Valve system for charge exchange control

Abstract

Valve system (1) for charge exchange control in an internal combustion engine with a valve seat ring (2) and a valve (3) with a valve seat (4), wherein the valve (3) consists entirely of a sintered material based on a steel powder and is a forged part, and wherein the valve seat (4) forms a sealing system with the valve seat ring (2), characterized in that the valve (3) of the valve system (1) is non-directional, that in addition to the valve seat (4) the entire valve body is nitrocarborized, and that the nitrocarborized surface layer is produced by means of a nitrocarborizing process in a salt bath.

Description

[0001] The invention relates to a valve system for charge exchange control in an internal combustion engine with a valve seat ring and a valve with a valve seat that forms a sealing system together with the valve seat ring.

[0002] A valve system, hereinafter referred to as the valve system, designed as an intake or exhaust system for charge exchange control in an internal combustion engine, comprises both intake and exhaust valves. The valve system determines when and how much fresh air-fuel mixture enters the combustion chamber and controls when the exhaust mixture is expelled. High material requirements are placed on all system components, but especially on the valve seat ring and the valve seat associated with it. Accordingly, these components must exhibit high fatigue strength, be highly resistant to mechanical and thermal stress, and possess high wear resistance. Depending on the specific requirements, different materials can be combined, utilizing their respective positive material properties to optimally support the individual components of a valve system.To reduce wear and increase service life, valve systems can be further optimized to meet specific requirements, for example by hardening the valve seat or welding on a special armor alloy to protect the valve seat. To improve the wear resistance of the valve stem, it is often nitrided or chrome-plated.

[0003] From US Patent 6,318,327 B1, a valve system for an internal combustion engine is known, comprising a valve seat and a valve element associated with the valve seat. The valve seat has a base element comprising a matrix of an iron-based sintered alloy and a powder of an intermetallic compound of the Si-Cr-Mo-Co group dispersed therein. The powder has a hardness of 600 to 1000 HV and an average particle diameter of 20 to 70 µm and is contained in the matrix in an amount of 10 to 50 percent by mass, based on the total mass of the base element. The valve system is characterized in that the valve element has a base element comprising a matrix of martensitic steel and a nitrided diffusion layer formed on a valve surface of the base element. The nitrided diffusion layer has a hardness of more than 500 HV and a thickness of more than 20 µm.

[0004] From DE 10 2008 061 237 A1, a multi-part gas exchange valve with a valve disc and a valve stem is known, wherein the gas exchange valve is made of a high-temperature resistant material and at least one other material, wherein a part of the gas exchange valve, which is made of the other material, is provided with a carbonitride layer, which is characterized in that a part of the gas exchange valve, which is made of the high-temperature resistant material, is also provided with a carbonitride layer.

[0005] Finally, a gas exchange valve with a valve cone comprising a valve stem and a valve disc is known from DE 10 2010 002 457 A1, wherein the valve stem is coated at least in the area of ​​the end face with a wear-resistant and hard coating which consists of a metallic material which is applied to the valve stem in particular by a nitriding process.

[0006] Although the nitriding diffusion layer on the valve element in the known valve system achieves improved wear and abrasion resistance, temporary adhesive adhesion or spot welding of the valve seat and valve seat ring cannot be prevented.

[0007] The object of the present invention is to provide a valve system for charge exchange control in an internal combustion engine comprising a valve seat ring and a valve with a valve seat forming a sealing system together with the valve seat ring, which exhibits a reduced overall wear taking into account the wear caused by the interaction between the valve seat and the valve seat ring.

[0008] The problem of a valve system for charge exchange control in an internal combustion engine is solved by a valve seat ring and a valve with a valve seat, wherein the valve consists entirely of a sintered material based on a steel powder and is a forged part, and wherein the valve seat forms a sealing system with the valve seat ring, characterized in that the valve of the valve system is non-directional and that, in addition to the valve seat, the entire valve body is nitrocarborized, and that the nitrocarborized surface layer is produced by means of a nitrocarborizing process in a salt bath.

[0009] The invention thus deliberately departs from the prior art, which is known to have a reground and therefore nitrided-layer-free valve seat of a nitrided valve. Due to tolerance requirements for the concentricity between the valve seat and valve stem, reground the valve seat after nitriding and the associated removal of the existing nitrided layer was absolutely necessary.

