EP1160349B1 - Process and apparatus for heat treating of metallic workpieces - Google Patents

Abstract

Apparatus for heat treating metallic workpieces comprises a heating chamber in which the workpieces are heated and subjected to a gas atmosphere containing nitrogen and a hydrocarbon; and devices for introducing nitrogen and hydrocarbons into the gas atmosphere depending on the carburizing index which represents the carbon content in the edge layer. An Independent claim is also included for a heat treatment process using the above apparatus. The apparatus preferably also has devices for introducing ammonia. The heating chamber is hermetically sealed and is flushed with an inert gas, preferably nitrogen.

Description

The invention relates to a method for heat treatment of metallic Workpieces, especially for carburizing or carbonitriding Ferrous materials. It also relates to a device with which one can carry out such procedure.

To create defined workpiece properties, such as a high one Wear or alternating strength, metallic workpieces become one subjected to thermochemical heat treatment. The result of Heat treatment is the enrichment of carburizing or carbonitriding Surface layer of the workpieces with carbon and / or nitrogen, due to the changed material composition of the workpieces after a subsequent hardening to give the required mechanical properties.

In addition to the, the treatment result is of particular influence general process parameters pressure, temperature and duration of treatment in primarily the composition of the enrichment of the boundary layer Carbon and / or nitrogen causing gas atmosphere. It can be about be required to add a reducing agent to the gas atmosphere in order to obtain a impermissibly high oxidation of the surface of the workpieces due to the Loading the workpieces in a heat treatment furnace due to the to avoid inevitable air intake noticeable oxygen.

The prior art also includes a number of procedures known to the composition of the gas atmosphere in terms of to adjust the desired treatment outcome. The gas atmosphere is in Depending on the carburizing temperature in its carburizing effect on the adapt to the respective requirements. This is a common practice So-called saturation compensation method, in which during a Enrichment phase the maximum possible amount of carbon in the Gas atmosphere is provided to saturate the surface layer with Reach carbon in a short time. To get the one you want Carbon content in the boundary layer will then increase during one subsequent diffusion phase, the exchange of substances with the gas atmosphere restricted or prevented by either using a gas atmosphere reduced carbon supply set or the gas atmosphere evacuated or is exchanged for an inert gas.

A method based on this is disclosed in DE-A-31 39 622, the Enrichment phase through different regulation of the Carbon levels of the gas atmosphere in a first and a second period is divided. During the first period, the maximum possible Carbon potential gradient between the gas atmosphere and the boundary layer Acceleration of the mass transfer and thus the diffusion exploited. With The beginning of the second period is the carbon potential gradient to avoid harmful carbide formation and the carbon level of the Gas atmosphere lowered so far that during the further diffusion phase a predetermined limit of the carbon content of the surface layer is exceeded.

Furthermore, EP-A-0 080 124 describes a case hardening process known metallic workpieces, in which by pulsating addition carbon-containing gas components achieved a high carbon potential gradient becomes. The supply of the carbon-containing gas takes place in several cycles instead of and is interrupted as soon as saturation occurs in the boundary layer Carbon is reached. In addition, GB-A-2 202 238 discloses a method for Carburizing workpieces, in which the enrichment phase and the Diffusion phase periodically until a desired carburization depth is reached alternate to the desired carbon content of the surface layer and the target carburization depth largely independent of each other and in to achieve as short a time as possible.

A method and a device for thermal or thermomechanical Treatment of steel is also known from US 5,139,584. Is described with this patent, a two-step treatment process which is as follows is carried out: A first phase at possible maximum temperature, defined based on metallurgical criteria, with control of the entry quantity of Hydrocarbons to get a maximum concentration of carbons on the surface, this maximum concentration being at least equal to that Saturation concentration of carbon of austenite; a second phase at the possible maximum temperature, defined based on metallurgical Criteria during which the carbon flow is transferred from the gas phase is zero is the carbon concentration profile and in particular the Set the concentration on the surface to a predetermined value, the is chosen based on metallurgical reasons. A value of 0.7% to 0.8% for conventional carburizing steels.

