Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Hardening steel for lifting, fastening, clamping and/or lashing means and
connecting
elements, component for lifting, fastening, clamping and/or lashing
technology,
connecting element and method of production thereof
The invention relates to a hardening steel for lifting, fastening, clamping
and/or lashing means of
quality grade 8 and above. In particular the invention relates to the use of
said steel for lifting,
fastening, clamping and/or lashing means, especially for chains and chain
links, and for
connecting elements, for example bolts. Further the invention relates to a
connecting element,
like a bolt, and component of the lifting, fastening, clamping and/or lashing
technology made
from said steel. Also the invention relates to a method for the production of
such a connecting
component, particularly the treatment of steel in a chain molding process and
hardening
process.
A plurality of compositions for steel is already known, also for steels that
are used in lifting,
fastening, clamping and/or lashing means. Under high safety requirements
lifting, fastening,
clamping and/or lashing means must lift or lash heavy loads or must enable
fastening or rather
securing of the lifting, fastening, clamping and/or lashing means at the load
or at fixing devices.
According to their mechanical resilience the lifting, fastening, clamping
and/or lashing means are
divided into different quality classes or grades. The quality classes or
grades are mostly
standardized, for example quality grade 8 according to ISO 3076, 3077 or in
Germany DIN-EN
818-2, 818-7 and quality grade 10 according to PAS 1061.
The higher a quality grade the heavier loads can be carried by the lifting,
fastening, clamping
and/or lashing means of identical cross sectional area. Hence a lifting,
fastening, clamping
and/or lashing means having a higher quality grade can, at identical weight,
carry a heavier load
than a lifting, fastening, clamping and/or lashing means having a lower
quality grade and is thus
easier to handle in operation.
For example, a quality grade 8 steel chain measuring 0 16 mm x 48 mm must have
a minimum
breaking force (BF) of 320 kilonewton, a corresponding chain of quality grade
10 must have a
minimum breaking force of at least 400 kilonewton. The same applies to the
classification into
"grade 8" and "grade 10" according to US standards.
For such structural elements of quality grade 8 and above previously steels
containing nickel
have been used. For example US 2007/0107808 Al discloses such steel. However,
since nickel
is expensive it is sensible from an economic point of view to avoid the use of
nickel or reduce the
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nickel content. In the light of the mechanical requirements defined for the
quality grades with
respect to the tempering resistance and the low temperature ductility the
avoidance of nickel is
not unproblematic. Nickel is particularly considered essential for steels for
lifting, fastening,
clamping and/or lashing means because the notched impact strength of nickel
alloyed steels (Ni
> 0.8 % by weight) is high enough that the strength is hardly impaired by
surface damages like
notches occurring in extreme use.
An annealed connecting element is known for example from DE 10 2008 041 391
Al. This bolt
attains its high strength as a result of a bainite structure. It is
disadvantageous that such a
microstructure can be formed only in accurately controlled cooling and
isothermal transformation
processes. DE 28 17 628 C2 relates to steel alloys having a bainite structure.
Other high-strength bolts are known for example from EP 1 728 883.
These known connecting elements are of high strength, however, at low
temperatures they are
very brittle. Thus they are unsuitable for use at a low temperature range, for
example in outdoor
use in mountain, winter or polar regions.
It is the object of the invention to provide a steel composition having a low
nickel content or
avoiding nickel that fulfils the requirements of quality grade 8 or above with
respect to the
mechanical properties. Particularly a steel is to be provided having a high
notch impact energy at
low temperatures, for example of -40 C, and at high temperatures, e.g. of +400
C, and at the
same time an adequate strength as well as tempering resistance. Since the
components of the
lifting, fastening, clamping and/or lashing technology possess surfaces prone
to wear the steel
must also be hardenable. Furthermore it should be forgeable in order to
produce particularly
resilient structural elements cost-effectively.
It is a further object of the invention to improve the connecting element
referred to above in such
a way that at a high resilience, i.e. a high tensile strength, at low
temperatures no brittle fracture
behaviour occurs.
