EP0655512B1 - Method for producing unitary oxidic layers on metallic substrates and apparatus for carrying out the process - Google Patents

Abstract

To achieve unitary (uniform) oxidic (oxidation) layers and defined surface properties, a process is provided for producing oxidic layers on metallic substrates subsequent to a nitridation (nitriding) or nitrocarburation process, in which the oxygen potential present in the oxidising atmosphere is continually determined, the measurement result is compared with a predetermined set value and, in the case of deviations, the oxidising atmosphere is modified until the measurement result agrees with the set value. <IMAGE>

Description

The invention relates to a method for producing uniform oxidation layers on metallic workpieces following a nitriding or Nitrocarburizing process, the workpieces after nitriding or Nitrocarburizing at a given temperature for a given time Oxidation atmosphere are exposed.

The generic methods are used to improve the wear or. the corrosion resistance of the surfaces of components made of ferrous materials. The nitrocarburizing process is carried out as a special nitriding process at temperatures between 570 ° C and 700 ° C. This minimizes the delay. After nitriding or nitrocarburizing, the workpieces are exposed to an oxidation atmosphere. One speaks of the so-called post-oxidation. The post-oxidation can significantly increase the wear resistance and in particular the corrosion resistance of the nitrocarburized surfaces. For this purpose, the workpieces are oxidized in an oxygen-containing atmosphere in an atmosphere-sphere furnace after the nitriding or nitrocarburizing. This post-oxidation normally takes place at temperatures between 500 ° C and 550 ° C, but is not limited to this range. Air, water vapor, CO ₂ , nitrous oxide and mixtures of these gases with nitrogen or ammonia are primarily used for the oxidation. The aim of the post-oxidation is to convert the outer edge layer of the workpieces, which essentially consists of iron nitrides or carbonitrides, into a thin iron oxide layer. Ideally, an iron oxide layer of the form Fe ₃ O _{4 is} sought, the so-called magnetite. Desired oxidation layer thicknesses are between 0.5 and 2 µm in the case of predominantly corrosive stress, or between 1 and 3 µm in the case of predominant wear and tear.

If the oxidation is too long, too strong or at too high temperatures, the oxide layers are too thick and can flake off. If the oxidation effect of the oxidation atmosphere is set incorrectly, other iron oxide modifications also arise, such as Fe ₂ O ₃ or FeO. These iron oxide modifications do not improve the corrosion and wear behavior of the workpieces, they can even worsen it. The setting of such iron oxides should therefore be avoided during post-oxidation. Generating Fe ₃ O ₄ layers that are too thick can also be harmful, particularly in the case of corrosion, as practice has shown. The targeted setting of the desired layer modification and thickness setting during post-oxidation is therefore of particular importance.

It is known in the prior art that this shift control over the Oxidation temperature and / or to carry out the oxidation duration. In the HTM 43, 1988, Issue 6, pages 365 to 372, is "NIOX - a modified one Nitrocarburizing process with subsequent oxidation "by S. Pakrasi described. A temperature of 520 ° C to be maintained is specified here, in which the workpieces are steamed for one hour become. According to another process, the so-called "NITROTEC" edge layer treatment process, published by C. Dawes in TZ for Metalworking, Issue 6, 1984, is a post-oxidation at 570 ° C to 580 ° C for 30 seconds. The post-oxidation is carried out in air becomes.

In practice it has been shown that the control of the layer modification and Layer thickness solely by setting the oxidation temperature and / or the duration of the oxidation and the chosen oxidation medium, i.e. for example Air or water, not enough to create uniform layers of oxidation and guarantee surface properties. Depending on the furnace, workpiece surface condition and other factors will have different post-oxidation results achieved. It has been shown that post-oxidation in air does not always lead to uniformly colored surfaces. It can be spotty trained Formed surfaces arise, although given oxidation temperatures and oxidation periods are observed.

Proceeding from this, the object of the invention is to improve a method for producing uniform oxidation layers of the generic type in such a way that defined, uniform oxidation layers and workpiece surface properties are achieved. Furthermore, an apparatus for performing the method is to be provided.

As a technical solution to this problem, the method according to the invention is further developed in that the oxygen potential present in the oxidation atmosphere is continuously determined, the measurement result is compared with a predetermined target value and, in the event of deviations, the oxidation atmosphere is modified until the measurement result matches the target value, the target value being so It is determined that the oxygen potential at the given oxidation temperature is always above the formation limit of Fe ₃ O ₄ oxide, but below the formation limit of other iron oxides.

