EP0415811A1 - Heat treatment furnace with cooling means - Google Patents

Abstract

The furnace according to the invention comprises an enclosure (1) of tubular shape closed on one side by a bottom and, on the other side, by a leakproof door (3) and a refractory casing (6) arranged inside the enclosure and fitted with a door (8) situated away from a bottom (7). A turbine (T) housed between the two bottoms delivers a flow of gas cooled with the aid of an exchanger (23, 24) into at least two injection conduits (12, 13) connected to two orifices provided at two opposite locations of the side wall of the casing (6). The intake orifice (21) of the turbine (T) is in communication with the internal space of the casing (6) by virtue of an exit orifice (22) made in the bottom of this casing (6). The invention makes it possible to obtain homogeneous cooling of the articles to be treated. <IMAGE>

Description

The present invention relates to an oven for the thermal or thermochemical treatment of metals such as, for example, general purpose steels, fine carbon steels and alloy steels.

Generally, it is known that for these metals, the heat treatment cycles usually follow a heating phase, an isothermal maintenance phase at the treatment temperature, and a cooling phase in one or more stages, the parameters of which essential are the cooling rate, the final temperature, the duration of isothermal maintenance and the cooling medium.

It turns out that certain treatments, such as stepped quenching and, in particular, bainitic quenching require relatively high cooling rates (speed greater than the critical quenching speed). On this subject, it is recalled that in the case of bainitic quenchings, this cooling at high speed is continued until a temperature lower than that of perlite formation, while being higher than the temperature at which the transformation of austenite begins. in martensite.

In treatment ovens in a rarefied atmosphere, the cooling phase is carried out by means of a cooling fluid which is circulated inside the furnace by means of a turbine.

This turbine is generally placed in the intermediate volume between the sealed enclosure of the furnace (possibly coated with an insulating layer) and the refractory box called "casing" which delimits the laboratory of the furnace. It generates a flow of coolant in closed cycle which is injected onto the parts to be treated thanks to at least one orifice made in the casing. This fluid then returns to the turbine after passing through a heat exchanger intended to evacuate the calories outside the oven.

It turns out that in conventional furnace structures, the flow of fluid enters the casing through an inlet port located on one side (generally on the upper face) and exits through a suction port located at the 'opposite. It is noted, in use, that this arrangement has drawbacks due to the fact that the cooling of the parts is not carried out homogeneously and that a temperature gradient occurs between the side of the parts oriented towards the orifice. intake side with the side toward the suction port. This drawback becomes particularly important in the case where it is desired to obtain relatively high cooling rates: in this case, the temperature gradients become relatively large and risk causing harmful deformations, or even deterioration (tapures) of the parts. .

The object of the invention is more particularly to eliminate these drawbacks by means of simple arrangements, inexpensive, but nevertheless effective.

To this end, it offers a furnace for the thermal or thermochemical treatment of metals of the type comprising a tubular enclosure closed, on one side, by a bottom and, on the other side, by a sealed door, a refractory box or casing arranged inside the enclosure, this box delimiting a laboratory accessible from the outside thanks to a door of refractory material located opposite a bottom, facing the sealed door of the enclosure, heating means arranged inside the box, means making it possible to generate a relative vacuum or to inject a gas inside the enclosure, and a turbine arranged coaxially with the enclosure in the interval between the two aforesaid funds and comprising an inlet orifice in communication with the interior volume of the box through an outlet orifice made in the bottom of said box, this turbine generating a circulation of gas inside the oven according to a ci rcuit closed including at least a first heat exchanger located between said outlet orifice and said inlet orifice.

According to the invention, this furnace is more particularly characterized in that the turbine delivers in at least two injection conduits respectively connected to two injection orifices provided at two opposite locations on the side wall of the box, each of these conduits being fitted with a closure device capable of dividing the gas flow it receives from the turbine into two complementary fractions, namely: a fraction injected into the box and a fraction injected into the intermediate volume between the box and the enclosure, and in that this intermediate volume is in communication with the intake orifice of the turbine, via a heat exchanger which may consist of all or part of said first exchanger.

Advantageously, the injection orifices are equipped with injection nozzles oriented obliquely, in the direction of the cabinet door, so as to generate a uniform distribution of the cooling gas flows over the parts.

