Note: Descriptions are shown in the official language in which they were submitted.
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OXIDATION RESISTANT ALLOY
The present invention relates to alloys used to prepare steel pipes i.e. tubes
for use in
chemical engineering applications. In particular, the invention relates to low
carbon
aluminium steel alloys and pipes made from such alloys. They may be used in
plant
such as ethylene cracker furnaces that need to be able to withstand elevated
temperatures for extended periods of time.
Frequently alloy materials for use at high temperatures, for example, alloy
tubes used
in ethylene pyrolysis cracker furnaces and to some extent steam methane
reforming,
suffer from insufficient oxidation and carburisation resistance. The industry
continues
to look for improved materials and other technologies to enable more efficient
ethylene
production under increasingly severe pyrolysis/cracking conditions (higher
temperatures, shorter residence times, and lower partial pressures of
product), leading
to increased ethylene yields. Current alloys have specific issues related to
their
corrosion resistance which causes failure at increasingly high design process
temperatures. This is the case currently for both castable alloy tubes and
wrought alloy
tubes.
It is necessary to heat the tubes to very high temperatures (above 900 C) to
allow the
steam cracking to take place continuously. This places stringent design
requirements
on the reactor and on the pipes used in its construction. A particular problem
concerns
oxidation and/or corrosion of the pipes as well as problems caused by coking
of the
pipes during use.
Pipes for steam cracker furnaces can be prepared by a centrifugal casting
process.
Centrifugal casting is a well-established process that is used to cast thin-
walled
cylinders, pipes and other axially symmetric objects. One benefit of this
process is that
it allows precise control of the metallurgy and crystal structure of the alloy
product. It is
generally used for casting iron, steel, stainless steels and alloys of
aluminium, copper
and nickel. The centrifugal casting process employs a permanent mould which is
rotated about its axis at high speeds of typically 300 to 3000 rpm as the
molten metal is
poured. The molten metal is centrifugally thrown towards the inside mould wall
where
it is able to solidify after cooling. The resulting cast cylinder i.e. tube,
has a fine grain
and the surface roughness of the outer surface of the cylinder is relatively
low.
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JP64-031931 describes the production of a curved tube made of heat-resistant
alloy.
The tube is prepared by centrifugal casting and the alloy of JP64-031931 is
made from
high strength and heat-resistant cast steel containing 15 to 30% chromium, 20
to 40%
nickel as well as the optional inclusion of smaller quantities of manganese
and
molybdenum. Small quantities of niobium and titanium are also added to the
alloy.
The cast tube is then subjected to the further step of an aging treatment at a
temperature of from 700 to 1100 C to deposit secondary carbide within the
grain
structure. This patent does not attempt to control the primary carbide
formation or to
control the relative amounts of niobium and titanium, or carbon and nitrogen.
Subsequently it is subjected to another processing step involving high
frequency
bending or die-bending at a temperature in the range of 550 to 1100 C.
W02012/121389 discloses an alloy intended for use in nuclear applications such
as in
heat exchanges in pressurised water reactors. The material is said to have
excellent
thin workability and corrosion resistance. This material is based on a nickel-
chromium-
iron alloy and contains small amounts of manganese, titanium, and optionally
aluminium as alloying elements.
W02000/22068 discloses anti-coking coatings for refractory alloys used in the
oil
industry. The coatings are obtained by a method which involves submitting the
refractory alloys surface to the action of oxygen and/or nitrogen gas plasma
at low
frequency and depositing on the treated surface coating based on silicon
oxide, nitride
or Oxy nitride by plasma enhanced chemical vapour deposition at low frequency.
US 2002192494 discloses a method for protecting low-carbon steel and stainless
steel
from coking and corrosion at elevated temperatures in corrosive environments
such as
during ethylene production by pyrolysis of hydrocarbons. The coating is an
alloy of
chromium, aluminium and silicon together with three other components one of
which is
selected from nickel, cobalt, iron or a mixture of these elements, the second
of which is
selected from yttrium, hafnium, zirconium, lanthanum, scandium or a
combination of
those elements, and the third is selected from tantalum, titanium, platinum,
palladium,
rhenium, molybdenum, tungsten, niobium, boron or a combination of those
elements.
The blended powder composition of this alloys applied to the surface and heat
treated
to bond the coating to the surface.
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US 2001001399 discloses austenitic nickel-chromium steel alloys based on steel
which
are particularly suitable for use as heat resistance and high strength
materials for parts,
such as pipes, in petrochemical cracking furnaces which are intended for the
production of ethylene or synthesis gas. The alloy is a low carbon steel
containing from
0.3 to 1.0% carbon
In addition to all of the usual technical issues associated with preparing a
steel pipe for
use in chemical plant, there are two particular problems which need to be
addressed
when fabricating pipes for this type of application. These issues arise
because of the
harsh working environment that the steel tubes will be exposed to and the fact
that any
'downtime' in plant operation is very costly in terms of lost production. The
pipes need
to be resistant to oxidation and/or corrosion during extended periods of use.
The pipes
also need to be capable of being in service for extended periods of time
without coking
or fouling. De-coking of reactor tubes is traditionally performed by passing
air and
steam through the pipes to burn the coke. However, this is at least a one or
two day
operation and therefore results in significant downtime and lost production
with the
attendant costs of this downtime.
The present invention aims to provide pipes which are resistant to oxidation.
