SE514833C2 - Method and medium for treating a steel tool with hardened surface layer - Google Patents

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

l0 15 20 25 30 35 š14 ~ ass' to harden the surface which is in contact with the aluminum and which is thus exposed to a great deal of wear. Such curing can and / or whereby an approximately 5-20 μm thick surface layer is usually formed, by nitration nitrocarburization, mainly consisting of s- and / or y '-phase and an approx. 0.05-0.3 mm thick diffu- in which the nitrogen is mainly in solution. tools that are hardened in this way show good wear properties and are thus suitable for extrusion of aluminum.

During the extrusion itself, the aluminum workpiece, the so-called ingot, has a temperature of about 450-520 ° C and the tool reaches a temperature of about GOOWI. In order to avoid that the tool is exposed to excessive temperature gradients, which is during an initial stage of the extrusion process, it could lead to different types of tool breakage, known to preheat the tool before the extrusion begins, preferably to a temperature of between 400 and 520 °. C. Such preheating can take place in a preheating oven. Due to various practical reasons, such as the fact that a large number of different tools are used in an extrusion plant, an individual tool will often remain in the preheating oven for a long time.

Applicants have done soul research showing that such a long preheating time results in a negative impact on the hardened surface layer of the tool. In doing so, two processes will take place. The first is that during the preheating of the tool, the temperature becomes so high that the steel surface begins to oxidize. A high-temperature corrosion is obtained, whereby oxides are formed in the nitrided surface layer. The second process is that which is present in the hardened portions of the tool since nitriding and / or nitrocarburization, will begin to diffuse again, with some of the nitrogen penetrating deeper into the tool and another part disappearing through the surface from 10 _15 20 25 30 35 tool. Thus, the properties of the tool will change and the wear resistance will deteriorate.

In connection with such preheating of extrusion tools, it is further known to place the tool during the preheating in a substantially inert or neutral atmosphere in order to prevent an oxidation of the nitrided hardened surface layer. An ordinary such atmosphere is achieved by supplying nitrogen gas.

SUMMARY OF THE INVENTION The object of the present invention is to obviate the above-mentioned problems and to create improved possibilities for maintaining the hardness of a nitrided hardened surface layer of a tool.

This object is achieved with the initially added process component, characterized in that the medium comprises an active comprising ammonia, in such an amount and with such properties that the amount of nitrogen in said surface layer is kept at least at a certain level during the treatment to maintain the hardness of said surface layer.

With such a main component, which constitutes the predominant part of the medium, an oxidation of the surface layer is effectively prevented. At the same time, with the active additive component, the diffusion of nitrogen atoms in the surface layer can be prevented and the concentration of nitrogen atoms in the surface layer can be kept substantially constant or possibly increase. The additive component, which constitutes only a small part of the total amount of the nodule and which, when it comes into contact with a metal surface at a certain temperature, is broken down into nitrogen atoms and hydrogen molecules. Such nitrogen in atomic form can be taken up by the tool steel and by selecting a correct amount of ammonia in the medium, a substantially constant nitrogen level can be maintained in the surface layer. It has been found that a further addition of an ammonia additive reduces the oxidation of the surface layer compared to preheating in a pure nitrogen atmosphere. The substantially neutral or inert main component may advantageously comprise at least one of the gases nitrogen gas, argon and helium. These three gases, which can be used individually or in admixture with each other, have an effective ability to displace oxygen and thus prevent an oxidation of the surface layer while the gases do not affect the nitrogen atoms in the surface layer.

According to an embodiment of the invention, the additive component may further comprise one of the gases hydrogen, carbon monoxide and carbon dioxide. With these gases, additional properties of the surface layer can be affected and / or maintained.

According to a further embodiment of the invention, the amount 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 amount of the additive component in the medium is <5%. According to a particularly advantageous embodiment of the invention, the amount of the additive component is preferably <2%. medium about 1%.

According to a further embodiment of the invention, the treatment comprises a preheating of the tool, which is followed by a working step during which the tool is used in the working operation for which the tool is intended. It should be noted here that the tool according to the invention is in the medium defined above during substantially the entire preheating. Within the scope of the invention, it is also possible to leave the tool in the medium even during the subsequent working step.