This object is achieved with the low-alloy steel according to the present
invention having the
composition below in weight percentage:
carbon 0.17 to 0.25, preferably 0.20 to 0.23,
nickel 0.00 to 0.25, preferably 0.00 to 0.15 or 0.10,
molybdenum 0.15 to 0.60, preferably 0.30 to 0.50,
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niobium 0.01 to 0.08 and/or titanium: 0.005 to 0.1 and/or vanadium: 5 0.16
wherein the
content of niobium can preferably be between 0.01 and 0.06,
aluminium 0 to 0.050, preferably 0.020 to 0.040,
chromium 0.01 to 0.50, preferably 0.20 to 0.40,
silicon 0.1 to 0.3, preferably 0.1 to 0.25,
manganese 1.40 to 1.60,
phosphorus less than 0.015,
sulphur less than 0.015,
copper less than 0.20,
nitrogen 0.006 to 0.014,
remainder of iron and unavoidable impurities.
The present steel fulfils in chains the mechanical requirements of quality
grade 8 and above,
especially of quality grades 8 and 10, with regard to the tensile strength and
the notch impact
energy. Thus experiments performed by the applicant have shown that a
component having a
diameter of 16mm, that has been kept at approximately 880 C for about half an
hour, quenched
and subsequently kept at approximately 450 C for about half an hour, in a
tensile test possessed
values of Rm (tensile strength) of just above 1200 MPa, A5 of about 14%
(breaking elongation),
Z of about 65 % (area reduction). The component has a notch impact energy of
approximately
140 Joule at room temperature.
A connecting element made of this steel possesses very high strengths and at
the same time an
extremely high low temperature ductility thus providing high safeties even in
a damaged
condition at very low temperatures.
The known bolts of the highest property classes 14.8, 15.8 and/or 16.8 even at
room
temperature mostly exhibit a low ductility. Inevitably at low temperatures
below the freezing point
they tend to exhibit brittle failure. This fracture behaviour is not tolerable
for a number of
applications, especially for bolts for fastening, lifting and lashing means,
because they are
frequently applied under very extreme climatic conditions as for example on
boats in the polar
region or for the transport of goods in mountain regions. Small damages can
thus result in an
immediate failure of the connecting element. This hazard is significantly
reduced with a
connecting element according to the present invention.
The steel can also be used for components of the fastening, lifting and
lashing technology,
especially chain links, and also for components of the connection technology,
i.e. for connecting
elements. This appears not readily obvious because the relevant standards for
connecting
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elements, e.g. ISO 898, require a carbon content of at least 0.28 weight
percent and the loads of
chain links and connecting elements, especially bolts, are different.
By means of the low material inventory of nickel or respectively the complete
avoidance of nickel
the present steel is cost-effective.
The steel according to the invention is particularly suitable for use in
components and work parts
that on the one hand must have high tensile strengths and at the same time are
exposed to
abrupt hard mechanical strains like shocks or impact occurrences, especially
when exhibiting
operational notches. These are in particular the already mentioned lifting,
fastening, clamping
and/or lashing means like for example steel chains or profile chains, chain
spanners, stop points,
hooks, etc., as well as in particular driving force transferring elements in
conveying systems and
conveying plants. These strains are dominated by high ductility values.
A steel chain measuring 0 16 mm x 48 mm, according to own experiments for
example,
possesses a breaking force of 320 kilonewton required for the quality grade 8
even after a
tempering treatment at a temperature of up to 550 C for one hour. Experiments
by the applicant
at 400, 450, 500 and 550 C have confirmed this. The present steel is denoted
20MnMoCrNb6-4.
From the state of the art a pressure vessel steel 18MnMo4-5 (EN 10028-2) is
known. This refers
to a coarse grained, heat-resistant steel that is used in a field which is
totally different from lifting,
fastening, clamping and/or lashing means and that does not fulfill the
requirements of the quality
grade 8 and above. Due to the requirement profile and coarseness of grains at
a high ductility
only low tensile strengths are attainable.
In particular a component made from the present steel can be treated in
different sections, for
example tempered, so that in these different sections different hardnesses,
ductilities and/or
notch impact energies are present.
In the following some preferred embodiments of the invention are exemplified.
The additional
features of these embodiments can be employed each individually or together
with the features
of other embodiments in arbitrary combination.
In a first preferred embodiment the content of nickel in weight percent is
less than 0.15 or 0.10%.
Since nickel is very expensive it is desirable to keep the nickel content in
the steel as low as
possible. By means of a content of 0.10% or less it is thus possible on the
basis of the remaining
alloy composition to further reduce the costs. It is possible in particular to
produce steel that
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irrespective of impurities does not contain any nickel. Such a nickel-free
steel furthermore saves
the additional working stage of adding nickel.
