Heat treatment of steel elements in fluidized beds

Abstract

 In the heat treatment of steel wires (W) in a patenting operation, the wires from an austenitizing furnace (1) are first quenched in a fluidized bed (Q). This bed (Q) is fluidized by hot gases from the furnace (1) and is also provided with a cooling system (28). The wires are then passed into a second fluidized bed (TR-S) where transformation takes place. This bed is fluidized by an independent source of hot gas (21) and is divided into regions (13) along its length which have independently controllable auxiliary heaters (14). The temperatures in the zone (Q) and the region (13) along zone (TR-S) are controlled to give a fine pearlite microstructure in the wire.

Description

[0001] The present invention relates to the heat treatment of steel in fluidized beds, and particularly but not exclusively to the quenching and subsequent isothermal transformation of wires in a patenting operation.

[0002] Patenting involves heating carbon steel wires into the austenitic phase, generally above 800°C, 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 550°C, with the intention being generally to provide a fine pearlitic structure. The wires will subsequently be drawn.

[0003] 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 0.25% to 1.25%. The wires may be of any cross-section, e.g. square or rectangular, but are preferably common wires with a circular cross-section whose area preferably exceeds 0.15 mm. The term "wire" is intended to extend to e.g. rods, strips and other elongate members.

[0004] In a conventional patenting operation the quenching and transformation steps are carried out in a bath of molten lead held at a constant temperature. Although this provides good results in view of the heat absorbing capacity of the molten lead, which gives rise to rapid cooling, there are problems. Apart from the environmental and safety problems of working with molten lead, there can be lead drag out and surface defects caused by lead contamination.

[0005] 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 2 mm.

[0006] It has also been proposed to use heated fluidized bed apparatus, where there are improved heat transfer properties with respect to forced gas or air treatment. A typical fluidized 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 possesses an elevated heat transfer coefficient which is situated between that of forced air cooling and molten lead.

[0007] 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 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.

[0008] 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 U.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 U.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 U.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.

[0009] 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.

[0010] 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 formation of undesirable structures, such as upper bainite.

[0011] 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 under- cooling 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.

[0012] 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.

[0013] The present invention aims to solve at least some of the problems associated with known fluidized bed techniques.

[0014] Thus having regard to the process disclosed in U.S. 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 fluidizing gas, the present invention is characterised in that the first fluidized bed zone is heated by its fluidizing gas and the temperatures of the two zones are controlled independently.

[0015] Apparatus in accordance with the invention is characterized 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] In many 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. The improved control over the second zone which is thus made possible, permits the soaking temperature to be maintained at a more constant level 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.

[0020] 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 provided with cooling means such as those mentioned above and/or the soaking zone could be provided with heating means, depending on the basic temperature of the hot gas.

[0021] 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 itself into a number of separate zones with auxiliary heating and/or cooling means.

[0022] 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 characterized in that the temperatures of separate zones of the bed are controlled by independently controlled auxiliary heating and/or cooling means.

[0023] 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 characterized by the provision of independently controlled auxiliary heating and/or cooling means for controlling the temperatures of separate zones of the bed.

[0024] 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 fluidizing gas, i.e. its inlet temperature, is set fairly low and the auxiliary heaters relied upon to bring the bed to the required tempo rature.

[0025] In all of the arrangements, regulation of the inlet tempera ture 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 exchanger between the plenum and the conbustor.

[0026] In a preferred embodiment of the present invention a fluidized bed soaking zone contains, in its longitudinal direction, a number of distinct heat transfer and control compartments, making it possible to adapt locally the energy balance resulting from work load heat, from the heat input by primary fluidization and by auxiliary heaters and from cooling and ambient heat losses, thereby 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.

[0027] Although the apparatus and processes in accordance with various aspects of the present invention 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. 400°C, 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 altogether, 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.

[0028] In the gradient patenting process the pearlite reaction commences at a low temperature level such as 540-560°C 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 600-650°C or more. Thus, the cementite growth rate is significantly slower. It is therefore possible to create fine structures, with a small interlamellar distance, without the growth defects encountered with fine pearlites reacted isothermally at higher rates (i.e. at constant lower temperatures).

[0029] 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.

[0030] 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 600°C and the wires could be allowed to increase in temperature by the effects of the transformation heat (with temperature rises up to 60-8D°C). Although the wire strength is less, the wire has good deformation properties.

