U.S. patent application number 12/519069 was filed with the patent office on 2010-04-08 for plate.
This patent application is currently assigned to Sandvik Intellectual Property AB. Invention is credited to Kenneth Goransson, Ulf Kivisakk.
Application Number | 20100084121 12/519069 |
Document ID | / |
Family ID | 39511973 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100084121 |
Kind Code |
A1 |
Kivisakk; Ulf ; et
al. |
April 8, 2010 |
PLATE
Abstract
The present invention relates to a plate of a plate heat
exchanger, said plate comprising a material which constitutes a
part of said plate with a surface made of this material positioned
to be in direct contact with a chloride-containing cooling liquid,
such as sea water, in which the material is a duplex stainless
steel alloy containing in weight-%: C max 0.06%, Si max 1.5%, Mn
0-3.0%, Cr 23.0-32.0%, Ni 4.9-10.0%, Mo 3.0-8.0%, N 0.15-0.5%, B
0.-0.010%, S max 0.030%, Co 0-3.5%, W 0-3.0%, Cu 0-2.0%, Ru 0-0.3%,
Al 0-0.2%, Ca 0-0.010% balance Fe and normal occurring impurities,
wherein the ferrite content is 35-70 volume-%.
Inventors: |
Kivisakk; Ulf; (Sandviken,
SE) ; Goransson; Kenneth; (Sandviken, SE) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W., SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Assignee: |
Sandvik Intellectual Property
AB
Sandviken
SE
|
Family ID: |
39511973 |
Appl. No.: |
12/519069 |
Filed: |
December 13, 2007 |
PCT Filed: |
December 13, 2007 |
PCT NO: |
PCT/SE2007/050986 |
371 Date: |
December 4, 2009 |
Current U.S.
Class: |
165/166 ;
165/167; 420/38 |
Current CPC
Class: |
C22C 38/44 20130101;
F28F 19/00 20130101; F28F 21/082 20130101; F28D 9/005 20130101;
C22C 38/001 20130101; C22C 38/04 20130101 |
Class at
Publication: |
165/166 ;
165/167; 420/38 |
International
Class: |
F28F 3/12 20060101
F28F003/12; F28F 21/08 20060101 F28F021/08; C22C 38/52 20060101
C22C038/52 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2006 |
SE |
0602697-5 |
Claims
1. Plate of a plate heat exchanger, said plate comprising a
material which constitutes a part of said plate with a surface made
of this material positioned to be in direct contact with a
chloride-containing cooling liquid, such as sea water, wherein said
plate material is a duplex stainless steel alloy containing in
weight-%: TABLE-US-00004 C max 0.06% Si max 1.5% Mn 0-3.0% Cr
23.0-32.0% Ni 4.9-10.0% Mo 3.0-8.0% N 0.15-0.5% B 0[[.]]-0.010% S
max 0.030% Co 0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.2% Ca
0-0.010%
balance Fe and normal occurring impurities, wherein the ferrite
content is 35-70 volume-%, and that the average Eq1-value of the
two phases of the alloy exceeds 40.5, whereby Eq1=% Cr+3.3% Mo,
wherein % is weight-%.
2. A plate according to claim 1, wherein said average Eq1-value of
the two phases of the alloy is higher than 41.
3. A plate according to claim 1, wherein the Eq1-value for both the
ferrite and the austenite phase is higher than 35.
4. A plate according to claim 1, wherein the content of Mo is
4.5-6.5 weight-%.
5. A plate according to claim 4, wherein the content of Mo is
4.5-5.5 weight-%.
6. A plate according to claim 1, wherein the content of Cr is
23.0-30.0 weight-%.
7. A plate according to any of the preceding claim 1, wherein the
average PRE-value of the two phases of the alloy is higher than 46,
whereby PRE=% Cr+3.3% Mo 1-16% N, wherein % is weight-%.
8. A plate according to claim 1, wherein the content of Al is 0-0.1
weight-%.
9. Plate heat exchanger adapted to utilise a chloride-containing
liquid, such as sea water, as cooling medium, wherein it is made of
plates according to claim 1.