According to another preferred embodiment the content of carbon in weight
percent is between
0.20 and 0.23 /0. Due to a higher content of carbon the steel becomes harder
however with an
increase in carbon content the ductility decreases. Therefore the preferred
range of the carbon
content is restricted to the present range.
In a further preferred embodiment of the invention the content of molybdenum
in weight percent
is between 0.30 and 0.50 %. This range is particularly advantageous for
obtaining the desired
tempering resistance at low costs.
In a further preferred embodiment of the invention the content of niobium in
weight percent is
between 0.01 and 0.06 %.
In a further preferred embodiment of the invention the content of aluminium in
weight percent is
at least 0.020 and/or at most 0.040 %.
In a further preferred embodiment the content of chromium in weight percent is
between 0.20
and 0.40 % Chromium increases the tensile strength, however, it reduces the
notched impact
strength. For the entire range these two effects are well balanced.
Preferably the content of silicon in weight percent is between 0.1 and 0.25 %.
It is of particular advantage if the sum of the doubled content of nickel or
niobium, the
approximately 1.6-fold content of titanium and/or the one-fold content of
vanadium is at most
approximately 0.16 % (each in weight percent).
In a particularly advantageous embodiment the steel having the composition
according to the
present invention is of an extremely fine grained form having a grain size
finer than 7. Fine
grained steel has a higher low temperature ductility together with a higher
tensile strength.
Preferably the present steel and respectively the present component has a
grain size of 9 to 10.
In a further advantageous embodiment for the production of a component from
the steel
according to the present invention this production comprises optionally
repeated hardening
and/or tempering.
In a further advantageous embodiment of the present steel a lifting,
fastening, clamping and/or
lashing means made from said steel possesses a tensile strength of at least
800 N/mm2, more
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preferably at least 1200 NI/mm2 at the ductility required for chains in the
respective quality
grades. By accepting a lower low temperature ductility, for example because
the steel is used
only at higher temperatures, higher tensile strengths can also be attained
with the present steel.
In a further advantageous embodiment the steel or a component made from the
steel possesses
a hardness of 400 to 450 HV30.
In a further advantageous embodiment the steel or, respectively, a component
made thereof
have in different sections a hardness difference of 80 to 120 HV30.
In a particularly advantageous embodiment the steel or a component made from
the steel
essentially retains its hardness at 380 C, even better at 400 C and even
better still at 410 C for
1 h.
The steel or, respectively, a component made from said steel possesses in a
preferred
embodiment a notch impact energy of at least 30 Joule, preferably at least 45
Joule, more
preferably from about 120 to 140 Joule at -40 C. This low temperature
ductility guarantees that
the component exhibits sufficient safeties even in cold surroundings. At -60 C
the notch impact
energy is at least about 50 Joule.
In a particularly advantageous embodiment of the method a component, for
example a chain
link, made from the present steel is treated differently in different
sections. In particular a
hardened component can be treated in different sections with different
tempering temperatures.
A chain link for example can be treated in the section of its limb with
another temperature than in
the section of the bow. By means of such a method sections having different
degrees of
hardness and ductility are produced. The harder sections constitute wear
surfaces while the less
hard but more ductile sections possess a particularly high resistance against
operational failure.
In a particularly advantageous embodiment of the method for the production of
a component, a
chain link especially, the chain link is treated in such a way that it
possesses in at least one
section a hardness of about 400 to 450 HV30 and in at least one other section
a hardness of
about 365 to 390 HV30 and at the same time fulfils the requirements of the
quality grade 8 with
respect to breaking force and breaking elongation. The sections having
different properties are
preferably connected, i.e. they are linked to one another.
According to another advantageous embodiment the difference in hardness of two
different
sections of a present component can be approximately 90 to 110 HV30. A chain
consisting of
such chain links has particularly advantageous properties. At sections exposed
to an increased
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frictional and/or shock load, for example at the bows, the chain can exhibit
an increased
hardness. In sections that are mainly exposed to mechanical tensile strain or
bending strain
when in operation, for example at the limbs, the chain link can exhibit an
increased resistance to
failure even under unfavourable conditions.
In a particularly advantageous embodiment a chain link has a hardness of about
430 to 470
HV30 in one section and of about 380 to 395 HV30 in another section and at the
same time
fulfils the requirements of quality grade 10.
The steel is particularly suitable for use in a component of the lifting,
fastening, clamping and/or
lashing technology, especially in a chain or a chain link, and/or in a
connecting element, for
example a bolt.