[0031] A further problem with the quenching of steel wires 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 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 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 characterized in that means are provided for supplying exhaust gases from the furnace to the bed so as to fluidize the bed.

[0032] Such a process and apparatus can be of use in many fields, but is of particular use in the patenting operations described earlier.

[0033] 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 the latter case, the exhaust gases may pass sequentially through the two beds.

[0034] 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%.

[0035] It Is conceivable that other types of non-oxidising gas could be used, even if not obtained from an austenifizing furnace.

[0036] In one preferred arrangement, the hot exhaust gas is pre- cooled in a recuperator, e.g. a waste heat boiler, to a level not exceeding 150°C 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 100-150°C to 450-500°C according to the operational stage (i.e. the highest temperature is required at start up) and the wire diameter.

[0037] In fluidized bed apparatus in accordance with the invention, a separate fluidizing gas make-up station is preferably located outside of the basic fluidized bed enclosure. Instead of employing conventional furnace designs (rigid constructions with fixed refractory / metal joints) for building the fluidized bed, it is preferred to use a modular and flexible construction as described in U.K. patent application No. 84.26455 although this choice 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 coveed 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 place (with apertures and/or nozzles for admittance of fluidizing gas) and is further improved in that the module parts are integrated 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.

[0038] 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 considerable 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.

[0039] 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, high repair costs and production loss. These persistent problems of widely divergent nature can be at least partiaHy resolved by preferred embodiments described herein.

[0040] 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 suitable gas mixtures prepared (at a regulable base temperature) 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. It also enables the gradual adaptation (from start-up to constant running) of the gas inlet temperature to a specified base temperature (selected as a function of wire type and process 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 quenching 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.

[0041] Some embodiments of various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, in which :

Figs. 1(a) and (b) and 2(a) and (b) show longitudinal sectional views respectively of a 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 example 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.

[0042] 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 1 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).

[0043] Figs. lb and 2b depict the changes in wire temperature as a function of time from the austenitizing temperature (Ta) until the patenting holding temperature (Tp) in both cases. Tq schematizes the course of wire temperature during quenching. From a comparison of Figs. lb and 2b it clearly appears that in a conventional FB-apparatus transformation start and real wire transformation temperatures shown by curve T 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 transformation) and of the lower heat transfer and heat capacity of a fluidized bed.

[0044] In Fig.3 the wire cooling-transformation curves (FB) obtained .by conventional fluidized bed patenting are represented in a T.T.T. diagram in comparison with lead patenting (Pb). The dashed curves (TR) and (TR) ₁₀₀ indicate start and end of austenite transformation, and the shaded area (OTB) illustrates the optimum transformation band for obtaining a fine pearlitic structure. It should be noticed that in the case of conventional FB-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 T₂ in Fig. 2b, are mostly too critical because of possible bainite formation caused by the degree of undercooling T₂ below T . P

[0045] In Fig. 4a a general embodiment of the present invention is schematized. There is shown an austenitizing heating furnace 1 and two-zone fluidized bed apparatus 2 with an independent quench zone Q and transformation-soaking zone TR-S. These zones each contain a modular assembly. 3, comprising essentially a particle container 4, 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, 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 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 designed 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 1 to the quench zone Q.

[0046] 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 1 is fed by pipe 8 to the fluidized-bed apparatus 2 by means of an extraction-blower 7¹. Base temperature adjustment of the gas, before its admittance to the soaking and quench zone modules, is carried out by means of individual appropriate heat exchangers 10 and 10¹, located at the entry of each zone.

[0047] Fig. 5a illustrates a preferred embodiment which is particularly advantageous. Here there is shown a gas fired austenitizing heating furnace 1 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 exhaust gas is precooled, preferably to below 150°C, in a furnace heat recuperator 11, 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 quench bed depending 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 present 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 gas is no longer demanded and the quench gas preheating battery can be switched-off. This will be described in more detail below.

[0048] 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 zone module. The gas inlet temperature 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.).

[0049] 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 600°C (to obtain a wire temperature between Ms and a given pearlite reaction temperature), while in the soaking zone the preset temperature can be selected within a range from 450 to 700 °C (to obtain a pearlitic structure of variable fineness).

