A METHOD AND A DEVICE FOR THE TREATMENT OF A TOOL, AND A MEDIUM

Abstract: The invention refers to a method and a device for the treatment of a tool (1) comprising a steel with a surface layer (3) which is hardened by nitriding and/or nitrocarburising. The tool is placed in a gaseous medium in a closed space (5) and heated to a raised temperature in this medium. The medium comprises a main component which is essentially neutral with regard to the substances comprised by the tool (1) and an active additive component in such a quantity and with such properties that the quantity of nitrogen in said surface layer (3) is at least maintained at a certain level during the treatment. The invention also refers to such a medium which comprises a gaseous main component and a gaseous additive component, wherein the gaseous main component consists of an essentially inert gas and the additive component comprises ammonia.

Description: A method and a device for the treatment of a tool, and a medium THE BACKGROUND OF THE INVENTION AND PRIOR ART The present invention refers to a method for the treatment of a tool comprising a steel with a surface layer which is hardened by nitriding and/or nitrocarburising, wherein the tool is placed in a gaseous medium and heated to a raised temperature in this medium. The invention also refers to a device for the treatment of a tool comprising a steel with a surface layer which is hardened by nitriding and/or nitrocarborising, wherein the device comprises a space, which is closed to the surroundings and arranged to receive the tool, means, which are arranged to maintain an atmosphere of a gaseous medium in the closed space, and a heating member which is arranged to heat the tool placed in the closed space to a raised temperature. In addition, the invention refers to a medium comprising a gaseous main component and a gaseous additive component, wherein the gaseous main component consists of an essentially inert gas. In connection with extruding of aluminium, a tool is used, by which the aluminium profile manufactured is given a desired cross-sectional shape. Such tools are normally manufactured by a tool steel, for instance SS 2242. It is known to harden the surface which is in contact with the aluminium and which thus is subjected to a significant wear. Such hardening may be obtained by nitriding and/or nitrocarborising, wherein a surface layer with a thickness of 5 - 20 μm usually is formed, which mainly consists of ε- and/or γ' -phase, and a diffusion layer laying inside the surface layer, which has a thickness of about 0,05 - 0,3 mm and in which the nitrogen essentially is in a solution. Tools hardened in this manner demonstrate good wear properties and are consequently suitable for extruding of aluminium. During the extruding proper, the aluminium work piece, the so-called billet, has a temperature of about 450 - 520°C and the tool reaches a temperature of about 600°C. In order to avoid that the tool is subjected to too large temperature gradients during an initial phase of the extruding process, which might lead to different types of tool breakage, it is known to preheat the tool before the extruding is started, preferably to a temperature of between 400°C and 520°C. Such a preheating may be performed in a preheating oven. Due to different practical reasons, for instance the fact that a large number of different tools are utilised in an extruding plant, an individual tool frequently will remain during a long time period in the preheating oven. The applicant has made investigations which show that such a long preheating period results in a negative influence on the hardened surface layer of the tool. Thereby, two processes will take place. The first is that during the pre- treatment of the tool, the temperature will be so high that the steel surface starts to oxidise. High temperature corrosion is formed, wherein oxides are formed in the nitrided surface layer. The second process is that the nitrogen, which is present in the hardened portions, will start to diffuse again, wherein a part of the nitrogen penetrates deeper into the tool and another part disappears away from the surface of the tool. Consequently, the properties of the tool will be changed and the wear strength is deteriorated. In connection with such a preheating of the extruding tool, it is further known to place the tool during the preheating in an essentially inert or neutral atmosphere in order to prevent an oxidation of the nitrided surface layer. Such a usual atmosphere is achieved by the supply of nitrogen. SUMMARY OF THE INVENTION The object of the present invention is to overcome the problems mentioned above and provide improved possibilities to maintain the hardness of a nitride hardened surface layer of a tool . This object is obtained by the method initially defined, which is characterised in that the medium comprises a main component, which is essentially neutral with regard to the substances comprised by the tool, and an active additive component in such a quantity and with such properties that the quantity of nitrogen in said surface layer is at least maintained at a certain level during the treatment. By such a main component, which forms the completely dominant part of the medium, an oxidation of the surface layer is prevented in an effective manner. By the active additive component, diffusion of nitrogen