Sliding elements, particularly piston rings, and methods for manufacturing the same.

Abstract

The present invention relates to a sliding element, in particular a piston ring, and a method for manufacturing the same, the sliding element comprising a substrate of martensitic or austenitic stainless steel having a chromium content of at least 6.0% by mass and a nitrided layer having a surface hardness of up to 950 HV1.

Description

Technical Field

[0001] The present invention relates to a sliding element, particularly a piston ring, which exhibits good wear resistance and improved fatigue strength as a whole, and a method for manufacturing the same.

Background Art

[0002] When reducing the carbon dioxide emissions of an internal combustion engine, fuel consumption plays an important role. This is particularly affected by the frictional losses of sliding elements in the engine, especially in the area of the piston. Sliding elements such as piston rings have a running surface that slides in contact with a mating surface. This tribological system is complex and is significantly influenced by the combination of the materials of the mating surfaces.

[0003] Sliding elements such as piston rings are increasingly being subjected to higher requirements, particularly in terms of fatigue strength, which is particularly caused by increased load conditions, such as an increase in cylinder peak pressure, and a decrease in piston ring dimensions (especially the axial ring height). On the other hand, particularly in recent engines, thermal and mechanical loads also occur on sliding elements such as piston rings, pistons, or cylinder liners of internal combustion engines, and thus high wear resistance is required over a long service life. To ensure this durability, a wear-resistant layer can be provided, for example, on the outer flank surface of a piston ring.

[0004] Therefore, in summary, sliding elements of internal combustion engines need to ensure the best possible frictional behavior throughout the entire service life while ensuring significantly increased fatigue strength and protection against necessary wear.

[0005] Piston rings with a flank that is partially or fully nitrided and a running surface that has at least a partially different coating are known from the prior art.

[0006] The DE 102 21 800 A1 discloses a steel piston ring having a running surface, an inner surface, and upper and lower flanks provided between them, wherein at least a portion of the running surface is provided with a thermal spray layer as a running surface coating, and at least the flanks are provided with a nitrided layer produced by plasma nitriding.

[0007] US 6 508 473 B1 describes a piston ring having a nitrided layer on the upper and lower flanks, or on the upper and lower flanks and the inner circumferential surface, and a hard coating on the outer circumferential surface formed by ion plating.

[0008] DE 10 2005 023 627 A1 discloses a steel piston ring having a running surface with a chamber on one side, the running surface being covered with a chromium ceramic-based wear-resistant layer having microcracks, and at least the flanks being provided with a wear-reducing nitride layer.

[0009] DE 10 2005 011 438 B3 discloses a method for manufacturing a wear-resistant layer on a piston ring substrate made of steel or cast iron, wherein first, at least a single-layer thermal spray layer based on a nitrogen affine metal element is provided on at least a portion of the running surface region, and then nitriding treatment is performed on at least the running surface and flank to which the thermal spray layer is applied.

[0010] Even if such sliding elements have a layer with sufficient wear resistance, they exhibit a decrease in fatigue strength under the aforementioned load conditions. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] West German Patent Application Publication No. 10221800 [Patent Document 2] U.S. Patent No. 6508473 [Patent Document 3] German Patent Application Publication No. 102005023627 Specification [Patent Document 4] German Patent No. 102005011438 Specification [Overview of the project] [Problems that the invention aims to solve]

[0012] The present invention aims to provide a sliding element, preferably a piston ring, that exhibits good wear resistance and improved fatigue strength overall, as well as a method for manufacturing the same. [Means for solving the problem]

[0013] This objective is achieved by the sliding element described in claim 1 and the method for manufacturing the sliding element described in claim 7.

[0014] Wear resistance is primarily ensured by providing a nitride layer on a martensitic or austenitic stainless steel substrate having a chromium content of at least 6.0% by mass. A chromium content of at least 11.0% by mass or at least 17.0% by mass, respectively, is advantageous in improving the wear resistance of the sliding element.

[0015] The fatigue strength of surface-treated parts largely depends on the brittleness of the surface layer region of each part. Nitriding of sliding elements is considered such a surface treatment. Various test series have shown that desired brittleness reduction can be achieved by lowering the hardness of the nitrided layer. In particular, this can be achieved by specific processes during nitriding.

[0016] Therefore, according to the present invention, it is proposed that nitriding of piston rings made of high-chromium steel be carried out in such a way that the brittleness of the nitrided layer is reduced. In particular, according to the present invention, the reduction of brittleness is achieved by lowering the hardness of the nitrided layer. It has been shown that a surface hardness of up to 900 HV1 measured perpendicular to the nitrided layer leads to a significant improvement in fatigue strength.

[0017] The structure of the sliding element described in claim 1 comprises, namely, a base material of martensitic or austenitic stainless steel having a chromium content of at least 6.0 mass%, and a nitrided layer having a surface hardness of up to 950 HV1, thereby ensuring desired wear protection while providing high fatigue strength.

