EP0451050A1 - Process and equipment for thermal treatment with gas quenching - Google Patents

Abstract

Heat treatment of objects with quenching in a gaseous medium with recirculation (8) on contact of the said treated objects with cooling of the quenching gas by means of a heat exchanger, of the kind in which helium is employed as quenching gas, which is stored under pressure while waiting in a buffer vessel (2) into which, at the end of a quenching operation, the helium charge is extracted (27-28) out of the treatment enclosure, as a final stage using a pump (5) until a primary vacuum is obtained, the said extracted helium is raised to a purification pressure by a compressor (14) used in combination with a mechanical filter (15), and the said helium is taken to purification pressure in a purifier for removing impurities (20-35) in order to transfer it, if appropriate, after recompression, into the buffer vessel (2). <IMAGE>

Description

The present invention relates to the heat treatment of objects with quenching in a gaseous medium. In this type of technique, the quenching gas is caused to recirculate in contact with said treated objects with cooling of the quenching gas by means of an external heat exchanger.

Heat treatments carried out under vacuum or under partial pressure of neutral or cementing gas, in so-called vacuum furnaces, often require rapid cooling of the charge at the end of the cycle. Quenching in a gaseous medium is an advantageous alternative to quenching in a liquid medium, allowing better control of the cooling kinetics, minimizing deformation of the treated parts and eliminating the necessary cleaning operations on parts quenched in a salt bath or oil bath.

From a practical point of view, vacuum ovens suitable for gas quenching are equipped with a powerful recirculation blower and the gas injected under pressure into the oven enclosure is successively moved through the charge during treatment. There is therefore convection cooling while the gas thus heated then passes through a heat exchanger, water for example, which cools it before it is returned to the load of objects being treated.

For a given furnace and charge to be treated, the rate of cooling of the parts in an atmosphere depends, among other factors, on the pressure, the recirculation speed and the nature of the gas.

For an existing oven, the gas speed is imposed by the recirculation blower. The pressure is controlled by the amount of gas injected into the oven; it is limited by the characteristics of the oven which define a maximum pressure admissible by the enclosure.

The gases commonly used for quenching are nitrogen and argon. In order to improve the cooling rates without modifying the pressure of the quenching gas or its recirculation speed, it has been proposed to use gases having better heat conductivity: hydrogen or helium. This promotes heat exchange by convection between the treated part and the cooling flow. The gain obtained on the cooling speed makes it possible to quench under gas a wider range of materials; for a given treatment, it is also possible to decrease the pressure of the gas in the furnace, lowering the stresses to which the material is subjected, or else to increase the mass of material which can be treated during a cycle.

The use of hydrogen, which is a flammable gas, requires adequate safety arrangements on quenching furnaces. Helium, which is a neutral gas, can instead be used in place of nitrogen or argon, without any technical modification of the oven. Its high cost price means that it is still little used.

The technical problem underlying the present invention is the use of helium, or a mixture based on helium, in gas quenching treatments, while remaining economically competitive with conventional treatments under argon or nitrogen and this objective is achieved in that at the end of a quenching operation, the helium charge is extracted from the treatment enclosure, in the final phase by pump until a primary vacuum is obtained, the helium extracted at a purification pressure is brought by compressor associated with a mechanical filter, and said helium is passed at purification pressure through a purifier for removing impurities, for transfer, if necessary, after recompression, in the buffer capacity. In this way, the end of quenching helium which is polluted by impurities, in particular oxygen and water, present on the charge or in the furnace, by possible air leaks and by the recovery or recompression equipment, is recovered. after careful purification.

According to a first embodiment, the helium at the purification pressure is collected in an intermediate capacity, after which it is sent to a permeation membrane separator delivering the purified helium under lower pressure, which is then dried and then discharged under pressure by the same compressor to the buffer capacity of the oven.

According to a second embodiment, the helium extracted, recompressed to a pressure higher than the waiting pressure of the buffer capacity and mechanically filtered, is brought to transfer into a device for purifying residual oxygen of the type to controlled addition of hydrogen for catalytic formation of water vapor, after which the gas is optionally cooled and dried, then transferred to said holding capacity.

According to a third embodiment, the helium extracted, recompressed at a pressure higher than the waiting pressure of the buffer capacity and mechanically filtered, is brought to transfer into a purification device of the catalytic type with oxygen scavenging. residual and regenerating the catalyst with a stream of hydrogen, after which the gas is optionally cooled and dried, then transferred to said holding capacity.

