U.S. patent application number 10/547572 was filed with the patent office on 2007-04-26 for duplex stainless steel alloy for use in seawater applications.
This patent application is currently assigned to Sandvik Intellectual Property AB. Invention is credited to Pasi Kangas, Ann Sundstrom.
Application Number | 20070089810 10/547572 |
Document ID | / |
Family ID | 20290561 |
Filed Date | 2007-04-26 |
United States Patent
Application |
20070089810 |
Kind Code |
A1 |
Sundstrom; Ann ; et
al. |
April 26, 2007 |
Duplex stainless steel alloy for use in seawater applications
Abstract
The present invention relates to a stainless steel alloy, more
precisely a duplex stainless steel alloy having ferritic-austenitic
matrix and having high corrosion resistance in combination with
good structural stability and hot-workability, in particular a
duplex stainless steel having a ferritic content of 40-65% and a
well-balanced composition, which gives the material corrosion
properties making it more suitable for use in chloride-containing
environments than what has been found possible previously. The
material according to the present invention has, in view of the
high alloy content thereof, extraordinarily good workability, in
particular hot-workability, and should thereby be very suitable to
be used for, for instance, the manufacture of bars: pipes, such as
welded and weld less pipes, weld material, construction parts, such
as, for instance, flanges and couplings. These objects are met
according to the present invention with duplex stainless alloy,
which contain (in % by weight): C more than 0 and up to max 0.03 Si
up to max 0.5 Mo 0-3.0 20 Cr 24.0-30.0 Ni 4.9-10.0 Mo 3.0-5.0 N
0.28-0.5 B 0-0.0030 25 S up to max 0.010 Co 0-3.5 W 0-3.0% Cu 0-2.0
Ru 0-0.3 3 0 Al 0-0.03 Ca 0-0.010%, balance Fe together with
inevitable contaminations.
Inventors: |
Sundstrom; Ann; (Sandviken,
SE) ; Kangas; Pasi; (Sandviken, SE) |
Correspondence
Address: |
WHITE, REDWAY & BROWN LLP
1217 KING STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Sandvik Intellectual Property
AB
Sandviken
SE
|
Family ID: |
20290561 |
Appl. No.: |
10/547572 |
Filed: |
February 19, 2004 |
PCT Filed: |
February 19, 2004 |
PCT NO: |
PCT/SE04/00223 |
371 Date: |
July 31, 2006 |
Current U.S.
Class: |
148/325 ;
148/327 |
Current CPC
Class: |
C22C 38/52 20130101;
C22C 38/001 20130101; C22C 38/44 20130101; C22C 38/04 20130101 |
Class at
Publication: |
148/325 ;
148/327 |
International
Class: |
C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 2003 |
SE |
0300574-1 |
Claims
1. Ferrite-austenitic duplex stainless steel alloy having the
following composition (in % by weight): TABLE-US-00015 C more than
0 and up to max 0.03% Si up to max 0.5% Mn 0-3.0% Cr 24.0-30.0% Ni
4.9-10.0% Mo 3.0-5.0% N 0.28-0.5% B 0-0.0030% S up to max 0.010% Co
0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.03% Ca 0-0.010%
as well as balance Fe together with normally occurring
contaminations and additives, the ferrite content being 40-65% by
volume and the relation PRE=% Cr+3.3% Mo+16% N exceeding 46 for the
total composition of the alloy, as well as that PRE in austenite
and ferrite phase exceeds 45 as well as that the yield point in
tension Rp.sub.0.2 of the alloy exceeds 720 N/mm.sup.2, as well as
that CPT>90.degree. C. as well as CCT.gtoreq.60.degree. C.
2. Alloy according to claim 1, wherein the chromium content is
between 26.5 and 29.0% by weight.
3. Alloy according to claim 1, wherein the manganese content is
between 0.5 and 1.2% by weight.
4. Alloy according to claim 1, wherein the nickel content is
between 5.0 and 8.0% by weight.
5. Alloy according to claim 1, wherein the molybdenum content is
between 3.6% and 4.9% by weight.
6. Alloy according to claim 1, wherein the nitrogen content is
between 0.35 and 0.45% by weight.
7. Alloy according to claim 1, wherein the ruthenium content is
between 0 and 0.3% by weight.
8. Alloy according to claim 1, wherein the cobalt content is
between 0.5 and 3.5% by weight.
9. Alloy according to claim 1, wherein the copper content is
between b 0.5 and 2.0% by weight.
10. Alloy according to claim 1, wherein the ferrite content is
between 42 and 60% by volume.
11. Alloy according to claim 1, wherein the total PRE or PREW value
of the alloy exceeds 46, wherein PRE=% Cr+3.3% Mo+16 N and PREW=%
Cr+3.3 (% Mo+0.5% W)+16 N, wherein % relates to % by weight.
12. Alloy according to claim 11, wherein the PRE or PREW value of
both the ferrite and austenite phase is greater than 45 and the PRE
or PREW value of the total alloy composition is greater than
46.
13. Use of an alloy according to claim 1 as umbilical cord pipe in
chloride-containing environments.
14. Use of an alloy according to claim 1 for the manufacture of
bars, pipes, weld material, and construction parts.
15. Alloy according to claim 7, wherein the ruthenium content is
greater than 0 and up to 0.1% by weight.
16. Alloy according to claim 8, wherein the cobalt content is
between 1.0 and 3.0% by weight.
17. Alloy according to claim 9, wherein the copper content is
between 1.0 and 1.5% by weight.
18. Alloy according to claim 10, wherein the ferrite content is
between 45 and 55% by volume.
19. Use of an alloy according to claim 13, wherein said
chloride-containing environments comprise sea-water
environments.
20. Use of an alloy according to claim 14, wherein said pipes
comprise welded and weldless pipes, and said construction parts
comprise flanges and couplings.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a stainless steel alloy,
more precisely a duplex stainless steel alloy having
ferritic-austenitic matrix and having high corrosion resistance in
combination with good structural stability and hot workability, in
particular a duplex stainless steel having a ferrite content of
40-65% by volume and a well-balanced composition that gives the
material corrosion properties making it more suitable able for use
in chloride-containing environments than what has previously been
found possible.
BACKGROUND OF THE INVENTION
[0002] In oil production in the sea, holes are drilled down from
the bottom of the sea to the oil deposit. At the bottom of the sea,
a unit is installed for control of the flow of the crude oil and
further transportation to the units that are to handle and refine
the crude oil to useful products or semi-finished products. At the
unit on the bottom of the sea there is, among other things, valves
that control extraction, pressure, flow rate, etc., and couplings
to pipes with possibility of injecting chemicals into the oil well.
