EP0213991B1 - Process for rapid cementation in a continuous furnace - Google Patents

Abstract

Process in which there are injected a carrier gas and a hydrocarbon capable of producing, at conventional carburizing temperatures, an atmosphere of predetermined composition having a nominal concentration of carbon monoxide, a door of the furnace being opened with a given periodicity to permit the passage of a charge to be carburized, the opening of this door resulting in particular in an increase in the concentration of the oxidizing species in the atmosphere of the furnace. According to the invention, the concentration of carbon monoxide of the atmosphere injected into the furnace is increased with the same periodicity so as to compensate for the increase in the concentration of the oxidizing species of the furnace and thus maintain the carbon potential of the carburizing atmosphere of the furnace substantially constant throughout the duration of the carburization of the workpieces of the charge.

Description

The present invention relates to a rapid case-hardening process in a continuous closed furnace into which a carrier gas and optionally a hydrocarbon are injected capable of generating, at the usual case-hardening temperatures, an atmosphere of predetermined composition, having a nominal concentration of carbon monoxide, an oven door being opened with a determined periodicity to allow the passage of a charge to be cemented, the opening of this door generating in particular an increase in the concentration of oxidizing species in the atmosphere of said oven.

A continuous closed furnace is a furnace into which are introduced at regular time intervals the charges to be treated which advance at low speed therein, successively passing through a zone of temperature rise of the charges, a zone of case-hardening of the pieces of the charge and a diffusion zone of said coins. A continuous closed oven may have entry and exit airlocks which partially decrease the increase in the concentration of oxidizing species in the atmosphere and may also have non-watertight separation doors between each zone.

The injection of carrier gas and of hydrocarbon generates an atmosphere of predetermined composition when the oven is in equilibrium, that is to say in particular when the doors of the oven are closed. This atmosphere consists of:

In a continuous oven, the introduction of a charge causes, when a door is opened, significant air intakes generating oxidizing species. The increase in the concentration of oxidizing species in the furnace atmosphere generates a rapid decrease in the carbon potential.

It has been proposed in American patent 4145 232 to multiply the flow rate of carrier gas by two when opening the oven door for the introduction of the charge and to return to the usual initial flow rate of carrier gas when closing Door.

Such a process is not satisfactory.

In fact, in such a process, whatever the high flow rate of carrier gas injected into the furnace, it is not possible to avoid a rise in the oxidizing species in the furnace, therefore an increase in their concentration and a correlative decrease in the carbon potential.

peut se définir par la relation:

k(T) = cte fonction de la température [CO] = concentration en monoxyde de carbone (CO_Zj = concentration en dioxyde de carbone.The carbon potential in the carburizing zone of the furnace, where the equilibrium reaction takes place:

can be defined by the relation:

k (T) = this function of the temperature [CO] = concentration of carbon monoxide (CO _Z j = concentration of carbon dioxide.

However, whatever the flow rate of gas injected into the furnace, the concentration of carbon monoxide in the furnace remains substantially constant. Consequently, an increase in the carbon dioxide concentration necessarily results in a decrease in the carbon potential.

The method according to the invention makes it possible to avoid these drawbacks. It is characterized in that the carbon monoxide concentration of the atmosphere injected into the furnace is increased with the same periodicity, so as to compensate for the increase in the concentration of oxidizing species in the furnace and thus maintain substantially constant the carbon potential of the furnace carburizing atmosphere throughout the carburizing duration of said parts. If carbon monoxide is formed in the furnace after cracking of one of the source elements of the carrier gas, the increase in concentration of carbon monoxide is understood as the correlative increase in the element. generator. Thus, in the most common practice, the carrier gas comprises nitrogen and an alcohol, preferably methanol (or ethanol). The increase in concentration of carbon monoxide signifies in this case a corresponding increase in the concentration of methanol in the carrier gas.

Preferably, as soon as the oven door is opened, the concentration of carbon monoxide in the atmosphere is increased, so as to compensate for the increase in carbon dioxide in order to maintain a substantially constant carbon potential. To ensure rapid renewal of the furnace atmosphere and therefore a more rapid increase in the carbon monoxide concentration, this increase in carbon monoxide concentration will preferably be accompanied by an increase in the flow rate of the carrier gas.

In this case, a carrier gas flow rate of 1.5 to 4 times the "nominal" carrier gas flow rate, corresponding to the charge treatment phase (case hardening and / or diffusion), will preferably be used.