According to a fourth embodiment, the helium extracted, recompressed to a pressure higher than the waiting pressure of the buffer capacity and mechanically filtered, is brought to transfer into an oxygen purification device and possibly residual water vapor, of the molecular sieve type allowing the adsorption of oxygen and possibly water vapor, the regeneration of which is ensured by depressurization or by increasing the temperature, after which the gas is if necessary cooled and dried, then transferred to said buffer capacity.

Usually, the helium extracted, brought to a cleaning pressure, passes directly and substantially in its entirety through a purifier for removing impurities, if necessary after being collected in an intermediate capacity. It is also possible, in particular when the content of impurities in the extracted helium, to purify only a fraction of said extracted helium or to purify it only after it has been used for at least two successive quenching operations.

The invention also relates to an installation for the thermal treatment of objects with quenching in a gaseous medium incorporating at least substantially helium, of the type comprising a gas recirculation blower oven connected by valve lines on the one hand to a capacity— buffer, on the other hand to a primary vacuum pump bypassed by a valve and expansion valve line, which is characterized in that it comprises, at the pump outlet, a small equalization capacity, a compressor, a filter mechanical, a helium purifier, and if necessary, a dryer preferably trapped in water vapor by molecular sieve.

The installation may, if necessary, also include a valve line bypassing the helium purifier.

According to one embodiment, the invention provides for the interposition of an intermediate tank upstream of the purifier which is of the membrane separator type, the outlet of the purified gas of which is connected to a dryer and then by valve upstream of said compressor, whose outlet is also and directly connected by valve to the buffer tank.

According to another embodiment, the helium purifier is of the type with controlled addition of hydrogen for catalytic formation of water vapor.

According to yet another embodiment, the purification of helium is of the type with catalytic elimination of residual oxygen and regeneration of the catalyst by a stream of hydrogen.

The purification of helium can also be of the type with elimination by passage and adsorption on molecular sieve of oxygen and possibly of water vapor, the regeneration of said molecular sieves being done by depressurization or increase in temperature, possibly accompanied by passing a stream of pure gas.

The characteristics and advantages of the invention will become apparent from the following description, by way of example, with reference to the accompanying drawings which show four variants of quenching furnace installations according to the invention.

Referring to Figure 1, a vacuum quenching furnace 1 is normally connected on the one hand to a buffer tank 2 by a pipe 3 with valve 4, on the other hand, to a vacuum pump 5 by a pipe of emptying 6 with valve 7. The gas quenching is carried out, at the end of the temperature rise of the objects in the oven put under vacuum by the pump 5 (valve 7 open), by sudden discharge of the buffer capacity 2 in oven 1 by opening valve 4 (valves 7 and 27 closed).

A recirculation blower 8 ensures the homogenization of the furnace atmosphere. For a new operation, the buffer capacity 2 is recharged to a maximum pressure via a charging line 9.

Applied to the quenching under helium, in accordance with the invention, the installation described above receives a helium recovery equipment at the end of quenching, which comprises, at the outlet of the emptying pump 5, a connecting pipe. 11 towards a waiting capacity 12, which successively incorporates a valve 10 on the discharge of the pump 5, a three-way valve 13, a compressor 14, an oil separator filter 15, a three-way valve 16.

The waiting capacity 12 is connected by a pipe 17 with a valve 18 to the upstream high pressure compartment 19 of a permeator 20 with permeation membrane 21 for separation from a downstream low pressure compartment 22. The high pressure compartment 19 is connected to an evacuation pipe 23 for impurities, while the purified helium arriving in the downstream compartment under low pressure 22 is dried at 35 and necessarily maintained under intermediate pressure via the compressor 14.

On the other hand, the three-way valve 16 is connected by a line 25 to the buffer capacity 2.

The vacuum pump 5 is by-passed through a line 26 with valve 27 and pressure regulator 28.

The installation described works as follows:

A helium charge is initially introduced via line 9 into the buffer capacity 2. The quenching is carried out in the usual manner by opening the valve 4 with unloading, for example, until a substantial equilibrium of part of the helium stored in the buffer capacity 2 and the quenching operation is carried out with recirculation of the helium by the blower 8 to an exchanger-cooler (not shown).

At the end of the quenching operation, we first transfer the polluted helium from the oven 1 to the holding capacity 12 (valve 27 open, valves 7 and 10 closed, pump 5 stopped), valve three tracks 13, the compressor 14, the oil separator filter 15, the three-way valve 16 open to the holding tank 12, where the partially purified helium gas is stored under pressure thanks to the compressor 14 in operation (valve 18 closed).