[0050] 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 treatment 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.

[0051] 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 the location of its temperature peaking effect in the soaking zone, may vary with wire diameter throughput speed and selected transformation curve.

[0052] 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 bed 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.

[0053] 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 combustion 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.

[0054] 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 (curve B), i.e. in the absence of individually regulated bed sections, it could escape to a given extent from the optimum transformation band, resulting in a partially annealed (coarser) pearlitic structure.

[0055] 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.

[0056] 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 integrated. The particle bed 4 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 adjacent plenum which is preferably a perforated plate having a large number of fluidizing nozzles 6' at regular, short distance from each other (for example in the range of 3 to 20 cm). The nozzles receive fluidizing gas from a plenum chamber, the gas admittance duct 5' of which is connected to a supply pipe 9 of the soaking bed make-up 20 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 for the soaking bed comprise a control device (not shown here) for regulating the make-up combustor 21 to establish and adjust the required soaking gas inlet temperature (primary soaking bed heating and holding at base temperature), and secondary control devices, as explained above in connection with Fig. 6, connected to auxiliary heaters of each soaking zone section to correct the local soaking bed temperature and to augment the base heat input of hot fluidizing gas to the soaking zone (especially useful in starting-up the fluidized bed apparatus).

[0057] The quench zone Q comprises one fluidized bed 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 the hot wires during quenching. Thus the exhaust gas mixture entering the quench module has an oxygen content of max. 2 vo1 X, 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.

[0058] An extraction-blower 8' 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 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.

[0059] 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 system 34 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.

[0060] In use in the patenting of carbon steel wires, the quench zone will be adjusted and maintained at a temperature within a range from 250 to 650°C, preferably from 350 to 550°C for a quench length of 0.5 to 2.5 m and the soaking zone temperature will be adjustable within a range from 450 to 700°C, and preferably a range from 500 to 650°C.'

[0061] The controls of the various heating and cooling means described above are preferably automatic.

[0062] Reference will now be made to certain examples :

Example 1

[0063] Steel wires of 1.50 mm diameter and 0.71% C were treated on different FB-patenting lines and compared with lead paten ting. Austenitization temperature and wire speed were the same in each case, namely 920°C and 24 m/minute.

[0064] Two different fluidized bed modes were used :

FB1 : conventional fluidized bed apparatus with one immersion zone; bed temperature setting at T_FB = 560°C.

_{FB2 :} fluidized bed in accordance with the invention with separate quench and soaking zones and individual fluidizing means and zone control.

[0065] Bed temperatures were adjusted as follows :

temperature control :

T_q = 500°C in the quench zone

T_FB = 560°C in the soaking zone

length of quench zone : 2.5 m

length of soaking zone : 4.5 m

[0066] The properties of the patented wires were as follows :

[0067] 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-1).

Example 2

[0068] 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 1.5m and a soaking zone of 5.5m length, each with individual temperature settings. The quench zone was fluidized with different gas mixtures.

[0069] Process conditions :

- wire diameter 1.3 mm; 0.69% carbon steel

- temperature of quench bed : 455°C

- temperature of soaking bed : 530°C

- aust. temp. : 900°C; wire speed : 30 m/min.

- quenching modes according to gas make-up and gas composition in quench zone :

. FB-3 : furnace exhaust gas % C0=0.15; % 0₂ 2

. FB-4 : combustion gas from external burner station % CO₂ 4; % ⁰₂ 5; % CO=0

. FB-5 : hot air.

[0070] The FB-patented wire results were compared to those of lead patented wire, isothermally transformed at 560 °C.

[0071] Wire properties are tabulated below :

It can be seen that the properties and microstructure of patented wire obtained according to the invention are close to lead patented wire, except in case of (less controled) hot air for quenching. The beneficial effect of using a non-oxidizing quench gas on wire surface oxidation is clearly recognizable.

Example 3

[0072] This involved the use of the same FB-patenting line as in Example 2, but 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.

[0073] Wire : diameter 1.25 mm; 0.73% C steel

[0074] Preset temperature : quench zone 550°C soaking zone 520°C

[0075] Running-in of line was compared under following circumstances :

A : heating elements of soaking sections switched-on

Al : inlet gas temperature adjusted at 400°C, sectional heaters of 12 kW total power

A2 : inlet gas temperature at 355°C; sectional heaters with increased heating power (25 kW) to enable both local temperature compensation and base heating support.