According to another characteristic of the invention, the above-mentioned injection nozzles are provided with valves (or registers) making it possible to close the injection orifices while putting the injection conduits in communication with the intermediate volume comprised between the box and the enclosure. In this case, means are also provided so that the inlet opening of the turbine is also in communication with the intermediate volume. These arrangements therefore make it possible to obtain an auxiliary fluid circuit which by-passes the box and, consequently, to produce a system for controlling the temperature of the parts contained in the furnace independent of the speed of rotation of the turbine.

Such an arrangement therefore makes it possible to avoid the problems due to start-ups and to the speed regulation of the engine driving the turbine, problems which are rarely resolved satisfactorily.

• La figure 1 est une coupe axiale schématique d'un four de traitement thermique selon l'invention ;
• La figure 2 est une coupe transversale du four repré­senté sur la figure 1 ; et
• La figure 3 est une représentation schématique du circuit d'asservissement associé au four.

An embodiment of the invention will be described below, by way of nonlimiting example, with reference to the appended drawings in which:

• Figure 1 is a schematic axial section of a heat treatment oven according to the invention;
• Figure 2 is a cross section of the oven shown in Figure 1; and
• Figure 3 is a schematic representation of the control circuit associated with the oven.

As shown in Figures 1 and 2, the oven firstly comprises a cylindrical enclosure 1 closed on one side by a curved bottom 2 and, on the other side, by a tin door che 3, the whole resting on the ground thanks to a base 4. Of course, this enclosure 1 can be single or double wall with intermediate water circulation, and possibly be coated internally with a layer of thermally insulating material. The interior volume of the enclosure 1 is connected to means 5 making it possible to achieve a relative vacuum as well as to means for injecting a treatment gas or a cooling gas.

Inside this enclosure 1 is disposed a box 6 of thermally insulating material which has a parallelepiped shape substantially coaxial with the enclosure 1 and which is closed, on one side, by a bottom 7 and, on the other side , by a door 8 substantially adjacent to the door 3 of the enclosure 1.

This box 6 delimits the laboratory inside which the parts P to be treated are deposited in order to be treated. In this example, the parts P are heated by radiation thanks to electric heating resistors 9 arranged inside the box 6.

The cooling of the parts P is, for its part, carried out by means of a cooling circuit involving a turbine T substantially coaxial with the enclosure 1 and disposed in the intermediate volume comprised between the bottom 2 of the enclosure 1 and the bottom 7 of the box 6. This turbine T is driven by an electric motor M housed in an external bell 11 fixed in leaktight manner on the enclosure 1, and the drive shaft 11 of which passes through the bottom 2.

This turbine T delivers in two gas injection circuits 12, 13 connected to two injection nozzles 14, 15 provided at two opposite locations on the lateral surface of the box 6.

In this example, these two injection circuits 12, 13 pass through two axial protrusions 16, 17 of the enclosure te, which have been designed so as to be able to reduce the diameter of the enclosure 1 to the strict minimum.

As previously mentioned, to obtain better temperature uniformity during the cooling phase, the nozzles 14, 15 are oriented obliquely to the longitudinal axis of the oven and are oriented towards the door 8 of the box 6.

Furthermore, these nozzles 14, 15 are fitted with valves 19, 20 made of thermally insulating material which make it possible to interrupt the flow of gas coming from the turbine T and to divert it into the intermediate space VI comprised between the enclosure 1 and the box 6.

The return of the air injected by the turbine T can take place in the following two paths.

In the case where the valves 19, 20 are in the open position, and where, consequently, the entire gas flow is injected into the box 6, the return of the gas to the inlet orifice 21 of the turbine T is effected by means of an outlet orifice 22 provided in the bottom 7 of the box 6 which opens into the intermediate volume VI by a first exchanger 23 in which a cooling fluid circulates. The gas flow, included in this intermediate volume VI, is then sucked through the inlet orifice 21 of the turbine T through a second exchanger 24 also traversed by a cooling fluid.

In the case where the valves 19, 20 are in the closed position and the gas flow is injected into the intermediate volume VI, this flow will be directly sucked in by the turbine T through the exchanger 24.