According to an advantageous application of the invention, the tool comprises an extrusion tool and the working operation includes extrusion of aluminum. According to a further embodiment of the invention, the elevated temperature is maintained during substantially the entire treatment and this temperature amounts to between 350 ° C and 600 ° C, preferably between 400 ° C and 550 ° C. The object is also achieved with the initially stated medium, which is characterized in that the medium comprises a gaseous additive component, which comprises ammonia, the amount of the additive component in the medium being less than 10%. Such a gaseous medium can be used in many different contexts to form an ambient atmosphere for a nitrided hardened tool or object such medium is heated to an elevated temperature. A suitable in connection with preheating of extrusion tools and other nitriding hardened tools such as forging tools. The medium is also suitable for other non-cured objects, for example cured components in machines in which an elevated temperature may prevail.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be further elucidated by a description of various embodiments and with reference to the accompanying drawings, in which Fig. 1 shows a sectional view of a nitrided hardened extrusion tool, Fig. 2 showing a schematic view. of an apparatus for treating a tool in accordance with an embodiment of the present invention, Fig. 3 shows a schematic view of an apparatus for treating a tool in accordance with a second embodiment of the present invention, and 4 shows a diagram of the concentration of nitrogen atoms in a surface layer of a nitrided core object.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION Fig. 1 shows an extrusion tool l which is intended to be used in a plant for extrusion of aluminum profiles (not shown). The tool 1 comprises a nozzle opening 2 through which the aluminum blank flows and obtains its cross-sectional shape. The nozzle opening 2 is defined by a surface of the tool 1. The tool 1 is nitrided hardened in a manner known per se, whereby a surface layer 3 is formed which consists mainly of s and y 'phase. Within the surface layer 3, a diffusion layer extends into which nitrogen atoms have diffused during the nitration and / or nitrocarburization process.

Before the tool J. is to be used in the extrusion plant (not shown), it is suitable to preheat the tool 1.

Such preheating can be done with the device shown 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 shown consists of tool 1. The device further comprises a heating means 6 which is arranged to heat and maintain an elevated temperature in the preheating furnace 4 and the closed space 5.

In accordance with the present invention, there is provided an apparatus for supplying a gaseous medium to the closed outlet space 5 to create a desired atmosphere in this space 5.

This device comprises a mixing means 7 with which a number of gas sources 8, 9, the example comprises each gas source a container, for example 10 are connected. In this, a pressure vessel is shown, wherein the pressure in the respective pressure vessel is so high that the gas in question can. is fed through the mixing means 7 into the closed space 5 in such a way that a higher pressure prevails in the closed space 5 than in the surrounding atmosphere. The gas sources 8, 9, 10 can also consist of plants for the production of the gas in question on site or a pipeline for supplying the gas in question from a remote position. It is also possible to arrange a supply of a desired gas pressure in the closed space 5. or more pump means which are arranged that the mixing means 7 is controlled by means of a control unit 11 which is arranged to control the amount of supplied gas, the mixing ratio between different components of the supplied gas, the heating means 6 for maintaining a desired temperature in the closed space 5 and an outlet valve 12 for enabling the discharge of increased amounts of undesired gases by exchanging the atmosphere in the space 5. The control unit 11 can then be connected to a several different types of, for example, one and one sensors, temperature sensor 13 pressure sensor 14.

In the example shown, the first gas source 8 contains nitrogen gas and the second gas source 9 ammonia. The control unit 11 is arranged so that a mixture of nitrogen gas and ammonia can be supplied to the closed space 5 and maintain such an atmosphere therein. The amount of ammonia is <10% of the total amount of nitrogen and ammonia. According to further embodiments, the total amount of ammonia is <5%, preferably <% of the total amount of nitrogen gas and ammonia. The applicant has carried out experiments in which the total amount of ammonia was about 1% similar to the preferred embodiment. In one of the ammonia,> ammonia should be> 0.1% and preferably> 0.2% of the total amount of nitrogen and ammonia. A preferred range could thus be 0.5% -1.5% ammonia of the total amount of nitrogen gas and ammonia in the atmosphere maintained in the enclosed space 5. The control unit 11 and the heating means 6 are arranged to maintain a temperature in front of the heating furnace 4 and the closed space 5 amounting to between 350 ° C and 600 ° C, preferably between 400 ° C and 550 ° C.

