EP0080124B1 - Process for case hardening metallic articles - Google Patents

Description

The invention relates to a method for case hardening metallic workpieces, the workpieces being exposed to the action of a carbon-containing gas mixture, to which one or more carbon-containing gas component (s) is (are) added in a pulsating manner during its action on the workpieces.

Such a method is described in EP-A-49 530. Due to the pulsating addition of the carbon-containing gas component, a large carbon potential gradient is achieved in a method of the aforementioned type. Pulsating addition of a carbon-containing gas component means adding it to the gas mixture during numerous cycles consisting of two different phases. In the first phase of a cycle, the carbon-containing gas is added to the gas mixture in a pulsed manner and the carbon potential of the gas atmosphere is raised to a certain level. In the second phase, the supply of carbon-containing gas is interrupted, that is, a carbon-containing gas is supplied to the gas mixture. This reduces the carbon potential of the gas atmosphere.

With this procedure, the workpiece is heavily carburized in a very thin layer. The diffusion that follows in the second phase now runs very quickly and creates new absorption capacity for carbon in the edge layer of the workpiece.

The pulsating addition of carbon-containing gas components enables a high, even carbon input up to and beyond saturation. This type of case hardening forms carbides, which dissolve to a certain extent during the second phase of a cycle. However, a complete dissolution of the very stable carbide compounds was not possible within one cycle.

This fact in particular prevented the use of the known method in a push-through furnace through which the workpieces (batches) pass in several, time-constant cycles.

From DE-C-618 026 a method for cementing iron and steel is known in which the supply of the cementing agent is interrupted from time to time. The interruption occurs when the penetration of the cementing agent slows down in the workpiece.

DE-A-2 851 982 relates to a method for controlling carburizing processes in the vacuum region. A carbon-containing gas is pumped out and renewed when a certain degree of decay of the gas has been reached.

DE-C-726 134 discloses a method for carburizing metallic workpieces. A certain gas atmosphere is supplied to the interior of the furnace. Pressure fluctuations are caused in the gas atmosphere in the furnace interior. For this purpose, a larger amount of gas is introduced into the interior of the furnace, so that the pressure increases abruptly. Gas escapes through openings in the furnace so that the pressure in the interior drops to the outlet pressure within a few minutes.

Finally, EP-A-49 531 describes a process for case hardening metallic workpieces, in which workpieces in a furnace are exposed to the action of a carbon-containing gas mixture at high temperatures and pressure fluctuations are generated in the gas mixture with a period that is considerably shorter compared to the duration of the heat treatment .

The invention is based on the object of specifying a method of the type mentioned at the outset with which workpieces can be carburized or carbonitrided uniformly, carbides formed being largely removed.

This object is achieved in accordance with the invention in that the addition of the carbon-containing gas component (s) is interrupted after several cycles, each consisting of a phase of the pulsed addition of carbon-containing gas and a subsequent phase without the addition of carbon-containing gas, and only after a pause has elapsed, which takes 10 to 100 times longer than a cycle to resume.

According to the invention, the carburizing or carbonitriding process is no longer divided exclusively into cycles consisting of two phases. Rather, the inventive method for case hardening is composed of several intervals, each of which consists of several cycles followed by a pause. Cycle - as in the known method - is to be understood as the brief addition of a hydrocarbon to the gas mixture with subsequent interruption of the hydrocarbon supply.

In the conventional process, the carbon potential never drops to zero during the case hardening process. The pulsating addition of hydrocarbons prevents the soot limit from being exceeded. The method according to the invention is now based on the knowledge that the duration of the phase in which no hydrocarbon is added to the gas mixture during a cycle is not sufficient for optimal diffusion and dissolution of the very stable carbide compounds.

Due to the very high reactivity of the split hydrocarbons, the actual carburization process is extremely short compared to the diffusion phases. Their length depends on the current carburization depth. The larger this becomes, the slower it diffuses the newly introduced carbon inside. According to the invention, a very high carbon potential is now reached at the beginning of an interval, as in the known method. After several cycles, however, there is a pause that is long compared to the duration of a cycle.

