Note: Descriptions are shown in the official language in which they were submitted.
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~E~T TREATMENT OF STEEL E~MEN~S IN ~LUIDIZED ~EDS
The present invention relates to the heat
treatment of steel in fluidised beds, and particularly
but not exclusively to the quenching and subsequent
isothermal transformation of wires in a patenting
operation.
Patenting involves heating carbon steel wires
into the austenitic phase, generally above 800C,
and then quenching the wires to a chosen temperature
at which the wires are held for a sufficient period
for generally isothermal decomposition of the austenite
to be completed. The temperature is usually in
the region of 5S0C, with the intention being generally
to provide a fine pearlitic structure. The wires
will subsequently be drawn.
In general the wires will be of a plain or
alloyed steel with a carbon content of from about
0.1% to more than 1% and preferably in the range
of about 0.2S~ to l.25~. The Wi res may be of anv
cross-section, e.g. square or rectangular, but
~0 are preferably common wires with a circular cross-
section whose area preferably exceeds 0.l5 mm2.
The term "wire" is intended to extend to e.g. rods,
strips and other elongate members.
In a conventional patenting operation the
2S quenching and transformation steps are carried
out in a bath of molten lead held at a constant
temperature. Although this provides good results
in ~iew of the heat absorhing capacitv of the molten
lead, which gives rise to rapid cooling, there
are problems. ~part from the environmental and
safety problems of working with molten lead, there
can be lead drag out and surface defects caused
by iead contamination.
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It has been proposed to replace the lead bath by
forced gas or air cooling, but this is insufficiently
reliable with wire diameters below 5 mm, i.e. the majority
of cases in wire drawing plants, and particularly with wire
diameters below 2mm.
It has also been proposed to use heated fluidized
bed apparatus, where there are improved heat transfer
2roPerties with respect to forced gas or air treatment. A
typical fluidised bed installation comprises a refractory
furnace construction with two compartments separated by a
fixed horizontal plate. The upper compartment forms a long
U-shaped vessel in which inert sand particles (silica,
alumina, zirconia, and the like) are fluidized and heated
by blowing a hot gas through its horizontal bottom plate
which for that purpose possesses a plurality of apertues
(i.e being of perforated or slitted metal) or is made of a
porous ceramic material such as asbestos sheets or ceramic
plate. The lower compartment below the separating gas
distribution plate is the gas plenum chamber from which the
fluidizing gas is admitted under pressure to the particle
container. The fluidized particulate medium, formed of
solid particles suspended in a fluidizing gas of adequate
velocity ~usually between 8 and 15 cm per second for an
average particle dimension ranging from 150 to 500
micrometer), behaves nearly like a liquid heat transfer
medium and posesses an elevated heat transfer coefficient
which is situated between that of forced air cooling and
molten lead.
It has been found, however, that the mechanical
properties and microstructure of wires treated in such
fluidized apparatus are still significantly inferior to
those obtained by lead bath treatment. There is a
significantly larger incidence of deviations from the ideal
fine pearlitic structure, with e.g. substantial amounts of
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coarse pearlite or bainite being formed. These problems
have generally been attributed to the lower heat capacity
and transfer properties of a fluidized bed compared to a
lead bath, which result in a slow cooling rate and the lack
of consistent isothermal transformation conditions.
In an attempt to overcome these problems,
particularly with rods or heavy wires, having e.g. a
diameter of more than 2.5 mm, it has been proposed in ~.S.
Patent 3,615,083 to use a separate precooling bed fluidized
by cold air, positioned between the austenitization furnace
and the heated fluidized bed. According to this ~.S.
Patent, a problem with the prior art is that the cooling
rate is not sufficiently rapid. Nevertheless, tests have
shown that the proposals in this ~.S. Patent do not provide
the necessary improvements in quality, particularly for
wires with a diameter of say, 3 mm or less and typically
0.7 to 1.5 mm.
We now believe that the problems associated with
fluidized bed processes lie not so much with the rate of
cooling but with the difficulty of choosing a bed
temperature which will be a satisfactory compromise between
the requirements of quenching, and soaking at an elevated
temperature.
During the soaking stage, substantially isothermal
transformation should take place. However, the
transformation is exothermic and the temperature of the
wires will tend to rise. With a lead bath of substantial
thermal capacity, the temperature can be kept almost
constant but with a conventional fluidized bed a significant
increase in temperature is encountered. This can lead to
the formation of coarse pearlite. On the other hand
significant under-cooling prior to soaking at an elevated
temperature in the transformation stage, may promote initial
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formation of undesirable structures, such upper bainite.
The temperature band over which fine pearlite
structures can be obtained reliably is relatively narrow
and for the optimum microstructures is narrower still. In
conventional heated fluidized beds used for treating
wires, the temperature variations may extend over a range
comparable with or larger than these preferred bands. If
the temperature of the fluidized bed is set sufficiently
low for the soaking temperature to be acceptable, taking
into account the exothermic nature of the transformation,
then there will be a risk of undercooling during the
quenching stage and undesirable formation of bainite. If
the bed temperature is increased to avoid this problem,
then there is a risk of overheating during the
transformation stage and undesirable formation of coarse
pearlite.
U.S. Patent 3,615,083 does not provide a solution
to these problems, since although two beds are provided,
the arrangement is likely to lead to undercooling
particularly in the case of thin wires.
