Note: Descriptions are shown in the official language in which they were submitted.
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Method for solution hardening of a cold deformed workpiece of a passive al-
loy, and a member solution hardened by the method.
Technical field
The invention relates to a method for solution hardening of a cold deformed
workpiece of a passive alloy. The method provides a hardened alloy in which
substantially no carbides and/or nitrides are formed. The method also pro-
vides a corrosion resistant surface while retaining the core strength of the
material obtained from the cold deformation. The invention further relates to
a member solution hardened by the method. Such members are particularly
relevant in the fields of medico, food, automotive, chemical, petrochemical,
pharmaceutical, marine, package, watches, cutlery/tableware, medical, en-
ergy, pulp & paper, mining, or waste water technology.
Background
Stainless steel and other passive alloys are typically materials with good cor-
rosion resistance, but with relatively poor tribological characteristics, e.g.
ad-
hesive wear characteristics. To solve this problem stainless steel and compa-
rable alloys can be surface hardened at low temperature (below 450-550 C)
by dissolution of nitrogen and/or carbon, by which is obtained a zone of so-
called expanded austenite or alternatively expanded martensite. This zone is
a supersaturated solution of carbon and/or nitrogen in austenite or marten-
site and is metastable with respect to carbide/nitride formation. Such low
temperature processes can be based on gas, plasma or molten salt; gas
processes require use of special activation techniques, whereas for plasma
and salt bath activation is immediately achieved and no special treatment is
necessary. Thereby a surface zone is obtained in the material, which surface
zone contains large amounts of nitrogen and/or carbon; this is due to the
relatively low process temperature. The material thereby becomes surface
hardened and retains its corrosion resistance. Most passive alloys, such as
stainless steel, however cannot immediately be solution hardened with nitro-
gen and/or carbon, since these passive alloys have an impermeable oxide
layer, also called the passive layer, which is the reason for the good
corrosion
characteristics, but which prevents solution of e.g. nitrogen and carbon. Spe-
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cial techniques for removal of this passive layer are therefore required.
These
techniques are known to the skilled person.
Most employed technological components are used in a machined condition,
which means that the material is inhomogeneously cold deformed (plastically
deformed). In many applications such cold deformation is desirable from a
component-strength-consideration; the component would not work if it did
not have the strength increase from the work hardening induced by cold de-
formation. This creates a big problem if such cold machined components are
surface hardened in a low temperature process, so that the surface is
changed to expanded austenite or martensite under uptake of nitrogen
and/or carbon. The presence of plastic deformation (defects in the micro-
structure) in the material implies that nitrides and carbides develop easier
by
reaction of nitrogen and carbon with e.g. chromium (Cr), which is an alloying
element in stainless steel. Consequently an amount of Cr is removed from
solid solution and bound as chromium nitride/chromium carbide. This implies
that the corrosion characteristics are deteriorated because less chromium is
available for maintenance of the passive layer. In local areas such Cr-
depletion can be pronounced and result in loss of corrosion protection at the
surface of the area. The precipitation of nitrides/carbides is called
sensitisa-
tion. In particular on dissolution of nitrogen this phenomenon is very pro-
nounced, because chromium nitrides are more stable than chromium carbides
and can be formed at lower temperature. This means that the temperature at
the low-temperature process must be lowered (further) to avoid sensitisation,
which is undesirable since the process thereby proceeds more slowly. For ex-
treme degrees of deformation in stainless steel there is perhaps not even a
lower limit to sensitisation.
At low-temperature hardening of cold deformed stainless steel workpieces
sensitisation will occur in connection with the low-temperature dissolution of
nitrogen and/or carbon, which takes place at temperatures below 550 C. To
solve the problem with sensitisation in cold deformed materials upon low-
temperature surface hardening a full annealing of the components has -
where possible - been made by a so-called austenitisation in vacuum or hy-
drogen atmosphere. Full annealing is a process, which is carried out at tem-
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peratures above 1020 C, typically in the range 1020-1120 C. Thereby the
cold deformation in the material is annihilated and the low-temperature dis-
solution can be carried out without the risk of sensitisation. However, the
process provides the problem that the strength of the cold-worked metal is
reduced - this is referred to as a so-called egg shell effect in the material,
i.e.
the material becomes soft with a hard thin surface, when the workpiece is
subsequently low-temperature hardened. By carrying out an austenitisation
the core strength of the material is reduced to that of annealed material, and
this process requires that the core strength of the treated component is a
design parameter of less importance.
Another possibility is to employ a carburising process where only carbon is
dissolved in the material at low temperature, i.e. formation of carbon ex-
panded austenite. Sensitisation is not as critical for carbon dissolution as
it is
for nitrogen dissolution (nitriding and nitrocarburising) and hence leads to
less influence on the corrosion resistance. However, for components with a
strong degree of cold deformation even this is considered detrimental. An-
other disadvantage by only employing carbon dissolution is that a lower sur-
face hardness is obtained than for nitrogen dissolution and that the composi-
tion profile (hardness) cannot be adjusted in the same way (see e.g. EP
1095170 B1 and WO 2006/136166 Al).
In e.g. Georgiev et al, Journal of Materials Science and Technology, Vol. 4,
1996, No. 4, pp. 28 and Bashchenko et al, Izvestiya Akademii Nauk SSSR.
MetaIly, no 4, 1985, pp. 173-178, it is shown that nitrogen and/or carbon can
be dissolved in stainless steel at high temperature (above about 1050 C) un-
der equilibrium conditions. It is shown that by employing high temperatures
the problem with permeation of the passive layer of stainless steel can be
bypassed, since this becomes unstable at these high temperatures. It is also
described that the solubility temperature for chromium carbide and chromium
nitride lies below this temperature. Consequently, carbides and/or nitrides
are not formed at these high temperatures. The solubility of nitrogen/carbon
is however relatively limited and for austenitic stainless steels no actual
sur-
face hardening occurs; this applies in particular for carbon. To avoid precipi-
tation of carbides/nitrides during cooling a fast cooling rate is required.
For
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martensitic stainless steel types a significant hardening of the surface can
take place by fast cooling; however, the hardening effect is at a
significantly
lower level than obtained by processes for formation of expanded austenite.