Of course, to ensure the continuity of the thermal insulation during the heating phase, the orifice of outlet is fitted with a thermally insulating door G, here of guillotine type with double covers 25, 26.

Advantageously, the two heat exchangers 23, 24 may be produced using a single coil arranged in the space between the door G and the turbine T. In this case, the separation between the two exchangers is simply carried out at by means of a radial partition 30 arranged coaxially between the door G and the turbine T, so as to force the flow of gas emanating from the box 6 to take a sinuous path passing through the two exchangers 23, 24 before reaching the orifice intake 21 of the turbine T.

The operation of the oven previously described will be described below, with reference to Figures 1 and 3 in which the corresponding elements have the same reference numbers.

During the heating phase of a treatment cycle, the valves 19, 20 and the door G are closed so as to excite as much as possible a heating contained in the intermediate volume VI situated between the box 6 and the enclosure 1 thus as the latter heating up. The parts P are then heated by means of the resistors 7 contained in the box. During this heating, the interior volume of the enclosure 1 can be brought to a low pressure so as to obtain a relative vacuum. Likewise, means (not shown) can be provided to produce, inside this enclosure 1 and, in particular, inside the box 6, a treatment atmosphere, for example by injection on the parts P d '' a process gas. Likewise, this furnace can also include equipment making it possible to carry out ion bombardment treatments and / or material deposits on the parts by PVD type processes (physical deposits and vapor phase).

During the cooling phase, a neutral gas, serving as a heat transfer vehicle, is injected into the oven; as a result of which the valves 19, 20 and the door G are open, while the turbine T is started. At the same time, a stream of refrigerant is circulated in the exchangers 23 and 24.

When the valves 19, 20 are fully open, the gas flow produced by the turbine is completely injected onto the parts P contained in the box 6. This flow then passes through the two exchangers before reaching the inlet orifice 21. It is clear that in this configuration, a maximum speed of cooling of the parts P is obtained without any disturbance of the discharge circuit on the intake circuit. The cooling gas passes entirely through the charge.

On the other hand, when the valves 19, 20 are closed, the parts P are no longer cooled and the entire gas flow passes through the intermediate volume VI before returning to the turbine T, after passing through the exchanger 24. It is advisable to note in this regard that, in this case, the air flow will only be very little heated by the box and does not require significant cooling means. Because it only crosses the exchanger 24, it therefore undergoes a less significant pressure drop, which results in a reduction in the drive torque of the turbine T and, consequently, a reduction in the electrical consumption of the engine. Mr.

It is thus possible to rapidly cool the load to a determined temperature (bainitic quenching) and maintain this temperature for the time necessary for transformation, by successive opening and closing of the valves and then cooling completely to defrost.

It is clear that between the two operating modes described above, there is an intermediate operating mode, in which the valves 19, 20 are partial Lely open and where a variable fraction of the air flow is injected on the parts P, while the remaining fraction returns to the turbine T via the intermediate volume VI and the exchanger 24.

The invention takes advantage of this particularity to effect a servo-control of the position of the valves as a function of the temperature of the parts, detected by one (or more) sensor 31 and a previously established cooling law.

In this case, the valves (which act as three-way valves) are equipped with a motorization so that they can be controlled, from outside the oven, by a servo circuit 32, preferably with microprocessor, which receives a signal representative of the temperature of the rooms as indicated in FIG. 3. An important advantage of this mode of control consists in that it does not intervene on the motor M of the turbine T, which rotates at constant speed, continuously during the entire cooling phase. It is therefore no longer necessary to provide variable speed drives or special provisions due to frequent starting and stopping of the M motor.

Of course, the invention is not limited to the embodiment described above.

Thus, for example, the oven may have a modular structure comprising at least one module whose cross section corresponds substantially from the point of view of form, to the longitudinal section of the oven shown in FIG.

More precisely, this module can have a tubular enclosure 1 oriented perpendicular to the plane of FIG. 1 and comprising two side walls instead of and in place of the bottom 2 and the door 5.

This enclosure can be closed at each of its ends by means of a sealed door or come to be connected by each of said ends to the enclosure of an adjacent module.

Similarly, the box 6 may have a tubular shape of rectangular section which extends coaxially to the enclosure and can be closed at each of its ends by a thermally insulating door. Of course, the references 7 and 8 designate here the side walls of the box and not a bottom and a door.

