EP0695807A1 - Process for heat treating especially carburizing, metallic workpieces - Google Patents

Abstract

The invention relates to a method for heat treatment, in particular carburizing, of metallic workpieces in a furnace in a treatment atmosphere at high temperatures, in which the atmosphere is converted by reacting at least one compound containing carbon and hydrogen and a further medium containing oxygen (in particular pure oxygen gas). is generated, the reaction optionally being supported by a generator or catalyst device in or in the furnace and, if appropriate, an enrichment agent being additionally added to the atmosphere and is characterized in that gaseous alkenes are used as the C- and H-containing compounds.

Description

The invention relates to a method for heat treatment, in particular carburizing, of metallic workpieces in a furnace in a treatment atmosphere at high temperatures, in which the atmosphere is converted by reacting at least one compound containing carbon and hydrogen and a further medium containing elemental oxygen (for example air). is produced.

In an already classic standard process, the so-called generator endogas process, for carburizing, carbon-neutral and, in certain cases, decarburizing heat treatment of goods, the required treatment gas is generated with large amounts of air, for example, under-stoichiometric combustion of natural gas, methane or propane with air at temperatures of around 1000 ° C Amounts of auxiliary energy generated. In other common processes, an identical treatment gas is produced by a catalyst retort arranged in the heat treatment furnace and by supplying the same starting materials (see, for example, technical article "Basic requirements for reducing gas consumption in controlled gas carburizing" from HTM 35/1980 No. 5, page 230 to 237, in particular point 1.1 and, for example, DE 36 32 577). At higher process temperatures, it is also known to introduce said media directly into the furnace, where they are in turn converted into a corresponding treatment gas
Still other processes start from different starting materials, namely in particular oxygen-bearing hydrocarbon compounds such as alcohols, and then come to a heat treatment atmosphere by introducing these oxygen-bearing compounds directly into heated heat treatment furnaces and these - due to the prevailing high temperatures (T> 800 ° C) -convert into the desired treatment gases. In these processes, nitrogen is often additionally provided as an atmospheric component, which leads to additional process engineering advantages, such as the possibility leads to the atomization of the often liquid, oxygen-bearing hydrocarbon compounds and also brings with it the general availability of an inert admixing and rinsing medium (see again technical article "Basic requirements for reducing gas consumption with regulated gas carburizing" from HTM 35/1980 No. 5, Pages 230 to 237, especially points 1.3 and 1.4).

If one considers the process categories described above, which largely include all processes that are widespread in practice, and analyzes them in particular with regard to the respective atmosphere formation reaction, that is, using the equations

CH₄ + 0.5 O₂ + 2 N₂ ---> CO + 2 H₂ + 2 N₂ and



CH₃OH + 2 N₂ ---> CO + 2 H₂ + 2 N₂,

it should be noted that these atmospheres have a CO to H₂ ratio of 1 to 2. It should be pointed out that atmospheres with a CO to H₂ ratio of about 1 to 1 are particularly advantageous for carburizing processes (see above quotation again, page 231), because these atmospheres have an extremely high carbon transition number. However, forming atmospheres with the CO-to-H₂ ratio mentioned in a simple and practical manner and on the basis of inexpensively available starting materials is a task which has hitherto not been satisfactorily solved (see, for example, DE 41 10 361 A1).

The invention was therefore based on the object of specifying a cost-acceptable and easy to implement option with which a treatment gas with a CO-to-H₂ ratio of approximately 1 to 1 can be produced.

This object is achieved according to the invention in that gaseous alkenes (ie hydrocarbon gases with a double bond) are used as the starting material for the formation of the atmosphere and these are used in accordance with appropriate, oxygen-containing media

C _n H _2n + n O -> n CO + n H₂ (n = 2,3,4)

converted into an advantageous atmosphere as described (CO: H₂ = 1: 1). This atmosphere can optionally be diluted with an inert gas - usually nitrogen - this being particularly the case with carburizing in the beginning Treatment part preferably does not happen or only to a small extent. Pure oxygen is preferably used as the oxygen-containing medium, but oxygen-containing gases, in particular air or O₂-enriched air, or also oxygen-containing, gaseous compounds are also suitable. According to the invention, it is primarily thought of carbon gases (CO₂, CO), nitrogen dioxide (NO₂) or nitrous oxide (N₂O). Propylene or ethylene are the most important alkenes of the invention.

