Patent Application
20090139610
Applicants claim priority under 35 U.S.C. 119 of German Application No. 10 2007 054 736.8 filed Nov. 16, 2007.
1. Field of the Invention
The invention relates to a method for hardening groove walls of the ring groove of a steel piston.
2. The Prior Art
German Patent No. DE 103 37 962 A1 describes a method for laser hardening of groove walls of the ring grooves of a steel piston, in which a laser beam having a wavelength of less than 5 μm is used in order to achieve a degree of absorption of the laser beam energy that is suitable for hardening. The laser beam furthermore has to be directed at the groove to be hardened at a sufficiently great incidence angle. It is disadvantageous, in this connection, that there is only a small selection of laser sources that generate laser radiation at a wavelength of less than 5 μm. Furthermore, the selection of a sufficiently great incidence angle is limited by the geometry of the ring groove, i.e. by the groove wall that lies opposite the groove wall to be hardened, depending on the position of the laser source relative to the steel piston.
It is therefore on object of the invention to avoid these disadvantages of the state of the art. This object is achieved by a method for hardening groove walls of the ring groove of a steel piston that put into rotation about its piston axis, by means of a laser beam is directed at a groove wall. The method includes the following steps:
In one embodiment, a mixture of argon and oxygen is used as the process gas. Alternatively, a mixture of nitrogen and oxygen can be used as the process gas. Or, a mixture of nitrogen, argon, and oxygen can be used as the process gas. Preferably, at least approximately 1% oxygen is mixed into the process gas.
Because the groove wall surface of the steel piston that is to be hardened is colored with iron oxide, its heat conductivity is increased to such an extent, and furthermore its reflectivity is reduced so greatly that the laser light can be radiated onto the groove wall surface to be hardened at any desired angle of incidence, independent of its wavelength.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.
In the drawings, wherein similar reference characters denote similar elements throughout the several views:
Referring now in detail to the drawings,
Furthermore, a gas line 8 is attached to holder device 1 and to a stand 9, by which line process gas is passed in the direction of upper ring groove 11, by way of a gas jet 4, which particularly surrounds the region of lower wall 10, which is to be laser-hardened, with the process gas.
A diode laser or an Nd:YAG laser can be used as the laser source 2, for example. Using this, in the present exemplary embodiment of the invention, lower wall 10 of ring groove 11 for the compression is hardened, because this wall is exposed to increased stress during engine operation, by the compression ring. However, any surface of piston 5 that is subject to great mechanical stress can be hardened using the method according to the invention.
In this connection, the small focus of laser beam 3 heats lower surface 10 of ring groove 11 of piston 5 that is put into rotation by means of turntable 6, in tracks. A cooling medium for bringing about the cooling of the heated track that is required for the hardening process is not necessary, since very rapid temperature equalization between the heated track and the adjacent piston material, steel, occurs, bringing with it automatic quenching, which brings about good hardening of lower groove wall 10.
During this hardening process, the process gas is passed onto lower groove wall 10 by way of gas jet 4; it can consist of a mixture of argon and oxygen, of nitrogen and oxygen, or of argon, nitrogen, and oxygen. Within the scope of comprehensive experiments, good results were achieved with a process gas mixture consisting of 1% oxygen and 99% nitrogen.
The groove wall surface hit by laser beam 3 colors as the result of oxide formation, whereby iron oxide (Fe2O3) is formed on the surface. As a result, an improvement of the heat conductivity of the surface irradiated by the laser occurs, for one thing, thereby accelerating the heating of this surface and thus improving the degree of effectiveness of the laser. For another thing, the coloring of the groove wall surface by iron oxide reduces the reflectivity of this surface, thereby reducing the intensity of the laser beam 3′ reflected by groove wall 10 and shown with a dotted line in
The method according to the invention for hardening groove wall 10 has the additional advantage that the angle of incidence α formed by groove wall 10 and laser beam 3 can be selected as desired, and is selected in such a manner that the size of the hardness penetration zone is optimally adjusted for the best possible hardening result.
Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 054 736.8 | Nov 2007 | DE | national |