In addition
the invention aims for the surfaces of these pipes to be resistant to coke
formation with
the intention of reducing the frequency of de-coking operations. It is an aim
of the
invention that when the alloy is made into pipes the surface of these have a
superior
resistance to coke formation compared with conventional pipes. Similarly, it
is an aim of
the invention that once formed into pipes and employed in a chemical reactor
or similar
environment that the surface of the pipes are stable and resistant to chemical
attack or
oxidation or corrosion etc. In addition, it is an aim of the invention that
the pipes should
also to have the appropriate mechanical properties in terms of creep
resistance and
ductility etc. as existing pipes so that they are suitable for use in high-
temperature
chemical reactors.
It is also an aim of the present invention to prepare a pipe which can be
produced in a
process which is convenient to run, so that the manufacturing process is
relatively
straight forward. It is also an aim to provide a process which is applicable
to the large
scale production of steel alloy pipes. The invention aims to provide a more
economic
production method and/or which is also more economic when the whole of life
use and
maintenance interruptions are considered. It is also an aim of the present
invention to
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provide a steel alloy which is economical to manufacture and which avoids or
reduces
the need for expensive alloying components.
It is also an aim to have pipes which can be prepared without the need for
further
subsequent processing steps.
The invention satisfies some or all of the above aims.
According to a first aspect of the present invention, there is provided an
alloy
comprising:
from 0.15 to 0.35 wt% carbon,
from 2.5 to 5.0 wt% aluminium,
from 40 to 45 wt% nickel,
from 25 to 35 wt% chromium,
from 0.50 to 1.50 wt% niobium and / or vanadium,
from 0.01 wt% to 0.25 wt% yttrium,
from 0.01 wt% to 0.25 wt% tungsten and / or tantalum,
from 0.01 wt% to 0.25 wt% in total of one or more of titanium and / or
zirconium and /
or hafnium,
up to 0.9 wt% manganese,
up to 0.9 wt% silicon, and
up to 0.10 wt% nitrogen,
with the balance of the composition being iron and incidental impurities.
In an embodiment, the alloy comprises:
from 0.15 to 0.35 wt% carbon,
from 2.5 to 5.0 wt% aluminium,
from 40 to 45 wt% nickel,
from 25 to 35 wt% chromium,
.. from 0.50 to 1.50 wt% niobium and / or vanadium,
from 0.01 wt% to 0.05 wt% yttrium,
from 0.05 wt% to 0.25 wt% tungsten and / or tantalum,
from 0.04 wt% to 0.15 wt% in total of one or more of titanium and / or
zirconium and /
or hafnium,
an amount of up to 0.9 wt% manganese,
an amount of up to 0.6 wt% silicon, and
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an amount of up to 0.10 wt% nitrogen,
with the balance of the composition being iron and incidental impurities.
The alloys of the invention may be used to fabricate steel pipes, particularly
steel pipes
5 which are intended to be used in chemical engineering applications such
as chemical
reactors and ethylene crackers.
It is the careful control of the metallurgical composition which gives
contributes to the
improved anti-coking properties of these alloys. Each of the elemental
components
described in the above steel compositions plays an important role in the
properties of
the steel and its resistance to coking and oxidation. VVithout wishing to be
bound by
theory, we believe that the particular combination of elements gives rise to
the superior
properties of the alloys of the invention because of the formation of a thin
but
continuous layer of alumina which is able to form on the surface of the alloy
at an early
stage in use. This layer is stable and contributes to a reduced level of coke
formation
because alumina is effectively non-catalytic and does not promote the
formation of
carbon deposits on the surface. On the contrary, the alumina layer which is
formed in
use generally provides an environment which is unfavourable to the deposition
of
carbon on the surface of the pipes.
In addition, because of the particular chemical composition of the alloys of
the invention
it is possible for this protective oxide layer to form under what would be
considered to
be very reducing conditions such as in situ in ethylene cracker. Indeed, the
alloys of the
present invention are able to form a stable protective surface layer even at
partial
pressures of oxygen as low as 10-27 atmospheres. This means that pipes made
from
alloys of the invention can be put into use directly without the need for
further treatment
and without any additional coatings being required. Hence a further benefit of
the steel
pipes of the invention (i.e. pipes made using steel alloys of the invention)
is that they
require no subsequent treating after production in order to be used in
chemical
engineering applications. This is a significant advantage both in terms of
time and cost
relative to pipes made from existing alloys.
The alloys of the invention also benefit from the fact that they contain
little or no nickel,
chromium or iron at the interface of alloy surface / cracking gas. These
elements
ordinarily behave as catalytic elements when in contact with the cracking gas
and
would promote coke formation and deposition under normal conditions of use.
The
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ability of the alloys of the present invention to be able during service to
avoid any
concentration of or the full presence of these elements on its surface and to
form an
alumina oxide layer containing little or none of these elements represents a
substantial
advantage relative to prior art alloys. The retention of these particular
elements, which
would otherwise be problematic in terms of carbon deposition, within the bulk
matrix of
the alloy is therefore an important feature of the alloys of the present
invention and
arises as a result of the carefully controlled metallurgy. It can therefore be
seen that the
combination of the thin surface layer of alumina and the absence of elemental
components such as nickel, chromium and iron at the surface together lead to
substantial improvements in resistance to coking, coke formation and
deposition.
Surprisingly, the relative proportions of the elements required to secure the
anti-coking
effect do not adversely affect the mechanical properties of the alloys. This
is important
because good mechanical properties are required of components in chemical
engineering applications. The inventors of surprisingly found that the
combination of
elements used in the alloys of the present invention also contributes to the
high-
temperature strength of the steel tube.
The individual elemental components in the alloy perform the roles discussed
below.
CARBON
Carbon is required in an amount of 0.15 to 0.35 wt.% in the alloys of the
invention. The
amount is carefully controlled because carbon has several different functions.
