U.S. patent application number 10/486103 was filed with the patent office on 2004-12-23 for aluminium-magnesium alloy product.
Invention is credited to Baekelandt, Jean Pierre Jules, De Smet, Peter, Schepers, Bruno, Van Der Hoeven, Job Anthonius, Zhuang, Linzhong.
Application Number | 20040256036 10/486103 |
Document ID | / |
Family ID | 27224305 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040256036 |
Kind Code |
A1 |
Van Der Hoeven, Job Anthonius ;
et al. |
December 23, 2004 |
Aluminium-magnesium alloy product
Abstract
Aluminium-magnesium alloy in the form of a rolled product or an
extrusion, having the composition in weight percent:--Mg 4.5-5.6 Mn
0.05-0.4 Zn 0.40-0.8 Cu 0.06-0.35 Cr 0.25 max. Fe 0.35 max. Si 0.25
max. Zr 0.12 max. Ti 0.3 max. others (each) max. 0.05, (total) max.
0.15 balance aluminium.
Inventors: |
Van Der Hoeven, Job Anthonius;
(Haarlem, NL) ; Zhuang, Linzhong; (Leiden, NL)
; Schepers, Bruno; (Brasschaat, BE) ; De Smet,
Peter; (Sint-Martens-Latem, BE) ; Baekelandt, Jean
Pierre Jules; (Lier, BE) |
Correspondence
Address: |
STEVENS DAVIS MILLER & MOSHER, LLP
1615 L STREET, NW
SUITE 850
WASHINGTON
DC
20036
US
|
Family ID: |
27224305 |
Appl. No.: |
10/486103 |
Filed: |
August 27, 2004 |
PCT Filed: |
July 31, 2002 |
PCT NO: |
PCT/EP02/08628 |
Current U.S.
Class: |
148/695 ;
148/439; 420/532 |
Current CPC
Class: |
B62D 29/008 20130101;
B62D 29/00 20130101; C22C 21/06 20130101 |
Class at
Publication: |
148/695 ;
148/439; 420/532 |
International
Class: |
C22C 021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 13, 2001 |
EP |
01203069.8 |
Jan 3, 2002 |
EP |
02075047.7 |
Jun 24, 2002 |
EP |
02077547.4 |
Claims
1. Aluminium-magnesium alloy in the form of a rolled product or an
extrusion, having the composition in weight percent: Mg 4.5-5.6 Mn
0.05-0.4 Zn 0.40-0.8 Cu 0.06-0.35 Cr 0.25 max. Fe 0.35 max. Si 0.25
max. Zr 0.12 max. Ti 0.3 max., impurities (each) max. 0.05, (total)
max. 0.15 balance aluminium.
2. Product according to claim 1, wherein the amount of Zr does not
exceed 0.05 wt. %.
3. Product according to claim 1, wherein the amount of Zr does not
exceed 0.02 wt. %.
4. Product according to any one of claim 1, wherein the amount of
Cu is more than 0.075 wt. %.
5. Product according to any one of claim 1, wherein the amount of
Cu is more than 0.10 wt. %.
6. Product according to claim 1, wherein the amount of Cu does not
exceed 0.24 wt. %.
7. Product according to claim 1, wherein the amount of Cu does not
exceed 0.18 wt. %.
8. Product according to claim 1, wherein the amount of Cu does not
exceed 0.15 wt. %.
9. Product according to claim 1, wherein the amount of Mg does not
exceed 4.8 wt. %.
10. Product according to claim 1, wherein the amount of Mn is in
the range of 0.1 to 0.2 wt. %.
11. Product according to claim 1, wherein the amount of Zn is in
the range of 0.4 to 0.75 wt. %.
12. Product according to claim 1, wherein the amount of Zn is in
the range of 0.4 to 0.6 wt. %.
13. Product according to claim 1, wherein the amount of Cr is in
the range of 0.06 to 0.2 wt. %.
14. Product according to claim 1, wherein the amount of Cr is in
the range of 0.11 to 0.2 wt. %.
15. Product according to claim 1, wherein the amount of Si is max.
0.2 wt. %.
16. Product according to claim 1, wherein the amount of Si is max.
0.12 wt. %.
17. Product according to claim 1, wherein the amount of Si is max.
0.10 wt. %.
18. Product according to claim 1, wherein the amount of Fe is max.
0.2 wt. %.
19. Product according to claim 1, wherein the amount of Ti is max.
0.15 wt. %.
