Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Austenitic stainless steel
Description
The present invention relates to a new austenitic
stainless steel with a low nickel content which has
special characteristics in terms of corrosion
resistance in given environments, deformability and
suitability for work-hardening. The steel according to
the present invention is characterized by the following
chemical composition:
0.03 0< carbon < 0.07 %
7.0 0< manganese < 8.5 %
0.3 0< silicon < 0.7 %
sulphur <_ 0.030 %
phosphorus <_ 0.045 %
16.5 0< chromium < 18.0 %
3.5 0< nickel < 4.5 0
0.1 0< molybdenum < 0.5 %
1 . 0 0< copper < 3.0 %
0.1 0< nitrogen < 0.3 %
the difference consisting in iron and common process
impurities.
A very important characteristic of the new steel
is the small amount of nickel it contains: it is in
fact well known that the price of this element is
unstable, with a continuous tendency to increase,
resulting in continuous variations in the costs of the
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articles produced with materials which contain this
element.
Background art
Austenitic stainless steel is an iron and carbon
alloy containing various other elements, the main ones
of which are chromium and nickel. The combination of
these elements gives the steel a basic property of
corrosion resistance owing to the formation of a
protective surface film which is due to the presence of
a chromium content of at least 1.11% and whose
qualities are improved by the presence of nickel and
other elements. Other typical properties of austenitic
stainless steels are the very low magnetic permeability
(non-magnetic property), heat resistance, cold
deformability and suitability for work-hardening.
Owing to these properties, austenitic stainless steels
are used in a very wide range of applications.
1.4301 steel
The most well known and widely used type of
austenitic stainless steel contains about 18% chromium
10% nickel and has always been referred to as 18/10
steel. In the European standard EN 10088-3 1997 this
steel has been called X5CrNi18-10 and has been
attributed the steel number 1.4301. In the United
States standard AISI this steel is called 304. The
percentage by weight chemical composition envisaged for
this steel by the European standard is as follows:
C = 0.07 max
Si = 1.00 max
Mn = 2.00 max
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P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 17.00 and 19.50
Ni = between 8.00 and 10.50
In the case of products which are intended to be
machined, the same standard envisages a variant whose
sulphur content is controlled (or "micro-
resulphurised") where
S = between 0.015 and 0.030
It should be noted that the maximum sulphur
content coincides with that of basic steel, so that in
fact this is not another steel, but only a variation
of the same type 1.4301 obtained by dividing the
analytical range permitted by sulphur. Sulphur has the
capacity to weaken the metallic matrix and therefore
improve the machinability during the swarf removal
operations. At the same time, however, sulphur, even
though present in limited amounts, modifies the
corrosion resistance. This micro-resulphurised variant
is cited here because below it will often be used for
comparison with the type 1.4301 steel and with the
steel of this invention.
1.4301 steel has extremely broad technological and
corrosion properties such it has been become very
widely established in the engineering sector as a
structural material as well as in the environmental
sector: it is in fact widely employed in the
transportation, architecture and the domestic sectors,
being used at high temperatures and in corrosive
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environments. The type 1.4301 is the most well known,
widespread and researched in the sector of austenitic
stainless steels and therefore is used as a reference
type for comparing the characteristics of other
austenitic stainless steels.