10. Plate heat exchanger according to claim 9, wherein said plates
are joined to each other in a stack for forming channels between
adjacent plates and that said channels formed in every second gap
between adjacent plates are configured to have a
chloride-containing cooling liquid flowing therein and said
channels formed in every second gap between adjacent plates are
configured to have a medium to be cooled flowing therein.
11. Use of a plate heat exchanger according to claim 9 for cooling
a medium to be cooled by a chloride-containing cooling liquid, such
as sea water.
12. Use according to claim 11, in which the temperature of said
cooling liquid is allowed to reach a temperature of at least
50.degree. C.
13. Use according to claim 12, wherein said cooling liquid is
allowed to reach a temperature of at least 60.degree. C.
14. A plate according to claim 2, wherein said average Eq1-value of
the two phases of the alloy is higher than 42.
15. A plate according to claim 3, wherein the Eq1-value for both
the ferrite and the austenite phase is higher than 36.
16. A plate according to claim 7, wherein the average PRE-value of
the two phases of the alloy is higher than 47.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a plate of a plate heat
exchanger, said plate comprising a material which constitutes a
part of said plate with a surface made of this material positioned
to be in direct contact with a chloride-containing cooling liquid,
such as sea water, and a plate heat exchanger adapted to utilize a
chloride-containing liquid, such as sea water, as cooling
medium.
BACKGROUND OF THE INVENTION AND PRIOR ART
[0002] In different plants, such as different types of machines,
electric converter stations and the like, located close to the sea
or on off-shore platforms it is necessary to use
chloride-containing liquid in the form of sea water as cooling
medium in heat exchangers used for cooling purposes in these
plants. The use of sea water in these heat exchangers puts high
demands on the material used due to the tough high corrosive
environment created by the sea water.
[0003] The crevice corrosion phenomena constitutes the main problem
in plate heat exchangers utilizing sea water as cooling liquid,
since it may not be avoided that connecting interfaces between
adjacent plates of the heat exchanger are located so that the sea
water will reach the crevices or joint thus formed between adjacent
plates, and they have also to be located where the cooling liquid
has a comparatively high temperature, which is also critical (see
below). This problem is much smaller for tube heat exchangers,
where such crevices or joints are less severe and may be located
where the risk of crevice corrosion is much lower. The problems of
crevice corrosion in plate heat exchangers may be reduced by
welding the plates to each other and connect them to each other by
sealings, but the problem will by that not disappear. Appended FIG.
1 schematically shows a plate heat exchanger PHE of this type
having a number of plates P joined to each other in a stack for the
flow of a cooling medium in the form of sea water SW in channels
formed in every second gap between adjacent plates P.sub.1, P.sub.2
and a medium to be cooled in adjacent channels in every second gap
between such plates. The crevice or joint sensitive to crevice
corrosion is indicated at C.
[0004] Crevice corrosion destroying the material is temperature
dependent, and when the material for a given cooling liquid, in
this case sea water, has a temperature below a critical crevice
temperature (CCT) substantially nothing will happen, but when the
temperature of the material is raised above this temperature the
corrosion of the material at said crevice will be very strong and
in a short time destroy the connection, so that temperatures above
said critical crevice temperature may not be accepted. This crevice
corrosion temperature should in a plate heat exchanger using a
chloride-containing cooling liquid, such as sea water, as cooling
medium be at least 50.degree. C., preferably at least 60.degree.,
for providing an acceptable cooling capacity of the heat exchanger.
Sea water used in plate heat exchangers may be chlorinated for the
purpose of killing micro organisms. If these microorganisms are not
killed by e.g. chlorination, their presence will cause an increase
in the corrosivity of the environment. At low temperatures, i.e.
below approximately 40.degree. C., the chlorination in itself does
not result in any increased corrosivity towards e.g. stainless
steels. At temperatures above 40.degree. C., the increase in redox
potential caused by the chlorination severely increases the
corrosivity of the water with respect to pitting and crevice
corrosion, thus limiting the choice of available construction
materials for heat exchangers. Plate heat exchangers of the type
defined in the introduction, i.e. utilizing chloride-containing
cooling liquid, such as sea water, as cooling medium, for use at
higher temperatures have for that sake so far almost exclusively
been provided with plates made of titanium, which has a critical
crevice temperature above 80.degree. C. in sea water. However,
titanium is a very expensive material and it is also not easy
available, so that it may sometimes be impossible to avoid waiting
times for deliveries thereof in the order of a year or more
irrespectively of the financial resources of the buyer.
