WO2002002840A1

WO2002002840A1 - Coil spring from an alloy steel and method for producing such coil springs

Info

Publication number: WO2002002840A1
Application number: PCT/DE2001/002402
Authority: WO
Inventors: Otto Krickau; Andreas Bernhardt; Nils Lippmann
Original assignee: Robert Bosch Gmbh
Priority date: 2000-07-04
Filing date: 2001-06-29
Publication date: 2002-01-10
Also published as: DE50107847D1; JP2004502032A; DE10032313A1; EP1301648A1; EP1301648B1

Abstract

The invention relates to a coil spring from an alloy steel and to a method for producing said coil springs. The aim of the invention is to produce coil springs that have a good core hardness, showing very little wear even under high constant loads, and to provide a simple method for producing such spring coils. To this end, the coil springs are produced from a steel that contains nitride-forming alloy components, wherein a nitride-containing diffusion layer forms at least part of the surface of the coil spring. According to the method for producing the inventive coil springs the surface of coil springs that are produced from a steel that contains nitride-forming alloy components is at least partially converted to a diffusion layer by nitrating it with a nitrogenous treatment gas. The nitrogen content in the treatment gas is selected such that no connecting layer is formed on the surface of the coil spring.

Description

Alloy steel coil springs and method of making such coil springs

State of the art

The invention relates to coil springs made of steel with nitride-forming alloy components and a method for producing such coil springs in particular.

Coil springs of the type mentioned include used as nozzle holder springs in diesel injection systems. In order to meet strict requirements with regard to environmentally friendly exhaust gas behavior and low fuel consumption, new diesel injection systems have to be operated with injection pressures between 1000 and 2500 bar. The nozzle holder used must keep the pressure constant. For nozzle holders on diesel injection systems, nozzle holder springs are made from a silicon-chromium-alloy spring steel wire with a fatigue strength of

KH> 900 N / mm ² and a maximum relaxation of 1% in continuous operation ^¬ used. The task of the nozzle holder springs is to close the solenoid valve that opens with each injection process. As a result of high shock, vibration and rotation stresses, wear phenomena occur in particular at high injection pressures on the end faces of the nozzle holder springs, which deteriorate the functionality of the injection system due to a drop in pressure, ie the solenoid valve is no longer closed tightly. The diesel engine "smokes".

To solve the problem, it has already been proposed to subject the spring surface to plasma nitriding with connection layer formation. But it turned out that the The nitride compound layer that forms on the metal surface tends to flake off under sudden stress and the resulting abrasion, which acts as an emery, further accelerates wear.

The invention and its advantages

It is the object of the invention to provide helical springs with good core hardness, which show very little wear even under heavy permanent load, and to provide a simple method for producing such helical springs in particular.

This object is achieved with coil springs of the initially mentioned kind ^• with the features of the characterizing portion of claim 1 and with a method of the initially mentioned type with the features of the characterizing portion of claim 10.

The coil springs according to the invention have a very high surface hardness, although the inventors have dispensed with the connecting layer and limited themselves to the diffusion layer, the first requirement for the exclusive production of the diffusion layer being that the spring steel contains nitride-forming alloy components. The nitride-forming alloy components form special nitrides in the diffusion layer with nitrogen. The surface hardened by the diffusion layer does not tend to flake off, like a connecting layer made of iron nitrides, and thus to increased wear.Rather, it has been shown that good wear resistance is obtained even if the diffusion layer is only of the order of 100 μm thick. Because such a thin diffusion layer is sufficient to achieve the desired wear resistance, the method according to the invention makes it possible to produce the diffusion layer with a reasonable effort in industrial production under conditions such that the core hardness does not deteriorate appreciably. The method according to the invention can be carried out using apparatus which the production of thin layers are common. The process is straightforward because, if coil springs made of the right material (see above) are used as the starting point, the exclusive formation of the diffusion layer is achieved simply by setting the ^ content in the treatment gas so low that there is no connecting layer arises, the upper limit can easily be determined by simple experiments.

In order to give the coil spring sufficient wear resistance, it is sufficient if the thickness of the diffusion layer is at least 20 μm with a hardness of> 750 HVO, 1 at a depth of 10 μm. A diffusion layer of> about 150 μm defined in this way brings no further improvement in wear resistance.