[0010] According to the invention, the valve of the valve system is non-directional, since straightening after forging the valves introduces material stresses into the valve body, including the valve seat, which are released again in the form of deformations during nitriding. Advantageously, the tolerance requirements for nitrided valve seats can thus be met. Straightening is understood to be the cold or hot forming of a workpiece with the aim of eliminating bulges and bends or restoring them to the desired shape. In particular, this refers to the cold forming of the valve with the aim of restoring the concentricity between the valve seat and valve stem to the desired shape.

[0011] According to the invention, in addition to the valve seat, the entire valve body is nitrocarborized. Advantageously, by nitrocarborizing not only the seat surface but also the entire valve body surface, weak points at the transition between a nitrocarborized and a non-nitrocarborized surface are avoided. This allows for an improvement in the wear properties of the valve system while maintaining the same wear requirements, as well as a reduction in manufacturing costs. The nitrocarborized surface layer of the valve body is created by enriching the surface layer of the material with nitrogen and carbon in a thermochemical treatment, whereby iron nitride is formed, among other things, in the compound layer. The nitrocarborized surface layer increases the resistance to adhesive wear.

[0012] According to the invention, the nitrocarborized surface layer is produced using a nitrocarborizing process in a salt bath. This allows for the use of lower nitriding temperatures and shorter treatment times. Very good results are achieved at temperatures of 500–550°C for a duration of 40–90 minutes. Furthermore, the decrease in the nitriding hardness profile from the surface to the depth of the component is not as steep as in steels that are only enriched with nitrogen, i.e., purely nitrided, as is the case, for example, with plasma-nitrided steels. Advantageously, this lower gradient of hardness decrease prevents spalling of the nitrocarborized layer. The flattening of the hardness decrease gradient is caused by the diffused carbon located beneath the nitrogen-richer layer.

[0013] Salt bath nitriding creates a diffusion layer > 15 µm thick and a compound layer up to 3 µm thick in martensitic steel; in austenitic steel, a diffusion layer > 5 µm thick and a compound layer up to 3 µm thick are created.

[0014] The diffusion layer consists of a very nitrogen-rich layer and an underlying carbon-rich layer. Nitrogen and carbon diffuse into the valve steel during the nitrocarburizing process. Above the diffusion layer, a much nitrogen-richer layer forms, the so-called compound layer. This layer consists of a chemical compound of the valve steel elements, especially Fe, Ni, and Cr, with the introduced nitrogen. The compound layer of martensitic steel itself can be subdivided into an outer compound layer with a porous border and a compact, white, or at least light-colored, compound layer. In austenitic steel, the compound layer appears as a gray zone. The compound layer no longer has a truly metallic character, but rather a more ceramic one.This ensures chemical separation of the contacting metal surfaces of the valve elements, in particular the valve seat and valve seat ring, the valve stem and valve guide, the valve grooves and cone pieces, as well as the valve stem end surface and the actuating element.

[0015] This separation of the surfaces reduces or prevents adhesive wear, i.e., micro-welding of the surfaces with subsequent tearing out of near-surface grains from the respective contacting valve section. Likewise, the high surface hardness of the nitrided layer reduces abrasive wear in the valve seat.

[0016] According to the invention, this also effectively reduces or prevents the adhesive bonding of parts of the valve seat or valve seat ring. This can reduce wear on a valve system according to the invention by up to 85% in the case of an intake system and by up to 90% in the case of an exhaust system. Furthermore, the overall wear of a valve system according to the invention for charge exchange control is reduced. To achieve sufficient hardness in addition to the good wear properties of the valve seat, alloyed valve steels are preferably used.

[0017] In a particularly preferred embodiment of a valve system according to the invention, at least the valve steel is a Cr-Mn-Ni alloyed austenitic valve steel, wherein a compound layer of up to 3 µm thick in the nitrocarborized valve, comprising the valve seat and the valve stem, consists of at least 10–65 wt.% Fe, 0.4–40 wt.% N, 10–22 wt.% Cr, 0.1–10 wt.% Mn, 0.1–5.5 wt.% Ni, and 0.45–16 wt.% C. Below the compound layer, the valve steel has a diffusion layer > 15 µm thick.

