Patent Publication Number: US-2011067784-A1

Title: Process for the low-pressure carburisation of metal workpieces

Description:
The invention relates to a process for the low-pressure carburisation of metal workpieces in an evacuable furnace, in which acetylene is added to the furnace atmosphere as the carburisation gas, wherein the partial pressure of the acetylene is varied intermittently and wherein the partial pressure of the acetylene in the furnace atmosphere is kept below 10 mbar. 
     Processes for the low-pressure carburisation of metal workpieces are known per se from the state of the art. 
     EP-B-0 882 811, for example, discloses a process for the carburisation of metal workpieces in a vacuum furnace with a furnace atmosphere containing a carbon carrier, wherein the process conditions in the furnace atmosphere are set such that the carbon carrier is split at negative pressure releasing pure carbon. The carbon carrier is one with a carbon-hydrogen ratio of 1:1. This is preferably acetylene. During implementation of the process, the partial pressure of the carbon carrier is varied intermittently, wherein the partial pressure of the carbon carrier is raised to 50 mbar with pressure pulses and is otherwise kept below 20 mbar. 
     Another low-pressure carburisation process is disclosed in EP-B-0 818 555. According to the process described here, acetylene is used as the carburising gas for the purposes of carburisation treatment and as the case-hardening gas for the purposes of case-hardening treatment. This is introduced into the heating chamber of the treatment furnace, wherein the partial pressure of the acetylene is set at under 10 mbar during implementation of the process. 
     The low-pressure carburisation processes known from the state of the art have proved successful in everyday practical application. Hence, low-pressure carburisation is particularly beneficial compared with traditional gas carburisation, because an oxidation-free surface can be achieved. In addition, it results in a shortening of the carburisation time and a avoidance of soot to a large extent. 
     Methane, ethane, propane and/or butane are used in the known manner as carburising gases, in other words, as the carbon carrier. Carbon carriers of this sort, which number among the saturated aliphatic hydrocarbons, are disadvantageously unable to produce a uniform carburisation result, particularly with geometrically complicated workpieces and/or densely packed batches of workpieces. An early dissociation of the carbon carrier in the case of these carbon carriers even at comparatively low temperatures means that when densely packed workpiece batches and/or workpieces with surfaces not readily accessible, such as blind bores, for example, are being treated the dissociated carbon is largely liberated at the outside of the batch, so that the effect of the carburisation at the centre of the batch is significantly less. Particularly in the case of blind bores, this may lead to the carbon passing into the workpiece, predominantly at the bore opening, whereas the inside of the bore is barely affected by the carburisation. 
     In order to overcome this problem, there has been greater emphasis on the use of acetylene than carburising gas in recent years. Due to the carbon-hydrogen ratio of 1:1, although the acetylene has a particular tendency to produce soot, it has emerged that this soot formation can be largely avoided if the acetylene process pressure, in other words, the partial pressure of the acetylene prevailing in the furnace chamber is set at a value of under 10 mbar. The particular advantage of acetylene is that a high carbon mass flow density or a high carbon transfer rate can be achieved, which leads to more uniform carburisation results, even with geometries that are not readily accessible. 
     A further improvement in the carburisation result can be achieved by not keeping the partial pressure of the acetylene constant. Instead, there will preferably be pulsing, which means the partial pressure is varied cyclically. 
     Although low-pressure carburisation processes have proved successful in everyday practical applications, even when using acetylene as the process gas, there is a need for improvement. The aim, in particular, is to achieve an even further improved carburisation result. 
     The object of the invention is therefore to specify a process for the low-pressure carburisation of metal workpieces, which brings with it an improvement over prior-art processes, particularly with regard to the carburisation result. 
     In order to achieve this object, the invention proposes a process of the generic type, which is characterised by the fact that a pot furnace is used as the furnace. 
     According to the state of the art, chamber furnaces are used. Consequently, EP-B-0 882 811 and EP-B-0 818 555 are also based on a low-pressure carburisation process according to which chamber furnaces are used as the furnace. 
     A “chamber furnace” is a furnace that is horizontally aligned. The furnace fittings are therefore also in horizontal alignment. A typical chamber furnace, including the charge module, is disclosed in EP-B-1 228 137, for example. 
