Piston ring for internal combustion engines with increased fatigue strength, and method for producing same

Abstract

A piston ring (2) having increased fatigue resistance includes a plastically deformable material. The piston ring (2) has a running face (4), which is delimited at the top by an upper running face edge (3) and at the bottom by a lower running face edge (1). Compressive stresses are introduced into the upper running face edge (3) and/or into the lower running face edge (1) along at least one part of the circumference, the compressive stresses having been produced by roller burnishing.

Claims

1. A piston ring (2) having increased fatigue resistance, comprising a ductile, plastically deformable material, the piston ring (2) having a running face (4), which is delimited at a top by an upper running face edge (3) and at a bottom by a lower running face edge (1), the upper running face edge (3) and/or into the lower running face edge (1) at least along a part of a circumference of the piston ring have roller burnished surfaces, and wherein compressive stresses from roller burnishing are present in the roller burnished surfaces, and wherein each running face edge (1, 3) into which the compressive stresses are introduced is also provided with a bevel or a radius of curvature between 20 and 100 μm caused by the roller burnishing.

2. The piston ring (2) according to claim 1, wherein compressive stresses are only introduced into the lower running face edge (1).

3. The piston ring (2) according to claim 1, wherein compressive stresses are only introduced into the upper running face edge (3).

4. The piston ring (2) according to claim 1, wherein the radius of curvature is between 30 μm and 80 μm, and preferably between 40 μm and 60 μm.

5. The piston ring (2) according to claim 1, wherein the running face (4) is further provided with at least one wear protection coating or a running-in coating.

6. The piston ring (2) according to claim 1, wherein the piston ring comprises an iron or steel material or consists substantially or entirely of an iron or steel material.

7. The piston ring (2) according to claim 1, wherein the piston ring is formed as at least one of a compression ring, rectangular ring, tapered compression ring, piston ring with inner bevel, piston ring with inner angle, tapered compression ring with inner bevel, tapered compression ring with inner angle, double-sided trapezium ring, single-sided trapezium ring or L-shaped compression ring.

8. The piston ring (2) according to claim 1, wherein the piston ring is formed as a scraper ring or tapered scraper ring, wherein compressive stresses are introduced into an outer edge (1) of a lower piston ring flank (8) of the piston ring (2) by roller burnishing.

9. The piston ring (2) according to claim 1, wherein the compressive stresses are introduced only into a part of the piston ring that has an angular distance from a piston ring gap of at least 45°, preferably at least 90°, further preferably at least 135°.

10. The piston ring (2) according to claim 1, wherein the compressive stresses are introduced along the entire upper and/or lower running face edge (3, 1) of the piston ring (2).

11. A piston ring including an outer-most running face having at least a portion of which provided with a burnished surface finish imparting residual compressive stresses to said at least said portion.

Description

THE DRAWINGS

(1) The invention is explained below using schematic figures of exemplary embodiments.

(2) FIG. 1 shows a perspective view of a piston ring.

(3) FIG. 2 shows an electron microscope image of a fracture in a conventional piston ring.

(4) FIG. 3 shows a cross section of a piston ring according to the invention, which is provided with a wear protection layer.

(5) FIG. 4 shows a cross-sectional view of a piston ring according to the invention during a roller-burnishing process according to the invention.

(6) FIG. 5 shows a diagram that shows the improvement in fatigue resistance of a piston ring according to the invention compared with a conventional piston ring.

DETAILED DESCRIPTION

(7) The same reference symbols are used for the same or similar components in the figures and in the description.

(8) FIG. 1 shows a perspective view of a piston ring 2. The piston ring 2 has a running face 4 on the outside. The lower side of the piston ring, which cannot be seen in the figure, forms the lower flank of the piston ring 2, which is provided with the reference symbol 8 in the other figures. The upper side of the piston ring is formed by the upper piston ring flank 6. The running face 4 meets the upper piston ring flank 6 at the upper edge 3 of the running face 4. The running face 4 meets the lower piston ring flank, the reference symbol 8 of which is not shown in this figure, at the lower edge 1 of the running face 4. The piston ring is delimited on the inside by the inner face 10 of the piston ring. The piston ring 2 is open at the ring gap 12.