Thus, in accordance with the present invention, it is possible to create an atmosphere consisting essentially of only a major component of nitrogen and an additive component of carbon dioxide. It is also possible to allow the additive component carbon dioxide. include additional gases, for example carbon monoxide, and / or hydrogen gas. these additional gases are preferably less than any additional additive component. The amount of ammonia in the total amount of gas. The main component can also be formed by other neutral or inert gases, primarily noble gases, such as argon and / or helium.

Fig. 3 shows a further embodiment of the invention which differs from the embodiment shown in Fig. 2 only in that the device uses a ready-mixed gaseous medium provided by means of a container 15 and a control valve 7. The container 15 can thus contain a pressurized medium comprising a main component and an additive component, for example nitrogen and ammonia. Also in this case it is possible to use a pump means as a complement or an alternative for the supply of the gaseous medium.

Fig. 4 shows a diagram of the nitrogen concentration N in samples as a function of the depth D from the surface and which has been measured after performed experiments. The samples have been in an oven for 30 hours at a temperature of 450 ° C. Curve a shows the nitrogen concentration in an atmosphere of nitrogen gas and 1% ammonia. Curve b shows the nitrogen gas concentration in an atmosphere of nitrogen gas alone and curve c shows the nitrogen concentration in an air atmosphere.

As is clear from the diagram, the nitrogen gas concentration can be maintained in a significantly much better way in an atmosphere of nitrogen gas and 1% ammonia than in previously used gas atmospheres.

The invention is not limited to the embodiments shown above. but may be varied and modified within the scope of the appended claims. The invention is also applicable to nitrated extrusion tools for extrusion other than aluminum, of other materials such as other nets or plastics.

Claims (14)

10 15 20 25 30 35 r514 ss3 10 A method of treating a tool (1) comprising a steel with a surface layer (3) which is hardened by nitration and / or nitrocarburization, the treatment comprising a preheating intended to be followed by a working step during which the tool is used in a working operation for which the tool is intended, wherein the preheating comprises placing the tool (1) in a gaseous medium and heating to an elevated temperature of between 350 ° C and 600 ° C in this medium and wherein the medium comprises a main component which is substantially neutral with respect to the substances comprised by the tool, characterized in that the medium comprises an active additive component, comprising ammonia, in such an amount and with such properties that the amount of nitrogen in said surface layer (3) is at least kept at a certain level during the treatment to maintain the hardness of said surface layer. A method according to claim 1, characterized in that the main component comprises at least one of the gases nitrogen gas, argon and helium. 3. A process according to claim 2, characterized in that the additive component further comprises at least one of the gases hydrogen, carbon monoxide and carbon dioxide. Method according to any one of the preceding claims, feel; tegknat_ay that the amount of the additive component in the needle is less than 10%. Process according to Claim 4, characterized in that the amount of the additive component in the medium is greater than 0.05%, preferably greater than 0.2%. 10 15 20 25 30 “i sagts 11 Process according to either of Claims 4 and 5, characterized in that the amount of the additive component in the medium is less than%, preferably less than 2%. A method according to any one of the preceding claims, felt; Lg fl g fifl; _ ay to the tool (1) tool. includes an extrusion 8. A method according to claim 7, characterized in that the working operation comprises extrusion of aluminum. A method according to any one of the preceding claims, felt; It is expected that the elevated temperature is maintained during substantially the entire treatment and is between 400%: and 550 ° C. A medium intended to form an ambient atmosphere in connection with preheating of a tool according to the method set out in any one of claims 1-9, wherein the tool comprises a steel with a surface layer which. is cured by nitration and / or nitrocarburization, the medium comprising a gaseous Inwud component, which consists of a substantially inert characteristic that the medium comprises a gaseous additive component, gas, which comprises ammonia, the amount of the additive component in the medium being less than 10%. A medium according to claim 10, characterized in that the main component comprises at least one of the gases nitrogen gas, argon and helium. Medium according to one of Claims 10 and 11, characterized in that the amount of the additive component is greater than 0.05%, preferably greater than 0.2%. 10 ° s14 ss3 12 Medium according to either of Claims 10 and 12, characterized in that the amount of the additive component is less than 5%, preferably less than 2%. A medium according to any one of claims 10 to 13, characterized in that the additive component further comprises at least one of the gases carbon monoxide, carbon dioxide and hydrogen gas.