The present invention relates to a device according to claims 1 to 9, and a method for using the device according to claims 10 to 17.

Background of the invention

It is known to improve surface properties of metallic workpieces by using different kind of surface treatment processes.

Processes in which the structural condition of the surface of an iron-based workpiece is changed by nitrogen and / or carbon enrichment with possibly subsequent oxidation with the aim of improving the strength behavior under static and oscillating loads and / or the wear behavior have proven to be particularly useful for improving deformation behavior and corrosion behavior of the surface of such kind of workpieces. Proven processes of this type are, for example, gas or liquid nitriding processes or plasma nitriding processes or a combination of the aforementioned processes and in particular nitrocarburizing processes or salt bath nitriding processes, which are widely used for finishing, hardening and corrosion protection or to improve the tribological behavior of stressed surfaces.

The problem that often arises by applying such kind of processes is that only a part of the surface of a workpiece is to be treated, for example in a gas atmosphere or in a liquid bath, while a certain other part of the surface of the workpiece not.

It is for example the case when the surface of ball studs need to be treated. The surface of ball studs for chassis systems, for example for transverse control arms of motor vehicles, is known to be treated in a gas nitrocarburizing process or in a liquid bath by salt bath nitriding.

Such ball studs are workpieces that essentially consist of an approximately cylindrical shaft that has a thread at one end over a certain area and is designed as i

CONFIRMATION COPY a ball at its other end so that the ball can interact positively with a ball socket of the chassis system. Since the ball is exposed to considerable forces and in particular tribological loads in the installed and operating state, it is necessary to treat at least the surface area of the ball of the ball stud, e.g. by hardening it in a gas nitrocarburizing process in a process chamber or in a liquid bath by salt bath nitriding.

It is common to treat the entire surface of the ball stud, including the thread area. However often the treatment of the ball stud in a nitriding or nitrocarburizing process has a negative effect in the thread area.

When a surface area is subjected to a nitrocarburizing or nitriding process, thin surface layers, so-called connecting layers, with thicknesses in the micrometer range are formed-. These layers can lead to the formation of microcracks when transverse loads are applied, for example when these treated areas are exposed to strong frictional forces during screwing the thread.

For avoiding the above mentioned drawback, it has been proposed to cover the thread area in such a manner that undesirable nitriding or nitrocarburizing can occur.

In this regard, for example:

- BODE proposes in EP1388592 to use a method for allowing a partial area of a surface of a workpiece to be treated, wherein a sealing element is used for avoiding penetration of medium used for nitriding or nitrocarburizing into the space, where the thread area is covered. One clear disadvantage of this method is the need of using an additional sealing element.

- WILDEN proposes in DE2006043067 a device consisting solely of a sleeve, whose mating thread rests over the entire surface to be protected in the area of the thread of the workpiece. In contrast to other also known devices of this type, by using this device there is no cavity between the workpiece surface to be protected and the inner wall of the device, therefore there is no cavity that must be sealed against the ingress of the fluid medium by means of any additional sealing means. The sealing effect of this device is established by the mechanical contact between the thread and the mating thread. Since this contact is given over the entire surface of the workpiece to be protected, further penetration of the fluid medium is effectively prevented. Furthermore, WILDEN explains that due to the large surface area between the workpiece and the device in the area of the thread and mating thread, efficient and sufficient protection of the workpiece surface against the fluid medium is achieved even if the local sealing effect is incomplete, and further suggests to screw the device till attaining a minimum tightening torque, wherein in order to know when the minimum tightening torque is attained, a stop should be put on the workpiece, so that by screwing the device until the stop on the workpiece, the specific minimum tightening torque is attained and in this manner an increased sealing effect between the thread of the workpiece and the counter-thread (or mating thread) of the device can be achieved. This minimum tightening torque should create a contact pressure between the threads that improves the mechanical contact between the thread and the counter-thread for preventing the penetration of fluid medium into the edge areas of the thread and mating thread. One clear disadvantage of using the above mentioned device belonging to the prior art is that it is necessary to design exact counter-threads for the devices to make perfect contact to the thread of the workpieces and/or stops for the workpieces for attaining the minimum tightening torque for helping to prevent penetration of fluid into the edge areas of the thread and mating thread (counter-thread) can be very complex and time consuming. Furthermore such a device is not suitable for preventing the penetration of gas medium, only for preventing penetration of fluid medium.

Objective of the present invention

It is therefore the main object of the invention to provide a method and a device which allow a partial area of a surface of a workpiece to be treated, in particular for treating ball studs without treating the thread of the ball studs, wherein the method and device allow avoiding the disadvantages known from the prior art, and which allow to treat workpieces having surfaces that should not be treated, which have threads that should not be subjected to the treatment but should be prevented from contact with the treatment medium, but not only in case of using fluid medium but especially also when using gas medium.