WO 2008/124239 suggests a hybrid carburisation process with intermediate
rapid quench, according to which a carbon hardened surface in a metal work-
piece can be formed without forming carbide precipitates by subjecting the
workpiece to both high temperature carburisation and low temperature car-
burisation, wherein immediately after high-temperature carburisation, the
workpiece is rapidly quenched to a temperature below which carbide precipi-
tates form. The rapid quenching may be accomplished using e.g. immersion
of the workpiece in water, oil or other cooling medium such as a gas or mol-
ten salt. WO 2008/124239 fails to recognize the issues of cold-deformation
and formation of carbides and/or nitrides during a subsequent low-
temperature hardening.
There is a need for a method which allows low temperature dissolution of ni-
trogen and/or carbon for hardening of passive alloys such as stainless steel,
where the problems with sensitisation and/or adjusting the composition pro-
file are solved.
To overcome the problem with sensitisation in connection with low tempera-
ture nitriding and/or carburising of cold deformed workpieces the prior art
suggests to anneal the material first, so that partial or full re-
crystallisation is
obtained; alternatively only a recovery of the material. Thereby the cold de-
formation in the material, and the strengthening obtained from the cold de-
formation, is annihilated, but on the other hand the low temperature dissolu-
tion can be carried out without problems with sensitisation. However, this
solution fails to provide components having high core strength.
The Danish patent application PA 2011 70208 discloses a method for
dissolution hardening of a cold deformed workpiece of a passive metal or a
passive alloy. The method comprises a first step in which nitrogen and/or
carbon is dissolved in the workpiece at a temperature higher than the
solubility temperature for carbide and/or nitride formation and lower than the
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melting point of the workpiece, and a subsequent second step, wherein
nitrogen and/or carbon are dissolved at a temperature where substantially no
formation of carbides and/or nitrides occurs. The method may also comprise
a quick cooling from the first to the second temperature. While treatment of
5 metals according to PA 2011 70208 provides superior characteristics com-
pared to other processes of the prior art it is suspected that further im-
provements in the characteristics of the metals may be achieved.
The aim of the present invention is to provide a method, which allows solu-
tion hardening of products shaped through cold deformation and prepared
from passive alloys, in particular stainless steel, without sensitisation
occur-
ring in the workpiece and thereby provide a better corrosion resistance. It is
a further object that the strengthening effect obtained is comparable to or
possibly even larger than the strengthening effect obtained by cold deforma-
tion.
Description of the invention
The present invention relates to a method for solution hardening of a cold
deformed workpiece of a passive alloy containing at least 10% chromium,
which method comprises
-dissolving at least nitrogen in the workpiece at a temperature Ti, which is
higher than the solubility temperature for carbide and/or nitride and lower
than the melting point of the passive alloy, wherein dissolution of nitrogen
at
temperature Ti is performed to obtain a diffusion depth in the range of
50 pm to 5 mm, and
-cooling the workpiece after the dissolution step at temperature Ti to a tem-
perature which is lower than the temperature at which carbides and/or ni-
trides form in the passive alloy, wherein the cooling step takes place in an
inert gas not containing nitrogen.
The method of the invention may also be viewed as a method for solution
hardening of a cold deformed workpiece of a passive alloy, which method
comprises the steps of:
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dissolving at least nitrogen in the workpiece at a temperature Ti, which is
higher than the austenisation temperature and lower than the melting point
of the passive alloy,
cooling the workpiece after the dissolution step to a temperature which is
lower than the temperature at which carbides and/or nitrides form in the pas-
sive alloy, wherein the cooling step takes place in an inert gas not
containing
nitrogen.
In a preferred example the first dissolution step is performed in a gas, such
as a gas containing N2, e.g. substantially pure N2 without other gasses than
unavoidable impurities, and the cooling step is also performed in a gas, which
is an inert gas not containing nitrogen (an nitrogen-free inert gas) with
argon
being particularly preferred. In the context of the invention an "inert gas"
is a
gas that does not contain any substantial amount of molecules which interact
with elements of the alloy; any inert gas not containing nitrogen is contem-
plated in the invention, or mixtures of gasses. When an inert gas is employed
in the cooling step it has surprisingly been found that the workpiece treated
in the method of the invention has a corrosion resistance, which is even su-
perior to the corrosion resistance obtained using other cooling gases, or when
the cooling step is performed using other methods. In particular, gasses con-
taining nitrogen are believed to accelerate formation of nitrides when the
cooling is performed in a gas containing nitrogen compared to cooling in an
inert gas, so that a more robust and flexible method is provided with a cool-
ing step using an inert gas. The partial pressure of nitrogen in the treatment
at temperature Ti determines the solubility of nitrogen, so that the higher
the partial pressure of nitrogen in the treatment at temperature Ti the more
pronounced is the effect of cooling in an inert gas not containing nitrogen.
Cooling in an nitrogen-free inert gas may also allow longer cooling times than
60 s, but preferably cooling is performed an nitrogen-free inert gas in less
than 30 s, such as in less than 10 s.
In a specific embodiment the method further provides formation of expanded
austenite and/or expanded martensite in the cold deformed workpiece of the
passive alloy. Thus, the method may further comprise a subsequent second
step of dissolving nitrogen and/or carbon in the workpiece at a temperature
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T2 of at least 300 C, which temperature T2 is lower than the temperature at
which carbides and/or nitrides form in the passive alloy.
The first step of dissolving nitrogen in the workpiece at a temperature higher
than the solubility temperature for nitride significantly improves the core
strength of the passive alloy, such as stainless steel, in comparison to only
re-crystallisation annealing of the material prior to low temperature harden-
ing. The high temperature dissolution of nitrogen is done at temperatures
above the austenisation temperature of the alloy, e.g. at least or above
1050 C and below the melting point of the alloy. The strengthening effect of
this high-temperature nitriding is, surprisingly, sufficient to compensate for
the loss of strength caused by annihilating the cold deformation while the
workpiece is kept at the high temperature during nitriding. Furthermore, the
high-temperature nitriding allows that low temperature hardening can be per-
formed at higher temperatures than usual without creating problems with
formation of nitrides and/or carbides, and that it is easier to activate the
pas-
sive surface on the material at the subsequent low temperature surface hard-
ening process. Thus, the formation of the hardened zone is accelerated. Fur-
thermore, better corrosion characteristics are obtained, since nitrogen exists
in solid solution.