According to a first variant embodiment of the invention, the oven door will be closed before the injection of carrier gas with a high concentration of carbon monoxide begins. In this way, a carrier gas saving is achieved since when the door is opened, the increase in concentration oxidizing species cannot be avoided.

According to a preferred variant of the invention, the opening of the oven door will be preceded for a few moments by an injection of carrier gas with a high concentration of carbon monoxide, this injection continuing at least until the door is closed. and possibly after closing thereof, under conditions of duration specified below. The carbon monoxide boost can be timed when the cycle is programmed. Thus it is easy to provide a time delay after closing the door, before returning to the "nominal" carbon monoxide flow. Likewise, provision may be made for a pre-triggering of the carbon monoxide supercharging synchronized with the opening of the door.

Of course, in all the cases described above, the injection of carrier gas with a high concentration of carbon monoxide may or may not be accompanied by an increase in the flow rate of carrier gas, preferably within the limits mentioned above.

In all the variants envisaged above, the duration of the injection of carrier gas having a concentration of carbon monoxide greater than the nominal value will be between 5% and 50% of the total duration of the treatment.

The carrier gas with a carbon monoxide concentration greater than the nominal value will preferably be obtained from a nitrogen-methanol mixture, with a volume ratio

The carrier gas with a carbon monoxide concentration equal to the nominal value will also be obtained from a nitrogen-methanol mixture in a volume ratio preferably having the value

• - les figures 1 et 2, les variations d'atmosphère selon l'art antérieur;
• - les figures 3 et 4, les variations d'atmosphère selon l'invention;
• - la figure 5, les variations du potentiel carbone selon l'art antérieur et selon l'invention.

The invention will be better understood with the aid of the following embodiments, given without limitation, together with the figures which represent:

• - Figures 1 and 2, the atmospheric variations according to the prior art;
• - Figures 3 and 4, the atmospheric variations according to the invention;
• - Figure 5, the variations of the carbon potential according to the prior art and according to the invention.

In a continuous oven (or "push-furnace" in English), a load consisting of steel parts to be cemented is introduced every few minutes (generally from 4 to 20 minutes). This oven generally comprises successively an entry door, an entry airlock, a carburizing zone and a diffusion zone, possibly separated by doors, an exit airlock with quenching tank.

The atmosphere generated in the furnace is of the "endothermic" type, that is to say rich mainly in hydrogen, carbon monoxide and nitrogen species, obtained from a generator, or nitrogen and bodies intended for create in the oven the CO and H ₂ species which may be methanol alone (preferred solution), ethanol-oxidant mixtures (H ₂ 0, Air, C0 _2, etc.) or equivalents, to which up to 10% may be added of hydrocarbon (CH ₄ , C ₃ H ₈ ...) to control the carbon potential and sometimes up to 5% of ammonia for specific treatments such as carbonitriding (cementation activated with ammonia).

For the introduction of a charge into the oven, the entry door is opened, which causes large uncontrolled entries of oxidizing species (0 ₂ or C0 ₂ , H ₂ 0, resulting from the combustion of the atmosphere of the oven with the outside air).

In currently known processes (Figures 1 and 2), the opening of the oven door, periodically, at times t _o , t ₁ , t ₂ , ..., causes (Figure 1) a very rapid increase in concentration into carbon dioxide from the furnace atmosphere, the latter passing almost instantaneously (a few tens of seconds or more) from a concentration [CO ₂ ] ₁ , for example from 0.15% to a concentration [CO ₂ ] ₂ up to 1%, about 6 times higher. (Values very variable depending on the ovens and the treatment).

Given the low concentration of carbon dioxide in the furnace atmosphere, the concentration of carbon monoxide can be considered constant throughout the process. Consequently, the carbon potential varies very strongly in the carburizing zone of the furnace, according to curve C, illustrated in FIG. 5. It can decrease up to a PC _M value of the order of 0.1 to 0.3 % for a case-hardening temperature of 920 ° C for example. (The setpoint for carbon potential at this temperature is often in the range of 0.8 to 1.0%.) Reconditioning the oven to the setpoint takes practically the entire time interval to to t , separating two successive introductions. Under these conditions, the carbon transfer which becomes efficient only around the PC _m value (defined below) value reached after a time interval T (T being able to represent up to half of the time interval t, - to separate the introduction of 2 charges), the case hardening of the parts during each of the periods T will be practically zero and there is even a risk, in certain cases, of causing decarburization of the parts in this period. Consequently, the cementation taking place only during the time intervals to + T to t ₁ , t, + T to t ₂ , etc., the depth of cementation for a determined hardness is small. By initially setting a given depth and hardness, the duration of the case hardening is therefore considerably lengthened.