Other comparison steels
There exist other steels with a similar
composition which differ owing to small analytical
variations of a certain element which give them an
improved property. Some of these steels are mentioned
here because below they have been used for comparison
with the steel according to the invention in order to
highlight its characteristics. The type 1.4307 -
X2CrNi18-9 (AISI 304L in the US standards) is a steel
similar to the preceding one, but with a limited carbon
content which improves the intergranular corrosion
resistance. The chemical composition of type 1.4307
steel is as follows:
C = 0.03 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 17.50 and 19.50
Ni = between 8.00 and 10.00
The type 1.4306 - X2CrNi19-11 is a further low-
carbon variant with a greater content of nickel which
is added in order to improve the cold deformability and
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the corrosion resistance. The chemical composition of
this type is as follows:
C = 0.03 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 18.00 and 20.00
Ni = between 10.00 and 12.00
The type 1.4567 - X3CrNiCu18-9-4 is a version with
the addition of copper in large amounts for the purpose
of improving the cold deformability: it is used for
those particular cold-pressed products where the
preceding types are unable to withstand the extreme
deformation, such as, for example, hexagonal socket
head screws. The chemical composition is as follows:
C = 0.04 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 17.00 and 19.00
Cu = between 3.00 and 4.00
Ni = between 10.00 and 12.00
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The characteristics of austenitic stainless steels
The main characteristics of an austenitic
stainless steel are its corrosion resistance, non-
magnetic nature, cold-deformability and suitability for
work-hardening. These characteristics are obtained by
modifying various factors, including the chemical
composition: in addition to chromium and nickel, the
other secondary elements have an important effect. The
effect of chromium, referred to as "alphagenic", tends
to stabilize the ferritic phase of the materials (alpha
phase): other elements, such as silicon and molybdenum,
behave in the same manner as chromium, although to a
lesser degree. The same applies to nickel, which is a
"gammagenic" element, and therefore has a stabilizing
effect on the austenitic phase (gamma phase) : various
elements such as carbon, nitrogen, copper and manganese
behave in the same manner as nickel.
The nickel content of austenitic stainless steels
Most of the known austenitic stainless steels used
on the market have nickel contents of about 8-10%, as
in the case of the types mentioned hitherto. During
the last few years, the worldwide economic situation
has resulted in the price of nickel being very
unstable, with a marked tendency to increase.
Manufacturers and retailers of stainless steels
therefore have difficulty in operating within a
fluctuating market, so much so that nowadays in Europe
the price of these products is composed of a base price
and an additional price, referred to as "alloy add-on",
which is defined at the time of delivery: the "alloy
add-on" varies with predefined mechanisms depending on
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the value of nickel on the world market. Steel product
processing companies, for their part, have difficulty
in establishing the prices of the parts produced since
they cannot know the exact price of the raw material
until the time of delivery.
For this reason, different austenitic stainless
steels with low nickel contents have been researched:
some of these, which are more widely used and have been
known for some time, are included in various standards
and used because of their specific characteristics.
Others have been recently developed with the aim of
obtaining some of the basic characteristics of
austenitic stainless steel. In fact, by suitably
increasing the content of the less costly "gammagenic"
elements (nitrogen, copper and manganese), it is
possible to obtain an austenitic stainless steel which
is equally stable, but has a low percentage content of
nickel (and therefore a price which is less dependent
on the fluctuations of the cost of nickel) and with one
or more technological properties the same as those of
normal conventional austenitic steels with a higher
nickel content. Austenitic steels with a low nickel
content are for example described in EP593158,
EP694626, EP896072, EP969113 e WO 00/26428.
Subject of the invention
The subject of the present invention is a steel
having a nickel content which is markedly lower than
that of basic steel type 1.4301 (AISI 304) and which,
with suitable balancing of the other elements, has many
properties similar to the corresponding properties of
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basic steel type 1.4301 (AISI 304); it has the
composition shown below:
0.03 0< carbon < 0.07 %
7.0 0< manganese < 8.5 %
0.3 0< silicon < 0.7 %
sulphur <- 0.030 %
phosphorus <- 0.045 %
16.5 0< chromium < 18.0 %
3.5 0< nickel < 4.5 0
0.1 0< molybdenum < 0.5 %
1 . 0 0< copper < 3.0 %
0.1 0< nitrogen < 0.3 %
where the difference consists in iron and common
process impurities.
The steel according to the present invention may
be obtained by means of the conventional processes for
the preparation of austenitic stainless steels, such as
those for example described in "ASM Specialty Handbook
- Stainless Steels" edited by "The Material Information
Society" - USA. Preferably it has the composition
indicated below:
0.04 0< carbon < 0.06 %
7.5 0<- manganese< 8.0 %
0.4 0< silicon 0.6 %
0.002 0< sulphur 0.004 %
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0.030 0< phosphorus < 0.035 %
17.0 0<_ chromium < 17.5 %
3 . 8 0< nickel < 4.2 %
0.1 0< molybdenum < 0.3 %
2.0 0<_ copper < 2.5 %
0.15 0< nitrogen < 0.2 %
According to one of the possible embodiments of
the invention, the sulphur is less than 0.005 %.