It is advantageous if the nitride-forming alloy components include at least one metal from the group Cr, Mo, V and Al, which can all improve the properties of steel at the same time.

It is advantageous if the coil spring according to the invention is a nozzle holder spring for a diesel injection system, in which wear resistance with high core strength is particularly important.

It is advantageous if the diffusion layer is produced by plasma nitriding. The use of plasma nitriding not only enables a defined treatment with regard to the structure of the diffusion layer, but rather the method can also be carried out in such a way that only certain areas of the spring surface are specifically nitrided. It is advantageous if the N content in the treatment gas is set to <approximately 13% by volume.

It is advantageous if nitriding is carried out at a temperature <440 ° C., because at this temperature the core hardness does not deteriorate, the formation of the diffusion layer in one reasonable time, and the tempering can be completed during the time required for nitriding.

It is advantageous if the plasma is operated at a voltage (between the anode (reactor wall) and cathode (parts to be nitrided)) of approximately 500 to approximately 580 volts.

It is advantageous to nitrate for between about 20 and about 30 hours.

It is advantageous if, for the partial nitriding of the spring surface, the surface areas which are not to be nitrided are mechanically covered by walls which are at a short distance from the spring surface. Advantageous devices that offer these requirements are, for example, metal plates with holes into which the coil springs for nitriding are inserted with one end face first in the direction of their longitudinal axis, the plates being approximately as thick as the coil springs are long and the holes unified Have a diameter that is only slightly larger than the spring outer diameter, and the end faces do not protrude from the plates or only slightly. With this device you can advantageously if the springs have an inner diameter of <about 7mm, at a pressure in the range between about 100 and 150 Pa or if the inner diameter of the coil springs> about 7mm and pins with a slightly smaller outer diameter than the inside diameter of the coil springs are inserted into the coil springs, at pressures up to about 300 Pa, restrict the nitriding to the end faces of the coil springs.

Further advantageous refinements of the coil springs according to the invention and of the method according to the invention are listed in the subclaims.

The drawing The invention is described in detail below with reference to exemplary embodiments explained by drawings. Show it

1 is a micrograph of the winding on the end face of a nozzle holder spring after plasma nitriding according to the invention,

2 shows a diagram of the hardness-depth curve on the end face of a nozzle holder spring treated according to the invention,

3 is a photograph of an embodiment of the charging of nozzle holder springs for the plasma nitriding according to the invention,

Fig. 4 is a photograph of another embodiment of the charging of nozzle holder springs for the plasma nitriding according to the invention and

Fig. 5 is a schematic representation of a further embodiment of the charging of nozzle holder springs for the plasma nitriding according to the invention.

The invention is described below primarily using the example of nozzle holder springs for diesel injection systems produced according to the invention and the method by which these coil springs are produced. However, it should be clarified that the invention can be used particularly advantageously in connection with such nozzle holder springs and can be explained particularly clearly, but that various deviations from this example are possible within the scope of the claims.

3 shows a nozzle holder spring 1 (hereinafter also referred to as a spring). The method according to the invention is applicable to springs which consist of a steel which contains nitride-forming alloy components such as Cr. Such springs preferably consist of a Cr-Si steel alloy, the Si improving the spring properties. Instead of Cr or in addition to Cr, the steel can contain at least one nitride-forming metal from the group Mo, V and Al.

1 shows in a micrograph the metal structure on the front winding 2 of a spring after it has been subjected to the plasma nitriding according to the invention. The cut shows that no connecting layer of iron nitrides has formed on the steel surface. The micrograph does not show that a diffusion layer has formed. The diffusion layer contains nitrides with the above-mentioned nitride-forming alloy components. The diffusion layer can be demonstrated using a diagram as shown in FIG. 2 (the hardness HVO, 1 is plotted against the distance (in mm) from the surface), which shows the hardness based on measurements of the Vickers hardness. Depth course on the front turn shows. In the spring examined, the diffusion thickness (nitriding hardness depth) is approximately 0.11 mm, as the diagram shows. The nitriding (Nht measured in HV0.1 (Vickers hardness at 100 g load)) to a maximum of about 150 microns and about 20 microns minimal amount ^'with a hardness of> 750 HV0.1 in 10 microns depth. The spring examined has a core hardness of about 610 HV0.1, which is about 50 HV0.1 above the mean value (560 HV0.1) of the measured core hardness. The diagram also shows that the core hardness has not been reduced by the nitriding.