A disadvantage of all these methods is that they are not always satisfactory Treatment outcome. This is mainly due to the fact that during a diffusion phase following the enrichment phase or Temperature compensation phase no longer has any direct influence on the There is carbon content in the surface layer. This comes in particular Dimensions to be taken into account when there are small amounts of carbon dioxide or Water vapor are in the gas atmosphere, for example due to a Air entry at leaks of a heat treatment furnace are formed and the cause an increased degradation of the carbon in the surface layer. About that also contain those used to regulate carbon levels during the Process gases used in the enrichment phase oxygen-containing components, such as carbon monoxide, carbon dioxide or Water vapor due to the partial pressure of oxygen in the outside Edge area of the carburized edge layer an undesirable edge oxidation cause.

The invention has for its object to provide a method and an apparatus for the heat treatment of metallic workpieces with which an improved treatment result can be achieved in a comparatively simple manner while avoiding carbide formation on the surface of the workpieces to be treated.

a) Erwärmen der Werkstücke auf eine bestimmte Temperatur in einer Stickstoff und einen Kohtenwasserstoff enthaltenden Gasatmosphäre während einer Aufheizphase, wobei die Zufuhr der Mengen an Stickstoff und Kohlenwasserstoff in die Gasatmosphäre derart geregelt wird, dass sich auf den Werkstücken eine Randschicht mit einem Kohlenstoffgehalt innerhalb eines vorgegebenen Bereichs bildet;

b) Halten der Werkstücke auf der am Ende der Aufheizphase erreichten Temperatur für eine bestimmte Zeitdauer während einer sich anschließenden Ausgleichsphase, wobei durch die mit der Gasatmosphäre zugeführten Mengen an Stickstoff und Kohlenwasserstoff der Kohlenstoffgehalt der Randschicht weiterhin innerhalb des vorgegebenen Bereichs liegt;

c) Variieren der Mengen an Stickstoff und Kohlenwasserstoff in der Gasatmosphäre während einer sich an die Ausgleichsphase anschließenden Anreicherungsphase und einer hierauf folgenden Diffusionsphase, wobei jeweils zunächst in einem ersten Abschnitt zum Erreichen einer vorgegebenen Obergrenze des Kohlenstoffgehalts der Randschicht der Anteil an Stickstoff reduziert und der Anteil an Kohlenwasserstoff erhöht wird und nachfolgend in einem zweiten Abschnitt zum Erreichen einer vorgegebenen Untergrenze des Kohlenstoffgehalts der Randschicht der Anteil an Stickstoff erhöht und der Anteil an Kohlenwasserstoff reduziert wird;

d) Abkühlen der Werkstücke auf Raumtemperatur während einer sich an die Diffusionsphase anschließenden Abkühlungsphase.

According to the invention, this object is achieved by a process for the heat treatment of metallic workpieces, in particular for carburizing or carbonitriding iron materials, which has the following process steps:

a) Heating the workpieces to a certain temperature in a gas atmosphere containing nitrogen and a hydrocarbon during a heating phase, the supply of the amounts of nitrogen and hydrocarbon in the gas atmosphere being regulated in such a way that an edge layer with a carbon content within a predetermined value is formed on the workpieces Area forms;

b) maintaining the workpieces at the temperature reached at the end of the heating phase for a certain period of time during a subsequent compensation phase, the carbon content of the surface layer still being within the predetermined range due to the amounts of nitrogen and hydrocarbon supplied with the gas atmosphere;

c) Varying the amounts of nitrogen and hydrocarbon in the gas atmosphere during an enrichment phase following the equalization phase and a subsequent diffusion phase, the proportion of nitrogen and the proportion being reduced in a first section in order to achieve a predetermined upper limit for the carbon content of the boundary layer is increased in hydrocarbon and subsequently in a second section to achieve a predetermined lower limit of the carbon content of the boundary layer, the proportion of nitrogen is increased and the proportion of hydrocarbon is reduced;

d) cooling the workpieces to room temperature during a cooling phase following the diffusion phase.

