EP0375491B1 - Method and installation for heat treatment for case-hardening, carbonitriding or heating before the quenching of metallic work pieces - Google Patents

Abstract

The invention is exclusively concerned with heat treatments before hardening, of metallic pieces, by cementation, carbonitridation and heating. The process concerns the feeding of a non muffle heat treatment furnace with various components including nitrogen which is produced by an adsorption or selective permeation generator and which has a residual oxygen content of the order of 2%. According to the invention, after the furnace has ceased to be in operation for a substantial period of time, it is reconditioned by injecting purer nitrogen which has a residual oxygen content lower than 0.3% and which is produced by said generator, adjusted at a lower extraction rate.

Description

The present invention relates to thermal treatments for carburizing, carbonitriding and heating before quenching steels intended to ensure surface hardening of metal parts.

In the past, the gaseous atmospheres used in case-hardening, carbonitriding and heating before quenching of steels were most often produced from endothermic type gas generators.

A typical example of a carburizing atmosphere composition is given below: nitrogen (N₂) 40% carbon monoxide (CO) 19% carbon dioxide (CO₂) 0.3% hydrogen (H₂) 35% methane (CH₄) 1% water vapor (H₂O) 0.6% oxygen (WHERE) traces

In carbonitriding, similar atmospheres are used, to which ammonia (NH₃) is added which allows the supply of nitrogen to the metal.

Currently, a significant proportion of carburizing, carbonitriding or heating workshops before quenching steels use industrial gases for the generation of their atmospheres, preferably in the solution of endothermic generators. Atmospheres are then produced in the ovens resulting from the injection of a mixture of N₂, CH₃OH (methanol), sometimes CH₄, and NH₃ in the case of carbonitriding.

• . d'une usine cryogénique située en général loin de l'utilisateur, et dans ce cas il est livré sous forme gazeuse (bouteilles comprimées) ou liquide (stockage liquide et vaporisation avant utilisation).
• . d'un générateur non cryogénique placé directement chez le client, qui est soit un générateur par adsorption connu sous la dénomination de "PSA", soit un générateur par perméation gazeuse, ou à "membranes" par exemple, ce qui conduit à une économie intéressante par rapport à l'azote d'origine cryogénique, mais aussi à des problèmes liés à une relative impureté du gaz produit, en particulier parce que la teneur en oxygène est relativement élevée, généralement de 0,1 à 5 %.

Nitrogen can come from:

• . from a cryogenic plant generally located far from the user, and in this case it is delivered in gaseous form (compressed bottles) or liquid form (liquid storage and vaporization before use).
• . a non-cryogenic generator placed directly at the customer, which is either an adsorption generator known under the name of "PSA", or a gas permeation generator, or with "membranes" for example, which leads to an advantageous economy compared to cryogenic nitrogen, but also to problems related to a relative impurity of the gas produced, in particular because the oxygen content is relatively high, generally from 0.1 to 5%.

If no further purification is done, the raw nitrogen produced is therefore impure, because it contains a little oxygen and traces of water. To limit the quantity of oxygen and water, it is necessary to lower the generator extraction factor (flow of nitrogen produced / flow of treated air), therefore its production capacity, which is obviously to the detriment of the cost price of the gas treated.

By way of example, a “PSA” type generator usually exhibits the following performances as a function of the oxygen content in the gas produced. Concentration WHERE (%) 5% 1% 0.1% Production (m³ / h) 180 100 35

The document Advanced Materials and Processes, vol. 134, No. 3, September 1988, pages 100 to 107, describes the use of membranes providing 95-96.5% nitrogen for the development of heat treatment atmospheres in ovens, the nitrogen produced passing through a buffer tank used for the oven purging phases.

However, in case-hardening, in carbonitriding, a residual oxygen concentration of the order of 2% in the nitrogen used for the N₃ - CH₃OH mixtures appears perfectly indicated, since a higher concentration would imply problems of obtaining a atmosphere with high carbon potential without soot formation, while a lower concentration would make the economic balance of the adsorption or permeation generator less attractive.