A significant improvement of the hardening of passive alloys can be obtained
by the high temperature dissolution of nitrogen followed by low temperature
nitriding, carburising or nitrocarburising. Any passive alloy in which
expanded
austenite or expanded martensite may form is relevant to the invention, and
stainless steel is preferred, in particular cold deformed austenitic stainless
steel.
The optional subsequent low temperature dissolution of nitrogen and/or car-
bon, which takes place at temperatures below the temperature at which car-
bides and/or nitrides form in the passive alloy, such as below 450-550 C de-
pendent on the process, may in the subsequent step be carried out on a ma-
terial, which does not contain plastic deformation, but which has a strength
on level with a plastically deformed workpiece. This means that the risk of
sensitisation is reduced significantly. The presence of nitrogen and
optionally
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carbon in solid solution in stainless steel have even been found to give a
faster low temperature process, than can be obtained using methods of the
prior art, since the diffusion coefficients of nitrogen and carbon increase
with
increasing carbon/nitrogen content. Thus, in certain examples the passive
alloy is a stainless steel containing nitrogen and/or carbon.
With the present invention it is possible to carry out a low temperature hard-
ening of passive materials, and in particular stainless steel, of even
strongly
cold deformed components without occurrence of sensitisation of the material
and without loss of strength. Cold deformed material treated with the method
of the invention can obtain a significantly better corrosion resistance than
untreated material. Conducted experiments have shown that the strength
which is obtained by dissolution of nitrogen and optionally carbon in
stainless
steel at high temperature, typically above 1050 C, may give a (core)
strength or substrate bearing capacity, which is sufficient to compensate for
the loss of strength which occurs when the cold deformation is removed by
recrystallisation while heating to and maintaining the high temperature dur-
ing nitriding. That is, although the strength obtained from cold deformation
is
lost, this loss is compensated by the strength obtained from solution harden-
ing with nitrogen and optionally carbon. Even relatively small amounts of ni-
trogen give a significant increase of strength to provide the bearing
capacity,
which is necessary for wear resistant expanded austenite.
The method of the present invention provides manufactured members having
at least the same strength as cold deformed members and at the same time
better corrosion resistance, and further provides the advantage of taking less
time to perform.
Dissolution at temperature Ti and at the optional temperature T2 may be
performed using any appropriate technology. For example dissolution at tem-
perature Ti and at temperature T2 may be performed in a gaseous process,
e.g. using a gas containing nitrogen, such as ammonia, preferably N2. Disso-
lution may also be performed using ion implantation, salt bath or plasma. It
is preferred that dissolution at temperature Ti and temperature T2 are car-
ried out using gas, since this is a cheap and efficient solution and because
all
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types of geometries may be treated uniformly, and there is a good tempera-
ture uniformity. Moreover, the use of a gas process means that the process is
within the framework of the laws of thermodynamics, which means that there
are very well controlled processing conditions. It is further an advantage to
employ gas because it has surprisingly been found that the high temperature
process of the invention makes the surface easier to activate using gas in the
low temperature process. It is thus easier to remove the impermeable oxide
layer (passive layer), which is found on passive materials after a high tem-
perature dissolution. It is assumed that this is attributable to the presence
of
nitrogen and optionally carbon which is dissolved at high temperature.
The optional low temperature process may be carried out immediately after
the high temperature process, but this is not mandatory. It is also possible
to
perform the two processes with an offset in time and place. If the processes
are carried out immediately after each other with the cooling step between
the first and the second dissolution step, it is possible to avoid that a
passiva-
tion of the surface occurs and hence activation prior to the low temperature
process is superfluous. Thus, the invention also relates to an example
wherein dissolution at temperature T2 takes place immediately after cooling
from temperature Ti without the passivation/activation of the surface in-
between the execution of the high temperature process and the low tempera-
ture process. This may be done in the same furnace. When using gas the
relevant gases containing nitrogen and/or carbon for use in the low tempera-
ture process may be supplied immediately when the material is cooled to
temperature T2. However, the cooling is advantageously done using argon
without any nitrogen present during cooling. An advantage of using gaseous
processing is that it is possible to use gases, which do not activate the sur-
face at temperature T2 in the low temperature process. Other advantages of
this example are that the hardening process thereby can be made cheaper
and quicker.
An advantage of the method of the invention is that better corrosion charac-
teristics are obtained, since nitrogen exists in solid solution. Dissolution
of
carbon does not change the corrosion characteristics. The material may be
considered to be a nitrogen-containing alloy, if the component is fully satu-
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rated with nitrogen. This will often be the case for thin-walled workpieces,
e.g. workpieces with a material thickness of up to 4 mm, such as a thickness
of 2-4 mm, which are treated with the method of the invention. Stainless
steel workpieces which are treated with the method of the invention therefore
5 have a far better corrosion resistance compared to workpieces, which
solely
are treated with the low temperature process (see the examples). An aspect
of the invention relates to a thin-walled component, or workpiece, of a cold
deformed metal or alloy treated according to the method of the invention.
10 For thin-walled components the material may be fully saturated with
nitrogen
by the high temperature process. In thick material a surface zone of up to
several millimetres, e.g. up to about 5 mm, may be obtained where nitrogen
is in solid solution. In both cases the bearing capacity of the material will
be
increased and comparable to what may be obtained by cold deformation. In
an example of the invention, which allows that workpieces with a thickness of
up to about 10 mm are fully saturated with nitrogen so that particularly
strong workpieces are obtained. In general, the method provides that a
thickness of expanded austenite or expanded martensite of at least 5 pm is
obtained in the workpiece, and the hardness of the expanded austenite zone
or the expanded martensite zone is at least 1000 HV, such as more than
1050 HV.
The method may further comprise that dissolution at temperature T2 takes
place immediately after cooling from dissolution at temperature Ti without
the occurrence of a passivation of the surface. In a certain example cooling
after the first dissolution process at temperature Ti takes place especially
quickly, e.g. in a period of no more than 60 second, in the temperature inter-
val in which there is the largest tendency for sensitisation and formation of
precipitations, such as nitrides and/or carbides, for the relevant alloy. For
stainless steel it has been found that this in particular takes place in the
in-
terval from 900 to 700 C where the material should be cooled quickly. In one
embodiment the workpiece is cooled from 900 to 700 C in less than
60 seconds. In a preferred embodiment the workpiece is cooled from 900 to
700 C in less than 30 seconds. Thereby the formation of carbides and/or ni-
trides is substantially avoided, and this is an advantage since these can
react
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with the alloying elements in stainless steel, such as chromium. The depletion
of alloying elements from solid solution and binding of these as nitrides
and/or carbides is suppressed and the corrosion resistance characteristics are
maintained.