(9) During loading, the piston ring 2 can be rotated outwards at the top. If the piston ring is rotated outwards at the top, this can also be referred to as a negative twist 22. The edge 3 of the running face 4 is in particular loaded in a tensile manner owing to this negative twisting. This tensile load can cause crack formation at the upper edge 3.

(10) The piston ring 2 can however also be twisted inwards at the top on loading, which corresponds to the movement direction for a positive twist 24. The lower edge 1 of the running face 4 is in particular loaded in a tensile manner owing to this positive twisting. This tensile load, together with severe loading owing to a pressure of the combustion gases, can cause crack formation at the lower outer edge 1 of the running face or piston ring 2. The lower edge 1 of the running face 4 is in this case much more susceptible to cracks than the upper edge 3 of the running face 4 owing to the greater load.

(11) The piston ring has been provided at the lower edge 1 of the running face 4 with compressive stresses by roller burnishing, as a result of which the lower edge 1 is rounded out and has compressive stresses. These compressive stresses can be detected by means of changes in the metal microstructure at the roller-burnished lower edge 1, for example during finishing grinding. This is possible because the roller-burnishing process effects a ductile material displacement, which is in turn detectable under a microscope during etched finishing grinding. The roller-burnished region can be very small and narrow, for which reason it is possible that the piston ring according to the invention cannot be distinguished from a conventional piston ring with the naked eye.

(12) FIG. 2 shows an SEM (scanning electron microscope) image of a fractured face of a conventional piston ring. In this case the different topographies within the fracture surface can be seen more clearly: Starting from the lower edge of the running face or running edge 1, the crack extends into the ring cross section. In the process, a very fine surface structure is produced, which is typical for a fatigue fracture under alternating load. If a weakness of approx. ⅓ of the ring cross section is reached, an overload fracture occurs, which can be recognised by the typical coarser surface structure. The surface of the overload fracture has a coarser structure than the region in which there is a fatigue fracture. The position of the piston ring can be seen clearly by means of the indicated upper running face edge 3, the running face 4 and the lower flank 8 of the piston ring as well as the inner bevel 26 of a piston ring.

(13) FIG. 3 shows a cross section of a piston ring according to the invention, which is provided with a wear protection layer. The shape of the lower running edge has considerable influence on the fatigue and fracture resistance of a piston ring. Coated edges (chromium, PVD) are more susceptible to crack formation than uncoated edges.

(14) Furthermore, thick layers consisting of hard materials are likewise more affected by crack formation than thin layers. Furthermore, high-strength wear protection layers tend to form cracks more than for example running-in layers consisting of lower-strength material. Sharp edges, i.e. edges with small radii of curvature tend to form cracks more than rounded edges with larger radii of curvature.

(15) The cross section of FIG. 3 through a piston ring according to the invention shows an embodiment that combines the lower edge 1 of the running face 4 that is provided with compressive stresses with the features of a configuration of a piston ring that is currently regarded as favourable in terms of fatigue fracture resistance.

(16) The ring 2 is manufactured from a high-quality cast material or a steel material. A coating 28 has the lowest possible thickness of at least 30 μm (which is in this case shown schematically with exaggerated thickness). The wear protection layer 28 is chambered, the wear protection layer 28 not extending as far as the lower edge 1 of the running face 4. The lower running edge 1 is exposed and can therefore be machined by roller burnishing without cracks being produced in the coating even if a very hard wear protection layer 28 is used. The lower edge 1 is roller-burnished, as a result of which compressive stresses are present in the lower edge 1. The roller burnishing produces a radius of curvature 42 of 45 μm. The radius of curvature is between 20 μm and 70 μm, preferably between 30 μm and 60 μm, and further preferably between 40 μm and 50 μm. The rounding can be achieved by roller burnishing or an already pre-rounded edge can be provided with compressive stresses by roller burnishing.