For
example, carbon is usually an important component of steel for providing
tensile
strength and resistance to creep rupture. This is because carbon is an
essential
component in the formation of carbides which normally provide steel with its
strength
due to the precipitation of the primary and secondary carbides. At the same
time, the
interplay between the amount of aluminium which is added and the amount of
carbon
must be closely controlled. Aluminium is generally incompatible with carbon as
aluminium rejects carbon in the sense of decreasing its solubility in the
austenitic
matrix. This has the effect of the carbon migrating to the grain boundaries
during
solidification with the consequence that the alloy precipitates an excessive
quantity of
primary chromium carbide, forming a thick network that makes it become
brittle.
Accordingly, it is necessary to have sufficient carbon in the alloy to ensure
sufficient
strength in the resulting alloy and this is the reason for the requirement to
have 0.15
wt% or more carbon in the alloy. At the same time it is important to ensure
that the
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upper limit of the amount of carbon is not too high so that it adversely
interacts with the
added aluminium. For this reason, the upper limit of carbon is 0.35 wt%. In a
preferred
embodiment, the amount of carbon is from 0.20 to 0.35%, and in a more
preferred
embodiment the amount of carbon is from 0.25 to 0.30 wt%.
ALUMINIUM
Aluminium is another very important component of the alloys of the invention.
Aluminium is present in an amount of from 2.5 to 5.0 wt%. Aluminium alloys are
generally difficult to cast and therefore can be hard and difficult work. The
steels of the
present invention are relatively easy to work despite the presence of
aluminium.
Aluminium is necessary for the formation of the alumina protective layer at
the surface.
At the same time aluminium and carbon compete for solubility and this can be a
problem. Furthermore, as mentioned above, aluminium also decreases the
solubility of
carbon but reciprocally the carbon decreases the solubility of the aluminium
in the
matrix leading to brittleness. Not only because of the precipitation of thick
chromium
carbide in the grain boundary during solidification, but also because of the
possibility of
precipitating in addition thick nickel-aluminide intermetallic. Indeed, the
aluminium
been rejected like the carbon during solidification too by the austenitic
matrix. This
involves the undesirable nickel-aluminide precipitation, known for its
embrittlement
properties, during solidification.
It is therefore important to ensure that the upper limit of aluminium does not
exceed 5
wt% so that the alloy does not become brittle forming uncontrolled excess
nickel
aluminide intermetallic at the inter-dentric space and grain boundaries. At
the same
time, a minimum amount of aluminium of 2.5wtc/o is required in order to ensure
that a
substantially continuous coating of alumina on the surface of the alloy can be
formed in
use.
The aluminium is therefore added to the alloy to allow the formation of a thin
protective
continuous dense homogenous and adherent alumina (aluminium oxide) layer on
the
surface of the alloy during service. This aluminium oxide layer is oxidation
resistant
(slow growth of the oxide layer during service prevents further reaction or
oxidation of
the alloy). The aluminium oxide is a surface stable barrier stopping the
diffusion of
carbon in the alloy. It serves to facilitate anti-coking (no catalytic
reaction involving
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formation of graphite/coke during cracking), protecting the alloy, and allows
its surface
to be case neutral when exposed to the gas to be cracked.
The higher the aluminium content, the higher the reservoir/stock of aluminium
which is
important in case the aluminium oxide layer needs to be re-formed. Conditions
under
which this might be an issue include alumimum oxide spallation due to over-
decoking,
or variation in the service conditions, or an uncontrolled event in the
cracking furnace.
The alumina also provides carburisation resistance as described above and
hence it is
important to ensure that there is sufficient aluminium in the alloy to provide
an effective
layer. This is the reason for the lower limit of aluminium which is specified.
The
minimum of 2.5wt wt% is important to ensure the formation of the layer at an
appropriate rate during service.
In the case of the alloys of the present invention, the aluminium content can
exceed the
usual levels that are normally included in aluminium steels specifically
because of the
deliberately low level of carbon that is added to the alloys of the invention.
It is
therefore possible for the amount of aluminium of reach levels much greater
than
3.5wt.c/o because of the relatively low content of carbon with amounts of
aluminium up
to 5.0wtc/o being achievable without damaging the properties of the alloy.
Above this
level there is a risk of the alloy becoming brittle such that for example it
can break
during manipulation or during following steps of manufacture like pipe
straightening
after casting.
The aluminium content is preferably in the range from 3.0 wt% to 5.0 wt%, and
more
preferably is in the range from 3.2 wt% to 4.6 wt%, such as in the range 3.5
wt% to 4.5
wt%.
It is important to note that the maximum and minimum aluminium content is
considered
relative to the carbon content. This is also relevant to the increase of
thermal
expansion of the alloy at high temperature. The relative proportions of carbon
and
aluminium, calculated in terms of their wt% in the alloy, ranges from about 1
: 33.3 to 1
: 7.1. Preferably, the relative amounts lie in the range of from about 1 : 17
to 1 : 22.
CHROMIUM
Chromium is present in an amount of from 25 wt% to 35 wt%. The chromium forms
a
primary carbide network during solidification (as described in the case of
carbon) which
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give primary strength to the alloy, and also forms secondary carbides during
service
with good creep resistance properties. In the situation in which the aluminium
has been
fully depleted after long service of the alloy, or following dramatic decoking
of the
furnace, the chromium will be able to form an oxide layer which will be able
to delay the
carburisation of the alloy. The chromium content of the matrix is able to
delay
carbonisation/carburisation of the alloy by trapping the carbon when it is in
the process
of diffusing thereby forming chromium carbides.
Chromium is not anti-coking but provides a well-documented and effective
corrosion
resistance and oxidation resistance effect in addition to acting as a carbide-
former.
