EP0049530A1 - Method and device for carbonizing metallic pieces - Google Patents

Abstract

A method for case hardening metallic workpieces is specified. The workpieces are brought to a high temperature in an annealing furnace, exposed to a carbon-containing gas mixture and hardened. In order to achieve a uniform carburizing of the workpieces in a reliable, fast and economical manner, one or more of the carbon-containing gas components of the gas mixture are or are added to the gas mixture in a pulsating manner during its action on the surface of the workpieces.

Description

The invention relates to a method and a device for carburizing metallic workpieces, in which the workpieces are brought to a high temperature in an annealing furnace, exposed to the action of a carbon-containing gas mixture and hardened.

Among the known carburizing processes, gas carburizing and carbonitriding are becoming increasingly important. The processes are carried out in closed annealing furnaces, which allow a controlled atmosphere to be set and maintained at a certain reaction temperature. The main problem with the gas carburizing process is the transfer of carbon from the gas atmosphere to the material, e.g. Steel, to be carried out in a controlled manner in order to achieve reproducible carburizing results on workpieces of different basic carbon content, different alloys and in particular different shapes.

It is known to form a carburizing gas atmosphere from a gas mixture of the three components nitrogen, coal Introduce hydrogen and oxygen dispenser into the furnace chamber (DE-OS 24 50 879 and DE-OS 28 18 558). The following are mentioned as hydrocarbons: paraffin or paraffinic hydrocarbons, methane, ethane, propane, butane or natural gas with a proportion of 7.5 to 38 grams of carbon. Oxygen, air, carbon dioxide, carbon monoxide, water vapor or mixtures thereof serve as oxygen donors. The gas components are fed to the furnace separately or in mixtures. The measurement and control of the gas atmosphere takes place via a dew point, infrared (C0 ₂ ) or oxygen measurement.

However, it has been shown that the above-mentioned relationship between carbon dioxide, oxygen content and dew point, on the one hand, and carbon potential, on the other hand, cannot readily be used for control purposes, since the gases introduced into the furnace are not in reaction equilibrium.

In this context, it is also known to add methane as a carbon-containing gas to a gas atmosphere. In this process, the carburizing process is divided into two or three intervals in which the carburizing agent is fed into the furnace (interval carbons). The intervals are separated in time by two or three phases in which no carbonizing agent is fed into the furnace. During the carbonization intervals, the carbon potential in the furnace rises sharply and soot is formed. In the subsequent phase, in which air is fed into the furnace instead of carbon dioxide, the carbon potential drops again to zero. Edge oxidations cannot be avoided with this procedure. However, if the proportion of carburizing agent in the gas mixture is reduced in order to avoid excessive soot formation, the carburizing time increases considerably.

The invention is therefore based on the object of a method for the uniform carburizing of workpieces that are more reliable, faster and therefore more economical.

This object is achieved in that one or more of the carbon-containing gas components of the gas mixture are added in a pulsating manner during its action on the surface of the workpieces.

The invention is based on the knowledge that a large carbon potential gradient between the workpiece surface and the core itself comes to the fore as an additional driving diffusion force from the beginning of carburization. In order to achieve this carbon potential gradient, the carbon-containing gas components of the gas mixture are added in a pulsating manner. This means that the carbon content of the gas mixture is raised to a certain level in various phases during the carburization, these phases being separated by time intervals in which the carbon content is not changed by the addition of carbon-containing gas components, and the carbon content of the gas mixture consequently decreases. The gas mixture has at least atmospheric pressure and rises rapidly during the intermittent addition of a carbon-containing gas component. This pressure increase is particularly pronounced when hydrocarbons with two or more carbon atoms are introduced into the furnace, since each of their molecules breaks down into several gas molecules with a corresponding pressure increase. The pressure of the gas atmosphere fluctuates in the same rhythm in which the pulsating addition of the carbon-containing gas components takes place ("breathing" of the gas atmosphere).

The main advantage of this process is that the glow time is reduced by up to 60% compared to the endogas process, while the duration of carburization takes place under glow time and understanding diffusion. Large hardening depths are also achieved. With the method according to the invention, a uniform carburization is achieved, which leads to the workpieces being free of soot and edge oxidation. Furthermore, the process is significantly more reliable, because of the low H ₂ content, the risk of explosion is lower and the furnaces can be gassed safely during idle and at the weekend.

In addition, gas generators are no longer necessary.

It has proven to be particularly advantageous to add one or more hydrocarbons in a pulsating manner, the duration of the addition period being short compared to the spaces between two addition periods (diffusion phase). The addition period is 2 to 200 seconds, preferably 15 to 60 seconds, while the duration of the diffusion phase is 10 to 500 seconds, preferably 50 to 200 seconds.