The present invention aims to solve at least some
of the problems associated with known fluidi.zed bed
techniques.
Thus having regard to the process disclosed in U.S.
~5 Patent 3,615,083, namely a process for heat treating steel
wires in a patenting operation in which the austenitized
wires are quenched in a first fluidized bed zone and
transferred to a second, fluidized bed zone where
transformation takes place, the second zone being heated
by the fluidi~ing gas, the present invention is
characterised in that the first fluidized bed zone is
heated by its fluidizing gas and the temperatures of the
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two zones are controlled independentl~.
Apparatus in accordance with the invention is
characterised by means for supplying heated fluidizing gas
to the first fluidized bed zone and means for controlling
independently the temperatures of the first and second
zones.
By means of the invention, it is not necessary to
find a compromise between the quenching and transformation
techniques. The temperature of the second zone can be
chosen, and the heat input controlled, to provide the
desired microstructure without interfering with the
quenching temperature in the first zone, and vice-versa.
In the first zone, the provision of a heated
fluidizing gas will make it possible to ensure that the
total heat input, including that from the wires being
treated, is such that the temperature of the wires does
not drop below a critical level at which formation of
bainite is promoted. This will be of particular advantage
in the case of thin wires where the heat stored by the
wires is not as great as with thicker wires. In general,
lamellar microstructures are desired but it may be
necessary to ensure that the wire temperature does not
rise to a level at which coarse pearlitic structures are
obtained in preference to fine structures. This can be
achieved by providing separately controllable cooling
means in the first fluidized bed zone. The balance
obtained between the heat input and cooling means makes it
easier to maintain a desired temperature.
These cooling means could comprise immersed cooling
tubes with a fixed or preferably regulated water flow
rate, or a regulatable water spray, or more preferably air
cooling of the fluidized bed surface.
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In rnany cases, the temperatures of the two
zones will be similar although the respective heat
inputs will be controlled independently to take
into account the different conditions and requirements.
S The improved control over the second zone which
is thus made possible, permits the soaking temperature
to be maintained at a more constant ]evel and this
further improves the microstructures which can
be obtained. Thus, another problem with prior
art fluidized bed systems is reduced. Coupled
with the possibilities of controlling the wire
cooling and the transformation start conditions,
significant improvements are obtained.
The two fluidized bed zones could be provided
by two separate fluidized beds with independently
controlled fluidization. Althernatively, a single
fluidized bed could be divided into two zones.
Whilst these two zones would be fluidized by a
single source of hot gas, at least one zone would
be provided with independently controlled auxiliary
heating and/or cooling means. Thus, the quenching
zone could be providec with cooling mean~ such
as those mentioned above and/or the soaking zone
could be provided with heating means, depending
on the ~asic temperature of the hot gas.
We have found that even in the soaking zone,
and with the improved performance obtained by means
of the invention, there can be variations from
the ideal temperature caused e g. by the exothermic
nature of the transformation. This can be corrected
by dividing the soaking zone ;tself into a number
of separate zones with auxiliarv heating and/or
cooling means.
Thus, viewed from another aspect of the invention,
a process for heat treating steel elements by passing
them through a single fluidized bed which is fluidized
and heated by a source of hot gas, is characteriseA
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in that the temperatures of separate zones of the
bed are controlled by independently controlled
auxiliary heating and/or cooling meansO
Apparatus for use in such a process can also
be of wider applicability and thus viewed from
a further aspect of the invention, a hot gas heated
fluidized bed is characterised by the provision
of independently controlled auxiliary heating and/or
cooling means for controlling the temperatures
of separate zones of the bed.
In the context of the two zone fluidized
bed used e.g. in patenting as described above,
it is not generally necessary for the soaking zone
to have auxiliary cooling means, whilst it may
be advantageous to have auxiliary heating means.
In a preferred arrangement, electric resistance
heaters are immersed in successive soaking bed
sections. These could be replaced by immersed
radiant tube heaters. With such arrangements,
the base heat input from the fluidizing gas, i.e.
its inlet temperature, is set fairly low and the
auxiliarv heaters r~]ied upon ~o brin~ the bed
to the required tem~erature.
In all of the arrangements, regulation of
~5 the inlet temperature of the fluidizing gas for
either zone can use lean to extra lean mixtures,
mix cooling air with the combustion gas, or provide
a regulate heat exchanqer between the plenum and
the conbustor.
In a preferred embodiment of the present
invention a fluidized bed soaking zone contains,
in its longitudinal direction, a numher of distinct
heat transfer and control compartments, making
it possible to aclapt locally the energy balance
resulting from work loacl heat, from the heat input
by primary fluidization and by auxiliary heaters
and from cooling and ambient heat losses, thereby
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enabling momentarily an improved accuracy of local bed
temperature, which temperature can be kept constant over
the entire soaking bed length or can be programmed to
impose and maintain a predetermined profile from soaking
zone entry to exit.
