Abstract:
The invention relates to a method for carbonitriding at least one component ( 12 ) in a treatment chamber ( 16 ), in which at least one process gas ( 28; 30 ) is introduced into the treatment chamber ( 16 ), wherein a hydrogen content ( 44 ) is detected in an atmosphere developing in the treatment chamber ( 16 ) and is maintained in a desired range ( 55; 57 ) at least at intervals by influencing of the amount of the process gas ( 28; 30 ) that is fed.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for carbonitriding at least one component in a treatment chamber, in which at least one process gas is introduced into the treatment chamber as well as to a treatment chamber and an open-loop and/or closed loop control device for such a treatment chamber. 
     Methods for carbonitriding metal components are known from the German patent publications DE 199 09 694 A1, DE 101 18 494 A1 and DE 103 22 255 A1. Carbonitriding of metal components is a thermochemical process, in which carbon and nitrogen are introduced into the surface layer of an iron-based material. It is a particular kind of “case hardening”. The German patent publication DE 199 09 694 A1 describes a carbonitriding method, in which the diffusion of nitrogen occurs during the entire process or when using nitrogen as the donating gas preferably in the last process phase. The German patent publication DE 101 18 494 C2 describes a low-pressure carbonitriding method, in which steel components are initially carburized and subsequently nitrided with a nitrogen-donating gas. The German patent publication DE 103 22 255 A1 describes a method for carburizing steel components, in which nitrogen producing gas is added during the heating-up phase as well as during the diffusion phase. The nitrogen diffused during carbonitriding leads to an improved resistance to wear due to friction and to an improved resistance to tempering in the surface layer. 
     The process control during carbonitriding takes place in at least one treatment chamber by the presetting of pressure, temperature, time, process gas composition and process gas flow volume. During carbonitriding, molecular hydrogen can develop as a by-product from the carbon-donating and nitrogen-donating gases. The hydrogen content can be detected by suitable sensors. The sensors being used must be designed for use in low-pressure or vacuum systems. 
     In conventional gas-nitriding processes, commercially available hydrogen sensors allow for the control of the process gas atmosphere with the aid of the nitriding index. The nitriding index is defined as follows:
 
 k   N   =p   NH3 ( p   H2 ) 1.5 , wherein
 
     k N =Nitriding index 
     P NH3 =Pressure of the ammonia, and 
     P H2 =Pressure of the hydrogen, 
     and describes the relationship between ammonia supply and ammonia conversion and consequently determines the excess supply of ammonia. In the case of gas-nitiriding processes exceeding the processing time, constant, reproducible nitriding conditions can be set independently of the size of the surface of the component charge by means of controlling in accordance with the nitriding index. 
     In the case of low-pressure carbonitriding processes, a control in accordance with the nitriding index is not possible because the carbon and nitrogen concentration and thereby the carbon and nitrogen absorption constantly change on the component surface while the process is being carried out, whereby no constant, reproducible carburizing and nitriding conditions can be set when the nitriding index is held constant. The progression of the gas decomposition or respectively of the resulting reactions occurs as a function of pressure, temperature and as a function of the reactive or catalytically active surface of the component charge or furnace lining. 
     The residence time of the gases in the treatment chamber resulting from the rate of flow is therefore crucial for the atmospheric composition in said treatment chamber. 
     For this reason, solid gas quantities for carrying out the process are in practice empirically ascertained through an elaborate series of tests, said solid gas quantities however only apply to the tested charge structure, the treatment chamber and the material of the metal components which is used. A transfer of the solid gas quantities to other process implementations, materials, charge structures and treatment chambers is not directly possible. These would have to again be empirically ascertained. The German patent publication DE 101 18 494 C2 describes a low-pressure carbonitriding using solid gases. 
     SUMMARY OF THE INVENTION 
     The problem underlying the invention is solved by a method according to the invention as well as by a treatment chamber and an open-loop and/or closed-loop control device according to the invention. Important features for the invention are also found in the subsequent description and in the drawings, wherein the features can be of importance to the invention by themselves as well as in different combinations without explicit reference again being made to this fact. 
     By detecting a hydrogen content in a treatment chamber, the method according to the invention has the advantage of being able to carry out an even, reproducible carbonitriding using low-pressure carbonitriding on at least one component located in the treatment chamber independently of a charge size or a furnace unit. 