Carbide formation ensures creep strengthening precipitations in the alloy. The
lower
limit of 25 wt% of chromium is required in order to ensure sufficient
oxidation resistance
and the upper limit of 35 wt% is determined by the fact that above this level
it is difficult
to obtain a stable austenite phase. In addition, too high a level of chromium
renders the
steel unworkable. In some embodiments, the chromium is present in the range of
from
26 wt% to 31 wt%, more particularly in the range of from 28 wt% to 31 wt%,
such as in
the range 28wtc/o to 30 wt%. In certain cases, the amount of chromium is in
the range
from 29 wt% to 30 wt%. In some alternative embodiments, the chromium is
present in
the range of from 30 wt% to 33 wt%, and more preferably in the range of from
31 wt%
to 32 wtc/o.
The chromium content is determined in balance with the nickel content for the
alloy so
that the ultimate alloy possesses a stable austenitic base matrix at elevated
temperature. It is important that there is a stable austenitic matrix at every
expected
service temperature to which the alloy is likely to be exposed. It is
therefore important
to consider the amount of chromium to be used in the context of the amount of
nickel
that is also present in the alloy. The relatively high quantity of chromium is
intended to
provide its availability to form chromium oxide on the surface and to allow
chromium to
trap carbon in the form of chromium carbide in case of carburisation not
contained by
the thin alumina oxide.
NICKEL
Nickel is present in an amount of from 40 wt% to 45 wt%. The nickel provides
the
stable austenitic matrix base of the alloy. Nickel is an element which is
essential in
order to obtain a stable austenite structure and improves the stability of
austenite and
supresses the generation of the sigma phase. Nickel is the austenitic
stabiliser
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element, allowing the alloy to be generally strong at above 8000. Therefore it
forms a
stable matrix with the iron which allows the possible precipitation of the
carbides/nitrides. The lower limit of the nickel content is chosen simply for
the reason
that this is a sufficient amount for improving the stability of austenite with
respect to the
5 lower limits of the other elements.
Nickel is also very important for cracking furnace applications as it give
better
carburisation resistance to the alloy, because of its slow carbon diffusion
kinetics. In
this respect, the kinetics of nickel in terms of carburisation are much slower
than in the
10 case of iron. On the other hand, nickel can in some circumstances be
deleterious
because it promotes the formation of catalytic coke when present in the
surface of the
alloy, and more precisely at the interface alloy / cracking gas. Thus, the
upper limit of
nickel is required in order to contribute to the carburisation resistance of
the alloy but at
the same time should not be exceeded because of its known property to promote
catalytic coking if it appears on the surface of the alloy. It is therefore
important to
ensure that not too much nickel is present in the alloy. In addition, nickel
is an
expensive material and consequently avoiding to hire nickel content also has
an
economic advantage. The lower limit of nickel is governed by the need to
provide an
adequate austenitic matrix. In some embodiments, the nickel is present in an
amount of
from 42 wt% to 45 wt%, and more preferably in an amount of from 42 wt% to 44
wt%.
In some particularly preferred embodiments, the nickel is present in an amount
of from
42.5 wt% to 43.5 wt%.
In some alternative embodiments of the invention, it is possible to replace
part of the
nickel content by cobalt. Cobalt is able to perform the same function as
nickel in the
alloys of the invention. In such embodiments, the total amount of cobalt and
nickel is
from 40 wt% to 45 wt%. Up to 75 wt% of the nickel can be replaced by cobalt,
and in
principle any amount from 1 wt% to 75 wt% of the nickel can be replaced by
cobalt;
however, in practice it is more usual for between 5 wt% and 15 wt% of the
nickel to be
replaced by cobalt on account of the higher cost of cobalt. The quantity of
nickel (and
cobalt, when present) that is included in the alloy of the present invention
is carefully
controlled relative to the amount of chromium which is present in order to
ensure that
the alloy is indeed a stable austenitic matrix at whatever service temperature
is
employed.
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NIOBIUM
Niobium is present in an amount of from 0.50 to 1.50 wt%. The function of the
niobium
is to form thin primary carbides during solidification, these primary carbides
have the
particularity in this specific alloy to re-dissolve at high temperature
(therefore during
service) and as a consequence to facilitate the precipitation of secondary
carbides and
ensure their dispersion. This contributes to the creep resistance of the
alloy.
The primary niobium carbides therefore act a reservoir of carbon, which can
then be
released at the service temperature to create more creep efficient carbides.
This role is
essential, as the Carbon is not in the usual excess that would be typical in a
high-
temperature alloy having a carbon content above 0.40wt.c/o. In the alloys of
the present
invention, the release of carbon by the niobium compensates for this and
therefore
provides carbon when it is required. In addition, during solidification the
niobium
carbides mainly forms at the interdendritic space or grain boundary thereby
participating predominantly in the primary carbides network. The amount of
niobium
employed is chosen on the grounds of its efficiency at disrupting the primary
chromium
carbide network. In some embodiments, the niobium is present in an amount of
from
0.80 wt% to 1.10 wt%, and preferably from 0.80 wt% to 0.90 wt% or from 0.90
wt% to
1.10 wt%. In other embodiments, the niobium is present in an amount of from
1.10
wt.% to 1.30 wt.%.
In some alternative embodiments of the invention, it is possible to substitute
the
niobium with vanadium. Vanadium is a carbide former from the same periodic
group
as niobium and can be employed in the same amounts as described above for
niobium. In addition, a mixture of the two can be employed subject to
satisfying the
ranges mentioned above for niobium for the mixture.