SUMMARY OF THE INVENTION
[0005] The object of the present invention is to provide a plate of
a plate heat exchanger being less costly and easier available than
such plates of titanium while still having sufficiently high
corrosion resistance for making it attractive to be used in a plate
heat exchanger using a chloride-containing cooling liquid, such as
sea water, as cooling medium.
[0006] This object is according to the invention obtained by
providing such a plate in which said plate material is a duplex
stainless steel alloy containing in weight %: C max 0.06%, Si max
1.5%, Mn 0-3.0%, Cr 23.0-32.0%, Ni 4.9-10.0%, Mo 3.0-8.0%, N
0.15-0.5%, B 0.-0.010%, S max 0.030%, Co 0-3.5%, W 0-3.0%, Cu
0-2.0%, Ru 0-0.3%, Al 0-0.2%, Ca 0-0.010% balance Fe and normal
occurring impurities, wherein the ferrite content is 35-70
volume-%. The steel may also contain impurities resulting from the
raw material used and/or the manufacturing process, and the average
Eq1-value of the two phases of the alloy exceeds 40.5, whereby
Eq1=% Cr+3.3% Mo, wherein % is weight-%. Examples of impurities
from the manufacturing process are Al and Mg. However, the content
of impurities are preferably kept at such a level that the
properties of the produced material is substantially unaffected
thereof.
[0007] The inventors have realized that for materials used for
plate heat exchanger using a chloride-containing cooling liquid as
cooling medium the critical crevice temperature is the main issue
and that the composition of the material may be determined with the
aim to raise this temperature to a level acceptable for a plate
heat exchanger without spending any particular efforts on other
resistance properties of the material. It has been found that
duplex stainless steel alloys with a composition within these
ranges has a critical crevice temperature in sea water well
exceeding 50.degree. C. and in fact exceeding 60.degree. C. Thus,
it has been found that not the PRE-value, also containing a factor
of 16% N, decides the crevice corrosion behaviour of the duplex
stainless steel alloy, but it is the content of Cr and Mo, that is
the determining factor for the critical crevice temperature of the
material. Furthermore, such a Eq1-value above 40.5 has turned out
to result in a critical crevice temperature of the steel alloy
exceeding 60.degree. C. For some compositions within these ranges
the critical crevice temperature may even be raised to the region
of 80.degree. C. This means that this material will constitute an
attractive substitution to titanium in a plate heat exchanger. The
cost thereof will be only a small fraction of the cost for
titanium, such as about 10-20% thereof, and it will be manufactured
at any time avoiding the long waiting times that may occur for
titanium.
[0008] According to another embodiment of the invention said
average Eq1-value of the two phases of the alloy is higher than 41,
preferably higher than 42. It has been found that such high levels
of the Eq1-value are influencing the critical crevice temperature
of the material in chloride-containing environment towards higher
levels.
[0009] According to another embodiment of the invention the
Eq1-value for both the ferrite and the austenite phase is higher
than 35, preferably higher than 36, which in combination with an
average Eq1-value exceeding 40.5 is preferred for keeping the
critical crevice temperature of the material at a required high
level. Mo and Cr will primarily choose the ferrite phase, so that
the Eq1-value of the austenite phase may be close to 35, although
said average Eq1-value is above 40.5.
[0010] According to another embodiment of the invention, the
content of Mo is 4.5-6.5 weight-%. It has turned out that for a
given value of Eq1 it is preferred to have a content of Mo being
high, namely within this range, since the content of Mo has turned
out to be the most important factor for the critical crevice
temperature of the material. For obtaining a given Eq1-value it is
also preferred to increase the content of Mo rather than that of
Cr, even though an increased content of Cr increases the
workability of the material when producing the plate, but it
increases at the same time the risk of formation of CrN. According
to another embodiment of the invention the content of Mo is 4.5-5.5
weight-%.