The treatment of the present steel in a cold working and hardening process for
the production of
a connecting element and/or a component having the aforementioned properties
is of particular
advantage.
According to a further advantageous embodiment the present connecting element
can contain
lath martensite having lancet packets within its structure. Lath martensite
induces a high
strength and contrary to other martensites, like mixed martensite or plate
martensite, does not
affect the low temperature ductility. Lath martensite is formed in the course
of the hardening and
tempering on abrupt quenching from a temperature above the AC3 point with
subsequent
tempering temperatures that remain below the temperature above which E-carbide
(transition
carbide Fe2C) decomposes. Consequently it is advantageous if in the production
of the
connecting element the tempering temperature lies below the decomposition
temperature of E-
carbide.
In a cross-section through the connecting element the area percentage of the
lath martensite
can be at least 85%, preferably at least 90 %. It should be hardly possible to
achieve area
percentages of more than 98 % of lath martensite in a cross-section so that
this value can be
considered the upper limit of the lath martensite percentage.
The tempering temperature in the course of the quenching and tempering of the
connecting
element can be between 180 C and 220 C, preferably around or exactly 200 C. At
these
tempering temperatures particularly advantageous combinations of a high low
temperature
ductility with a high tensile strength are obtained.
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The connecting element can contain cold-worked sections, for example one or
more shaped or
rolled thread sections. Preferably cold forming is performed before quenching
and tempering.
On the basis of the aforementioned steel alloy, compared to the known
connecting elements the
present connecting element is characterized by special combinations of
mechanical parameters
that can be adjusted essentially by means of the tempering temperature. It
appears to apply to
the present steel alloy at tempering temperatures up to a maximum of 250 C
that the higher the
tempering temperature is the lower the attainable tensile strength and the
higher the low
temperature ductility will be. In the following the notch impact energy KV
serves as a parameter
for the low temperature ductility, as determined for example with a notch bar
impact test on V-
notch samples according to ISO 148-1.
According to an advantageous embodiment at a temperature of -40 C the notch
impact energy
KV is at least 55 Joule with a tensile strength Rm of at least 1400 N/mm2. An
upper limit of the
notch impact energy KV at a temperature of -40 C and a tensile strength of at
least 1400 N/mm2
can be 70 J.
The notch impact energy KV at even lower temperatures, especially at -60 C,
and at least 1400
N/mm2 can be at least 45 J. An upper limit of the notch impact energy KV at -
60 C can be 60 J.
At a notch impact energy KV of at least 55 J at -40 C, and preferably no more
than 70 J at -
40 C, the tensile strength Rm can preferably be between 1500 and 1600 N/mm2.
The present connecting element can possess a hardness of 450 to 480 HV30.
The connecting element preferably has a fine grained microstructure having a
grain size of 9 or
finer. The grain size can preferably be 10. The grain size can be determined
for example
according to ASTME E 112.
According to a most preferred embodiment the connecting element is
manufactured preferably
from steel 20MnMoCrNb6-4.
In order to guarantee a sufficient hardness characteristic, in one embodiment
the diameter of the
present connecting element is at most between 20 and 25 mm, corresponding to
bolt diameters
of at most M20 to M25.
Most preferably the connecting element is a bolt, preferably for a fastening
means, of property
classes 14.8, 15.8 and/or 16.8.
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The tempering resistance, as required for example in PAS 1061 for chains, can
be over 1 hour
at a tempering temperature of at least 380 C, preferably at least 400 C, more
preferably at least
410 C. At such tempering temperatures, however, the tensile strength required
for connecting
elements of property class 14.8 and above can no longer be achieved.
The invention also relates to the use of steel having one of the above-
mentioned compositions
for the production of a quenched and tempered connecting element and
preferably a connecting
element at least tempered in sections, preferably a bolt.
The invention further relates to a method for the production of a connecting
element, preferably
a bolt, from such steel comprising the additional step of quenching and
tempering. As already
described above, in the course of the quenching and tempering the connecting
element can be
tempered at temperatures between 180 C and 220 C, preferably around or at 200
C.
In the following the invention is exemplarily described by means of one
example. In the light of
the above explanations, the embodiments described in connection with this
example can be
arbitrarily combined with one another or be omitted if the advantage linked
with the respective
feature should not matter in one embodiment.
