Title of Invention	COIL SPRING FROM AN ALLOY STEEL AND METHOD FOR PRODUCING SUCH COIL SPRINGS
Abstract	A coil spring made from a steel which contains nitride-forming alloying constituents, characterized in that a nitride-containing diffusion layer forms the sruface of the end-side turns of the coil spring.

Full Text

Prior Art The invention relates to coil springs made from steel comprising nitride-forming alloying constituents, and to a process for producing in particular coil springs of this type. Coil springs of the abovementioned type are used, inter alias, as nozzle holder springs in diesel injection systems. To satisfy stringent demands with regard to environmentally friendly exhaust emissions and a low fuel consumption, new diesel injection systems have to be operated at injection pressures of between 1000 and 2500 bar. The nozzle holders used have to keep the pressure constant. Nozzle holder springs made from a silicon/chromium-alloyed spring steel wire with a fatigue strength of KH > 900 N/mm^ and a maximum relaxation of 1% in long-term operation are used for nozzle holders on diesel injection systems. The nozzle holder springs have the role of closing the solenoid valve, which opens for each injection operation, tightly again. As a result of high impact, dynamic and rotational loads, particularly at high injection pressures wear phenomena occur at the end faces of the nozzle holder springs, having an adverse effect on the ability of the injection system to function, on account of a pressure drop, i.e. the solenoid valve is no longer tightly closed. The diesel engine develops soot. To solve this problem, it has already been proposed to expose the spring surface to plasma nit riding with the formation of a compound layer. However, it has been found that the nitride compound layer which then forms on the metal surface tends to flake off in the event of a sudden impact load, and the abraded material which is formed, and acts as an abrasive, still accelerates wear. The invention and its advantages The object of the invention is to provide coil springs with a good core hardness which are subject to very little wear even under high levels of long-term load, as well as a simple process for producing in particular coil springs of this type. This object is achieved by coil springs of the type described in the introduction having the features of the characterizing clause of Claim 1 and by a process of the type described in the introduction having the features of the characterizing clause of Claim 10. The coil springs according to the invention have a very high surface hardness, even though the inventors have dispensed with the compound layer and have restricted themselves to the diffusion layer, the first condition for generating only the diffusion layer being that the spring steel contains nitride-forming alloying constituents. The nitride-forming alloying constituents form special nitrides with nitrogen in the diffusion layer. The surface which has been hardened by the diffusion layer does not tend to flake off - unlike a compound layer consisting of iron nitrides - and therefore to increase the wear, but rather it has been found that a good wear resistance is obtained even if the diffusion layer is only of the order of magnitude of 100 aim thick. Since even such a thin diffusion layer is sufficient to achieve the desired wear resistance, the process according to the invention makes it possible to produce the diffusion layer with a level of outlay which is acceptable for industrial production under conditions which are such that the core hardness does not significantly deteriorate. The process according to the invention can be carried out using equipment which is customary for the production of thin films. The process is uncomplicated, since, if the starting point is coil springs which consist of the correct material (cf. above), the exclusive formation of the diffusion layer is easily achieved through the fact that the N2 content in the treatment gas is set to be so low that a compound layer is not formed, it being easy to determine the upper limit by means of simple tests. To impart sufficient wear resistance to the coil spring, it is sufficient if the thickness of the diffusion layer is at least 20 fem. with a hardness of > 750 HVO.l at a depth of 10 |a.m. A diffusion layer of > approximately 150 defined in this way does not provide any further improvement to the wear resistance. It is advantageous if the nitride-forming alloying constituents include at least one metal selected from the group consisting of Cr, Mo, V and Al, which are all simultaneously able to improve the properties of steel. It is advantageous if the coil spring according to the invention is a nozzle holder spring for a diesel injection system in which in particular wear resistance with a high core strength is important. It is advantageous if the diffusion layer is produced by plasma nitriding. The