atoms outwardly in the surface layer may at the same time be prevented and the concentration of nitrogen atoms in the surface layer may be kept substantially constant or may possibly increase. The active additive component, which merely is a small part of the total quantity of the media, may advantageously comprise ammonia, which when it contacts a metal surface at a certain temperature, is split to nitrogen atoms and hydrogen molecules . Such nitrogen in the atomic form may be picked up by the tool steel and by choosing a correct quantity of ammonia in the medium, an essentially constant nitrogen level may be maintained in the surface layer. Furthermore, it has appeared that such an ammonia addition reduces the oxidation of the surface layer in comparison with preheating in a pure nitrogen gas atmosphere. The substantially neutral or inert main component may advantageously comprise at least one of the gases nitrogen, argon and helium. These three gases, which may be utilised separately or in a mixture with each other, have an effective capability of removing oxygen gas and in such a manner prevent an oxidation of the surface layer at the same time as the gases do not influence the nitrogen atoms in the surface layer. According to one embodiment of the invention, the additive component may in addition comprise one of the gases hydrogen, carbon monoxide and carbon dioxide. By these gases, further properties of the surface layer may be influenced and/or maintained. According to a further embodiment of the invention, the quantity of the additive component in the medium is < 10% and > 0,05%, preferably > 0,2%. According to a more specific embodiment of the invention, the quantity of the additive component in the medium is < 5%, preferably < 2%. According to an especially advantageous embodiment of the invention, the quantity of the additive component in the medium is about 1%. According to a further embodiment of the invention, the treatment includes a preheating of the tool, which is followed by a working step during which the tool is utilised in the working operation for which the tool is intended. It is here to be noted that the tool in accordance with the invention is located in the medium defined above during essentially the whole preheating. Within the scope of the invention, it is also possible to let the tool be located in the medium also during the following working step. According to an advantageous application of the invention, the tool comprises an extruding tool and the working operation comprises extruding of aluminium. According to a further embodiment of the invention, the raised temperature is maintained during essentially the whole treatment and this temperature amounts to between 350°C and 600°C, preferably between 400°C and 550°C. The object is also obtained by the initially defined device which is characterised in that the medium comprises a main component, which is essentially neutral with respect to the substances which are comprised by the tool, and an active additive component in such a quantity and with such properties that the quantity of nitrogen in said surface layer is at least maintained at a certain level during the treatment. Said means may advantageously comprise a source, which contains the medium, and are arranged to supply the medium from the source to the closed space in such a manner that there is a higher pressure in the space than in the surroundings . The object is also obtained by the initially defined medium which is characterised in that the additive component comprises ammonia. Such a gaseous medium may be utilised in many different connections in order to form a surrounding atmosphere for a nitride hardened tool or object, which is heated to a raised temperature. Such a medium is suitable in connection with preheating of extruding tools and other tools hardened by nitriding, such as forging tools. The medium is suitable also for other objects hardened by nitriding, for instance hardened components in machines in which a raised temperature may prevail. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now to be explained more closely by a description of different embodiments and with reference to the drawings attached, in which Fig 1 discloses a sectional view of an extruding tool hardened by nitriding, Fig 2 discloses ' schematically a view of a device for the treatment of a tool in accordance with an embodiment of the present invention, Fig 3 discloses schematically a view of a device for the treatment of a tool in accordance with a second embodiment of the present invention and Fig 4 discloses a diagram over the concentration of nitrogen atoms in a surface layer of an object hardened by nitriding. DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION Fig 1 discloses an extruding tool 1 which is intended to be used in a plant for extruding aluminium profiles (not disclosed) . The tool 1 comprises a nozzle aperture 2 through which the aluminium billet flows and obtains its cross- sectional shape. The nozzle aperture 2 is defined by a surface of the tool 1. The tool 1 is nitride-hardened by nitriding in a manner known per se, wherein a surface layer 3 is formed, which mainly consists of ε- and γ' -phase. Within the surface layer 3 a diffusion layer extends, into which nitrogen atoms have diffused during the nitriding and/or nitrocarburising process. Before the