TITANIUM, ZIRCONIUM and HAFNIUM
Titanium and / or zirconium may be present independently of one another in a
total
amount of 0.01 wt% to 0.25 wt%, and preferably in an amount of from 0.04 wt%
to 0.15
wt%. In other words, titanium can be present within the range stated, with no
zirconium
being present; or zirconium can be present within the range stated, with no
titanium
being present; or both titanium and zirconium can be present and in this case
each
may be present in any amount within the range stated, subject to not exceeding
the
total limit. Both elements function in a similar manner by forming carbides.
It is
generally preferred to have titanium present. In some embodiments, the amount
of the
element or elements (usually but not necessarily only titanium) is from 0.08
wt% to 0.15
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wt%, and more preferably is from 0.08 wt% to 0.13 wt%. In some embodiments,
only
titanium is present in this amount and there is substantially no zirconium or
hafnium
present.
Titanium and zirconium are stable carbide-forming elements and are used either
singly
or together to improve the creep properties of the alloy. Either element
individually or
both elements together do this by precipitating secondary carbides during
service.
Furthermore, as carbide forming elements they not only form their respective
carbides
but also are able to form a titanium / zirconium-niobium double carbide
precipitates
which improves creep strength.
In addition, titanium and zirconium have a second function in that they are
very active
scavengers for oxygen and consequently they help to protect the aluminium
present in
the melt from oxidation. These elements are therefore oxidised in preference
to the
aluminium. Titanium / zirconium are added to melt as a deoxidiser.
The quantities of one or both elements that is / are used represents a balance
between
obtaining an improvement in the creep properties of the alloy (which decrease
due to
the addition of aluminium) and undesirable oxide, carbides, nitrides formation
when too
much is added. Indeed, the addition of too high an amount of titanium or
zirconium can
lead to the undesirable formation of oxide, carbide, nitride, hardening too
much the
alloy.
Cost is also an issue since both of these elements are expensive raw
materials.
In some alternative embodiments of the invention, it is possible to substitute
either or
both amounts of titanium and zirconium with hafnium.
Thus, in certain embodiments of the invention hafnium can be employed in
addition to
or in place of one or both of titanium and zirconium. In other words, in some
embodiments of the invention there may be no titanium or zirconium present in
the
alloy and hafnium is present instead of these elements. In other embodiments,
some or
all of the titanium and / or zirconium is replaced by hafnium, with the
consequence that
all three elements are present. In this circumstance, the minimum amount of
all three
elements together would be 0.04 wt% and the maximum amount of all three
elements
together would be 0.15 wt%. Hafnium is effective at increasing the creep
resistance
properties of the alloy but has the disadvantage of being an expensive raw
material.
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In certain embodiments of the invention, it is preferable that titanium is
used in the
alloys of the invention without zirconium or hafnium being present. In an
alternative
embodiment of the invention, both titanium and zirconium can be used without
hafnium
being present.
TUNGSTEN
Tungsten is present in an amount of from 0.01 wt% to 0.25 wt%, and is
preferably
present in an amount of from 0.05 wt% to 0.25 wt%. Tungsten is a large element
in
terms of its atomic size and it is also a carbide-forming element. Both of
these factors
contribute to an improvement of creep properties of the alloy when tungsten is
added.
The amount of tungsten that is added is a balance between improving the creep
properties, and limiting the high temperature resulting plastic deformation
elongation
involved by its presence, There are diminishing returns in this improvement,
on the one
hand and the expense of adding tungsten, on the other hand. In some
embodiments,
the amount of tungsten is from 0.05 wt% to 0.15 wt%, and more preferably is
from 0.05
wt% to 0.10 wt%. In certain embodiments, the amount of tungsten is preferably
from
0.01 wtcY0 to 0.05 wtcYo.
In certain alternative embodiments of the invention, the tungsten can be
replaced by
tantalum for the same purpose. In this circumstance, there would be no
tungsten
present in the alloy and the amount of tungsten would then be replaced by the
same
amount of tantalum. In these alternative embodiments, the amount of tantalum
would
therefore be from 0.01wtc/o to 0.25 wt%, with similar sub-ranges being
preferred for
tantalum as described above in relation to tungsten. However, it is preferable
that
tungsten is used instead of tantalum. In certain embodiments, both tungsten
and
tantalum could be used in amounts such that the total amount of both elements
is from
0.01 wt% to 0.25 wt%.
YTTRIUM
The yttrium is present in an amount of from 0.01 wt.% to 0.25 wt.%, and
preferably is
present in an amount of from 0.01 wt.% to 0.05 wt.%. A particularly preferred
amount
is from 0.03 wt% to 0.05 wt%. Yttrium performs several roles in the alloys of
the
invention. Firstly, it serves to protect the addition of the aluminium in the
melt, because
the melt is formed in air melt rather than under a special protective
atmosphere.
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Yttrium is more reactive with oxygen than aluminium so it acts as a scavenger
for
oxygen.
In addition, yttrium, when present in small quantities modifies the primary
carbide
network shape making the resulting carbides thinner and more discontinuous,
and
consequently less brittle. Yttrium also has the benefit of improving the
adherence of
the surface oxide layer, acting as 'sink' for cavities.
It is not necessary to have a very large quantity of yttrium (another very
expensive
element) in the alloy to achieve these beneficial effects and an amount within
the
stated range is sufficient for this purpose. In some embodiments, the amount
of yttrium
is from 0.01 wt% to 0.03 wt%.
