EP0206873B2 - Method of heat treating, gas curtain device and its use in heat treating furnaces - Google Patents

Abstract

The process comprises creating a screen of non-reactive or inert gas at least at one of the two ends of the heat treating zone or cooling zone of a furnace, the screen being substantially homogeneous and laminar throughout its height. The use of this process permits, on the one hand, a reduction in the flows of gas necessary for the heat treating and, on the other hand, the division of the heat treating furnace into precise zones. The invention is particularly applicable to heat treating furnaces.

Description

The present invention relates to a method of heat treatment of objects in a continuous oven comprising at least one heat treatment zone, method in which an atmosphere of non-reactive gas is created under the treatment conditions, at at least one of the ends of said treatment area.

In heat treatment processes such as carburizing, nitriding, sintering, annealing, etc., it is generally desirable to maintain a reducing or non-oxidizing atmosphere in the treatment furnace. For large series of parts, the ovens are generally continuous and open at their ends. They include an entry zone for the objects to be heat treated, a heat treatment zone as well as generally a cooling zone, and an exit zone for the objects. The oven has a system for advancing objects to the heat treatment zone, the temperature of the objects gradually increasing as they advance into the oven. When the treatment is finished, the object generally crosses a cooling zone in which it is cooled to a temperature such that no oxidation of this object will occur in the ambient air.

The required heat treatment atmosphere, generally reducing or neutral, is supplied to the furnace via endothermic or exothermic generators or by direct injection of suitable liquid-gas mixtures. The injection of this atmosphere is generally carried out in or near the heat treatment zone. It is necessary to carry out an overpressure of the atmosphere-generating gas at its injection point in order to try to avoid the return in the furnace of the oxidizing species contained in the air.

A first solution to this problem of ascent of the oxidizing species in the furnace has been described in US Pat. No. 3,467,366. There is provided at the entrance and at the exit of the oven, a confined zone made up of a plurality of curtains. defining a plurality of chambers. In the central chamber is injected an atmosphere of inert gas, using a perforated tube placed at the base thereof, so as to create a plug preventing the ambient air from rising into the atmosphere of the oven and to oxidize the parts being treated. In the chamber adjacent to the oven and to the central chamber, suction means are provided which cooperate with those arranged in the central chamber, so as to aspirate the atmosphere of this chamber, possibly polluted by the oxidizing species coming from the chamber. central. The air drawn in is released into the outside atmosphere.

The system described in this patent also makes it possible to avoid the gaseous atmosphere of the oven from being ejected from the oven and to mix with the ambient air, which leads, of course, to reducing the amount of gas injected into the treatment oven for a specified time interval.

The Applicant has found that such a system has many drawbacks. First, the injection of inert gas through a perforated tube creates a vortex current in the chamber: for the perforations located on the same circumference of the tube, the geometry tends to create a first vortex zone around the tube. Furthermore, the supply of inert gas being carried out at one end of the perforated tube, the other end of which is closed, the gas will tend, with equal perforation diameter, to escape through the end situated at near the closed part and on the contrary create a suction through the perforations located near the arrival of the inert gas, thus creating a second vortex current in the chamber.

This explains the need for a suction system located downstream of this chamber, taking into account the fact that the vortices generated in said chamber necessarily create an air suction in the oven. The suction system allows the air-inert gas mixture to be evacuated before it can enter the heat treatment zone of the oven.

The system described in this patent therefore requires both the use of a containment chamber provided with curtains and filled with an inert atmosphere and with a suction system combined therewith.

The document EP-A-75 438 describes a method of heat treatment of objects in a continuous oven, in which the objects to be treated are successively introduced by a mobile support in the oven comprising at least one heat treatment zone into which is injected a atmosphere of determined composition, the inlet and / or outlet zones of the oven comprising means generating a substantially laminar flow of inert or non-reactive gas under the treatment conditions so as to prevent the entry of air into the oven.

In this document, the inlet and outlet zones of the furnace comprise a plurality of inclined curtains arranged parallel to each other defining a plurality of chambers into which an inert gas such as nitrogen is injected. This injection is carried out through a perforated wall located above and / or below said chambers. The injection of gas through these perforated walls is carried out using a conduit in front of which a deflector is placed, the gas bypassing the latter before entering through the perforations in said chambers.

This produces an overpressure in said chambers, relative to the pressure of the atmosphere of the furnace cooling zone, the pressure of which is itself greater than the pressure of the furnace heat treatment zone, the latter being greater than atmospheric pressure.