20. Product according to claim 1, wherein the product is provided
in an O-temper condition.
21. Product according to claim 1, wherein the product is provided
in an H-temper condition.
22. Product according to claim 1, wherein the product is a rolled
product having a gauge up to 200 mm.
23. Product according to claim 1, wherein the product is a rolled
product having a gauge in the range of 0.5 to 5 mm.
24. Product according to claim 1, wherein the product has a weight
loss after sensitising for 100 hours at 100.degree. C. less than 25
Mg/cm.sup.2 when tested against intergranular corrosion in
accordance with ASTM G67.
25. Product according to claim 1, wherein the product has a weight
loss after sensitising for 100 hours at 100.degree. C. less than 15
Mg/cm when tested against intergranular corrosion in accordance
with ASTM G67.
26. Product according to claim 1, wherein the product has an
elongation A50 of at least 24%.
27. Aluminium-magnesium alloy in the form of a rolled product or an
extrusion, having the composition consisting of, in weight
percent:
7 Mg 4.5-5.6 Mn 0.05-0.4 Zn 0.40-0.8 Cu 0.06-0.35 Cr 0.25 max. Fe
0.35 max. Si 0.25 max. Zr 0.12 max. Ti 0.3 max.,
impurities (each) max. 0.05, (total) max. 0.15 balance
aluminium.
28. Welded structure, comprising at least one section of the
product according to claim 1.
29. Welded pressure vessel comprising a shell that comprises the
wrought aluminium-magnesium alloy product according to claim 1.
30. Method of producing an aluminium alloy rolled product
comprising the steps of: (i) providing an intermediate alloy
product having a composition according to the composition according
to claim 1; (ii) cold working the intermediate alloy product to a
final gauge to obtain an intermediate wrought product; (iii)
annealing the intermediate wrought product by heating the product
at a heating rate in the range of 2 to 200.degree. C./sec, holding
the product at a soaking temperature in the range of 480 to
570.degree. C. for a duration of up to 100 sec, followed by a
cooling at a cooling rate in the range of 10 to 500.degree. C./sec
to a temperature below 150.degree. C.
31. Method according to claim 30, wherein the product is held at
the soaking temperature in the range of 480 to 570.degree. C. for a
duration of up to 40 sec.
32. Method according to claim 30, wherein the soaking temperature
during step (iii) is in the range of 520 to 550.degree. C.
33. Method according to claim 30, wherein the heating rate during
processing step (iii) is at least 50.degree. C./sec.
34. Method according to claim 30, wherein the heating rate during
processing step (iii) is at least 80.degree. C./sec.
35. Method according to claim 30, wherein the final gauge of the
aluminium sheet is in a range of 0.5 to 5 mm.
36. Method according to claim 30, wherein processing step (iii) is
carried out in a continuous annealing facility.
Description
[0001] The invention relates to an aluminium alloy product in the
form of a rolled product or an extrusion. In another aspect, the
invention relates to a welded structure, comprising such an alloy
product.
[0002] Aluminium-magnesium alloy products are known to be used in
the form of sheets or plates or extrusion in the construction of
welded or joined structures such as marine and automotive
applications, storage tanks, pressure vessels, vessels for land or
marine structure. Wrought products are products that have been
subjected to mechanical working by such processes as rolling,
extruding, or forging. Rolled products may have a gauge typically
to about 200 mm.
[0003] A known aluminium alloy having appropriate formability and
weldability, is the Aluminium Association (AA)5454 alloy. Although
the formability and weldability of the AA5454 alloy are sufficient
for many applications, the alloy does not meet the desired higher
strength levels. There is a constant drive toward down-gauging, for
which a basic requirement is to increase the strength. With such
fairly low Mg-level in the range of 2.4 to 3.0 wt. %, the alloy
product is not susceptible to intergranular corrosion ("IGC").
[0004] The aluminium alloy AA5083, which has a Mg content in the
range of 4.0 to 4.9 wt. %, having a higher strength level than
AA5454, is known to be susceptible to IGC. This susceptibility to
IGC is highly undesirable, because an alloy product that has low
resistance against IGC cannot be used always in a reliable manner,
in particular at service temperatures above 65.degree. C.
[0005] The aluminium alloy AA5059, which has a Mg content in the
range of 5.0-6.0 wt. %, a Mn content in the range of 0.6-1.2%, a Zn
content in the range of 0.4-1.5 wt. %, and a mandatory Zr addition
in the range of 0.05-0.25%, has an improved resistance to amongst
others IGC, and provides a high strength also in the welded
condition.
