EP0738785A1 - Process and apparatus for adjusting the carbon monoxide concentration of a furnace atmosphere for carburizing or nitrocarburizing of metallic parts - Google Patents

Abstract

The CO content of the atmosphere of an oven (2) supplied with a mixture of air, hydrocarbon and, if required, NH3 and used for carburisation or carbonitriding of metal workpieces is regulated by a process in which the CO level is determined and a CO forming material is introduced when a min. CO content, which is freely adjustable, has been reached. Also claimed is an appts. for carrying out this process.

Description

The invention relates to a method for controlling the CO content of a furnace atmosphere for carburizing and carbonitriding metallic workpieces in a furnace, the furnace atmosphere by directly feeding a mixture of an oxidizing agent, eg. B. air, and a hydrocarbon-containing fuel and optionally ammonia (NH ₃ ) is generated in the furnace.

The invention further relates to an apparatus for performing this method.

In the carburizing or carbonitriding processes, the required carburizing atmosphere is generated either in separate inert gas generators (endogas) or by feeding nitrogen with methanol into the furnace. In both methods of protective gas production, the furnace has a relatively stable CO value, which in the first case results from the setting of the protective gas generator and the fuel used in the protective gas generator, and in the second case from the percentage of methanol fed into the furnace. A third variant is direct gassing with hydrocarbons and an oxidizing gas component, such as. B. air or CO ₂ . With this technique, liquid or gaseous fuels are mixed with the oxidizing agent and fed into the furnace. The CO content required for the carburization is generated in the furnace by direct reaction of the fuel with the oxygen of the oxidizing agent. Of these direct fumigation methods, natural gas-air fumigation is currently the most widespread. This is due to the high availability and low price of natural gas.

statt.The conversion of the natural gas in the furnace with the atmospheric oxygen takes place according to the equation $${\text{CH}}_{\text{4th}}{\text{+ 0.5 O}}_{\text{2nd}}{\text{+ 1.88 N}}_{\text{2nd}}{\text{→ CO + 2H}}_{\text{2nd}}{\text{+ 1.88 N}}_{\text{2nd}}$$

instead of.

With complete conversion of the methane in the furnace with the atmospheric oxygen, this results in a maximum CO content in the furnace atmosphere of 20.5% by volume. This high proportion of CO is only achieved under ideal conditions (very high furnace temperature).

At low furnace temperatures, especially below about 870 ° C, the above reaction is very slow and the conversion of methane into CO is correspondingly low.

In addition, the CO formation reaction mentioned above is hampered by the presence of ammonia (necessary for carbonitriding).

Low CO contents have the effect that the carbon transfer decreases and the furnace atmosphere for carburizing or carbonitriding can hardly be regulated and the furnace also becomes sooty very quickly. The sooting of the furnace in turn results in production downtimes, since the soot has to be shut down and burned out to remove the soot.

The invention has for its object to provide a control for the CO content of the furnace atmosphere, which ensures continuous and trouble-free operation of the carburizing and carbonitriding furnaces even at low carburizing temperatures (≦ 870 ° C) and also in the presence of ammonia (carbonitriding) .

gespalten (bei Ofentemperaturen ≧800°C), wodurch der CO-Gehalt in der Ofenatmosphäre wieder über den eingestellten minimalen CO-Gehalt ansteigt.The task of continuous and trouble-free operation with a regulated CO content is achieved in that the CO content of the furnace atmosphere is determined and a CO-forming substance is added to the furnace atmosphere when a freely adjustable minimum CO content is reached. According to a preferred embodiment, methanol is used as the CO-forming substance. The methanol fed into the furnace atmosphere becomes after the reaction $${\text{CH}}_{\text{3rd}}{\text{OH → CO + 2H}}_{\text{2nd}}$$

split (at furnace temperatures ≧ 800 ° C), whereby the CO content in the furnace atmosphere rises again above the set minimum CO content.

An alternative CO-forming substance is CO ₂ .

In order to keep the amount of CO-forming substance to be added small and to keep the process inexpensive, an upper value for the CO content can additionally be set, upon reaching which the addition of the CO generator is stopped again until the CO content in the course the process has dropped back to the minimum CO content.

A CO content of around 12% has been found to be the minimum CO content in the furnace atmosphere, since falling below this value results in heavy soot formation and, moreover, the furnace atmosphere can no longer be precisely controlled. A bandwidth between approximately 12% and 15% CO in the furnace atmosphere has proven to be particularly suitable as a bandwidth for the minimum and the upper CO content. Since below a CO content of 15% the course of the CO decrease is very flat, an increase in the CO content by adding the CO former up to this limit of about 15% is sufficient to keep the process going for a long time drive a CO content above the minimum limit. In addition, this narrow range means that only a little CO generator has to be used to increase the CO content, which in turn can keep the costs of the process low.

The device for carrying out the described method has a CO analyzer for determining the CO content in the furnace atmosphere and a programmable CO controller in order to control a valve and possibly a pump as a function of the CO content. The valve and if necessary, the pump is switched to flow or operation when the set minimum CO content is reached. When the set upper limit of the CO content is reached, the valve is closed again and the pump is switched off if necessary.

