The present invention relates to a gas exchange valve of an internal combustion engine having a valve cone essentially made of a valve shaft, which passes into a valve disk while forming a hollow cone.
Gas exchange valves, i.e., inlet and outlet valves for opening and closing the gas channel of the internal combustion engine, are subjected to great mechanical and thermal strains and corrosion attacks by the combustion gases. Only high-alloy steels of great heat resistance and good scaling resistance may meet the strains, in particular of the outlet valves.
Multiple measures have already become known for increasing the service life of such gas exchange valves. Thus, for example, the valve disk is armored on the sealing face using an especially resistant CrNi alloy.
As suggested in DE 43 41 811 A1, for example, in addition to the armoring described above, the service life of the outlet valve may be increased multiple times in highly-strained engines by a rotating device in the form of a propeller, which is attached to the valve shaft. Because of the forced rotation due to the outflowing exhaust gas, which excites the propeller, the valve shaft ends and disks remain free of deposits and single-sided heating may not cause leaks of the disk.
However, the other parts of the gas exchange valve are also subject to varying requirements in regard to heat, fatigue, and corrosion resistance.
Different requirements in regard to the heat, fatigue, and corrosion resistance in the various temperature zones of the valve cone are known to be taken into consideration in that the valve disk is produced from a material having high temperature and burn-off resistance, while the valve shaft including the propeller comprises a material having lower notch sensitivity and higher fatigue resistance, i.e., has sufficient toughness to counter the bending stresses occurring in this area. A material made of a typical valve steel or made of a super alloy, such as NiCr2OTiAl, is preferably used for the valve disk and a material made of a typical valve steel, such as X45CrSi9-3, is preferably used for the valve shaft including propeller. This is because steels made of a nickel-based alloy used to avoid corrosion are known to be very expensive, so that the gas exchange valve is extensively manufactured from the typical valve steel, such as X45CrSi9-3, where this is acceptable.
Furthermore, it has also already been recognized that a further aspect of the strain of a gas exchange valve comprises valve shaft and hollow cone being attacked by wet corrosion (condensation) because the combustion gases fall below the dew point during the engine shutdown.
However, hardening methods in the form of plasma nitration or plasma nitro-carburization in nitride-forming steels are already known.
In general, plasma nitration/plasma nitro-carburization are understood as hardening of surface layers of steels, nitrogen and/or carbon atoms diffusing in and reacting in a thin surface layer with iron to form nitrides and/or carbon nitrides, the bonding layer (VS). In the adjoining diffusion layer (DS), the nitrogen is first partially precipitated as a nitride upon cooling and then causes the hardness increase. The hardness itself is a function of the types of nitrides. Nitration times and layers differ depending on how the nitrogen is caused to react with the steel. In other words, there is diffusion saturation of the boundary layer of a material with nitrogen to increase hardness, wear resistance, fatigue strength, or corrosion resistance. The boundary layer comprises an external nitride and/or carbon nitride layer (bonding layer) and an adjoining layer made of mixed crystals enriched with nitrogen and precipitated nitrides (diffusion layer) after the nitration/nitro-carburization.
The nitration times may be shortened by ionization of the nitrogen by glow discharge, so-called plasma nitration (plasma nitration at 450° C. to 550° C.).
In nitro-carburization, in which the treatment agent also contains components discharging carbon in addition to nitrogen, nitro-carburization may be performed in powder, salt bath, gas, or plasma (plasma nitro-carburization at 500° C. to 590° C., preferably at approximately 520° C.).
Proceeding from this, it is the object of the present invention to refine a gas exchange valve forming the species in such a way that its parts which comprise the typical valve steels described at the beginning also have good corrosion protection.
This object is achieved by the characterizing features of claim 1 for a gas exchange valve of the type according to the species.
If the valve body of the gas exchange valve is implemented in one piece and the valve disk is armored on the sealing face and/or on the seat area as described at the beginning, the nitride and/or carbon nitride layer is preferably provided completely on the valve shaft and the hollow cone up to the armored sealing face.
The present invention is explained on the basis of the single FIGURE:
The gas exchange valve, in particular an outlet valve (1) for an internal combustion engine, has a rotating device in the form of a propeller (3) situated on its valve shaft (2). The valve disk (4) is armored on its sealing face (5). The wings (6) of the propeller (3) are milled out of the rotating shape of the propeller.
Furthermore, different requirements in regard to heat, fatigue, and corrosion resistance in the various temperature zones of the valve cone (1) are taken into consideration in that the valve disk (4) is produced from a material having high temperature and burn-off resistance, while the valve shaft (2) including the propeller (3) comprises a material having lower notch sensitivity and higher fatigue strength, i.e., having sufficient toughness to counter the bending stresses occurring in this area. A material made of a typical valve steel or a super alloy, such as NiCr2OTiAl, is preferably used for the valve disk (4) and a material made of a hot forming steel, such as X45CrSi9-3, is preferably used for the valve shaft (2) having propeller (3). The valve disk (4) is connected to the valve shaft (2) by a friction weld (7).
In the exemplary outlet valve (1) shown here, the valve cone is provided in at least partial areas with a corrosion protection layer in the form of a nitration layer (8), the corrosion protection layer being generated by reacting the nitride-forming base alloy by plasma nitration or plasma nitro-carburization in a nitrogen or nitrogen-carbon atmosphere.
In a preferred way, in the two-part embodiment of a gas exchange valve here, the complete valve shaft (2) up to the friction weld (7), at which the hot working steel material is also delimited, is provided with the nitration layer (8), the area of the hollow cone (11) remains open. Of course, in general, both in the one-part and also in the multipart embodiment of a gas exchange valve, the two front faces on the valve disk (4) and on the valve shaft (2) may remain left out in regard to the nitride and/or carbon nitride layer (8).
However, preferably in the one-part embodiment, the complete valve cone (1) up to the armored valve seat area (5) of the valve disk (4) and also up to its disk floor and the valve shaft front side, may be provided with the nitration layer (8).
In the gas exchange valve according to the present invention, the surface is converted in such a way that a hard, wear-resistant boundary layer results. For this purpose, the valve cone blank is processed, either in the form of the valve shaft or in its entirety with valve shaft (2) and valve plate (4), over all manufacturing steps in such a way that it is provided in its final surface roughness, and the plasma nitration and/or plasma nitro-carburization is subsequently performed. Post-treatment of the nitrated gas exchange valve is possible but not necessary (however, the post-treatment is not to be performed in the areas of the gas exchange valve to be protected from corrosion so as not to remove the bonding layer generated). For example, the valve cone (1) may be ground after the nitration.
It may be specified as characteristic for the generated corrosion protection layers that the nitration layer has a diffusion layer (9) having a thickness (nitration hardness depth) of 0.1 mm to 0.3 mm and a bonding layer (10) built up thereon of 3 μm to 15 μm and offers a surface hardness greater than 750 HV (Vickers).
If a thickness around 10 μm is to be achieved for the bonding layer, plasma nitro-carburization with the addition of carbon is preferred.
Significantly improved corrosion protection and an increase of the alternating fatigue strength are achieved by the plasma nitration and/or plasma nitro-carburization of the valve cone (1), i.e., longer maintenance intervals and/or component service lives are achieved and cracking due to bending strains is counteracted.