use of plasma nitriding not only allows a defined treatment with regard to the structure of the diffusion layer, but rather also allows the process to be carried out in such a way that only certain regions of the spring surface are nitrided in a targeted fashion. In this context, it is advantageous if the N content in the treatment gas is set to It is advantageous if nitriding is carried out at a temperature of It is expedient if the plasma is operated at a voltage (between anode (reactor wall) and cathode (parts to be nitrided)) of approximately 500 to approximately 580 volts. It is advantageous if nitriding is carried out for between approximately 20 and approximately 30 hours. It is advantageous if, for the only partial nitriding of the spring surface, the surface regions which are not to be nitrided are mechanically covered by walls which are at a short distance from the spring surface. Advantageous devices which offer these conditions are, for example, metal plates with holes into which the coil springs to be nitrided are fitted, with one end side at the front in the direction of the longitudinal axis thereof, the thickness of the plates being approximately equal to the length of the coil springs, and the holes having a diameter which is only slightly greater than the external diameter of the springs, and the end sides not projecting out of the plates or doing so only to an insignificant extent. With this device, it is advantageously possible, if the springs have an internal diameter of approximately 7 mm and pins with a slightly smaller external diameter than the internal diameter of the coil springs are fitted into the coil springs, to limit the nitriding to the end faces of the coil springs at pressures of up to approximately 300 Pa. Further advantageous configurations of the coil springs according to the invention and of the process according to the invention are given in the subclaims. The drawing In the text which follows, the invention is described in detail with reference to exemplary embodiments explained by drawings, in which: Fig. 1 shows a micro section through the turn at the end side of a nozzle holder spring after the plasma nitriding in accordance with the invention. Fig. 2 shows a diagram illustrating the hardness/depth profile at the end side of a nozzle holder spring which has been treated in accordance with the invention. Fig. 3 shows a photograph of an exemplary embodiment of the loading of nozzle holder springs for the plasma nitriding according to the invention. Fig. 4 shows a photograph of another exemplary embodiment of the loading of nozzle holder springs for the plasma nitriding according to the invention, and Fig. 5 diagrammatically depicts a further exemplary embodiment of the loading of nozzle holder springs for the plasma nitriding according to the invention. In the text which follows, the invention is described primarily with reference to the example of nozzle holder springs which have been produced in accordance with the invention for diesel injection systems and the process with which these coil springs are produced. However, it should be noted that although the invention can be used particularly advantageously in connection with nozzle holder springs of this type and can be explained particularly clearly with reference thereto, numerous deviations from this example are possible within the scope of the claims. Fig. 3 shows a nozzle holder spring 1 (also referred to below as a spring for short) . The process according to the invention can be applied to springs of this type, which consist of a steel which contains nitride-forming alloying constituents, such as Cr. Springs of this type preferably consist of a Cr-Si steel alloy, the Si improving the spring properties. The steel may contain at least one nitride-forming metal selected from the group consisting of Mo, V and Al instead of Cr or in addition to Cr. Fig. 1 shows a microsection through the metal structure at the end-side turn 2 of a spring after it has been subjected to the plasma nitriding according to the invention. The microsection shows that no compound layer of iron nitrides has formed on the steel surface. It is impossible to see from the microsection that a diffusion layer has formed. The diffusion layer contains nitrides with the nitride-forming alloying constituents mentioned above. The diffusion layer can be detected on the basis of a diagram as shown in Fig. 2 (which plots the hardness HVO.1 against the distance (in mm) from the surface) , this figure reproducing the hardness/depth profile, based on measurements of the Vickers hardness, at the end-side turn. In the case of the spring studied, the diffusion layer thickness (nitriding hardness depth) is - as shown in the diagram - approximately 0.11 mm. The nitriding hardness depth (NHD measured in HVO.1 (Vickers hardness under 100 g load)) should be at most approximately 150 |im and at least approximately 20 |j.m for a