[0018] Surprisingly, the significantly lower hardness in high-chromium steel compared to conventional nitrided layers is achieved by the remarkably high temperature during nitriding.

[0019] When a mixture of ammonia and ammonia decomposition gas is supplied at elevated temperatures, the ammonia decomposes on the surface of the metal sliding element until atomic nitrogen is absorbed. This absorbed nitrogen diffuses onto the surface of the metal piston ring due to the nitrogen concentration gradient, forming a nitride layer. The formation of the nitride layer is determined by the solubility of the high-chromium piston ring steel.

[0020] According to the present invention, the nitriding process conditions are selected to exceed the nitrogen solubility of the substrate, and precipitates of iron nitride and chromium nitride may already be formed during nitriding and continue to grow over the course of further events. The gradually growing precipitates of iron nitride and chromium nitride affect the metallic strain of the iron lattice so as to suppress the increase in lattice strain. These reduced lattice strains are directly related to the brittleness and hardness of the nitrided layer. We have now found, surprisingly, that the above effects can be achieved by nitriding the substrate at a temperature between at least 600°C and a maximum of 700°C without forming an undesirable, so-called brownite phase in the diffusion region of the nitrided layer. These advantageous effects are particularly pronounced at temperatures of at least 630°C and a maximum of 650°C, respectively. The aforementioned upper temperature limits ensure that the risk of brownite formation is avoided.

[0021] Nitriding is preferably carried out for 15 to 60 minutes.

[0022] Preferred developments of the sliding element and the corresponding method according to the present invention are described in further claims.

[0023] Preferably, the fatigue strength is further improved by a surface hardness of at least 700 HV1 and / or a maximum of 900 HV1. Similarly, a chromium content of at least 11.0% by mass or at least 17.0% by mass advantageously improves the wear resistance, respectively.

[0024] According to an embodiment of the present invention, the sliding element additionally includes, as an outermost layer, a wear-resistant layer preferably selected from a PVD layer or an electroplated layer, particularly preferably a DLC layer, on at least a part of the surface of the sliding element. Such a wear-resistant layer further improves the wear protection of the sliding element. Furthermore, when the nitride layer according to the present invention is combined with the wear-resistant layer, the risk of crack king in the high-pressure nitride layer, which is also caused by a pre-ignition process in the engine or a very high dynamic gas pressure due to so-called knocking, is synergistically reduced.

[0025] Advantageously, the sliding element is a piston ring, and the wear-resistant layer is applied to the outer peripheral surface and / or the flank of the piston ring. The cited area of the piston ring particularly benefits from the protection against wear provided by the wear-resistant layer.

[0026] According to an advantageous embodiment, the nitride layer forms the outermost layer on at least a part of the surface of the sliding element, preferably on the outer peripheral surface and / or the flank of the piston ring. Such a sliding element exhibits sufficient characteristics in terms of wear resistance and fatigue strength, while being particularly easy to manufacture.

[0027] Preferably, the nitride layer has a nitriding hardness depth Nht 700 HV0.1 between 2 inches and 100 inches measured in accordance with section 4.2.15 of ISO 6621-2. The said nitriding hardness depth ensures the desired wear resistance and fatigue strength.

[0028] Advantageously, the thickness of the wear-resistant layer is at least 3 inches, preferably at least 10 inches. In this value range, a particularly high wear resistance of the wear-resistant layer can be achieved.

[0029] Preferably, the nitrided layer consists only of a single region of nitrided layer in which the hardness continuously decreases from the outer surface toward the substrate that does not contain the nitrided layer. That is, the nitrided layer does not exhibit multi-stage discontinuous transient nitrided layer formation. This embodiment is characterized by excellent wear resistance and fatigue strength.

[0030] Advantageously, the substrate of the sliding element has a uniform, fine-grained tempered structure with no carbide accumulation, and the maximum carbide particle size is 50 μm. This advantageously improves the fatigue strength of the sliding element.

[0031] According to an advantageous embodiment, the substrate is subjected to a cleaning treatment before nitriding. This allows for the removal of surface impurities.

[0032] Preferably, prior to nitriding, the substrate is heated in a gas nitriding apparatus to a pretreatment temperature between 450°C and 550°C while nitrogen gas is added.

[0033] Advantageously, the substrate is subjected to a single-step or multi-step etching process with the addition of an etching agent and ammonia in a solid or liquid state prior to nitriding. This removes the passivation oxide film formed by chromium and oxygen elements. Furthermore, the initial nitride nucleation occurs on the piston ring surface.

[0034] According to a favorable embodiment, nitriding is carried out with the addition of ammonia, and optionally nitrogen and / or hydrogen.

[0035] Preferably, during heating to the nitriding temperature, at least one holding step is provided in which the substrate is held at a temperature lower than the nitriding temperature.

