Gas hardening cell

Abstract

A cell for hardening steel parts by circulation of a gas in a tight enclosure, including at least one static aspirator for circulating the gas, the gas present at the inlet of the static aspirator being at a pressure greater than the atmospheric pressure.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the processing of steel parts, and more specifically to the hardening of parts having undergone thermal processings, especially of cementation, that is, of introduction of carbon in the surface of the parts to improve their hardness.

[0003] 2. Discussion of the Related Art

[0004] A cementation processing consists of submitting the parts to be processed, in an airtight chamber, to an alternation of steps of enrichment in the presence of a cementation gas and of steps of diffusion under vacuum or under a neutral atmosphere. The respective durations of the enrichment and diffusion steps as well as their number especially depend on the desired carbon concentration and case depth in the parts, and such processings are well known in the art. An example of a low-pressure cementation process is described in French patent application N.sup.o 2,678,287 of the applicant.

[0005] Any cementation processing is followed by at least one hardening step performed either in oil, or in a gas. A main purpose of the hardening is to obtain a fast cooling down of the cemented parts without altering the obtained surface state. A gas hardening is often preferred since it enables directly obtaining clean cemented parts.

[0006] The present invention also relates to the carbonitriding, the only difference with respect to the cementation lying in the enrichment gas used, to which ammonia is generally added. The known result thereof is the forming of nitride (instead of carbide for the cementation) at the part surface. It should thus be noted that all that will be discussed hereafter in relation with cementation also applies to carbonitriding.

[0007] The hardening processing that follows the cementation or the carbonitriding must respect several constraints, among which a fast cooling of the part to avoid damaging its surface. To increase the hardening rate with a given gas, the gas mass flow must be increased, that is, the speed and/or the static pressure of the hardening gas must be increased.

[0008] Hardening and cementation processings are generally performed in thermal processing installations by batches in which loads or batches of parts to be processed are significant (often several hundreds of kilograms).

[0009] FIG. 1 is a simplified cross-section view showing a conventional example of a thermal processing cell 1 of a cementation installation of the type to which the present invention applies. Cell 1 illustrated in FIG. 1 is a dual cell that can be used to heat up a batch 2 of parts to be processed in a cementation processing and to submit this batch to a hardening, that is, a fast cooling down. Cell 1 essentially includes a tight external enclosure 3 (most often tubular for improving the mechanical resistance to pressure differences between the outside and the inside of the enclosure) in which a thermal processing chamber 4 is defined by appropriate walls, and which generally rests on the ground via a base 10. Heating elements 5 (for example, electric resistive bars) are distributed inside the processing chamber in which load 2 is placed. Enclosure 3 is also equipped with a cooling turbine 6 that can be driven by an engine 7 to stir air or gas within enclosure 3 during the hardening step. To enable circulation of air, chamber 4 is equipped, for example in its upper and lower walls, of mobile thermal flaps 8, which are intended to be closed during thermal cementation processings and to be opened during hardening processings. Turbine 6 sends the air generally outside of chamber 4 to have it cross a heat exchanger 9 interposed between the external walls of chamber 4 and the internal walls of enclosure 3. The hardening gas comes into processing chamber 4 through the bottom of the enclosure and comes out of it at the level of turbine 6 placed at its top, as illustrated by the arrows in FIG. 1. This is of course an example of arrangement and other structures are also known; in particular, the gas circulation may be inverted. To simplify, the gas inlets/outlets in enclosure 3 have not been shown.

[0010] FIG. 2 is a simplified cross-section view showing another known example of a hardening cell 1' which is here dedicated, that is, which is only used for the hardening of cemented parts. Such a cell 1' is, for example, provided in an on-line installation for receiving batches 2 of parts to be processed having undergone, in neighboring cells, thermal cementation or carbonitridation processings. Cell 1' essentially includes, like the previously-described cell 1, a tight enclosure 3 in which load 2 to be processed is placed. A hardening chamber 4' is defined by walls which are here permanently open, at the top and at the bottom of enclosure 3. An engine 7 is intended for driving, via its shaft 7', a turbine 6 in charge of sending air or gas towards a thermal exchanger 9 arranged between the external wall of chamber 41 and the internal wall of enclosure 3. The gas then follows the path indicated by the arrows in FIG. 2 to enter processing chamber 4' through the bottom of enclosure 3 and come out of it at the level of turbine 6. Other types of turbines allow for an inverted gas circulation.