     According to the invention, a hydrogen content of a process gas atmosphere is monitored in the treatment chamber with the aid of a hydrogen sensor for the purpose of carbonitriding components. A carbon- or nitrogen-donating gas supply is thereby set or controlled using predefined limit values for the hydrogen content. This is based on the consideration that using the measured value of the hydrogen content, a carbon- or nitrogen-donating gas supply in the process gas atmosphere can be inferred independently of the structure of the component charge and/or the treatment chamber. Based on this inference, the quantity of the process gas flowing into the treatment chamber can be controlled with respect to point in time and/or period of time and/or quantity. The method can basically be used with a variety of components and is especially well suited for metallic components, in particular for iron-based materials. The use of the method is therefore described below with regard to metallic components. 
     In one process phase of the so-called “low-pressure carburization”, for example using acetylene, a maximum hydrogen content in the process gas atmosphere (“atmosphere”) of, for example, 75 vol % should not be exceeded. In a process phase, in which a nitrogen-donating gas, for example ammonia, is introduced into the treatment chamber, a maximum hydrogen content in the atmosphere of, for example, 50 vol % should not be exceeded due to the known nitrogen effusion. The advantage of the invention is that comparable carbon or nitrogen supplies are present locally at one or several metal components of a charge, and therefore a uniform carbon or nitrogen input into the surface(s) of the metal components is made possible. The method generally makes provision for the individual process phases to be carried out for any desired number and/or in any desired order. 
     In addition, the carbon or the nitrogen absorption changes during the processing time due to the already absorbed carbon or nitrogen and on account of the limited solubility of the two elements in the metallic matrix of the surfaces of the components. By means of the closed-loop control of the process or the process gas atmosphere with the aid of the hydrogen content detected by the hydrogen sensor, an unnecessarily high carbon or nitrogen supply can be prevented, whereby a process gas application which is efficient as possible and therefore a reduction in the processing costs are achieved. 
     The formation of toxic compounds, as e.g. cyanides, can be minimized or prevented by a monitoring of the process gas atmosphere, which is made possible by the detection of the hydrogen content. The treatment chamber is preferably evacuated or purged with an inert gas, such as nitrogen or argon in order to prevent a simultaneous presence, for example, of carbon- and nitrogen-donating gas. In so doing, undesired chemical reactions, as for example the formation of cyanides, can be prevented. The hydrogen content in the atmosphere of the treatment chamber, which was detected during the process gas exchange, can also be indirectly used as the measured value for the contents of the carbon- or nitrogen-donating gases. If a treatment chamber is purged with an inert gas during a process gas exchange, it can be assumed for hydrogen contents less than 5 vol %, preferably less than 1 vol %, that the concentrations of carbon- or nitrogen-donating gases are sufficiently small to adequately reduce or prevent environmental damage. If the treatment chamber is evacuated during a process gas exchange, it is required for a pressure in the treatment chamber of at least less than 1×10 −1  mbar, preferably less than 1×10 −2 , to be undershot, whereby it can be assumed that the concentration of carbon- or nitrogen-donating gases is sufficiently small to adequately reduce or prevent environmental damage. 
     The method is especially simple to implement if a flow volume of the process gas introduced into the treatment chamber is controlled. For example, the quantity of the introduced process gas can be open-loop and/or closed-loop controlled by means of an adjustable valve at the inlet of the treatment chamber. 
     The method furthermore makes provision for the process gas to comprise a carbon-donating gas. In so doing, a first gas or a first gas composition for a process phase for the carbonitriding of components is provided, with which the carbon content important for the carbonitriding is directly influenced, a fast and precise open-loop or closed-loop control being thereby facilitated. 
     It is particularly favorable if the carbon-donating gas comprises a compound which is selected from a group consisting of acetylene, ethylene, propane, propylene, methane, hexanaphthene, cyclopentane or mixtures thereof. For this reason, a selection of commercially available and thereby comparatively inexpensive gases is available as the carbon-donating gas for implementing the method. 
     Provision is further made in the method for the process gas to comprise a nitrogen-donating gas. In so doing, a second gas or a second gas composition for a process phase for carbonitriding of components is provided, with which the nitrogen content important for the carbonitriding is directly influenced, a fast and precise open-loop or closed-loop control being thereby facilitated. 