Frequently, methanol is used for injection with the object of
avoiding that crude oil coagulates and causes undesired stops in
the production pipes.
[0003] Valves and couplings on the unit at the bottom of the sea
are controlled hydraulically and electrically from a platform, a
production ship or another unit on the surface of the sea or on
land. An umbilical cord pipe, a so-called umbilical, couples
together the guiding unit with the units on the bottom of the sea.
The part of the umbilical that lies on the bottom of the sea, for
instance, between two underwater units on different extraction
sites, is called static umbilical since the same only to a
relatively small extent is effected by the motions of the sea. The
part of the umbilical, that is situated between the bottom of the
sea and the surface, is called dynamic umbilical and is effected to
a large extent by motions in the water and on the surface. Examples
of such motions are flows in the water, wave motions as well as
motions of the platform and the production ship.
[0004] The demands that are made on the pipes in an umbilical are
foremost related to corrosion and mechanical properties. The pipe
material has to be resistant to corrosion in sea water, which
surrounds the outer surface of the pipes. This property is what is
regarded as being most important, since sea water has a very
corrosive impact on stainless steel. Furthermore, the material has
to have high corrosion resistance to the possible corrosive
solutions that are injected in the oil well. The material has to be
compatible with hydraulic liquids without contaminating the liquid.
Possible contamination may affect the service function of the
control unit at the bottom of the sea very negatively.
[0005] The mechanical properties of the used pipe material are very
important for the application of umbilical pipes. Since the depth
may be considerable on the site of the oil production, the dynamic
part of the umbilical generally becomes long, and thereby heavy.
The weight has to be carried by the platform or the floating
production ship. In practice, there is two ways to decrease the
weight of an umbilical having a given configuration. It is possible
to choose a lighter material or a material having the same density
but having higher tensile yield limit and ultimate tensile
strength. By choosing a material having higher strength, pipes
having thinner wall may be used, and thereby the total mass of the
umbilical is reduced. The deeper the sea at the site of extraction,
the more important the total weight per unit of length of umbilical
of the material will be.
[0006] During the most recent years, when the environments in which
corrosion-resistant metallic materials are used have become more
heavy-duty, the requirements on the corrosion properties of the
materials as well as on their mechanical properties have increased.
Duplex steel alloys that were established as an alternative to
hitherto used types of steel, such as, for instance, ferritic steel
that previously were used in this application, nickel base alloys
or other high-alloy steels, are not excepted from this
development.
[0007] Furthermore, the latest development on the market for
umbilical pipes implies additionally increased demands on the
performance of the materials. The demands that hitherto have been
made regarding strength and corrosion resistance have been able to
be met by existing alloys. The new demands that are made on
construction materials in the future for umbilicals mean, however,
considerable exacting demands on corrosion resistance, by virtue of
plants being projected in warmer waters as well as by virtue of
process solutions in the umbilical will have higher temperatures.
The new demands that are made may involve that the alloy must have
resistance to crevice corrosion in sea water at temperatures up to
70-90.degree. C. Today's construction materials do not meet these
requirements with sufficient reliability against corrosion. It is
this problem that has to be solved. However, hitherto all feasible
alloys that have been evaluated, have had a weak point. An alloy of
higher resistance to chloride-induced corrosion that also meets
other demands such as increased strength and good structural
stability, would, on the other hand, mean greater possibilities to
meet the new demands made on umbilical pipes.
[0008] A recognized measure for the corrosion resistance in
chloride-containing environments is the so-called Pitting
Resistance Equivalent (abbreviated PRE), being defined as PRE=%
Cr+3.3% Mo+16% N where the percentage figures of each element refer
to percentage by weight.
[0009] A higher numerical value indicates a better corrosion
resistance, in particular to pitting. The principal alloying
elements that affect this property are, according to the formula,
Cr, Mo, N. An example of such a steel grade is seen in EP 0 220
141, which through this reference hereby is included in this
description. This steel grade, having the trade mark of SAF 2507
(UNS S32750), has essentially been alloyed with high contents of
Cr, Mo and N. Thus, it is developed towards this property with,
above all, good corrosion resistance in chloride environments.
Recently, also the elements Cu and W have turned out to be
efficient alloying additives for additional optimization of the
corrosion properties of the steel in chloride environments. The
element W has, on that occasion, been used as substitution for a
part of Mo, as for instance in the commercial alloys DP3W (UNS
S39274) or Zeron100, which contain 2.0% and 0.7% of W,
respectively. The latter also contains 0.7% of Cu with the purpose
of increasing the alloy's corrosion resistance in acid
environments.
[0010] Addition of tungsten led to a further development of the
measure for the corrosion resistance, and thereby the PRE formula
to the PREW formula, which also elucidates the relation between the
impact of Mo and W on the corrosion resistance of the alloy: PREW=%
Cr+3.3(% Mo+0.5% W)+16% N, as described, for instance, in EP 0 545
753, which relates to a duplex stainless alloy having generally
improved corrosion properties.
[0011] The above-described steel grades have a PRE number,
irrespective of method of calculation, which is above 40 but the
PRE number is limited upwards to about 43 since higher values mean
that the alloys obtain inferior structural stability. A higher
degree of alloying increases the risk of precipitation of
internietallic phase, and therefore the level of alloying in duplex
steel is regarded as limited to achieve PRE values around a maximum
of about 43, irrespective of method of calculation.
[0012] Of the alloys having good corrosion resistance in chloride
environments, SAF 2906 should also be mentioned, the composition of
which is seen in EP 0 708 845. This alloy, which is characterized
by higher contents of Cr and N in comparison with, for instance,
SAF 2507, has turned out to be especially suitable for use in
environments where the resistance to intercrystalline corrosion and
corrosion in ammonium carbamate is of importance, but it has also a
high corrosion resistance in chloride-containing environments.
[0013] The alloy has a corrosion resistance in chloride environment
corresponding to the alloy UNS S32750, but simultaneously a higher
yield point in tension Rp.sub.0.2. This makes that this alloy has
advantages in comparison with UNS S32750 as umbilical material,
since lower weight of the umbilical can be obtained. The corrosion
resistance gives, however, no improvements in comparison with UNS
S32750, which means considerable limitations in umbilical pipes
that are exposed to higher temperatures in future plants.