Such a device has a number of drawbacks. First, the overpressure imposed on the chambers relative to all of the different parts of the oven requires the use of a large volume of nitrogen. Furthermore, it can also be seen that there are vortex currents between the different chambers. Indeed, the stream of nitrogen which bypasses the deflector, arrives on the outside of the perforated area with a greater speed than on the central area. The pressure drop inflicted on the gas when passing through the openings is therefore lower in this central zone than in the external parts of the perforated plate. Under these conditions, nitrogen tends to penetrate into the central chambers creating a suction through said openings at the outer parts of the perforated plate, thus inducing a vortex of nitrogen inside said system. This is particularly troublesome in the first chamber which is located directly in contact with the outside air. The air is thus sucked into the system and then redistributed with the nitrogen in the different rooms. This stream of nitrogen and air is then entrained towards the interior of the furnace, in the heat treatment zone. It follows that the treatment atmosphere includes a non-negligible part of oxidizing species coming from the air aspirated outside the oven. It is therefore necessary to associate with this system a distribution of pressure of the gases decreasing from the oven outlet to the central part of it.

In the two systems analyzed above, there are therefore the same drawbacks, namely essentially the suction of air towards the heat treatment zone of said oven.

Although these systems present improvements compared to the previous system, in which the ends of the oven were open, it can be seen that the problem of air inlets into the oven is not completely solved by these. This means in particular that the solutions set out in the two previous patents cannot be applied to certain heat treatments such as annealing of stainless steel, since it is necessary for this type of application to have a quantity of extremely low oxygen in the oven as well as at the start of the cooling zone, taking into account the avidity of chromium for oxygen.

The object of the invention is to propose a method making it possible to avoid these drawbacks. To this end, the method according to the invention is characterized in that the flow of inert or non-reactive gas at the end of the furnace is in the form of a single curtain, homogeneous, with vertical flow in a transverse plane of a horizontal end portion of the furnace and traversed by the direction of advance of the parts to be treated, the injection of inert or non-reactive gas being effected, after homogenization of its speed and pressure, under conditions such as substantially laminar flow regime is maintained over the entire height of the gas curtain.

The Applicant has in fact demonstrated that the use of a homogeneous and laminar gas curtain over its entire height, avoids the phenomena of air suction. It can thus be seen that the method according to the invention makes it possible to considerably simplify the devices for implementing it, since it is then neither necessary to add a suction system to the assembly nor necessary to provide a plurality of inert gas curtains.

Preferably, the substantially homogeneous gas curtain is generated at each end of the furnace, the pressure losses induced by these being different from each other, so as to modify the relative value of the incoming gas flows and out of the oven.

The use of the method according to the invention allows in particular the zoning of heat treatment furnaces. In the case where the furnace has several injection points of different atmospheres, the presence of the homogeneous curtain of inert gas at one and / or the other end of the furnace makes it possible, depending on the modulation of the flows of neutral gas injected into each curtain, to modify in a separate manner the conditions of exit of the gases at each end of the furnace, and this in a significant way compared to the pressure losses imposed on the moving gas inside the furnace. This results in a modification of the gas flows on either side of the gas injection points and makes it possible, in particular, to create between two injection points an area where the average gas circulation speed is zero, resulting from a substantially identical pressure at these two points. In this case, we see that the atmospheres injected at these two points diverge from each other.

When there is a gas injection point at a higher pressure than that of the gases injected at the other points, this injection point will make it possible to orient the gas flow rates in the furnace. If it is located towards the entrance of the oven, the gas flow will be the same as the direction of advance of the parts. Conversely, if it is located near the outlet of the oven, the gas flow will be in the opposite direction to the direction of advance of the parts in the oven.

In particular, it can be seen that the zone at maximum pressure of the furnace can be better located in the desired location, in the case of a plurality of injections at different points without thereby increasing the flow rates of the active gases.

The term “non-reactive gas” used in the present application naturally means an inert or non-reactive gas with respect to the other constituents of the atmosphere of the oven as well as the parts which must be treated in it. Generally, nitrogen will be used as the non-reactive gas, although in some cases it is preferable to use argon or possibly helium.

The term “active gas” designates the gas or gases from the heat treatment atmosphere.

The term "heat treatment" includes all the heat treatments that are usually subjected to metals, ceramics, etc., but is particularly intended for the annealing of metal parts such as stainless steel.

The term "heat treatment zone" means one or more parts of the oven in which heating means are optionally arranged, in which identical or different atmospheres are created, each atmosphere preferably being homogeneous. It also includes the case where the heat present in this zone comes from the part itself which enters the heat treatment zone to undergo a transformation therein such as hot rolling, etc.

Of course, the method according to the invention can be used in all continuous ovens of the horizontal or vertical type. However, in the case of vertical ovens, the conditions of homogeneity imposed on the inert gas curtains are such that the entry and / or exit zones provided with the homogeneous gas curtains according to the invention must be located in parts not vertical from the oven.