Description of the present invention

The aforementioned problem is solved by a device with the features of claim 1 , a use with the features of claim 10 as well as a method with the features of claim 11.

In a first aspect of the invention, a device for holding a workpiece during surface treatment of the workpiece by using a fluid and/or a gaseous medium is provided. The workpiece comprises a treatable surface, a non-treatable surface and an intermediate surface in between of the treatable surface and the non-treatable surface. Further, the device comprises a covering component for covering the non- treatable surface area of the workpiece, said covering component comprising a hollow chamber and an opening for receiving the non-treatable surface, wherein the covering component comprising a sealing-acting surface that is in contact with the intermediate surface of the workpiece when the non-treatable surface is placed in the interior of the hollow chamber. The sealing-acting surface of the covering component is designed to fit the intermediate surface of the workpiece in such a manner that the interaction between the sealing-acting surface and the intermediate surface has a self-sealing effect, which results in sealing of the hollow chamber, so that penetration of the fluid and/or gaseous medium into the hollow chamber is prevented. In this manner, contact of the fluid and/or gaseous medium with the non- treatable surface of the workpiece is prevented and contact of the fluid and/or gaseous medium with the treatable surface of the workpiece is allowed.

Advantageously, it can be provided that the workpiece is a ball stud having a ball pin, wherein the ball pin surface is a non-treatable surface and the covering component is a cover nut.

It can also be provided that the cover nut is designed comprising a radius resulting in the design of the contact surface , so that the sealing-acting surface has a curvature that produces that the hollow chamber is sealed in interaction with the intermediate surface of the ball stud , when the ball stud is fixed in the cover nut.

Further, it can be provided that the radius R is in a range between R 0.1 - R 5, i.e. from 0.1 mm up to 5 mm.

It can also be provided that the ball pin of the ball stud has a thread and the hollow chamber of the cover nut has a thread, wherein the thread of the ball pin and the thread of the cover nut have the same size.

Advantageously, it can be provided that the material of the ball pin is 41CrS4.

It can also be provided that the material of the cover nut is stainless steel 1.4305.

Further, it can be provided that the thread of the ball pin and the thread of the cover nut are connected by screwing with torque in a range between 5 Nm and 25 Nm.

It can also be provided that pretension force from the torque is selected to create enough stress in the material of cover nut and in the material of the ball pin at the same time that the tangent connection does not allow any fluid and/or gaseous medium getting in contact with the surface of the ball pin comprising threads that are inside of the hollow chamber of the cover nut.

In a second aspect of the invention, a use of a device according to the invention for conducting a nitriding process by using a nitriding gas for nitriding the treatable surface of the workpiece is provided.

In a third aspect of the invention, a method for the surface treatment of a ball stud by means of a device according to the invention is provided, wherein the device is used to isolate a non-treatable surface area of a ball stud from a gaseous medium, wherein the method comprises following steps: a. the ball stud is partially brought through an opening into a hollow chamber of a cover nut, b. the cover nut is used for isolation, c. the ball stud is being fixed to the interior of the cover nut d. a treatable surface of the surface of the ball stud is nitrided or nitrocarburized by using a gas nitriding process and /or a plasma nitriding process and/or a gas nitrocarburizing process and/or a plasma nitrocarburizing process at a temperature between 350°C and 900°C, wherein the cover nut is designed comprising a radius forming a contact surface CScs having a curvature that produces that the hollow space is sealed in interaction with an intermediate surface of the ball stud BScs , when the ball stud is fixed in the cover nut.

The method steps can run at least partially simultaneously or chronologically one after the other, whereby the sequence of the method steps is not limited to the sequence defined by the numbering, so that individual steps can be carried out in different sequences.

Advantageously, it can be provided that the radius R is in a range between R 0.1 - R 5, i.e. from 0.1 mm up to 5 mm.

It can also be provided that the ball pin has thread with the same size as cover nut.

Further, it can be provided that the material of the ball pin is 41CrS4.

It can also be provided that the material of the cover nut is stainless steel 1 .4305.

Advantageously, it can be provided that the ball pin and cover nut are connected by screwing with torque range from 5 Nm up to 25 Nm.

It can also be provided that the pretension force from the torque creates enough stress in material of cover nut and ball pin that tangent connection does not allow to get nitriding gas in covered area of the threads inside of cover nut. Further measures improving the invention will result from the following description of some embodiments of the invention, which are shown schematically in the figures. All features and/or advantages arising from the claims, the description or the drawings, including constructional details, spatial arrangements and process steps, may be essential to the invention both individually and in the most diverse combinations. It should be noted that the figures are descriptive only and are not intended to limit the invention in any way. They show schematically:

Figure 1 : a drawing of a device according to the present invention, in which the workpiece is a ball stud 1 having a ball pin (not shown, because it is into the hollow chamber of the covering component that is a cover nut 7,

Figure 2: a drawing of the device shown in Figure 1 , showing parts and surfaces of the ball stud 1 and of the cover nut 7 in more detail,

Figure 3: a schematic view of a radius to Radius contact,

Figure 4: a schematic view of a radius to conus contact, and

Figure 5: a schematic view of a flat contact.

