DE10056842A1 - Process for surface treating a spring steel wire comprises nitriding the wire surface in such a way that a diffusion layer is formed on the whole surface of the wire without using a connecting layer between the diffusion layer and the wire - Google Patents

Abstract

Process for surface treating a spring steel wire comprises nitriding the wire surface in such a way that a diffusion layer is formed on the whole surface of the wire without using a connecting layer between the diffusion layer and the wire. Preferred Features: The duration and temperature of the nitriding are determined based on the spring geometry and the diameter of the spring steel wire. The spring steel material is made from a silicon-chromium alloy.

Description

The invention relates to a method for the surface treatment of pressure spiral springs, which are used in particular in diesel injection systems and a compression coil spring for injectors.

Such compression springs serve to bias a nozzle needle within the Nozzle holder system of injection nozzles against a nozzle needle seat for Ver close the injector. As soon as an injection pressure the spring preload exceeds, the injector is opened and then again by the spring closed when the pressure drops below the nozzle opening pressure. On Such compression springs are therefore very demanding with regard to their Fatigue strength and their wear, as they are at high injection pressures ken of up to 1000 to 2500 bar very high shock, vibration and rotation are exposed to stress.

Furthermore, such compression springs in nozzle holder systems for one spray nozzles over a longer period of time, the spring preload pressure if possible can not keep constant to the functionality of the injection system adversely affect. The problem with producing such compression springs is, therefore, the highest possible fatigue strength at the same time as possible to achieve low spring wear.

To increase the fatigue strength and reduce wear me metallic materials are so-called mechanical surface hardening processes  known, such as B. shot peening or deep rolling (see VDIW, VDI Society for Materials Technology, 1984/1985, page 25 ff.). There are also procedures ren known, in which by means of thermal or thermochemical edge layer hardening an increase in the fatigue strength and a higher Hardness of the processed material can be achieved. A disadvantage of these ver driving is, however, that usually also by the surface treatment Core hardness of the material affected, d. H. is reduced.

It is the object of the present invention to provide a method for the treatment of To provide springs with which high-durability and low-wear springs 4 are producible, while largely avoiding a change in their core hardness te. Another object of the invention is to provide springs for use in this To provide injection systems which are subjected to increased stress high injection pressures can be exposed, and in their mechanical properties changed as little as possible despite a strengthening treatment.

In the method according to the invention according to claim 1, the surface of a spring steel wire treated by targeted nitriding so that a nitride Diffusion layer is formed, the nitriding layer substantially without a connection layer, or with only a very small connection layer is formed on the wire. In this way, heavy-duty springs are targeted adjustable which improved properties regarding the fatigue strength and the Constant pressure, as the spring characteristics, d. H. their mechanical properties are retained. Through the targeted, d. H. in the process parameters of the nitriding process, action can be produced largely wear-free springs, in particular wear on the end faces of the compression coil spring and between their Prevents turns, which with a pressure drop and thus a Ver deterioration of the function of the injection system would be connected. Also the dhow The shock resistance of compression springs can be significantly increased.  

According to an advantageous embodiment of the invention, the method comprises the steps

• a) tempering the spring steel wire wound as a spring;
• b) cleaning the spring steel wire;
• c) nitriding the spring steel wire in such a way that a nitride Diffusion layer is brought into the surface, with extensive Ver avoidance of connection layers;
• d) post-treatment of the spring steel wire.

This has the advantage of significantly improving the functionality of the treatment springs through a targeted build-up of the layer characteristics on the ge entire surface of the spring steel wire. Through the nitriding treatment in step c) a diffusion layer is brought into the spring wire surface at weitge avoiding the formation of connecting layers on the surface. The core hardness of the spring is not ver through the targeted temperature control decreases.

According to a further advantageous embodiment of the invention, the nitriding duration and the nitriding temperature on the spring geometry and the diameter of the Spring steel wire matched. This ensures that the surfaces treatment consistently effective even with different springs is feasible and qualitative constant compression springs for injection systems are adjustable.

According to a further advantageous embodiment of the invention, the spring Annealed for a maximum of ten minutes at a temperature of up to 440 ° C. By such a short-term tempering cracks on the inner edge of the gewic cold spring steel wire avoided and still cheaper Relaxati ons values (values of maximum 1% in continuous operation) can be achieved because of a length res tempering is advantageously contained in the nitriding step itself.  

According to a further advantageous embodiment of the invention, the spring steel wire gas nitrided or gas oxynitrided. The advantage here is the univer general applicability for all possible spring geometries and the possibility to charge the compression coil springs as bulk material in the manufacturing process. One advantage of gas nitriding is the possibility of adding oxidizing agents Media such as B. oxygen or water vapor, the so-called "gas oxynitrie ren ".