Usually, the non-reactive gases as well as the reactive gases intended for the heat treatment of the parts are injected directly into the heat treatment zone of the furnace, or in the vicinity thereof. It is however possible to introduce these gases into part of the cooling zone or possibly into or near the zone of entry into the furnace. In all cases, the use of the method according to the invention will make it possible to direct the flow of these gases towards the interior of the furnace and achieve zoning thereof.

According to another aspect, the method according to the invention is characterized in that said atmosphere of inert or non-reactive gas is created by a stream of inert gas injected vertically at the inlet of the substantially homogeneous furnace, according to a laminar flow regime with a flow rate equal to the air flow entering the oven in the absence of injection of inert gas.

Of course, the injection of a homogeneous and laminar flow of inert gas over the entire width of the oven and in particular in the zone located near the conveyor belt for objects entering the oven requires particularly suitable devices, such as the hood which will be described later.

In the absence of special measures according to the method of the invention, the air enters the oven, by natural convection phenomena, through the lower part of the inlet zone, because this air is much colder than the out of the oven. Under these conditions, it has been found that when the curtain of inert or non-reactive gas is injected from top to bottom, the presence of curtains, preferably refractory, on either side of the gas curtain, is necessary, these curtains are 'extend substantially to the conveyor belt of objects in the oven.

Conversely, when the gas is injected from the bottom up, it was found that the presence of said refractory curtains was not necessary. On the other hand, the presence of these refractory curtains may prove necessary to allow the creation of a zoning in the oven, that is to say successive zones of determined atmospheres. These refractory curtains indeed generate a sufficient pressure drop at the inlet and / or outlet of the oven to control the gaseous streams of atmosphere, from their injection points to the inlet or the outlet of the oven.

The use of the process according to the invention proves to be particularly effective when the continuous ovens have a short inlet zone and / or a significant temperature difference between the gases leaving the oven and the ambient temperature (for example, a difference above 300 ° C).

According to a preferred embodiment, the homogeneous curtain of inert gas will be created using a hood making it possible to maintain the flow of non-reactive gas in laminar mode and substantially homogeneous at all points of the gas curtain.

• -des moyens d'injection de gaz inerte dans une chambre d'admission dont le fond est perforé,
• -des moyens perméables au gaz inerte, disposés sur le fond perforé de la chambre d'admission, permettant de donner une vitesse très faible au flux de gaz inerte à la sortie de la plaque perforée sans provoquer de perte de charge sensible au niveau du flux de gaz,
• -au moins un rideau de part et d'autre du flux de gaz, mobile autourd'un axe situé dans le plan du rideau, et disposé dans le passage des pièces à traiter.

To achieve this result, the hood according to the invention comprises:

• means for injecting inert gas into an intake chamber, the bottom of which is perforated,
• means permeable to inert gas, arranged on the perforated bottom of the intake chamber, making it possible to give a very low speed to the flow of inert gas at the outlet of the perforated plate without causing a significant pressure drop at the level of the flow gas,
• at least one curtain on either side of the gas flow, movable around an axis located in the plane of the curtain, and arranged in the passage of the parts to be treated.

avec

• n = viscosité du gaz non réactif utilisé dans la hotte à température ambiante;
• p = masse volumique dudit gaz non réactif dans les conditions normales;
• a = largeur du four et longueur de la plaque perfo rée rectangulaire;
• b = profondeur de la plaque rectangulaire perforée (distance entre les deux rideaux).

Preferably, the intake chamber will have a substantially rectangular perforated bottom, the length of which is equal to the width of the bottom on which the hood is intended to be mounted, the speed of the non-reactive gas having to be substantially identical at all crossing points. of the perforated plate and less than:

with

• n = viscosity of the non-reactive gas used in the hood at room temperature;
• p = density of said non-reactive gas under normal conditions;
• a = oven width and length of the rectangular perforated plate;
• b = depth of the perforated rectangular plate (distance between the two curtains).

The curtains used in this hood will preferably take the form of those described in the American patent cited above, this form of curtains made up of a plurality of elements of different lengths being better suited in particular to ovens in which objects of different forms are treated. Of course the material constituting said curtains must be on the one hand without action on the flow of non-reactive gas from the hood and on the other hand must resist the temperatures to which it is subjected.

As a means permeable to inert gas and having the properties mentioned above, it has been found that sintered materials, such as materials of the rock wool, quartz wool, or glass wool type, having a thickness of at least two centimeters , were particularly suitable in this application.

The inlet chamber of the inert or non-reactive gas, generally has a rectangular shape, the base of which is formed by the perforated plate. It has been found that the best results of continuity and uniformity of the gas curtain were obtained when the height of this intake chamber was at least twice the thickness of the material permeable to neutral gas. In this way, the pressure gradients and therefore the turbulence inside this intake chamber are practically avoided.