tool 1 is to be used in the extruding plant not disclosed, it is suitable to preheat the tool 1. Such a preheating may be performed by the device disclosed in Fig 2. This device comprises a preheating oven or chamber 4 which forms a closed space 5. The space 5 is arranged to receive an object to be treated and which in the example disclosed is formed by the tool 1. Furthermore, the tool comprises a heating member 6 which is arranged to heat and maintain a raised temperature m the preheating oven 4 and the closed space 5. In accordance with the present invention, there is an arrangement for the supply of a gaseous medium to the closed space in order to create a desired atmosphere in this space 5. This arrangement comprises a mixing member 7 to which a number of gas sources 8, 9, 10 are connected. In the example disclosed, each gas source comprises a container, for instance a pressure vessel, wherein the pressure m the respective pressure vessel is so high that the gas in question may be fed through the mixing member 7 into the closed space 5 m such a manner that a higher pressure prevails in the closed space 5 than in the surrounding atmosphere. The gas sources 8, 9, 10 may also be formed by plants for the production of the gas in question in situ or a pipe conduit for the supply of the gas in question from a remote position. It is also possible to provide one or several pump members which are arranged to provide a desired gas pressure m the closed space 5. The mixing member 7 is controlled by means of a control unit 11 which is arranged to control the quantity of the gas supplied, the mixing relation between the different components of the gas supplied, the heating member 6 for maintaining a desired temperature in the closed space 5, and an outlet valve 12 for enabling discharge of raised quantities of not desired gases by an exchange of the atmosphere in the space 5. The control unit 11 may thereby be connected to a plurality of different types of sensors, for instance a temperature sensor 13 and a pressure sensor 14. In the example disclosed, the first gas source 8 contains nitrogen and the second gas source 9 ammonia. The control unit 11 is arranged in such a manner that a mixture of nitrogen gas and ammonia may be supplied to the closed space 5 and maintain such an atmosphere therein. Thereby, the quantity of ammonia is < 10% of the total quantity of nitrogen gas and ammonia. According to further embodiments, the total quantity of ammonia is < 5%, preferably < 2% of the total quantity of nitrogen gas and ammonia. The applicant has performed trials, in which the total quantity of ammonia was about 1% of the total quantity of nitrogen gas and ammonia. According to the preferred embodiments, the quantity of ammonia is thus to be > 0,1% and preferably > 0,2% of the total quantity of nitrogen gas and ammonia. A preferred interval thus would be 0.5% - 1,5% ammonia of the total quantity of nitrogen gas and ammonia in the atmosphere which is maintained in the closed space 5. The control unit 11 and the heating member 6 are arranged to maintain a temperature in the preheating oven which amounts to between 350°C and 600°C, preferably between 400°C and 550°C. In accordance with the present invention, it is thus possible to create an atmosphere which consists essentially of only one main component of nitrogen gas and an additive component of ammonia. It is also possible to let the additive component contain further gases, for instance carbon monoxide, carbon dioxide and/or hydrogen. The disclosed third gas source 10 may contain such a further additive component. The quantity of these further gases are preferably less than the quantity of ammonia in the total gas quantity. The main component may also be formed by other neutral or inert gases, in the first place noble gases, such as argon and/or helium. Fig 3 discloses a further embodiment of the invention which differs from the embodiment disclosed in Fig 2 merely in that the device utilises a premixed gaseous medium which is provided by means of a container 15 and a control valve 7. The container 15 may thus contain a pressurised medium containing a main component and an additive component, for instance nitrogen gas and ammonia. Also in this case it is possible to utilise a pump member, as a complement or an alternative, for the supply of the gaseous medium. Fig 4 discloses a diagram of the nitrogen concentration N in samples as a function of the depth D from the surface and which have been measured after trials performed. The samples have been in an oven during 30 hours at a temperature of 450°C. The curve a discloses the nitrogen concentration in an atmosphere of nitrogen gas and 1% ammonia. The curve b discloses the nitrogen gas concentration in an atmosphere of merely nitrogen gas and the curve c discloses the nitrogen concentration in an air atmosphere. As appears clearly of the diagram, the nitrogen gas concentration may be maintained in a significantly very much better manner than in an atmosphere of nitrogen gas and 1% ammonia than in the previously used gas atmospheres. The present invention is not limited to the embodiments disclosed above but may be varied and modified within the scope of the following claims. The invention is also applicable to nitrided extruding tools for extruding other materials than aluminium, for instance other metals or plastics .