Fig. 1

ein Diagramm über den Verlauf des CO-Gehaltes in der Ofenatmosphäre bei dem erfindungsgemäßen Verfahren und

Fig. 2

eine schematische Darstellung einer Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens.

Further details and advantages result from the following description of the associated drawings, in which the method according to the invention and an apparatus for carrying out this method are shown schematically. The drawings show:

Fig. 1

a diagram of the course of the CO content in the furnace atmosphere in the inventive method and

Fig. 2

is a schematic representation of an apparatus for performing the method according to the invention.

gespalten wird. Durch diese CO-Bildung infolge der Methanolspaltung steigt der CO-Gehalt in der Ofenatmosphäre sehr schnell an, welches der steile Anstieg der CO-Kurve in Fig. 1 verdeutlicht. Bei Erreichen einer frei einstellbaren oberen Grenze - in Fig. 1 15 % - wird die Methanolzufuhr wieder abgestellt, so daß der CO-Gehalt in der Ofenatmosphäre infolge des kontinuierlich weiterlaufenden Prozesses wieder abnimmt.The diagram in FIG. 1 shows the course of the CO content during a carbonitriding process. By adding ammonia to the furnace atmosphere, the CO content drops sharply in the course of the process. As can be seen from FIG. 1, the curve of the CO content curve runs very flat below 15% CO. Below the 12% line shown as the minimum CO content limit, the too low CO content in the furnace atmosphere causes the furnace to soot quickly. When this lower limit is reached, the furnace atmosphere becomes a CO-forming substance, e.g. B. added methanol, which due to the high process temperature after the reaction $${\text{CH}}_{\text{3rd}}{\text{OH → CO + 2H}}_{\text{2nd}}$$

is split. Due to this CO formation due to the splitting of methanol, the CO content in the furnace atmosphere rises very quickly, which is illustrated by the steep rise in the CO curve in FIG. 1. When a freely adjustable upper limit is reached - 15% in FIG. 1 - the methanol supply is switched off again, so that the CO content in the furnace atmosphere decreases again as a result of the continuously ongoing process.

From Fig. 1 it can be seen that even the slight increase in the CO content from 12% to 15% for a longer period of time a trouble-free process of the process above the soot limit, since the course of the CO curve is very flat below 15%.

2 schematically shows the structure of a device for carrying out the method described above. The CO content of the furnace atmosphere in a furnace chamber 2 is determined by means of a CO analyzer 1. The control unit also has a programmable CO controller 3, to which freely adjustable values for the upper and lower CO content can be entered.

Via the control system shown in dashed lines, the CO controller 3 controls a valve 4 and possibly a pump 5 as soon as a comparison of the CO content determined by the CO analyzer 1 with the minimum CO content entered in the CO controller 3 has shown that this minimum CO content has been reached.

The pump 5, which is controlled by the CO controller 3, then conveys the CO generator from a tank 6 through the valve 4, which is switched to a passage, into the furnace chamber 2. In the furnace chamber 2, the CO generator is split as described above, so that the CO Content in the furnace atmosphere rises again. If the continuous comparison of the CO content determined by the CO analyzer 1 in the furnace atmosphere with the values stored in the CO controller 3 shows that the entered upper CO content has been reached, the valve is activated via the CO controller 3 4 closed and, if necessary, the pump 5 turned off again.

The process described above begins again as soon as the CO analyzer 1 and CO controller 3 determine that the minimum CO content set has been reached again.

With a process controlled in this way, it is ensured on the one hand that the CO content of the furnace atmosphere never drops below the set minimum CO content causing strong sooting of the furnace and on the other hand only as much CO former is added to the furnace atmosphere as is necessary for an inexpensive and trouble-free operation of the process is necessary.

Reference list

1

CO analyzer

2nd

Furnace chamber

3rd

CO regulator

4th

Valve

5

pump

6

tank

Claims (6)

A process for controlling the CO content of an oven atmosphere for carburizing and carbonitriding metallic workpieces in an oven, the oven atmosphere by directly feeding a mixture of an oxidizing agent, e.g. B. air, and a hydrocarbon-containing fuel and optionally ammonia (NH ₃ ) is generated in the furnace,
characterized,
that the CO content of the furnace atmosphere is determined and a CO-forming substance is supplied to the furnace atmosphere when a freely adjustable minimum CO content is reached. Process according to Claim 1, characterized in that methanol or CO _{2 is} used as the CO-forming substance. A method according to claim 1, characterized in that the CO-forming substance is fed into the furnace atmosphere until a freely adjustable upper CO content is reached. Method according to one of claims 1 to 3, characterized in that the minimum CO content is about 12% CO. A method according to claim 3, characterized in that the control range of the CO content is preferably between about 12% and about 15% CO. Device for carrying out the method according to one of claims 1 to 5,
marked by
a CO analyzer (1) for determining the CO content in the furnace atmosphere and a programmable CO controller (3) for controlling a valve (4) and optionally a pump (5) depending on the CO content in the furnace atmosphere.

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