[0011] Be it in dual cells or in dedicated cells, several engines and several turbines are most often aligned in the high portion of the enclosure to increase the air flow which conditions the hardening speed.

[0012] The present invention more specifically applies to hardening cells such as illustrated in FIGS. 1 and 2 where the gas recirculates on the load to be processed in a closed circuit, heating up at the contact of the load, then loosing these calories through an exchanger. Such cells are especially used when the hardening gas is not air but a gas (for example, nitrogen or another neutral gas), the used quantities of which are desired to be spared.

[0013] A disadvantage of conventional processing cells is that the flow rates required for a fast hardening strain the engines that must rotate at very high speeds to drive the turbines. For example, to obtain a hardening pressure on the order of 20 bars with a flow rate of approximately 5 m.sup.3/s, engines of a power greater than 100 kW that rotate at several thousands of turns per minute are used. Such rotation speeds accelerate the wearing of the engines, in particular of the mechanical rolling parts.

SUMMARY OF THE INVENTION

[0014] The present invention aims at overcoming the disadvantages of known hardening and/or thermal processing cells.

[0015] The present invention more specifically aims at avoiding the problems associated with the engine wearing in the driving of the turbines of a thermal cooling cell.

[0016] The present invention also aims at providing a solution which is compatible with a closed circuit operation of the hardening cell, in particular, if the hardening gas used is not air and must thus be saved.

[0017] The present invention also aims at maintaining, or even improving, the hardening speed.

[0018] To achieve these objects, the present invention provides a cell for hardening steel parts by circulation of a gas in a tight enclosure, which includes at least one static aspirator for circulating the gas, the gas present at the inlet of the static aspirator being at a pressure greater than the atmospheric pressure.

[0019] According to an embodiment of the present invention, the gas circulation in the enclosure occurs in closed circuit except for a secondary air flow injected as an inductor fluid into the static aspirator.

[0020] According to an embodiment of the present invention, the closed circuit circulation of the gas passes through thermal exchange means in charge of cooling it down.

[0021] According to an embodiment of the present invention, the cell includes means for recycling the secondary inductor air flow.

[0022] According to an embodiment of the present invention, the inductor fluid is injected into the static aspirators with a pressure ranging between 20 and 80 bars.

[0023] According to an embodiment of the present invention, the cell includes an air-tight enclosure; a cooling chamber within said enclosure, intended for receiving a load of steel parts to be cooled down; a thermal exchange means on the circulation path of a cooling gas between an external wall of the cooling chamber and an internal wall of the tight enclosure; a plurality of static gas aspirators in an upper wall of the processing chamber, the latter being opened in an opposite wall to evacuate the processing gas; and inductor gas ducts under a pressure greater than the pressure of the gas contained in the enclosure.

[0024] According to an embodiment of the present invention, the cell is associated with a compressor for injecting the inductor fluid into the static aspirators.

[0025] According to an embodiment of the present invention, the gas is chosen from among nitrogen, hydrogen, helium, and air.

[0026] The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a simplified cross-section view of an example of a conventional thermal processing cell of the type to which the present invention applies;

[0028] FIG. 2 is a simplified cross-section view of an example of a conventional hardening cell of the type to which the present invention applies;

[0029] FIG. 3 shows, in a very simplified cross-section view, a first embodiment of a hardening cell according to the present invention;

[0030] FIG. 4 is a cross-section view of a static aspirator of a hardening cell according to an embodiment of the present invention; and

[0031] FIG. 5 shows, in a view similar to that of FIG. 3, a second embodiment of the present invention.

DETAILED DESCRIPTION

[0032] The same elements have been designated with the same references in the different drawings. For clarity, only those elements of a hardening cell, and more generally of a thermal installation, that are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter.

[0033] A feature of the present invention is to use, as elements for forcing the air or gas circulation in the hardening cell, flow multipliers associated with a compressed air source providing a relatively low flow of acceleration gas. According to the present invention, so-called "venturi" effect flow multipliers are used, which are known to increase ambient air flows by means of compressed air. This type of device is also known as a static aspirator.

[0034] FIG. 3 is a simplified cross-section view showing an embodiment of a cooling cell according to the present invention.