The means for injecting the inert gas into the intake chamber will generally be in communication with the latter on the face opposite to its perforated face. It was found that it was preferable to have the arrival of neutral gas substantially in the center of this plate, so as to create symmetry in the injection of said neutral gas.

However, it is not always possible, given the geometry of the heat treatment oven, to inject the gas into the upper part of the intake chamber. In this case, we are therefore forced to carry out this injection on one of the side faces of the intake chamber. It is then preferable for the inert gas supply channel to be connected to the intake chamber by means of a pre-admission chamber which is substantially symmetrical about the axis of arrival of the inert gas. Preferably, the connection zone between this pre-admission chamber and the intake chamber will be constituted by means permeable to neutral gas identical in their nature and structure to those described above. This allows in particular a gas supply, although not symmetrical, at particularly low speeds, without turbulence, as well as a uniformity of pressure and speed of the inert gas in the intake chamber, which is reflected, taking into account the symmetry of the assembly, by a homogeneity of the curtain of inert gas injected at the inlet and / or the outlet of the heat treatment oven.

The invention also relates to the use of the method in a heat treatment oven, comprising a hood as defined above, at least at the inlet and / or the outlet thereof. This hood will preferably be arranged with its intake chamber placed above the parts to be treated. it is also possible to place this hood in the lower part of the oven. Of course, in this case, the perforated plate of the intake chamber will be opposite the passage of the objects to be treated, while the curtains which allow the confinement of the flow of homogeneous laminar gas will be suspended from the upper part of the oven. In other cases, it is possible or desirable to use a hood placed in the upper part of the oven and provided with its curtains, while a second intake chamber is placed in the lower part of the oven so as to that the flow of inert gas leaving the perforated plate of this second chamber is located between the curtains of the upper hood.

According to a preferred embodiment, there will be a hood at each end of the oven, the pressure of inert gas injected into each of the hoods being different, the pressure losses induced by each curtain of gas being different from each other, so as to modify the relative value of the gas flows entering and leaving the furnace. It is thus possible to orient the flow of said heat treatment gases in the desired direction relative to the direction of advance of the parts to be treated. In particular, it is possible to direct the flow of gases against the current in the direction of advance of the parts, according to the type of heat treatment to which said parts are subjected. In some cases, this pressure difference may result in the absence of injection of inert gas into one of the hoods.

• la figure 1, les variations de pression dans un four de traitement thermique avec et sans hotte,
• la figure 2, une disposition schématique d'un four ouvert,
• la figure 3, une vue de face et une vue de coupe d'une hotte utilisée dans le procédé suivant l'invention,
• la figure 4, les différentes dispositions possibles des hottes dans un four selon l'invention,
• la figure 5, une courbe montrant l'influence d'une hotte sur la concentration en espèces oxydantes à l'entrée d'un four ouvert continu de recuit de tubes en acier,
• la figure 6, une courbe montrant l'influence d'une hotte sur la répartition des gaz à l'intérieur d'un four,
• la figure 7, une courbe montrant des profils de concentration en gas carbonique et en eau à l'entrée d'un four de recuit en continu de feuillards,
• la figure 8 illustre un exemple de réalisation du procédé selon l'invention, avec zonage du four,
• la figure 9 représente une variante préférentielle de réalisation de l'invention.

The invention will be better understood with the aid of the following embodiments, given without limitation, together with the figures which represent:

• FIG. 1, the pressure variations in a heat treatment oven with and without a hood,
• FIG. 2, a schematic arrangement of an open oven,
• FIG. 3, a front view and a sectional view of a hood used in the method according to the invention,
• FIG. 4, the different possible arrangements of the hoods in an oven according to the invention,
• FIG. 5, a curve showing the influence of a hood on the concentration of oxidizing species at the entrance to a continuous open furnace for annealing steel tubes,
• FIG. 6, a curve showing the influence of a hood on the distribution of gases inside an oven,
• FIG. 7, a curve showing profiles of carbon dioxide and water concentration at the inlet of a continuous strip annealing furnace,
• FIG. 8 illustrates an exemplary embodiment of the method according to the invention, with zoning of the oven,
• Figure 9 shows a preferred embodiment of the invention.