     Unlike chamber furnaces, pot furnaces are vertically aligned. Consequently, unlike chamber furnaces, charging and removal also take place in a vertical direction. Pot furnaces have been known from the state of the art for some time, for example from DE-A-1 046 082 or DE-A-1 104 088. 
     The special feature of the process according to the invention lies in the combination of a low-pressure carburisation process, on the one hand, and a pot furnace, on the other. There has hitherto been prejudice among those skilled in the art that it was not possible to conduct a low-pressure carburisation process in a pot furnace. A particular reason for this reservation was the assumption that the process control within a vertically aligned pot furnace would be insufficient. In actual fact, however, the process according to the invention leads to improved carburisation results, which had not been expected in this manner. 
     A prejudice that is overcome with implementation of the process according to the invention results from the process control. Unlike traditional gas carburisation processes, there are no in-situ facilities available for low-pressure carburisation processes facilitating in-situ process monitoring. For this reason, low-pressure carburisation processes have only been used hitherto to achieve comparatively small case-hardening depths of 1 mm, for example. Although greater case-hardening depths should also be achievable with a low-pressure carburisation process, to date those skilled in the art have refrained from using corresponding low-pressure carburisation processes to achieve greater case-hardening depths due to the lack of available in-situ process monitoring. Surprisingly, the use of a pot furnace provides a solution in this case, since unlike a chamber furnace, a pot furnace facilitates the use of carburisation samples in the form of inspection hole samples, for example, which are included during implementation of a process and facilitate something identical to in-situ process monitoring. Use of a pot furnace to implement a low-pressure carburising process is advantageous to the extent greater case-hardening depths can also be achieved, namely with simultaneous process testing during implementation of the process. Implementation of the process according to the invention therefore permits a safe, reproducible and, while the process is being implemented, regularly checkable creation of carburised or case-hardened workpieces, specifically also with great case-hardening depths in particular. 
     The quenching stage that follows the heat treatment has also hitherto militated against the use of a pot furnace to implement a low-pressure carburisation process. Those skilled in the art have endeavoured to combine low-pressure carburisation processes with gas quenching, preferably with high-pressure gas quenching, specifically to avoid a purification stage, which is necessary if oil quenching is used instead of gas quenching. Implementation of the process according to the invention may also be used in a particular way to handle large-scale components. However, these sorts of components make quenching in a liquid medium necessary, such as water, a polymer and/or oil, for example, since gas quenching does not produce the desired quenching results in the case of large-scale components. Implementation of the process according to the invention therefore makes quenching with a liquid medium necessary, particularly with the heat treatment of large-scale components. The combination of a low-pressure carburisation process, on the one hand, and particularly oil quenching, on the other, had to be avoided previously. With the process implementation according to the invention, this problem which had hitherto been seen as detrimental is being deliberately tackled, because the advantages gained with implementation of the process according to the invention in relation to the carburisation result are overwhelming. Also, a prejudice that has existed hitherto in the state of the art is thereby removed. 
     Tests have shown that a very much better temperature distribution can be achieved in a pot furnace than in a chamber furnace. Hence, temperature gradients within the furnace atmosphere of ±5° C., preferably ±3° C., can be set. Chamber furnaces do not allow low temperature gradients of this sort. The result of the improved temperature distribution is an improvement in the carburisation result. The reason for the improved temperature distribution within a pot furnace is to be found in its geometry or its vertical alignment. Hence, due to its geometric design or alignment, once a pot furnace has been duly charged with a workpiece batch for processing, the distance between the workpiece batch and the furnace chamber wall is uniform across the cross-section, in other words, an annular space is created between the workpiece batch and the inner surface of the chamber wall, which favours even temperature distribution. In the case of a horizontally aligned chamber furnace, this sort of space profile in the form of an annular space does not exist. On account of its construction, it is far more likely in the case of a chamber furnace, that with regard to the batch when the furnace chamber is open, the upper outer edge of the batch and also the lower outer edge of the batch are at a greater distance from the inner chamber surface than is the case in relation to the lateral outer edges. In this case, the greater distance between the upper outer edge of the batch and the lower outer edge of the batch from the inner chamber wall is necessary, so that the batch can be manipulated using lifting and transport equipment for loading and unloading. As a result of these different distances between the inner chamber surface and the batch, poorer temperature distributions occur in a chamber furnace than in a pot furnace. This problem that has existed to date involving the use of chamber furnaces to implement a low-pressure carburisation process is overcome through implementation of the process according to the invention. 