In general, the features of the methods of the invention may be combined
freely, and all such combinations are contemplated in the present invention.
For example, all features and variations discussed for the first dissolution
step at temperature Ti are relevant also when the method comprises a sec-
ond dissolution step at temperature T2. Likewise, all features discussed for
the subsequent step of dissolving nitrogen and/or carbon in the workpiece at
a temperature T2, which is lower than the temperature at which carbides
and/or nitrides form in the passive alloy are relevant for any combination of
features for the first dissolution step at temperature Ti and the cooling in
an
inert gas.
In another aspect the invention relates to a member solution hardened by the
method of the invention. Any workpiece may be treated in the method, al-
though it is preferred that the workpiece has a thickness of up to about
10 mm, since this will provide that the resulting member is fully saturated
with nitrogen. Members which are solution hardened according to a method
of the invention may be used in any technological field. Fields of particular
relevance comprise members for use in the technical areas of medico, food,
automotive, chemical, petrochemical, pharmaceutical, marine, package,
watches, cutlery/tableware, medical, energy, pulp & paper, mining or waste
water technologies. Members of particular interest comprise valves (butterfly
valves, ball valves, control valves), steering bolts, nuts, washers,
fasteners,
nozzles, pumps, machinery components, semiconductor ASML, ferrule parts,
ball bearings and bearing gages, pneumatic parts, membranes etc.
In a further aspect the invention relates to a member solution hardened by
the method according to the invention, where the member is a valve part or
a part used in a valve.
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In a further aspect the invention relates to a member solution hardened by
the method according to the invention, where the member forms an outer
surface area of a design object, such as a clips for holding paper or notes, a
sign plate, a holder, a lid of a box, cutlery, a watch, or a plate mounted to-
gether with a handle or a plate forming part of a lamp.
In a further aspect the invention relates to a member solution hardened by
the method according to the invention, where the member is part of a bear-
ing, such as a part of a ball bearing, a part of a roller bearing, or a
bearing
cage.
In a further aspect the invention relates to a member solution hardened by
the method according to the invention, where the member is part of medical
equipment, or medical instruments, or dental equipment, or dental instru-
ments, or is a medical instrument or a dental instrument.
In a further aspect the invention relates to a member solution hardened by
the method according to the invention, where the member is part of pharma-
ceutical equipment, such as a plate, a nozzle, a shim, a pipe, or a grid.
In a further aspect the invention relates to a member solution hardened by
the method according to the invention, where the member is part of a car,
such as a plate, a part in the exhaust system, a filter part, an engine part,
a
fixture, a handle, or a part having a decorative surface.
Figures of the drawings
Fig. 1 shows an isothermal transformation diagram (TTT diagram) for a nitro-
gen-containing austenitic stainless steel.
Fig. 2a shows a set of lock washers.
Fig. 2b shows a set of lock washers with a bolt and nut.
Fig. 3 shows photomicrographs of a lock washer treated in two prior art
methods.
Fig. 4 shows photomicrographs of a lock washer treated in two prior art
methods.
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Fig. 5 shows photomicrographs of a sample of AISI 316 treated in two prior
art methods.
Fig. 6 shows photomicrographs of a sample of AISI 304 treated in two prior
art methods.
Fig. 7 shows hardness profiles of stainless steel treated in a prior art
method
and by the method of the invention.
Fig. 8 shows lock washers treated in the method of the invention and in a
prior art method.
Fig. 9 shows photomicrographs of samples of AISI 316 treated in a prior art
method (right) and by the method of the invention (left).
Definitions
In the context of the present invention the terms "expanded austenite" and
"expanded martensite" describe an austenite or martensite, respectively,
which has been supersaturated with nitrogen or carbon, or nitrogen and car-
bon (with respect to nitride or carbide formation). Expanded austenite and
expanded martensite may be specified as nitrogen-expanded or carbon-
expanded, or the expansion may be specified as nitrogen- and carbon-
expanded. However, in the context of the invention "expanded austenite" and
"expanded martensite" generally refer broadly to austenite or martensite,
respectively, expanded with nitrogen, carbon or any combination of nitrogen
and carbon. A review of expanded austenite is provided by T.L. Christiansen
and M.A.J. Somers (2009, Int. J. Mat. Res., 100: 1361-1377), the contents of
which are hereby included by reference. Any alloy in which "expanded aus-
tenite" or "expanded martensite" may be formed is contemplated for the
method of the invention. Expanded austenite or expanded martensite may
form in the surface of an alloy when the alloy is subjected to solution of ni-
trogen or carbon, or nitrogen and carbon, and the expanded austenite or ex-
panded martensite may also be referred to as a "zone" of expanded austenite
or expanded martensite. In the context of the present invention the term
"zone" should be understood in relation to the thickness of the treated mate-
rial so that "zone" is comparable to the thickness of expanded austenite or
expanded martensite. The method of the invention provides that a thickness
of expanded austenite or expanded martensite of at least 5 pm is obtained in
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the workpiece; the thickness of the expanded austenite or expanded marten-
site may be up to about 50 pm or higher.
In terms of the invention an "alloying element" may refer to a metallic com-
ponent or element in the alloy, or any constituent in the analysis of the
alloy.
In particular, alloys of relevance in the method of the invention comprise an
element that may form nitrides and/or carbides with present nitrogen and
carbon, respectively. The method of the invention advantageously provides a
surface free from nitrides and carbides of alloying elements. It is however
also contemplated in the invention that an alloy may comprise only a single
metallic element capable of forming nitrides and/or carbides. An alloy may
also comprise other elements, such as semi-metallic elements, inter-metallic
elements, or non-metallic elements. Alloying elements capable of forming
nitrides and/or carbides may typically be metallic elements providing corro-
sion resistance to the alloy due to formation of a passive oxide layer with
the
alloying element. The terms "nitride" and "carbide" as used in the context of
the invention refer to nitrides and carbides formed between alloying elements
and nitrogen and carbon, respectively. An exemplary nitride is chromium ni-
tride, CrN or Cr2N although terms "nitride" and "carbide" are not limited to
nitrides and carbides with chromium.