Although the apparatus and processes in accordance
with various aspects of the present invenion are
particularly of use in a patenting operation using
conventional quench and soaking temperatures, other
possibilities are envisaged. Thus, "step patenting" could
be undertaken. In this, the quench temperature is lower,
e.g. 400C, whilst still above Ms, and this is followed
by rapid heating to the selected transformation
temperature "Gradient patenting" could also be
undertaken by quenching and then transforming through a
chosen temperature gradient using separate temperature
control of various zones of a fluidized bed. The
apparatus could also be used in other processes
altoyether, such as the formation and subsequent tempering
~ of martensite to produce hard structures. In such
processes, the quench temperature will be below Ms. Other
possible processes are precipitation hardening, quench
hardening and so forth.
In the gradient patenting process the pearlite
reaction commences at a low temperature level such as
540-560C and continues to a given degree. This
initiates formation of fine sorbite. Thereafter, and e.g.
after 10-20~ transformation the remaining austenite is
decomposed at a higher temperature level such as
600-650C or more. Thus, the cementite growth rate is
signficantly slower. It is therefore possible to create
fine structures, with a small interlamellar distance,
without the growth defects encountered with fine pearlite
reacted isothermally at higher rates (i.e. at constant
lower temperatures).
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Wires produced in this manner have improved
drawability and strength properties. In fact, the
fluidized bed apparatus and method of the preferred
embodiments allow the selection of any convenient cooling-
transformation curve in the T.T.T.-diagram or the carrying
out of a patenting treatment according to a specific
curve, e.g. to obtain special effects or particular wire
properties. This is not known with common fluidized bed
plants nor with lead baths.
One possibility is to take full advantage of the
exothermic nature of the reaction so as to form uniform
pearlitic structures with a larger than usual inter-
lamellar distance. Thus, the reaction could start at 580
to 600C and the wires could be allowed to increase in
15 temperature by the effects of the transformation heat
(with temperature rises up to 60-80C). Although the
wire strength is less, the wire has good deformation
properties.
A further problem with the quenching of steel wires
20 in a fluidized bed such as the cold air bed of the prior
art, is oxidation of the surfaces of the wires, producing
undesirable scale. We therefore propose using a
substantially non-oxidising hot gas to fluidize (and heat)
the quenching zone. Viewed from this aspect~ the
25 invention provides an improvement in a process for heat
treating of steel in which steel from an austenitizing
furnace is-quenched in a fluidized bed, the improvement
being characterized in that the bed is fluidized by
substantially non-oxidising exhaust gases from the
30 austenitizing furnace. Apparatus for heat treating steel
in accordance with this aspect of the invention comprises
an austenitizing furnace and a quenching fluidized bed,
and is characterised in that means are provided for
supplying exhaust gases from the furnace to the bed so as
35 to fluidize the bed.
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Such a process and apparatus can be of use
- in many fields, but is of particular use in the
patenting operations described earlier.
Where two fluidized bed zones are used, the
exhaust gases can be passed through both zones,
either by fluidizing a single bed divided into
zones, or by being passed through two separate
beds. In ~he latter case, the exhaust gases may
pass sequentially through the two beds.
The exhaust gas preferably has an oxygen
content of 5% by volume or less and preferably
no more than 2~ with a target of 1% maximum. Preferably
the content is not more than 0.5~ or most preferably
0.1 or 0.2~, with a residual carbon monoxide content
of not less than 0.1% and preferably in the range
of 0.5 to 2~.
It is conceivable that other types of non-
oxidising gas could be used, even if not obtained
from an austenifizing furnace.
In one preferred arrangement, the hot exhaust
gas is precooled in a recuperator, e.g. a waste
hi-at koiler, tc a level not exceedins ]5nC and
subsequently heated to the desired input temperature.
This can be done by means of a battery of variable
power electric heaters. The inlet temperatures
may vary from 100-150C to 450-500C according
to the operational stage li.e. the highest temperature
is required at start up) and the wire diameter.
In fluidized bed apparatus in accordance
with the invention, a separate fluidizing gas make
up station is preferably located outside o~ the
basic fluidized bed enclosure~ Instead of employing
conventional furnace designs (rigid constructions
with fixed refractory / metal ~oints) for building
the fluidized bed, it is preferred to use a modular
and flexible construction as described in Spanish Patent
No. 547,978 granted June 22, 1~87 to N.V. Bekaert S.A.
although this choice
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is not essential for putting the various aspects of the
invention into effect. More in particular a preferred
construction comprises a main steel-backed refractory
enclosure, forming a tunnel-like space covered by a
removable or liftable roof, in which at least two
separate fluidized bed modules (without incorporated
burners) are disposed, respectively a quenching module
and one or more soaking modules. A distinct module is
preferably made in the form of a two-chamber metal
assembly comprising an open vessel for containing the
particles and an adjacent gas plenum chamber underneath
separated from the particle vessel by a gas distribution
bottom plate (with apertures and/or nozzles for
admittance of fluidizing gas) and is further improved in
that the module parts are intergrated in a distinct
one-piece assembly. Such modular design, in which
combustion heaters are absent, is advantageous in terms
of exploitation and maintenance : the individual zone
modules are easily mounted in the apparatus enclosure,
~ and if needed, they can be detached from the main frame
(such as e.g. for repair) and replaced by other modules.
The soaking zone may comprise one fluidized bed
module of suitable length, or a number of smaller modules
linked together if a soaking zone of considereable length
is desired. Admittance of fluidizing gas to the soaking
zone with one or more modules can be by means of a
central inlet from a soaking gas station to a common
plenum duct extending below the adjoining plenum chambers.