[0014] The alloy 19D (UNS S32001) is a duplex alloy characterized
by the composition 19.5-21.5% of Cr, 0.05-0.17% of N and max 0.6%
of Mo. This alloy has a PRE number of about 22, and therefore the
alloy is unsuitable in sea-water applications such as umbilicals.
Accordingly, in order to achieve a sufficient corrosion resistance
in this alloy, a cathode protection has to be applied in the form
of a zinc layer on the outer surface of the umbilical pipe. If the
zinc layer is consumed or if a greater surface becomes damaged, the
corrosion protection is, however, ruined and a fast corrosion
process may occur, which means expensive repairs and down
periods.
[0015] A problem with the above-described alloys, all having high
PRE numbers, is the appearance of hard and brittle intermetallic
precipitations in the steel, such as, for instance, sigma phase,
especially after heat treatment, such as, for instance, upon
welding during later working. This results in a harder material
having worse workability and finally a deteriorated corrosion
resistance.
[0016] Another group of alloys having good corrosion resistance is
austenitic steels, with PRE numbers of up to 55 having been made
possible by the addition of high contents of Cr, Mo and NJ combined
with high contents of Ni. Said alloys should work very well to the
new tougher corrosion conditions in umbilicals. The disadvantage of
the same alloys is that they have considerably lower yield point in
tension than duplex steel and are, furthermore, considerably more
expensive to manufacture, foremost by virtue of their high
percentage of Ni, which is an expensive alloying material. Examples
of austenites having good resistance in chloride environment are
UNS S32654 having a PRE number of about 55, and UNS S34565 having a
PRE number of about 45. These have, however, too low a strength and
high a cost in order to be a realistic alternative for umbilical
pipes.
[0017] In order to additionally improve, among other things, the
pitting resistance of duplex stainless steel, an increase of the
PRE number is required in both the ferrite phase and the austenite
phase without, because of this, jeopardizing the structural
stability or the workability of the material. If the composition in
the two phases is not equivalent in respect of the active alloying
components, one of the phases becomes more susceptible to pitting
and crevice corrosion. Thus, the more corrosion-susceptible phase
controls the resistance of the alloy, while the structural
stability is controlled by the highest alloyed phase.
[0018] The demands that may be made on an alloy that shall meet the
requirements in the future for umbilical pipes, can be summarized
in table 1, with examples of the best various alternative alloys
existing on the market in the present situation being included. It
is clear that all existing alloys on at least one point do not meet
the new stiffer demands that are made on umbilical pipes.
TABLE-US-00001 TABLE 1 Demands = alloy according UNS UNS UNS UNS
Property to the invention S32750 S32906 S32654 S32001 PRE Min 46
42.5 42 55 22 Yield point in 720 550 650 430 450 tension Rp.sub.0.2
(N/mm.sup.2) Pitting CPT in >90.degree. C. 50 50 >95 <20
.degree. C. Crevice 60.degree. C. 35 35 60 <20 corrosion CCT in
.degree. C. Structural Max 0.5% OK OK OK OK stability sigma phase
Manufacture Weldable by OK OK OK OK means of conventional
technique
SUMMARY OF THE INVENTION
[0019] Therefore, it is an object of the present invention to
provide a duplex stainless steel alloy, which has high corrosion
resistance in combination with improved mechanical properties and
simultaneously having good structural stability and that is most
suitable for use in environments where a high resistance is
required to general corrosion and local corrosion, such as, for
instance, in chloride-containing environments.
[0020] It is an additional object of the present invention to
provide a duplex stainless steel alloy having a Critical Pitting
Corrosion Temperature (henceforth abbreviated CPT) value greater
than 90.degree. C., preferably greater than 95.degree. C. and a
Critical Crevice Corrosion Temperature (henceforth abbreviated CCT)
value of at least 60.degree. C. in 6% FeCl.sub.3.
[0021] It is an additional object of the present invention to
provide an alloy having an impact resistance of at least 100 J at
room temperature and a yield point in tension Rp.sub.0.2 of at
least 720 N/mm.sup.2 and an elongation upon tensile testing of at
least 25% at room temperature.
[0022] The material according to the present invention has, in view
of the high alloy content thereof, extraordinarily good
workability, in particular hot-workability, and should thereby be
very suitable to be used for, for instance, the manufacture of
bars, pipes, such as welded and seamless pipes, weld material,
construction parts such as, for instance, flanges and
couplings.
[0023] These objects are met according to the present invention by
means of duplex stainless steel alloys, which contain (in % by
weight) TABLE-US-00002 C more than 0 up to max 0.03% Si up to max
0.5% Mn 0-3.0% Cr 24.0-30.0% Ni 4.9-10.0% Mo 3.0-5.0% N 0.28-0.5% B
0-0.0030% S up to max 0.010% Co 0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3%
Al 0-0.03% Ca 0-0.010%
balance Fe together with inevitable contaminations.
BRIEF DESCRIPTION OF THE FIGURES
[0024] FIG. 1 shows CPT values from test of the experimental
charges in the modified ASTM G48C test in the "Green Death"
solution in comparison with the duplex steels SAF 2507, SAF
2906.
[0025] FIG. 2 shows CPT values produced by means of the modified
ASTM G48C test in "Green Death" solution for the experimental
charges in comparison with the duplex steel SAF 2507 as well as SAF
2906.
[0026] FIG. 3 shows the mean value of the corrosion in mm/year in
2% HCl at the temperature of 75.degree. C.
[0027] FIG. 4 shows the results from hot ductility test for most of
the charges.
DETAILED DESCRIPTION OF THE INVENTION
[0028] A systematic development work has surprisingly shown that by
a well balanced combination of the elements Cr, Mo, Ni, N, Mn and
Co, an optimum distribution of the elements in the ferrite and in
the austenite can be obtained, which enables a very corrosion
resistant material having only a negligible quantity of sigma phase
in the material. The material also gets good workability, which
enables the extrusion to weldless pipes. In order to obtain the
combination of high corrosion resistance in connection with good
structural stability, a very narrow combination of the alloying
elements in the material is required. Therefore, the alloy
according to the invention contains (in % by weight):
TABLE-US-00003 C more than 0 up to max 0.03% Si up to max 0.5% Mn
0-3.0% Cr 24.0-30.0% Ni 4.9-10.0% Mo 3.0-5.0% N 0.28-0.5% B
0-0.0030% S up to max 0.010% Co 0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3%
Al 0-0.03% Ca 0-0.010%
balance Fe together with normally occurring contaminations and
additives, the ferrite content being 40-65% by volume.