As soon as a pressure close to atmospheric pressure is reached in the furnace 1, the vacuum pump 5 is put into operation (valves 27 closed, valves 7 and 10 open) with delivery to the compressor 14 which acts as previously. Once the primary vacuum has been reached in the furnace 1, the pump 5 is stopped (valves 7 and 10 closed) and the three-way valves 13 and 16 switched to their second positions, where they respectively connect the downstream compartment 22 of the permeator 20 with the inlet of compressor 14 (via valve 13), the outlet of the oil separator filter with buffer capacity 2 (via valve 16). The gaseous purification of the helium stored in the holding tank 12 can then be carried out by opening the valve 18, at the same time as the helium purified under low pressure collected and transported by line 24 is recompressed by the compressor 14 to be stored in the buffer capacity 2. Once this operation is complete, the valve 18 is closed and the three-way valves 13 and 16 switched to their initial positions of free passage from the oven 1 to the waiting capacity 12.

The buffer capacity 2 having collected only a portion, which is however very substantial, of the helium used during quenching, additional filling is carried out via line 9 to reach normal operating pressure and a new operation helium quenching can then take place.

According to an alternative embodiment, when the helium extracted from the furnace has only a low content of impurities, it is possible, before purification, to reuse the helium for one or more quenching operations. To do this, at the end of a quenching operation, the slightly polluted helium is transferred from the furnace 1 to the open valve 16 towards the buffer tank 2, this after passage of the helium in the pump 5, the compressor 14 and the oil separator filter 15 as indicated above; the helium stored in buffer capacity 2 can then be reused for a new quenching operation. Once the helium has reached a sufficient level of impurities, the valve 16 is switched so as to allow the installation to operate according to the embodiment described above, and allowing its purification.

Example of realization :

The recycling equipment is installed on a vacuum furnace with an internal volume of 10 m³ in which treatments of nickel-based alloy parts are carried out. After a plateau at a temperature of 1300 ° C., quenching is carried out under pure helium at a pressure of 2.5 bar absolute.

At the end of the treatment, the helium is cold in the oven, at a pressure of 2.5 bar absolute. The valve 27 is then opened and the gas is expanded at 28 in a circuit bypassing the primary vacuum pump of the oven; the gas is then recompressed 14, filtered and de-oiled at 15 and stored in a holding capacity 12 at a pressure greater than the buffer capacity 2 of the furnace. When the gas in the oven 1 is at atmospheric pressure, the valve 27 is closed, the valves 7 and 10 are open; the primary pump 5 is put into operation and sucks the gas to return it to the inlet of the compressor 14. The oven 1 is then put into the air, then opened and ready for unloading-recharging.

The helium in the holding capacity 12 is already filtered and de-oiled beforehand; it is then purified in the membrane separator 20, recompressed at 14 and returned to the buffer tank 2 of the oven. You just need to fill in 9 the amount of helium lost during recovery, before starting another treatment cycle.

• soit un appareil d'élimination de l'oxygène résiduel 31 par réaction avec de l'hydrogène adjoint à débit contrôlé en 32 ;
• soit un appareil d'élimination catalytique de l'oxygène avec régénération par un courant d'azote circulant selon 33/33′;
• soit des tamis moléculaires pour l'élimination par adsorption de l'oxygène et de la vapeur d'eau.

Referring to Figure 2, we find the assembly oven 1, buffer capacity 2, vacuum pump 5 bypassed by a regulator 28, connected to an equalization capacity 29. This is connected to the compressor 14 followed by a mechanical filter 15 and from there directly to a helium purifier 31. The compressor 14 can possibly be bypassed by a line 37 comprising a regulator 38, starting downstream of the compressor 14 and returning to the capacity d equalization 29. The helium purifier can be of one of the following three types:

• either an apparatus for removing residual oxygen 31 by reaction with assistant hydrogen at a controlled flow rate at 32;
• either a catalytic oxygen elimination device with regeneration by a current of nitrogen flowing according to 33/33 ′;
• or molecular sieves for elimination by adsorption of oxygen and water vapor.

In the first two cases, it is preferable to have a cooler 34 followed by a dryer 35 of the type with water vapor trapping by molecular sieve and purged by a stream of nitrogen 36-36 ′, before reinjection in buffer capacity 2 under waiting pressure.

