EP1101826A1 - Quenching process after low pressure carburising - Google Patents

Abstract

A method for quenching steel components after being subjected to a heat treatment process, such as cementation, at low pressure (Pcem), is characterised in that it consists of submitting the components to a flow of air at an elevated pressure (Ptrem), of between 5 and 50 bars. The duration of the hardening process is below 15 minutes and preferably less than 2 minutes. The heat treatment installation using this method for treating steel components is also claimed and incorporates a plurality of treatment cells and handling equipment for transferring the components between the different treatment cells.

Description

The present invention relates to the treatment of parts steel, and more particularly the hardening of parts which have undergone heat treatments, in particular case hardening, that is to say introduction of carbon into the surface of the parts in order to improve hardness. The invention relates more particularly quenching of parts having undergone a case hardening treatment vacuum or under low gas pressure (below pressure atmospheric).

A low pressure carburizing treatment consists of to submit the parts to be treated, in a sealed enclosure to the air, alternating enrichment stages in the presence a low pressure carburizing gas and diffusion steps vacuum or neutral atmosphere at low pressure. The respective durations of enrichment and dissemination stages as well as their number depend in particular on the concentration in carbon and depth of cementation desired in the parts, and these treatments are well known in the art. A example of low pressure carburizing process is described in French patent application No. 2,678,287 of the applicant.

Any cementation treatment is followed by at least one quenching step carried out either under oil or under gas. A main purpose of quenching is to get cooling rapid hardened parts without altering the surface finish got. Gas quenching is often preferred because it allows obtain dry and clean parts directly. We are looking for usually to get the most cooling speed fast possible. To increase the quenching speed with a given gas, we must increase the mass flow rate of the gas, i.e. increase the speed and / or static pressure of the gas quenching.

Among the quenching gases generally used, nitrogen conventionally constitutes an acceptable compromise in terms of cost and performance. Nitrogen is often preferred over gases neutrals such as helium and hydrogen which, although more light, therefore easier to transport under relatively pressure high, too expensive (helium) or too dangerous (hydrogen).

It would however be desirable to reduce the cost of the quenching step which, due to the gaseous atmosphere which is seeks to maintain and required mass flow, is not negligible in the overall cost of processing the parts.

In addition, a disadvantage of using a gas such as nitrogen or other is, in addition to the cost, the need for transportation and storage of large volumes. Indeed, the speakers of industrial gas quenching often have volumes of several cubic meters, or even several tens of cubic meters.

The quenching treatment must respect several constraints, in particular, in connection with the above cementation. First of all, quenching should not affect the hardness of the surface of the hardened part. In addition, the quenching must be rapid to satisfy the rapid cooling of the room and not degrade its surface. In addition, we must most often satisfy to an imperative of appearance of the part obtained which, not only must generally present a surface state without roughness, but also be the color of the steel (gray). In particular, it is generally considered unacceptable that a part has a blackened appearance, suggesting an oxidation.

The invention also refers to carbonitriding whose only difference compared to the case hardening comes from enrichment gas used to which is generally added ammonia. The perfectly known result is the formation of nitrides (instead of carbides for cementation) on the surface of the room. It will therefore be noted that all that will be exposed subsequently in connection with case hardening also applies to carbonitriding.

An object of the present invention is to provide a new quenching process which overcomes the disadvantages of the processes known.

The invention aims, in particular, to allow the realization a particularly economical quenching treatment.

Another object of the invention is to propose a method which is compatible with conventional treatments of carburizing at low pressure.

Another object of the invention is to propose a method which respects the surface appearance of the finished parts.

To achieve these objects, the present invention provides a process for tempering treated steel parts low pressure thermal, which consists in subjecting the parts to a high pressure air flow.

According to an embodiment of the present invention, the air pressure is between 5 and 50 bars.

According to an embodiment of the present invention, the quenching time is less than 15 minutes and, preferably, less than 2 minutes.

According to an embodiment of the present invention, parts are not released to air at atmospheric pressure between the low pressure heat treatment and the air quenching under high pressure.

The invention also provides a method of treatment parts comprising a low pressure carburizing treatment, followed by a quenching step.

According to an embodiment of the present invention, cementation treatment includes alternating steps enrichment at low pressure in the presence of a carburizing and diffusion stages in the presence of a neutral gas at substantially the same pressure as the enrichment steps.

According to an embodiment of the present invention, the parts are subjected, after the quenching step, to a shot blasting step to, in particular, remove the roughness of unwanted surface.

The invention further provides a treatment installation thermal comprising means for the implementation of the above processing method.