According to a further advantageous embodiment of the invention, the spring steel wire from a winding ratio of the spring of w <5 plasma nitrided. here can nitrate at temperatures above 350 ° C and others on the other hand, in the case of springs with a winding ratio w <5, there is a targeted layer construction better possible by adjusting the nitriding process parameters in the plasma.

According to yet another advantageous embodiment of the invention, the Fe the steel wire after the nitriding treatment by means of blasting is. Shot peening increases the fatigue strength of the compression springs increased and possibly locally created connection layers are advantageous is hereby eliminated.

The compression coil spring according to claim 12, which is particularly suitable for use is provided in an injection nozzle holder, points below the surface a diffusion layer made of special nitrides. The compression coil spring is thereby not only high-strength and also wear-free over a long period of time, but has improvements over the previously known springs their resistance to impact, vibration and rotation as well as their permanent stigkeit.

According to an advantageous aspect in this regard, the spring consists of a sili zium-chromium alloy. The alloy components are advantageous in terms of selectable for improved nitridability of the spring steel wire.  

The following is an embodiment of the method according to the invention described for surface treatment.

The compression coil springs, which are used in nozzle holders of diesel injection systems, consist of a silicon-chromium-alloy spring steel wire and normally have a fatigue strength of τ _KH <900 N / mm ² and a maximum relaxation in continuous operation of 1%. The fatigue strength is increased by nitriding. The surface treatment method according to the invention uses a targeted layer structure by means of a nitriding treatment, as a result of which a layer of special nitrides can be formed in a diffusion zone in the surface of the entire spring. This is achieved by a choice of process parameters coordinated in this regard, in particular the process parameters in the nitriding step. The method also brings an improvement in terms of the economy of the production of compression coil springs, since an otherwise usual tempering after winding the spring steel wire to the coil spring with a winding ratio of w <5 is replaced by a short-term tempering for a maximum of 10 minutes becomes. Tempering to achieve the specific and required relaxation values is included in the nitriding step (e.g. at a temperature of 420 ° C over a period of 24 hours). The spring wire material used is normal Cr-Si wire according to DIN 17223 and Cr-Si wire with additional alloy components, such as. B. Mo, V, Al and the like, whereby advantages in the nitridability of the spring can be produced. In order to increase the fatigue strength as far as possible, a peeled wire rod is preferably used for the spring production.

The targeted build-up of a diffusion layer by nitriding the spring steel wire is completely wrapped within the entire wire surface condition, the nitriding time and to a limited extent the nitriding temperature (up to approx. 420 ° C) can be varied and in order to achieve a ge desired nitriding hardness depth (Nht) in coordination with the respective spring geometry  can be set. This advantageously influences the Core hardness of the compression spring largely avoided.

A spring treated with the method according to the invention has, for example It has a nitriding depth of 0.11 mm and an average core hardness of more times 560 HV0.1.

The treatment parameters in detail are riveting temperatures of up to 420 ° C, this temperature from the previous nitriding treatment at 440 ° C, taking into account the time and temperature-dependent starting effect. The nitriding reduces the Core hardness of the spring maximum by 40 HV0.1 and a core hardness of <530 HV0.1 remains at least. In the case of gas nitriding, the temp the ammonia is thermally pre-split and nitrided is carried out with a nitriding index of approx. 2.5. By adding oxi media, such as B. oxygen or water vapor can be the surface of the The spring can also be activated (so-called gas oxynitriding). With gas nitrie a nitriding depth of about Nht = 120 µm is achieved by nitriding 24 hours at a nitriding temperature of 410 ° C. Essential before Part of gas nitriding is the possibility of charging those to be treated Springs as bulk goods and a very universal applicability for all possible Spring geometries.

In the case of plasma nitriding, however, the springs must be charged individually, e.g. B. hanging on a batch carrier. Here is also on a sufficient down stood between the pressure coil springs to be treated to pay attention to a good one To achieve surface treatment result. The measurement and control of the Nitriding parameters are carried out by a contacted thermocouple, which is directly the spring is attached. Precautions must be taken in order to Avoid overheating during nitriding as the ones to be treated Compression coil springs have a very large surface area in relation to a very small one  Have volume. The gas must be closed before the plasma is switched on composition and gas pressure can be adjusted. To be as verbal as possible To achieve nitriding in the plasma that is free of the coating layer is the nitrogen content in the treatment gas to about 10%. By adding argon the width of the glowing seam can be further reduced. For a nitriding depth of about Nht = 100 µm is a nitriding temperature of 410 ° C and a nitriding time of about 24 hours is required. Here is the set voltage at 500-580 volts and the pulse / pause ratio at 1: 1 to 1: 2. The pressure to be set depends on the spring geometry. With narrower spring winding ratios, for example, nitriding pressures of up to to 500 Pa required to ensure sufficient nitriding of the entire spring top reach area.