[0011] According to another embodiment of the invention the content
of Cr is 23.0-30.0 weight-%. It has been found that a content of Cr
within this range is suitable for obtaining a crevice corrosion
behaviour of the alloy in a chloride-containing environment aimed
at.
[0012] According to another embodiment of the invention the average
PRE-value of the two phases of the alloy is higher than 46,
preferably higher than 47, whereby PRE=% Cr+3.3% Mo+16% N, wherein
% is weight-%. It has turned out to be advantageous to have such a
high PRE-value of the material, although the Eq1-value is more
important for the crevice corrosion resistance of the material.
[0013] According to another embodiment of the invention the content
of Al is 0-0.1 weight-%.
[0014] The invention also relates to a plate heat exchanger adapted
to utilize the chloride-containing liquid, such as sea water, as
cooling medium, which is characterized in that it is made of plates
according to the invention. The advantageous features and
advantages of such a plate heat exchanger appear clearly from the
above discussion of the plate according to the invention. The
invention also relates to use of a plate heat exchanger according
to the invention for cooling a medium to be cooled by a
chloride-containing cooling liquid, such as sea water, and such a
use in which the temperature of said cooling liquid is allowed to
reach a temperature of at least 50.degree. C., preferably at least
60.degree. C. It is pointed out that the medium to be cooled by the
heat exchanger may be of any type, and it may be a gas or gas
mixture, such as air, just as well as for example a liquid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a very simplified view showing the general
structure of a plate heat exchanger with a portion thereof enlarged
for explaining the problems to be solved by the present
invention,
[0016] FIG. 2 is a graph of critical crevice temperatures versus
PRENW-value for alloys according to the invention and reference
alloys,
[0017] FIG. 3 is a graph of critical crevice temperatures versus
Eq1-value for alloys according to the invention and reference
alloys,
[0018] FIG. 4 is a simplified view illustrating how a material test
has been carried out for alloys according to the invention and
reference alloys, and
[0019] FIG. 5 is a graph corresponding to the graph of FIG. 3 based
on another testing method.
DETAILED DESCRIPTION OF THE INVENTION
[0020] A high critical crevice temperature in chloride-containing
environment is obtained by the combination of elements in a duplex
stainless steel alloy according to the invention. The alloy
according to the invention contains (in weight-%):
TABLE-US-00001 C max 0.06% Si max 1.5% Mn 0-3.0% Cr 23.0-32.0% Ni
4.9-10.0% Mo 3.0-8.0% N 0.15-0.5% B 0.-0.010% S max 0.030% Co
0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.2% Ca 0-0.010%
balance Fe and normal occurring impurities, wherein the ferrite
content is 35-70 volume-%.
[0021] Carbon (C) has limited solubility in both ferrite and
austenite. The limited solubility implies a risk of precipitation
of chromium carbides and the content should therefore not exceed
0.06 weight-%, preferably not exceed 0.02 weight-%.
[0022] Silicon (Si) is utilized as desoxidation agent in the steel
production and it increases the flowability during production and
welding. However, too high contents of Si lead to precipitation of
unwanted intermetallic phase, and the content thereof is limited to
1.5 weight-%.
[0023] Manganese (Mn) is added in order to increase the
N-solubility in the material. However, it has shown that Mn only
has a limited influence on the N-solubility in the type of alloy in
question. Instead there are found other elements with higher
influence on the solubility. Besides, Mn in combination with high
contents of sulfur can give rise to formation of manganese
sulfides, which act as initiation-points for pitting corrosion. The
content of Mn should therefore be limited to between 0-3.0
weight-%.
[0024] Sulfur (S) influences the corrosion resistance negatively by
forming soluble sulfides. Furthermore, the hotworkability
deteriorates, for what reason the content of sulfur is limited to
max 0.030 weight-%, preferably max 0.010 weight-%.