By the term "passive" in connection with alloys or metals is to be understood
an alloy, which has an oxide layer on the surface. The alloy can be both self-
passivating or be passivated as a consequence of a process to which the alloy
is subjected. Belonging to the group of self-passivating alloys are those,
which have a strong affinity to oxygen (e.g. Cr, Ti, V), including alloys con-
taining such alloying elements (e.g. stainless steel which essentially is an
Fe-
based alloy containing at least 10.5 % Cr).
By the term "cold deformation" (also named "cold working") is to be under-
stood a plastic deformation induced in the material by external forces at a
temperature below the recrystallisation temperature of the material. Cold
deformation may be provided by an actual plastic shape change, such as
forging, extrusion, shaping, drawing, pressing, or rolling, and may also be
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caused by machining such as turning, milling, punching, grinding or polishing
etc., or by a combination of these processes.
By the term "sensitisation" is to be understood that nitrogen or carbon have
5 formed nitrides and carbides, respectively, by reaction with one or more al-
loying elements otherwise utilized to form the protective oxide layer on the
surface, as for example chromium in stainless steel. When sensitisation oc-
curs, the free content of the alloying element, such as chromium, in solid so-
lution is lowered to a level, which is no longer sufficient to maintain a com-
10 plete protective oxide layer, which means that the corrosion
characteristics
are deteriorated.
By the term "solubility temperature for carbide and/or nitride" is to be under-
stood the temperature at which nitrides/carbides are not stable, and where
15 already formed nitrides/carbides are dissolved. In general, alloys
comprising
metallic alloying elements capable of forming nitrides and/or carbides will
have a temperature interval in which nitrides and/or carbides may form when
nitrogen and carbon, respectively, are present. Thus, above this temperature
interval, nitrides and carbides will not form, and already formed ni-
trides/carbides are dissolved. When nitrides or carbides exist, i.e. sensitisa-
tion has occurred, these carbides can generally only be removed by exposing
the sensitised metal to a temperature above the austenisation temperature.
Furthermore, such alloys have a temperature below the temperature interval,
where nitrides and carbides will not form, although nitrides or carbides al-
ready formed in an alloy cannot be removed at the low temperature.
The "austenisation temperature" is typically the temperature used when heat
treating an alloy in order to dissolve carbides, and "austenisation tempera-
ture" may thus correspond to the "solubility temperature for carbide". At the
austenisation temperature the alloy is in the austenitic phase. The tempera-
ture at which a steel alloy changes phase from ferrite to austenite is
typically
at a somewhat lower temperature than the austenisation temperature.
The austenisation temperature as well as the temperature at which carbides
and/or nitrides form in a passive alloy are generally well-known to the
skilled
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person. Likewise the temperature below which nitrides or carbides will not
form is generally known to the skilled person. Furthermore the melting tem-
perature of the alloy is generally known to the skilled person. The tempera-
tures may depend on the composition of the passive alloy, and for any given
composition these temperatures are furthermore easily determined experi-
mentally by the skilled person.
The alloying contents mentioned are expressed in percent by weight. With
respect to compositions of alloys or of gas unavoidable impurities may natu-
rally also be present, even if this is not specifically mentioned.
Further description of the invention
Fig. 1 shows an example of an isothermal transformation diagram (TTT dia-
gram) for a nitrogen-containing austenitic stainless steel; the stainless
steel
has the composition Fe-19Cr-5Mn-5Ni-3Mo-0.024C-0.69N (from J.W. Sim-
mons, PhD thesis, Oregon Graduate Institute of Science and Technology
1993). In Fig. 1 the temperature interval in which nitrides may begin to form
is indicated with "Cr2N". In the method of the invention the step of
dissolving
nitrogen in the passive alloy is thus performed at a temperature Ti above the
austenisation temperature and the workpiece is cooled to a temperature,
which is lower than the temperature at which carbides and/or nitrides form in
the passive alloy in an inert gas not containing nitrogen. The method may
comprise a second step of dissolving nitrogen and/or carbon, which is per-
formed at a temperature T2 below the temperature interval where nitrides
and/or carbides can form. Thus, temperature Ti is higher than temperature
T2. The workpiece is cooled, e.g. within a time span of 60 seconds, after the
first dissolution step at temperature Ti to a temperature which is lower than
the temperature at which carbides and/or nitrides form in the passive alloy.
The passive alloy of the workpiece will thus be stabilised with respect to for-
mation of nitrides and/or carbides, and the optional second dissolution step
may then be performed as desired. The austenisation temperature may also
be referred to as "high" temperature in the context of the invention.
Likewise,
the temperature below the temperature at which carbides and/or nitrides
form is also referred to as "low" temperature.
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The method of the invention comprises steps of dissolving nitrogen and/or
carbon in the passive alloy. The step of dissolving nitrogen may also be re-
ferred to as the "dissolution of nitrogen" or "nitriding", and likewise step
of
dissolving carbon may also be referred to as the "dissolution of carbon" or
"carburising". When both nitrogen and carbon are dissolved in the same
process step may be referred to as "nitrocarburising".
In a certain aspect the invention relates to a member solution hardened by
the method of the invention. In the contexts of the invention "treated" should
be understood broadly. In particular, the term "treated" means that method
of the invention has been employed in the manufacture of the member. Thus,
the invention also relates to a member manufactured using the method of the
invention and the terms "treated in" and "manufactured using" may be used
interchangeably. The method of the invention may be the last step in the
manufacture of the member or a member treated by the method may also be
subjected to further processing steps to provide the final member.
In the context of the present invention a "thin-walled component" is a com-
ponent of a size allowing the component to be fully saturated with nitrogen
and/or carbon in the method of the invention. Thus, a "thin-walled compo-
nent" may have a material thickness, e.g. in its smallest dimension, of up to,
and including, about 10 mm, such as a thickness of about 2 mm to about 4
mm or a thickness in the range from 0.2 mm to 8 mm, or a thickness in the
range from 0.4 mm to 6 mm, or a thickness in the range from 0.5 mm to 5
mm, or a thickness in the range from 1.5 mm to 4.5 mm. The method may
be used with any thin-walled component.
