EP0882811B2 - Method of carburizing metallic workpieces in a vacuum furnace - Google Patents

Abstract

The method concerns carburisation of metal workpieces in a vacuum furnace, according to which the furnace atmosphere contains a carbon carrier which under the process conditions is split for delivery of pure carbon. The carbon carrier used is a hydrocarbon with a carbon to hydrogen ratio of 1:1, preferably acetylene.

Description

The invention relates to a method of carburizing metallic workpieces in a vacuum furnace, wherein the furnace atmosphere contains a carbon support which is split under the carburization process conditions to release pure carbon.

Among the thermochemical treatment processes for case hardening of metallic workpieces, in addition to conventional gas carburization, carburization processes have increasingly become established in vacuum plants in recent years, since carburization without edge oxidation is feasible only with these processes. These carburizing processes in vacuum plants are vacuum and plasma carburizing. Since working with these carburizing without oxygen-containing reaction gases, no C-level control can be done; In this process, the decisive parameter for the carbon transition is the carbon mass flow density, which is defined as the amount of carbon that passes into the material per unit time and area. This carbon needed for carburizing is provided by a carbonaceous carrier in the furnace atmosphere, usually a hydrocarbon, which is split at the given process conditions with release of pure carbon.

Vacuum carburization processes are, for example, from the documents SU-A-668978 . WO-A-96/3056 and GB-A-226127 known.

In the known vacuum carburizing process, the carbon carrier used is generally propane (C ₃ H ₈ ), which is split in the course of the so-called propane pyrolysis according to the following reaction equations:

C ₃ H ₈ → CH ₄ + C ₂ H ₄

C ₂ H ₄ → 2C + 2H ₂

CH ₄ → C + 2H ₂

In the case of plasma carburization, the carbon carrier used is usually methane (CH ₄ ), which is obtained by methane pyrolysis according to the equation

CH ₄ → C + 2H ₂

is split. In plasma carburization, however, it is also possible to use propane instead of methane.

The use of methane or propane as a carbon carrier is associated with various advantages and disadvantages. For example, propane is a more effective carbon carrier than methane because of its greater number of carbon atoms - 3 C atoms for propane versus 1 C atom for methane. On the other hand, however, propane has the disadvantage that it is already thermally cracked in the temperature range above 600 ° C, which can lead to sooting of the furnace and tar formation in the oven. The early dissociation of the propane even at low temperatures also has the consequence that in the treatment of densely packed batches as well as workpieces with difficult to access surfaces, such as blind holes, the dissociated carbon is released mainly outside of the batch, so that the carburizing effect in the middle of the batch is lower. The same is true not only for densely packed batches, but also for drilling, in particular blind holes, in which the carbon is discharged predominantly at the hole opening and in the interior of the bore barely a carburizing effect can be detected.

Although methane has only one carbon atom, the methane molecule is so stable that it is not already split at the necessary carburizing temperature. The cleavage takes place only in the plasma and thus really only on the workpiece surface. Since the carbon mass density in the cleavage of methane is low, large quantities of process gas must be supplied to the furnace in large-scale batches.

As already indicated above, another problem with the carburization methods known from the prior art is that soot formation in the furnace increases with increasing carbon-hydrogen ratio (C / H) of the carbon support. For methane, CH ₄ (C / H = 0.25), the soot accumulation is low, for ethane, C ₂ H ₆ (C / H = 0.33), the soot accumulation is medium, for propane, C ₃ H ₈ ( C / H = 0.375) and very high for butane, C ₄ H ₁₀ (C / H = 0.4). Thus, in optimizing the carburizing processes in vacuum furnaces, there are two conflicting demands and processes, namely, on the one hand, the requirement for increasing the carbon-to-hydrogen ratio in the carbon support to increase the carbon mass flow density to achieve a better carburizing effect and, on the other hand the increasing soot formation in increasing the carbon-to-hydrogen ratio of the carbon support. An increase of the partial pressure of the carbon carrier aimed at increasing the carbon mass flow density additionally increases soot formation in the furnace.

To reduce the soot formation with increasing partial pressure of the carbon carrier, it is for example from U.S. Patent 3,796,615 known to vary the partial pressure of the carbon carrier pulsating to higher partial pressures, so that the carbon mass flow density-increasing partial pressure of the Carbon support is available only for a short time to increase the carburizing effect, but then decreases again, so that the soot formation can be kept within limits. Due to the partially high partial pressure of the carbon support of up to 100 mbar, however, even in this operated with pressure pulses process is a gradual sooting of the furnace, so that it still has to be turned off for cleaning purposes.

In view of the above-described prior art, it is an object of the present invention to provide a method of carburizing metallic workpieces in a vacuum furnace which ensures consistent carburizing with a high carbon mass flow density without the risk of fouling of the furnace ,

Surprisingly, it has been found that this object is achieved in that as the carbon carrier is a hydrocarbon with a carbon-hydrogen ratio of 1: 1, preferably is used under the conditions stated in claim 1 acetylene.

Surprising in the use of acetylene as a carbon carrier is not only the very good and uniform carburizing effect even with difficult to access workpieces, but in particular the fact that despite the high carbon-to-hydrogen ratio of 1: 1 as well as no soot and Tar formation occurs. The good Aufkohlungswirkung when using acetylene as a carbon support can be explained by the fact that due to the high carbon-to-hydrogen ratio even at low partial pressures of the carbon support sufficient carbon mass flow density is available to a consistent and sufficient carburization to achieve.