The springs, which are to be hardened according to the invention, are nitrided in the manufacturing process after the processing steps of winding, tempering and end-face grinding. The tempering which serves. Relieving stresses that build up when the spring wire is wound and that can cause stress cracks on the inner edge of the wire is shortened to a short-term tempering (at about 440 ° C. for 10 minutes) in the method according to the invention (starting time: 1 hour at 440 ° C.), that is initially sufficient to avoid stress cracks. The final tempering takes place during plasma nitriding. By using the Short-term tempering shortens the overall process time, which is prolonged by plasma nitriding, and thus improves the economics of the process.

The springs are preferably cleaned before plasma nitriding so that there is a grease-free surface. The cleaning can be carried out, for example, with alkaline aqueous cleaners or with alcohol. If the springs have been stored for 24 hours, the springs are preferably blasted with glass beads such as the Bailotini MGL (trade name, manufactured by Eisenwerke Würth GmbH + Co. KG) or with beads made of an equivalent material such as a ceramic material (pressure: 4 bar , 10 min).

The diffusion layer is intended to prevent the springs from wearing out. Such wear only occurs on the end faces of the springs. A diffusion layer is therefore only required on the end faces. As such, it would not be critical if the diffusion layer were present on the entire surface of the spring. However, since the springs have to be held and electrically contacted during plasma nitriding, the spring surface cannot be completely nitrided but only largely and - which is disadvantageous - the extensive nitriding would not be defined. It is therefore advantageous to limit the nitriding to the end faces of the springs, preferably to their end turns.

Devices to cover the spring surface to such an extent that nitriding is restricted to the end faces are shown in FIGS. 3 to 5.

In the device shown in FIG. 3, the springs 1 are inserted into the holes 3 of plates 4 in the direction of their longitudinal axis. The plates are made of a conductive material because they make electrical contact with the springs. The plates are about as thick as the feathers are long. The diameter of the holes 3 is approximately 0.1 mm larger than the outside diameter of the springs. In the device shown in FIG. 4, the springs are tied to form a “bundle” 5, ie a larger number of springs are arranged parallel (with respect to the longitudinal axis) to one another and with the lateral surfaces 6 lying against one another and wrapped with a steel band 7. The bundle is held together with a steel wire 8. A steel loop 9 is provided to hang the bundle in the plasma reactor and to ensure the electrical contact. The steel strip is approximately as wide as the springs are long and is designed so that the springs do not protrude or only slightly protrude beyond the edge of the steel strip.

In the device shown in FIG. 5, a larger amount of springs are stacked parallel (with respect to the longitudinal axis) to one another and with the lateral surfaces abutting one another in a frame 10, the dimension of which is approximately as long as the length of the springs parallel to the screw axes , The outer surfaces of the outer springs lie against or almost touch the frame, and the end faces of the screws protrude slightly beyond the frame.

In the three devices described, at least the outer surfaces of the springs are adequately protected against the action of the plasma under the nitriding conditions used.

Equipped with springs, at least one device according to one of the alternatives described is placed or hung in the plasma reactor.

During the plasma treatment, a sputtering step is carried out before the nitriding, in which the accessible surface is cleaned with a plasma generated in an H2 ~ or an H2-Ar atmosphere.