B : soaking zone as usual (without using auxiliary heaters; fluidizing gas supplied at about 500°C.

[0076] In case A1 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.

[0077] 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.

Note^* temperature of last zone section : temperature drop influenced by FB-furnace exit.

[0078] The favourable effect of separate soaking zone control sections on bed temperature equalization is apparent from cases A1 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 A2 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 prescribed level.

[0079] The wire properties obtained after case Al, A2 and 8 (with lead patenting as reference) were as follows :

A1 : Tensile strength = 1217 N/mm² mean spread between wires ₌ 1₂.₇ N/mm 2

A2 : Tensile strength = 1234 N/mm² = 10.2 N/mm²

B : Tensile strength = 1192 N/mm² = 1₉.5 N/mm² Lead (560°C) : Tensile strength : 1247 N/mm² = 12.4 N/mm²

[0080] In Figs. 9(a) and (b) the tensile strength distribution of treated wires (related to their furnace position) according to A1 and B are compared with lead patented wires. The improved consistency of wire properties obtained by conditions Al are apparent.

[0081] 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 illustrates FB-patenting with transformation start at a first temperature and transformation progress and finish at a selected higher temperature which can be imposed 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.

[0082] 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 soft quench to below Ms (martensite start temperature) without intersecting the pearlite nose of the T.T.T.-curve, the quench zone being long enough or, if needed, there being and additional cold bed module, to ensure complete transformation of austenite to martensite before entering the soaking zone, where martensite is to be tempered at a preset holding temperature.

[0083] An arrangement for patenting steel wires, in particular of small diameter, may use apparatus with only one common particle immersion bed which is fluidized by a gas mixture (supplied from furnace exhaust or make-up burner) at a de- lierately 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 independant systems, resp. for quenching and for transformation or soaking.

[0084] 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 extent that these retain the advantageous results of the invention it is intended that they be considered as incorporated herein.

Claims

1. A process for continuously heat treating steel wires in multiwire patenting operation with essentially parallel and rectilinear wire displacement in which the austenitized wires are quenched to a predetermined temperature in a first fluidized bed zone (Q) and transferred to a second adjacent fluidized bed zone (TR-S) where soaking-transformation takes place, the second zone being heated by fluidizing gas, characterized in that the first bed zone is heated and fluidized by its fluidizing gas and the temperatures of the two bed zones and of their fluidizing gas supply are controlled independently.

2. A process as claimed in claim 1, characterized in that the first and second zones (Q, TR-S) are fluidized by separate and independently controlled supplies of gas (7).

3. A process as claimed in claim 1 or 2, characterized in that the temperature of the second zone (TR-S) is controlled at least in part by auxiliary heating means (14) in the bed.

4. A process as claimed in claim 3 characterized in that the temperature of individual regions (13) along the second zone (TR-S) are controlled at least in part by individual heating means (14) for each region.

5. A process as claimed in claim 4 characterized in that the temperatures of the individual regions (13) are controlled so as to provide a temperature gradient along the second zone (TR-S).

6. A process as claimed in claim 5 characterized in that the temperature gradient is such that tranformation is commenced at first temperature and is subsequently continued at a second, higher temperature.

7. A process as claimed in claim 6 characterized in that transformation at the second temperature is initiated after between about 10 and 20% of transformation has taken place.

8. A process as claimed in any of claims 1 to 5 characterized in that there is rapid undercooling of the austenitized wire followed by rapid heating to a temperature suitable for transformation.

9. A process as claimed in any preceding claim, characterized in that the temperature of the first zone (Q) is controlled at least in part by auxiliary cooling means (28).

10. A process as claimed in claim 9 characterized in that the first zone (Q) is subjected to continuous cooling by first cooling means and variable cooling by second cooling means (28, 27).

11. A process as claimed in any preceding claim, characterized in that the first zone (Q) is fluidized by substantially non-oxidising exhaust gases from an austenitizing furnace (1).

12. A process as claimed in claim 11, characterized in that the exhaust gases are cooled and/or heated by auxiliary means

(11, 12) before entering the first zone (Q).

₁₃. A process as claimed in claim 11 or 12 characterized in that the exhaust gases have an oxygen content of 2% or less by volume.