[0006] In spite of these references, there is still a great need
for an improved aluminium alloy product having improved balance of
strength, high formability and a good corrosion resistance, in
particular against IGC.
[0007] It is an object of the present invention to provide an
Al--Mg alloy sheet, plate or extrusion with improved formability as
compared to those of the standard AA5083 alloy in the same temper.
It is another object of the present invention to provide alloy
sheets, plates or extrusions which can offer IGC resistance at
least equivalent or better to those of AA5083, in combination with
an elongation A50 of 24% or more. It is another object of the
present invention to provide a method of manufacturing such alloy
products.
[0008] According to the invention in one aspect there is provided
an aluminium-magnesium alloy in the form of a rolled product or an
extrusion, having the composition in weight percent:
1 Mg 4.5-5.6 Mn 0.05-0.4 Zn 0.40-0.8 Cu 0.06-0.35 Cr 0.25 max. Fe
0.35 max. Si 0.25 max. Zr 0.12 max. Ti 0.3 max.
[0009] others (each) max. 0.05, (total) max. 0.15 balance
aluminium.
[0010] By the invention can be provided an alloy product in the
form of rolled product, sheet or plate, or extrusion that has a
higher formability than AA5083when using the same or similar temper
material.
[0011] Surprisingly, the alloy product according to the invention
has good resistance against corrosion, in particular against IGC.
It has been thought in the past that resistance against IGC is
normally reduced when the Mg content exceeds about 3.0 wt. %, but
the resistance against IGC of the alloy product according to the
invention is high compared to most conventional AA5000-series alloy
products with a Mg content of more than 4 wt. %. It has been found
that the alloy product according to the invention has a weight loss
of less than 25 mg/cm.sup.2 when tested after sensitising at a
temperature of 100.degree. C. during 100 hours in accordance with
ASTM G67, and has a weight loss of less than 15 mg/cm.sup.2 when
tested after sensitising at a temperature of 85.degree. C. during
100 hours in accordance with ASTM G67, resulting in that the alloy
product may be used at a service temperature of 65.degree. C. or
more without any problems, e.g. typically at a service temperature
of 80 to 100.degree. C.
[0012] It is believed that the improved balance of properties
available with the invention, particularly the higher strength and
good formability in combination with the improved corrosion
resistance, in particular against IGC, results from the balanced
combination of the alloying elements Mg, Mn, Zn, and Cu in the
given ranges. Particularly, it is believed that the Cu and Zn
contents in the ranges according to the invention at such
relatively high Mg levels optimise the resistance against
corrosion, in particular the resistance against IGC and exfoliation
corrosion, whereas the Mg and Mn contents in the given ranges
optimise strength and formability of the alloy product.
[0013] Magnesium is the primary strengthening element in the alloy
product. Mg levels above 4.5 wt. % do provide the required
strength. The amount of Mg should not exceed 5.6 wt. %, in order to
ensure an acceptable corrosion performance and workability, e.g. by
means of rolling, of the alloy product as such high Mg levels.
Preferably, the Mg content in the alloy product is more than 4.8
wt. %. by which the alloy product is provided with a better
optimised balance of tensile strength, yield strength, formability
as measured by its elongation (A50), and its corrosion
resistance.
[0014] Manganese is an essential additive element also. In
combination with Mg, Mn provides the strength and formability in
the alloy product as well as in the welds of the alloy product. A
preferred range for the Mn content is 0.1 to 0.2 wt. %, and thereby
providing a balance in providing sufficient grain size control and
a good formability and in particular in achieving an elongation A50
of 24% or more in the final product.
[0015] Zinc is an important alloying element for achieving
sufficient corrosion resistance in combination with a good
formability of the alloy product. At least 0.40 wt. % Zn addition
is required in order to achieve sufficient resistance against IGC.
It has been found that for this alloy that at a Zn content above
0.8 wt. %, the uniform elongation is significantly reduced and
thereby adversely affecting the formability of the alloy product,
e.g. the reverse bendability is adversely affected. Preferably, the
amount of Zn does not exceed 0.75 wt. %, and it is more preferred
that the content of Zn does not exceed 0.6 wt. %, in order to
optimise the balance of desired characteristics of the alloy
product, and to further optimise the uniform elongation. The most
preferred range for the Zn addition is in the range of 0.4 to 0.6
wt. %.