     It is thereby favorable if the nitrogen-donating gas comprises a compound which is selected from a group consisting of ammonia, nitrogen or mixtures thereof. In so doing, a selection of commercially available and therefore inexpensive gases is available as the nitrogen-donating gas for implementing the method. 
     The method works especially advantageously if at least two chemically different process gases act chemically in succession on the one component and if the treatment chamber is at least partially evacuated between the gaseous process phases. As a result of the different gas compositions, which act successively in different process phases on the at least one component—that is to say, for example, a carbon-donating gas and a nitrogen-donating gas—chemical effects with respect to the method can in each case be achieved. Here it is useful not to mix these gas compositions with each other when a change in the process phases occurs. This can be simply achieved by at least partially evacuating the treatment chamber. 
     Provision is alternatively made for at least two chemically different process gases to chemically act in succession on the component and for the treatment chamber to be purged with an inert gas between the gaseous process phases. In so doing, the change between two gaseous process phases can occur, wherein the prevailing pressure in the treatment chamber can substantially remain unchanged. 
     The method according to the invention furthermore makes it possible for the purging or evacuating of the treatment chamber to end if the detected hydrogen content or the overall pressure of the atmosphere does not meet a predefined threshold value. The detection of the hydrogen content can thereby also be used between the process phases in order to ascertain the effect of the evacuation or the purging. Unwanted chemical reactions can thereby be prevented and the quantity of the required purging gas or the strength and/or duration of the evacuation can be limited. 
     Provision is made in one embodiment of the method for two process phases having a similar process gas to be implemented and for an evacuation or purge to be carried out between the two process phases. In this way, a so-called diffusion phase is set up between said two process phases. 
     Provision is made in a further embodiment of the method for two process phases having a similar process gas to be implemented and for a process gas not to be delivered to the treatment chamber and an evacuation or purge not to be carried out between the two process phases. In this way, a diffusion phase is set up between said two process phases, in which the process gas still remaining in the treatment chamber can continue to remain reactively active on the component surface. 
     The method works especially effectively if the treatment chamber, the process gas and/or the atmosphere are heated. The desired chemical reactions on the surfaces of the components generally take place more intensively and faster at higher temperatures of the atmosphere resulting in the treatment chamber. The treatment chamber can thereby itself be heated or heated up as well as a heater situated therein, the atmosphere and/or the process gas fed thereto. 
     Provision is made in one embodiment of the method for the treatment temperature to lie in a range between 750° C. to 1050° C. A favorable temperature range is thereby specified for many applications. 
     In addition, the method provides for the flow volume of the carbon-donating gas to be dropped to such an extent that a soot formation inside of the atmosphere of the treatment chamber is reduced or prevented. In this way, the hydrogen content ascertained can advantageously be used to reduce or prevent sooting of the treatment chamber or the elements or components situated therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and advantageous embodiments of the method according to the invention or the devices according to the invention are illustrated by means of the drawings and explained in the description below. It should thereby be noted that the drawings serve only a descriptive purpose and are not to be considered in a way that limits the invention in any manner. In the drawings: 
         FIG. 1  shows a schematic view of a treatment chamber for low-pressure carbonitriding of components; 
         FIG. 2  shows a time diagram of a low-pressure carbonitriding method comprising a depiction of process phases and process temperatures; and 
         FIG. 3  shows a time diagram of the low-pressure carbonitriding method comprising a depiction of a hydrogen content in an atmosphere of the treatment chamber. 
     
    
    
     DETAILED DESCRIPTION 
     The same reference numerals are used for functionally equivalent elements and sizes in all of the figures even when the embodiments are different. 
       FIG. 1  shows a schematic view of a layout  10  for the low-pressure carbonitriding of metallic components  12 , which are disposed on a support plate  14  in a treatment chamber  16 . The components  12  can be heated up by means of a heating device  18  situated in the lower region of the drawing. A first inlet  20  and a second inlet  22  having associated flow control valves  24  and  26  facilitate the introduction of carbon-donating gas  28  and nitrogen-donating gas  30 . A temperature sensor  32 , a pressure sensor  34  and a hydrogen sensor  36  suitable for the low-pressure carbonitriding are disposed in the drawing in the upper region of the treatment chamber  16 . An open-loop and/or closed loop control device  38 , which is depicted above the aforementioned sensors receives among other things signals coming from the temperature sensor  32 , the pressure sensor  34  and the hydrogen sensor  36 . An outlet  40  of the treatment chamber  16  leads to the entrance of a pump  42 . 