Claims: Claims 1. A method for the treatment of a tool (1) comprising a steel with a surface layer (3) which is hardened by nitriding and/or nitrocarburising, wherein the tool (1) is placed in a gaseous medium and heated to a raised temperature in this medium, characterised in that the medium comprises a main component being essentially neutral with regard to the substances comprised by the tool, and an active additive component in such a quantity and with such properties that the quantity of nitrogen in said surface layer (3) is at least maintained at a certain level during the treatment. 2. A method according to claim 1, characterised in that the additive component comprises ammonia. 3. A method according to any one of claims 1 and 2, characterised in that the main component comprises at least one of the gases nitrogen, argon and helium. 4. A method according to claim 3, characterised in that the additive component in addition comprises at least one of the gases hydrogen, carbon monoxide and carbon dioxide. 5. A method according to any one of the preceding claims, characterised in that the quantity of the additive component in the medium is less than 10%. 6. A method according to claim 5, characterised in that the quantity of the additive component in the medium is greater than 0,05%, preferably greater than 0,2%. 7. A method according to any one of claims 5 and 6, characterised in that the quantity of the additive component in the medium is less than 5%, preferably less than 2%. 8. A method according to any one of the preceding claims, characterised in that the treatment comprises a preheating of the tool (1), which is followed by a working step during which the tool is utilised in the working operation for which the tool is intended. 9. A method according to any one of the preceding claims, characterised in that the tool (1) comprises an extruder tool. 10. A method according to claims 8 and 9, characterised in that the working operation comprises extruding aluminium. 11. A method according to any one of the preceding claims, characterised in that the raised temperature is maintained during essentially the whole treatment and amounts to between 350°C and 600°C, preferably between 400°C and 550°C. 12. A device for the treatment of a tool (1) comprising a steel with a surface layer (3) which is hardened by nitriding and/or nitrocarburising, wherein the device comprises a space (5) , which is closed to the surroundings and arranged to receive the tool (1), means (7-10), which are arranged to maintain an atmosphere of a gaseous medium in the closed space, and a heating member (6) which is arranged to heat the tool (1) placed in the closed space (5) to a raised temperature, characterised in that the medium comprises a main component which is essentially neutral with regard to the substances comprised by the tool (1), and an active additive component in such a quantity and with such properties that the quantity of nitrogen in said surface layer (3) is at least maintained at a certain level during the treatment. 13. A device according to claim 12, characterised in that said means comprises a source (8 - 10), which contains said medium, and is arranged to supply said medium from the source to the closed space (5) in such a manner that a higher pressure prevails in the space than in the surroundings . 14. A device according to any one of claims 12 and 13, characterised in that the heating member (6) is arranged to maintain a temperature in the space, which amounts to between 350°C and 600°C, preferably between 400°C and 550°C. 15. A medium comprising a gaseous main component and a gaseous additive component, wherein the gaseous main component consists of an essentially inert gas, characterised in that the additive component comprises ammonia . 16. A medium according to claim 15, characterised in that the main component comprises at least one of the gases nitrogen, argon and helium. 17. A medium according to any one of claims 15 and 16, characterised in that the quantity of the additive component in the medium is less than 10%. 18. A medium according to claim 17, characterised in that the quantity of the additive component is greater than 0,05%, preferably greater than 0,2%. 19. A medium according to any one of claims 17 and 18, characterised in that the quantity of the additive component is less than 5%, preferably less than 2%. 20. A medium according to any one of claims 15 - 19, characterised in that the additive component in addition comprises at least one of the gases carbon monoxide, carbon dioxide and hydrogen. 21. A medium according to any one of claims 15 - 20, characterised in that it is intended to form a surrounding atmosphere in connection with heat treatment of a tool comprising a steel with a surface layer which is hardened by nitriding and/or nitrocarburising.

Inventors: SPROGE LARS, SPROGE, LARS
Applicants: AGA AKTIEBOLAG PUBL, SPROGE LARS, AGA AKTIEBOLAG (PUBL), SPROGE, LARS
Application year: 1999
Main classification: C23C8
IPC codes: B21C3, B21C25, C23C8