In the following, the identical reference signs are used for the same technical features even of different embodiment examples.

The inventive method is concretely a method for the surface treatment of a ball stud 1 by means of a cover nut 7 that is used to isolate a surface area of the ball stud 1 from a gaseous medium, wherein the inventive method comprises following steps:

(a) the ball stud 1 is partially brought through an opening 6 into a hollow space 8 of the cover nut 7,

(b) a cover nut 7 is used as the apparatus for isolation,

(c) the ball stud 1 being fixed to the interior of the cover nut 7

(d) an area intended to be treated (treatable area 2 or treatable surface 2) of the surface of the ball stud 1 is nitrided or nitrocarburized through a gas nitriding process and /or a plasma nitriding process at a temperature between 350°C and 900°C, wherein the cover nut ’ is designed comprising a surface 10, which exhibits a contact surface (hereafter also called contact surface of the cover nut CSCS) having a curvature that produces that the hollow space 8 is sealed in interaction with a contact surface of the ball stud 1 (hereafter also called contact surface of the ball stud BSCS), when the ball stud 1 is fixed in the cover nut 7.

The inventive design of the cover nut 7 ensures the right contact between the special shape of the cover nut 7’ with the ball stud 1 , so that not any gas leakage to the thread surface area 3 of the ball studs 1 occurs, and in this manner the surface areas of the ball stud 1 that should not be nitrided and/or not nitrocarburized remain non-nitrided and/or non-nitrocarburized.

The inventive cover nut 7 can be designed for all types of ball studs 1 , for example for ball studs with contact surface BScs (for sealing in contact with the contact surface of the cover nut CScs) being flat or conus-shaped.

According to the present invention there are three preferred embodiments, corresponding to three preferred types of connections between ball studs and cover nuts without using any sealing ring as sealing element as they are shown in Figures 3 to 5.

A radius to radius contact as it is schematically shown in Figure 3 means tangent contact between the intermediate surface 10 that is the contact contour of the ball stud 1 which is between the area of the ball stud to be treated and the area of the ball stud that should not be treated, which has radius shape and the sealing-acting surface 6 is the contact contour of the cover nut 7 which has radius shape as well. The contact between the ball stud 1 and the cover nut 7 results in this manner in an interaction creating a sealing circular area.

A radius to conus surface contact as it is schematically shown in Figure 4 means tangent contact between the intermediate surface 10 that is the contact contour of the ball stud 1 which is between the area of the ball stud to be treated and the area of the ball stud that should not be treated, which has cone shape and the sealingacting surface 6 is the contact contour of the cover nut 7 which has radius shape. This contact, between the ball stud 1 and cover nut 7, results in this manner in an interaction creating a sealing circular area.

A flat surface contact as it is schematically shown in Figure 5 means plane contact between the intermediate surface 10 that is the contact contour of the ball stud 1 which is between the area of the ball stud to be treated and the area of the ball stud that should not be treated, and the sealing-acting surface 6 is the contact contour of the cover nut 7 which has flat surface shape and the contact contour of ball stud has flat surface shape as well. This contact, between the ball stud 1 and the cover nut 7, results in in this manner in an interaction creating a sealing flat plane circular area.

Contact surfaces of both ball stud and cover nut are perpendicular to axis of thread rotation of both parts.

Inventive Example:

According to two concrete examples of the present invention, cover nuts were designed for ensuring sealing during nitriding of ball studs with M10, M12 and M14 threads, respectively. In this examples the ball studs were made of 41Cr4+QT and 41CrS4+QT, respectively.

The inventive cover nuts in these examples were made of stainless steel 1.4305. More exactly cover nuts with thread M14 x 1 ,5 and contact radius RO, 5 mm were used in the examples.

For flat contact zone radius (i.e. contact radius) is not important.

The other radius from R 0.1 - R 5 (i.e. from 0.1 mm up to 5 mm) work for other contact zones, more specifically for conus contact zones.

The inner hole of the cover nuts was ended with radius R = 0.5 mm in the first examples mentioned above. This radius is in tangent contact with the ball pin which on nitride layer should be applied. Ball pin has thread with the same size as cover nut and material of ball pin is 41CrS4. They are connected by screwing with max torque 20 Nm (the range 5-25Nm). Pretension force from specified torque is creating enough stress in material of cover nut and ball pin that tangent connection will not allow to get nitriding gas in covered area of the threads inside of cover nut.

The foregoing explanation of the embodiments describes the present invention exclusively in the context of examples. Of course, individual features of the embodiments may be freely combined with one another, provided that this is technically expedient, without departing from the scope of the present invention.