According to a further embodiment of the method according to the invention, the methane and / or the soot content in the gas mixture is measured, the measured variables are fed to a control unit and the addition of one or more of the hydrocarbons by the control unit after comparison of the values of the methane and / or soot content with a setpoint set in each case.

According to the invention, the setpoints are selected so that over-carbonization, ie, soot formation, does not take place. The surface of the metallic workpieces therefore always remains soot-free. Rather, the carbon potential is kept almost constant at a value, the working line, around which the carbon potential fluctuates. The carbon potential rises briefly during the pulsed addition of hydrocarbons and falls below the almost constant mean value given during the subsequent diffusion phase. The coal However, material potential never drops to zero. It is crucial here that the working line of the process is at a high carbon potential above the classic soot limit, without producing soot, since the stay in the soot area is only very short. This fact has proven to be an essential feature of the method according to the invention, on which the high carburizing rate can be attributed.

The method according to the invention can also be used for carbonitriding the workpieces. For this purpose, ammonia is added pulsating to the gas mixture while it is acting on the surface of the workpieces.

An annealing furnace for carrying out the method according to the invention essentially has a device for supplying and discharging gases into and from the annealing furnace and an associated control device. The control device is advantageously connected to a soot sensor and a gas analyzer for determining the methane content in the gas mixture, the control unit responding to the difference between measured value signals and a setpoint signal for the soot and / or methane content that can be set in one or two setpoint transmitters Valve for the addition of hydrocarbons is connected.

In such an annealing furnace, it is also advantageous if the device for supplying the carbon-containing gas components opens into the interior of the annealing furnace in the lower region and in the immediate vicinity of the carburized workpieces located in the annealing furnace.

• Figur 1 die schematische Darstellung einer Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens;
• Figur2 den zeitlichen Verlauf der Änderung des Kohlenstoffgehaltes im Aufkohlungsgas;
• Figur 3 einen schematischen Ausschnitt aus einem Doppelstirnrad;
• Figur 4 den Härteverlauf gemäß Beispiel 1;
• Figur 5 den Härteverlauf gemäß Beispiel 2;
• Figur 6 den Verlauf der Einhärtetiefe mit der Zeit.

The sequence of the method according to the invention is to be demonstrated below with reference to the drawing. Show it:

• Figure 1 is a schematic representation of an apparatus for performing the method according to the invention;
• Figure 2 shows the time course of the change in the carbon content in the carburizing gas;
• Figure 3 shows a schematic section of a double spur gear;
• Figure 4 shows the hardness curve according to Example 1;
• Figure 5 shows the hardness curve according to Example 2;
• Figure 6 shows the course of the hardening depth with time.

An annealing furnace 1 is connected via a line 2 to a control unit consisting of a soot sensor 3, a gas analyzer 4 and a controller 5. The gas components nitrogen, carbon dioxide, hydrocarbon, and which come directly from storage bottles, are fed via valves 7, 8 and 9 and a feed 6 into the annealing furnace, which is connected to an outflow line 10 for the exhaust gases. Valve 7, which regulates the supply of nitrogen, is open for driving in and heating up the batch in the furnace chamber, while valve 8 (carbon dioxide) and valve 9 (hydrocarbon, for example propane) are closed. When a temperature of 800 ° - 1000 ° C is reached in the annealing furnace, depending on the alloy of the workpieces and the desired hardening depth, the carburization of the batch begins. For this purpose, valves 7 and 8 are open and also valve 9 for a short time, for example 20 seconds. The gas mixture passed into the furnace chamber accordingly consists of inert nitrogen, CO ₂ and propane.

The propane is unstable at the high temperatures and breaks down into radicals, some of which are highly reactive, which cause rapid oversaturation of the workpiece surface with carbon. Because of the resulting significant carbon potential gradient between the workpiece surface and the core, this potential gradient comes to the fore as an additional driving diffusion force from the beginning of the carbonization.

Because of the carbon supersaturation, both methane and soot are generated in the gas mixture. A part of this gas mixture is drawn off via line 2 and the soot content in the soot sensor and the methane content in the gas analyzer 4, for example an infrared analyzer, are measured. In the controller 5, the output values of these two measuring devices are compared with predetermined target values, and if these target values are exceeded, the valve 9 is closed via a relay 11, i.e. the supply of e.g. Interrupted propane. The addition period for the propane is, for example, 20 seconds.

During the subsequent diffusion phase, which is, for example, 60 seconds, the supersaturation of the workpiece surface is continuously reduced by the carbon diffusing further towards the core of the workpiece. At the same time, the iron carbides formed due to the high supersaturation are redissolved from the material. Small amounts of amorphous soot are removed from the workpiece surface by reaction with CO ₂ to CO. During this period, the methane and soot content in the gas mixture decrease again. When a predetermined minimum setpoint is reached, the controller 5 opens the valve 9 again via the relay 11 and a new cycle begins.