It is shown by:
Fig. 1 a schematic diagram of a chain link that has been made from the
steel according
to the present invention,
Fig. 2 a schematic diagram of a connecting element according to the present
invention,
Fig. 3 a schematic, qualitative diagram of the results of a tension test
with a round
sample,
Fig. 4 a schematic, qualitative diagram of the results of a static bending
test with stud-
bolts of different property classes,
Fig. 5 a schematic, qualitative diagram of the notch impact energy KV at -
40 C for bolts
having different property classes,
Fig. 6 a schematic, qualitative diagram of the energy absorbed on breaking
threaded
bolts M20 at a temperature of -40 C,
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Fig. 7 a schematic, qualitative diagram of the results of tension tests
(SOD tests) with
slotted head screws of different property classes with two different slot
depths at
-40 C,
Fig. 8 a schematic qualitative diagram of the breaking force and breaking
nominal
voltage after the tension tests (SOD tests) with slotted head screws according
to
the present invention at -40 C and at -60 C as a function of the slot depth.
Representative for components of the lifting, fastening, clamping and/or
lashing technology, in
Fig. 1 a chain link 1 is shown that is made from the present steel. It can be,
for example, a steel
chain link. The chemical composition that can be determined for example by
chemical analysis
of the melt, according to the invention is as follows in weight percent:
carbon 0.17 - 0.25 %,
nickel 0.00 - 0.25 `)/0, molybdenum 0.15 -0.60 %, niobium 0.01 -0.08 % and/or
titanium 0.005 -
0.1 % and/or vanadium 5. 0.16 %, aluminium 0.020 - 0.050 %, chromium 0.10 -
0.50 %, silicon
0.1 - 0.3 %, manganese 1.40 - 1.60 %, phosphorus < 0.015 %, sulphur <0.015 %,
copper
<0.20 %, nitrogen 0.006 - 0.014 %, remainder of iron and unavoidable
impurities.
Preferably the nickel content can be smaller than 0.10 weight percent, the
carbon content
between 0.20 and 0.23 weight percent, the molybdenum content between 0.30 and
0.50 weight
percent, the niobium content between 0.01 and 0.06 weight percent, the
aluminium content
between 0 or respectively 0.020 and 0.040 weight percent, the chromium content
between 0.20
and 0.40 weight percent and/or the silicon content between 0.1 and 0.25 weight
percent.
In particular the sum of the doubled content of nickel (in weight percent),
the approximately 1.6-
fold content of titanium (in weight percent) and/or the one-fold content of
vanadium (in weight
percent) should be at most approximately 0.16 % weight percent.
The chain link 1 made from the present steel exhibits mechanical properties
constituting a good
compromise of tensile strength and notch impact energy. As is shown by the
experimental runs,
the present steel fulfills the requirements of quality grades 8 and 10 without
any difficulty. The
production is cost-effective due to the low content of nickel or the avoidance
of nickel in the
production process because nickel is expensive. Especially the present steel
can possess a high
notch impact energy at a low temperature range, for example at -40 C, and a
high tempering
resistance at high temperatures, for example at 400 C.
In order to determine the properties of the steel, as required by the relevant
steel standards, e.g.
DIN 17115, firstly a 0 16mm cylinder was examined as a reference. After
annealing at 880 C for
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about 1/2 h it was quenched in water and subsequently tempered at 450 C for 1
h and cooled in
air. Thereafter the sample had an Rm value of 1213 MPa, an A5 value of 13.1%
and a Z value of
64%. At room temperature it had a notched impact strength of about 140 J.
A sample that had been heat-treated at 930 C for approximately 4 h and
quenched in water
possessed a grain size of 8-9. Hence the steel is fine grain stable.
In order to demonstrate that components made of the present steel meet the
requirements of
different quality grades the applicant has carried out various experiments.
All experiments were
conducted using a steel chain measuring 0 16 mm x 48 mm. The results for the
steel chains can
be transferred to other typical fastening, lashing and lifting means as for
example stop points,
clamps, chain locks etc.
Test series 1
In a first part, a chain was tempered after hardening from a temperature above
the critical point
AC3 of the iron carbon diagram and then tempered for about one hour at various
temperatures.
Here, after various tempering temperatures for one hour each, the chain
exhibited the values of
tensile strength and breaking elongation shown in Table 1.