On the other hand, it should be remembered that most of the carburizing, carbonitriding and heating treatments before quenching of steels take place in non-reeved ovens, that is to say single wall of refractory bricks, without wall metallic, or muffle, so that the interior atmosphere of the furnace is in direct contact with the refractory bricks which constitute the thermal insulation of the furnace. However, refractory bricks are themselves porous and behave like sponges vis-à-vis the atmosphere.

When such an oven is in operation, the residual oxygen is transformed into CO, H₂O and CO₂. The additional hydrocarbon makes it possible in particular to maintain a low H₂O and CO₂ content, despite the presence of oxygen in the nitrogen, provided that the oxygen content is not too high. If this is not the case, an additional hydrocarbon content qualified as excessive must be injected, since it can cause soot formation, heterogeneous carburizing, drops in the CO content. Ultimately, obtaining a high potential carbon in the atmosphere may be impossible, which is obviously contrary to good treatment.

The maximum oxygen content compatible with the majority of the treatment cycles provided for in case hardening, carbonitriding and heating before quenching of steels is of the order of 2% in nitrogen. In this case, the residual H enO and CO₂ contents can be contained at low values, generally less than 0.6% for H₂O and 0.3% for CO₂.

However, the atmosphere formed inside the furnace diffuses into the refractory bricks and at the bricks / atmosphere interface, a balance is reached when the furnace is in continuous operation, but a significant problem remains during the periods of non-operation of the oven. It is indeed more and more common that the heat treatment workshop undergoes interruptions of operation of relatively long durations, for example during the weekend rest. In this case, the treatment atmosphere is of course no longer injected into the oven, not only for reasons of economy but also of safety, since this atmosphere is potentially explosive (high hydrogen and CO content) and toxic (high CO level). In addition, the oven temperature is often lowered somewhat.

If no atmosphere is injected into the oven, it tends to fill with air which then diffuses through the refractory bricks. When the treatment must be resumed, the air contained in the oven, as well as that present in the refractory bricks, must first be purged. This operation is particularly expensive if, also for purging, high purity cryogenic nitrogen is used, as described in the documents Iron & Steel Engineer, vol. 39, N ° 8, August 1962, pp. 124-134 and Steel in the USSR, vol. 15, N ° 12, December 1985, pp. 610-611.

It is therefore common to try rather to protect the furnace from this air pollution during the period of non-production, and for this purpose, the orifices of the furnace are closed and a low flow of nitrogen generally between 1 / 6 and 1/3 of the nominal flow rate is permanently injected into the oven to ensure an overpressure preventing air from entering. This procedure is also expensive.

If the nitrogen used comes from a cryogenic source, the residual oxygen content in the oven and in the refractory bricks remains very low, and the restart of the oven in production, the so-called reconditioning period, is then very short, generally 15 minutes to a few hours, depending in particular on the oven temperature, but the cost is also important.

If the nitrogen comes from another source and contains for example 2% oxygen, a value compatible with the subsequent treatment and particularly economical, the reconditioning of the oven can take considerably more time, thus penalizing the productivity of the installation. Indeed, it will not only be necessary to purge the interior atmosphere of the oven, but also the atmosphere contained in the refractory bricks, a particularly long operation, because these bricks behaving like sponges, it is difficult to diffuse gas there. In addition, the purge is conventionally done from the treatment atmosphere injected again into the oven. This contains in particular a high hydrogen content. This gas, which consists of a very "small" molecule diffuses very quickly, so that the hydrogen transforms the oxygen contained in the refractory bricks into water vapor, so that the water vapor content thus produced reaches 4%. This 4% water vapor content is incompatible with the subsequent treatment which requires values of less than 0.6%. We must therefore chemically destroy or purge this water vapor. Purging water vapor is always a difficult operation because this polar molecule has the property of being very easily adsorbed on the surface of solids. However, refractory bricks, by their porosity, have a very large specific surface.