[0016] Surprisingly, in a narrow range copper has been found to
increase the resistance against IGC even though the Mg content is
relatively high. Normally in the art, a deliberate Cu addition is
avoided in alloys of this type, since it is thought to harm the
resistance against corrosion. When Cu is present above 0.06 wt. %
in combination with the zinc, a positive effect has been found on
the resistance against IGC. However, Cu should be kept below 0.35
wt. % in order to avoid an adverse effect on the resistance against
corrosion, in particular in the resistance against pitting
corrosion. In an embodiment, the lower limit of Cu is more than
0.075 wt. %, and more preferably more than 0.10 wt. %. Herewith a
good resistance against IGC is better save guarded. Preferably, the
amount of Cu does not exceed 0.24 wt. %. Herewith the balance of
desired characteristics is better achieved. More preferably, the
amount of Cu not exceeding 0.18 wt. %, in order to preserve the
corrosion resistance in a weld zone also. It is more preferred if
Cu does not exceed 0.15 wt. %, to better ensure good corrosion
resistance in a weld zone. Also, the general resistance against IGC
in the alloy product is optimised.
[0017] Fe is not an essential alloying element, and tends to form
for example Al--Fe--Mn compounds during casting, thereby limiting
the beneficial effects of Mn. Therefore Fe must not be present in
an amount of 0.35 wt. % or more. For the mechanical properties of
the product, in particular to improve the formability of the alloy
product, the amount of Fe is preferably to be kept below 0.2 wt.
%.
[0018] Si is not an essential alloying element. It also combines
with Fe to form coarse Al--Fe--Si phase particles which can affect
the fatigue life and fracture toughness of for example the welded
joints of the alloy product. For this reason, the Si level is kept
to a maximum of 0.25 wt. %. Preferably the amount of Si is kept to
a maximum of 0.2 wt. % and more preferably of 0.12 wt. %, and most
preferably at a maximum of 0.1 wt. % in order to better ensure
favourable formability characteristics of the alloy product.
[0019] Zirconium is not essential for achieving the improved
corrosion performance in the alloy product according to the
invention, but it can have an effect to achieve a more fine grain
refined structure in the fusion zone of welded joints. Zr levels of
0.15 wt. % or more are to be avoided, and should be less than 0.12
wt. %, since this tends to result in very coarse needle-shaped
primary particles with decrease in ease of fabrication of the alloy
product and in the formability of the alloy product. Zr may cause
to form undesirable coarse primaries, in particular together with
Ti. In a preferred embodiment, the amount of Zr does therefore not
exceed 0.05 wt. %. Moreover, it may be favourable to keep Zr out of
scrap source material for specific recycling reasons. To this
extend, it is more preferred to limit the presence of Zr to less
than 0.02 wt. %.
[0020] Titanium is often used as a grain refiner during
solidification of both cast ingots and welded joints produced using
the alloy product of the invention. This effect is obtained with a
Ti content of less than 0.3 wt. %, and preferably less than 0.15
wt. %. Ti may be replaced in part or in whole by V in the same
compositional range to achieve a similar effect.
[0021] Chromium is an optional alloying element, that may improve
further the corrosion resistance and strength of the alloy product.
However, Cr limits the solubility of Mn and, if present, also that
of Zr. Therefore, to avoid formation of undesirable coarse
primaries, the Cr level must not be more than 0.25 wt. %.
Preferably, the Cr is present in a range of 0.06 to 0.2 wt. %, and
more preferred range is 0.11 to 0.2 wt. %.
[0022] The balance is Al and inevitable impurities. Typically each
impurity element is present at 0.05% maximum and the total of
impurities is 0.15% maximum.
[0023] The aluminium alloy in the form of a rolled product may be
provided in a wide range of gauges, for example up to 200 mm, but a
preferred gauge for the alloy product according to the invention is
in the range of 0.5 to 5 mm.
[0024] The alloy product according to the invention can be
delivered in various temper conditions. However, for the group of
applications for which the alloy product is ideally suited,
preferably it should be a temper similar to a soft worked temper,
also known in the art as an "O"-temper, or, in case of thin plates,
a light "H"-strain hardened temper such as for example H111.
[0025] The invention further relates to a welded structure
comprising at least one section of the product according to one of
the above described embodiments. The alloy product according to one
or more embodiments of the invention is eminently suitable for
application in such a welded structure due to its excellent
weldability, and its high strength in a weld zone in combination
with its improved corrosion performance.