By regulating the oxygen potential in the oxidation atmosphere can be ensured that no undesirable iron oxide modifications be formed. In addition, the results achieved are always uniform. The oxide layers have a uniform appearance and that Surface properties are defined and can be easily reproduced.

There is a range for setting the target value of the oxygen potential between 1100 mV and 800 mV at temperatures from 450 ° C to 650 ° C as proven suitable.

It is advantageously stated that when the measured oxygen potential is below the specified target value, further oxidizing agent is fed. If the measured oxygen potential value is above of the predetermined target value, then according to a proposal of the invention the supply of the oxidant stopped. Alternatively or additionally, the Oxidation atmosphere can also be supplied with a reducing agent.

Combinations of the proposed measures are also part of the Invention possible. According to a proposal of the invention, when the setpoint is exceeded the oxidant path connection and the reducing agent connection made both simultaneously and alternately. At a According to a proposal of the invention, the oxidant connection is undershot and the reducing agent path circuit also both made simultaneously and alternately.

Air, water, CO ₂ , N ₂ O and the like are advantageously supplied as the oxidizing agent, these oxidizing agents mentioned being able to be supplied individually but also in the context of gas mixtures from these components.

H ₂ , NH ₃ , CH ₄ or CO are advantageously proposed as reducing agents. These components can also be used individually or as a gas mixture.

On the device side, a device for post-oxidation after nitrocarburizing comprises or nitriding an atmosphere furnace, which is a heater and has at least one gas supply line. Usually point known atmosphere furnaces on circulating devices, which the the atmosphere forming gas from a batch room and along heaters or return heat exchangers to the batch room.

The device according to the invention for carrying out the method of manufacture uniform oxidation layers on metallic workpieces further developed in that an oxygen probe is arranged in the furnace chamber and that a control device is connected to the oxygen probe which is, depending on the measured value, the gas supply to the atmosphere furnace regulates.

According to the method, the oxygen potential can be determined with this oxygen probe determined at any time and depending on the time evaluate the temperature and the like. According to an advantageous embodiment According to the invention, the control device acts in the range of Valves in the gas supply lines.

A particularly advantageous device-side solution for controlling the Oxygen potential during post-oxidation is through the use of a Endexo retort marked. Such retorts are used to generate Carrier gas in nitrocarburizing furnaces already known. Using the Endexo retort Endogas is usually produced during nitrocarburization. To the Using Endexo retort for post-oxidation then only needs that Nitriding agent, for example ammonia, to be switched off. The Endexo retort is then carried out in such a way that when the target value is undershot of the oxygen potential in the oxidation atmosphere is the natural gas supply switched off, or if the setpoint is exceeded, the natural gas supply is switched on again with constant air supply to the retort.

Fig. 1 eine schematische Darstellung einer erfindungsgemäßen Vorrichtung zur Durchführung des erfindungsgemäßen Nachoxidationsverfahrens;

Fig. 2 eine schematische Darstellung eines weiteren Ausführungsbeispieles einer Vorrichtung zur Durchführung des erfindungsgemäßen Nachoxidationsverfahrens und

Fig. 3 ein Schaubild zur Darstellung des angestrebten Regelbereiches.

Further advantages and features of the invention result from the following description with reference to the figures. Show:

1 shows a schematic representation of a device according to the invention for carrying out the postoxidation process according to the invention;

Fig. 2 is a schematic representation of a further embodiment of a device for performing the post-oxidation process according to the invention and

Fig. 3 is a graph showing the desired control range.

1 shows an atmosphere furnace 1 known per se, consisting of a furnace housing 2, which is arranged on a frame 3. Internally the furnace 1 has a furnace chamber 4 which comprises a batch space 5. A circulation fan 6, which is driven by a thermally insulated drive 7 is driven, sucks the atmospheric gas from the batch space and it rolls into the space surrounding the furnace chamber, in which not shown Heating elements are arranged. The gas then comes out with the right one Put the temperature back into batch room 5 from below.

The atmospheric furnace 1 comprises a gas supply line 8, into which the gases required in each case are fed into the furnace space via corresponding valves. For example, the regulating valve 9a NH ₃ , the regulating valve 9b, the endogas or exogas, the regulating valve 9c, nitrogen and the regulating valve 9d, air, water or another oxidizing agent can be supplied.