[0025] Chromium (Cr) is an active element in order to improve the
crevice corrosion resistance. Furthermore, a high content of
chromium implies that one gets a very good N-solubility in the
material. Thus, it is desirable to keep the Cr-content as high as
possible in order to improve the corrosion resistance. For good
crevice corrosion resistance the content of chromium should be at
least 23 weight-%. However, high contents of Cr increase the risk
for intermetallic precipitations and the formation of CrN, for what
reason the content of chromium should not exceed 32 weight-%,
preferably not 30 weight-%.
[0026] Nickel (Ni) is used as austenite stabilizing element and is
added in suitable contents in order to obtain the desired content
of ferrite. In order to obtain the desired relationship between the
austenitic and the ferritic phase with between 40-65 volume-%
ferrite, an addition of 4.9-10.0 weight-% nickel is required.
[0027] Molybdenum (Mo) is an active element which improves the
crevice corrosion resistance in chloride environments. The
Mo-content in the present invention should lie in the range of
0-8.0 weight-%, preferably above 4.5 weight-%. The content of Mo in
combination with the content of Cr is the determining factors for
obtaining a high critical crevice temperature of the alloy.
[0028] Tungsten (W) increases mainly the resistance to pitting
corrosion. But the addition of too high contents of tungsten in
combination with that the Cr-contents as well as Mo-contents are
high, means that the risk for intermetallic precipitations
increases. The W-content in the present invention should lie in the
range of 0-3.0 weight-%.
[0029] Copper (Cu) may be added in order to improve the general
corrosion resistance in acid environments such as sulfuric acid. At
the same time Cu influences the structural stability. However,
thigh contents of Cu imply that the solid solubility will be
exceeded. Therefore the Cu-content should be limited to max 2.0
weight-%.
[0030] Cobalt (Co) has properties that are intermediate between
those of iron and nickel. Therefore, a minor replacement of these
elements with Co, or the use of Co-containing raw materials (Ni
scrap metal usually contains some Co, in some cases in quantities
greater than 10%) will not result in any major change in
properties. Co can be used to replace some Ni as an
austenite-stabilizing element. Co is a relatively expensive
element, so the addition of Co is limited to be within the range of
0-3.5 weight-%.
[0031] Aluminium (Al) and Calcium (Ca) are used as desoxidation
agents at the steel production. The content of Al should be limited
to max 0.2 weight-%, preferably max 0.1 weight-%, in order to limit
the forming of nitrides. Ca has a favourable effect on the
hotductility. However, the Ca-content should be limited to max
0.010 weight-% in order to avoid an unwanted amount of slag.
[0032] Boron (B) may be added in order to increase the
hotworkability of the material. At a too high content of Boron the
weldability as well as the corrosion resistance could deteriorate.
Therefore, the content of boron should be limited to max 0.010
weight-%.
[0033] Nitrogen (N) is a very active element, which increases the
corrosion resistance, the structural stability as well as the
strength of the material. Furthermore, a high N-content improves
the recovering of the austenite after welding, which gives good
properties within the welded joint. In order to obtain a good
effect of N, at least 0.15 weight-% N should be added. At high
contents of N the risk for precipitation of chromium nitrides
increases, especially when simultaneously the chromium content is
high. Furthermore, a high N-content implies that the risk for
porosity increases because of the exceeded solubility of N in the
smelt. For these reasons the N-content should be limited to max
0.50 weight-%.
[0034] The content of ferrite is important in order to obtain good
mechanical properties and corrosion properties as well as good
weldability. From a corrosion point of view and a point of view of
weldability a content of ferrite between 35-70% is desirable in
order to obtain good properties.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] Table 1 below shows the composition of alloys 1-25 according
to the invention and reference alloys being not according to the
invention as well as results of a testing, Testing 1.
Testing 1
[0036] The crevice corrosion resistance of 25 alloys according to
the invention and 7 reference alloys was tested according to MTI-2.
The critical crevice temperature (CCT) was determined for all the
32 alloys for two different samples. The average value of CCT for
each alloy is indicated in Table 1. Furthermore, the conventional
expression for "pitting resistance equivalent" in the alloys is
given in Table 1 (PRENW=% Cr+3.3% Mo+0.5% W)+16% N, as well as the
Eq1-value defined as Eq1=% Cr+3.3% Mo.