According to another possible embodiment, which does
not exclude the previous embodiment, the nickel is
higher than 4.0 %. According to the best
embodiment of the invention, the carbon is about 0.055
%, the manganese is about 7.50 %, the silicon is about
0.52 %, the sulphur is about 0.003 %, the phosphorus is
about 0.032 %, the chromium is about 17.0 %, the nickel
is about 4.0 %, the molybdenum is about 0.19 %, the
copper is about 2.0 % and/or the nitrogen is about 0.17
0
o.
In order to define the characteristics of the
product obtained with the newly invented steel, its
main performance features have been studied and
compared with those normally encountered in basic
1.4301 steel and similar steels: the results have
proved to be very positive since, for the same
functional characteristics, the cost of the steel is
decidedly lower than that of basic steel type 1.4301
and in any case not so closely dependent on the nickel
market.
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The characteristics considered on the pages below
have been obtained by means of varying castings of the
new steel, all carried out with analyses similar to
that of the best embodiment mentioned above.
Stress corrosion cracking
The steel according to the present invention
presents a higher resistance to "stress corrosion
cracking" (also called "delayed corrosion") than the
steels commonly known in the art and, in particular,
than those disclosed by WO 00/26428, FP896072 or
EP969113. Such a. higher resistance can be expl.ained
through the selected nickel range of between 3.5 and
4.5% by weight, as for instance subsequently
demonstrated. by J. Charles, Stainless Stee.l. 05,
Proceedings of the 5 'h European Congress SLainless
Steel Science and Yiarket, Seville, September 27-30,
2005 (pages 1_9-26).
This improved resistance to "stress corrosion
cracking" makes the steel. of the present invention
particularly suitable for the manufacture of wires
having a "deep drawing ratio" and which could be
exposed to aggressive e.nv.i.ro.nments as for instance
wires for agricultural use, elect::ric household
appliances, bicycle spokes; wires for laundry drying
frames; w.i.res for a.rchitecture, for meshwork and for
hooks used on slate roofs.
Cold deformability by means of drawing
For the reduction in cross-section r the
following relation is applicable:
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=A -'4'
r 100
A0
where:
Ao = Initial cross-sectional area
A1 = Final cross-sectional area
The drawing of the rolls is performed by means of
successive passes through the tools (drawing dies)
which deform the product, gradually decreasing its
cross-section.
During deformation, a phenomenon called work-
hardening, proportional to the reduction, occurs, said
phenomenon resulting in an increase in the tensile
properties of the material (Rm , Rp(o,2) ) and a decrease
in the plastic properties (A, Z), up to the point where
the material is no longer deformable. When work-
hardening is such that the material no longer possesses
plasticity, the wire breaks during further passes
through the drawing dies and the product can no longer
be drawn.
Under normal conditions with multiple-pass drawing
machines operating at suitable industrial speeds, the
reference stainless steel 1.4301 (AISI 304) is able to
withstand drawing reductions of up to 88%. Beyond
these values the work-hardening is such that the
material breaks and is no longer capable of being
deformed.
The stainless steel according to this invention,
under identical conditions, is able to withstand
drawing with reductions in the cross-section in the
region of 92-94%.
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This data is very important for detailed work
where small diameters of the drawn wire are required,
with the result that a certain amount of annealing
during the reduction cycles may be dispensed with.
Table 1 shows the tensile strength and elongation
at break values of the steel according to the invention
for various degrees of reduction during drawing,
compared with two reference steels: steel type 1.4307
with a low carbon content (about 0.02%) and steel type
1.4301 with a slightly higher carbon content (0.040).
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Table 1: Mechanical properties depending on work-hardening
New steel 1.4307 low C 1.4301
0
R A R A R A
reduction
MPa % Mpa % MPa %
0 659 42 580 42 569 42
17.4 770 23 810
35.1 1045 12 952 12
56.4 1390 3.5 1140 4.0
67.9 1420 4.0
70 1583 2.5 1320 3.0
76 1610 1.5
84 1803 1.5 1490 2.5 1700 1.5
87.7 1750 1.5
90.3 1932 1.2
92 2000 1.0
Figure 1 shows in graph form the tensile strength
values as a function of the drawing reduction for these
steels, while Figure 2 shows the same type of
comparative graph relating this time to the percentage
elongation at break value.