     During operation, the carbon-donating gas  28  or the nitrogen-donating gas  30  is successively introduced in different process phases into the treatment chamber  16  by means of the flow control valves  24  and  26 . The open-loop and/or closed-loop control device  38  monitors and controls in an open loop or in a closed-loop among other things the process or rather the individual process phases using the sensors  32 ,  34  and  36 . A hydrogen content  44 , which is detected by the hydrogen sensor  36  and which results in an atmosphere  46  of the treatment chamber  16 , is particularly important as will be explained further in regard to the following  FIGS. 2 and 3 . The pump  42  acts simultaneously as a valve at the outlet  40  and is actuated in a process-oriented manner to partially evacuate the treatment chamber  16  or to let out or exchange the gases situated therein. The flow control valves  24  and  26  are controlled among other things by the open-loop and or closed-loop control device  38  as a function of hydrogen content  44  detected by the hydrogen sensor  32 . 
     A time diagram of a process implementation of a low-pressure carbonitriding is depicted in  FIG. 2 , said diagram, for example, being used in the layout  10  shown in  FIG. 1 . The time t is plotted on the abscissa of the diagram and the temperature T of the atmosphere  46  on the ordinate. A curve  48  shows the temporal profile of the temperature T. The low-pressure carbonitriding comprises a heating-up phase A, a temperature equalization phase B, three nitriding phases C 1 , C 2  and C 3 , three carburizing phases D 1 , D 2  and D 3 , four process gas exchange phases E 1 , E 2 , E 3  and E 4  as well as a diffusion phase F and a cooling-down phase G. Two discontinuities  50  indicate that the process phases that are depicted do not have to have the respectively designated durations but can also deviate as desired from the depiction of  FIG. 2 . 
     The difference between the diffusion phase F and the process gas exchange phases designated by the reference numerals E 1  to E 4  is that the detected hydrogen content  44  is used during the process gas exchange phases E 1  to E 4  to monitor and thus to reduce or prevent undesirable reaction products, as e.g. cyanide, wherein a process gas is not fed and a process gas exchange does not take place. The process or the method can therefore be interrupted in the case of a malfunction, e.g. if the pump  42  or the flow control valves  24  and  26  break down in order to reduce or eliminate a danger to the environment. In the entire depicted time period of  FIG. 2 , the hydrogen content  44  is detected by the hydrogen sensor  36  and used for the process control. 
       FIG. 2  shows that during the heating-up phase A, the temperature T with an approximately constant heat-up rate is continually increased up to a treatment temperature of approximately 950°. The temperature T is thus located in an optimal range of 750° C. to 1050° C. 
     In the temperature equalization phase B subsequent to the heating-up phase A, the treatment temperature is constantly maintained at approximately 950° C. Neither a nitrogen-donating gas  30  nor a carbon-donating gas  28  is supplied during the heating-up phase A and the temperature equalization phase B. 
     In the first nitriding phase C 1  immediately subsequent to the temperature equalization phase B, a nitrogen-donating gas  30 , for example ammonia, having a nitrogen-donating gas partial pressure of approximately 50 mbar is supplied. This is displayed on the right vertical axis of the diagram of  FIG. 2 . Thereafter a first process gas exchange E 1 , in which the treatment chamber  16  is evacuated or purged with an inert gas, e.g. nitrogen or argon, takes place. In this process phase, the overall pressure in the treatment chamber  16  or the detected hydrogen content is used for the purpose of monitoring the still remaining content of the nitrogen-donating gas  30  from the nitriding phase C 1  in order to be able to reduce or prevent environmentally damaging reaction products such as, for example, cyanide during the subsequent carburizing phase D 1 . If the treatment chamber  16  is purged during the process gas exchange phase E 1  with an inert gas and if the hydrogen content  44  is smaller than 5 vol % ideally smaller than 1 vol %, the carburizing phase D 1  can begin. If the treatment chamber  16  is evacuated during the process gas exchange phase E 1  and the overall pressure of said treatment chamber  16  becomes less than 1×10 −1 , ideally less than 1×10 −2 , the carburizing phase D 1  can begin. Otherwise a warning indication is produced by the open-loop and/or closed-loop control device  38  and an intervention by the unit&#39;s operator must take place. 
     A carburizing phase D 1 , which has a partial pressure of the carbon-donating gas  28  of approximately 10 mbar, follows the first process gas exchange E 1 . 