Table 1
T in C Fma, in kN A in %
450 372.8 26.1
500 354.8 28.6
550 346.3 30.2
The tempering experiments demonstrate that the mechanical properties required
in quality grade
8 are maintained also at high temperatures up to the beginning of the creep
range. At a
tempering temperature of 400 C for approximately one hour it possessed a notch
impact energy
of about 130 j at -40 C. The present steel consequently is both low
temperature ductile and
creep resistant.
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Hence, according to Table 1, the chain achieves the minimum breaking force of
320 kN and
minimum breaking resistance of 800 N/mm2, respectively, required for such a
steel chain for
quality grade 8.
According to this test series a chain made from the present steel meets the
mechanical
requirements of quality grade 8 according to ISO 3076 and DIN EN 818-2.
Test series 2
The chain was hardened from a temperature above AC3, tempered and subsequently
tempered
again at 380 C for about one hour. Thereby a tensile strength of approximately
435 kN was
attained at a breaking elongation of 31 %. Thus the chain exhibits the
required minimum
breaking strength of 400 kN and 1000 N/mm2, respectively, after reheating.
Test series 2 demonstrates that a chain made of the present steel also meets
the requirements
of PAS 1061 and thus it is suitable for chains having the quality grade 10.
Test series 3
In a third test series a chain link was tempered at a temperature between 180
C and 220 C after
hardening from a temperature above AC3. A chain link treated that way had a
notched impact
bending energy of more than 50 J at -40 C and of about 50 J at -60 C. The
minimum breaking
force was significantly higher than 420 kN at just above 490 kN. Hence such a
chain link can be
used for applications at very low temperatures.
Figure 2 shows in an exemplary way a connecting element 10 in the form of a
bolt. The bolt is
fitted and annealed with a cold-formed, especially rolled thread section 20.
The connecting element 10 is made from steel having the following alloy
components:
carbon: 0.15 to 0.25 % by weight, preferably 0.20 to 0.23 % by weight,
nickel 0.00 to 0.25 % by weight, preferably 0.00 to 0.10 % by weight,
molybdenum: 0.15 to 0.60 % by weight, preferably 0.30 to 0.50 A by weight,
niobium: 0.01 to 0.8 % by weight and/or titanium: 0.005 to 0.01 % by weight
and/or
vanadium: 0.16%, wherein niobium can preferably be from 0.01 to 0.06% by
weight,
aluminium: 0 to 0.050 % by weight, preferably 0.020 to 0.040 % by weight,
chromium: 0.10 to 0.50% by weight, preferably 0.20 to 0.40% by weight,
silicon: 0.1 to 0.3 % by weight, preferably 0.1 to 0.25 % by weight,
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manganese: 1.40 to 1.60 % by weight,
phosphorus: <0.015 % by weight,
sulphur: <0.015 A) by weight,
copper: <0.20 % by weight,
nitrogen: 0.006 to 0.14 % by weight.
The remainder of the steel is iron and unavoidable impurities.
The sum of the doubled content in weight percent of nickel, the approximately
1.6-fold content of
titanium and/or the one-fold content of vanadium, in weight percent
respectively, should
preferably be at most approximately 0.16% by weight.
The first test series with that steel was conducted with regard to its
suitability for the production
of chain links. These tests also suggest the suitability for the production of
connecting elements
10.
Experiment 1.1
Firstly according to DIN 17115 as a reference a cylinder having a diameter of
16mm made from
the steel described above was examined. After annealing at 880 C for about
half an hour the
cylinder was quenched in water and subsequently tempered at 450 C for one hour
and cooled in
air. Thereafter this sample had a tensile strength 13, of 1213 N/mm2, a value
of A5 of 13.1% and
a value of Z of 64%. At room temperature the sample possessed a notched impact
strength of
about 140 J.
It can be concluded from that experiment that at a tempering temperature of
between 180 C an
220 C, preferably around 200 C, significantly higher values of the tensile
strength 13, are
achieved. The tensile strength at such a tempering temperature should be at
least 1400 N/mm2,
especially above 1500 N/mm2 up to about 1600 N/mm2, possibly slightly above,
so that bolts
having a property class of 14.8, 15.8 and 16.8. can be obtained.
Experiment 1.2
A sample that had been heat-treated at 930 C for about four hours and quenched
in water
showed an ASTM grain size of 8 to 9. Therefore, the steel is fine grain
resistant.
At tempering temperatures of 180 C to 220 C and shorter tempering periods, of
for example one
hour, even finer grain sizes, about ASTM 10, are anticipated. Grain sizes of
about ASTM 10 are
also attainable with a heat treatment at lower temperatures and/or a shorter
period.