Such a procedure adopts the knowledge that one can achieve improved treatment results of the workpieces if both during the enrichment phase and during the diffusion phase the carbon content of the boundary layer by varying that of the gas atmosphere supplied volume flows of nitrogen and hydrocarbon in one predetermined interval between a lower limit and an upper limit is set. By first in each case in a first section Nitrogen is reduced and the proportion of hydrocarbon is increased high carbon supply in the gas atmosphere, which in a relatively short Time the carbon content of the boundary layer up to the specified upper limit accumulates. By subsequently increasing the proportion of nitrogen and Reduction of the hydrocarbon content in a second section the carbon potential difference between the gas atmosphere and the Surface layer with the consequence that the carbon content of the surface layer as a result of Diffusion of the carbon into the interior of the workpieces to the given Lower limit drops. In this way, the surface of the Workpieces to be treated optimized carbon supply in the Provide gas atmosphere while it is also possible to create an undesirable one Avoid carbide or soot formation on the surface of the workpieces.

The temperature to which heating takes place in the heating phase, the length of time for which the workpieces are kept at this temperature during the compensation phase, the temperatures which are set in the enrichment phase and the diffusion phase, and the amounts of nitrogen supplied in the respective process stages and carbon depend primarily on the material of the workpieces to be treated, the specific composition of the gas atmosphere required to achieve the desired carbon content of the surface layer and the desired treatment success, such as the desired carburization depth. Since the process parameters of the material properties depend on the workpieces to be treated, can be obtained from publicly available databases, such as the Calphad (Cal culation of pha se d iagrams), Stockholm, necessary for a particular steel composition parameter values with respect to the desired carbon content in the Remove surface layer.

It is particularly advantageous to repeat the first one alternately and second sections during the enrichment phase and / or the Diffusion phase corresponding to the enrichment of the surface layer with carbon the absorption capacity of the depending on the respective material To adapt workpieces. With regard to that required for carbonitriding Release of diffusible nitrogen can also be advantageous during the enrichment phase and / or the diffusion phase of the Ammonia gas atmosphere, preferably in the order of 2% by volume up to 20 vol% of the total gassing rate. Has proven to be beneficial also highlighted the amounts of nitrogen and hydrocarbon in the Such gas atmosphere during the enrichment phase and the diffusion phase to vary that the lower limit of the carbon content of the surface layer is between 0.5% and 0.8% by weight. With such a carbon content the Boundary layer, the proportion of nitrogen in the gas atmosphere is still sufficient high to ensure safe heat treatment.

To prevent undesirable carbide or soot formation on the surface of the Definitely exclude workpieces, the amounts of nitrogen and Hydrocarbon in the gas atmosphere during the enrichment phase and / or the diffusion phase advantageously varies such that the Upper limit of the carbon content of the surface layer slightly below the due to carbide formation occurring at the prevailing temperature marked saturation of austenite. The upper limit of Carbon content of the surface layer can be so during the diffusion phase be chosen to meet the requirements of one of the following Cooling phase taking place corresponds to the hardening process.

A particularly advantageous procedure is also given if the workpieces during the heating phase to one for carburizing or Carbonitriding favorable temperature between 750 ° C and 1050 ° C to be heated. A carbon content of the surface layer is also of particular advantage during the heating phase in the range between 0.2 wt .-% and 0.5 wt .-%. Possibly existing oxide or passive layers on the workpieces are eliminated in this way or at least converted so that a uniform diffusion of carbon into the material is favored. To for this purpose it is also useful to remove the workpieces during the Compensation phase for a period of 0.1 h to 1 h at the end of the Keep heating temperature reached. With regard to one desired treatment success corresponding procedure management also a reduced temperature during the enrichment phase during the diffusion phase be useful.