14. A process as claimed in claim 13 characterized in that the exhaust gases include a residual carbon monoxide content to further promote non-oxidising conditions.

15. A process as claimed in claim 14 characterized in that the carbon monoxide content is between 0.5 and 2 %.

16. A process as claimed in any preceding claim characterized in that the conditions are so controlled as to produce a substantially entirely lamellar pearlitic microstructure.

17. A process as claimed in claim 16 characterized in that the conditions are so controlled as to produce a homogeneous pearlitic microstructure of desired fineness and strength for further wire drawing.

18. Fluidized bed apparatus for heat treating steel wires comprising a first fluidized bed zone (Q) for quenching wires, a second, heated fluidized bed zone (TR-S), and means (2) for fluidizing and heating the second zone, characterized by means (8') for fluidizing and heating the first fluidized bed zone in substantially non-oxidizing conditions and means (34, 15) for controlling independently the temperatures of the first and second zones.

19. Apparatus as claimed in claim 18 characterized in that means (28) are provided for cooling the first zone (Q).

₂0. Apparatus as claimed in claim 18 characterized in that the cooling means comprises fixed cooling means and additional variable cooling means (28, 27).

21. Apparatus as claimed in claim 18, 19 or 20, characterized in that means (14) are provided for independently controlling the temperatures of separate regions (13) along the second zone.

22. Apparatus as claimed in claim 21 characterized in that separately controlled heating elements (14) are provided in the separate regions (13) of the bed.

23. Apparatus as claimed in any of claims 18 to 22 characterized in that the first zone (Q) is supplied with exhaust gas from an austenitizing furnace (1).

24. Apparatus as claimed in claim 23 characterized in that a pre-cooler (11) and an auxiliary heater (12) are provided for the exhaust gas before it is fed to the first zone (Q).

25. Apparatus as claimed in claim 23 or 24 characterized in that means are provided for passing the exhaust gas sequentially through the first and second zones (Q, TR-S) and separate temperature control means (10, 10¹) are provided to control the temperature of the exhaust gas entering the respective zones.

26. Apparatus as claimed in any of claims 10 to 24 characterized in that the first and second zones (Q, TR-S) are fluidized by completely independent sources of gas (8, 21).

₂₇. A process for the heat treating steel elements by passing them along essentially parallel and straight paths through a single fluidized bed chamber (TR-S) which is fluidized and heated by a source of hot gas (21) of regulable base temperature, characterized in that the temperatures of separate zones (13) of the bed is controlled by independently controlled auxiliary heating and/or cooling means (14).

28. A fluidized bed (TR-S) comprising two or more successive zones in its lengthwise direction which are fluidized and heated by a common source of hot gas (21) of regulable base temperature characterized by the provision of independently controlled auxiliary heating and/or cooling means (14) for controlling the temperatures of separate zones (13) of the bed.

29. A process for heat treating steel In which steel from an austenitizing furnace (1) is quenched in a fluidized bed (Q), characterized in that the bed is fluidized by substantially non-oxiding exhaust gases from the austenitizing furnace.

30. Apparatus for heat treating steel, comprising an austenitizing furnace (1) and a quenching fluidized bed (Q), characterized in that means (8^f) are provided for supplying exhaust gases from the furnace (1) to the bed (Q) so as to fluidize the bed.

31. A process as claimed in claim 27, characterized in that the steel elements, having been austenitized and quenched, are passed through the bed (TR-S) which has the temperatures of its zones (13) controlled so as to produce a temperature gradient such that transformation of the austenitized elements is commenced at a first temperature and continued at a second, higher temperature.

₃₂. A process for the heat treatment of steel elements in a patenting operation wherein the elements are austenitized quenched, and passed through heated fluidized bed apparatus (TR-S) where transformation takes place, characterized in that the temperature along the apparatus (TR-S) is controlled by independent heating and/or cooling means so as to produce a temperature gradient such that transformation of the austenitized elements is commenced at a first temperature and continued at a second, higher temperature.

33. A process as claimed in claim 31 or 32 characterized in that transformation is commenced at a temperature in the range of 540-600°C to initiate the production of fine pealite or sorbite and is continued at higher temperature such that cementite growth is considerably slower.

Drawing