The work-hardening is due to the partial and
progressive transformation of part of the austenite
into martensite, which is the hardest component of
steel. A metallographic study was carried out on
samples taken from materials in the annealed and work-
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hardened state, these revealing both the deformation of
the grain, with elongation in the drawing direction,
and the austenite-martensite transformation.
Figure 3 shows a longitudinal metallographic
cross-section through the product in an ultra work-
hardened state of the wire obtained with the new steel,
in which the work-hardening lines due to the
martensitic transformation are clearly visible.
Figure 4 shows the same type of cross-section
carried out on a sample of the reference steel type
1.4301 (AISI 304).
Relative magnetic permeability r
The relative magnetic permeability measures the
ratio between the magnetic permeability of a material
and that of a vacuum go.
gr 9
o
The magnetic permeability of a material
(measured in Henry/metre [H/m]) is defined by the ratio
between the magnetic induction value B and the value of
the magnetizing force H.
The magnetic permeability of the vacuum is equal
to o = 1.256 x 10-6 H/m.
The magnetic permeability of a material basically
measures the ferromagnetism, i.e. the property of a
steel to react with a magnetic field of given value.
In the case of stainless steels, the martensitic
structure is ferromagnetic ( r=700-1000), while
austenite is practically non-magnetic ( r<1,2).
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An austenitic steel in the solubilized state, and
hence with a totally austenitic structure, is
completely non-magnetic: when it is subjected to a
magnetic field, for example that of a magnet, it does
not react.
An austenitic steel in the work-hardened state,
for example after undergoing drawing reductions, is
increasingly more magnetic depending on the percentage
of austenite transformed into martensite (basically
dependent on the drawing reduction and the chemical
composition).
For this reason, a steel type 1.4301 (AISI 304)
which in the solubilized state is non-magnetic, after
reductions with value of about 65%, has a structure
which is partially ferromagnetic with a relative
magnetic permeability of about gr = 1.50 (with a
magnetic field of 4000 A/m); after reductions of 85%,
its relative magnetic permeability rises to 2.20
with the same magnetic field.
The steel according to the present invention
remained perfectly non-magnetic also following numerous
drawing operations: under the same test conditions,
with reductions of 65%, we obtained a permeability r =
1.10, while with reductions of 85% the permeability
rose only to gr = 1.30.
The magnetic permeability in a stainless steel
assumes particular importance both in the case of more
complex applications (e.g. solenoid valve bodies, where
the part must not be influenced by the magnetic field
of excitation of the valve), but also for more
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straightforward applications, where recognition of the
material is simply carried out by means of a magnet, as
in the case of laundry drying frames sold at markets or
in supermarkets: if the wire of the laundry drying rack
is not attracted by the magnet, it is recognised as
being austenitic stainless steel and is much more
highly valued than the corresponding wire made of
ferritic stainless steel or even galvanized iron, which
are both highly ferromagnetic. The possibility of
obtaining drawn wires with high work-hardening values
(required by the product itself in order to withstand
the load of wet laundry), without any significant
variation in the magnetic permeability, results in the
invention being particularly suitable for this type of
use.
Cold deformability by means of pressing
Tests for the production of screws by means of
cold deformation were carried out as follows:
= Hexagonal-head screws (DIN 933 M5 x 25) : for
this product a steel type 304L with Cu content of
about 0.9% is used.
= Socket-head cap screws (DIN 912 M5 x 12): for
this type of product normally a steel type 304Cu is
used, with the addition of 3-4% Cu in order to
improve the deformability.
The characteristics of the screws produced were
determined by means of tensile tests carried out in
accordance with the standard UNI EN ISO 3506 part 1
edition February 2000 and HV 500 microhardness tests.
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The results of the tensile test are shown in Table
2
Table 2: Results of tensile tests carried out on cold-
pressed screws
Breaking Upper Elongation
yield
Type of load at break
Dimensions Material point
screw Rm A
RPC0,2>
Mpa %
Mpa
DIN 933 1.4306
967 754 2.7
Sheared (304L)
hexagonal M 5 x 25
head New steel 1137 887 2.8
screw
DIN 912 1.4567
865 675 2.3
Socket M 5 x 12 (304Cu)
head
New steel 1160 905 2.2
screw
Figure 6 shows the microhardness values
determined at various points in the longitudinal
section of the screws DIN 933 M5 x 25 produced.
