp 0.2% tensile stress (J) A Chargenr (MPa) Rm
(MPa) RT -50.degree. C. (%) 5536 660 880 245.0 61.5 46.2 5537 787
955 83.5 28.5 36.9 5539 793 986 78.0 21.0 29.9 5542 826 984 *88.7
*33.3 36.4 5543 756 959 128.7 46.0 38.0 5544 757 937 45.0 17.0 37.5
5546 637 916 65.0 24.7 43.4 5548 839 1014 104.3 33.0 33.3 5549 849
1017 *50.3 *28.3 32.6 5550 763 972 *19.5 7.0 23.3 5552 na na 5.5
5.0 na 5553 na na 4.0 4.0 na 5554 714 905 49.0 10.5 37.8 5556 820
1063 5.0 4.5 8.2 5557 820 1003 21.5 8.0 22.5 5558 780 1033 4.0 4.5
10.5 *Rem. Cracks in the test bars
TABLE-US-00008 TABLE 8 Chargenr CPT (.degree. C.) CCT (.degree. C.)
5536 80 40 5537 **75 50 5538 5539 75 *45 5541 5542 *40 35 5543 55
40 5544 *70 45 5546 65 40 5547 5548 80 40 5549 **50 *42.5 605550
*40 30 605552 40 30 605553 40 *30 605554 75 40 605556 45 30 605557
65 35 605558 40 30 *Rem. Cracks in the test bars **Rem. Wide spread
of the test results
TABLE-US-00009 TABLE 9 % Cr % Co % Ni % Cu % Mo % W % C % N PRE PRE
aust ferr aust ferr aust ferr aust ferr aust ferr aust ferr aust
ferr aust ferr aust ferr Total 5536 26.98 29.06 0.00 0.00 8.43 5.70
0.01 0.01 2.75 4.47 0.06 0.09 0.016 0.014 0.645 0.070 46.5 45.1
47.4 5537 29.44 31.39 0.95 0.81 8.68 5.97 1.08 0.77 1.75 2.78 2.21
3.50 0.025 -0.008 0.705 0.061 50.1 47.3 49.7 5539 29.25 30.88 0.98
0.92 8.51 5.96 1.17 0.84 1.78 2.76 2.44 3.84 0.017 -0.013 0.732
0.070 50.9 47.4 49.8 5542 27.52 30.00 0.00 0.00 9.10 6.00 2.21 1.62
1.06 1.71 2.95 4.73 0.11 0.01 0.65 0.08 46.4 44.7 47.2 5543 28.33
33.06 1.98 1.63 9.69 6.02 2.31 1.49 2.51 3.97 0.09 0.14 0.08 0.05
0.62 0.16 46.7 49.0 50.3 5544 26.20 29.01 0.91 0.79 9.17 5.81 1.12
0.82 1.54 2.51 2.35 3.75 0.09 0.04 0.70 0.09 46.4 45.0 50.0 5546
27.08 30.16 0.00 0.00 8.01 4.97 2.16 1.49 2.74 4.09 0.09 0.16 0.06
0.02 0.58 0.07 45.5 45.0 49.0 5548 28.94 33.24 0.03 0.02 11.63 7.21
2.38 1.53 1.10 1.80 3.06 5.07 0.03 0.02 0.60 0.08 47.2 48.8 49.5
5549 29.76 32.69 0.00 0.00 10.34 7.02 2.23 1.60 2.50 3.87 0.43 0.65
0.04 0.03 0.71 0.07 50.1 47.6 52.1
TABLE-US-00010 TABLE 10 Corrosion Yield point Impact Charge
Structural CPT in tension toughness Nr Forg stab. ASTM G48C Rp0.2
-50.degree. C. 5536 5 ++ + - + 5537 3 0 + 0 0 5538 0 5539 3 0 + 0 0
5541 0 5542 4 0 - + + 5543 2 - - - + 5544 4 - 0 - 0 5546 1 + 0 - 0
5547 0 5548 1 0 + + + 5549 2 -- - + 0 5550 1 0 - - - 5552 5 + - -
5553 4 - - - 5554 2 + + - - 5556 4 0 - + - 5557 3 + 0 + - 5558 5 0
- 0 -
* * * * *