The novel and unique way in which one or more of the above aims is ob-
tained, is by the provision of a method for solution hardening of a cold de-
formed workpiece of a passive alloy containing at least 10% chromium, which
method comprises
-dissolving at least nitrogen in the workpiece at a temperature Ti, which is
higher than the solubility temperature for carbide and/or nitride and lower
than the melting point of the passive alloy, wherein dissolution of nitrogen
at
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temperature Ti is performed to obtain a diffusion depth in the range of
50 pm to 5 mm, and
-cooling the workpiece after the dissolution step at temperature Ti to a tem-
perature which is lower than the temperature at which carbides and/or ni-
trides form in the passive alloy, wherein the cooling step takes place in an
inert gas not containing nitrogen. The method may further comprise a subse-
quent second step of dissolving nitrogen and/or carbon in the workpiece at a
temperature T2 of at least 300 C, which temperature T2 is lower than the
temperature at which carbides and/or nitrides form in the passive alloy.
The invention is especially suitable for stainless steels and comparable
alloys,
where expanded austenite or martensite can be obtained in a low tempera-
ture dissolution process. In general, alloys based on iron, nickel and/or
cobalt
comprising chromium are relevant for the method. The chromium content
may vary and may as an example be up to about 10 %. In other examples
the chromium content may be at about 10 % or at least 10%. Thus, the in-
vention in one example relates to a method for solution hardening of a cold
deformed workpiece of stainless steel. Nitrogen and optionally also carbon
can be dissolved in the stainless steel at a temperature, which is higher than
the austenisation temperature of the stainless steel, e.g. the solubility tem-
perature for carbide and/or nitride for present alloying elements, such as
chromium. Even relatively small amounts of nitrogen give a significant in-
crease in strength to provide a load bearing capacity, which is necessary for
wear resistant expanded austenite. In an example of the invention the hard-
ness of the expanded austenite zone or the expanded martensite zone is at
least 1000 HV.
In an example of the invention the stainless steel is an austenitic steel.
This
material is relatively soft compared to e.g. martensitic stainless steel.
There-
fore, it is especially advantageous for this material that nitrogen and option-
ally carbon is dissolved at the high temperature process. Thereby, it is ob-
tained that the austenitic steel receives a sufficient core strength to compen-
sate for the loss of strength, which takes place when the cold deformation is
annihilated and that it is then possible to dissolve nitrogen and/or carbon at
low temperature without problems with precipitation, such as nitrides and/or
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carbides. In further examples of the invention the passive alloy is selected
from the group comprising stainless steel, austenitic stainless steel, marten-
sitic stainless steel, ferritic stainless steel, precipitation hardenable (PH)
stainless steel or ferritic-austenitic stainless steel; a ferritic-austenitic
stainless steel may also be referred to as a duplex stainless steel.
The content of nitrogen and optionally carbon, which is dissolved at the high
temperature process in stainless steel will typically be less than 1 % by
weight, but may, if desired, be higher. This may e.g. be obtained by applying
a higher nitrogen and optionally carbon activity, for example in the form of a
higher partial pressure of N2 in a gaseous process. The content of nitrogen
and/or carbon, which is obtained in stainless steel at the low temperature
dissolution may be as high as 14 % by weight and 6 % by weight, respec-
tively.
In a preferred example the above dissolution of nitrogen and/or carbon takes
place at the temperature Ti using gas, which contains nitrogen and optionally
carbon, but it may also be performed by ion implantation, plasma assistance
or by salt bath. In a preferred example a nitrogen containing gas, such as N2,
is used. The pressure of the gas may be up to several bar, but it may also be
below 1 bar, such as 0.1 bar. It is an advantage to employ gas, since all
types of geometries may be treated uniformly and there is a good tempera-
ture uniformity.
In an example of the invention dissolutions are performed at temperature
Ti and temperature T2 using gas. The gasses contain nitrogen and/or car-
bon, and the gas employed in the cooling step is an inert gas not containing
nitrogen. In certain examples dissolution at temperature T2 is performed in a
process selected from the group comprising a gas-based process, ion implan-
tation, salt bath or plasma.
In an example of the invention a diffusion depth of 50 pm to 5 mm is ob-
tained by dissolution of nitrogen and optionally carbon at temperature Ti.
This provides both a hard surface and a strengthening of the core of the ma-
terial. Thereby a full hardening of thin-walled components with a material
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thickness comparable with, or up to about twice the dissolution depth, may
be obtained since dissolution normally takes place from both sides of the
workpiece. For thicker components a relatively thick surface zone where ni-
trogen and optionally carbon is in solid solution is obtained. This provides
5 support for the expanded austenitic layer, which is formed in the surface
in
the subsequent low temperature process. For thin-walled workpieces a full
nitriding/carburising/nitrocarburising of the workpiece may thus be obtained.
Even if this is not fully obtained the dissolution will be a significant advan-
tage, especially for thin-walled workpieces, where strict requirements to the
10 corrosion resistance, and to the bearing capacity, are relevant, since
these
are improved significantly in the method of the invention.
In an example of the invention the temperature Ti is above 1000 C, such as
at least 1050 C, or it may be at least 1100 C, such as 1120 C or 1160 C, at
15 least 1200 C, or at least 1250 C. The upper limit for the temperature is
be-
low the melting point of the treated materials. For stainless steel the
melting
point is about 1600 C. In an example of the invention temperature Ti is be-
low 1600 C, such as below 1500 C, or below 1400 C, such as below 1350 C.
In an example of the invention temperature Ti is in the range of 1050 and
20 1300 C, such as at about 1150 C. It is important that the temperature
is
higher than the solubility temperature for the relevant carbides and/or ni-
trides, which may potentially be formed in the material, but however below
the melting point of the treated material. When gas is employed in dissolution
at temperature Ti the employed temperature may be chosen with considera-
tion to the gas mixture and the applied gas pressure.
In another example of the invention carbon is dissolved at temperature T2,
and temperature T2 is below 550 C, preferably the range of 300 - 530 C
during carburising.
In yet another example of the invention nitrogen is dissolved at temperature
T2, and temperature T2 is below 500 C, such as below 470 C, preferably
the range of 300 - 470 C during nitriding.
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In yet another example of the invention nitrogen and carbon are dissolved at
temperature T2, and temperature T2 is below 500 C, such as below 470 C,
preferably the range of between 300 - 470 C during nitrocarburising.