ou

In FIG. 1, a heat treatment furnace is shown diagrammatically comprising successively an inlet zone H ₁ followed by the hot heat treatment zone HZ, followed by a cooling zone CZ at the end of which is the zone H ₂ outlet. In this example, the injection of heat treatment gas takes place at the point GI substantially in the zone of separation of the hot zone HZ and the cooling zone CZ. The curves shown above the schematic view of this furnace show the pressure on the ordinate and the distance from the point considered with respect to the inlet zone of the furnace on the abscissa. The curve Ci represents the pressure variations of the heat treatment gas injected at point GI for a conventional open oven according to the prior art. In this case, the maximum pressure of the heat treatment gas is located in GI, point of injection of this gas, the pressure of the gas, which moves away on the one hand towards the hot zone and on the other hand in the direction of the cooling zone, being equal in the zones H ₁ and H ₂ to atmospheric pressure. Curve C ₃ shows the profile of the pressures in the oven after having placed a homogeneous gas curtain according to the invention at the ends thereof. The pressure is then maintained at a maximum at the gas injection points to decrease to a value which remains above atmospheric pressure in the vicinity of the inlet and / or outlet zones of the furnace. If Pa denotes atmospheric pressure, Ph _max the maximum pressure in the hood, Pt _max the maximum pressure in the heat treatment zone and Pf _max the maximum pressure in the cooling zone of the oven the process according to the invention , in a preferred mode, is characterized by one of the following relationships:

or

In practice, Pt Pf max or max is in the order of 10- ¹ to 10- ² Pascal above atmospheric pressure.

FIG. 2 represents a schematic view of an open furnace with a stainless steel annealing mat, according to the invention. this oven successively comprises an inlet hood H ₁ described in more detail below, a zone for introducing IZ of the parts to be treated, of length L _l , a heat treatment zone HZ, of length L ₂ , then a zone cooling unit CZ, of length L ₃ which ends with a Hz hood identical to the hood H ₁ . Different gas injection points are provided in particular substantially in the middle of the cooling zone CZ, the injection point Gl ₁ , at the limit of the cooling zones CZ and of the heat treatment HZ the injection point G1 ₂ , at the entrance to the heat treatment zone HZ the injection point G1 ₃ and at the entrance to the zone IZ the injection point G1 ₄ .

Figure 3 shows on its part 3A a front view and on its part 3B a sectional view of a hood according to the invention. It consists of a supply channel 100 of inert gas connected to the inlet of the preadmission chamber 103. The latter, of substantially cylindrical shape, of diameter substantially equal to that of the height of the zone 107 of the chamber intake (see below) comprises two zones having substantially the same volume, a first zone 120, followed for a second zone delimited by two perforated plates 101, 102 between which is disposed a rock wool mattress 104. The wall perforated 102 opens into the intake chamber 105 of substantially parallelepiped shape. It has an upper wall 106 and a lower wall 109 perforated, this wall being coated with a rock wool mattress 110, itself covered by a second perforated wall 108. Between the wall 108 and the upper wall 106 of this chamber there is a gas expansion chamber 107. The height of this expansion chamber is at least equal to the height of the rock wool carpet 110. The intake chamber 105 is bordered laterally by walls 111 and 112 as well as 121 and 122. Towards the lower part of said walls 111 and 112 are located two fixing strips 115, 116 parallel to said walls to which are hung two refractory curtains 113, 114. The height of these curtains is such that those these come into contact with the advance conveyor of objects in the oven.

FIG. 4 represents different possibilities of fixing the hoods in an oven, the same elements as those of the preceding figures bearing the same references.

FIG. 4A schematically represents a hood fixed in the upper part of the oven, FIG. 48 represents a hood fixed in the lower part of the oven, while FIG. 4C represents a variant with two diffusion chambers and a single pair of curtains.

In FIG. 4A, 150 and 151 respectively represent the upper and lower walls of the furnace. The refractory curtains 113 and 114 extend substantially to the bottom wall 151 of the oven.

In FIG. 48, the refractory curtains 113, 114 are fixed by their fixing strips 115, 116 to the wall upper 150 of the oven, while the expansion chamber 205 (identical to the chamber 105 previously described) is fixed to the lower wall 151 of the oven, the perforated plate of said chamber 105 being well oriented towards the upper wall 150 of the oven. The gas is injected into the chamber 205 via the pipe 203, the ends of the curtains 113 and 114 arriving substantially at the level of the perforated wall of the chamber 205.

FIG. 4C shows a variant with a single pair of curtains and two intake chambers 105 and 205 respectively. The relative arrangements of the two chambers 105 and 205, substantially identical to each other, are such that the refractory curtains 113 and 114 in a vertical position surround the intake chamber 205, so as to maintain the gas injected through the pipes 103 and 203 between said curtains 113 and 114.

Example 1

The example below relates to a continuous open furnace for annealing steel pipe. The atmosphere used in this annealing furnace has substantially the following composition: 10% of H ₂ , 8% of CO, 4% of CO ₂ , 78 of N ₁ (by volume), dew point: approximately 0 ° C.