SILICON
Silicon is present in an amount up to 0.9 wt%. In some embodiments, silicon is
present
in an amount up to 0.6 wt%. In certain embodiments, the amount of silicon is
from 0.6
wt% to 0.9 wt%. Silicon provides the function of a deoxidiser and is usually
an
essential component in an austenite stainless steel. Silicon may also
contribute to
increasing the stability of any surface oxide film. Silicon also provides some
fluidity to
the melt bath before the addition of the aluminium to the melt. On the other
hand, if the
content of silicon is too high the workability of the steel is reduced. A high
Si content
can also cause the formation of a detrimental phase known as the G phase which
is
composed of nickel, silicon and niobium (Ni16Nb6Si7). Consequently, the amount
of
silicon must be carefully controlled. In some embodiments, the silicon is
present in an
amount of from 0.3 wt% to 0.6 wt%, and more preferably from 0.5 wt% to 0.6
wt%. In
certain embodiments, the silicon may be absent or only a lower limit of
silicon of 0.1
wt% is needed.
MANGANESE
Manganese is present in an amount of up to 0.9 wt%. Manganese can improve the
workability of the alloy and it is also an effective de-oxidant and
contributes to austenite
formation in the steel. However, the high coefficient of diffusion of the
manganese at
high temperature means that it competes with the aluminium. Furthermore, the
addition of too much manganese can result in a reduction in high-temperature
strength
and also toughness over an extended period of time. Consequently, the amount
of
manganese must be limited to 0.9 wt%. In some embodiments, the manganese is
present in an amount from 0.4 wt% to 0.8 wt%, and more preferably from 0.4 wt%
to
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0.6 wt%. In certain embodiments, the manganese is present in an amount of from
0.7
wt% to 0.9 wt%. In certain embodiments, the manganese may be absent or only a
lower limit of manganese of 0.1 wt% is needed.
5 NITROGEN
Nitrogen is inevitably present in the alloys of the invention. It is present
in an amount of
up to 0.10wt.c/o. Nitrogen may be found in the alloy because the alloy mix is
prepared
under an air atmosphere so it is often the case that nitrogen can diffuse into
the liquid
alloy during its production. However, it is not always possible to determine
the amount
10 of nitrogen in the alloy because the amount is insignificant and / or
unmeasurable and
in these cases the effective amount of nitrogen is almost zero. The absence of
measurable nitrogen is not a detriment. Equally, in certain cases there is a
measurable
amount of nitrogen and in these cases the upper limit is 0.10 wt%.
15 Nitrogen forms austenite together with carbon and it contributes to high-
temperature
strength. Nitrogen allows the dilution, dispersion, and the homogenisation of
the
carbon. However, careful control of the amount of nitrogen is important
because it
slows the precipitation of primary chromium carbides when it is added in a
suitable
quantity. In effect, the nitrogen helps to control the 'behaviour' of the
carbon so to
control its several precipitations. The nitrogen participates in the
precipitation of
secondary niobium carbides, niobium carbido-nitrides, and niobium nitrides
during
ageing which are more stable than carbides at high temperature and therefore
have
better creep properties. However, if the quantity of nitrogen is too large
then an
excessive amount of nitrides are produced as chromium nitrides and/or
aluminium
nitrides which reduces the ductility and the toughness of the alloy over an
extended
period of time and prevent the rnitrided' elements to fulfil their role in the
alloy as they
should. It is therefore essential to keep the amount of nitrogen present to a
maximum
of 0.10 wt%. There is however no practical lower limit to the measurable
nitrogen in an
alloy sample of the invention.
Alloys according to the present invention are produced in a conventional
furnace and
without the need for a special atmosphere. The first stage of preparing the
alloy
involves working out the relative proportions by weight of the various
component
minerals (which are the source of the various elements required in the final
alloy) in
order to achieve the desired amounts of the various elements which are
required in the
final alloy. The solid minerals are added to the hot furnace. Heating is
continued in
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order to melt all of the mineral components together and ensure a thorough
mixing of
the minerals in the furnace so that the elements are properly distributed
within the
matrix.
In circumstances in which the addition of a particular essential element in
the alloy
composition of the invention also results in the addition of another essential
element
(because the other essential element is perhaps present as an impurity in the
addition
agent for the first essential element) then the overall composition must be
carefully
monitored to ensure that all of the essential components remain within the
desired
parameters. If necessary, this can be compensated for by adjusting the
relative
proportions of additional materials used for each of the essential components
in the
alloys of the invention. Sometimes elements are added in the form of preformed
alloys,
for example ferrotitanium might be used as a source of titanium and
consequently the
elemental calculations are adapted as necessary so that the required amount of
the
element (titanium in that particular example) is correctly added. The skilled
person will
be able to analyse and compensate as necessary for variations in the essential
elemental components due to the presence of incidental impurities using known
analytical techniques and by varying the amounts of the usual addition agents
for each
essential component.
A number of elements will be present in the alloy as inevitable impurities.
Such
incidental elements will not have any discernible technical benefit or adverse
effect on
the alloys of the present invention. In some cases, the presence of such
elements, as
the nitrogen can be tolerated in relatively large amounts provided that they
do not affect
the desired properties of the alloy. For example, although not specifically
envisaged in
the alloys of the present invention, it is conceivable that an element may
arise as an
incidental impurity as a consequence of its occurrence as an impurity in one
of the
deliberately added elemental components. This is acceptable provided that the
presence of such an element does not have any deleterious effects on the
alloy. In
certain cases, deliberately added elemental components such as titanium may
bring
with them other incidental elements. These can be generally tolerated as
incidental
impurities at low levels. Where analysis reveals that such impurities are
unacceptable,
an alternative source of the desired elemental component (free of damaging
impurities)
is used.