In an example of the invention the high temperature dissolution is carried out
at temperature Ti for at least 20 min, such as for at least 30 minutes, or for
at least 1 hour, or for at least 1.5 hours, or for at least 2 hours or for at
least
3 hours, or for at least 4 hours, or for at least 5 hours, or for at least
hours or for at least 15 hours. In principle there is no upper time limit,
10 since no nitrides or carbides are formed at temperature Ti. At extended
treatment the material may, depending on its thickness, be saturated with
nitrogen and optionally carbon, i.e. be fully nitrided or nitrocarburised.
In an example of the invention the method comprises cooling the material to
ambient temperature after the dissolution at temperature Ti. It is
particularly
preferred that the second dissolution step at temperature T2 is performed
immediately after the cooling step; this will avoid passivation of the work-
piece, i.e. formation of an oxide layer. In an example of the invention the
cooling takes place under high pressure, such as in the range of 6 and
10 bar, such as at 7 bar or at 8 bar, or at 9 bar. The cooling takes place in
an
inert gas not containing nitrogen, such as a noble gas, e.g. helium (He), neon
(Ne), argon (Ar), krypton (Kr), xenon (Xe), or radon (Rn), or any mixture of
these, with argon being particularly preferred. In another example cooling
takes place in argon at high pressure, e.g. in the range of 4 and 20 bar, such
as in the range of 6 and 10 bar, such as at 7 bar or at 8 bar, or at 9 bar.
The invention further relates to a lock washer (see Fig. 2a and 2b) of
stainless steel for securing bolts and nuts, which is dissolution hardened us-
ing the method of the invention. The lock washer is relatively thin-walled, so
that by hardening the lock washer using the method of the invention a sig-
nificant and necessary improvement of both strength and corrosion resistance
of the lock washer is obtained. In an embodiment of the invention the lock
washer has a first side with radial teeth and an opposite other side, the cam-
side, with cams. The lock washers are used in pairs mounted with the cams
against each other to obtain a key lock effect. They are especially suitable
to
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effectively lock bolt assemblies which are exposed to extreme vibrations or
dynamic loads and to corrosive environment, such as salt water. There are
therefore strong requirements to the strength and corrosion resistance of
these washers.
The invention is especially suitable for stainless steels and comparable
alloys,
in which expanded austenite or martensite can be obtained at a low tempera-
ture dissolution process. The invention is, however, generic in nature: a high
temperature dissolution process with nitrogen and optionally carbon in pas-
sive alloys, such as iron-based alloys, cobalt-based alloys, nickel-based
alloys
or chromium-based alloys, which provides strength and an improved low
temperature dissolution process with respect to corrosion, processing rate
and strength.
The following examples and prior art examples with accompanying figures
explain the invention in further detail.
Prior Art Example 1
Hardening of key lock washers of cold deformed austenitic stainless steel,
AISI 316, by two methods of the prior art.
Two identical key lock washers of cold deformed austenitic stainless steel
AISI 316L were hardened. Fig. 2 shows a key lock washer set 1 of said key
lock washers 2 and illustrates the use of these. Each washer 2 has a first
side
3 with radial teeth 4 and an opposite other camside 5 with cams 6. During
use of the key lock washer set 1 the washers 2 are placed as shown with the
camsides 5 facing each other. The two key lock washers were solution hard-
ened with nitrogen and carbon at a temperature of 440 C. One washer was
hardened by a method disclosed in PA 2011 70208, i.e. in a high temperature
process and subsequently in a low temperature process, and the other
washer was directly surface hardened with the same low temperature proc-
ess, i.e. of the prior art. The washers were analysed with optical microscopy.
Fig. 3 and Fig. 4 in the left panel show the washer, which was only surface
hardened with a nitrocarburising process conducted using a gas containing
nitrogen and carbon at a temperature of 440 C for 16 hours at atmospheric
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pressure. The outer surface in the nitrogen containing zone appears partly
sensitised (chromium nitride precipitations). The deformed substrate appears
strongly deformed and becomes clearly influenced by the employed etching
liquid to development of the micro structure. Fig. 4 shows an enlarged ver-
sion of Fig. 3.
Fig. 3 and Fig. 4 in the right panel show the washer treated by the method
disclosed in PA 2011 70208. The washer was exposed to a nitrogen contain-
ing atmosphere (N2 gas) at a temperature above 1050 C and was subse-
quently quickly cooled in the same gas. Thereby the material was austeni-
tised completely and the material was fully saturated with nitrogen. Then the
washer was surface hardened with a nitrocarburising process conducted using
a gas containing nitrogen and carbon at a temperature of 440 C for 16 hours
at atmospheric pressure, whereby expanded austenite was formed in the sur-
face in a zone with a thickness of at least 5 pm. The nitrocarburised nitrogen-
containing zone was not sensitised and the substrate was clearly without cold
deformation. The substrate hardness (260-300 HVO.5) and the surface hard-
ness (1200-1400 HVO.005) in the two washers are however practically identi-
cal. The corrosion resistance (exposure time in salt spray chamber (ISO
9227)) of the washer, where the method disclosed in PA 2011 70208 was
employed, is many times better than for the washer which was only surface
hardened (time in the chamber until corrosion was observed). The washer
which was treated with the method disclosed in PA 2011 70208 did not ex-
hibit corrosion after 400 hours whereas the washer which was directly low
temperature hardened did exhibit clearly visible corrosion already after
20 hours. A further improvement in the corrosion resistance can be obtained
while retaining the other advantageous characteristics by exposing the
washer to a nitrogen containing atmosphere (N2 gas) at a temperature above
1050 C and subsequently quickly cooling in an inert atmosphere not contain-
ing nitrogen, e.g. argon, instead of cooling in the nitrogen containing atmos-
phere.
Prior Art Example 2
Hardening of cold deformed austenitic stainless steel, AISI 316, by a method
of the prior art and a method disclosed in PA 2011 70208.