According to an embodiment of the present invention, the installation includes several clean treatment cells to be sealed from the outside, and means for handling to transfer a load from one cell to another, one of these cells constituting a quenching cell specific to be further isolated from the rest of the installation for setting up work of an air quenching.

According to an embodiment of the present invention, the quenching cell also serves as an unloading cell for the load at the end of treatment.

les figures 1A et 1B illustrent, par des caractéristiques de pression et de température en fonction du temps, un exemple de mise en oeuvre du procédé de traitement thermique selon l'invention ; et

la figure 2 représente, de façon très schématique, un mode de réalisation d'une installation de traitement adaptée à la mise en oeuvre du procédé selon l'invention.

These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular modes of implementation and embodiment given without limitation in relation to the attached figures, among which:

FIGS. 1A and 1B illustrate, by pressure and temperature characteristics as a function of time, an example of implementation of the heat treatment method according to the invention; and

FIG. 2 very schematically shows an embodiment of a treatment installation adapted to the implementation of the method according to the invention.

For the sake of clarity, the diagrams in the figures 1A and 1B are not to scale. In addition, only the elements necessary for understanding the invention have been shown in the figures and will be described later. In particular, in figure 2, we just represented the multicellular structure of an installation without worrying about the constituent details cells which, unless otherwise specified, are conventional. In addition, reference will be made to request publications of patents to which we can refer and whose contents are fully incorporated by reference into the present description.

A feature of the present invention is provide quenching under air flow of parts having undergone carburizing or carbonitriding operation at low pressure. According to the invention, this quenching in air is carried out under high pressure (greater than 5 bars).

One advantage of using air is that it is a free gas source, available everywhere without packaging particular, and inexhaustible. Thus, we considerably reduce the cost of the quenching steps compared to the processes classics.

However, the use of air was classically excluded for several reasons in gas quenching processes in low pressure cementation plants or the like.

First, the presence of oxygen just after the cementation leads to an oxidation that was thought to be harmful in more ways than one on the final part after quenching. Oxidation of the part changes its hardness and makes its surface grainy.

In addition, in a vacuum furnace where the case hardening steps and gas quenching are carried out successively in the same enclosure, the introduction of air is not possible because oxidation of the hot parts of the oven.

In addition, a ventilation of the room for the whole the duration of the heat treatments has the effect of blackening its surface by oxidation, which was considered unacceptable from the point of view of the appearance of the room.

The present invention provides, unlike all of these prejudices, to use air for the gas quenching of the room case-hardened. According to the invention, this air is used under high pressure (greater than 5 bars and, preferably between 5 and 50 bars). Another advantage of the present invention is that the air can be used with very high pressures without difficulty. However, the fact of using a high pressure makes it possible to shorten the duration of the quenching step since the mass flow is improved. According to the invention, the duration of the air quenching step is limited to a few minutes (typically, less than 15 minutes) and is preferably less 2 minutes. The shorter the duration, the more the thickness of the surface oxidation of the part is low. Note that the finding the highest possible pressure is compatible with the search for a minimum duration. With durations too thin, the oxidation thickness linked to the presence of air during quenching is limited to a few micrometers. Such a thickness is negligible compared to the carburizing thicknesses generally carried out (from several hundred micrometers to a few millimeters).

By accepting an oxidation on a very small thickness (less than 5 micrometers), the use of air in the quenching treatment without altering the properties of the part final. Indeed, a loss of hardness on such a small thickness is generally perfectly negligible since the layer of desired hardness is immediately below and is not altered.

In addition, the parts are most often subjected to a so-called blasting step which consists in causing erosion mechanics of their surface. Conventionally, this blasting step aims to eliminate burrs and surface irregularities related to the molding, forging or machining of parts and that is easier to remove after case hardening due to the more great hardness of the part. According to the invention, this step eliminates also the few micrometers of oxidation linked to quenching under air. We thus find the metallic aspect of the surface of the part as at the end of a conventional quenching under nitrogen.

Figures 1A and 1B show the respective developments pressure and temperature during a mode implementing the heat treatment process of the invention, applied to an example of processing a grade steel 20MnCr5.

According to this example, a load consisting of a batch of toothed crowns representing a total weight of 300 to 350 kg is introduced into a cementation treatment installation at low pressure. We want to obtain, for these parts, a content 0.36% carbon up to 700 µm deep.