During this pause, the carbon concentration in the edge area of the workpieces drops in every interval. At the beginning of the next interval, the gradient of the carbon content and therefore the carbon transition in the workpiece is very high. The amount of carbon-containing gas supplied in a cycle or the number of cycles is selected so that the supply of the carbon-containing gas is cut off as soon as carbon saturation is reached in the edge layer of the workpieces. The total carburizing time is made up of the periods in which carbon is introduced into the workpiece and the periods in which the carbon diffuses in the workpiece. According to the invention, the duration of the carbon introduction is considerably shorter than that of the diffusion.

It was found that the process according to the invention causes the very stable carbides to dissolve during an interval, so that an excellent, surface oxidation-free surface quality of the workpieces is achieved. Because of this feature, the proposed method can also be used in push-through furnaces.

Furthermore, it was found that, compared to the known method, a significantly smaller number of cycles is sufficient to achieve the same hardening depth. In addition, the average consumption of carbon-containing gas, for example propane, is lower than in the known method.

In an advantageous embodiment of the inventive concept, in which nitrogen is added to the gas mixture, the amount of nitrogen supplied per unit of time is increased for the duration of the pulsating addition of the carbon-containing gas component (s).

In this variant, the furnace in which the workpieces are treated is constantly gassed with nitrogen to ensure an inert basic gas atmosphere in the furnace chamber.

When the selected carburizing temperature is reached, the amount of nitrogen is increased for the duration of the pulsating hydrocarbon addition in order not to let the concentration of the hydrocarbon radicals become too high. Otherwise they would react with each other and form soot and would no longer contribute to carburization.

According to one feature of the method according to the invention, it is particularly economical if the pause lasts a factor of 10 to 100 longer than the previous cycles.

In this variant, carbide compounds are still largely eliminated, but the duration of the case hardening process is minimal.

Due to the decreasing carbon potential gradient, the diffusion process takes place more slowly with increasing depth of carburization. The breaks are therefore advantageously extended with increasing duration of the carburizing process.

A particularly simple regulation is possible in the method according to the invention if, according to an advantageous variant, the duration of the cycles is kept constant during case hardening. The carbon-containing gas is consequently pulsed into the furnace chamber at constant time intervals.

In a further embodiment of the invention, the gas atmosphere is formed exclusively from an inert gas, in particular nitrogen, and a carbon-containing gas, in particular propane.

An exemplary embodiment of the method according to the invention is to be explained below with the aid of schematic sketches.

• Figur 1 eine schematische Darstellung einer Anlage, in der das erfindungsgemäße Verfahren durchgeführt werden kann,
• Figur 2 den zeitlichen Verlauf des Kohlenstoffgehaltes im Aufkohlungsgas.

Show it:

• FIG. 1 shows a schematic representation of a plant in which the method according to the invention can be carried out,
• Figure 2 shows the time course of the carbon content in the carburizing gas.

An annealing furnace 1 is connected via a line 2 to a soot sensor 3 and a control unit 4. According to the invention, the gas components nitrogen and a hydrocarbon, in the exemplary embodiment propane, are now introduced into the annealing furnace 1. Supply lines 5 (nitrogen) and 6 (propane) are used for this. Valves 7, 8, 9, 10 and 11 control the addition of nitrogen and propane from the appropriate tanks in which these components are stored in liquid form. Nitrogen and propane are mixed before entering the annealing furnace 1 and passed into the furnace via a line 12. When the batch is moved into the furnace chamber, valve 9 in a line 13 is open, while valves 7 and 10 are closed. At this point, the furnace atmosphere consists essentially of nitrogen. The carburizing or carbonitriding process begins as soon as a suitable temperature of approx. 800 to 1000 ° C has been reached in the furnace. In order to expose the workpieces in the furnace chamber to a gas mixture with a high carbon potential, solenoid valve 10 is opened according to the invention. Valve 10 is opened and closed by the control unit 4 at constant time intervals. According to the invention, a certain amount of nitrogen is always passed into the furnace per unit time during case hardening. During the phases in which propane is briefly fed into the furnace, the amount of nitrogen supplied is increased in order not to make the concentration of the hydrocarbon radicals too great. Otherwise the radicals would react with one another and form soot. For this purpose, solenoid valve 7 in one Bypass line 14 opened by the control unit 4. The propane is unstable at the high temperatures prevailing in the interior of the annealing furnace and disintegrates spontaneously. The fission products are very reactive and enable the workpiece surface layer to be saturated quickly with carbon. Because of the resulting significant carbon potential difference between the workpiece surface and the core, the carbon quickly diffuses into the edge layer of the workpieces.