According to the procedure of the invention, a partial pressure of the carbon support of less than 20 mbar, preferably 10 mbar, is advantageously maintained in order to achieve a high carbon mass flow density or carbon transfer rate without soot formation. In this case, the partial pressure of the carbon carrier is varied in a pulsating manner, the partial pressure of the carbon carrier reaching values of up to 50 mbar.

In addition to the hydrocarbon with a carbon-to-hydrogen ratio of 1: 1, the furnace atmosphere may additionally contain other gases, in particular hydrogen and / or argon, which should additionally prevent the oxidation of the workpieces as inert gases.

In a development of the method according to the invention, the splitting of the carbon carrier can be assisted by a plasma.

Fig. 1

einen schematischen Längsschnitt durch ein Probewerkstück mit zugehörigem Tabellenwerk, die Oberflächenhärtewerte auf der Innenseite des Probewerkstücks bei verschiedenen Kohlenstoff-Trägern wiedergebend;

Fig. 2

eine Seitenansicht des Probewerkstücks gemäß Fig. 1 mit der Angabe verschiedener Meßpunkte für den Härteverlauf an der Außen- und Innenseite des Probewerkstücks;

Fig. 3

eine graphische Darstellung des Härteverlaufs an den Meßpunkten A, C und E gemäß Fig. 2 an der Außenseite des Probewerkstücks nach der Einsatzhärtung mit Acetylen und

Fig. 4

eine graphische Darstellung des Härteverlaufs an den Meßpunkten B, D, F und H gemäß Fig. 2 an der Innenseite des Probewerkstück nach der Einsatzhärtung mit Acetylen.

Features of the method without the pressure pulsation according to the invention will become apparent from the following explanations, which refer to the accompanying drawings. In the drawing shows:

Fig. 1

a schematic longitudinal section through a sample workpiece with associated tables, the surface hardness values on the inside of the sample workpiece in different carbon carriers reproduced;

Fig. 2

a side view of the sample workpiece according to Fig. 1 with the indication of different measuring points for the hardness profile on the outside and inside of the sample work piece;

Fig. 3

a graphical representation of the hardness curve at the measuring points A, C and E according to Fig. 2 on the outside of the trial workpiece after case hardening with acetylene and

Fig. 4

a graphical representation of the hardness curve at the measuring points B, D, F and H according to Fig. 2 on the inside of the sample piece after case hardening with acetylene.

In the drawing with the corresponding tables, the course of the surface hardness on the inside of the tube after the vacuum carburizing with the carbon carriers acetylene, propane and ethane is shown comparatively for a tube made of the material 16 MnCr 5 with a stepped through hole.

The vacuum carburization with the carbon carriers propane and ethane was carried out at 860 ° C and with a partial pressure of the carbon carrier of 10 mbar. The vacuum carburization with the carbon carrier acetylene was carried out at 930 ° C and a partial pressure of the carbon carrier of 10 mbar over a period for the carburizing and diffusion phase of 260 min.

As can be seen from the figure and the accompanying tables, with the known from the prior art carbon carriers propane and ethane surface hardness of about 60 HRC and more only in the edge regions of the holes, that is up to a hole depth of about 50 mm seen from both bore openings, scored. By contrast, when using propane as the carbon support, the surface hardness value at the center of the through hole at a depth of 110 mm was only 36.0 HRC. So there was almost no carburisation here. When using ethane as a carbon support, which due to its lower carbon-to-hydrogen ratio at the same partial pressure of the carbon support only can achieve a lower carbon mass density, the surface hardness value in the middle of the through-hole was as low as 25.9 HRC.

In comparison with these known carbon carriers propane and ethane was achieved with the new carbon carrier acetylene almost consistently good carburization over the entire through hole. As can be seen from the table, the surface hardness value at the inner surface of the through hole is almost continuously at 60 HRC or more.

The above-described uniform carburization on the outer and inner surface of the sample workpiece also illustrate the illustrations Fig. 2 to 4 in which the surface hardness and the case hardening depth (HV 1.0) are shown at different measuring points. A comparison of the graphics in Fig. 3 and 4 shows that when acetylene is used as the carbon support, not only a nearly uniform surface hardness along the inner and outer workpiece surfaces is achieved, but also the case hardening depth (HV 1.0) at the inner and outer workpiece surfaces coincide almost at all measurement points.

Thus, by using a hydrocarbon having a carbon-to-hydrogen ratio of 1: 1, preferably acetylene, as the carbon support, it is possible to use the carburizing effect of carburizing metallic workpieces in a vacuum furnace even on workpieces To increase significantly with hard to reach surfaces without the risk of fouling of the furnace.

Claims (4)

1. Method for carburizing metallic workpieces in a vacuum furnace with a furnace atmosphere, which contains a carbon carrier, wherein the process conditions in the furnace atmosphere are set such that the carbon carrier is decomposed while releasing pure carbon at a reduced pressure,
   characterised in that the carbon carrier has a carbon-hydrogen ratio of 1:1 and the partial pressure of the carbon carrier is varied in a pulsating fashion, wherein the partial pressure of the carbon carrier is raised in pressure pulses to 50 mbar and otherwise maintained below 20 mbar.
2. Method according to Claim 1, characterised by acetylene as the carbon carrier.
3. Method according to Claim 1 or 2, characterised in that hydrogen and/or argon is/are contained in the furnace atmosphere in addition to the carbon carrier.
4. Method according to any one of Claims 1 to 3, characterised in that the carbon carrier is decomposed with the assistance of a plasma.

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