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Experiment 1.3
In order to demonstrate that steel chains and other typical fastening, lashing
and lifting means,
as for example stop points, clamps, chain locks etc., made from the above
steel meet the
relevant quality grades for these components, further tests have been carried
out. These
experiments were conducted using a steel chain measuring 0 16 mm x 48 mm.
Test series 1.3.a
In a first series of tests after hardening the steel chain 16 x 48 was
annealed at a temperature
above the critical point AC3 of the iron carbon diagram and thereafter
tempered for
approximately one hour at various temperatures. Here, after various tempering
temperatures,
the chain had the values shown in Table 2 with respect to tensile strength and
breaking
elongation.
Table 2
T C Frna, in kN Ar %
450 372.8 26.1
500 354.8 28.6
550 346.3 30.2
The tempering experiments demonstrate that the steel is creep resistant (heat-
resisting). Since
at a tempering temperature of 400 C for about one hour a notch impact energy
of about 130 J at
-40 C could be verified the above steel for the connecting element 1 is both
low temperature
ductile and creep resistant. As is proved by Table 2, the breaking load
increases with a decrease
in tempering temperature while the breaking elongation decreases.
Within the chain link a minimum breaking strength of 800 NI/rnm2 is achieved.
Test series 1.3.b
The chain was hardened from a temperature above AC3, tempered and subsequently
tempered
again at 380 C for about one hour. Here the chain link showed a tensile
strength of about 435
CA 02848823 2014-03-14
kN at a breaking elongation of 31%. Thus on reheating the chain has a minimum
breaking
strength of 1000N/mm2.
Test series 1.3.c
In a third series of tests a chain link was tempered at a temperature between
180 C and 220 C
after hardening from a temperature above AC3. A chain link treated that way
had a notched-bar
impact energy KV according to DIN EN 10045 of more than 50 J at -40 C and of
about 50 J at
-60 C. The minimum breaking strength was just above 490 kN within the chain
link. Also from
this a tensile strength Rm of more than 1400 N/mm2, especially between 1500
N/mm2 and 1600
N/mm2, can be concluded for a connecting element like a bolt if one allows for
the more
multidimensional stress condition of the connecting element.
In a second series of tests the steel was quenched from a temperature above
the AC3 point and
then tempered between 180 C and 220 C. After this quenching and tempering, in
a cross
section or micrograph, respectively, the samples possessed a lath martensite
area percentage
of between 85% or 90%, respectively, and 98%.
Starting material for all samples was a threaded bolt M20.
Experiment 2.1.a
A tensile test according to ISO 6892-1 at a temperature of 20 C with the round
specimen turned
off from the threaded bolt M20 having an outside-diameter of 15 mm
qualitatively results in the
distribution shown in Fig. 2.
The tensile elastic limit Rp0,2 of the turned off round specimen thus is
between 1250 and 1350
N/mm2. The tensile strength Rm is above 1400 N/mm2, between 1500 and 1600
N/mm2.
The breaking elongation A5 is from above 13 % to a maximum of 18%, in the
region of about
15%. The area reduction Z is higher than 48% up to about 55%, in the region of
51%.
Experiment 2.1.b
If a round specimen having an outside diameter of 15 mm turned off from the
stud bolt M20 was
tempered at about 200 C in order to obtain a high tensile strength Rm, the
following values would
be obtained: Rm = 1550 ... 1600 N/mm2, Rp0,2= 1300 ... 1350 N/mm2, A5 = 8 ...
12%, Z = 40 ...
50%.
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From the results of the tensile test according to Figure 3 it can be concluded
that the connecting
element according to the present invention exhibits a very high tensile
strength and at the same
time a high ductility at room temperature. In the light of the results of the
tensile test, as far as
hereinafter tests are carried out with a stud bolt according to the present
invention, it is assigned
to property class 15.8.
Experiment 2.1.c
In a further experiment, after a tempering temperature of 300 C a further
round specimen
equally cut from a bolt M20 made from the present steel had an area reduction
Z in the range of
60 ... 70%. Due to the high tempering temperature the tensile strength Rm was
1425 ... 1475
N/mm2.
Experiment 2.2
In Figure 4 the results of a static bending test with stud bolts M20 of
property classes 8.8, 10.9,
12.9 and 15.8 are presented qualitatively and, on the bottom right, the
samples at the end of the
experiment are shown. The connecting element according to the present
invention of property
class 15.8 has been compared with commercial connecting elements of property
classes 8.8,
10.9 and 12.9.