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Two identical components (back ferrules) of cold deformed austenitic
stainless steel AISI 316 were solution hardened with nitrogen and carbon at a
temperature of 440 C. One component was hardened by a method disclosed
in PA 2011 70208, i.e. in a high temperature process and subsequently in a
low temperature process and the other component was directly surface hard-
ened with the same low temperature process. Fig. 5 in the left panel shows
the microstructure analysed with optical microscopy of a component, which
was only surface hardened with a nitrocarburising process conducted using a
gas containing nitrogen and carbon at a temperature 440 C for 12 hours. The
outer surface in the nitrogen containing zone appears partly sensitised with
clear precipitations of CrN in the outermost surface. Fig. 5 in the right
panel
shows a component treated with the method disclosed in PA 2011 70208. The
component was exposed to a nitrogen containing atmosphere (N2 gas) at a
temperature above 1050 C and was subsequently quickly cooled in the same
gas. Then the component surface was hardened with a nitrocarburising proc-
ess in a low temperature process conducted using a gas containing nitrogen
and carbon at a temperature of 440 C for 12 hours. The nitrocarburised ni-
trogen containing zone was not sensitised. The substrate hardness (260-
300 HV0.5) and the surface hardness (1200-1400 HVO.005) in the two com-
ponents are, however, practically identical. The total layer thickness of the
expanded austenite zone is in both cases approximately 20 pm. The outer-
most layer is nitrogen expanded austenite, and the innermost layer is carbon
expanded austenite. The corrosion resistance for both components was tested
in a 14 % by weight sodium hypochlorite solution. The component which was
treated with the method disclosed in PA 2011 70208 did not exhibit corrosion
after 24 hours, whereas the component, which was directly low-temperature
hardened exhibited clear corrosion after only 10 minutes. The component
where the method disclosed in PA 2011 70208 was employed thus differs in
having a significantly better corrosion resistance than the workpiece, which
was directly nitrocarburised. A further improvement in the corrosion resis-
tance can be obtained while retaining the other advantageous characteristics
by exposing the ferrule to a nitrogen containing atmosphere (N2 gas) at a
temperature above 1050 C and subsequently quickly cooling in an inert at-
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mosphere not containing nitrogen, e.g. argon, instead of cooling in the nitro-
gen containing atmosphere.
Prior Art Example 3
5 Hardening of cold deformed Austenitic Stainless steel AISI 304 plate by a
method of the prior art and a method disclosed in PA 2011 70208.
Two identical components of cold rolled (deformed) austenitic stainless steel
plate, AISI 304, were solution hardened with nitrogen and carbon at a tem-
10 perature of 440 C. One component was hardened by a method disclosed in
PA 2011 70208, i.e. in a high temperature process and subsequently in a low
temperature process and the other component was directly surface hardened
with the same low temperature process. Fig. 6 in the left panel shows a com-
ponent, which was only surface hardened with a nitrocarburising process
15 conducted using a gas containing nitrogen and carbon at a temperature of
440 C for 20 hours and subsequently corrosion tested by exposure to 14 %
by weight sodium hypochlorite solution for 70 minutes. Fig. 6 in the right
panel shows the component hardened with the method disclosed in
PA 2011 70208. The component was exposed to a nitrogen containing at-
20 mosphere (N2 gas) at a temperature of 1150 C for 30 minutes and was sub-
sequently cooled quickly in the same gas. Then the component was surface
hardened with a nitrocarburising process conducted using a gas containing
nitrogen and carbon at a temperature of 440 C for 20 hours. Finally the com-
ponent was exposed to corrosion test by exposure to 14 % by weight sodium
25 hypochlorite solution. The surface appears unaffected by the corrosion
test
even after 16 hours of exposure. In the component which was directly low
temperature hardened clear corrosion attacks are seen after short term expo-
sure/corrosion test (70 minutes). The component where the method disclosed
in PA 2011 70208 was employed thus differs in having a much better corro-
sion resistance. A further improvement in the corrosion resistance can be ob-
tained while retaining the other advantageous characteristics by exposing the
component to a nitrogen containing atmosphere (N2 gas) at a temperature
above 1050 C and subsequently quickly cooling in an inert atmosphere not
containing nitrogen, e.g. argon, instead of cooling in the nitrogen containing
atmosphere.
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Example 1
Hardness profiles of cold deformed Stainless steel treated by a method of the
prior art and a method of the invention.
Two identical components of cold deformed austenitic stainless steel were
treated in a method of the prior art and according to the method of the in-
vention. The samples were exposed to a nitrogen containing atmosphere
(N2 gas) or to an atmosphere of hydrogen (H2) at a temperature above
1050 C and were subsequently cooled quickly in the argon (for the N2-
treated sample) or H2 gas. The component surfaces were then hardened by
nitrocarburising in a low temperature process conducted using a gas contain-
ing nitrogen and carbon at a temperature of 440 C for 12 hours. The nitro-
carburised zones were not sensitised. The hardness profiles of the samples
were analysed and the results are shown in Fig. 7. It is evident from Fig. 7
that the sample treated at high temperature in the nitrogen containing at-
mosphere ("EXPANITE ON HTSN") retained the core strength of the material
whereas the core strength was annihilated in the high temperature annealing
in hydrogen ("EXPANITE ON ANNEALED").
Example 2
Argon cooling following high-temperature solution hardening with nitrogen.
Lock washers of cold deformed austenitic stainless steel, AISI 316L, as de-
scribed in Prior Art Example 1 and illustrated in Fig. 2 were exposed to a ni-
trogen containing atmosphere (N2 gas) at a temperature above 1050 C be-
fore quickly cooling to ambient temperature in either the same atmosphere or
an atmosphere of argon. The samples were not subjected to further surface
hardening. The corrosion resistance of the components was tested in a 14 %
by weight sodium hypochlorite solution. Fig. 8 shows three exemplary lock
washers cooled in argon (left side) and three lock washers cooled in nitrogen
(right side). The argon cooled lock washers had far superior corrosion resis-
tance than lock washers cooled in nitrogen, which showed clear signs of cor-
rosion.
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Example 3
Hardening of cold deformed austenitic stainless steel, AISI 316, component
by a method of the prior art and a method of the invention.
The corrosion resistance of cold deformed austenitic stainless steel AISI 316
treated according to the invention was compared with a similar component
treated with a process of the prior art. The corrosion testing was performed
by submerging the two surface hardened components into 14% by weight
sodium hypochlorite solution for 18 hours.
Fig. 9 in the left panel shows the component treated according to the inven-
tion, i.e. in a high temperature process and subsequently, after cooling in
argon, in a low temperature process and the other component in the right
panel was directly surface hardened solely with a low temperature process.
The surface of the component treated according to the invention appears un-
affected by the corrosion test even after 18 hours of exposure. In the compo-
nent which was treated according to the prior art, corrosion attacks were ob-
served after short term exposure (7 minutes). The component where the
method of the invention was employed thus differs in having a much better
corrosion resistance.