hardness of > 750 HVO. 1 at a depth of 10 fim. The spring analysed has a core hardness at the NHD of approximately 610 HVO.l, which is therefore approximately 50 HVO.l above the mean (560 HVO.l) of the measured core hardnesses. The diagram also shows that the core hardness has not been reduced by the nitriding. The springs which are to be hardened in accordance with the invention are nitrided after the working steps of coiling, tempering and end-side grinding in the production sequence. The tempering, which is used to reduce stresses which build up during the coiling of the spring wire and may cause stress cracks at the inner edge of the wire, is shortened in the process according to the invention compared to the known process (tempering time: 1 hour at 440°C) to rapid thermal tempering (10 minutes at approximately 440°C) which is initially sufficient to avoid stress cracks. The final tempering takes place during the plasma nitriding. The use of the rapid thermal tempering shortens the overall process duration, which is lengthened by the plasma nitriding, and therefore improves the economic viability of the process. The springs are preferably cleaned prior to the plasma nitriding, so that a grease-free surface is present. The cleaning may be carried out, for example, using aqueous alkaline cleaners or using spirit. If the springs have been stored for more than 24 hours, the springs are "preferably blasted with glass beads, such as Ballotini MGL (trade name, produced by Eisenwerke Wiirth GmbH + Co. KG) or with beads of an equivalent material, such as a ceramic material (pressure: 4 bar, 10 min). The diffusion layer is intended to prevent wear to the springs. Wear of this type takes place only at the end sides of the springs. A diffusion layer is therefore only required at the end sides. It would not inherently be critical if the diffusion layer were present over the entire spring surface. However, since it is necessary to hold and make electrical contact with the springs during the plasma nitriding, the spring surface cannot be completely nitrided, but rather can only be substantially nitrided, and the substantial nitriding would be undefined, which is unfavourable. Therefore, it is advantageous if the nitriding is restricted to the end side of the springs, preferably to the end-side turns thereof. Devices for covering the spring surface to such an extent that the nitriding is restricted to the end sides are shown in Figs 3 to 5. With the device shown in Fig. 3, the springs 1 are inserted in the direction of their longitudinal axis into the holes 3 in plates 4. The plates consist of a conductive material, since they are used to make electrical contact with the springs. The thickness of the plates is approximately equal to the length of the springs. The diameter of the holes 3 is approximately 0.1 mm greater than the external diameter of the springs. With the device shown in Fig. 4, the springs are tied together to form a "bundle" 5, i.e. a relatively large number of springs are arranged parallel to one another (in terms of the longitudinal axis) and with the lateral surfaces 6 bearing against one another and a steel strip 7 is wrapped around them. The bundle is held together by a steel wire 8. A steel loop 9 is present in order to suspend the bundle in the plasma reactor and to ensure that electrical contact is made. The width of the steel strip is approximately equal to the length of the springs, and the steel strip is laid in such a way that the springs do not project beyond the edge of the steel strip or do so only to an insignificant extent. In the case of the device shown in Fig. 5, a relatively large quantity of springs are stacked parallel to one another (in terms of the longitudinal axis) and with the lateral surfaces bearing against one another in a frame 10, the dimension of which parallel to the spring axes is approximately equal to the length of the springs. The lateral surfaces of the outer springs bear against the frame or almost bear against the frame, and the end sides of the springs project at most to an insignificant extent beyond the frame. With the three devices described, at least the lateral surfaces of the springs are sufficiently protected against the action of the plasma under the nitriding conditions employed. After it has been fitted with springs, at least one device in accordance with one of the alternatives described is placed or suspended in the plasma reactor. During the plasma treatment, prior to the nitriding a sputtering step is carried out, in which the accessible surface is finely cleaned using a plasma generated in an H2 or an H2-Ar atmosphere. The nitriding lasts for between approximately 20 and approximately 30 hours and preferably approximately 24 hours. It is carried out in an atmosphere which contains approximately 13% by volume. The H2 content is not critical. The. Ar content should be at most 10% by volume. The nitriding is carried