[0035] A gas hardening cell 20 includes an air-tight enclosure 23 supported by a base 30 and intended for receiving a load 2 to be cooled down. Load 2 is introduced into a chamber 24 which is opened on a single side, for example, at its low portion. On the side opposite to the opening of chamber 24, at least one static aspirator 26 is provided, in which the inlet for gas to be drawn in is located at the outside of chamber 24 and the induced gas outlet is directed towards load 2 to be processed. Preferably, several static aspirators are used due to the volumes to be processed. Aspirators 26 receive, as a working fluid, air or gas under pressure coming from ducts 27' originating from an air or gas compressor 27. Compressor 27 is preferentially external to enclosure 23. The cell 20 also includes a heat exchanger 9 of conventional structure. Although this has not been shown in the drawings, exchanger 9 uses, as a heat-conducting fluid, a gaseous or liquid fluid and communicates with the outside of enclosure 23 to cool down this heat-conducting fluid.

[0036] The flow of air or gas under pressure in a cell 20 according to the present invention is performed from static aspirators 26, which inject the gas flow into chamber 24, from which it comes out through the opening at the lower portion and then flows through heat exchangers 9 to be drawn back in by flow multipliers 26.

[0037] According to the embodiment illustrated in FIG. 4, a controlled-flow air vent 28 is further provided, having the function of evacuating the excess gas injected into enclosure 23 through ducts 27' to be used as a working fluid.

[0038] FIG. 4 shows, in a simplified cross-section view, a static compressed air aspirator usable in a hardening cell according to the present invention. Such a static aspirator, also called a venturi nozzle, conventionally has the function of converting a low gas flow at medium pressure into a very large induced atmospheric flow. According to the present invention, this aspirator is used to convert a low gas flow at very high pressure into a very large gas flow at medium pressure.

[0039] Such a device uses a primary supply gas flow introduced by an inductor 41 into a annular chamber 42. Chamber 42 opens by an annular slot 43 into inlet 44 of a venturi nozzle. The working gas follows the surface of the venturi nozzle by surface effect, while being accelerated. The working gas flow creates a high depression in groove 45 of the venturi nozzle, which results in drawing in the gas present in front of the inlet by the center. The induced and inductor gas flows mix in a diverging nozzle 46, to be ejected at the nozzle outlet with a high speed. At the outlet of diverging nozzle 46, gas external to envelope 47 of the aspirator is also moved along. Inductor flow annular chamber 42 may include an additional annular slot 48 at the periphery of the outlet of diverging nozzle 46 to further accelerate the phenomenon. The static flow multipliers enable obtaining induced flows having rates from 5 to 30 times greater than the gas flow at the inlet of the venturi nozzle.

[0040] The operating principle and the structure of a venturi-effect nozzle or air aspirator are known. FIG. 4 shows a conventional example thereof but other structures may be used in a cell according to the present invention.

[0041] The use of static aspirators in a steel part hardening cell takes advantage of the fact that enclosure 23 of the hardening cell is designed to stand strong pressure differences between the outside and the inside of the cell. Thus, while static aspirators are conventionally used to accelerate an ambient air flow by means of compressed air, the present invention provides using them to accelerate a gas flow inside the cell, which already is at a pressure greater than the atmospheric pressure, and using very high gas pressures for the inductor fluid.

[0042] Another feature of the present invention is to recycle the gas fluid induced by the static aspirators. Indeed, most often, the gas used in a hardening cell is not air but is an inert gas, which should preferably not be consumed in too large amounts. Accordingly, it is provided to enclose aspirators 26 within the cell to organize the flowing of the gas fluid in closed circuit.

[0043] According to a preferred embodiment of the present invention, a gas compressed at a pressure ranging between 20 and 80 bars is used as an inductor flow to obtain an induced flow at a pressure ranging between 10 and 20 bars.

[0044] An advantage of the present invention is that instead of imposing an air flow by the speed of an engine, a compressed gas source and flow multipliers are used. Accordingly, for a same cooling rate, the engine of compressor 27 rotates slower than that of a conventional turbine. Due to the flow multipliers, a coefficient on the order of 5 to 15 in terms of gas flow is gained in the cell.