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The minerals and elements of the alloy composition are careful added in an
appropriate
sequence in order to obtain the desired content for the final composition. It
is also
important to protect the aluminium when added to the melt and avoid its
oxidation
because the melting is performed in air.
Once melting and mixing has been achieved, any slag is decanted from the
furnace in
order to remove impurities and clean the bath of liquid alloy in the furnace.
A sample of
the molten alloy is then removed from the furnace, allowed to cool and
analysed by x-
ray fluorescence in order to determine its elemental composition. An
adjustment to the
composition may or may not be required at this stage to accommodate for any
elemental mass loss due to volatility. The composition is adjusted by the
addition of
further minerals as necessary, and optionally re-analysed to ensure that the
desired
composition has been achieved.
After the desired composition has been achieved, the temperature is further
raised
above the melting temperature to a tapping temperature in order to ensure easy
pouring of the melt. At the same time, the mould is prepared for centrifugal
casting.
The mould is a conventional centrifugal casting mould and this type of mould
is well
known to the skilled person. The process of preparing the mould involves
washing the
mould with water/steam to clean it and to remove any old mould wash or coating
that
might have been used in a previous casting process. The washed mould is then
coated
with an insulating/release agent which is required to prevent the alloy from
sticking to
the mould after casting. A typical insulating/release agent is silica or
alumina.
A disc of ceramic is then added to the centrifugal casting mould in the manner
known in
the art in order to ensure that the mould is liquid tight and ready for
casting. This
prevents any alloy leakage during the casting process. The mould temperature
is
adjusted in preparation for the casting and may be in the range of 200 to 300
C. The
mould is then rotated at high speed to obtain usually the range of 80g to
120g, with a
rotation providing 100g being typical for a centrifugal casting speed.
A ladle is then brought to the furnace and a desired weight of molten alloy is
tapped off
for the purposes of casting including the relevant additions, such as titanium
and
yttrium, in order to reach the final desired quantity of aluminium and other
reactive
elements of the alloy composition. The ladle itself is preheated to a
temperature in the
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region of 800 to 1000 C in order to minimise cooling of the alloy after
pouring. Alloy is
then transferred to the hot ladle. At this stage, a further analysis of the
alloy may be
performed.
The molten alloy in the ladle is then transferred to a pouring cup. The nose
of the
pouring cup has previously been adjusted to ensure that it mates with and
properly fits
the size of the input tube for the centrifugal casting mould. The level of
molten alloy in
the pouring cup is maintained in order to maintain adequate flow of alloy into
the mould
which is in effect fed by gravity. This provides a continuous flow of alloy
into the mould
until all of the weight of the alloy has been poured into the mould. The mould
is rotated
at high speed i.e. maintained at the centrifugal casting speed during the
process and
whilst the alloy is molten. The length of time the casting process takes
depends
ultimately on the desired thickness of the tube required and the skilled
person is able to
determine a suitable rotation time for a particular thickness of tube and
weight of alloy.
The mould is gradually slowed down as the alloy cools from its solidification
point.
Generally speaking, a "fast" solidification process is one in which the alloy
is cast and
then cools at a rate of more than about 100 C per minute and a "slow"
solidification
process is one in which the alloy is cast and then cools at a rate of about 50
C or
greater per minute. The casting process is usually completed in less than
about 10
minutes. The tube is extracted after the mould stops and the process may be
repeated
again.
The steel tubes of the present invention show excellent high-temperature
strength and
high creep resistance. The tubes also display exceptional corrosion resistance
at
elevated temperatures over an extended period of time. Consequently, these
steels
are particularly suited to use in chemical plant under demanding environments
such as
steam cracking. In addition, it is expected that steel tubes according to the
invention
may be used in a variety of other applications such as steam reformers and in
nuclear
applications in heat exchanges and the like, such as those found in
pressurised water
reactors.
The alloys of the invention were tested to demonstrate effective formation of
alumina
(aluminium oxide) on the surface in the hostile environment of a pack-
carburisation test
at 1100C over a period of 200hr5 for an alloy containing 3.86 wt% aluminium.
The alloy
composition is shown in Table 1 below:
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Table 1
Ni Cr Nb Si Mn W Ti Y Al
wt% 0.25 43.3 29.6 0.81 0.56 0.54 0.05 0.11 0.01 3.86
The alloy was analysed by E.D.X (Energy Dispersive X-Ray). EDX cross section
surface mapping identifies the oxide layer elements in the composition across
its entire
thickness and enables comparison with the alloy composition immediately below.
Each
of the elements in the alloy composition possess a characteristic X-ray line
as Ka, and
in the EDX map a greater quantity of a particular element in an area is
indicated by
increased brightness in the area.
Some of the key technical benefits of the invention can be seen from the
following
Figures in which:
Figure 1 shows an alloy according to the invention which has been subjected to
EDX
cross section surface mapping at the relatively low magnitude of 500X.
It can be observed from Figure 1 that the aluminium and oxygen appear in the
alloy
sample according to the invention as one very bright and continuous line. This
indicates two things. Firstly, the aluminium and oxygen are present in a very
high
concentration, as alumina, at the exact same position where the surface oxide
layer is
observed by SEM photo (Scanning Electron Microscope). Secondly, there is a
continuous layer of alumina at the surface of the material.