[0037] All the alloys were produced by melting, hot working and
annealing followed by water quenching.
TABLE-US-00002 TABLE 1 % Cr + CCT/ 3.3% .degree. C. PRENW Mo C Si
Mn Cu Co Alloy 1 77.5 49.9 42.8 0.015 0.16 0.86 0.01 1.5 Alloy 2 65
49.0 42.7 0.017 0.18 1.06 0.01 1.5 Alloy 3 77.5 50.5 42.8 0.017
0.23 0.99 0.01 1.5 Alloy 4 82.5 50.8 42.8 0.019 0.23 1.12 0.03 0.6
Alloy 5 65 48.2 42.1 0.017 0.22 1.01 0.03 1.5 Alloy 6 70 48.2 42.3
0.018 0.2 1.1 0.03 0.5 Alloy 7 75 48.4 42.5 0.012 0.15 1.04 0.01
1.4 Alloy 8 80 50.4 42.7 0.016 0.2 1.07 0.03 1.0 Alloy 9 65 46.9
41.9 0.017 0.21 1.02 0.04 3 Alloy 10 67.5 50.3 43.0 0.02 0.25 1.1
0.14 1.5 Alloy 11 85 50.2 43.0 0.02 0.23 1.1 0.14 0.6 Alloy 12 70
48.5 41.4 0.018 0.25 1.1 0.13 1.5 Alloy 13 77.5 48.6 41.7 0.019
0.23 1 0.13 0.6 Alloy 14 82.5 47.9 40.5 0.017 0.21 2.7 0.12 1 Alloy
15 75 50.0 41.9 0.018 0.2 1 0.13 1 Alloy 16 77.5 49.3 43.3 0.02
0.25 1.1 0.14 <0.1 Alloy 17 90 50.5 41.6 0.019 0.25 1 0.18 1
Alloy 18 85 49.9 41.4 0.02 0.28 1.1 1 1 Alloy 19 80 49.2 42.7 0.021
0.24 1 0.14 1.5 Alloy 20 72.5 49.3 43.3 0.019 0.23 1.1 1.5 <0.1
Alloy 21 65 47.8 41.4 0.019 0.22 0.5 0.02 <0.1 Alloy 22 62.5
49.8 43.1 0.019 0.62 0.47 0.02 <0.1 Alloy 23 67.5 49.8 43.2
0.017 0.21 0.49 <0.01 <0.1 Alloy 24 62.5 49.3 43.2 0.021 0.61
0.48 0.01 1.0 Alloy 25 62.5 48.6 42.0 0.019 0.24 0.51 <0.01 1.0
Ref 1 50 50.1 38.3 0.034 0.42 0.86 1.0 1.0 Ref 2 35 47.4 34.2 0.055
0.89 0.93 2.0 <0.1 Ref 3 45 50.3 38.0 0.035 0.48 0.93 1.0 1.0
Ref 4 40 49.4 36.5 0.007 0.12 0.9 2.0 0.1 Ref 5 30 49.9 34.5 0.006
0.12 0.95 2.0 2.0 Ref 6 35 51.9 36.8 0.06 0.11 1.09 <0.01
<0.1 Ref 7 40 47.3 40.3 0.008 0.14 1.07 <0.01 <0.1 % Cr +
CCT/ 3.3% .degree. C. PRENW Mo Cr Ni Mo W N Alloy 1 77.5 49.9 42.8
28.9 6.6 4.2 0.01 0.44 Alloy 2 65 49.0 42.7 28.8 6.5 4.2 0.01 0.39
Alloy 3 77.5 50.5 42.8 28.8 7.0 4.2 1 0.38 Alloy 4 82.5 50.8 42.8
28.8 7.6 4.2 0.99 0.4 Alloy 5 65 48.2 42.1 28.1 6.5 4.2 0.01 0.38
Alloy 6 70 48.2 42.3 28.4 6.9 4.2 <0.01 0.37 Alloy 7 75 48.4
42.5 28.8 7.0 4.2 <0.01 0.37 Alloy 8 80 50.4 42.7 26.9 6.5 4.8
1.01 0.38 Alloy 9 65 46.9 41.9 28.6 6.5 4.0 0.01 0.31 Alloy 10 67.5
50.3 43.0 29.0 6.5 4.2 <0.01 0.46 Alloy 11 85 50.2 43.0 29.0 6.8
4.2 <0.01 0.45 Alloy 12 70 48.5 41.4 27.5 5.9 4.2 <0.01 0.44
Alloy 13 77.5 48.6 41.7 27.8 6.1 4.2 <0.01 0.43 Alloy 14 82.5
47.9 40.5 27.6 6.9 3.9 1 0.36 Alloy 15 75 50.0 41.9 28.7 6.6 4.0 1
0.4 Alloy 16 77.5 49.3 43.3 30.0 7.1 4.0 <0.01 0.38 Alloy 17 90