In the same manner, Figure 7 shows the
microhardness values detected at various points of the
cross-section of screws DIN 912 M5 x 12.
Before commenting on these results, it should be
noted that the reference standard for stainless steel
screws (UNI EN ISO 3506-1 "Mechanical properties of
corrosion-resistant stainless steel connecting elements
- screws and stud screws") does not permit at the
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moment this type of austenitic steel. It may, however,
be possible to apply for and obtain inclusion of the
newly invented type in the future standard for screws,
thus allowing its use.
The comparisons have been made, as always, with
screws made of normal steel type 1.4301 (AISI 304).
The screws made with the steel according to the present
study had a higher tensile strength of about 70 MPa in
the case of hexagonal head screws and 95 MPa in the
case of socket head screws: this greater difference is
due to the very poor work-hardening property of the
304Cu steel used for the comparison. Likewise, the
hardness values are about 100 HV points higher in the
case of the steel according to the invention. All the
mechanical properties recorded are, however, within the
limits stipulated by the standard for quality A4 screws
(corresponding to the reference steel 1.4301) with
strength class 70 or 80 (relating, therefore, to "work-
hardened" or "ultra work-hardened" materials).
These results of pressability must be related to
the technical possibility of producing screws by means
of cold deformation using the new steel. Considering
the corrosion-resistance properties of this steel
(described in the following paragraphs), it seems
possible to request, in due course, broadening of the
range of steels accepted for the production of screws,
at least as regards the strength class 80 (that of
ultra work-hardened steels), which is sometimes
difficult to achieve with normal austenitic steels.
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Corrosion resistance of the semifinished starting
product
Corrosion-resistance tests were carried out using
samples obtained by means of machine-tool processing of
solubilized wire rod.
The types of steel which underwent the tests were,
in addition to the steel of the present invention, also
two castings of austenitic steel type 1.4307 consisting
of the micro-resulphurised variant (S=0.030 for
machine-tool processing) and the variant with a very
low sulphur content (0.003).
The tests carried out and the corresponding
reference standard, where applicable, are listed in
Table 3.
Table 3: Corrosion tests carried out on samples obtained
from solubilized wire rod
Test in 20% 1 cycle of 96 hours
sulphuric acid at +20 C
3 cycles at 48 hours
at boiling
Test in 65% nitric ASTM A262 test
temperature - change
acid C
of solution with
each new cycle
Test in 6% ferric 1 cycle of 72 hours
ASTM G-48
chloride at 22 C +/-2
The results for the test with 20% sulphuric acid
are shown in the graphs of Figure 9. Similarly, Figure
shows the results of the test in 65% nitric acid
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carried out on the same steels. In Figure 11, the
corrosion test was carried out in 6% ferric chloride.
From the results it can be easily understood that
in this type of test, the progression of the corrosion
is greatly influenced by the sulphur content of the
steel, while the decidedly lower nickel content did not
result in a substantial deterioration.
The new steel in fact has a performance perfectly
in keeping with that of the reference types and only in
the nitric acid test is the corrosion value slightly
higher than that of the type 1.4307 micro-resulphurised
steel.
Before reaching conclusions in connection with
these tests it is necessary to point out again that
both the steels used for comparison had an extremely
low carbon content (type 1.4307 corresponds to the type
AISI 304L, Low Carbon) : the new steel is therefore not
affected, all other conditions being equal, by the C
content which is higher than in the basic comparison
steels.
By way of conclusion, these tests show that the
sulphur content in a steel type 1.4307 (with a
corrosion-resistance considerably higher than the basic
type 1.4301) has a decisive influence on the corrosion
resistance. Both the compared types (1.4307 steel with
low sulphur content and micro-resulphurised steel) are
able to form part of a perfectly compliant supply of
"normal" 1.4301 steel since this type envisages only a
maximum limit for the elements C (0.07 max) and S
(0.030 max).
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The steel according to the invention, in the
solubilized state and on test pieces obtained by means
of machining, has corrosion-resistance properties which
are practically the same as those of the reference
steels.