     Further implementation of the process is carried out analogously, wherein a diffusion phase F without a process gas exchange takes place between the two carburinzing phases D 2  and D 3 . The treatment chamber  16  is evacuated in the diffusion phase F or alternatively purged with an inert gas, e.g. nitrogen or argon. 
     After the last nitriding phase C 3 , the temperature T of the atmosphere  46  (treatment temperature) of 950° C. is no longer maintained and a swift cooling down to room temperature is carried out in the cool-down phase G in order to set the desired structural composition of the metallic components  12 . 
     It goes without saying that numerous methods for controlled carbonitiriding or controlled low-pressure carbonitriding are possible and the invention is not limited to the sequence and number of three nitriding phases C 1 , C 2  and C 3 , three carburizing phases D 1 , D 2  and D 3 , four process gas exchanges E 1 , E 2 , E 3  and E 4  as well as a diffusion phase F as presented in  FIG. 2 . 
     In  FIG. 3 , a time diagram for controlling a carbon- and nitrogen-donating gas supply during a carburizing phase D and a subsequent nitriding phase is depicted. The abscissa of the diagram of  FIG. 3  depicts the time t and the ordinate depicts the volumetric content of the hydrogen (H 2 ) in vol %. The scale covers thereby the range from 0% to 100%. A curve  43  then reflects the temporal profile of the hydrogen content  44 . A horizontal line indicates a threshold value  45  for the hydrogen content  44 . A process gas exchange phase E occurs after the carburizing phase D and prior to the nitriding phase C. 
     At the start of the carburizing phase D, carbon-donating gas  28  is introduced into the treatment chamber  16 . As a result of the breakdown of the carbon-donating gas  28  on the surface of one or a plurality of metallic components  12 , hydrogen is released and the measured hydrogen content  44  in the atmosphere  46  (process gas atmosphere) consequently increases. At the same time, the content of the carbon-donating gas  28  in the treatment chamber  16  drops. In order to prevent an uneven carburizing of one or a plurality of metallic components  12  as a result of too small a content of carbon-donating gas  28 , the content of said carbon-donating gas  28  is, for example, adjusted or controlled by varying the flow control valve  24 . This is depicted in  FIG. 3  by an arrow  51 . A range provided in  FIG. 3  for the hydrogen content  44  extends between 60 vol % and 70 vol %. 
     Following the carburizing phase D, the treatment chamber  16  is evacuated or purged with an inert gas, e.g. nitrogen or argon. This is illustrated by an arrow  52 . The measured hydrogen content  44  (arrow  53 ) is thereby reduced. If said hydrogen content  44  falls under 5 vol %, ideally under 1 vol %, when purging with an inert gas under 5 vol %, ideally under 1 vol %, during the process gas exchange phase E or the overall pressure becomes less than 1×10 −1  mbar, ideally less than 1×10 −2  when evacuating the treatment chamber during the process gas exchange phase E, the nitriding phase C can begin. This is depicted by the arrow  54 . 
     At the start of the nitriding phase C, nitrogen-donating gas  30  is introduced into the treatment chamber  16 . As a result of the breakdown of the nitrogen-donating gas  30  on the surface of one or a plurality of metallic components  12 , hydrogen is released and consequently the measured hydrogen content  44  increases in the atmosphere  46 . At the same time, the content of the nitrogen-donating gas  30  drops in the treatment chamber  16 . In order to prevent an uneven nitriding of one or a plurality of metallic components  12  as a result of too small a content of nitrogen-donating gas  30 , the flow capacity of the nitrogen-donating gas  30  is controlled by means of the flow control valve  26  with the aid of the detected hydrogen content  44 , cf.  FIG. 1 . This takes place in  FIG. 3  in the section indicated by the arrow  54 , which has a range  57  for the hydrogen content  44  between 40 vol % and 50 vol %. The control of the nitrogen-donating gas flow capacity on the basis of the measured hydrogen content  44  therefore ensures an even nitriding of one or a plurality of metallic components. After the nitriding has taken place, the treatment chamber  16  is either evacuated or purged with a suitable inert gas. 
     It goes without saying that in this manner, numerous methods for controlled nitriding are possible and the invention is not limited to the sequence and number of a carburizing phase, a process gas exchange and a nitriding phase, which are explained in  FIG. 3 .