A particularly advantageous procedure is also given when unsaturated hydrocarbons of the type C _n H _2n , preferably alkenes such as ethylene (C ₂ H ₄ ) and propylene (C ₃ H ₆ ), saturated hydrocarbons of the type C _n H _{2n + 2} , preferably alkanes such as ethane (C ₂ H ₆ ) and propane (C ₃ H ₈ ), or alkynes such as acetylene (C ₂ H ₂ ) are added to the gas atmosphere. Primarily alkynes, which are characterized by a low proportion of hydrogen, offer the advantage that no further fission products are generated and the proportion of hydrogen in the gas atmosphere accordingly remains low. In order to obtain a reducing and thus avoiding oxidation of the workpieces to be treated, a reducing agent, preferably hydrogen, is advantageously added to the gas atmosphere in the case of oxygen present in the gas atmosphere, which stems, for example, from an air inlet at leaks in a heat treatment furnace constant or variable amount added. The point in time and the amount of reducing agent added depend on the prevailing circumstances.

In a particularly advantageous development of the method according to the invention, the supply of the amounts of nitrogen and hydrocarbon into the gas atmosphere is regulated or controlled in a database-dependent manner as a function of a characteristic value representing the carbon content in the surface layer, preferably the carbonation index K _c . The database-related control is advantageous if a characteristic value representing the carbon content in the surface layer is not available, for example due to measurement difficulties. The regulation of the supply as a function of a characteristic value representing the carbon content in the surface layer, on the other hand, enables the desired carbon content of the surface layer to be set precisely, without the size of the surface of the workpieces to be treated being particularly important. The carbon index K _c , which gives information about the carbon content in the surface layer, is calculated, for example, by the quotient of the partial pressure of methane (p _CH4 ) and the square of the partial pressure of hydrogen (p _H2 ² ) (K _c = p _CH4 / p _H2 ² ).

The workpieces are expediently removed during the cooling phase a reducing or neutral gas atmosphere or in a liquid Quench medium cooled to room temperature.

To solve the above object, a device for heat treatment of metallic workpieces is also proposed, which has a heating chamber in which the workpieces can be heated and exposed to a gas atmosphere containing nitrogen and a hydrocarbon, and is characterized in that means are provided with which the supply of the Amounts of nitrogen and hydrocarbon in the gas atmosphere as a function of a characteristic value representing the carbon content in the boundary layer, preferably the carbon index K _c , can be regulated or controlled in a database-related manner, with a means arranged within the heating chamber as a means of measuring the characteristic value representing the carbon content in the boundary layer Wire sensor is provided, and that means are provided with which the supply of the amount of a reducing agent, preferably hydrogen, and / or ammonia can be regulated or controlled in a database-related manner. The method according to the invention can be carried out in a reliable manner with such a device.

In a further development of this device, it is also proposed that the Heating chamber is hermetically sealed to prevent air from entering and thus avoid oxygen in the gas atmosphere. Eventually proposed that the heating chamber be filled with an inert gas, preferably nitrogen, is flushable, so that regardless of the volume of the heating chamber the content of Oxygen in the atmosphere of the heating chamber after introduction of the treating workpieces in a comparatively short time to <1% by volume reducible and undesirable oxidation of the workpieces to be treated this is avoidable.

Ein den Verlauf von Temperatur, Kohlenstoffgehalt und verschiedener Volumenströme über der Zeit veranschaulichendes Diagramm.

Details and further advantages of the objects of the present invention result from the following description of a preferred exemplary embodiment. The only figure in the accompanying drawing shows:

A diagram illustrating the course of temperature, carbon content and different volume flows over time.