This oven has a P.H.Z. 3.50 meters in length followed by a heat treatment area at around 900 ° C. In the preheating zone, the steel tubes are gradually brought to the temperature of the hot zone.

FIG. 5 illustrates, using the curves J ₁ and J _{2 respectively} , the ratio of the concentrations of carbon dioxide and carbon monoxide as a function of the distance in the furnace relative to the inlet zone. In this comparative example, a hood having the structure shown in FIG. 3 with the dimensions given below had been installed at the inlet of the oven, the outlet of the latter taking place directly on the ambient atmosphere. Curve J ₁ represents the ratio of CO / CO ₂ concentrations in the absence of a homogeneous laminar flow of nitrogen in the hood, while curve J ₂ represents the same concentration ratio with a homogeneous and laminar flow of nitrogen between the refractory curtains of said hood. It is evident that the ratio of said concentrations is substantially constant over the entire length of the preheating zone of the oven, when a homogeneous and laminar curtain of nitrogen circulates between the refractory curtains. This shows the advantage of using a hood according to the invention, since there is thus found at the entrance of the oven the reducing nature of the atmosphere vis-à-vis the treated metal.

• Largeur: 1 m
• Profondeur: 0, 15 m
• Epaisseur matelas de laine de roche: 0,05 m
• Hauteur chambre d'expansion: 0, 10 m
• Diamètre perforations: 2 mm
• Entre-axes de deux perforations successives: 4 mm
• Pas de chambre de pré-admission.

The geometry of the hood used was as follows:

• Width: 1m
• Depth: 0.15 m
• Rock wool mattress thickness: 0.05 m
• Expansion chamber height: 0.10 m
• Perforation diameter: 2 mm
• Between axes of two successive perforations: 4 mm
• No pre-admission room.

The nitrogen flow in the hood was 10 N _M ³ per hour.

Example 2

This example was carried out using the furnace shown in FIG. 2.

The oven is an open oven with stainless steel annealing mat. The different atmospheres injected at points GI ₁ , G1 ₂ , G1 ₃ , G1 ₄ of the oven are shown in the table below:

Figure 6 shows the hydrogen concentrations in the furnace.

Curve D ₁ represents the hydrogen concentration in the furnace in the absence of a hood, while curve D ₂ represents the hydrogen concentration in the furnace using the method according to the invention, summarized in the table above. The injection point G1 ₂ is located at the limit of the heat treatment heating zone and the furnace cooling zone. According to the invention, the hydrogen is almost exclusively directed to the cooling zone of the furnace. The parts taken out of the oven show no trace of oxidation.

Curve D ₁ (oven without hood) shows that, practically over the entire length of the hot zone HZ of the treatment oven, (4 meters in this example), there is a significant concentration of hydrogen. This varies from approximately 25% at the injection point (7 meters from the inlet area) to approximately 1% at 3 meters from the oven inlet area. In the middle of this hot zone, there is a concentration of about 10% in hydrogen.

Curve D ₂ (oven with hoods according to the invention) shows that the hydrogen concentration is of the order of 1% at about 6 meters from the inlet of the oven, 3/4 of the hot zone not having d 'hydrogen. On the other hand, the hydrogen concentration profile with or without a hood in the CZ cooling zone is substantially identical.

This example shows the possibilities of precise zoning of heat treatment furnaces using the method according to the invention.

Example 3

This example was carried out in the furnace of FIG. 1. The heat treatment zone HZ was at a temperature of 800 ° C., with an injection of gas at the point GI at the limit between the hot zone HZ and the cooling zone CZ . In the present case, a hood was placed only at the inlet H ₁ of the hot zone, no hood being arranged at the outlet. The atmosphere injected is identical to that of Example 1, an atmosphere well known to those skilled in the art for the annealing of steel strips.

FIG. 7A represents the concentration of carbon dioxide in the atmosphere of the furnace respectively without hood (E1) and with hood (E2), as a function of the abscissa of the measurement point in the furnace relative to the inlet thereof. this.

It can be seen that at about 6 meters from the entrance to the furnace, for a heat treatment zone with a total length of 20 meters, the concentration of C0 ₂ is the same in both cases, while there is a decrease of half the concentration of C0 ₂ at 1 meter from the inlet, in the case of an oven provided with an inlet hood according to the invention.

In the latter case, the concentration of C0 ₂ at the inlet of the oven is substantially identical to that of the atmosphere injected into the oven, which shows the absence of entry of oxidizing species into the oven using the process according to the invention.

The curves F ₁ and F ₂ of FIG. 7B represent the variations of the dew point in ° C in an oven respectively without hood and with hood relative to the abscissa of the measurement point thereof relative to the input . The dew point is significantly lowered, with a hood (curve F ₂ ) which is substantially identical in both cases 8 meters from the entrance to the oven. Consequently, the concentration of H ₂ 0, oxidizing species, in the oven using the method according to the invention is also kept constant until the inlet of the oven.