This analysis demonstrates that there is a dense, adherent, thin, continuous
alumina
(aluminium oxide) layer at the surface of the alloy even after pack-
carburisation at
.. 11000 over a period of 200hr5. It can also be seen that the alumina layer
stays very
stable and therefore does not get transformed in aluminium carbide, and acts
as a
protective layer i.e. a barrier against carburisation
Figure 2 shows EDX surface mapping at the higher magnification of 2000X for an
alloy
.. according to the invention having the composition shown in Table 1. In
Figure 2, it can
be seen that the aluminium and oxygen appear as the brightest components, and
hence are present in very high concentration. In addition, it can be seen that
these
elements appear at the exact same position where the surface oxide layer is
observed
by SEM. A further important feature is the dark areas in the Figure which
indicate the
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absence of elements such as nickel, iron and chromium. Figure 2 also
illustrates the
dense, adherent, thin, continuous alumina surface layer and indicates that it
is pure,
and free of highly catalytic elements as nickel and iron. The continuous
surface layer
and the absence of catalytic elements contribute to the anti-coking effect of
the alloy
5 when exposed to an external source of carbon. It can be seen from Figure
2 that the
alumina layer is anti-coking, and does not bond or react catalytically with
carbon during
pack-carburisation at 11000 over a period of 200hr5.
Figure 3 shows is a High Magnification SEM Photo of the carburised surface of
the
10 alloy according to the invention having the composition of Table 1 (an
alloy containing
3.8 wt.% Aluminium). The photograph clearly illustrates a continuous
protective
alumina layer at the surface. The thickness of the alumina layer was
determined to be
4.28 1.21 pm on the basis of 100 measurements.
15 It can therefore be seen that the alloys of the invention demonstrate
superior properties
in terms of the carburisation resistance and also exhibit resistance to
oxidation on
account of the alumina layer which can be formed in situ on the alloys in use.
Alloys of the present invention can be tested using a thermal oxidation cycle
test. This
20 test involves cycling the alloy through high and low temperatures over
an extended
time period in order to expose the alloy to oxidative stress. This test is
frequently used
for studying the oxidation characteristics of materials such as the
superalloys /
superalloys coating employed in gas turbines which are intended for high
temperature
use and / or use under extreme condition.
The test machine consists of a furnace brought to the test temperature and a
sample
handling apparatus which is able to introduce and remove the sample from the
furnace
very quickly in order to facilitate rapid heating and rapid cooling. The
alloys of the
present invention were tested at 1150 C. The higher the temperature, the
faster
growth of the oxide layer, so a thick oxide layer is likely to be formed.
The sample handling apparatus is a mechanism for bringing the test sample
quickly in
and out the furnace. The higher the furnace temperature, more extreme the
thermal
shock on the sample as it is introduced into and removed from the furnace.
Repeated
exposure of the sample under these conditions leads to the potential
spallation of the
oxide layer. The adhesion and thickness of the resulting oxide layer on the
surface can
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then be investigated and used as a basis for establishing the performance of
the alloys
of the invention relative to those of the prior art.
The alloys of the invention were tested by repeating the heat-shock exposure
through
50 cycles. Figure 4 of the drawings illustrates the temperature profile in the
thermal
oxidation test. The samples were pre-oxidised at 875 C over a period of 48
hours.
Subsequently, the samples were exposed to 50 cycles in and out of the furnace
and
the temperature profile is shown in Figure 4. Each cycle consists of heating
the sample
under test from room temperature by introducing it into the furnace at 1150
C, keeping
sample in the furnace for 45 minutes at that temperature in an atmosphere of
air, and
then removing the sample from the furnace and cooling it to room temperature
over a
period of 15 minutes. In the graph of Figure 4, the vertical axis represents
both the
temperature of the sample in degrees centigrade and also the air pressure in
millibars.
It can be seen from the graph that the air pressure in millibars is just
slightly less than
1000 millibars (approximately 975 mbar). It can also be seen from the graph
that the
maximum temperature achieved in the heating cycles is 1150 C and that the
minimum
temperature, represented by the inverse peaks in the graph, is effectively
room
temperature. The horizontal axis in the graph indicates the time in hours
throughout the
test.
The repetitive nature of the test effectively exposes the sample to both
mechanical
fatigue and thermal fatigue so that the effects at the interface between bulk
sample and
its oxide surface can be investigated. The rate of growth of the oxide layer
is
measured as the sample is exposed to high temperature. The amount of stress
and
spalling of the oxide layer can therefore be viewed as the oxide layer
develops.
The mass change of the sample is recorded before and after the test in order
to assess
the stability of the oxide layer. It is expected that the mass of any sample
exposed to
these harsh conditions will change because the test is very severe. However, a
smaller variation in the mass before and after the test is indicative of a
more stable and
adherent oxide layer. This in turn is indicative of stability of the alloy
over an extended
period such as during long term service. The results achieved using alloys
according to
the invention are very impressive.
Figure 5 shows the compositions of three alloys according to the invention
which were
subjected to the thermal oxidation test.
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Figure 6 shows photographs of the three samples after the Thermo-oxidation
test and
the variation of mass for each of the samples. It can be seen from Figure 6
that the
variation of mass after test is negligible (all show a mass change of less
than 1.15mg)
and consequently that the alloys of the invention demonstrate excellent
stability under
.. these conditions. This also demonstrates that the alloys will be stable
during an
extended period in use.
Figure 7 illustrates the cross section of the surface of Sample C of Figure 5
after 50
thermo-oxidation cycles. This is investigated by X-ray spectroscopy using
copper Ka
radiation. It can be seen that there is a continuous oxide layer which is
present across
the majority of the sample. The oxide thickness is 3.61pm 0.61pm (based on
137
measurements). Figure 7 also highlights the scale of the aluminium and oxygen
components of the alloy, as well as the scale of the nickel plating in the
alloy.
The thermo-oxidation test results demonstrate that the alloys of the invention
have a
good resistance to oxidation under extreme conditions. These results also show
that
the alloys will have a long service lifetime.