In the diagram shown in FIG. 1, the time t on the abscissa and the temperature ϑ on the ordinate, the carbon content w _c resulting in the surface layer of workpieces to be treated, and the volume flows of hydrogen V ˙ _H supplied to a gas atmosphere for this purpose ₂ , Nitrogen V ˙ _{N 2} , Ammonia V ˙ _NH , and the hydrocarbon acetylene V ˙ _{C 2 H 2} ablated. The heat treatment method shown in Fig. 1 is primarily used for carburizing metallic workpieces, which can consist of different steels. The entire process can be divided into five phases. In a first phase, the heating phase A, the workpieces are heated to a carburizing temperature ϑ of, for example, 930 ° C. The heat treatment furnace used for this purpose, an atmosphere furnace, was flushed with nitrogen in a short time after the workpieces had been introduced, and was then flooded with a gas atmosphere containing nitrogen and acetylene. The size of the volume flows of nitrogen V ˙ _N supplied to the gas atmosphere ₂ and acetylene V ˙ _{C 2 H 2} is such that an edge layer with a carbon content w _c of approx. 0.35% by weight is obtained on the workpieces. The supply of the amounts of nitrogen V ˙ _{N 2} and acetylene V ˙ _{C 2 H 2} The gas atmosphere is regulated as a function of a characteristic value representing the carbon content w _c in the surface layer, which is determined by a measuring device. A wire sensor arranged in the heat treatment furnace as used in the article by K.-H. Weissohn in HTM 49 (1994) 2, pages 118 to 122. As can also be seen in FIG. 1, the gas atmosphere becomes a constant volume flow of hydrogen V ˙ _H during the heating phase A. ₂ of approx. 0.3 m ³ / h to put the gas atmosphere in a sufficiently reducing state.

During a compensation phase B following the heating-up phase A, the workpieces are kept at the temperature ϑ reached at the end of the heating-up phase A of approx. 930 ° C. for approx. 20 min. The quantities of nitrogen V ˙ _N fed into the gas atmosphere ₂ and acetylene V ˙ _{C 2 H 2} are further regulated so that there is a carbon content w _c in the surface layer of the workpieces of approx. 0.35% by weight. The volume flow of hydrogen V ˙ _{H 2} is, however, reduced to a value of less than 0.2 m ³ / h.

In an enrichment phase C following the equalization phase B, the volume flows of nitrogen V ˙ _N supplied to the gas atmosphere become ₂ and acetylene V ˙ _{C 2 H 2} varies in such a way that a carbon content w _{c of} the surface layer - occurs in an interval between a lower limit G _u of approximately 0.7% by weight and an upper limit G _o of approximately 1.2% by weight. At the temperature ϑ of approx. 930 ° C which continues to exist in the enrichment phase C, the upper limit G _o of approx. 1.2% by weight is below the saturation limit of austenite which is characterized by carbide and soot formation for unalloyed case-hardened steels. In order to first reach the upper limit G _o , the volume flow of nitrogen V ˙ _{N 2} reduced and the volume flow of acetylene V ˙ _{C 2 H 2} elevated. As soon as the carbon content w _{c has reached} the upper limit G _o , the volume flow of nitrogen V ˙ _N is reached in a second section in order to reach the lower limit G _{u 2} increased again, whereas the volume flow of acetylene V ˙ _{C 2 H 2} is reduced. The first and second sections are repeated alternately within the enrichment phase C, whereby an adaptation of the enrichment of the surface layer with carbon is achieved in accordance with the absorption capacity of the workpieces depending on the respective material. For this purpose, the duration and thus the respective size of the volume flows of nitrogen is V ˙ _{N 2} and acetylene V ˙ _{C 2 H 2} during the first and second sections unevenly, as can be seen from the steep slopes of the curve of the carbon content w _c in FIG. 1. The system and process-adapted volume flow of hydrogen V ˙ _{H 2} is increased to approximately 0.2 m ³ / h during the enrichment phase C.

The enrichment phase C is followed by a diffusion phase D in which the temperature ϑ is reduced to approximately 880 ° C. With regard to a subsequent hardening process, the upper limit G _{o is} reduced to approximately 0.8% by weight. According to the procedure during the enrichment phase C, the thermodynamic equilibrium between the gas atmosphere and the boundary layer of the workpieces to be treated is varied by varying the volume flows of nitrogen V ˙ _N supplied to the gas atmosphere ₂ and acetylene V ˙ _{C 2 H 2} set in first and second sections such that the carbon content w _{c of} the boundary layer lies in the interval between the lower limit G _u of approximately 0.7% by weight and the upper limit G _o of approximately 0.8% by weight. The system and process-adapted volume flow of hydrogen V ˙ _{H 2} is again reduced to a value below 0.2 m ³ / h during the diffusion phase D. Finally, the workpieces are cooled to room temperature during a cooling phase E following the diffusion phase D.