Moreover, the unfavourable prior art installation
design and apparatus construction associated with
the presence of internal combustors, heat sensitive
parts (exposed to direct flame heat) and of fixed
joints between dissimilar metal and~refractory
components, gave rise to frequent apparatus downtime,
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high repair costs and production loss. These persistent
problems of widely divergent nature can be at least
partially resolved by preferred embodiments described
herein.
In the preferred arrangements, each zone
is equipped with its own fluidization circuit and
integrated heat control system. Accordingly the
separate quench zone and the soaking zone are individually
fluidized by means of suitahle gas mixtures prepared
(at a regulable base temperature3 outside the apparatus
in the gas make-up station of each zone, and there
are independent heat input regulation and bed temperature
control systems. Such an integrated system per
zone is effective in practice with respect to starting
and operating a fluidized bed line. Thus, it allows
the use of an appropriate gas mixture in each zone
and preferably a non-oxidizing gas in the quench
zone for scale-free cooling the hot wires. ~t
also enables the gradual adaptation (from start-
up to constant running) of the gas inlet temperatureto a specified base temperature (selected as a
functior o~ wire type and procesC conditions~ as
required in each zone, from which base level the
temperature inside the fluidized bed is further
more accurately adjusted in the preferred embodiments
by specific secondary control devices incorporated
respectively in the auenching and in the soaking
zone. In addition, since there are no burners
(for heating and fluidizing) in the zone modules,
direct thermal damage is reduced and access, repair
and replacement of the module parts is easier.
Some embodiments of various aspects of the
invention will now be described by way of example
only and with reference to the accompanying drawings,
~5 in which:
~ igs. l(a) and (b) and ~(a) and (b) show
longitudinal sectional views respectively of a
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standard lead and a conventional fluidized bed patenting
installation, and the corresponding wire
cooling~transformation curves;
Fig. 3 is a diagrammatic illustration of the
relationship between the temperature-time-transformation
(T.T.T.) diagram and the cooling-transformation curve of a
lead patented and a conventionally fluidized bed patented
carbon steel wire;
Figs. 4(a) and (b) show first and second examples
of fluidized bed apparatus in accordance with the
invention;
Figs. 5(a) and (b) show a schematic view of a third
e~ample of apparatus in accordance with the invention,
together with the achievable patenting curves;
Fig. 6 shows further details of apparatus in
accordance with the invention;
Fig. 7 shows wire cooling and transformation curves
obtainable by fluidized bed patenting process in
accordance with the invention;
Fig. 8 shows further details of apparatus in
accordance with the invention;
Figs. 9(a) and (b) compare the fluctuation of
patented wire strength in lead and fluidized
bed-patenting; and
Fig. 10 illustrates a number of specially selected
fluidized bed-patenting curves.
Referring to Figs. la and 2a there are
schematically shown a lead (Pb) and a prior art fluidized
bed (FB) patenting line, whereby a wire material W, after
heating in an austenitization furnace l enters a lead bath
2', or a FB-apparatus 2 of usual single zone construction,
kept at a constant temperature by suitable means (not
shown).
Fig. lb and 2b depict the changes in wire
temperature as a function of time from the austenitizing
temperature (Ta) until the patenting holding temperature
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- (Tp) in both cases. Tq schematizes the course
of wire temperature during quenching. From a comparison
of Fiqs. lb and 2b it clearly appears that in a
conventional FB-apparatus transformation start
and real wire transformation temperatures shown
by curve Tl and the shading considerably depart
from the preferred temperature (Tp), and that the
pearlite reaction may occur over a broad range
of temperatures. These tend to rise excessively
during reaction progress, due to the combined effect
of wire recalescence (heat release by teansformation)
and of the lower heat transfer and heat capacity
of a fluidized bed.
In Fig. 3 the wire cooling-transformation
curves (FB) obtained by conventional fluidized
bed patenting are represented in a ~.T.T. diagram
in comparison with lead patenting (Pb). The dashed
c~urves (TR) and (TR)100 indicate start and end
of austenite transformation, and the shaded area
(OT8) illustrates the optimum transformation band
for obtaining a fine pearlitic structure. It should
be noticecl that in the case of conventional ~s-
patenting the temperature departs from the OTB-
region. Prior art attemps to remedy this situation,
for example by using a precooling unit such as
a cold air FB-zone, or by drastically lowering
the fluidized bed soaking temperature so as to
provide a temperature curve such as T~ in Fig.
2b, are mostly too critical because of possible
bainite formation caused by the degree of undercooling
T~ below Tp.
In Fig. 4a a general embodiment of the present
invention is schematized. There is shown an austenitizing
heating furnace 1 and a two-zone fluidized bed
apparatus 2 with an indepenclent quench zone Q and
trans~ormation-soaking zone TR-S. ~hese zones
each contain a modular asembly 3, comprising essentially
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a particle container ~, a plenum chamber 5, a gas
distribution plate 6 (such as a perforated plate,
preferably with gas pipes or nozzles) which links
the container bottom and the plenum upper part,
S and a gas admittance duct 5' connected to the plenum
bottom. A (desirably detachable~ pipe connection
8 joins each module to the gas supply duct of a
fluidizing gas make-up station 7 (not shown here
in detail) where the required gas (in terms of
volume and composition) is prepared at a regulable
base temperature. This base temperature is determined
for each zone according to wire type and selected
process and is adjusted during processing according
to the prevailing bed conditions related e.g. to
lS start-up or running, change of wire diameter, etc.