     In order to achieve the most uniform temperature distribution possible, a further feature of the invention envisages the use of a control mechanism, which uses a measuring device to measure the temperature in the furnace and gives off a signal according to the temperature measured, which uses a comparator circuit to compare the signal supplied by the measuring device with a presettable target value and, depending on the result of the comparison, triggers a tracking of the temperature, so that the temperature gradient within the furnace atmosphere is set at ±5° C., preferably at ±3° C. This automatic temperature tracking makes it possible for an exact temperature path to be followed over the entire duration of the process, so that a low temperature gradient within the furnace atmosphere is guaranteed, contrary to the state of the art. 
     A further disadvantage of the chamber furnaces used to implement the process according to the state of the art is their size. Due to their design, chamber furnaces can only be operated economically up to a given size. To this extent, chamber furnaces reach the limits of their feasibility in the case of large components. Pot furnaces do not suffer from this disadvantage. Due to their design and alignment, they are able to provide a very much greater intake capacity than chamber furnaces in the prior art. Implementation of the process according to the invention therefore also enables comparatively large workpieces to be handled, such as the wheels of gear assemblies for wind turbines, for example. Low-pressure carburisation of such workpieces was not hitherto possible, which means that implementation of the process according to the invention has produced a solution to this extent. 
     There is also a further reason why implementation of the process according to the invention is advantageous. The use of a pot furnace enables samples to be used, which are exposed to the furnace atmosphere during normal operation of the pot furnace. Such samples undergo the process according to the invention and may be removed for testing during routine operation of the pot furnace. The results of this sort of testing may then be used in a targeted manner to influence the current cycle, in other words, the process currently underway. The advantage of this is that the way in which the process is implemented can be deliberately influenced during implementation of the process, with the specific aim of optimising the carburisation result. To this extent, implementation of the process according to the invention is characterised in that there is in-situ monitoring. This sort of in-situ monitoring has not been possible hitherto with low-pressure carburisation processes. In this case, the synergetic effect following implementation of the process according to the invention produces a solution, specifically due to the positive effect that for the first time it is possible to influence the process implementation based on measurements and test results obtained while the process is being implemented and, to this extent, control the process. In this respect, implementation of the process according to the invention enables comparatively large components to be handled for the first time and also to achieve great case-hardening depths with the possibility of simultaneous process control. 
     In the case of chamber furnaces, the inclusion of samples removed during the cycle is not established practice. This is due firstly to the fact that chamber furnaces have limited space available on account of their design. To this extent, there is no space available for including samples, particularly not samples similar to components in the form of inspection hole samples, for example. Film samples are therefore used in the case of chamber furnaces, but unlike samples that are similar to components, they display a comparatively large spread in relation to test results, because they are only used to check the preset C-level (atmospheric monitoring) and not the carbon transfer into the workpiece. On the other hand, the aim with chamber furnaces is for as few through-holes as possible to be made in the furnace chamber wall, to prevent the in any event only a comparatively poor temperature distribution from being liable to yet more negative influences. To this extent, there are fundamental reservations to removing samples during the routine cycle, in other words, during the course of a normal process implementation. Here, too, the pot furnace construction offers a solution, so that a process implementation is made possible in an advantageous way, which permits the use of samples similar to the component. As a consequence, the process implementation may be tracked and, in particular, tested far more effectively, which enables process parameters to be reset during implementation of the process, which can also take place automatically by means of corresponding regulating mechanisms, if necessary. 
     According to the invention, there is preferably a process implementation in which at least three samples are included. Implementation of the entire process may take 35 hours, for example. In this case, for instance, a first sample is removed after approx. 10 hours and tested. Depending on the test result, individual process parameters may be reset, such as the process duration and/or temperature, for example. A second sample may be removed when approx. 98% of the CHD (case-hardening depth) has been achieved. Depending on the test result, additional process tracking may be undertaken, if necessary. A third and final sample is preferably taken shortly before the workpieces are removed from the furnace, in other words, just before the workpieces are quenched. The advantage of removing a final sample at such a late stage in the process is that adjustments can still be made in relation to the process duration, before the completely processed workpieces are quenched. 