If carbonitriding takes place instead of a pure carburizing process, ammonia V ˙ _{NH can} additionally be present in the gas atmosphere during the enrichment phase C and the diffusion phase D, as can be seen in FIG. 1 ₃ are supplied, the amount of ammonia supplied V ˙ _{NH 3} in the order of 2 vol.% to 20 vol.% of the total gassing rate.

The method described above is characterized by the volume flow regulation of the quantities of nitrogen V ˙ _N supplied to the gas atmosphere ₂ and acetylene V ˙ _{C 2 H 2} in the individual stages of the process, whereby an improved treatment result is achieved in comparison with conventional carburizing processes. The main reason for this is that the carbon supply in the gas atmosphere can be adapted in this way according to the material and the surface of the workpieces to be treated and thus the absorption capacity of the surface layer. Last but not least, the possibility to provide alternating first and second sections of different carbon contents w _c during the enrichment phase C and the diffusion phase D also contributes to this result to a particular extent.

LIST OF REFERENCE NUMBERS

A

heating phase

B

balancing phase

C

enrichment phase

D

diffusion phase

e

cooling phase

t

time

θ

temperature

w _c

Carbon content

G _o

Upper carbon limit

G _u

Lower limit carbon content

V ˙ _{C 2 H 2}

Volume flow acetylene

V ˙ _{N 2}

Volume flow nitrogen

V ˙ _{H 2}

Volume flow hydrogen

V ˙ _{NH 3}

Volume flow ammonia

Claims (16)

1. Method of heat treatment of metal workpieces, particularly for carburising or carbonitriding iron workpieces, comprising the following method steps:

   a) heating of the workpieces to a specific temperature (ϑ) in a gaseous atmosphere containing nitrogen and a hydrocarbon during a heating phase (A), wherein the supply of quantities of nitrogen (V ˙ _{C 2 H 2} ) and hydrocarbon (V ˙ _{C 2 H 2} ) into the gaseous atmosphere is regulated in such a way that an edge layer forms on the workpieces with a carbon content (w_c) within a predetermined range;

   b) keeping the workpieces at the temperature (ϑ) reached at the end of the heating phase (A) for a specific period of time during a subsequent balancing phase (B), wherein the carbon content (w_c) of the edge layer still lies within the predetermined range due to the quantities of nitrogen (V ˙ _{N 2} ) and hydrocarbon (V ˙ _{C 2 H 2} ) supplied with the gaseous atmosphere;

   c) varying the quantities of nitrogen (V ˙ _N2 ) and hydrocarbon (V ˙ _C2H2 ) in the gaseous atmosphere during an enrichment phase (C) which follows the balancing phase (B) and during a subsequent diffusion phase (D), wherein in each case first of all in a first period in order to reach a predetermined upper limit (G_o) of the carbon content (w_c) of the edge layer the proportion of nitrogen ( V ˙ _N2 ) is reduced and the proportion of hydrocarbon (V ˙ _C2H2 ) is increased and then in a second period in order to reach a predetermined lower limit (G_u) of the carbon content (w_c) of the edge layer the proportion of nitrogen (V ˙ _N2 ) is increased and the proportion of hydrocarbon (V ˙ _C2H2 ) is reduced;