In these two examples, the flow rate of neutral gas in the hoods, that is to say nitrogen in the present case, was 2.5 m ³ / hour.

• Pt_maxi ⁼ Pf_maxi ^> Ph_maxi ^> Pa

FIG. 8 illustrates a preferred embodiment of the method according to the invention, requiring at least two points for injecting gas into the treatment oven. This variant is characterized by equal pressures at the injection points GI and G'.I '. from the oven. This makes it possible to obtain a zone CD in the oven in which the gas pressure is substantially identical. Consequently, an excellent “zoning” of the oven is thus achieved since the gas coming from point G. 1. will go almost exclusively towards the outlet AB of the oven, while the gas coming from injection point G '. the. will go almost exclusively to the EF outlet of the oven. Only a diffusion of gases takes place in the CD zone, diffusion at very low speed. If, for example, this alternative embodiment is applied to the furnace of FIG. 2, by choosing to inject gas in G.1 ₂ . and G.1 ₃ . only, that is to say at the entry and exit of the hot zone HZ, this will have the characteristics of the zone CD described above. In particular, note the following relationships concerning pressures:

• Pt _max ⁼ Pf ^_max> Ph ^_max> Pa

Of course, we can keep the CD area properties while achieving other gas injections into the oven in areas 8C and DE oven at lower pressure Pt and Pf _{max maxi-}

Figures 9A and 9B show a preferred variant of the invention in which a gas curtain inert or inactive (N ₂ in the figure) is used at the entrance of the oven only.

In FIG. 9A, the oven is shown diagrammatically, seen in section, only at its inlet 303 and its outlet 304. At the inlet 303 of the oven is placed a hood 305 provided with refractory curtains 306 and 307, such as 'illustrated in Figures 3 and 4, this hood being integral with the upper part 301 of the oven. The refractory curtains have their lower end located near the lower part 302 of the oven, generally provided with a conveyor belt for advancing objects such as 308. A distance of the order of a few centimeters between the lower end of the curtains 306 and 307 and the lower part 302 of the oven is well suited in practice. No particular device is placed at the outlet 304 of the oven. To determine the flow of inert or inactive gas (generally nitrogen) which must be injected into the hood 305 as described above, first of all, measurements are made at the curtains 306 and 307, in the absence nitrogen injection, the air flow that enters the oven by natural convection phenomena. This measurement is carried out using a hot wire, in a manner known per se.

The same flow of nitrogen is then injected into the hood. It can be seen, as shown by the arrows in the figure, that the nitrogen flows between the curtains, then enters the oven in place of the air. This, although drawn towards the entrance, flows along the curtain 306 without penetrating between them. It is easy to verify the significant decrease in the oxygen level in the oven by measuring the concentration thereof using a probe placed in the oven, beyond curtain 307.

In FIG. 9B, the same elements as those in FIG. 9Aa have the same references. The hood 305 is placed, in this variant, in the lower part of the oven, without refractory curtains. The nitrogen flow is adjusted as described above. It is noted as above that the air arriving near the inlet of the oven does not penetrate into it but is entrained upwards by the atmosphere current leaving the upper part of the inlet of the oven.

The use of the method illustrated in FIG. 9 makes it possible to reduce the flow rates of atmosphere used in the heat treatment furnaces, whatever the number and the nature of the gas injection points therein, for its rate of oxygen determined in the hot zone of the oven. For example, a continuous oven having an inlet area of 2m, a hot area at 800 ° C of 5m and a water cooling area of 10m, as well as an inlet section of approximately 0.2 m ² , consumed when its two ends were open 100 Nm ³ / h of nitrogen to achieve a protective atmosphere intended for the annealing of copper parts. After placing two refractory curtains (whose lower end is less than 5 cm from the bottom of the oven) and the appropriate hood at the entrance to the entry area, the air speed is measured at l entry of the oven, in the absence of nitrogen in the hood. This is 37 cm / s. Nitrogen is then injected at 37 cm / s into said hood, which corresponds to a flow rate of 30 Nm ³ / h of nitrogen. The nitrogen flow rate in the oven can then be reduced to 20 Nm ³ / h, for an identical quantity of the products leaving the oven. There is therefore an overall reduction of 50% in the nitrogen flow rates in this furnace.