[0026] The invention further relates to a pressure vessel, in
particular a welded pressure vessel, comprising a shell that
comprises the rolled aluminium-magnesium alloy product as is
described above. Due to the increased strength, such pressure
vessel can be down-gauged to have a lower weight. Moreover, the
corrosion properties can be improved. The pressure vessel, e.g. for
a braking system, according to this aspect of the invention can be
used at a higher service temperature, in particular above
65.degree. C.
[0027] The alloy product in accordance with the invention may be
employed also very successfully for automotive applications, in
particular as body panels, and structural parts such as suspension
systems and wheels.
[0028] In another aspect, the invention relates to a method of
producing an aluminium alloy product comprising the sequential
processing steps:
[0029] (i). providing an intermediate alloy product having a
composition according to the mentioned above and set forth in the
claims;
[0030] (ii). cold working the intermediate alloy product to a final
gauge to obtain an intermediate wrought product;
[0031] (iii). annealing the intermediate wrought product by heating
the product at a heating rate in the range of 2 to 200.degree.
C./sec, holding the product at a soaking temperature in the range
of 480 to 570.degree. C. for a duration of up to 100 sec, followed
by a cooling at a cooling rate in the range of 10 to 500.degree.
C./sec to below a temperature of 150.degree. C.
[0032] By this method it is achieved that the positive influence of
Cu on the resistance against IGC is fully exploited. Although the
alloy product has good properties when other annealing schemes are
applied, it is believed that the positive influence of Cu on the
corrosion properties is in particular enhanced by the annealing
scheme of processing step (iii).
[0033] The aluminium alloy as described herein can be provided in
process step (i) as an ingot or slab for fabrication into a
suitable wrought product by casting techniques currently employed
in the art for cast products, e.g. DC-casting, EMC-casting,
EMS-casting. Slabs resulting from continuous casting, e.g. belt
casters or roll casters, may be used also.
[0034] In order to obtain an intermediate product suitable for cold
working, preferably by means of cold rolling, the provided
intermediate alloy product can be hot worked by means of hot
rolling or hot rolling in combination with one or more forging
steps.
[0035] The annealing scheme of processing step (iii) can be applied
in a continuous annealing facility. The required heating rates can
be achieved, for example, by homogeneous heating by means of
inductive heating. This gives further improved mechanical
properties in the sheets or plates.
[0036] Particularly favourable results have been obtained in an
embodiment of the method wherein the soaking temperature is in the
range of between 520 and 550.degree. C.
[0037] The balance of characteristics of the alloy product produced
by the method is found to be better optimised in the embodiment
wherein the product is held at the soaking temperature for a
duration of up to 40 sec.
[0038] In an embodiment of the method, the heating rate is at least
50.degree. C./sec, and preferably at least 80.degree. C./sec.
Herewith, the balance between the mechanical properties and the
resistance against IGC has been found to be more favourable. This
is especially the case when the cooling rate after soaking is at
least 100 .degree. C./sec.
[0039] The invention will now be explained with reference to
laboratory experiments.
[0040] Various slabs were cast having chemical compositions as
shown in the following Table 1, balance aluminium. Slab A
corresponds to a standard AA5083 alloy, and Slabs B and C are
according to the invention.
2TABLE 1 compositions (in wt %) of the cast slabs (Balance Al and
impurities) Slab Inv. Mg Mn Zn Cu Cr Fe Si Zr Ti A No 4.5 0.50 0.03
0.005 0.10 0.31 0.16 0.001 0.015 B Yes 5.23 0.17 0.51 0.12 0.16
0.23 0.10 <0.01 0.02 C Yes 5.23 0.17 0.51 0.12 0.16 0.23 0.10
<0.01 0.02 D No 5.36 0.50 0.50 0.12 0.15 0.20 0.11 <0.01
0.02
[0041] The processing of the slabs A and B comprised a
homogenisation anneal during 10 hours at a temperature of
510.degree. C., hot rolling whereby the exit temperature was about
330.degree. C., followed by cold rolling with 60% cold reduction
and finally soft annealing in batch anneal at a temperature of
330.degree. C. during 1 hour. The processing of slabs C and D was
identical to those of A and B, with the exception of the final soft
anneal, which was a continuous anneal for 10 sec. at 530.degree. C.
Final gauges were 3 mm, and the plates were delivered in
H111-temper.
[0042] These products were tensile tested according to EN 10002,
and the results for the parallel (.parallel.) and perpendicular
(.perp.) directions are given in Table 2.