The charge, introduced at room temperature Tamb (Figure 1B) in the installation, is first brought to a temperature Tcem from 920 to 1000 ° C in 1 to 2 hours (instants t0 to t1). At the same time, or separately if an airlock is used as intended will see later in relation to FIG. 2, the pressure is lowered to a Pcem value of 5 to 20 mbar (Figure 1A). Then, the load is subjected to five enrichment stages (E) under carbon atmosphere, alternated with as many diffusion stages (D) under nitrogen. The respective durations of the enrichment stages and of diffusion are chosen in a conventional way and are, of preferably decreasing for the enrichment stages (by example, respectively, of approximately 5 min, 2 min, 1 min 40 s, 1 min 35 s and 1 min 30 s) and increasing for the diffusion stages (e.g., approximately 5 min, 10 min, 15 min, respectively, 25 min and 40 min). The total duration of the carburizing step is, for example, around 97 min (instants t1 to t2) and we obtain, in end of carburizing, a carbon content greater than 0.36% up to a depth of 775 µm.

The load is then subjected to a quenching according to the invention (instants t3 to t4) under an air pressure Ptrem of approximately 16 bars for 30 seconds. Preferably, the case hardening steps and quenching are carried out in separate cells. This is why, in FIG. 1A, we have indicated a time of transfer (T, instants t2 to t3) from the cementation cell to the quenching cell. At the end of treatment, a steel is obtained having a surface hardness of 62-64 Hrc and a hardness of feet 300-320 HV20 tooth.

The quenching step has the effect of oxidizing the surface on a thickness of less than 5 µm. Besides that this depth is too weak to influence the hardness of the part, it is preferably eliminated by a shot blasting step implemented later outside the enclosure. Note that the depths carbon diffusion are usually planned with a margin allowing the shot peening to leave a thickness conforms to that intended. Thus, the invention does not require extend the enrichment and dissemination stages to increase cementation depth to account for the low oxidation.

To simplify, we considered above that the pieces were brought to room temperature by the step of quenching. In practice, the parts are generally removed from installation while they are still at a higher temperature high. However, this does not change the principles of the invention.

For comparison, the quenching of such a charge to bring the temperature down to about 100 ° C takes about 2 minutes under an air pressure of 20 bars, and about 2.5 minutes under an air pressure of 10 bars.

FIG. 2 represents an exemplary embodiment of a treatment installation adapted to the implementation of a treatment air quenching according to the invention. The embodiment of figure 2 is inspired by a modular installation such as described in European patent application No. 0 922 778 of the Applicant to whom we can refer for further information details.

A basic module 6 includes a sealed enclosure 10 in the form of a cylinder (not necessarily circular in cross-section) with horizontal axis. The two ends of this cylinder 10, provided flanges, are sealed with removable waterproof covers 12. Treatment cells are connected laterally to the cylinder 10 and lie in the same horizontal plane. Through example, two heat treatment cells 14 (for example, to contain two charges to be cemented) are arranged one in facing each other by being connected to a first transfer case 10-1 constituting the cylinder 10. A loading-unloading cell 15 is arranged opposite a quenching cell 16, these cells being connected to a second transfer case 10-2, itself axially connected to the box 10-1.

A handling device is in the form of a carriage 18 moving parallel to the axis of the cylinder 10, from one transfer case to another. This carriage moves, by example, on rails 20 extending all along the cylinder 10. The carriage is provided with a telescopic fork 22 which is capable of to stretch on either side of the carriage 18 to the center from each of cells 14 to 16 to pick up and drop off a load 24 being processed. In Figure 2, in lines full, the carriage 18 is located at cells 15 and 16, and the telescopic fork 22 enters the cell 15 to there take charge 24. Of course, cell 15 has been previously setting the enclosure 10 to low pressure to be able to open door 15-1 which constitutes, with the exterior door 15-2, an entrance airlock. In dotted lines, the carriage 18 is located at level of cells 14. An installation as illustrated in Figure 2 is modular, i.e. one or more modules additional 8 each consisting of a transfer case 10-3 provided with 20 'rails and one or two 14' cells can be axially connected to one of the boxes 10-1 or 10-2 to complete cylinder 10.

The only change you need to make to an installation as described in the patent application European EP-A-0 922 778 mentioned above, for the implementation of the invention is to provide means for organizing a circulation of pressurized air in the quench cell 16 and, depending a preferred embodiment, means for resetting this vacuum cell before the introduction of a new charge and / or before the load can return to the transfer 10-2. Cell 16 can be isolated from the rest of the installation by a watertight door 16-1.

Alternatively, the quench cell is also a cementation cell. However, we generally prefer provide separate cells and thus reduce the time treatment. Indeed, we can then predict that a load or more charges are being cemented in a cell adapted while another previous charge is being quenching.