Claims (18)

1. Process for the heat treatment of objects in a continuous oven, wherein the objects to be treated are introduced successively by a movable carrier into the oven comprising at least one heat treatment section (H.Z.) into which is injected an atmosphere, of specific composition, the inlet and/or outlet sections of the oven comprising means producing a substantially luminar stream of inert or non-reactive gas under the conditions of treatment so as to prevent the entry of air into the oven, characterised in that the stream of inert or non-reactive gas at the end of the oven is in the form of a single homogeneous curtain with vertical flow in a transverse plane of a horizontal end portion of the furnace and traversed by the direction of advance of the workpieces to be treated, the injection of the inert or non-reactive gas occurring, after homogenisation of its speed and of its pressure, under conditions such that a substantially laminar flow state is maintained over the full height of the gas curtain.

2. Process according to claim 1, characterised in that the curtain of inert or non-reactive gas extends over substantially the full height of the horizontal end portion of the oven.

3. Process according to claim 2, characterised in that the means generating an atmosphere of inert or non-reactive gas comprise two screens of refractory material extending substantially as far as the movable carrier, between which the curtain of inert or non-reactive gas is injected downwards from above.

4. Process according to one of claims 1 and 2, characterised in that the gas curtain is established by the upward injection from below of inert or non-reactive gas.

5. Process according to claim 4, characterised in that the injection of inert or non-reactive gas occurs in a substantially vertical plane.

6. Process according to one of claims 1 to 5, in which means generating an atmosphere of inert or non-reactive gas are situated at least at the inlet of the continuous oven, characterised in that the flow rate of inert or non-reactive gas injected by the said means is equal to the rate of flow of air penetrating into the oven measured in the absence of a flow of inert or non-reactive gas in the means generating the atmosphere of the said gas.

7. Process according to one of claims 1 to 6, in which the heat treatment oven comprises two gas injection points, characterised in that the gas injection is effected at identical pressure at these two points in such a way as to maintain a section of equal pressure between them the gases injected flowing on each side of this section.

8. Process according to one of claims 1 to 7, characterised in that the gas pressures within the oven are linked by one of the following relationships:

or

in which

-Pa is the atmospheric pressure;

-Ph max is the maximum pressure of the laminar stream of inert or non-reactive gas;

-Pt max is the maximum pressure in the heat treatment section;

-Pf max is the maximum pressure in the cooling section of the oven.

9. Hood for application of the process according to one of the claims 1 to 8, wherein are provided:

-means for injecting non-reactive gas into an intake chamber (105) of which the bottom (109) is perforated,

-at least one refractory screen (113,114) on each side of the gas flow, movable around an axis (115,116) situated in the plane of the screen (113,114) and oriented so as to be placed in the passage of the workpieces which are to be treated.

characterised in that it also comprises:

-means (110) permeable to the non-reactive gas situated on the perforated bottom (109) of the intake chamber (105) allowing a very low speed to be imparted to the flow of gas emerging from the perforated plate (109), without causing a substantial pressure drop at the level of the said flow, in such a way as to generate between the refractory screens (113,114) a curtain of gas which is substantially homogeneous and laminon over its whole height.

10. Hood according to claim 9, in which the intake chamber (105) comprises a substantially rectangular perforated bottom (109), the length of which is equal to the width of the oven on which the hood is intended to be installed, characterised in that the speed of the inert non-reactive gas is substantially identical at every point of traversal of the perforated plate and lower than:

with

n = viscosity of the neutral gas which is inert at ambient temperature

p = volumetric mass of the neutral gas in normal conditions,

a = width of the oven,

b = depth of the diffuser plate

11. Hood according to one of claims 9 or 10, characterised in that the injection of gas into the intake chamber (105) is effected substantially symmetrically with respect to the direction of intake of the said gas into the said chamber (105).

12. Hood according to one of claims 9 or 10, characterised in that the injection into the intake chamber (105) is effected via a pre-intake chamber (103), the said pre-intake chamber (103) being separated from the intake chamber (105) by means permeable to the inert gas (110), allowing a very low speed to be imparted to the gas during its penetration into the intake chamber (105) without causing a substantial pressure loss at the level of the gas flow.

13. Hood according to claim 12, characterised in that the pre-intake chamber (103) also comprises two perforated walls (120,102) between which are situated means (104) permeable to the gas.

14. Hood according to one of claims 9 to 13, characterised in that the height of the intake chamber (105) is at least greater than twice the thickness of the gas permeable means (110).

15. Hood according to one of claims 9 to 14, characterised in that the said gas-permeable means (110) are selected from calcined materials, rock wool, glass wool, quartz wool.

16. Hood according to one of the claims 9 to 15, characterised in that the thickness of the gas-permeable means (110) is substantially identical over the whole surface of the perforated bottom (109) of the intake chamber (105) and is no smaller than 2 centimetres.

17. Hood according to one of claims 9 to 16, characterised in the height of the intake chamber (105) is substantially constant.

18. Utilisation of the hood according to one of the claims 9 to 17 in heat treatment ovens.

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