An oxygen probe is used to determine the oxygen potential in the furnace chamber 4 10 provided with which the oxygen potential is measured becomes. From a control device, not shown, the measured value with compared to a target value. If a discrepancy is found, the Control device 13 controls the respective valves 9a, 9b and 9d to modify the oxidation atmosphere in the furnace chamber 4.

A corresponding embodiment is shown in FIG. 2. The atmosphere furnace 1 is constructed in the manner described. The figure shows the use a so-called endexo retort. Instead of the shown Oxygen probe 10 is also possible by means of a gas discharge line 12 to discharge so-called sample gas and outside the atmosphere furnace 1 to be measured using an oxygen probe (λ probe), not shown. About the Control valve 9a can, for example, ammonia via a separate Feed line 8 are introduced into the furnace. During post-oxidation this valve remains closed. Natural gas is produced via the control valve 9b fed. If the measured oxygen potential value exceeds the setpoint, valve 9b is opened. If the value falls below the setpoint, the Natural gas supply switched off. Via the oxygen source 11 and the valve 9c only oxidant is supplied to the furnace chamber 4.

The diagram shown in FIG. 3 shows the control range according to the invention, which is to be maintained by the setpoint determination. The diagram shows the oxygen probe voltage mV over the temperature in ° C. The desired range, in which in the oxidation atmosphere at the given oxidation temperature the oxidation is always just above the formation limit of Fe ₃ O ₄ oxide, but regularly below the formation limit of the other iron oxides, can be shown, for example, in FIG specify the temperature range from approximately 450 ° C to approximately 650 ° C with an oxygen probe voltage between approximately 1100 mV and 800 mV (hatched area).

Reference list

1

Atmospheric furnace

2nd

casing

3rd

frame

4th

Furnace chamber

5

Batch room

6

Circulation blowers

7

drive

8th

Gas supply line

9a

Valve

9b

Valve

9c

Valve

9d

Valve

10th

Oxygen probe

11

Oxygen source

12th

Gas discharge line

13

Control device

Claims (13)

1. Process for producing uniform oxide layers on metallic workpieces following a nitration or nitrocarburisation process, wherein the workpieces are exposed to an oxidising atmosphere at given temperature for a preset time after nitration or nitrocarburisation, characterised in that the oxygen potential existing in the oxidising atmosphere is determined continuously, the measured result is compared with a preset theoretical value and in the case of deviations the oxidising atmosphere is modified until the measured result agrees with the theoretical value, wherein the theoretical value is fixed so that the oxygen potential at the given oxidising temperature always lies above the formation limit of Fe₃O₄ oxide, but below the formation limit of other iron oxides.
2. Process according to claim 1, characterised in that the theoretical value for the oxygen potential is set between 1,100 mV and 800 mV at temperatures between 450°C and 650°C.
3. Process according to claim 1 or 2, characterised in that oxidising agent is supplied when the theoretical value is not reached.
4. Process according to one of the preceding claims, characterised in that the supply of oxidising agent is stopped when the theoretical value is exceeded.
5. Process according to one of the preceding claims, characterised in that a reducing agent is supplied when the theoretical value is exceeded.
6. Process according to one of the preceding claims, characterised in that the supply of reducing agent is stopped when the theoretical value is not reached.
7. Process according to one of the preceding claims, characterised in that when the theoretical value is exceeded the oxidising agent is disconnected and the reducing agent connected at the same time or alternately.
8. Process according to one of the preceding claims, characterised in that when the theoretical value is not reached the oxidising agent is connected and the reducing disconnected at the same time or alternately.
9. Process according to one of the preceding claims, characterised in that the oxidising agent contains air H₂O, CO₂ and/or N₂O.
10. Process according to one of the preceding claims, characterised in that the reducing agent contains H₂, NH₃, CH₄ and/or CO.
11. Device for carrying out the process according to one of claims 1 to 10, wherein the nitration or nitrocarburisation and the subsequent oxidisation is carried out in an atmospheric furnace (1) which has at least one gas supply line (8), characterised in that an oxygen probe (10) is arranged in the furnace chamber (4) and in that a control device is connected to the oxygen probe and controls the gas supply to the atmospheric furnace (1) as a function of the measured value.
12. Device according to claim 11, characterised in that the control device controls the gas supply via valves (9a, 9b, 9d).
13. Device according to one of claims 11 or 12, characterised in that an endexo retort is arranged in the region of the gas supply (8).

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