FIG. 2, by way of indication represents the flow rate of carrier gas injected into the furnace according to the known solution of the American patent cited above, this flow rate normally having the value DL when the door is closed, and a value DH when the door of the furnace is open, substantially equal to 2 times DL or more.

According to the invention (FIGS. 3 and 4), the concentration of carbon monoxide in the atmosphere injected into the oven is increased during the charging of a new charge (or during the charging, if the latter causes a disturbance similar) or shortly before in order to anticipate the increase in concentration of oxidizing species, without reaching an atmospheric carbon activity equal to 1, which would generate soot on the parts. This increase in concentration is generally carried out during the entire duration of the opening of the oven door. It generally continues after the closing of this door in order to return more quickly to the set carbon potential. This measure is doubly favorable because it allows, on the one hand, to maintain the carbon potential of the atmosphere at a sufficient value for there to be carbon transfer from the atmosphere in the room but it also allows other part to accelerate this transfer to the part, since the carbon transfer speed depends, in the carburizing phase, on the product pH ₂ x pCo, which are the respective partial pressures of H ₂ and CO in the furnace (here equal to concentrations).

This increase in carbon monoxide concentration is effected by injection into the furnace of carbon monoxide or, preferably, of a product liable to decompose, in the atmosphere of the furnace to generate this carbon monoxide.

In "normal" mode (doors closed), the atmosphere injected into the oven is either that of an endo generator at constant flow rate, or preferably, a nitrogen / methanol mixture or equivalent as described above. Thus, according to the invention (FIGS. 3, 4 and 5), the injection is increased, during the time at t ′, of carbon monoxide whose concentration changes from [CO], (which is generally of the order of 20% by volume) to [CO] ₂ (which is around 27% by volume).

This is reflected (Figure 5) by a carbon potential, the variations of which are represented by the curves C ₂ . We will regulate the flow rate of this supercharging of carbon monoxide (or of the body which generates it) and its duration so as not to fall significantly below the value PC _m of the carbon potential, value below which the atmosphere would not be cementing . For example, for a 16NC6 type steel and a carburizing temperature of 920 ° C, these various parameters will be adjusted so as not to fall below a value of approximately 0.4% of carbon potential. Thus, also thanks to the increase in the carbon transfer speed, the speed of the continuous carburizing processes is improved, all other things being equal.

The simplest method for implementing the invention is to use a nitrogen-methanol mixture to generate the atmosphere of the oven, and to vary the relative proportions of nitrogen and methanol.

During the period corresponding to the opening, the proportion of methanol in the mixture is increased, this increase being able to go as far as the introduction of pure methnol during or during this brief period. However, it is preferable to maintain at least 10% and preferably at least 20% nitrogen in the mixture injected into the oven.

For simplicity, the flow rate of the mixture and the proportions thereof can be varied simultaneously, so as to keep the nitrogen flow rate substantially constant. This variant is that shown in FIG. 4 with a flow rate D ' _H from to to to + At', etc. of a mixture comprising 20% nitrogen and 80% methanol and a flow rate D ' _L , lower to D ' _H of a mixture containing 40% nitrogen and 60% methanol.

The invention will be better understood with the aid of the following comparative examples:

Example 1:

This example represents the prior art typically used to date.

In a continuous push furnace, cementing of steel transmission parts of grade 16NC6 is carried out, for which the cementation depth sought at 550 HV1 is from 0.7 to 0.9 mm. The oven temperature is 920 ° C, the charges, introduced every 7 minutes, being 150 kg. The carbon potential that we are trying to maintain, in the cementation zone, is 0.8%. The duration of the opening of the loading door, at the entrance of the oven, is 27 seconds.

The atmosphere injected into the oven is obtained using a nitrogen-methanol mixture, in the 40/60 ratio (so-called "endothermic" atmosphere). The flow rate of the injected atmosphere is 19 m ³ / hour. The consumption of atmosphere per cycle (of 7 minutes) is therefore 2.22 _m 3).

The variations in carbon potential noted in the oven are shown in FIG. 6. The carbon potential, which was 0.8% before the door was opened, drops to 0.1% after one minute and then gradually rises to 0, 8% (0.4% after 3 minutes).