After a predetermined carburizing time or after reaching a predetermined hardening depth, the valves 8 and 9 close and the charge is lowered to the hardening temperature.

In Figure 2, the carbon content in the gas mixture is plotted in volume percent against time in seconds. The working line "dynamic equilibrium" of the method according to the invention is shown in dashed lines in this diagram, while the classic soot limit thermodynamic equilibrium is shown as a solid line.

A double spur gear made of 20 Mn CrS, module 5 is sketched in the detail in FIG. 3. The broad dashed line outside the body edges marks the surface hardened area, the narrow dashed line its position and its course. The course of the hardened surface layer requires the determination of the measuring points, which are designated M1 and M2, for the hardening depth.

example 1

A production batch having a surface area of about 11 m ² and a weight of 500 kg is at a temperature Glühtem ^e-p 130 min of 945 ° C in a Aichelin multipurpose chamber furnace. Carburized long. The addition period for propane is approximately 20 seconds and the duration of the diffusion phase is approximately 60 seconds.

Figure 4 shows the hardness curve on a double spur gear (material 16 Mn Cr5) from this batch. The surface hardness in HV according to DIN 6773 is plotted against the hardening depth in mm. Curve 1 relates to measuring point 1 and curve 2 to measuring point 2. The workpiece is free of carbides and residual austenite, the carburized steel is martensite. It can clearly be seen that there is constant surface hardening down to a hardening depth of approx. 0.45 mm. However, this then decreases with the hardening depth, due to the shape of the workpiece at measuring point M1 more slowly than at measuring point M2. A specified surface hardness of 610 HV (solid line) corresponds to a hardening depth of 0.83 mm at measuring point M1 and 0.68 mm at measuring point M2. This means that the indentation in the workpiece is sufficiently hardened to avoid deformation.

Example 2

Analogous to Example 1, with the difference that the annealing temperature is 960 ° C., the annealing time is 30 minutes, and the addition period for Propane on average 90 seconds and the diffusion time is approximately 200 seconds.

Figure 5 shows the hardness curve. In this case, a surface hardness of 610 HV corresponds to a hardening depth of 0.45 mm at measuring point M1 and 0.36 mm at measuring point M2. So after a short time (30 min) a considerable hardening depth was reached.

FIG. 6 shows the course of the case hardening depth at a limit hardness of 610 HV1 as a function of the annealing time at different annealing temperatures. The almost linear course of the curve from a hardening depth of approx. 0.45 mm is noteworthy. Curves for annealing temperatures between 930 ° C and 960 ° C lie between the two curves shown. The results of 60 tests under production conditions are incorporated into the curve.

Claims (7)

1. A process for case hardening of metallic workpieces, in which the workpieces are brought to a high temperature in an annealing furnace, exposed to the action of a carbon-containing gas mixture and hardened, characterized in that one or more of the carbon-containing gas components of the gas mixture during its action on the surface of the workpieces is added to the gas mixture in a pulsating manner. 2. The method according to claim 1, characterized in that one or more hydrocarbon (s) - in particular hydrocarbons with two or more carbon atoms - is (are) added, the duration of the addition period is short compared to the intervals between two addition periods (diffusion phases) . 3. The method according to any one of claims 1 or 2, characterized in that the addition period 2 to 200 sec., Preferably 15 to 60 sec. And the period from the end of an addition period to the beginning of the next (diffusion phase) 10 to 500 sec., is preferably 50 to 200 sec. 4. The method according to any one of claims 1 to 3, characterized in that the methane and / or the continuously The amount of soot in the gas mixture is measured, the measured quantities are fed to a control unit and the addition of one or more of the hydrocarbons is controlled by the control unit after comparing the values of the methane and / or the soot content with a respectively predetermined target value. 5. The method according to any one of claims 1 to 4, characterized in that the mixture is additionally added pulsating ammonia during its action on the surface of the workpieces. 6. annealing furnace for performing the method according to any one of claims 1 to 5 with means for supplying and discharging gases into or from the interior of the annealing furnace and an associated control device, characterized in that the control device with a soot sensor and a gas analyzer for determining the methane content in is connected to the gas mixture, the control device being responsive to the difference between measured value signals and a setpoint signal for the soot and / or methane content which can be set in one or two setpoint generators and being connected to a valve for the addition of carbon-containing gas components. 7. annealing furnace according to claim 6, characterized in that the device for supplying the carbon-containing gas components in the lower region and in the immediate vicinity of the workpieces located in the annealing furnace, to be carburized opens into the interior of the annealing furnace.

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