50.5 41.6 28.5 7.0 4.0 1 0.45 Alloy 18 85 49.9 41.4 28.2 6.6 4.0 1
0.43 Alloy 19 80 49.2 42.7 28.8 7.0 4.2 <0.01 0.41 Alloy 20 72.5
49.3 43.3 29.3 6.5 4.2 <0.01 0.38 Alloy 21 65 47.8 41.4 25.8 7.1
4.7 <0.01 0.4 Alloy 22 62.5 49.8 43.1 26.1 7.0 5.2 <0.01 0.42
Alloy 23 67.5 49.8 43.2 26.1 7.1 5.2 <0.01 0.41 Alloy 24 62.5
49.3 43.2 26.3 7.0 5.1 <0.01 0.38 Alloy 25 62.5 48.6 42.0 26.2
6.5 4.8 <0.01 0.41 Reference 50 50.1 38.3 30.8 7.5 2.2 2.7 0.46
1 Reference 35 47.4 34.2 29.2 7.6 1.5 3.72 0.44 2 Reference 45 50.3
38.0 30.6 7.7 2.2 2.88 0.47 3 Reference 40 49.4 36.5 31.6 9.4 1.5
3.86 0.41 4 Reference 30 49.9 34.5 29.3 6.2 1.6 3.94 0.56 5
Reference 35 51.9 36.8 31.9 6.3 1.5 3.85 0.55 6 Reference 40 47.3
40.3 28.7 7.4 3.5 <0.01 0.44 7
[0038] Appended FIG. 2 shows the relationship between PRENW and CCT
and FIG. 3 the relationship between Eq1 and CCT for Testing 1. FIG.
2 shows that the CCT is for all the alloys according to the
invention above 60.degree. C., whereas it is not above 50.degree.
C. for any of the reference alloys in spite of high PRENW-values
thereof. It is shown that the PRENW-value as long as it is in a
region above 46 has no real influence upon the critical crevice
temperature of the alloy. FIG. 3 shows that Eq1 should be higher
than 40.5 for obtaining a CCT above 60.degree. C. It should be
noted that the experimental spread in determining the CCT-value is
great (about .+-.10.degree. C.), and that it is not only the total
value of Eq1 that is determining but also how it is distributed
between the two phases of the material (ferrite and austenite).
Testing 2
[0039] An electrochemical test according to a modified version of
ASTM G-150, modified in the way that the samples examined were
provided with a crevice former in PVDF mounted in approximately the
same way as in MTI-2 in Testing 1. A constant tightening moment of
3 Nm was used for mounting crevice formers. A constant crevice
pressure was maintained by means of four spring washers 1 mounted
according to FIG. 4, in which 2 shows the sample and 3 and 4 the
crevice formers.
[0040] The test was carried out for four different constant
potentials: 0, +200, +400 and +700 mV.sub.SCE. The start
temperature was 20.degree. C., and the temperature increased during
the experiment 1.degree. C. per minute. The CCT for each sample has
been defined as the temperature resulting in a corrosion current
density of at least 0.1 mA/cm.sup.2 in 60 seconds.