For the external gas make-up stations, possible
installations are gas generators, suitable make-
up burners supplying a (preferably lean) combustion
mixture, forced air heaters and combinations thereof.
The two zones Q and TR-S are separated by a heat
insulating wall suitably apertured to permit the
passage of wires. The apparatus is dexigned to
handle a number of wires travelling in straight
and parallel paths. The wires may pass through
a protective hood or the like from the furnace
l to the quench zone Q.
In Fig~ 4b there is shown an alternative
embodiment of a two-zone fluidized bed, in which
austenitizing furnace exhaust gas is employed for
fluidizing first the soaking zone and next the
quench zone (or vice versa when using precooled
furnace exhaust gas). In this case the exhaust
gas from austenitization furnace L is fed by pipe
8 to the fluidized-bed ap~aratus 2 by means of
an extraction-blower 7'. Base temperature adjustment
of the gas, before its adrnittance to the soaking
and quench zone modules, is carried out by means
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of individual appropriate heat exchangers 10 and
lO', located at the entry of each zone.
Fig. 5a illustrates a preferred embodiment
which is particularly advantageous. Here there
is shown a gas fired austenitizing heating furnace
l and a two-zone fluidized bed 2 with separate
quench and soaking modules Q and TR-S, in which
the quench zone is fluidized by means of ~preferably
non-oxidizing) furnace exhaust gas 8 whereas the
soaking zone TR-S is equipped with an independent
gas generator 7, for example a suitable combustor
(e.g. a make-up burner). In this particular case
the fluidizing base temperature at the quench zone
inlet is preferably controlled as follows. First
the extracted furnace e~haust gas is precooled,
preferably to below ]50C, in a furnace heat recuperator
ll, and then it is blown to a regulable heat exchanger
12 (for example an electrical gas heater) to adjust
actual gas temperature to an instantly required
inlet temperature level which may vary according
to momentarily prevailing heat conditions inside
the ~uench bed dependina on operational regime,
heat input from hot wires, throughput speed, etc.
The primary adjustment of quench gas inlet temperature
is supplemented by a secondary control system for
accurately regulating the temperature inside the
quench bed to maintain any desired prsent value.
In practice, the secondary control system takes
over completely once full time running operation
is fully established, that is when additional heat
input from the fluidizing yas is no longer demanded
and the quench gas preheating battery can be switched-
off. This will be described ;n more detail ~elow.
The soaking zone TR-S is fluidized and heated
by means of hot gas derived from station 7, e.g.
a make-up combustor, which supplies a gaseous combustion
mixture at a given base temperature to the soaking
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zone module. The gas inlet ternperature level, needed for
heating and holding the soaking bed at a constant present
(average) temperature, is automatically adapted as a function
of actual soaking bed heat balance (work load, recalescence,
heat losses, etc.).
Thus both the quench and soaking bed are individually
fluidized, heated and temperature controlled in such a way as
to maintain a constant bed temperature, which is
characteristic for each zone and is adapted according to the
wire and desired properties for a given process. In wire
patenting for example, the internal quench bed temperature
may be varied from 250 to 600C (to obtain a wire
temperature between Ms and a given pearlite reaction
temperature), while in the soa~ing zone the preset
temperature can be selected within a range from 450 to
700C (to obtain a pearlitic structure of variable
fineness).
Fig. 5b shows a set of wire cooling-transformation
curves obtained on wire patenting by means of an apparatus
and process of preferred embodiments of this invention
(curves FB-IN) as compared to prior art fluidized bed
patenting using a single zone (curves FB-PA). As can be seen
from the diagram the curves FB-IN correspond to a much more
closely controlled patenting treament than possible with the
prior art process, given the better adjustment of wire
cooling and transformation start conditions combined with a
more precise control of pearlite reaction temperature.
The local bed temperature, may have a tendency to rise
at some places above the optimum level at a given
transformation stage owing to the previously mentioned
recalescence effect (release of transformation heat). From
experiments we have found that the degree of recalescence and
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the location of its temperature peaking effect
in the soaking zone, may vary with wire diameter
throughput speed and selected transformation curve.
Accordingly, in preferred embodiments there
are provided auxiliary heating elements and temperature
sensors in the particle bed of the soaking zone
module, which elements are grouped and operated
in a number of distinct zone compartments making
up the complete soaking-transformation zone length.
The groups are regulated independently by compartment
to correct the local soaking zone temperature in
combination with the control of primary fluidization
heat. To solve the problem of unequal heat losses
in the presence of a variable release of transformation
heat, the average heat input is divided into a
primary and a secondary fraction, with the primary
fraction being deliberately chosen below the constant
running heating needs In this way, the auxiliary
heaters not only deliver the necessary power to
compensate for local heat deficiency, but also
a part of the primary heat. As a result possible
local beà overheating owing to the wire recalescence
peak (which may exceed the average bed heat loss)
can still be counteracted without affecting the
adjacent transformation zones. An additional advantage
of this measure is the possibility of having a
programmed pearlite reaction, e.g. in steps of
different temperature levels and reaction speeds.