The nitriding takes between about 20 and about 30 hours, and preferably about 24 hours. It is performed in an atmosphere that is <about 13 volumes. % N2 and also preferably argon and H2. contains. The ^ content is preferably between about 8 and about 12 vol% and most preferably at <about 10 vol. -%. At ^ contents> about 13 vol.%, The formation of a connection layer on the spring surface begins. The H2 portion is not critical. The Ar content should not exceed 10% by volume. The nitriding is carried out at substrate temperatures between approximately 350 and approximately 420 ° C. and preferably at approximately 420 ° C. No nitriding takes place at temperatures below about 350 ° C. At temperatures> around 440 ° C or, if the treatment lasts for many hours,> around 420 ° C, the core hardness of the spring deteriorates and relaxation increases. At temperatures <approx. 420 ° C the usual core hardness of the springs of> approx. 550 HV0.1 is retained during nitriding (maximum drop due to nitriding 40 HV0.1). The exact determination of the temperature takes place within the specified range depending on the desired temperature and time-dependent starting effect. The voltage between the anode (reactor wall) and the parts to be nitrided (cathode), at which the plasma is operated, must be so high that an abnormal glow discharge is ignited. This ignition voltage, which also depends on the reactor geometry, is between 380 and 420 volts. On the other hand, the voltage must not be so high that an arc discharge forms, which melts the spring steel wire. This voltage is typically above about 600 volts. The voltage in the method according to the invention is preferably between approximately 500 and approximately 580 volts. It depends on the plasma current density in the reactor whether nitration takes place at all. The plasma current density depends on the voltage, the gas composition and the pressure. Since the applicable ranges of gas composition and voltage are determined for other reasons, the pressure to ensure a reasonable plasma flow density should not be at

<about 100 Pa. The upper limit of the pressure is the requirement that only the surfaces of the wire windings on the end faces should be exposed to the plasma. If the inside diameter of the springs is not larger than 7 mm, the pressure can

<150 Pa without the inside of further turns being exposed to nitriding. With larger inside diameters It is also necessary to insert pins, the diameter of which is only slightly smaller than the inside diameter of the springs, into the screws. If this additional measure is taken, the pressure can be increased to about 300 Pa without the plasma acting on the inside of the spring.

To check the nitriding achieved, the springs are separated axially. A hardness-depth profile in HV0.1 (because of the low sewage is only measured with a load of 100 g) is created from one of the metallographically prepared separating surfaces, as shown in FIG. 2. In addition, the core hardness is determined and it is checked whether nitriding has actually only taken place on the end faces of the springs.

The hardness-depth profile shown in FIG. 2 was obtained after nitriding under the following conditions (the measured values which gave the hardness-depth profile are mean values which were obtained when measurements were carried out on a larger number of samples) :

Pressure: 120 Pa

Temperature: 420 ° C

Treatment gas nitrogen content 10% by volume

Voltage: 540 volts (pulse: pause ratio: 1: 1.5)

Duration: 24 hours. The diagram shows the diffusion layer about 0.11 mm thick and the core hardness that has remained constant.

After nitriding, the springs are blasted with spherical grain from a special spring wire for at least 45 minutes (the process parameters: grain size <approximately 0.4 mm discharge speed, maximum approximately 60 m / s). R _z (average roughness value) after blasting may not exceed 15μm.

A comparison between the springs tempered according to the invention and the springs according to the St.dT shows that in the case of the latter, areal wear occurs during use, whereas only wear occurs with the springs produced according to the invention dotted or linear.

Claims

Expectations

1. Coil spring made of a steel containing nitride-forming alloy components, characterized in that a nitride-containing diffusion layer forms at least partially the surface of the coil spring.

2. Coil spring according to claim 1, characterized in that the alloy components include at least one metal from the group Cr, Mo, V, and AI.

3. Coil spring according to claim 1 or 2, characterized in that the steel contains Si.

4. Coil spring according to one of claims 1 to 3, characterized in that the coil spring is a nozzle holder spring for diesel injection systems.

5. Coil spring according to one of claims 1 to 4, characterized in that the diffusion layer forms exclusively the surface on the end faces of the coil spring.

6. Coil spring according to claim 5, characterized in that the diffusion layer forms the surface of the end windings of the spring.

7. Object according to one of claims 1 to 6, characterized in that the diffusion layer is between about 20 and about 150 microns thick with a hardness of> 750 HV0.1 in 10 microns depth,

8. The article of claim 7, characterized in that the diffusion layer is between about 50 and about 120 microns thick.

9. Article according to claim 8, characterized in that the diffusion layer is about 100 microns thick.

10. A method for producing a coil spring in particular according to one of claims 1 to 9, characterized in that the surface of a coil spring made of steel containing nitride-forming alloy components is at least partially converted into a diffusion layer by nitriding in a nitrogen-containing treatment gas, the nitrogen content in the treatment gas being so determined is that no connection layer forms on the spring surface.

