EP0779376A1 - Plasma carburizing of metallic workpieces - Google Patents

Abstract

In plasma carburisation of metal work pieces in a furnace containing an atmosphere of carbon carrier under plasma carburisation conditions to produce pure C, a mixture of methane and propane is used as C carrier.

Description

The invention relates to a process for the plasma carburization of metallic workpieces in a furnace, the furnace atmosphere containing a carbon carrier which is split under the process conditions of the plasma carburization with the release of pure carbon.

In addition to conventional gas carburizing, the carburizing processes in vacuum systems have become more and more common among thermochemical treatment processes for case hardening of metallic workpieces, since only these processes can be used to carry out carburizing without edge oxidation. These carburizing processes in vacuum plants are low pressure and plasma carburizing. Since these carburizing processes work without oxygen-containing reaction gases, no C level control can take place; The decisive parameter for the carbon transition in these processes is the carbon mass flow density, which is defined as the amount of carbon that passes into the material per unit of time and area. This carbon required for the carburization is made available by a carbon carrier in the furnace atmosphere - usually a hydrocarbon - which is split under the given process conditions with the release of pure carbon.

$${\text{C}}_{\text{2}}{\text{H}}_{\text{4}}{\text{→ 2C + 2H}}_{\text{2}}$$

$${\text{CH}}_{\text{4}}{\text{→ C + 2H}}_{\text{2}}$$

In the known low-pressure carburizing processes, propane (C ₃ H ₈ ) is generally used as the carbon carrier, which is split in the course of the so-called propane pyrolysis according to the following reaction equations: $${\text{C.}}_{\text{3rd}}{\text{H}}_{\text{8th}}{\text{→ CH}}_{\text{4th}}{\text{+ C}}_{\text{2nd}}{\text{H}}_{\text{4th}}$$

$${\text{C.}}_{\text{2nd}}{\text{H}}_{\text{4th}}{\text{→ 2C + 2H}}_{\text{2nd}}$$

$${\text{CH}}_{\text{4th}}{\text{→ C + 2H}}_{\text{2nd}}$$

gespalten wird. Bei der Plasmaaufkohlung ist es jedoch auch möglich, anstelle von Methan Propan zu verwenden.In plasma carburizing, methane (CH ₄ ) is mostly used as the carbon carrier, which is obtained by methane pyrolysis according to the equation $${\text{CH}}_{\text{4th}}{\text{→ C + 2H}}_{\text{2nd}}$$

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

The use of methane or propane as a carbon carrier has various advantages and disadvantages. For example, propane is a more effective carbon carrier than methane due to its larger number of carbon atoms - 3 carbon atoms for propane versus 1 carbon atom for methane. On the other hand, however, propane has the disadvantage that propane is thermally split in the temperature range above 600 ° C., as a result of which carburization takes place in the furnace, which leads to sooting of the furnace. Methane, on the other hand, has only one carbon atom, but the methane molecule is so stable that it is not already split at the necessary carburizing temperature. Rather, the splitting takes place only in the plasma and therefore really only on the workpiece surface. Since the carbon mass flow density is only very low when splitting methane, large-scale batches are very difficult to carburize evenly with methane.

In view of the prior art described above, the invention has for its object to provide a method for the plasma carburizing of metal workpieces, which ensures carburizing with a high carbon mass flow density, without at the same time the risk of sooting the furnace.

Surprisingly, the experiments has been found in the course that this object is inventively achieved in that a mixture of methane and propane is used as carbon support.

The achievement of the high carbon mass flow density on the one hand and the avoidance of sooting on the other hand is due to the fact that propane can provide much more carbon than methane due to its three carbon atoms during thermal and electrical splitting in the plasma. The methane on the other hand hardly splits at carburizing temperatures between 800 ° C and 1000 ° C. The methane is only split in the plasma, ie really only on the workpiece surface, so that these released carbon atoms can only contribute to carburizing the workpieces, but not to sooting the furnace.

The tests have shown that a methane / propane mixture with up to 60 vol.% Propane, in particular a propane fraction of 5 to 50 vol.%, Is particularly suitable for achieving a high carbon mass flow density or soot formation without soot formation To get carbon transfer rate.

According to a preferred embodiment of the method according to the invention, the gas pressure in the furnace atmosphere is below 10 mbar, since thermal splitting of the methane is almost impossible in this pressure range.

In addition to the methane-propane mixture, the furnace atmosphere can also contain other gases, in particular hydrogen and / or argon, which, as inert gases, are also intended to prevent oxidation of the workpieces.

The drawing shows the hardness curve for the material 27 CrMo 4 according to the plasma carburizing process with a methane-propane mixture as a carbon carrier.

• zehnminütiges Aufkohlen bei einer Aufkohlungstemperatur von 940°C.
• Die anschließende Diffusionsphase betrug 51 Minuten,
• woran anschließend nach dem Absenken auf die Härtetemperatur von 860°C die Charge mittels Hochdruckgasabschreckung abgeschreckt wurde.

The process parameters for the plasma carburizing process shown in the figure were:

• carburizing for ten minutes at a carburizing temperature of 940 ° C.
• The subsequent diffusion phase was 51 minutes,
• after which the charge was quenched by high-pressure gas quenching after lowering to the hardening temperature of 860 ° C.

As a result of this process, a case hardening depth (550 HV 1) of 0.7 mm was achieved on the tooth flank.

With the method described above, it is thus possible, by using the methane-propane mixture as the carbon carrier, to significantly increase the carbon mass flow density in the plasma carburizing, without the risk of sooting the furnace.

Claims (6)

Process for plasma carburizing metallic workpieces in a furnace, the furnace atmosphere containing a carbon carrier which is split under the process conditions of plasma carburization with the release of pure carbon,
characterized,
that a mixture of methane and propane is used as the carbon carrier. A method according to claim 1, characterized in that the methane-propane mixture contains up to 60 vol .-% propane. A method according to claim 1 or 2, characterized in that the propane content in the methane-propane mixture is 5 to 50% by volume. Method according to one of claims 1 to 3, characterized in that the gas pressure of the furnace atmosphere is below 10 mbar. Method according to one of claims 1 to 4, characterized in that other gases are contained in the furnace atmosphere in addition to the carbon carrier. A method according to claim 5, characterized in that the furnace atmosphere additionally contains hydrogen and / or argon.

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