This has several advantages in practice, such as
increased flexibility to carry out patenting right
on target (possibly even better than lead patenting),
the ability to control the patenting reaction beyond
the usually adopted cooling-transformation curves
and better productivity in terms of apparatus used
due to shorter start-ups and a quicker transition
to desired regime operation.
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- 19 -
Fig. 6 illustrates how the optimum reaction
temperature may be precisely adjusted during transformation
progress according to the above principles, on
a wire W. For this purpose the soaking bed TR-S
has been divided into a number of sections 13 each
of which comprises a set of individual heating
elements 14 inside the fluidized bed, a suitable
temperature sensor 16 and a heating power regulator
17, connected to a control panel 15. The heating
elements are operated at a given base power to
keep the soaking bed at a preset temperature, in
combination with the heat input of the hot fluidizing
gas supplied by the soaking bed gas make-up station.
They are further actuated in an increasing or decreasing
power sense when local bed temperature drops below
or exceeds the prescribed soaking temperature.
The heating and fluidizing gas make-up station
is disposed outside the main apparatus enclosure.
The station is here essentially a combustion device,
arranged to prepare a combustion gas mixture at
desired rate, temperature and pressure, and comprises
a comhustion chamber 20 and a gas burner 21 with
supply of preferably gaseous fuel 23 (e.g. natural
gas) and forced air 22 from blower 7. The gas
inlet temperature is fed by line 18 to panel 15.
The gas for the quench zone Q, e.g. pre-cooled
from a furnace, passes through a heater 12.
Fig. 7 illustrates the effect of additional
temperature correction within the soaking zone
on the position of the patenting curves in a T.T.T.
diagram. As can be seen wire transformation temperature
or pearlite reaction can be forced entirely into
the required optimum OTB-region (curve A), by instant
correction of local soaking bed temperature whereas
otherwise (curves B), i.e. in the absence of individually
- . .
. .. :
: '~ ' ,.. : '
~ ~7~3~
-- ~o --
regulated bed sections, it could escape to a given
extent from the optimum transformation band, resulting
in a partially annealed (coarser) pearlitic structure.
Fig. 8 shows a more detailed view of a preferred
embodiment of a fluidized bed plant utilizing the
principles of Fig. 6. Wire W, austenitized in
a gas fired furnace 1, passes successively through
a quench compartment Q and a separate cooling zone
TR-S of fluidized bed apparatus 2. The soaking
zone, contains a number of sections 13 with immersed
auxiliary bed heaters and related control devices
(depicted in Fig. 6 but not again represented here~.
The combustion air for burner 21 is preferably
preheated and for that purpose fed by a blower
7 over a heat recuperator 24 located in the soaking
bed exhaust 25.
From combustion chamber 20 the prepared fluidizing
gas is piped to the soaking zone module TR/S, which
is essentially a metallic assembly disposed in
the U-shaped inner space of the FB-furnace, in
which assembly the particle vessel, plenum chamber
and gas admittance duct are inteqrated ~he parti~]e
bed ~ contained in vessel 3 is fluidized. There
is also shown a gas plenum 5 with gas admittance
duct 5' and a gas distribution device 6 between
the vessel bottom and the ad~acent plenum which
is preferably a perforated plate having a large
number of fluidizing nozzles ~' at regular, short
distance from each other (for example in the range
of 3 to 20 cm). The nozzles receive f]uidizing
gas from a plenum chamber, the gas admittance duct
5' of which is connected to a suPply pine ~ of
the soaking hed make-up 2n and make it possible
to obtain and maintain an optimum fluidizing velocity
(usually around 10-12 cm per second) and stable
bed conditions. Control means ~or the soaking
bed comprise a control device (not shown here)
` `' ' ':,
- 21 -
for regulating the make-up combustor 21 to establish
and adjust the required soaking gas inlet teMperature
(primar~ soaking bed heating and holding at base
temperature~, and secondary control devices, as
explained above in connection with Fig. 6, connected
to the auxiliary heaters of each soaking zone section
to correct the local soaking bed temperature and
to augment the base heat input cf hot fluidizing
gas to the soaking zone (especially useful in starting-
up the fluidized bed apparatus).
The quench zone Q comprises one fluidizedbed module of the same type as described above
for the soaking zone, but of shorter length, preferably
between 50 and 250 cm. In principle the zone can
be fluidized in the same way as the soaking zone,
that is by means of a separate external combustion
gas make-up station connected to the quench module.
In this embodiment, however, the quench gas is
derived from the exhaust of the preceding gas fired
austenitizing furnace. The composition of the
exhaust gas is adapted so as to reduce and even
avoid oxidation of ~he hot wires durina quenching.
Thus the exhaust gas mixture entering the quench
module has an oxygen content of max. 2 vol ~, and
preferably not more than 0.5~ to slow down or prevent
undesirable surface oxidation. More specifically
the oxygen content is preferably limited to 0.1~
max. for oxidation free quenching, in combination
with a small amount of CO of between 0.5 and about
2~ to ensure that oxidation free conditions are
met. In the latter case, energy consumption is
slightly increased due to non-stoichiometric combustion
in heating furnace.
An extraction-blower ~' supplies exhaust
gas which passes through a precooler or exhaust
heat recuperator (not shown) to lower the gas temperature,
and a regulable electrical gas heater 12 allowing
:.