   d) cooling of the workpieces to room temperature during a cooling phase (E) following the diffusion phase (D).
2. Method as claimed in Claim 1, characterised by an alternating repetition of the first and second periods during the enrichment phase (C) and/or the diffusion phase (D).
3. Method as claimed in either Claim 1 or Claim 2, characterised in that during the enrichment phase (C) and/or the diffusion phase (D) ammonia (V ˙ _NH3 ) is added to the gaseous atmosphere, preferably in the order of magnitude of 2% to 20% by volume of the total quantity of gas injected.
4. Method as claimed in any one of Claims 1 to 3, characterised in that the quantities of nitrogen (V ˙ _{N 2} ) and hydrocarbon (V ˙ _C2H2 ) in the gaseous atmosphere are varied during the enrichment phase (C) and the diffusion phase (D) in such a way that the lower limit (G_u) of the carbon content (w_c) of the edge layer is between 0.5% and 0.8% by weight.
5. Method as claimed in any one of Claims 1 to 4, characterised in that the quantities of nitrogen (V ˙ _N2 ) and hydrocarbon (V ˙ _C2H2 ) in the gaseous atmosphere are varied during the enrichment phase (C) in such a way that the upper limit (G_o) of the carbon content (w_c) of the edge layer is slightly below the austenite saturation characterised by a carbide formation at the respective prevailing temperature (ϑ).
6. Method as claimed in any one of Claims 1 to 5, characterised in that the workpieces are heated during the heating phase (A) to a temperature (ϑ) between 750°C and 1050°C.
7. Method as claimed in any one of Claims 1 to 6, characterised by a carbon content (w_c) of the edge layer during the heating phase (A) in the range between 0.2% by weight and 0.5% by weight.
8. Method as claimed in any one of Claims 1 to 7, characterised in that during the balancing phase (B) the workpieces are kept for a period of 0.1 h to 1 h at the temperature (ϑ) reached at the end of the heating phase (A).
9. Method as claimed in any one of Claims 1 to 8, characterised by a temperature (ϑ) during the diffusion phase (D) which is reduced relative to the enrichment phase (C).
10. Method as claimed in any one of Claims 1 to 9, characterised in that unsaturated hydrocarbons of the C_nH_2n type, preferably alkenes such as for instance ethylene (C₂H₄) and propylene (C₃H₆), saturated hydrocarbons of the C_nH_2n+2 type, preferably alkanes such as for instance ethane (C₂H₆) and propane (C₃H₈), or alkynes such as for instance acetylene (C₂H₂) are added to the gaseous atmosphere.
11. Method as claimed in any one of Claims 1 to 10, characterised in that a reducing agent, preferably hydrogen ( ^.V _H2 ), is added to the gaseous atmosphere in a constant or variable quantity.
12. Method as claimed in any one of Claims 1 to 11, characterised in that the supply of quantities of nitrogen (V ˙ _N2 ) and hydrocarbon (V ˙ _C2H2 ) to the gaseous atmosphere is regulated as a function of a characteristic value representing the carbon content (w_c) in the edge layer, preferably the carbonisation characteristic quantity K_c, or is controlled in relation to a database.
13. Method as claimed in any one of Claims 1 to 12, characterised in that the during the cooling phase (E) the workpieces are cooled to room temperature in a reducing or neutral gaseous atmosphere or in a liquid quenching medium.
14. Apparatus for the heat treatment of metal workpieces, particularly for carrying out the method as claimed in any one of Claims 1 to 13, with a heating chamber in which the workpieces can be heated and can be exposed to a gaseous atmosphere containing nitrogen and a hydrocarbon, characterised in that means are provided by which the supply of quantities of nitrogen (V ˙ _N2 ) and hydrocarbon (V ˙ _C2H2 ) to the gaseous atmosphere is regulated as a function of a characteristic value representing the carbon content (w_c) in the edge layer, preferably the carbonisation characteristic quantity K_c, or is controlled in relation to a database, wherein a wire sensor disposed inside the heating chamber is provided as means for measuring the characteristic value representing the carbon content in the edge layer, and that means are provided by which the supply of the quantity of a reducing agent, preferably hydrogen (V ˙ _H2 ), and/or ammonia (V ˙ _NH3 ) can be regulated or can be controlled in relation to a database.
15. Apparatus as claimed in Claim 14, characterised in that the heating chamber is constructed so that it can be hermetically sealed.
16. Apparatus as claimed in either one of Claims 14 or 15, characterised in that the heating chamber can be flushed with an inert gas, preferably nitrogen.

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