Example 2:

In the same oven, all other things being equal, the same parts are treated to obtain the same final conditions as in Example 1. The atmosphere injected into the oven in the previous example is replaced by an atmosphere of variable composition, for variable durations, shown in FIG. 7.

Thirty seconds before opening the door and for two minutes, the atmosphere Atm (2) is injected with a nitrogen / methanol ratio equal to 20/80 at a flow rate of 24 m ³ / h. Then the atmosphere Atm (1) is injected at a flow rate of 12 m ³ / h, for 3 minutes and 50 seconds. The gas consumption during a cycle is 1.57 m ³ . The variations in carbon potential are shown in Figure 8 to scale. (Note that on the time scale (Figures 6, 7 and 8), F represents the instant of closing of the oven door). The depth cemented at 550 HV1 of the parts in the batch is between 0.7 and 0.9 mm.

Thus, the duration of the cylce was reduced by 17% (from 7 min to 5 min 50 s) and the consumption of atmosphere by 29%. Such a reduction in cycle times, all other things being equal, represents a considerable gain for those skilled in the art.

Claims (12)

1. A rapid casehardening process within a closed continuous furnace, into which are injected a carrier gas and a hydrocarbon which at the normal casehardening temperatures are liable to generate an atmosphere of predetermined composition having a nominal concentration of carbon monoxide, one furnace door being opened with a specific repetition rate to allow the passage of a charge to be casehardened, the opening of this door in particular generating an increase in the concentration of the oxidizing species in the atmosphere of the said furnace, characterised in that the carbon monoxide concentration of the atmosphere injected into the furnace is increased with the same repetition rate, in such a way as to compensate for the increase in the concentration of the oxidizing species of the furnace and thus to keep the carbon potential of the casehardening atmosphere of the furnace constant throughout the duration of the casehardening of the said workpieces.

2. A rapid casehardening process within a closed continuous furnace according to claim 1, characterised in that the increase of the carbon monoxide concentration takes place from the opening of the door.

3. A rapid casehardening process within a closed continuous furnace according to claim 2, characterised in that immediately upon closing the furnace door, the carbon monoxide concentration of the atmosphere injected is restored to its nominal value in the predetermined cmposition.

4. A rapid casehardening process within a closed continuous furnace according to claim 2, characterised in that immediately upon closing the furnace door but with an adjustable time-lag, the carbon monoxide concentration of the atmosphere injected is restored to its nominal value in the predetermined composition.

5. A rapid casehardening process within a closed continuous furnace according to claims 1 to 4, characterised in that the injection of carbon monoxide precedes the opening of the door by a few instants.

6. A rapid casehardening process within a closed continuous furnace according to claim 1, characterised in that the increase of the carbon monoxide concentration takes place upon closing the door or precedes the said closing by a few instants when the opening period of the door is predetermined, during a predetermined time interval, prior to reverting to the nominal concentration.

7. A rapid casehardening process within a closed continuous furnace according to claims 1 to 6, characterised in that the return to the nominal value of the carbon monoxide concentration of the injected atmosphere occurs when the carbon potential measured in the enclosure has returned substantially to a predetermined reference value.

8. A rapid casehardening process within a closed continuous furnace according to claims 1 to 7, characterised in that the rate of flow of the atmosphere injected into the furnace is raised during at least a part of the period of injection of an atmosphere having a higher carbon monoxide concentration than the nominal value.

9. A rapid casehardening process within a closed continuous furnace according to claim 8, characterised in that the increase in the carrier gas flow rate corresponds to 1.5 to 4 times the nominal value of the flow rate.

10. A rapid casehardening process within a closed continuous furnace according to claim 9, characterised in that the duration of injection of the carrier gas having a higher carbon monoxide concentration than the nominal value is comprised between 5 and 50 % of the total duration of the treatment.

11. A rapid casehardening process within a closed continuous furnace according to claims 1 to 10, characterised in that the carrier gas having a CO concentration higher than the nominal value is obtained at least partially from a nitrogen- methanol mixture having a volume ratio

N₂ and MeOH respectively representing the nitrogen and methanol concentrations.

12. A rapid casehardening process within a closed continuous furnace according to claims 1 to 11, in which there is used a mixture of nitrogen and of methanol to generate the carrier gas, characterised in hat the nitrogen flow rate remains constant throughout the duration of the process, the rate of flow of methanol varying according to the variations in carbon monoxide concentration of the atmosphere.

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