The bending test has been carried out using 120 mm long stud bolts and a steel
prop having a
radius of 20 mm. The stud bolts rested on the inclined plane of a 90 prism.
It turns out that not only can the connecting element according to the present
invention absorb a
significantly higher bending load, but also the deformability of the present
connecting element
exceeds the deformability of the bolts of the lower property classes 12.9 and
10.9. Thus a bolt
M20 according to the present invention survives a bending deformation of 24mm.
At that
deformation the bolts of property classes 12.9 and 10.9 had already broken.
Experiment 2.3
In further experiments the low temperature ductility of a threaded bolt M20
according to the
present invention was examined. For that purpose at -40 C a notched-bar impact
test according
to ISO 148-1 was carried out. The qualitative results, again in comparison
with connecting
elements of lower property classes, here 10.9 and 12.9, are shown in Figure 5.
The notch impact energy KV at -40 C of above 60 J up to about 69 J obtained
according to
these experiments with the present connecting element 15.8 is significantly
above the values of
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17
the notch impact energy KV for the otherwise identical ISO-V-samples of the
stud bolts of
property classes 10.9 and 12.9.
The connecting elements according to the present invention thus possess a high
low
temperature ductility that exceeds the low temperature ductility of the lower
property classes.
Experiment 2.4
The high low temperature ductility of a present connecting element which
despite a significantly
higher strength exceeds the low temperature ductility of lower property
classes, can also be
seen from Figure 5. Figure 6 shows qualitatively the absorbed energy when
breaking threaded
bolts M20 at a test temperature of -40 C.
Hence at -40 C a present threaded bolt M20 absorbs significantly more energy
than the
threaded bolts M20 of the property classes 10.9 and 12.9. At low temperature
applications the
excess of absorbed energy of the connecting element according to the present
invention
provides higher safety in operation.
Experiment 2.5
In a further series of tests the ductility behavior of present connecting
elements was examined in
comparison with commercial connecting elements of lower property classes at -
40 C by means
of SOD experiments.
In SOD ("Slit Opening Displacement") experiments a slot parallel to the
secant, in some samples
having a depth of 3.4mm and in other samples having a depth of 6.8 mm measured
from the
core depth of the thread, is driven into the bolts M20 (see Fig. 6). The slot
depth thus
corresponds to 20% (slot depth 3.4 mm) and 40% (slot depth 6.8 mm),
respectively, of the
diameter. Subsequently the bolts are strained under tension. Via a strain
gauge at the outer
diameter opposite to the deepest point of the slot the opening of the slot
with an increase in
tensile strain is monitored.
The result of the SOD experiments is qualitatively shown in Figure 7.
It can be seen that in comparison to the bolts 10.9 and 12.9 up to the
breakage of the bolt the
highest absolute energies can be absorbed by the present connecting element.
Further it can be seen from the test results in Figure 7 that breakage occurs
with the present
connecting elements only after the slot has been considerably widened. While a
bolt M20 of
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property class 12.9, more or less regardless of the slot depth, breaks after
widening of the slot
by about 0.3 mm and a bolt of property class 10.9, equally more or less
regardless of the slot
depth, breaks at a slot widening by about 0.5 mm, a connecting element
according to the
present invention made from the steel above tolerates a widening of the slot
by significantly
more than 0.5 mm, namely up to more than 0.7 mm.
It can be concluded from the SOD experiments that even at a temperature of -40
C a connecting
element according to the present invention will not fall below the admissible
working load limit
WLL for stop bolts even at -40 C and when damaged. Thus a safety factor of 6
applies to stop
bolts with respect to the tensile strength for being an admissible working
load limit. A bolt of
property class 15.8 with a tensile strength of 1500 kN when used as stop bolt
consequently must
be loaded at most with only 1500 kN/6 = 250 kN. However, at -40 C the
connecting element
according to the present invention has a more than three-fold remaining safety
with respect to
the working load limit. This safety exists even at -60 C. In Figure 8 the
tensile strength, the
working load limit WLL and the three-fold working load limit are marked by
chain dotted lines.
All of the experiments show that the connecting element according to the
present invention
combines an extremely high strength with an extremely high low temperature
ductility. With
regard to bending strength as well as notch impact energy and breaking
strength of the SOD
samples, the present connecting elements are superior to the known connecting
elements.