4~
- 22 -
the fluidizing gas to be supplied to the quench
zone at any required inlet temperature level.
The primary control contains a control device 34
which regulates power supply 36 of preheater 12
as a function of quench bed temperature and inlet
temperature supplied by lines 33 and 35.
Additional cooling and bed control means
are provided to adjust and to maintain a preset
temperature inside the quench bed during constant
running operation, that is when the heat input
of the hot wires largely exceeds the heat removal
capacity of the fluidized quench bed with inlet
gas preheater switched off. These supplementary
cooling means comprise fixed bed cooling means
such as immersed water coils (not shown) and regulable
bed cooling means. The latter comprises a blower
28 which directs a variable amount of cooling air
from a source 29 through pipe 26 onto the surface
of the quench bed or even inside the bed. A motorized
valve 27 adjusts the rate of cooling air by means
of the suitable control system 34 to which it is
connected by line 30. The control svstem 3~ measures
actual bed temperature by means of sensor 33, compares
it with the quench bed temperature and accordingly
regulates the motorized valve of the cooling air
supply. Alternatively regulable water cooling
may be used with heat exchanging coils (pressurized
water or boiling water) located inside the Particle
bed, a variable water flow rate being obtained
by means of a motorized control valve.
In use in the patenting of carbon steel wires,
the quench zone will be adjusted and mai.ntai.ned
a a temperature within a range from 250 to 650C,
preferably from 350 to 550C for a quench length
of ~.5 to 2.5 m and the soaking zone temperature
will be adjustable within a range Erom ~50 to 7nooc,
and pre~erably a range El-om 5()(~ ~o 65()U(.
.
. .
The controls of the various heating and cooling
means described above are preferably automatic.
Reference will now be made to certain examples:-
Example lSteel wires of 1.50 mm diameter and 0.71%
C were treated on different FB-patenting lines
and compared with lead patenting. Austenitization
temperature and wire speed were the same in each0 case, namely 920C and 24 m/minute.
Two different fluidized bed modes were used:
FBl: conventional fluidized bed apparatus with
one immersion zone; bed temDerature setting
at TFB = 560C.5 FB2: fluidized bed in accordance with the invention
with separate quench and soaking zones and
individual fluidizing means and zone control.
Bed temperatures were adjusted as follows:
temperature control:
Tq = 500C in the quench zone
TFB = 560C in the soaking zone
length of quench zone: 2.5m
length of soaking zone: 4.5m
The properties of the patented wires were
as follows:
:,. . ..~ :
:: , . ...
~70
-- 24 --
Table 1
Tensile strength Max. spread*
N/mm2 on wires Microstructure
N/mm2
Lead1240-1255 15 Fine pearlite
patenting (100~)
FBl (prior art) 1140-1204 64 Mixed, up to 20~
coarser pearlite
FB2 1186-1222 36 Fine pearlite +
(invention) some coarse
lamellar areas
(5-10~)
(*) max. spread measured on the same wire and between
different wires according to their position in the furnace.
The results indicate the beneficial effect of the
invention (FB-2) on the properties of patented wire as
compared to prior art fluidized bed patenting (FB-l).
Example 2
A FB-patenting line of 36 wires was equipped with
two-zone fluidized bed apparatus in accordance with the
invention comprising a quench zone of 1.5 m and a soaking
zone of 5.5 m length, each with individual temperature
settings. The quench zone was fluidized with different gas
mixtures.
Process conditions:
- wire diameter 1.3 mm; 0.69~ carbon steel
- temperature of quench bed: 455C
- temperature of soaking bed: 530C
- aust. temp.: 900C; wire speed: 30 m/min.
`
- - -
- 25 -
- quenching modes according to gas make-up
and gas composition in q~ench zone:
. FB-3: furnace exhaust gas % CO=0.15; % 2 2
. FB-4: combustion gas from external burner
station % CO2 4; % 2 5; % CO=O
. FB-5: hot air.
The FB-patented wire results were compared to those
of lead patented wire, isothermally transformed
at 560C.
Wire properties are tabulated below:
Table 2
T.S. Striction Microstructure surface oxidation:
N/mm2 % scale thickness in
micrometer
FB-3 1207-1221 56.5-53.5 fine sorbite + 0.6-0.9
traces lamellar
pearlite
FB--4 1205-1222 52-57 fine sorblte + 1.2-1.5
traces lamellar
pearlite
FB-5 1191-128] 41-54 fine sorbite + 1.5
coarse pearlite
+ ferrite
Lead 1224-1238 48-55 fine soebite1.0-1.2
560~C
It can be seen that the properties and rnicrostructure
of patented wire obtained according to the invention
are close to lead patented wire, except in case
. ~ .
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,
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- 26 -
f tless controled) hot air for quenching. The
beneficial effect of using a non-oxidizing quench
gas on wire surface oxidation is clearly recognizable.
Example 3
This involved the use of the same FB-patenting
line as in ExamPle 2, hut with extra temperature
regulation of the soaking-transformation zone which
was divided into 5 subsections with individual
heating elements for auxiliary heating and correction
of local soaking zone temperature.