[0029] The impact of the alloying elements is described in the
following:
[0030] Carbon (C) has limited solubility in both ferrite and
austenite. The limited solubility means a risk of precipitation of
chromium carbides and therefore the content should not exceed 0.03%
by weight, preferably not exceed 0.02% by weight.
[0031] Silicon (Si) is utilized as deoxidizer in the steel
production and increases the flowability in production and upon
welding. However, too high contents of Si lead to precipitation of
undesired intermetallic phase, and therefore the content should be
limited to max 0.5% by weight, preferably max 0.3% by weight.
[0032] Manganese (Mn) is added in order to increase the solubility
of N in the material. However, it has turned out that Mn only has a
limited impact on the solubility of N in the alloy type in
question. Instead, there are other elements having higher impact on
the solubility. Furthermore, Mn may in combination with high
sulphur contents give rise to the formation of manganese sulphides,
which work as initiation spots for pitting. Therefore, the content
of Mn should be limited to between 0-3.0% by weight, preferably
0.5-1.2% by weight.
[0033] Chromium (Cr) is a very active element in order to improve
the resistance to the majority of corrosion types. Furthermore, a
high chromium content means that a very good solubility of N is
obtained in the material. Thus, it is desirable to hold the content
of Cr as high as possible in order to improve the corrosion
resistance. For very good values of the corrosion resistance, the
chromium content should be at least 24.0% by weight, preferably
27.0-29.0% by weight. However, high contents of Cr increases the
risk of intermetallic precipitations, and therefore the chromium
content has to be limited upwards to max 30.0% by weight.
[0034] Nickel (Ni) is used as austenite-stabilizing element and is
added in suitable contents so that the desired ferrite content is
attained. In order to achieve the desired relation between the
austenitic and the ferritic phase of between 40-65% by volume of
ferrite, an addition of between 4.9-10.0% by weight of nickel is
required, preferably 4.9-9.0% by weight, in particular 6.0-9.0% by
weight.
[0035] Molybdenum (Mo) is an active element that improves the
corrosion resistance in chloride environments as well as preferably
in reducing acids. Too high a content of Mo, in combination with
the contents of Cr being high, means that the risk of intermetallic
precipitations increases. The content of in the present invention
should be in the interial of 3.0-5.0% by weight, preferably
3.6-4.9% by weight, in particular 4.4-4.9% by weight.
[0036] Nitrogen (N) is a very active element that increases the
corrosion resistance, the structural stability as well as the
strength of the material. Furthermore, a high content of N improves
the reformation of austenite after welding, which gives good
properties of welded joints. In order to achieve a good effect from
N, at least 0.28% by weight of N should be added. At high contents
of N, the risk of precipitation of chromium nitrides increases,
especially when the chromium content simultaneously is high.
Furthermore, a high content of N means that the risk of porosity
increases by virtue of the solubility of N in the charge being
exceeded. The content of N should, for these reasons, be limited to
max 0.5% by weight, preferably is >0.35-0.45% by weight of N
added.
[0037] Too high a chromium as well as a nitrogen content result in
the precipitation of Cr.sub.2N, which is to be avoided since it
deteriorates the properties of the material, especially upon heat
treatment, for instance welding.
[0038] Boron (B) is added in order to increase the hot workability
of the material. At too high a boron content, the weldability and
the corrosion resistance may be deteriorated. Therefore, the boron
content should be greater than 0 and up to 0.0030% by weight.
[0039] Sulphur (S) affects the corrosion resistance negatively by
forming easily soluble sulphides. Furthermore, the hot workability
is deteriorated, and therefore the sulphur content is limited to
max 0.010% by weight.
[0040] Cobalt (Co) is added foremost in order to improve the
structural stability as well as the corrosion resistance. Co is an
austenite stabilizer. In order to have an effect, at least 0.5% by
weight, preferably at least 1.0% by weight should be added. Since
cobalt is a relatively expensive element, the cobalt addition is
therefore limited to max 3.5% by a weight.
[0041] Tungsten increases the resistance to pitting and crevice
corrosion. But addition of too high contents of tungsten in
combination with the contents of Cr and contents of Mo being high,
means that the risk of intermetallic precipitations increases. The
content of W in the present invention should be in the interval of
0-3.0% by weight, preferably between 0-1.8% by weight.
[0042] Copper is added in order to improve the corrosion resistance
in acid environments such as sulphuric acid. Cu also affects the
structural stability. However, high contents of Cu means that the
solid solubility is exceeded. Therefore, the content of Cu is
limited to max 2.0% by weight, preferably between 0.1 and 1.5% by
weight.
[0043] Ruthenium (Ru) is added in order to increase the corrosion
resistance. Ruthenium is a very expensive element, and therefore
the content is limited to max 0.3% by weight, preferably greater
than 0 and up to 0.1% by weight.
[0044] Aluminum (Al) as well as Calcium (Ca) are utilized as
deoxidizers in the steel production. The content of Al should be
limited to max 0.03% by weight in order to limit nitride formation.
Ca has a favourable effect on the hot ductility but the content of
Ca should, however, be limited to 0.010% by weight in order to
avoid undesired quantity of cinder.
[0045] The ferrite content is important in order to obtain good
mechanical properties and corrosion properties as well as good
weldability. From a corrosion and a weldability point of view, it
is desirable having a ferrite content of between 40-65% in order to
obtain good properties. Furthermore, high ferrite contents means
that the low-temperature impact resistance as well as the
resistance to hydrogen embrittlement risk being deteriorated.
Therefore, the ferrite content is 40-65% by volume, preferably
42-60% by volume, in particular 45-55% by volume.
DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES
[0046] In the examples below, the composition of a number of
experimental charges is given, which illustrate the impact of
various alloying elements on the properties. Charge 605182
represents a reference composition and is accordingly not included
in the field of this invention. Neither should other charges be
regarded as limiting the invention but only states examples of
charges that illustrate the invention according to the claims.
[0047] Given PRE numbers or values always relate to values
calculated according to the PREW formula, even if not explicitly
stated.
EXAMPLE 1
[0048] Experimental charges according to this example were produced
by laboratory casting of 170 kg of ingot that was hot-forged into
round bar. The same was hot extruded into bar (round bar as well as
flat bar), where test material was sampled from round bar.