Soot can now be formed in the gas mixture. The amount of propane is reduced before a minimum soot value is reached. The value determined by the sensor 3 is sent to the control unit 4, in which the measured value is compared with a predetermined setpoint signal. A difference between the measured value and setpoint signal is converted into a signal by which a control valve 11 in the supply line 6 for propane is controlled.

When the soot value corresponding to the target value is exceeded, the amount of propane is reduced by control valve 11 until the actual and target values match again.

After several cycles in which propane was briefly fed into the furnace in a first phase or the propane supply was interrupted in a second phase, valve 10 - triggered by control unit 4 - is closed for a pause which lasts longer than the previous cycles and only nitrogen is fed via line 13 with valve 9 into the furnace. During this pause, the carbon concentration in the edge region of the workpiece drops again, so that at the beginning of the interval consisting of several cycles with a subsequent pause, the gradient and thus the carbon transition into the workpiece is high. During the breaks, the carbon not only diffuses into the workpiece, but also the unwanted carbides on the workpiece surface are redissolved and removed.

After several intervals, valves 7 and 10 are closed and the workpieces are lowered to the hardening temperature.

In Figure 2, the carbon content in the gas mixture (e.g. in% by volume) is shown schematically against time (in seconds).

Essentially, an interval of the method according to the invention is shown which lasts from the time zero to the time T '. At the beginning of the interval, the carbon content of the gas mixture is briefly increased in several cycles, each of which takes time t. A cycle consists of phase I (duration _e ) and phase 11 (duration t ₂ ). As already described, in phase I propane is added to the gas mixture in pulses, while in phase 11 no propane is added to the gas mixture. In phase I increases, in phase 11 the carbon content therefore decreases. During the cycles, however, the carbon content does not drop to 0. It is only in the pause (duration T) after several cycles (only the first four cycles in the first interval are shown in the diagram) that no propane is fed into the furnace the carbon content to 0.

For example, a typical interval contains up to 15 cycles. The addition periods depend on the furnace volume and the tightness of the furnace. For example, phase I lasts 5 to 15 seconds, phase 11, on the other hand, 15 to 60 seconds. Several cycles (e.g. 10 with a total duration of 5 minutes) are followed by a break of e.g. T - 30 minutes on so that an interval lasts about 35 minutes. With the same hardening depth, the number of cycles of the method according to the invention is reduced by a factor of 3 compared to the known method.

In summary, it can be stated that the method according to the invention enables rapid case hardening of workpieces with low consumption of carbon carriers. Since carbides formed are redissolved by the process according to the invention, push-through furnaces can also be used in this process.

Claims (5)

1. A method of case-hardening metallic workpieces, in which the workpieces are subjected to the action of a carbonaceous gas mixture, to which one or more carbonaceous gas component(s) is(are) added in pulsating manner during the action on the workpieces,
characterised in
that the addition of the carbonaceous gas component(s) is interrupted after a plurality of cycles which in each case consist of a phase of the pulsating addition of carbonaceous gas and a subsequent phase without the addition of carbonaceous gas, and is only resumed after the expiry of a pause which lasts longer than a cycle by a factor of 10 to 100.

2. A method as claimed in Claim 1, comprising an N-containing gas mixture, characterised in that the amount of nitrogen added per unit of time, is increased for the duration of the pulsating addition of the carbonaceous gas component(s).

3. A method as claimed in one of Claims 1 to 3, characterised in that the pauses are extended with increasing duration of the carburizing process.

4. A method as claimed in one of Claims 1 to 4, characterised in that the duration of the cycles is kept constant during the case-hardening.

5. A method as claimed in one of Claims 1 to 5, characterised in that the gas atmosphere is formed exclusively from an inert gas, in particular nitrogen, and a carbonaceous gas or a plurality of carbonaceous gases, in particular, propane.

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