In the third case, a cooler 34 may optionally also be available. Preferably, at least two molecular sieves arranged in parallel are used, one being in operation and the other in regeneration by depressurization or increase in temperature, possibly accompanied by the passage of a stream of pure gas.

Claims (12)

Process for the heat treatment of objects with quenching in a gaseous recirculating medium (8) in contact with said treated objects with cooling of the quenching gas by means of a heat exchanger, of the type in which helium is used as quenching which is stored under waiting pressure in a buffer tank (2), characterized in that at the end of a quenching operation, the helium charge is extracted (27-28) from the enclosure treatment, in the final phase by pump (5) until a primary vacuum is obtained, said helium extracted by a compressor (14) associated with a mechanical filter (15) at a purification pressure, and l 'said helium is passed at a cleaning pressure through an impurity removal purifier (20-35) (31-35), to transfer it, if necessary after recompression, into the buffer tank (2). A method of heat treating objects according to claim 1, characterized in that the helium at the cleaning pressure is collected in an intermediate capacity (12), after which it is sent to a separator (20) with a membrane of permeation (21) delivering the purified helium under lower pressure, which is then dried (35) and discharged under pressure by the same compressor (14) to the furnace buffer capacity (2). Process for the heat treatment of objects according to claim 1, characterized in that the helium extracted, recompressed (14) at a pressure higher than the waiting pressure of the buffer capacity (2), and mechanically filtered (15), is brought to transfer into a device residual oxygen (31) of the type with controlled addition of hydrogen (32) for catalytic formation of water vapor, after which the gas is optionally cooled (34) and dried ( 35), then transferred to said buffer capacity (2). Process for the heat treatment of objects according to claim 1, characterized in that the helium extracted, recompressed (14) at a pressure higher than the waiting pressure of the buffer capacity (2), and mechanically filtered (15), is brought to transfer into a purification device (31) of the catalytic type with trapping of residual oxygen and regeneration of the catalyst by a stream of hydrogen (33-33 ′), after which the gas is optionally cooled (34) and dried (35), then transferred to said buffer capacity (2). Process for the heat treatment of objects according to claim 1, characterized in that the helium extracted, recompressed (14) at a pressure higher than the waiting pressure of the buffer capacity (2), and mechanically filtered (15), is transferred to a device for purifying oxygen and possibly residual water vapor, of the molecular sieve type for adsorption of oxygen and optionally water vapor, regenerating by depressurization or increasing the temperature, possibly accompanied by the passage of a stream of pure gas, after which the gas is optionally cooled (34) and dried (35), then transferred to said buffer capacity (2). A method of heat treatment of objects according to one of claims 1 to 5, characterized in that only a fraction of said extracted helium passes through the impurity removal purifier. Process for the heat treatment of objects according to one of Claims 1 to 5, characterized in that the said helium extracted passes through the purifier for removing the impurities after having been used for at least two successive quenching operations. Installation for the heat treatment of objects with quenching in a gaseous medium incorporating at least substantially helium, of the type comprising a furnace (1) with a gas recirculation turbine (8) connected by valve lines on the one hand to a capacity buffer (2), on the other hand to a primary vacuum pump (5) bypassed by a valve line (27) and regulator (28), characterized in that it comprises, at the pump outlet (5) , a small equalization capacity (29), a compressor (14), a mechanical filter (15), a helium purifier (20-31), and, if necessary, a dryer (35) preferably trapped water vapor by molecular sieve. Installation for heat treatment of objects according to claim 8, characterized by the interposition of an intermediate tank (12) upstream of the purifier (20), which is of the membrane separator type (21), the outlet of which purified and dried gas (35) is connected by valve (13) upstream of said compressor (14), the outlet of which is also connected by valve (16) to the buffer tank (2). Installation for the heat treatment of objects according to claim 8, characterized in that the helium purifier (31) is of the type with controlled addition of hydrogen (32) for catalytic formation of water vapor. Installation for the heat treatment of objects according to claim 8, characterized in that the helium purifier (31) is of the type with catalytic elimination of residual oxygen and regeneration of the catalyst by a stream of hydrogen (33) (33 ′). Installation for the heat treatment of objects according to claim 8, characterized in that the helium purifier (31) is of the molecular sieve type for adsorption of oxygen and optionally of water vapor and for regeneration by depressurization or an increase in temperature, possibly accompanied by the passage of a stream of pure gas.

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