Wire: diameter 1.25 mm; 0.73 % C steel
Preset temperature: quench zone 550C
soaking zone 52noc
Running-in of line was compared under following
circumstances:
A: heating elements of soaking sections switched-on
Al:inlet gas temperature adjusted at 40noc; sectional
heaters of 12 kW total power
A2: inlet gas temperature at 355C;
sectional heaters with increased heat;ng
power (25 kW! to enahLe both local tem~eratllr~
compensation and base heating support.
B: soaking zone as usual (without using auxiliary
heaters; fluidizing gas supplied at about 500C.
In case Al effective running was reached
in less than 40 minutes and in case A2, less than
30 minutes. In case B the time for attaining the
required temperature profile in the transformation
zone was more than one hour.
In addition, the distribution and spread
of temperature during normal running operation
was compared in the different bed sections. The
results of temperature measurements are summarised
in Table 3.
,
, : ~ .
..
:
Table 3
TemPerature distribution over lenqth of fluidized hed
soaking zone
quench sect.1 sect.2 sect~3 sect.4 sect.5*
zone
Case Al 440-450 495-510 515-525 510-520 510-515 485-500
Case A2 ~40-450 515-525 520 520 520 515-520
10 Case B 440-460 490-530 520-550 525-580 540-570 450-490
Note* temperature of last zone section: temperature
drop influenced by FB-furnace exit.
15 The favourable effect of separate soaking
zone control sections on bed temperature equalization
is apparent from cases Al and A2. In case B local
particle bed temperatures continue to rise (real
wire or transformation temperature is even a bit
higher), possibly above optimum level. These unwanted
temperature fluctuations could become considerable,
such as e.g. on changing wire diameters and when
intermittent (stop and go) operation occurs (for
example in case of line troubles), which could
lead to inferior wire quality and to a larger amount
of scrapped wire as is frequently the case with
prior art fluidized bed patenting. It also appears
from case A~ that a judicious choice of auxiliary
heating power (which must be large enough to encompas
a broad compensation range) and a lower than usual
primary gas temperature gives an excellent flexibility
and makes it possible to keep the local temperature
very close to the presecribed level.
The wire properties obtained after case ~1,
A2 and B (with lead patenting as reference) were
as follows:
- : " '' ,' ,:
.; , ~
' :,:, ' ' ' : . ,
, ,, : :
,
.: :. : ~
.
.,
:~æ7~
- ~8 -
Al: Tensile strength = 1217 N/mm2 mean spread
between wires: = 12.7 N/mm
A2: Tensile strength = 123a N/mm2 = 10.2 N/mm
B: Tensile strength = 1192 N/mm2 = = 19.5 N/mm2
Lead (560C): Tensile strength: 1247 N/mm2 = 12.4 N/mm2
In Figs. 9(a) and (b) the tensile strength
distribution of treated wires (related to their
furnace position) according to Al and B are compared
with lead patented wires. The improved cons;stercy
of wire properties obtained by conditions Al are
apparent.
Fig. 10 schematically shows a variety of
patenting modes which can be selected and carried
out correctly when using two-zone fluidized in
accordance with the invention including distinct
soaking-zone control compartments. In the T.T.T.-
diagram curves 1 and 2 illustrate FB-patenting
at two different temperature levels; curve 3 illusttrates
FB-patenting with transformation start at a first
temperature and transformation progress and finish
at a selected higher temperature which can be impose~
from any transformation fraction (TR)x onwards
(3a, 3b, 3c). Curve 4 is an example of step patenting
with austenite undercooling before rapid heating
to a suitable temperature for isothermal transformation
to pearlite.
A special adaptation relates to continuous
martensitic hardening of steel wire by means of
a two-zone fluidized bed, which for that purpose
is provided with an adapted quench zone for deep
cooling, making it possible to carry out a so~t
quench to below Ms (martensite start temperature)
without intersecting the pearlite nose of the T.T.T.-curve,
the quench zone heing long enough or, if needed,
there heing an additional cold hed module, to ensure
complete transformation of 3ustenite to rnartensite
. . , ,: ., . :
~, .. .
, . ~ . ,: .
- :
- ., :.
- .: ,,
- 29 -
before entering the soaking zone, where martensite
is to be tempered at a preset holding temperature.
An arrangement for patenting steel wires,
in particular of small diameter, may use apparatus
with only one common particle imersion bed which
is fluidized by a gas mixture (supplied from furnace
exhaust or a make-up burner) at a delierately chosen
"low"base temperature. The immersion or module
length is then subdivided in a number of separate
control sections in which the first section, used
for quenching, is further equipped with fixed cooling
as well as with regulable cooling means to remove
the excess quenching heat. The second and following
module sections, forming the proper transformation
zone, are provided with regulable internal heaters
of sufficient power for establishing and maintaining
a prescribed transformation temperature. In this
case the fluidized bed hardware is integrated in
one modular construction whereas the heat control
and temperature compensation devices form two independent
systems, resp. for quenching and for transformation
or soak,ng
It will be appreciated that, at least in
the case of certain aspects of this invention it
may not be significant whether a particular installation
is considered as a number of separate fluidized
beds or as a single bed divided into separate zone.
Gradient patenting might be carried out using a
number of adjacent, separately fluidized, beds,
for example. Modifications of the principles and
embodiments disclosed herein may be apparent to
those skilled in the art and to the extent that
these retain the advantageous results of the invention
it is intended that they be considered as incorporated
herein.