The invention is explained in more detail below using an exemplary embodiment with the aid of the figure:
The figure shows a chamber furnace 1 in which a batch 2 of steel parts to be carburized is located. In the chamber furnace 1 there is also a double jacket converter K consisting essentially of two concentric tubes, the external dimensions of which correspond to a conventional radiant tube. The converter K is connected with its inner tube 3 to feed lines 4 and 5, which are used to supply the starting media for atmospheric formation, the atmosphere essentially formed in the inner tube finally flowing into the furnace interior via a nozzle 7. The free space 6 between the outer and inner tube of the converter K is connected to a feed line 8, with which a heating medium for heating the inner tube 3 can optionally be supplied. Furthermore, the furnace is equipped with - but not shown - jet pipes for its heating and with a fan 10 which serves to circulate the treatment gas. Furthermore, a gas analyzer 11, here in particular a so-called C-flow probe, is connected to the furnace interior, which regulates the oxygen supply quantity via the associated signal processing unit 12 via an adjusting element 14 arranged in the oxygen supply line 4b. Likewise, a quantity control unit 13 is arranged in the feed line 4a, which serves to supply the propylene to the line 4. However, their setting remains largely constant in the running process.

According to the invention, a heat treatment is now carried out in the chamber furnace described as follows: Workpieces made of 16Mn5Cr steel with a carbon content of approximately 0.16% by weight are to be carburized to an edge carbon content of approximately 0.9% C, with a hardening depth of 0 .5 mm (C content there = 0.35%) is to be achieved. For this purpose, the workpiece batch 2 - as shown in the figure - is placed in the chamber furnace 1 and the batch is heated up via the feed line 5 and the converter K started supplying nitrogen to the furnace. When a temperature of about 500 ° C is reached - the final temperature for the carburizing treatment that is ultimately to be achieved is approximately 900 to 950 ° C - the nitrogen supply is terminated and a stoichiometrically coordinated propylene-oxygen mixture is fed to the double-jacket converter K via the feed line 4 started and thus the formation of a treatment gas atmosphere according to

C₃H₆ + 3/2 O₂ ---> 3 CO + 3 H₂

initiated. The starting temperature for this reaction is about 450 ° C., so that the reaction proceeds above 500 ° C. and in particular at normal carburizing temperatures without any problems. The converter heating takes place from the interior of the furnace and is supported by the fact that hot combustion exhaust gases originating from the radiant heat pipes are introduced via line 8 into the free space 6 between the inner and outer pipes of the converter. The conversion taking place in converter K and its inner tube 3 provides, according to the reaction equation shown, an approximately 50% CO and 50% H₂ treatment gas, that is to say a treatment gas whose CO-to-H₂ ratio is exactly "1".

As discussed at the beginning, this is the optimal CO-to-H₂ ratio for carburization, because it provides the greatest possible carbon transition number β of around 310 * 10 ⁻⁷ cm / s (cf. article mentioned at the beginning, page 231).

This carbon transition number β of an atmosphere is of great importance for a carburizing process and especially its initialization phase, since in this initial phase the respective workpieces still have relatively low C components in the surface layers and therefore the C absorption of these workpieces in this phase is very important depends on the C delivery, for which in turn the said C transition number is a measure. In the later stages of a carburization - usually at the latest after approx. 70% of the treatment period - the importance of this C delivery increasingly takes a back seat, since the outer layers of the workpieces to be carburized have reached a saturated carbon content and then the carburization speed is essentially dominated by the diffusion of carbon from the surface into the interior of the workpiece. Therefore, in later phases of carburization, the C transition number β of an atmosphere is no longer relevant for the carburization rate so pervasive meaning, and it can, as also partially provided according to the invention, then be converted to an atmosphere with a lower β number. This is accomplished, for example, by a correspondingly arranged reduction in the propylene and oxygen supply according to the invention, with the simultaneous start of an adapted nitrogen supply. It is advantageous to provide nitrogen proportions of more than 20% up to 50% in the resulting atmosphere, it being important to ensure that no unacceptable decarburization of the workpieces occurs with this final phase atmosphere. For this purpose, their C level - as well as the C level of the initial atmosphere - must be set appropriately. With an atmosphere continuously set to a C level of approximately 1.0% according to the invention, an edge carbon content of approximately 0.9% is achieved in the present example case in particular in an approximately 90-minute treatment period (in the example case considered in more detail here with a relative low hardening depth of 0.5 mm and a relatively short treatment time, the maintenance of the pure propylene-oxygen atmosphere is basically justifiable over the entire treatment time without nitrogen dilution in the final phase).