(17) The rounding of the lower running edge 1 with a larger radius of 50 μm compared with the conventional 20 μm allows the fatigue fracture resistance to be further increased.

(18) According to previous experience, a radius of curvature of the lower edge 1 of the running face 4 within a range between 50 and 80 μm is still acceptable in terms of the oil-scraping effect. Limitations of the oil-scraping effect are to be expected with radii of curvature upwards of 100 μm.

(19) The application of large radii is associated with a clear increase in costs in the ring production process, since defined large radii can only be achieved by machining rings individually. Smaller radii of curvature to approx. 20 μm on average can be achieved by inexpensive polishing of the running faces by machining in bundles.

(20) FIG. 4 shows a cross-sectional view of a piston ring according to the invention during a roller-burnishing process according to the invention. Here, the burnishing roll 40 is pressed against the lower edge 1 between the running face 1 and the lower flank 8 of the piston ring 2 in order to produce a rounded portion 42, build up compressive stresses in the edge and smooth. The present invention aims to further increase fatigue strength. According to the invention, an edge rounding on the rings is achieved not only by material removal but also by material compression and displacement (roller burnishing) at least of the lower edge 1 between the running face 4 and the lower piston ring flank 8 of the piston ring. Owing to the roller burnishing and the material compression during roller burnishing, compressive stresses are introduced into the piston ring into the material in the region of highest load 42 at the running face edge 1 by the burnishing roll 40. Compressive stresses in this critical ring region counteract the occurrence of cracks in the edge region, as a result of which the fatigue strength of the ring is considerably further increased. The effect is achieved inter alia by displacing the tensile stresses from the surface with its surface roughness to below the surface, in which an ideally pore-free, compressed metal microstructure is present. A stress crack cannot therefore propagate starting from a small surface depression with the aid of the notch effect, since there are no surface structures in the microstructure that can act as a starting point for a stress crack. Furthermore, a notch acts only on a fracture line in the material directly on the surface of the edge, while two sides of the propagating crack run on the surface. In contrast, a defect under the surface of the material is held at its entire outer edge by the material. Therefore, cracks preferably propagate inwards from an outer surface of the material using the notch effect.

(21) FIG. 5 shows a diagram that shows the improvement in fatigue resistance of a piston ring according to the invention compared with a conventional piston ring. In the diagram, temperatures between 0° C. and 600° C. are plotted along the x-axis. The y-axis shows the fatigue resistance using the unit of 200 MPa to 550 MPa at 10.sup.6 cycles. The lower curve 44 connects the measurement points of a conventional piston ring. The fatigue resistance decreases with increasing temperature. The upper curve 46 connects the measurement points of a piston ring according to the invention, in which the lower edge between the running face and the lower piston ring flank has been provided with compressive stresses by roller burnishing. It can be seen clearly that the piston ring according to the invention has a much increased fatigue resistance compared with a conventional piston ring. The fatigue resistance decreases more with increasing temperature, since the higher temperatures have the same effect as stress-free annealing in metal materials. The effects achieved by the compressive stress occur more in colder piston rings, since a cold piston ring has a higher strength overall.

(22) An increase in the fatigue resistance of approx. 30% was achieved using roller burnishing, with a local plastic deformation of the metal from which the piston ring is manufactured. The present invention thus allows an increase in the fatigue and fracture resistance of a piston ring without resorting to expensive materials or to designs or layouts that are expensive to produce.

(23) The present invention has been illustrated with reference to the figures using embodiments that should not be used to define or limit the scope of protection. The claims define the scope of protection of the invention. In addition to the disclosure of individual combinations of features in the figures, other embodiments that can be produced by a simple combination of the features of the embodiments shown should also be regarded as disclosed.