Furthermore, flat bar was annealed before cold rolling took place,
and then additional test material was sampled. The process may,
from a material technology point of view, be regarded as
representative for the manufacture on a larger scale, for instance
for the manufacture of seamless pipes by means of the extrusion
method followed by cold rolling. Table 2 shows composition of
experimental charges of the first batch. TABLE-US-00004 TABLE 2
Charge Mn Cr Ni Mo W Co V La Ti N 605193 1.03 27.90 8.80 4.00 0.01
0.02 0.04 0.01 0.01 0.36 605195 0.97 27.90 9.80 4.00 0.01 0.97 0.55
0.01 0.35 0.48 605197 1.07 28.40 8.00 4.00 1.00 1.01 0.04 0.01 0.01
0.44 605178 0.91 27.94 7.26 4.01 0.99 0.10 0.07 0.01 0.03 0.44
605183 1.02 28.71 6.49 4.03 0.01 1.00 0.04 0.01 0.04 0.28 605184
0.99 28.09 7.83 4.01 0.01 0.03 0.54 0.01 0.01 0.44 605187 2.94
27.74 4.93 3.98 0.01 0.98 0.06 0.01 0.01 0.44 605153 2.78 27.85
6.93 4.03 1.01 0.02 0.06 0.02 0.01 0.34 605182 0.17 23.48 7.88 5.75
0.01 0.05 0.04 0.01 0.10 0.26
[0049] With the purpose of examining the structural stability,
samples from each charge were annealed at 900-1150.degree. C. with
steps of 50.degree. C. and were quenched in air and water,
respectively. At the lowest temperatures, intermetallic phase was
formed. The lowest temperature where the amount of intermetallic
phase became negligibly small, was determined by means of studies
in light-optical microscope. New samples from the respective charge
were then annealed at said temperature during five minutes, and
then the samples were cooled down by the constant cooling rate of
-140.degree. C./min to room temperature. The area fraction of sigma
phase in the materials was then determined by means of digital
image processing of images recorded by mean of back-scattered
electrons in scanning electron microscope. The results are seen in
Table 3.
[0050] T.sub.max sigma is calculated by means of Thermo-Calc (T-C
version N the thermodynamic database of steel TCFE99) based on
guiding values of all stated elements in the different variants.
T.sub.max sigma is the resolution temperature of the sigma phase,
with high resolution temperature indicating lower structural
stability. TABLE-US-00005 TABLE 3 Charge Heat treatment Quantity of
.sigma. [% by vol.] Tmax .sigma. 605193 1100.degree. C., 5 min 7.5%
1016 605195 1150.degree. C., 5 min 32% 1047 605197 1100.degree. C.,
5 min 18% 1061 605178 1100.degree. C., 5 min 14% 1038 605183
1050.degree. C., 5 min 0.4% 997 605184 1100.degree. C., 5 min 0.4%
999 605187 1050.degree. C., 5 min 0.3% 962 605153 1100.degree. C.,
5 min 3.5% 1032 605182 1100.degree. C., 5 min 2.0% 1028
[0051] The object of this investigation is to be able to rank
materials in respect of the structural stability, i.e. this is not
the actual content of sigma phase in the test pieces that have been
heat treated and quenched before, for instance, corrosion test. It
is evident that T.sub.max sigma that has been calculated by means
of Thermo-calc does not directly corresponds with measured quantity
of sigma phase, but in this investigation it is, however, clear
that the experimental charges having the lowest calculated
T.sub.max sigma contain the lowest quantity of sigma phase.
[0052] The pitting properties of all charges have been tested for
ranking in the so-called "Green Dealth" solution that consists of
1% FeCl.sub.3, 1% CuCl.sub.2, 11% H.sub.2SO.sub.4, 12% HCl. The
test procedure corresponds to the pitting testing according to ASTM
G48C, but is carried out in the more aggressive "Green Death"
solution. Furthermore, some charges have been tested according to
ASTM G48C (2 experiments per charge). Also electrochemical testing
in 3% NaCl (6 experiments per charge) has been carried out. The
results in the form of critical pitting temperature (CPT) from all
experiments are seen in Table 4, such as the PREW number
(Cr+3.3(Mo+0.5W)+16 N) of the total alloy composition as well as of
austenite and ferrite. The indexing alpha relates to ferrite and
gamma relates to austenite. TABLE-US-00006 TABLE 4 CPT .degree. C.
Modified CPT .degree. C. ASTM CPT .degree. C. 3% ASTM G48C G48C 6%
NaCl (600 mv Charge PRE .alpha. PRE .gamma. PRE .gamma./PRE .alpha.
PRE Green Death FeCl.sub.3 SCE 605193 51.3 49.0 0.9552 46.9 90/90
64 605195 51.5 48.9 0.9495 48.7 90/90 95 605197 53.3 53.7 1.0075
50.3 90/90 >95 >95 605178 50.7 52.5 1.0355 49.8 75/80 94
605183 48.9 48.9 1.0000 46.5 85/85 90 93 605184 48.9 51.7 1.0573
48.3 80/80 72 605187 48.0 54.4 1.1333 48.0 70/75 77 605153 49.6
51.9 1.0464 48.3 80/85 85 90 605182 54.4 46.2 0.8493 46.6 75/70 85
62 SAF2507 39.4 42.4 1.0761 41.1 70/70 80 95 SAF2906 39.6 46.4
1.1717 41.0 60/50 75 75
[0053] It is recognized that there is a linear relation between the
lowest PRE value in the austenite or the ferrite and the CPT value
in duplex steels, but the results in Table 4 show that the PRE
number not solely explains the CPT value.
[0054] It is clear from these results that all test materials have
better CPT in the modified ASTM G48C than SAF 2507 and SAF 2906.
Test charge 605 183 alloyed with cobalt shows good structural
stability at controlled cooling rate (-140.degree. C./min), in
spite of it containing high contents of chromium as well as
molybdenum, has better results than SAF 2507 as well as SAF 2906.
In this investigation, it is seen that a high PRE not solely
explains the CPT values, but the ratio of PRE austenite/PRE ferrite
is of utmost importance for the properties of higher alloyed duplex
steels, and a very narrow and accurate levelling between the
alloying elements is required in order to obtain this optimal
ratio, which is between 0.9-1.15; preferably 0.9-1.05 and
simultaneously obtain PRE values above 46. The ratio of PRE
austenite/PRE ferrite versus CPT in the modified ASTM G48C test for
the experimental charges are accounted for in Table 4.
[0055] The strength at room temperature (RT), 100.degree. C. and
200.degree. C. and the impact resistance at room temperature (RT)
have been determined for all charges and are shown as mean value of
three experiments.