[0036] The basic concept of the present invention will be explained in more detail below with reference to the drawings. The drawings shown are as follows. [Brief explanation of the drawing]

[0037] [Figure 1]This shows a comparison of the surface hardness of a conventional nitrided piston ring (Var.1) and a nitrided piston ring (Var.2) according to the present invention, measured according to HV1 and HV0.5. [Figure 2] This shows a comparison of the inherent fatigue strength of a conventionally nitrided piston ring (Var.1) and a piston ring nitrided according to the present invention (Var.2). [Figure 3] This shows a comparison of the microstructure cross-sections of a conventional nitrided piston ring (Var.1) and a nitrided piston ring (Var.2) according to the present invention, both of which have an additional PVD wear-resistant layer. [Modes for carrying out the invention]

[0038] (Detailed explanation of the drawing) The expected relationship between the surface hardness of the nitrided layer and the fatigue strength of the nitrided piston ring is demonstrated by the results shown in Figures 1 and 2: on the one hand, the method according to the present invention leads to a significantly reduced surface hardness of the nitrided layer (see Figure 1). This reduced surface hardness, in turn, leads to a significantly increased fatigue strength, as shown in Figure 2. The piston ring's inherent fatigue strength is measured using the method underlying Figure 2, where fatigue strength - standard load cycle 10 7 This was derived by determining the mean stress and stress amplitude for [the material]. The preferred growth of iron nitride and chromium nitride precipitates according to the present invention is further demonstrated by the improved etching properties of the nitride layer with a 1% alcohol-nitric acid solution in the metal microstructure cross-sectional microsection, as shown in Figure 3.

[0039] The following additional embodiments again demonstrate the effect of nitriding according to the present invention on hardness. The surface hardness shown in Table 1 was measured on the nitrided layer of a sliding element nitrided by a standard method. In contrast, the surface hardness shown in Table 2 was measured on the nitrided layer of a sliding element nitrided by the method of the present invention. As is clear from comparing the two tables, the method according to the present invention leads to a significantly reduced surface hardness.

[0040] Table 1

[0041] Table 2

Claims

1. A sliding element, A base material of martensitic or austenitic stainless steel having a chromium content of at least 6.0% by mass, A nitrided layer having a surface hardness of less than 900 HV1, Includes, A sliding element wherein the nitrided layer is composed only of a single region of nitrided layer in which the hardness continuously decreases from the outer surface toward the substrate that does not contain the nitrided layer.

2. The sliding element according to claim 1, wherein the sliding element further includes an abrasion-resistant layer as the outermost layer on at least a portion of the surface of the sliding element.

3. The sliding element according to claim 2, wherein the sliding element is a piston ring, and the wear-resistant layer is applied to the outer circumferential surface and / or flank of the piston ring.

4. The sliding element according to claim 1, wherein the nitrided layer constitutes the outermost layer on at least a portion of the surface of the sliding element.

5. The sliding element according to any one of claims 1 to 4, wherein the nitrided layer has a nitrided hardness depth Nht700 HV0.1 between 20 and 100 μm as measured in accordance with ISO 6621-2, section 4.2.

15.

6. The sliding element according to claim 2 or 3, wherein the wear-resistant layer has a thickness of at least 3 μm.

7. The sliding element according to any one of claims 1 to 6, wherein the substrate is uniform, has a fine-grained tempered structure without carbide accumulation, and has a maximum carbide particle size of 50 μm.

8. A method for manufacturing a sliding element, The steps include providing a base material of martensitic or austenitic stainless steel having a chromium content of at least 6.0% by mass, The steps include nitriding the substrate at a temperature between at least 600°C and a maximum of 700°C, Includes, The nitrided substrate has a surface hardness of less than 900 HV1. A method for manufacturing a sliding element, wherein the nitrided layer consists only of a single region of nitrided layer in which the hardness continuously decreases from the outer surface toward the substrate that does not contain the nitrided layer.

9. The method for manufacturing a sliding element according to claim 8, wherein the substrate is subjected to a cleaning treatment before nitriding.

10. The method for manufacturing a sliding element according to claim 8 or 9, wherein, prior to nitriding, the substrate is heated to a pretreatment temperature between 450°C and 550°C in a gas nitriding apparatus while nitrogen gas is added.

11. A method for manufacturing a sliding element according to any one of claims 8 to 10, wherein the substrate is subjected to a single-stage or multi-stage etching treatment in which an etching agent and ammonia in a solid or liquid state are added before nitriding.

12. A method for manufacturing a sliding element according to any one of claims 8 to 11, wherein nitriding is carried out by adding ammonia, and optionally nitrogen and / or hydrogen.

13. A method for manufacturing a sliding element according to any one of claims 8 to 12, wherein at least one holding step is provided during heating to the temperature in the nitriding step, the substrate is held at a temperature lower than the temperature in the nitriding step.

14. A method for manufacturing a sliding element according to any one of claims 8 to 13, wherein the nitriding process exceeds the solubility limit of nitrogen in the substrate.

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