[0045] Another advantage of the present invention is that it enables transferring most of the mechanical parts (compressor engine) outside of the enclosure, which enables not only suppressing moving components within the hardening cell, but also reducing the hardening cell volume for a given air flow. Accordingly, the present invention enables, by an induced advantage, decreasing the gas consumption in a hardening cell.

[0046] It should be noted that the present invention applies whatever the gas fluid used. It may even be air in some applications. However, the working or inductor fluid is, preferably, of same nature as the ambient fluid in the enclosure.

[0047] It should also be noted that the acceleration of the ambient gas in the cementation enclosure 23 preferably occurs after cooling down of the gas in exchanger 9. The system output is thus optimized by accelerating cooled gas rather than hot gas. However, an alternative embodiment consists of placing the flow multipliers at the outlet of chamber 24, that is, on the hot gas.

[0048] FIG. 5 shows a second embodiment of a hardening cell according to the present invention. Within cell 23, this embodiment shows the same elements as those described in relation with FIG. 3. The compressed air static aspirators have been shown more schematically in FIG. 5. The difference between the embodiments of FIGS. 3 and 5 is that, according to the second embodiment, it is provided to reuse the gas flow discharged through vent 28. For this purpose, this secondary flow is recycled by using a buffer reservoir 51 which receives the gas fluid coming out of vent 28 via a compressor 57 and, if necessary, an additional heat exchanger 58 for cooling down the air discharged through opening 28. The outlet of reservoir 51 is connected to ducts 27' supplying venturi nozzles 26 with inductor fluid.

[0049] An advantage of the embodiment of FIG. 5 is that it further reduces the volume of used gas.

[0050] The number of static aspirators 26 in a hardening cell depends, in particular, on the cell dimensions and on the desired cooling rate. The sizing of the static aspirators and their number and distribution are within the abilities of those skilled in the art based on the functional indications given hereabove and on the application to a given hardening cell. In particular, it should be noted that the present invention only requires slight modifications of a conventional hardening cell. In an extreme case, the present invention may be implemented without modifying the thermal exchange network in a hardening cell and by simply replacing the turbine and the engine with static aspirators while of course ascertaining to maintain the enclosure tightness.

[0051] Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, although the present invention has been described in relation with dedicated hardening cells, it should be noted that it applies to dual cells of the type of that illustrated in FIG. 1 where the hardening is performed in the very enclosure where the cementation or carbonitridation thermal processing has been carried out. Further, other static aspirators than those indicated as an example may be used, provided to respect the essential feature of the present invention, which is to enable acceleration, by means of a working gas, of the ambient gas of the cementation enclosure. Among hardening gases likely to be used, neutral gases such as nitrogen, helium, or hydrogen should be mentioned.

Claims

What is claimed is:

1. A cell (20) for hardening steel parts by circulation of a gas in a tight enclosure (23), including at least one static aspirator (26) for circulating the gas, the gas present at the inlet of the static aspirator (26) being at a pressure greater than the atmospheric pressure.

2. The hardening cell of claim 1, wherein the gas fluid circulation in the enclosure (23) occurs in closed circuit except for a secondary air flow injected as an inductor fluid in the static aspirator (26).

3. The hardening cell of claim 2, wherein the closed circuit circulation of the gas passes through cooling thermal exchange means (9).

4. The hardening cell of claim 2, including means (51, 57, 58) for recycling the secondary inductor gas flow.

5. The hardening cell of claim 2, wherein the inductor fluid is injected into the static aspirators (26) with a pressure ranging between 20 and 80 bars.

6. The hardening cell of claim 1, including: an air-tight enclosure (23); a cooling chamber (24) within said enclosure, intended for receiving a load (2) of steel parts to be cooled down; a thermal exchange means (9) on the path of the flowing of a cooling gas between an external wall of the cooling chamber and an internal wall of the tight enclosure; a plurality of gas static aspirators (26) in an upper wall of the processing chamber, the latter being opened in an opposite wall to evacuate the processing gas; and inductor gas ducts (27') under a pressure greater than the pressure of the gas contained in the enclosure.

7. The hardening cell of claim 6, wherein the cell is associated with a compressor (27, 57) for injecting the inductor fluid into the static aspirators (26).

8. The hardening cell of claim 1, wherein the gas is chosen from among nitrogen, hydrogen, helium, and air.