Before the actual nitriding treatment, the springs go through the manufacturing process steps winding, tempering or short-term tempering as well as grinding on the face. For an effective surface treatment, the Springs may only have new hardness zones of <1 µm. Before nitriding Springs are cleaned so that they have a grease-free surface. This is done, for example, with alkaline aqueous cleaners or with alcohol. After a subsequent storage of over 24 hours, the springs are with Blasted glass beads. The radiation parameters are P = 4 bar, irradiated processing time: ten minutes, radiation: MS245B, coated.

The aftertreatment by shot peening of the springs is carried out over a period of at least 45 minutes with balls with a degree of arundundation G3 and spherical grain made of spring wire. The grain size is <0.4 mm, the discharge speed is a maximum of 60 m / s and R _z after blasting is a maximum of 15 µm.

In the case of surface treatment using plasma nitriding, before the step of Nitriding inserted a sputtering step in which by means of a hydrogen or a hydrogen-argon mixture created the surface of the springs  is cleaned. Shot peening can be carried out after nitriding by which residual compressive stresses are generated in the springs. This has the advantage of a further increase in fatigue strength and elimination possible local connection layers.

For an examination of the result of the surface treatment by the inventor The method according to the invention separates the springs axially and on the metallograph the hardness-depth profile is measured in HV0.1. The Hardness measurement takes place at the test points: outside turn, turn inside Inner spring area, evidence of nitriding between the windings and Proof of core hardness.

In the drawing, the hardness-depth curve of a spring is shown, which with the method according to the invention was treated. It is clearly recognizable that by the surface treatment because of the targeted temperature control the core hardness of the spring was not reduced and the mean value of the core hardness is more than 560 HV0.1.

Claims (15)

1. A method for the surface treatment of spring steel wire, in particular for the manufacture of compression coil springs for injection systems, characterized by targeted nitriding of the wire surface in such a way that ei ne diffusion layer is formed on the entire surface of the spring steel wire substantially without a connecting layer between the diffusion layer and the wire. 2. A method for the surface treatment of spring steel wire according to claim 1, characterized by the steps:

• a) tempering the spring steel wire wound as a spring;
• b) cleaning the spring steel wire;
• c) nitriding the spring steel wire in such a way that a nitride diffusion layer is brought into the surface with extensive avoidance of connecting layers;
• d) post-treatment of the spring steel wire.

3. The method according to claim 1 or 2, characterized in that the nitriding duration and the nitriding temperature on the spring geometry and the diameter of the spring steel wire. 4. The method according to claim 2, characterized in that in step a. the Spring annealed for a maximum of 10 minutes at a temperature of up to 440 ° C will. 5. The method according to claim 2, characterized in that in step c. the Spring steel wire is gas nitrided or gas oxynitrided.   6. The method according to claim 1 or 2, characterized in that the spring steel wire from a winding ratio of the spiral spring of w <5 plasma nitrided becomes. 7. The method according to any one of the preceding claims, characterized in that that is nitrided at a temperature of up to 420 ° C. 8. The method according to any one of the preceding claims, characterized in that that is plasma nitrided with the addition of argon. 9. The method according to any one of the preceding claims, characterized in that that the spring steel wire after nitriding by means of a beam processing is treated. 10. The method according to any one of the preceding claims, characterized in that that between steps b. and c. the spring steel wire with a gas in it Plasma is cleaned in a sputtering step. 11. The method according to any one of the preceding claims, characterized by a Special shot peening after step c. 12. The method according to any one of the preceding claims, characterized by the Use silicon-chromium alloys as spring steel material as well Additives. 13. The method according to any one of the preceding claims, characterized by forehead side special grinding of the wound spring steel wire in front of the nitrie ren. 14. Compression coil spring made of spring steel wire for holding injection nozzles in one injection systems, in particular according to a method according to one of the claims  1 to 13 manufactured compression coil spring, characterized in that that the surface of the spring steel wire a special diffusion layer Nitride has only a slight connection layer. 15. Compression coil spring according to claim 14, characterized in that the spring steel material is a silicon-chromium alloy.