Corrosion resistance of drawn and solution
annealed steels
Corrosion tests were carried out, in different
work-hardening conditions, on some samples of drawn
wire and drawn + solution annealed wire made from the
new steel and, by way of comparison, various other
qualities of stainless steel.
Most of the tests were carried out in accordance
with international standards which describe the methods
to be applied, but do not describe the threshold values
(exposure time or the like) which must be surpassed:
these threshold values are established contractually in
each case during placing of the order. In the present
test program only comparative tests were carried out
between the new steel and some reference steels,
subjecting all the parts together to variable exposure
times, until oxidation appeared in some of the parts or
for time periods which were sufficiently long to
guarantee the applicability thereof.
Table 4 lists the types of materials which
underwent this type of test, their diameters and the
associated working conditions.
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Table 4: Wire samples subjected to corrosion tests
Quality Diameter
European AISI State Reference
standard standard nun number
1.4301 304 2.30 Partially work- 1
hardened
1.4301 304 2.00 Solution annealed 2
1.4301 304 1.30 Work-hardened 3
New steel 1.40 Solution annealed 4
New steel 1.40 Work-hardened 5
New steel 2.25 Solution annealed 6
New steel 2.00 Partially work- 8
hardened
Table 5 instead lists the tests which these
samples underwent and the reference standards.
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Table 5: Corrosion tests on wire
Test Reference Duration
standard
Neutral saline UNI ISO 9227 168 / 400 hours
mist NSS
Copper acetic acid UNI ISO 9227 120 hours
mist CASS
Kesternich cycles DIN 50018 21 4 cycles of 24 hours
(corrosion in an consisting of 8
industrial hours exposure to SO2
atmosphere) and 16 hours
exposure to the
laboratory air
Immersion test in -- 168 hours
a solution of NaCl
2M with pH 6.6
Intercrystalline ASTM A262 24 hours in
corrosion test test E copper/copper
sulphate/sulphuric
acid solution
Outcome of tests:
= Neutral saline mist test
After exposure for 150 hours, no sample showed
signs of corrosion.
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Only after 200 hours were some spots of rust
detected on the surface of samples 5 and 6 and some
more extensive areas found on ferritic sample 7.
After 400 hours these rust spots were
extensive, so much so that the ferritic steel was
widely oxidised, while some rust areas affected the
new steel (the extent of these areas is proportional
to the degree of work-hardening); at the same time
only small sporadic spots appeared on the type
1.4301 steel in the work-hardened state.
= Copper acetic acid mist test
After 120 hours exposure, the behaviour of the
various wires was sufficiently varied and it was
possible to detect that the ferritic steel 1.4016
had the most area covered by corrosion products
(about 4 0 0 ) .
The behaviour of the new steel and the 1.4301
steel is instead greatly influenced by the degree of
work-hardening: as known from the literature, the
best corrosion resistance is obtained with the
material in the solution annealed state, while it is
worsened by work-hardening. It was noted, however,
that the behaviour of the steel considered in this
study is midway between the type 1.4301 and the type
14016.
= Corrosion tests in an industrial
atmosphere using Kesternich cycles
After 4 cycles the behaviour of the new steel
was entirely similar to all the other types of
austenitic steel, there being no appreciable
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corrosion (the surface remained substantially
unchanged).
= Tests with immersion in a solution of NaCl 2M
with pH 6.6:
In this case the best behaviour was that of the
type 1.4301, followed very closely by the new type,
while the type 1.4016 had various rust spots.
= Intercrystalline corrosion tests
After attack, all the test pieces were able to
be bent through 180 without any signs of cracking
or flaking on the surface subject to tensile stress.
The corrosion tests carried out were particularly
numerous and covered all the possible ranges of
applications such that it was possible to determine the
characteristics of the new material with a wide series
of tests.
The tests were carried out on products in the wire
state, in various finishing conditions, and confirmed,
as is well known in the literature, that materials in
the work-hardened state behave in general less well
when subjected to aggressive agents: the explanation of
this phenomenon is due mainly to the tensioning of the
grains and the grain edges which make the individual
points more unstable and therefore more prone to attack
and also the partial martensitic transformation, since
this structure has a corrosion resistance which is less
than that of austenite.