According to another variant, provision may be made for the cell quenching system constitutes the exit airlock of an installation multicellular. Indeed, the quenching step is generally the last stage of treatment within the installation. In the case of an installation such as that of the patent application European EP-A-0 922 778 already cited, this is compatible with the quenching of a charge at the same time as the case hardening of one or several subsequent charges. The only change to make concerns the quenching cell (16, figure 1) to which then adapt an unloading door to the outside.

An advantage of using the quench cell as exit airlock is that the transfer cases which constitute large volumes (several tens of cubic meters) can thus remain under vacuum or under controlled atmosphere at low pressure. In addition, you save time by not doing iron the load once cooled in the transfer boxes.

In addition, like the classic cell structure of loading-unloading 15 does not need to be changed, can still use the latter as an exit airlock, for example, if the quenching step is not the last treatment applied inside the installation. An advantage that there is to to dissociate the entry and exit airlocks is to facilitate the organization handling of loads outside the installation and the association of this facility with the rest of the parts manufacturing line.

Of course, the present invention is capable of various variants and modifications which will appear to the man of art. In particular, although the invention has been described in relationship with low pressure carburizing treatment, it more generally applies to any processing in which similar problems arise, in particular in which one today provides for quenching under neutral gas, under nitrogen or similar, following treatment at low pressure. He will be able to for example, carbonitriding, brazing and others partial vacuum applications before quenching.

In addition, the adaptation of the implementation data of the quenching method of the invention depending on the type of parts, of the load volume, and previous treatments is at the scope of the skilled person from the indications given above. In particular, it should be noted that the figures given of the particular example indicated above have only one virtue illustrating the feasibility of the invention, and that others values may be adopted including for the treatment of this type of steel. It will also be noted that the composition of the air is generally not critical. Indeed, the compositions of atmospheric air from different parts of the world are few different (at least with regard to the compounds of interest invention) and in most cases do not require, no particular adaptation. In the extreme, we can adapt the quench time and / or air pressure and / or circulation speed to the oxygen content of the air. Of course, the air used is, however, filtered to avoid introducing impurities into the installation. In addition, the air will be dried if necessary to reduce the risks of oxidation.

In addition, the practical realization of an installation of processing of the invention and its adaptation to the application concerned is within the reach of the skilled person from the indications functional data given above. In particular, the choice of loading / unloading of parts depends on the application and, generally, of a compromise between the bulk overall installation and processing time reported to the room. Finally, it should be noted that the invention can also be implemented in a treatment plant type of that described in European Patent No. 0 388 333 of the Applicant where several vertical processing cells are distributed over a sealed enclosure for the transfer of load on both sides of the quenching cell. Adaptation of such an installation according to the invention simply requires, as for the installation described in relation to FIG. 2, to associate with the quenching cell means to organize the pressurized air circulation and preferably also for vacuum this cell.

Claims (10)

Quenching process for steel parts which have undergone low pressure heat treatment (Pcem), characterized in that it consists in subjecting the parts to a flow of air under pressure high (Ptrem). Quenching method according to claim 1, characterized in that the air pressure (Ptrem) is between 5 and 50 bars. Quenching method according to claim 1 or 2, characterized in that the quenching time (t4-t3) is less than 15 minutes and preferably less than 2 minutes. Quenching method according to any one of the claims 1 to 3, characterized in that the parts are not released to air at atmospheric pressure (Patm) between treatment thermal at low pressure and quenching in pressurized air high (Ptrem). Parts treatment method comprising treatment low pressure carburizing, followed by a step of quenching, characterized in that the quenching step conforms to any one of claims 1 to 4. Treatment method according to claim 5, characterized in that the carburizing treatment comprises a alternation of enrichment steps (E) at low pressure (Pcem) in the presence of a carburizing gas and diffusion stages (D) in the presence of a neutral gas at substantially the same pressure as enrichment stages. Treatment method according to claim 5 or 6, characterized in that the pieces are submitted, after the step of quenching, at a blasting step to, in particular, remove the unwanted surface roughness. Heat treatment plant, characterized in that it includes means for implementing the treatment method according to any one of claims 5 to 7. Heat treatment installation according to claim 8, characterized in that it comprises several cells treatment (14, 15, 16)) suitable for being isolated from the outside tightly, and handling means (18, 20, 22) to transfer a charge (24) from one cell to another, one of these cells constituting a quenching cell (16) capable of being additionally isolated from the rest of the installation for implementation air quenching according to any one of the claims 1 to 4. Treatment installation according to claim 9, characterized in that the quenching cell (16) also serves load unloading cell (24) at the end of treatment.

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