For comparison: In a carburization with an endogas atmosphere generated from natural gas (composition: about 20% CO, 40% H₂, 40% N₂), a treatment time of about 110 minutes was required for a corresponding carburizing result, according to the experience of the applicant.

Otherwise, the C level setting of the atmosphere according to the invention can be carried out in the usual way by an independent and controlled supply of an enrichment gas, e.g. of natural gas or acetylene, with the usual indicator of the C level being e.g. the dew point or the CO₂ content of the atmosphere are applicable.

Acetylene has the advantage that the CO to H₂ ratio of the atmosphere is not "detuned". There is also the possibility of regulating the C level via the addition of propylene and / or oxygen via lines 4a and 4b with the setting members 12 and 14. If the addition ratio of propylene and oxygen differs from "one", the C level can be set via the O₂ partial pressure of the atmosphere. The amount of the O₂ excess determines the C level.

If a mixture of propylene and oxygen which differs from the stoichiometric ratio and has an excess of propylene is introduced into the converter 3, the excess propylene content does not contribute directly to the formation of CO-H₂ carrier gas, but rather leads to an increase in the carbon level of the atmosphere which is formed. The gas analyzer, designated 11 in the figure, is used to regulate this process. the O₂ potential of the atmosphere, that is to say all the O ions contained in the atmospheric constituents CO, CO₂ and H₂O, is determined and on this basis together with the processing unit 12 determines the appropriate amount of oxygen and adjusts this by means of the volume controller 14.

Finally, a variant of the invention based on NO is to be described in more detail: The atmosphere-forming reaction is involved

C₂H₄ + 2 NO ---> 2 CO + 2 H₂ + N₂

given. In turn, a treatment atmosphere is obtained which has an advantageous CO to H₂ ratio of approximately 1: 1. With this atmosphere, any carburization can be carried out according to the above pattern with suitable enrichment.

In summary, it can be stated that the processes described here give novel and effective carburizations which work on the basis of raw materials which have hitherto not been used - ethylene, propylene, butylene. These processes are not limited to the discontinuous chamber furnace described above, but can also be used in other furnaces, for example also in continuous and piercing furnaces.

Claims (5)

Process for heat treatment, in particular carburizing, of metallic workpieces in an oven in a treatment atmosphere at high temperatures, in which the atmosphere is generated by reacting a compound containing at least carbon and hydrogen with an oxygen-containing medium (for example air),
where appropriate, the implementation, supported by a generator or catalyst device, is carried out at or in the furnace
and where appropriate, an enrichment agent is additionally added to the atmosphere, characterized in that
that gaseous alkenes are used as carbon- and hydrogen-containing compounds A method according to claim 1, characterized in that an oxygen-containing gas or an oxygen-containing, gaseous compound is supplied as the oxygen-containing medium, these oxygen media in accordance with the alkenes

C _n H _2n + n O -> n CO + n H₂ (n = 2,3,4)

are reacted and the basis of the treatment atmosphere is formed, which can optionally be diluted with an inert gas, in particular nitrogen. A method according to claim 2, characterized in that pure oxygen gas is used as the oxygen-containing medium. Method according to one of claims 1 to 3, characterized in that propylene or ethylene is used as the C- and H-containing compound. Method according to one of claims 1 to 4, characterized in that in the case of a carburizing treatment after an initial period of approximately 10 to 70% of the total duration of treatment with a high CO and H2 content, one which is diluted with inert gas or - if already diluted - is used - Further dilute atmosphere is formed and with this the carburizing treatment is completed.

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