Overall the corrosion behaviour of the new steel
was scarcely inferior to that of the reference type
1.4301, for the same work-hardening conditions.
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Particularly positive was the behaviour in
relation to atmospheric corrosion and intergranular
corrosion, where no differences were noted compared to
the reference type.
Additional tests in acid environment (H2SQ4 0.2 M
+ NaC.1_ 1q/1.) evidenced that the steel. of the present
invention also presents better anodic polarization
curves than similar steels having a lower nickel
content.
Hot tensile strength
One of the main characteristics of stainless
steels is the possibility of use at high temperatures.
Rapid hot tensile tests were carried out in order to
verify the mechanical properties at temperatures higher
than room temperature. The samples of the new steel
which underwent this test, in the form of 3 mm diameter
solubilized wires, were compared with identical samples
of 1.4307, 1.4310 and 1.4301 steel.
The tests were carried out at 900 C in accordance
with the standard EN 10002 part 5, giving the results
listed in Table 6.
Table 6: Mechanical properties during high temperature
tests
Material Test Cross Test Rpo,2 Rm
European AISI piece sectional temperature
diameter area oC
standard Standard MPa Mpa
mm mm2
1.4301 304 3 7.1 900 91 141
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1.4307 304L 3 7.1 900 94 155
1.4310 302 3 7.1 900 103 169
New steel 3 7.1 900 90 155
The rapid hot tensile tests were carried out at a
decidedly high temperature (900 C) compared to the
operating temperatures normally permitted. The results
show that the new steel has a behaviour very similar to
that of the normal reference steel, type 1.4301, while
only the type with a higher carbon content (1.4310) has
a slightly higher hot strength, even though it as of
the same order of magnitude.
High temperature stay test
The basic stainless steel 1.4301 (AISI 304) is
resistant for fairly long periods in a high temperature
oxidising environment: in particular the most common
uses for this material are those which envisage stays
in air up to about 500 C. The new steel was also
tested for its resistance to temperatures higher than
room temperature by means of air heating tests inside a
muffle furnace. The results can be seen in Figure 10.
The resistance was evaluated by measuring the
depth of surface oxidation, i.e. the loss of diameter
as a result of oxidation. It is possible to note that
the new steel behaves in a manner perfectly similar to
that of the of the various types with a high nickel
content up to a temperature of higher than 800 C. As
mentioned, the temperatures commonly used for normal
austenitic steels (belonging to the family of 1.4301
steel) are about 500 C, while for higher temperatures
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refractory alloys (with high nickel contents) or
superalloys (nickel based alloys, not belonging to the
family of steels) are used. The new steel is therefore
perfectly utilisable at the same temperatures at which
the basic type is used since there is no variation in
its characteristics.
Conclusions
The new stainless steel according to the present
invention with a low nickel content possesses technical
characteristics similar or comparable to those of steel
type 1.4301.
The main advantage of this new steel from the
commercial point of view is its lesser dependency on
the nickel market and therefore its greater stability
from a price point of view. From the technical point
of view, the main advantage is the extremely high
suitability for drawing which allows a large reduction
during drawing and a small number of intermediate
annealing operations.
The new material is particularly suitable as a
substitute for traditional types of steel in certain
specific applications
= agricultural wire, owing to its optimum
atmospheric corrosion resistance and the excellent
mechanical properties which can be obtained;
= glossy wire for domestic use, electric
household appliances, gratings, luggage racks,
bicycle spokes, owing to the optimum combination of
corrosion resistance and mechanical strength in the
work-hardened state;
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= wire for laundry drying frames, owing to the
good resistance to saline mist (traces of chlorides
may remain on the washed laundry) and also good
mechanical strength and non-magnetic property;
= special wires and screws for electronic
components, owing to its non-magnetic property in
the deformed state and good cold deformability;
= wires for architecture, for meshwork and for
hooks used on slate roofs, owing to the mechanical
strength and resistance to environmental corrosion;
= wire and tie-rods for industrial furnaces
operating at a medium to low temperature (up to
550 C, for treatment of copper, aluminium and other
alloys), owing to the excellent resistance to
temperatures up to 800 C.