The cooling circuit fitted to the module can be substantially identical to that previously described and will therefore not be explained again.

Claims (10)

1. Furnace for the thermal or thermochemical treatment of metals of the type comprising an enclosure (1) of tubular shape closed, on one side, by a bottom (2) and, on the other side, by a sealed door (3), a refractory box or casing (6) disposed inside the enclosure (1), this box (6) delimiting a laboratory accessible from the outside by a door (8) made of refractory material located opposite d 'a bottom (7), facing the sealed door (3) of the enclosure (1), heating means (9) arranged inside the box (6), means (5) allowing generate a relative vacuum or inject a gas inside the enclosure (1), and a turbine (T) arranged coaxially with the enclosure (1) in the interval between the two aforesaid funds (2, 7) and comprising an inlet orifice (2) in communication with the interior volume of the casing by means of an outlet orifice (22) produced in the bottom (7) of said casing (6), this turbine generates t a gas circulation inside the furnace according to a closed circuit including at least a first heat exchanger (23) located between said outlet orifice and said inlet orifice,
characterized in that the turbine (T) delivers in at least two injection conduits (12, 13) respectively connected to two injection orifices provided at two opposite locations on the side wall of the box, each of these conduits being provided with '' a shutter device capable of dividing the gas flow it receives from the turbine into two complementary fractions, namely: a fraction injected into the box and a fraction injected into the intermediate volume between the box (6) and the enclosure (1), and in that this intermediate volume (VI) is in communication with the intake orifice (21) of the turbine (T), via a heat exchanger which may consist in whole or in part of said first exchanger. 2. Oven according to claim 1,
characterized in that the injection orifices are equipped with injection nozzles (14, 15) oriented obliquely, in the direction of the door (8) of the box (6). 3. Oven according to claim 1,
characterized in that the intermediate volume (VI) is in communication with the internal volume of the box (6) via a first exchanger (23), and with the intake orifice (21) of the turbine ( T) via a second exchanger (24), the air flow from the box (6) passing through the two exchangers (23, 24) before entering the turbine. 4. Oven according to claim 3,
characterized in that the above exchangers (23, 24) are produced using a coil arranged in the space between the bottom (7) of the box (6) and the intake orifice (21) of the turbine (T), and in that the separation between the two exchangers (23, 24) is carried out by means of a radial partition (30) disposed in the central region of said space. 5. Oven according to one of the preceding claims,
characterized in that the outlet orifice (22) of the box (6) is equipped with a door (G). 6. Oven according to one of the preceding claims,
characterized in that the aforementioned closure devices (19, 20) are actuated by means controllable from outside the oven, as a function of the temperature of the parts (P) and of a previously established cooling law. 7. Furnace for heat or thermochemical treatment of metals comprising at least one module comprising a tubular enclosure (1), a box (6) of tubular shape which extends inside the enclosure (1), means for heater arranged inside the box (6), means making it possible to generate a relative vacuum or injecting a gas inside the enclosure (1) and a turbine (T) making it possible to circulate a gas inside the module, characterized in that the turbine (T) is arranged in the interval between the side walls of the enclosure (1) and of the box (6), in that this turbine (T) delivers in at least two injection conduits (12, 13) respectively connected to two orifices provided in two opposite locations of the wall of the box (6), and in that the inlet orifice (21) of the turbine ( T) is in communication with the interior volume of the box (6) through a third orifice made in the box at a location remote from that of the first two orifices. 8. Oven according to claim 7,
characterized in that the enclosure (1) can be closed at each of its ends by a sealed door. 9. Oven according to claim 7,
characterized in that the enclosure (1) is connected by at least one of its ends to the enclosure of an adjacent module. 10. Oven according to one of claims 7 to 9,
characterized in that the aforesaid injection orifices are provided with shutters (19, 20) allowing them to be closed while putting the injection conduits (12, 13) in communication with the intermediate volume comprised between the box and the enclosure (1), and in that this intermediate volume (VI) is in communication with the interior volume of the box (6) and with the intake orifice (21) of the turbine (T) by means of at least one heat exchanger in which a refrigerant circulates.

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