     This process implementation makes it possible in an advantageous manner to intervene early on in the process cycle, so that aberrations can be avoided, which is not possible in this form where implementation of the process involves using a chamber furnace. 
     A further feature of the invention envisages setting the carburisation temperature at 350° C. to 1,050° C., preferably 800° C. to 1,050° C., more preferably at 950° C. to 1,050° C. An accelerated implementation of the process can thereby be achieved in an advantageous way. In the case of heavy components, it is preferable, however, to provide for a carburisation temperature of under 950° C., in order to prevent any distortion. 
     The flow rate of the acetylene used as the carbon carrier is preferably set at 150 l/m 2  to 500 l/m 2  h. The flow rate may vary depending on pressure pulses and/or their duration. Hence, for example, it may be anticipated that the flow rate is not kept constant during a pulse. According to this sort of process implementation, a certain flow rate is used at the start of a pressure pulse. Over the pulse duration, this flow rate then drops off. In addition and/or as an alternative to this, when using several pressure pulses, provision may be made for later pressure pulses to be operated at a reduced flow rate compared with earlier pressure pulses. It may be envisaged, therefore, that a first pressure pulse will be started with an initial flow rate of 400 l/m 2 . At a later stage in the process, the flow rate is then reduced to 300 l/m 2  until the end of the pulse. At the start of a second pressure pulse, the flow rate is increased again, but to a lower value than at the start of the first pressure pulse, for example to a value of 390 l/m 2  h. During the course of the second pressure pulse, the flow rate is then also reduced, for example, to a final value identical to the first pressure pulse. Through this pressure pulse variation, whether at the start of the respective pressure pulses and/or over the duration of a respective pressure pulse, the carburisation result may be advantageously influenced. In particular, it is possible to influence implementation of the process, to the extent that unwanted soot production is avoided in just the same way as the creation of unwanted carbides on the workpiece, in other words, the batch surface. To this extent, an optimised carburisation result is the advantageous consequence. 
     The process implementation described earlier in relation to flow rate and pressure pulses or pulse duration has a particularly advantageous effect with a view to the use of a pot furnace according to the invention. It has emerged, namely, that the workpiece arrangement within the pot furnace leads to the surface of the workpiece batch being treated being covered more favourably flow-wise than in the case of chamber furnaces with the circulating furnace atmosphere, where appropriate, because of the furnace&#39;s vertical alignment. Combined with the variation in flow rate and/or pressure pulses or else the pulse duration, this thereby produces a synergetic effect, which leads to an improved carburisation result. 
     The pulse duration over the course of the process implementation may be set at two to six minutes. The pulse duration is preferably configured depending on the batch size and/or geometry. In each case, the pulse duration must be determined in such a way that soot formation and/or carbide formation are avoided. 
     Implementation of the process according to the invention uses in a synergetic fashion the advantages of the low-pressure carburisation process, on the one hand, and the advantages of using a pot furnace, on the other. Consequently, a process implementation is thereby achieved, which is superior to that in a chamber furnace. Improved carburisation results can be achieved particularly with large-scale workpieces. The prejudice against this among those skilled in the art was that a low-pressure carburisation process could not be carried out satisfactorily in a vertically aligned pot furnace. This prejudice is refuted by implementation of the process according to the invention. 
    
    
     
       Additional features and advantages of the invention are revealed in the following description with the aid of figures. In the figures: 
         FIG. 1  shows a schematic sectional view of a pot furnace, which acts as the furnace for implementing the low-pressure carburisation process according to the invention and 
         FIG. 2  shows a partial section of a pot furnace in side view, according to a possible embodiment. 
     
    
    
     The pot furnace  1  only depicted schematically in  FIG. 1  provides a possibly insulated chamber wall  2 . On the inside, the chamber wall  2  supports an intake device  3 , which in turn serves the arrangement of a heating mechanism  4 . 