3TABLE 2 Tensile strength ("UTS"), 0.2% Proof strength ("PS"),
Elongation ("A50") Direction of UTS PS A50 Alloy Testing [MPa]
[MPa] [%] A .parallel. 299 149 19 (AA5083) .perp. 293 147 21 B
.parallel. 311 146 22 .perp. 310 147 24 C .parallel. 317 153 25
.perp. 314 152 26 D .parallel. 332 166 23 .perp. 332 163 23
[0043] The elongation A50 is considered to be a measure for the
formability. The results in table 1 indicate that the formability
of the alloys B and C is improved when compared to alloys A
(AA5083) or D. This effect is contributed to the lower amounts of
Mn in alloys B and C.
[0044] The alloy products have been subjected to a weight loss test
according to ASTM G67 after sensitising at 100.degree. C. for a
duration of 100 hours in H111-temper condition. Results are shown
in Table 3.
4TABLE 3 Weight loss (in mg/cm.sup.2) after sensitising. A B C D
100 hr at 100.degree. C. 36 17 13 20
[0045] This indicates that the corrosion resistance of products B
and C is much better than of the standard AA5083 alloy (A). Product
C is below 15 mg/cm.sup.2, which is according to ASTM-G67 the upper
limit for a product quality not susceptible to IGC, and product B
is already close to this limit.
[0046] The resistance against IGC of product C in the presently
used sensitising conditions show an improvement over that of B,
apparently by using the continuous anneal the corrosion resistance
of the product is improved. It is expected that under more severe
sensitising conditions the difference is more clearly visible.
[0047] Alloy products B and C were welded without any problem using
TIG welding under standard conditions.
[0048] In an additional test series, the influence of Cu on the
corrosion resistance was tested. Some additional slabs were cast
having the chemical compositions as shown in the following table 4,
balance aluminium.
[0049] The processing of the additional alloys was identical to the
processing of alloy C, i.e. with a final soft anneal as a
continuous anneal.
5TABLE 4 composition (in wt %) of the additional cast slabs Slab
Inv. Mg Mn Zn Cu Cr Fe Si Zr Ti E No 5.58 0.16 0.51 0.02 0.15 0.21
0.11 <0.01 0.02 F Yes 5.49 0.16 0.51 0.09 0.16 0.20 0.11
<0.01 0.02 G Yes 5.41 0.16 0.50 0.21 0.16 0.20 0.11 <0.01
0.02 H Yes 5.42 0.15 0.50 0.30 0.15 0.20 0.11 <0.01 0.02 J No
5.51 0.17 0.51 0.41 0.16 0.20 0.11 <0.01 0.02
[0050] The alloy products have been subjected to a weight loss test
according to ASTM G67 after sensitising at 100.degree. C. for a
duration of 100 hours in H111 temper condition. The alloy products
have also been subjected to an ASSET test according to ASTM G66
after welding, followed by sensitising at 100.degree. C. for a
duration of 100 hours. The weld was a TIG weld using AA5183 as
filler wire. Results are shown in table 5. The ASSET results
correspond to the Heat Affected Zone, because here the most severe
attack is found.
6TABLE 5 Weight Loss (in mg/cm.sup.2) and ASSET result after
sensitising ASSET result Alloy % Cu WL [mg/cm.sup.2] in HAZ E 0.02
37 N F 0.09 21 PA C 0.12 13 PA G 0.21 13 PB H 0.30 11 PB J 0.41 12
PC
[0051] According to ASTM G67 the upper limit for a product quality
not susceptible to IGC is 15 mg/cm.sup.2. In ASTM G66 the range to
classify the results is given, but limits for acceptable or not
acceptable are not specified. However, for a person skilled in the
art, it is clear that pitting A is still acceptable whereas pitting
C in unacceptable. Pitting B is for most applications still
acceptable.
[0052] The results indicate that the resistance against IGC
increases with increasing Cu content, but at the same time the
resistance against pitting decreases. For a Cu level of 0.30 wt. %
and lower, the resistance against pitting is acceptable or better
than acceptable. The weight loss is thought to measure below 15
mg/cm.sup.2 when the Cu level is above about 0.11 wt. %.
[0053] Based on these results it is concluded that the broadest
operational window is found with Cu levels between 0.06 and 0.35
wt. %. Preferably the amount of Cu does not exceed 0.18 wt. % in
order to preserve the corrosion resistance in a weld zone.
* * * * *