[0056] Tensile test pieces (DR5C50) were produced from extruded
bars .0.20 mm, which were heat treated at temperatures according to
Table 2 for 20 min followed by cooling down in either air or water
(605 195, 605 197, 605 184). The results of the investigation are
presented in Tables 5 and 6. The results of the tensile strength
investigation show that the contents of chromium, nitrogen and
tungsten strongly affect the tensile strength in the material. All
charges except 605 153 meet the requirement on a 25% elongation
upon tensile testing at room temperature (RT). TABLE-US-00007 TABLE
5 R.sub.p0.2 R.sub.p0.1 R.sub.m A5 Z Charge Temperatur (MPa) (MPa)
(MPa) (%) (%) 605193 RT 652 791 916 29.7 38 100.degree. C. 513 646
818 30.4 36 200.degree. C. 511 583 756 29.8 36 605195 RT 671 773
910 38.0 66 100.degree. C. 563 637 825 39.3 68 200.degree. C. 504
563 769 38.1 64 605197 RT 701 799 939 38.4 66 100.degree. C. 564
652 844 40.7 69 200.degree. C. 502 577 802 35.0 65 605178 RT 712
828 925 27.0 37 100.degree. C. 596 677 829 31.9 45 200.degree. C.
535 608 763 27.1 36 605183 RT 677 775 882 32.4 67 100.degree. C.
560 642 788 33.0 59 200.degree. C. 499 578 737 29.9 52 605184 RT
702 793 915 32.5 60 100.degree. C. 569 657 821 34.5 61 200.degree.
C. 526 581 774 31.6 56 605187 RT 679 777 893 35.7 61 100.degree. C.
513 628 799 38.9 64 200.degree. C. 505 558 743 35.8 58 605153 RT
715 845 917 20.7 24 100.degree. C. 572 692 817 29.3 27 200.degree.
C. 532 611 749 23.7 31 605182 RT 627 754 903 28.4 43 100.degree. C.
493 621 802 31.8 42
[0057] TABLE-US-00008 TABLE 6 Impact Impact resis- Annealing
Cooling resistance Annealing Cooling tance Charge [.degree. C./min]
down [J] [.degree. C./min] down [J] 605193 1100/20 Air 35 1100/20
Water 242 605195 1150/20 Water 223 605197 1100/20 Water 254 1130/20
Water 259 605178 1100/20 Air 62 1100/20 Water 234 605183 1050/20
Air 79 1050/20 Water 244 605184 1100/20 Water 81 1100/20 Air 78
605187 1050/20 Air 51 1100/20 Water 95 605153 1100/20 Air 50
1100/20 Water 246 605182 1100/20 Air 22 1100/20 Water 324
[0058] This investigation shows very clearly that water quenching
naturally is required in order to obtain the best structure and
accordingly good impact resistance values. The requirement is 100 J
upon testing at room temperature and this do all charges manage
except charge 605 184 and 605 187, where, however, the
last-mentioned one is very near the requirement.
[0059] Table 7 shows results from Tungsten Inert Gas remelting test
(henceforth abbreviated TIG), with the charges 605 193, 605 183,
605 184 as well as 605 253 having a stable structure in the heat
affected zone (henceforth abbreviated HAZ). The Ti-containing
charges have TiN in HAZ. TABLE-US-00009 TABLE 7 Charge
Precipitations Protective gas Ar (99.99%) 605193 HAZ: OK 605195
HAZ: Large amounts of TiN and a phase 605197 HAZ: Small amounts of
Cr.sub.2N in .delta. grains, however not much 605178 HAZ: Cr.sub.2N
in .delta. grains, otherwise OK 605183 HAZ: OK 605184 HAZ: OK
605187 HAZ: Cr.sub.2N fairly close to the melting boundary, no
precipitations farther out 605153 HAZ: OK 605182 HAZ: TiN as well
as decorated grain boundaries .delta./.delta..
EXAMPLE 2
[0060] In the example below, the composition is given of an
additional number of experimental charges manufactured with the
intention of finding the optimal composition. Said charges are
modified, based on the properties of the charges having good
structural stability as well as high corrosion resistance, from the
results that were shown in Example 1. All charges in Table 8 are
comprised of the composition according to the present invention,
with charges 1-8 being included in a statistical experimental plan,
while charges e to n are additional experimental alloys within the
scope of this invention.
[0061] A number of experimental charges were produced by casting a
270 kg casting, which was hot-forged into round bar. This was
extruded to bar, from which test materials were sampled. Then the
bar was annealed before cold rolling of flat bar took place and
then additional test materials were sampled. Table 8 shows the
composition of the same experimental charges. TABLE-US-00010 TABLE
8 Charge Mn Cr Ni Mo W Co Cu Ru B N 1 605258 1.1 29.0 6.5 4.23 1.5
0.0018 0.46 2 605249 1.0 28.8 7.0 4.23 1.5 0.0026 0.38 3 605259 1.1
29.0 6.8 4.23 0.6 0.0019 0.45 4 605260 1.1 27.5 5.9 4.22 1.5 0.0020
0.44 5 605250 1.1 28.8 7.6 4.24 0.6 0.0019 0.40 6 605251 1.0 28.1
6.5 4.24 1.5 0.0021 0.38 7 605261 1.0 27.8 6.1 4.22 0.6 0.0021 0.43
8 605252 1.1 28.4 6.9 4.23 0.5 0.0018 0.37 e 605254 1.1 26.9 6.5
4.8 1.0 0.0021 0.38 f 605255 1.0 28.6 6.5 4.0 3.0 0.0020 0.31 g
605262 2.7 27.6 6.9 3.9 1.0 1.0 0.0019 0.36 h 605263 1.0 28.7 6.6
4.0 1.0 1.0 0.0020 0.40 i 605253 1.0 28.8 7.0 4.16 1.5 0.0019 0.37
j 605266 1.1 30.0 7.1 4.02 0.0018 0.38 k 605269 1.0 28.5 7.0 3.97
1.0 1.0 0.0020 0.45 l 605268 1.1 28.2 6.6 4.0 1.0 1.0 1.0 0.0021
0.43 m 605270 1.0 28.8 7.0 4.2 1.5 0.1 0.0021 0.41 n 605267 1.1
29.3 6.5 4.23 1.5 0.0019 0.38
[0062] The distribution of alloying elements in the ferrite and