out at substrate temperatures of between approximately 350 and approximately 420°C and preferably at approximately 420""C. No nitriding takes place at temperatures below approximately 350"C. At temperatures of > approximately 440°C or, if the treatment lasts a large number of hours, greater than > approximately 420°C, the core hardness of the spring deteriorates and therefore the relaxation increases. At temperatures of approximately 550 HVO,1 is retained during nitriding (maximum drop caused by nitriding 40 HVO.l). The precise setting of the temperature takes place, within the stipulated range, as a function of the desired temperature- and time-dependent tempering action. The voltage between the anode (reactor wall) and the parts to be nitrided (cathode) at which the plasma is operated must be sufficiently high for an anomalous glow discharge to be ignited. This ignition voltage, which is also dependent on the reactor geometry, is between 380 and 420 volts. On the other hand, the voltage must not be high enough for an arc discharge which melts the spring steel wire to form. This voltage is typically above approximately 600 volts. In the process according to the invention the voltage is preferably between approximately 500 and approximately 580 volts. Whether nitriding takes place at all is dependent on the plasma current density in the reactor. The plasma current density is dependent on the voltage, the gas composition and the pressure. Since (cf. above) the ranges of the gas composition and voltage which can be used are fixed for other reasons, the pressure, in order to ensure an appropriate plasma current density, should not be If the internal diameters are greater, it is necessary for pins whose diameter is only insignificantly smaller than the internal diameter of the springs to be introduced into the coils. If this additional measure is taken, the pressure can be increased to approximately 300 Pa without the plasma acting on the spring inside. To test the success of the nitriding, the springs are cut open in the axial direction. A hardness/depth profile in HVO. 1 (on account of the low NHD, measurement is carried out only with a load of 100 g) is prepared for one of the metallographically prepared cut surfaces, as shown in Fig. 2. Moreover, the core hardness is determined and it is checked whether nitriding has actually taken place only at the end sides of the springs. The hardness/depth profile illustrated in Fig. 2 was obtained after nitriding under the following conditions (the measured values which resulted in the hardness/depth profile are means which were obtained from measurements on a relatively large number of specimens): pressure: 120 Pa temperature: 420°C nitrogen content of the treatment gas 10% by volume voltage: 540 volts (duty ratio 1:1.5) duration: 24 hours. The diagram shows the approximately 0.11 mm thick diffusion layer and the fact that the core hardness has remained constant. After the nitriding, the springs are blasted for at least 45 min with spherical shot made from a special spring wire (process parameters: shot size average) after the blasting must be at most approximately 15 }im. A comparison between the springs which have been treated in accordance with the invention and the springs in accordance with the prior art shows that in the latter case, in use, surface wear occurs, while with the springs which have been produced in accordance with the invention wear occurred only in punctiform or linear fashion. WE CLAIM: 1. A coil spring made from steel which contains nitride-forming alloying constituents, characterized in that a nitride-containing diffusion layer forms the surface of the end-side turns of the coil spring. 2. The coil spring according to Claim 1, wherein the alloying constituents comprise at least one metal selected from the group consisting of Cr, Mo, V and Al. 3. The coil spring according to Claim 1 or 2, wherein the steel contains Si. 4. The coil spring according to any one of Claims 1 to 3, wherein the coil spring is a nozzle holder spring for diesel injection systems. 5. The coil spring according to any one of Claims 1 to 4, wherein the diffusion layer is between 20 and 150 |jinn thick with a hardness of 750 HVO.l at a depth of 10 jimmy. 6. The coil spring according to Claim 5, wherein the diffusion layer is between 50 and 120 thick.

Documents:

0019-chenp-2003-abstract-duplicate.pdf

0019-chenp-2003-abstract.pdf

0019-chenp-2003-claims-duplicate.pdf

0019-chenp-2003-claims.pdf

0019-chenp-2003-correspondence-others.pdf

0019-chenp-2003-correspondence-po.pdf

0019-chenp-2003-description-(complete)-duplicate.pdf

0019-chenp-2003-description-(complete).pdf

0019-chenp-2003-drawings-duplicate.pdf

0019-chenp-2003-drawings.pdf

0019-chenp-2003-form-1.pdf

0019-chenp-2003-form-18.pdf

0019-chenp-2003-form-26.pdf

0019-chenp-2003-form-3.pdf

0019-chenp-2003-form-5.pdf

0019-chenp-2003-other-document.pdf

0019-chenp-2003-pct.pdf