Chemical destruction of water vapor is possibly done by reaction with a hydrocarbon such as methane, but this reaction is very slow or even almost nonexistent when the temperature is below 600 ° C, which is quickly the case in bricks refractory, since there is a strong temperature gradient between the interior of the furnace and the exterior wall of the furnace, the temperature of which is generally less than 100 ° C. in a normal furnace.

The object of the present invention is to propose a method and an installation making it possible to optimize the costs of use while greatly reducing the investment costs.

To do this, the process of heat treatment of carburizing, carbonitriding or heating before quenching of metal parts, of the kind in which an auxiliary gas mixture based on nitrogen, methanol, if necessary ammonia is used, to constitute a treatment atmosphere in an unmuffled oven, the nitrogen being crude nitrogen resulting from the separation of air by an adsorption or permeation separator, providing in nominal operation a nominal flow of nitrogen having a content oxygen residual of the order of 2%, and where a resumption of treatment, after a significant interruption phase, is preceded by a purge by injecting nitrogen into the oven, is characterized in that purge nitrogen is supplied, at a flow rate much lower than the nominal flow rate, by the separator, set at a lower extraction rate, such that the residual oxygen content in the purge nitrogen flow rate is between 0.1 and 0.3 %.

Experience shows that a residual oxygen content of the order of 0.1 to 0.3%, typically between 0.1 and 0.2%, in purge nitrogen is not likely to form with hydrogen a water vapor content incompatible with the subsequent treatment.

The process according to the invention has the double merit of requiring no other source of gas for purging and of ensuring this purging under economic conditions that are the least detrimental to the operating efficiency of the cementation, carbonitriding, or heating before quenching.

The invention also relates to an installation for heat treatment of carburizing, carbonitriding or heating before quenching of metal parts, of the type comprising a treatment furnace which is not reeved, at least two sources of constituents in the fluid state of production of an atmosphere carburizing or carbonitriding heat treatment, among which, for the nitrogen component, a nitrogen separator from the air by adsorption or selective permeation, characterized in that it comprises means for adjusting the extraction rate of l production nitrogen supplied by the separator on at least two levels, namely a high level with residual oxygen content of the order of 2% and a low level with residual oxygen content of between 0.1 and 0.3%.

Preferred embodiments of this installation are the subject of claims 3 to 7.

• En marche normale, le débit gazeux produit par le générateur 1 passe par le limiteur de débit 2, une vanne trois voies 3, un débitmètre 4, une seconde vanne trois voies 5, un réservoir-tempon principal 6, et une troisième vanne trois voies 7.
• En marche à débit réduit de purge, lors du reconditionnement du four, le débit gazeux produit passe par le limiteur de débit 2 la vanne trois voies 3, un réducteur de débit 8, le débitmètre 4, la vanne trois voies 5, un réservoir auxiliaire de gaz de purge 9, les vannes trois voies 5 et 7 étant alors positionnées de façon à interdire le passage par le réservoir 6.
• Un stockage d'azote liquide 10, muni de son dispositif de vaporisation 11 et d'un détendeur 12, débouche sur la conduite d'alimentation directement en amont du débitmètre 4 et sert à assurer l'écrêtage des pointes extrêmes et le secours en cas d'arrêt du générateur.
• Les réservoirs-tampon 6 et 9 sont utilisés pour absorber les variations de débit appelé par l'utilisateur, en marche normale ou en marche réduite respectivement. Ils ne sont pas nécessaires si le débit appelé est stable.

A schematic example of this type of installation is given in the appended drawing where it is specified:

• In normal operation, the gas flow produced by the generator 1 goes through the flow limiter 2, a three-way valve 3, a flow meter 4, a second three-way valve 5, a main tank-tempon 6, and a third three-way valve 7.
• When operating at reduced purge flow, during reconditioning of the oven, the gas flow produced passes through the flow limiter 2 the three-way valve 3, a flow reducer 8, the flow meter 4, the three-way valve 5, an auxiliary tank purge gas 9, the three-way valves 5 and 7 then being positioned so as to prevent passage through the tank 6.
• A liquid nitrogen storage 10, provided with its vaporization device 11 and with a pressure reducer 12, opens onto the supply line directly upstream of the flow meter 4 and serves to ensure the clipping of the extreme peaks and the rescue in the event of generator stop.
• The buffer tanks 6 and 9 are used to absorb variations in the flow rate called by the user, in normal operation or in reduced operation respectively. They are not necessary if the called flow is stable.