austenite phase was examined by means of micro probe analysis, the
result is seen in Table 9. TABLE-US-00011 TABLE 9 Charge Phase Cr
Mn Ni Mo W Co Cu N 605258 Ferrit 29.8 1.3 4.8 5.0 1.4 0.11 Austenit
28.3 1.4 7.3 3.4 1.5 0.60 605249 Ferrit 29.8 1.1 5.4 5.1 1.3 0.10
Austenite 27.3 1.2 7.9 3.3 1.6 0.53 605259 Ferrite 29.7 1.3 5.3 5.3
0.5 0.10 Austenite 28.1 1.4 7.8 3.3 0.58 0.59 605260 Ferrite 28.4
1.3 4.4 5.0 1.4 0.08 Austenite 26.5 1.4 6.3 3.6 1.5 0.54 605250
Ferrite 30.1 1.3 5.6 5.1 0.46 0.07 Austenite 27.3 1.4 8.8 3.4 0.53
0.52 605251 Ferrite 29.6 1.2 5.0 5.2 1.3 0.08 Austenite 26.9 1.3
7.6 3.5 1.5 0.53 605261 Ferrite 28.0 1.2 4.5 4.9 0.45 0.07
Austenite 26.5 1.4 6.9 3.3 0.56 0.56 605252 Ferrite 29.6 1.3 5.3
5.2 0.42 0.09 Austenite 27.1 1.4 8.2 3.3 0.51 0.48 605254 Ferrite
28.1 1.3 4.9 5.8 0.89 0.08 Austenite 26.0 1.4 7.6 3.8 1.0 0.48
605255 Ferrite 30.1 1.3 5.0 4.7 2.7 0.08 Austenite 27.0 1.3 7.7 3.0
3.3 0.45 605262 Ferrite 28.8 3.0 5.3 4.8 1.4 0.9 0.08 Austenite
26.3 3.2 8.1 3.0 0.85 1.1 0.46 605263 Ferrite 29.7 1.3 5.1 5.1 1.3
0.91 0.07 Austenite 27.8 1.4 7.7 3.2 0.79 1.1 0.51 605253 Ferrite
30.2 1.3 5.4 5.0 1.3 0.09 Austenite 27.5 1.4 8.4 3.1 1.5 0.48
605266 Ferrite 31.0 1.4 5.7 4.8 0.09 Austenite 29.0 1.5 8.4 3.1
0.52 605269 Ferrite 28.7 1.3 5.2 5.1 1.4 0.9 0.11 Austenite 26.6
1.4 7.8 3.2 0.87 1.1 0.52 605268 Ferrite 29.1 1.3 5.0 4.7 1.3 0.91
0.84 0.12 Austenite 26.7 1.4 7.5 3.2 0.97 1.0 1.2 0.51 605270
Ferrite 30.2 1.2 5.3 5.0 1.3 0.11 Austenite 27.7 1.3 8.0 3.2 1.4
0.47 605267 Ferrite 30.1 1.3 5.1 4.9 1.3 0.08 Austenite 27.8 1.4
7.6 3.1 1.8 0.46
[0063] The pitting properties of all charges have been tested in
the "Green Death" solution (1% FeCl.sub.3, 1% CuCl.sub.2, 11%
H.sub.2SO.sub.4, 1.2% HCl) for ranking. The test procedure is the
same as pitting testing according to ASTM G48C, but the testing is
carried out in a more aggressive solution than 6% FeCl.sub.3, the
so-called "Green Death" solution. Also general corrosion test in 2%
HCl (2 experiments per charge) has been carried out for ranking
before dew point testing. The results from all experiments are seen
in Table 10, FIG. 2 and FIG. 3. All tested charges perform better
than SAF 2507 in the Green Death solution. All charges are within
the identified interval of 0.9-1.15; preferably 0.9-1.05 as regards
the ratio PRE austenite/PRE ferrite at the same time as PRE in both
austenite and ferrite is higher than 44 and for most of the charges
also substantially higher than 44. Some of the charges even reach
the limit total PRE 50. It is very interesting to note that charge
605 251, alloyed with 1.5% by weight of cobalt, performs almost
equivalent to charge 605 250, alloyed with 0.6% by weight of
cobalt, in "Green Death" solution, in spite of the lower chromium
content in charge 605 251. It is particularly surprisingly and
interesting when charge 605 251 has a PRE number of approx. 48,
which is higher than any commercial super duplex alloy today at the
same time as the T.sub.max sigma value below 1010.degree. C.
indicates a good structural stability based on the values in Table
2 in example 1.
[0064] In Table 10, also the PREW number (% Cr+3.3%(Mo+0.5% W)+16%
N) is given for the total alloy composition and PRE in austenite as
well as ferrite (rounded) based on phase composition being measured
by means of micro probe. The ferrite content is measured after heat
treatment at 1100.degree. C. followed by water quenching.
TABLE-US-00012 TABLE 10 PREW PRE PRE PRE.gamma./ CPT .degree. C.
Charge .alpha. content Total .alpha. .gamma. PRE.alpha. the Green
Death 605258 48.2 50.3 48.1 49.1 1.021 65/70 605249 59.8 48.9 48.3
46.6 0.967 75/80 605259 49.2 50.2 48.8 48.4 0.991 75/75 605260 53.4
48.5 46.1 47.0 1.019 75/80 605250 53.6 49.2 48.1 46.8 0.974 95/80
605251 54.2 48.2 48.1 46.9 0.976 90/80 605261 50.8 48.6 45.2 46.3
1.024 80/70 605252 56.6 48.2 48.2 45.6 0.946 80/75 605254 53.2 48.8
48.5 46.2 0.953 90/75 605255 57.4 46.9 46.9 44.1 0.940 90/80 605262
57.2 47.9 48.3 45.0 0.931 70/85 605263 53.6 49.7 49.8 47.8 0.959
80/75 605253 52.6 48.4 48.2 45.4 0.942 85/75 605266 62.6 49.4 48.3
47.6 0.986 70/65 605269 52.8 50.5 49.6 46.9 0.945 80/90 605268 52.0
49.9 48.7 47.0 0.965 85/75 605270 57.0 49.2 48.5 45.7 0.944 80/85
605267 59.8 49.3 47.6 45.4 0.953 60/65 CPT CCT Charge Average
Average RP0.12 RT Rm RT A RT Z RT 605258 84 68 725 929 40 73 605249
74 78 706 922 38 74 605259 90 85 722 928 39 73 605260 93 70 709 917
40 73 605250 89 83 698 923 38 75 605251 95 65 700 909 37 74 605261
93 78 718 918 40 73 605252 87 70 704 909 38 74 605254 93 80 695 909
39 73 605255 84 65 698 896 37 74 605262 80 83 721 919 36 75 605263
83 75 731 924 37 73 605253 96 75 707 908 38 73 605266 63 78 742 916
34 71 605269 95 90 732 932 39 73 605268 75 85 708 926 38 73 605270
95 80 711 916 38 74 605267 58 73 759 943 34 71
[0065] In order to more closely examine the structural stability,
the samples were annealed for 20 min at 1080.degree. C.,
1100.degree. C. and 1150.degree. C., and then they were quenched in
water.