[0018] In a further particularly preferred embodiment of a valve system according to the invention, at least the valve comprising the valve seat is made of a Cr-Si alloyed martensitic valve steel, wherein a compound layer of up to 3 µm thick of the nitrocarborized valve, in particular of the valve seat and the valve stem, consists of at least 10–90 wt.% Fe, 10–30 wt.% N, 2.5–10 wt.% Cr, and 0.5–10 wt.% Ni. Below the compound layer, the valve steel has a diffusion layer > 15 µm thick.

[0019] The valve steel is preferably X45CrSi9-3, X50CrMnNiNbN21-9, or NIREVA 3015 (UNS-# 66315). A plating material is, for example, X180Fe-CoNiMo50-28 12 5. The aforementioned valve steels are particularly well-suited for achieving a wear-resistant surface layer through nitrocarburizing, as they contain at least one nitride former, such as Fe and Cr.

[0020] An advantageous embodiment provides that the nitrocarburized surface layer has a hardness of 400 to 1200 HV. Such a hardness results in very high resistance to adhesive wear for a nitrocarburized surface.

[0021] The principle and other characteristic features of the present invention will become clearer from the following descriptions. Furthermore, other embodiments, which individually or in combination lead to the solution according to the invention, are presented.

[0022] Fig. Figure 1 shows a valve system according to the invention for charge exchange control in an internal combustion engine. The valve system 1 comprises a valve seat ring 2 and a valve 3 with a valve seat 4. The valve seat 4 forms a sealing system together with the valve seat ring 2. The valve seat 4 has a substantially rotationally symmetrical shape and is nitrocarborized according to the invention.

[0023] In a closed valve position, the valve seat 4 rests against the valve seat ring 2, thus closing the combustion chamber (not shown in the figure). The in Fig. The valve system 1 shown, and in particular the valve 3, consists entirely of a sintered material based on steel powder. The valve 3 is a forged component, which allows for tight tolerances during manufacturing and may eliminate the need for straightening. In a final machining process involving grinding and turning all valve surfaces, the functionally relevant surfaces being primarily the valve stem end face, the valve stem grooves, and the valve seat 4, the entire valve 3 is nitrocarborized in a salt bath according to the invention and finally measured and inspected.

[0024] The valve is then ready for installation in the engine. In some cases, the valve stem is polished to reduce roughness. However, no further production process takes place after nitriding that completely removes the nitrided layer.

[0025] The valve consists, for example, of a Cr-Mn-Ni alloyed austenitic steel or a Cr-Si alloyed martensitic steel, optionally with additional nitrite-forming elements. Alloying elements such as Fe, Cu, Ni, Cr, Mo, Co, W, V, C, Mn, or Si may be added to the seat ring material.

Claims

[1] Valve system (1) for charge exchange control in an internal combustion engine comprising a valve seat ring (2) and a valve (3) comprising a valve seat (4), wherein the valve (3) is made entirely of a sintered material based on steel powder and is a forged part, and wherein the valve seat (4) forms a sealing system with the valve seat ring (2), characterized by , that the valve (3) of the valve system (1) is non-directional, that the entire valve body is nitrocarborized in addition to the valve seat (4), and that the nitrocarborized surface layer is produced by means of a nitrocarborizing process in a salt bath. [2] Valve system according to claim 1, characterized by, that at least the valve steel is a Cr-Mn-Ni alloyed austenitic valve steel, that a 3 µm thick compound layer of the nitrocarborized valve seat (4) consists of at least 10 - 90 wt.% Fe, 10 - 30 wt.% N, 2.5 - 10 wt.% Cr and 0.5 - 10 wt.% Ni and that a > 15 µm thick diffusion layer is formed below the compound layer. [3] Valve system according to claim 1, characterized by , that at least the valve steel is a Cr-Si alloyed martensitic valve steel, that a 3 µm thick compound layer of the nitrocarborized valve seat (4) consists of at least 10 - 90 wt.% Fe, 10 - 30 wt.% N, 2.5 - 10 wt.% Cr and 0.5 - 10 wt.% Ni and that a > 15 µm thick diffusion layer is formed below the compound layer. [4] Valve system according to one of the preceding claims characterized by , that the nitrocarborized surface layer has a hardness of 400 to 1200 HV.

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