• soit opérées manuellement par l'utilisateur en fonction de ses besoins
• soit opérées automatiquement par un dispositif approprié (minuterie, détection de charge du client, ...).

Note that the three-way valves 3, 5 and 7 can be

• either operated manually by the user according to his needs
• either operated automatically by an appropriate device (timer, customer load detection, etc.).

The installation described makes it possible to guarantee a high instantaneous flow rate thanks to the emergency regulator 12 whatever the flow rates in the tanks 6 and 9.

It can be seen that a single flow meter is used, as well as a single pressure reducer and that this flow meter remains protected from over-flows by the tanks 6 and 9 placed downstream.

A single buffer tank may be sufficient, but then it should be possible to purge it for approximately the time necessary for the generator to change from normal nitrogen quality to purge nitrogen quality.

As the reduced step during the reconditioning of the oven requires a little less compressed air to supply the generator, the excess of compressed air compared to the normal step is either vented, with no energy saving effect, or the compressor vacuum device switches on at regular intervals, resulting in significant energy savings.

As an example, we can expect the following values: O₂ content in nitrogen 2% 0.1% nominal flow rate of a "PSA" type generator (m³ / h) 100 25 nominal power of generator type "PSA" (kW) Pn 90% Pn

Claims (7)

1. Process for the carburizing, carbonitriding or heating before quenching heat treatment of metal components, of the kind where an extra gaseous mixture based on nitrogen, methanol or, if appropriate, ammonia is used to constitute a treatment atmosphere in a non-muffled furnace, the nitrogen being crude nitrogen resulting from the separation from air by an adsorption or permeation separator, providing, in nominal operation, a nominal nitrogen flow rate having a residual oxygen content of the order of 2 %, and where a repeat of the treatment, after a phase of interruption of significant length, is preceded by a purge by injection of nitrogen into the furnace, characterized in that the purging nitrogen is provided, at a flow rate markedly less than the nominal flow rate, by the separator, adjusted to a lower degree of extraction, such that the residual oxygen content in the purging nitrogen flow rate is between 0.1 and 0.3 %.
2. Plant for the carburizing, carbonitriding or heating before quenching heat treatment of metal components, of the kind comprising a non-muffled treatment furnace, at least two sources of constituents in the fluid state for development of a carburizing or carbonitriding heat treatment atmosphere, among which there is, for the nitrogen constituent, a separator of the nitrogen from the air (1) by selective adsorption or permeation, characterized in that it comprises means (2, 8) for adjusting the degree of extraction of production nitrogen provided by the separator on at least two levels, namely a high level containing a residual oxygen content of the order of 2 % and a low level containing a residual oxygen content between 0.1 and 0.3 %.
3. Plant according to Claim 2, characterized in that it comprises, in parallel, downstream of the separator (1), at least one first (6) and one second (9) storage tanks for storing nitrogen volumes having different oxygen contents.
4. Plant according to Claim 3, characterized in that the separator (1) services a production pipe, incorporating successively a first flow rate limiter (2), a second flow rate limiter (8) in parallel via a three-way valve, and the first storage tank (6).
5. Plant according to Claim 4, characterized in that the second tank (9) is placed in parallel with the first tank (6) via three-way valves (5) and (7).
6. Plant according to either of Claims 4 or 5, characterized in that the production pipe contains a flow rate meter (4) placed upstream of the tanks (6) and (9).
7. Plant according to one of Claims 4 to 6, characterized in that it includes a device for supplying stand-by and/or peak-flow nitrogen, comprising a liquid nitrogen storage (10), an evaporator (11) and a pressure reducing valve (12), and emerging into the production pipe upstream of the first tank (6).

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