[0066] The temperature where the amount of intermetallic phase
became negligibly small was determined by means of investigations
in light-optical microscope. A comparison of the structure of the
charges after annealing at 1080.degree. C. followed by water
quenching indicates which of the charges that are more inclined to
contain undesired sigma phase. The results are seen in Table 11.
Structural control shows that the charges 605 249, 605 251, 605
252, 605 253, 605 254, 605 255, 605 259, 605 260, 605 266 as well
as 605 267 are free from undesired sigma phase. Furthermore, charge
605 249, alloyed with 1.5% by weight of cobalt, is free from sigma
phase, while charge 605 250, alloyed with 0.6% by weight of cobalt,
contains a little sigma phase. Both charges are alloyed with high
percentage of chromium, almost 29.0% by weight, as well as
molybdenum content of almost 4.25% by weight. When the compositions
of the charges 605 249, 605 250, 605 251 and 605 252 are compared
considering the sigma phase content, it is very clear that the
composition interval for the optimal material in respect of, in
this case, structural stability, is very narrow. Furthermore, it is
evident that charge 605 268 contains only occasional sigma phase in
comparison with charge 605 263, which contains much sigma phase.
What essentially separates these charges, is addition of copper to
charge 605 268. In charge 605 266 as well as 605 267, the sigma
phase is free in spite of high chromium content, the later charge
is alloyed with copper. Furthermore, the charges 605 262 and 605
263, having the addition of 1.0% by weight of tungsten, have a
structure with much sigma phase, while it is interesting to note
that charge 605 269, also having 1.0% by weight of tungsten but of
a higher nitrogen content than 605 262 and 605 263, has a
considerably smaller quantity of sigma phase. Thus, a very
well-adjusted balance between the various alloying elements is
required at these high alloy contents for, e.g., chromium and
molybdenum, in order to obtain good structural properties.
[0067] Table 12 shows the results from the light optical
investigation after annealing at 1080.degree. C., 20 min, followed
by water quenching. The amount of sigma phase is indicated by means
of values from 1 to 5, with 1 representing that no sigma phase has
been detected upon the investigation, while 5 representing that a
very high percentage of sigma phase has been detected upon the
investigation. TABLE-US-00013 TABLE 12 Charge Sigma phase Cr Mo W
Co Cu N Ru 605249 1 28.8 4.23 1.5 0.38 605250 2 28.8 4.24 0.6 0.40
605251 1 28.1 4.24 1.5 0.38 605252 1 28.4 4.23 0.5 0.37 605253 1
28.8 4.16 1.5 0.37 605254 1 26.9 4.80 1.0 0.38 605255 1 28.6 4.04
3.0 0.31 605258 2 29.0 4.23 1.5 0.46 605259 1 29.0 4.23 0.6 0.45
605260 1 27.5 4.22 1.5 0.44 605261 2 27.8 4.22 0.6 0.43 605262 4
27.6 3.93 1.0 1.0 0.36 605263 5 28.7 3.96 1.0 1.0 0.40 605266 1
30.0 4.02 0.38 605267 1 29.3 4.23 1.5 0.38 605268 2 28.2 3.98 1.0
1.0 1.0 0.43 605269 3 28.5 3.97 1.0 1.0 0.45 605270 3 28.8 4.19 1.5
0.41 0.1
[0068] Table 13, results are shown from impact resistance testing
of some of the charges. The results are very good, which indicates
a fine structure after annealing at 1100.degree. C. followed by
water quenching and the requirement of 100 J is met by a large
margin by all tested charges. TABLE-US-00014 TABLE 13 Impact Impact
Impact Annealing resistance resistance resistance Charge [.degree.
C./min] Cooling down [J] [J] [J] 605249 1100/20 Water >300
>300 >300 605250 1100/20 Water >300 >300 >300 605251
1100/20 Water >300 >300 >300 605252 1100/20 Water >300
>300 >300 605253 1100/20 Water 258 267 257 605254 1100/20
Water >300 >300 >300 605255 1100/20 Water >300 >300
>300
[0069] FIG. 4 shows the results from hot ductility test of most of
the charges. A good workability is naturally crucial in order to be
able to manufacture the material into product shapes such as bars,
pipes, such as welded and seamless pipes, thread, weld material,
construction parts such as, for instance, flanges and couplings.
The charges 605 249, 605 250, 605 251, 605 252, 605 255, 605 266 as
well as 605 267, most having a nitrogen content of around 0.38% by
weight, have somewhat better hot ductility values.
[0070] The strain controlled fatigue properties give information
about how much, and how many times, a material may be elongated,
before strain controlled fatigue cracks arise in the material.
Since umbilical pipes are welded together into long lengths, are
reeled on drums before the are twisted into the umbilical, it is
not unusual that a number of operations occurs where certain
plastic deformation arises before the umbilical starts function.
The strain controlled fatigue data that has been established
emphasize that the risk of rupture as a consequence of strain
controlled fatigue in an umbilical pipe borders on zero.
SUMMARY
[0071] The demands that are made on umbilical pipes in the future
and that are met by an optimised alloy according to above, is that
PRE of min 46 in the alloy combined with the fact that PRE in
austenite or ferrite exceeds 45 is required in order to obtain
sufficiently good pitting and crevice corrosion properties. Thus,
it is required that: CPT in 6% FeCl.sub.3>90.degree. C. CCT in
6% FeCl.sub.3>60.degree. C.
[0072] The strength that is required for being able to
substantially reduce the weight of an umbilical is: Yield point in
tension Rp.sub.0.2 min 720 N/mm.sup.2
[0073] In order to be able to manufacture umbilical pipes and in
order to guarantee that pitting and crevice corrosion resistance as
well as mechanical properties being preserved, the following is
required regarding the structural stability: [0074] The alloy shall
be weldable by means of conventional welding methods [0075]
Maximally 0.5% sigma phase in the structure [0076] Maximum
resolution temperature of sigma phase is 1010.degree. C.
[0077] The material according to the present invention has, in view
of the high alloys content thereof, extraordinarily good
workability, in particular hot-workability, and should thereby be
very suitable to be used for, for instance, the manufacture of
bars, pipes, such as welded and weldless pipes, weld material,
construction parts, such as, for instance, flanges and
couplings.
* * * * *