     The chamber wall  2  is closed up to the charge opening and provides a space acting as a furnace chamber or furnace space  23 . This is designed to permit vacuum-tight closing at the charging end by means of a cover  5 . In the furnace chamber, feet  8  are constructed at the bottom end in the embodiment shown. These feet  8  are used during the course of a normal process implementation to hold a batch frame  6 , which takes the workpieces to be treated in the form of a batch  7 . The batch frame  6  may have guide plates  9 , which ensure a targeted redirection of the preferably circulated furnace atmosphere, for example, by splitting the batch workpieces arranged spaced from one another, which is symbolised by the arrows  16 . 
     In the exemplary embodiment shown, the pot furnace  1  is fitted with a fan  19 , which is disposed at the bottom end. The fan wheel  19  is driven via a shaft  18  and an external motor  17 , which causes the furnace atmosphere to circulate. Instead of a fan, the provision of nozzle arrangements close to the incoming gas openings may also be envisaged, by means of which a homogenised furnace atmosphere can be created. 
     As can be seen from the drawing in  FIG. 1 , the cover  5  of the pot furnace has through-holes  10 ,  12  and  14 . In the exemplary embodiment shown, the through-hole  12  is used as a duct, in other words, for the introduction of process gas into the furnace chamber, which is symbolised by the arrow  13 . Acetylene is used as the process gas, in other words, as the carbon carrier. 
     The opening  10  is used to dissipate the furnace atmosphere, for which purpose the opening  10  may be connected to a vacuum device. This is symbolised by the arrow  11 . 
     A further opening  14  is provided for sample removal. This is symbolised by the arrow  15 . 
     The pot furnace according to  FIG. 1  is used for the implementation of a low-pressure carburisation process for metal workpieces. Implementation of the process involves the use of acetylene as the carbon carrier, wherein the furnace atmosphere is varied intermittently and the acetylene&#39;s partial pressure is set at a value of under 10 mbar. 
       FIG. 2  shows a possible embodiment of a pot furnace  1  as a partial section in side view, which is used as a furnace to implement a process for the low-pressure carburisation of metal workpieces. 
     The pot furnace  1  according to  FIG. 2  is in the form of a retort furnace. The retort  20  is designed as a cylindrical casing, which is inserted into the furnace space  23 . The retort  20  is equipped with a vacuum-tight compensator  24  in the lower, cold section, which supports the retort  20 . During normal use, the compensator  24  absorbs the thermally triggered expansion in length of the retort  20  and is not loaded by the batch weight. A correct alignment of the retort  20  is thereby constantly guaranteed. 
     Due to the arrangement of the retort  20  in the furnace chamber  23 , this furnace chamber is firstly subdivided into an annular space  21  and secondly a batch space  22 . In this case, the annular space  21  is located between the retort  20  and the chamber wall  2 . The batch space  22  is determined by the space defined by the retort  20 . 
     When the process is implemented according to the invention, the furnace atmosphere to be set as required prevails in the batch space  22 . A vacuum is produced, particularly in the batch space, which preferably lies below 10 mbar, more preferably below 6 mbar. 
     In the annular space  21 , a so-called support vacuum is produced, namely with a pressure of, for example, 15 mbar to 25 mbar. The arrangement of the support vacuum brings with it the advantage that the retort&#39;s walls can be comparatively thin, without this having any negative effects on the retort&#39;s stability, which advantageously favours a transfer of heat from the heating mechanism  4  to the batch space  22 . Without this sort of support vacuum in the annular space, the walls of the retort  20  would have to be far thicker in the interests of stability, which then in turn would impede a transfer of heat into the batch space  22 . To this extent, the arrangement of the support vacuum also serves to optimise the heat distribution within the batch space. 
     REFERENCE LIST 
     
         
           1  Pot furnace 
           2  Chamber wall 
           3  Intake device 
           4  Heating mechanism 
           5  Cover 
           6  Batch frame 
           7  Batch 
           8  Foot 
           9  Guide plate 
           10  Through-hole 
           11  Arrow 
           12  Through-hole 
           13  Arrow 
           14  Through-hole 
           15  Arrow 
           16  Arrow 
           17  External motor 
           18  Shaft 
           19  Fan wheel 
           20  Retort 
           21  Annular space 
           22  Batch space 
           23  Furnace space 
           24  Compensator