[0041] CCT was determined for 4-6 samples per material and
potential. Table 2 below shows the compositions and the results in
the form of min values of CCT for all potentials over 0 mV versus
SCE. The variations in CCT between different potentials were small.
In most cases no crevice corrosion was noted at 0 mV.
TABLE-US-00003 TABLE 2 Alloy Cr Ni Mo W N PRENW Eq1 CCT Alloy 26
31.71 7 3.45 <0.01 0.50 51.095 43.1 82.7 Alloy 27 26.62 7 4.73
<0.01 0.38 48.309 42.2 86.7 Alloy 28 26.02 9 5.06 <0.01 0.4
49.118 42.7 87.1 Reference 6 31.9 6 1.47 3.85 0.55 51.9035 36.8
67.1 Reference 7 28.71 7 3.51 <0.01 0.44 47.333 40.3 76.3
Reference 8 29.88 7 1.5 1.98 0.44 45.137 34.8 58.4 Reference 9
29.36 8 2.51 2.1 0.44 48.148 37.6 73.1 Reference 10 25 7 4.00
<0.01 0.28 42.68 38.2 70.6 Reference 11 29.21 7 2.32 <0.01
0.37 42.786 36.9 72.3
[0042] FIG. 5 shows the relation between Eq1 and CCT according to
this modified ASTM G-150. Some of the alloys are present both in
Testing 1 and Testing 2. The two different testing methods give
somewhat different values of CCT, which is considered to be normal
as a consequence of the differences of the methods.
[0043] However, the relationship between the composition and the
CCT-value is mainly the same in the two Testings.
[0044] This Testing 2 measures primarily the tendency to initiate
crevice corrosion, while the Testing 1 primarily measures the
tendency to propagation of crevice corrosion. This means that the
CCT-values will in the Testing 2 be somewhat higher than in the
testing 1. This is the reason while the lowest CCT-value allowed
for use in a plate heat exchanger according to the Testing 2 should
be set to 80.degree. C.
Testing 3
[0045] This testing was an electrochemical testing with constant
potential and temperature in 24 hours. The same type of crevice
former and the same crevice pressure as in the Testing 2 were used.
New samples were used for each temperature-/potential-combination.
The potential was: +200 mV versus SCE.
[0046] The CCT-value for each sample was defined as the temperature
resulted in a corrosion current density of at least 0.01
mA/cm.sup.2 in 60 s. This testing is extremely tough, since the
material is tested in an active state, whereas it in the preceding
testings was passivated before testing, when the temperature was
low. The ability of the material to form a passivating oxide layer
at certain temperature was measured in this Testing 3. The
passivating temperature is for most of the materials considerably
lower than the CCT.
[0047] A comparison of the alloy 26 and 27 in the testing 2 shows
that the alloy 26 was not passivated at 45.degree. C., whereas the
alloy 27 was passivated at 45.degree. C. as well as at 55.degree.
C. However, it may not be passivated at 65.degree.. This means that
if a corrosion attack has been started as a consequence of for
example an over temperature, the alloy 27 will be able to be
passivated already when the temperature falls down below 55.degree.
C., which remarkably reduces the risk for propagating attacks upon
disturbances in the operation of the plate heat exchanger.
[0048] Thus, the alloy 27 having a higher content of Mo has a
better ability to be passivated than the alloy 26 in spite of the
fact that the alloy 26 has a higher Eq1-value. The conclusion is
that the Eq1-value well describes the initiation and the
propagation of crevice corrosion, while passivation against crevice
corrosion is mainly controlled by the material content of Mo.
[0049] An alloy according to another embodiment of the invention
has the following approximate composition: C 0.017%, Si 0.2%, Mn
0.5%, P 0.005%, S 0.006%, Cr 26%, Ni 7%, Mo 5.2%, W<0.01%,
Cu<0.01%, Co<0.010%, Ti<0.005%, Al 0.004%, B 24 ppm, Ca 22
ppm, N 0.41%. Testing of critical crevice temperature (CCT)
according to MTI-2 for two samples resulted in 65.degree. C. and
70.degree. C.
* * * * *