11. The method according to claim 10, characterized in that nitriding in a plasma.

12. The method according to claim 10 or 11, characterized in that the ^ portion in the treatment gas to a value <about

13 vol .-% is determined.

13. The method according to claim 12, characterized in that the N2 content in the treatment gas is set to a value between about 8 and about 12 vol .-%.

14. The method according to claim 13, characterized in that the N2 content in the treatment gas is set to <about 10 vol.%.

15. The method according to any one of claims 10 to 14, characterized in that the treatment gas consists essentially of 2, H2 and Ar.

16. The method according to any one of claims 11 to 15, characterized in that the plasma nitriding is carried out at a temperature <about 440 ° C.

17. The method according to claim 16, characterized in that nitriding in the range between about 350 and about 430 ° C.

18. The method according to claim 17, characterized in that is nitrided at about 420 ° C.

19. The method according to claim 18, characterized in that the plasma is operated at a voltage in the range between about 500 and about 580 V.

20. The method according to claim 19, characterized in that the plasma is operated at a voltage of about 550 volts.

21. The method according to claim 19 or 20, characterized in that with a pulse duration of at least 50 μsec, the pulse / pause ratio is between approximately 1: 1 and approximately 1: 2.

22. The method according to any one of claims 11 to 21, characterized in that at least one pressure is set in the plasma reactor at which the plasma current density, which is also dependent on the voltage and the gas composition, is sufficient to ensure nitriding.

23. The method according to claim 22, characterized in that a pressure of> about 100 Pa is set.

24. The method according to any one of claims 11 to 23, characterized in that nitriding between about 20 and about 30 hours.

25. The method according to claim 24, characterized in that nitriding is carried out for about 24 hours.

26. The method according to any one of claims 11 to 25, characterized in that for the only partial nitriding of the spring surface, the surface areas which are not to be nitrided are mechanically covered by walls which are at a short distance from the spring surface.

27. The method according to claim 26, characterized in that, if the end faces of the coil springs are to be selectively nitrided, metal plates with holes are provided, into which the coil springs are inserted for nitriding with one end face ahead in the direction of their longitudinal axis, the plates being approximately as thick as the coil springs are long, and that Holes have a diameter that is only slightly larger than the outer spring diameter, and the end faces do not protrude from the plates, or only slightly.

28. The method according to claim 26, characterized in that when the end faces of the coil springs are to be selectively nitrided, the coil springs are bundled for nitriding, i.e. that a number of coil springs is ßt so that areas of the outer surface abut each other, and that the resulting bundle is wrapped around the end faces leaving a steel band, the steel band is about as wide as the coil springs are long, and the end faces are not or protrude only slightly beyond the edges of the steel strip.

29. The method according to claim 26, characterized in that if the end faces of the coil springs are to be selectively nitrided, the coil springs are stacked in a frame, the stacked coil springs with regions of the lateral surfaces abutting one another or optionally on the frame, the dimension of the frame parallel to the spring axis is approximately as large as the coil springs are long, and not ^'or protrude the ends of the coil springs only insignificantly from the frame.

30. The method according to any one of claims 26 to 29, characterized in that nitriding is carried out at a pressure in the range between about 100 and about 150 Pa.

31. The method according to claim 30, characterized in that nitriding is carried out at a pressure of about 120 Pa.

32. The method according to any one of claims 26 to 29, characterized in that nitriding is carried out at a pressure in the range between about 100 and about 300 Pa, pins with a diameter which is only slightly smaller than the inside diameter of the coil springs, in the coil springs are introduced.

33. The method according to any one of claims 11 to 32, characterized in that at a pressure of about 120 Pa, a temperature of about 420 ° C, a voltage of about 550 volts is nitrided for about 24 hours.

34. The method according to any one of claims 10 to 33, characterized in that the coil springs are degreased before nitriding.

35. The method according to any one of claims 10 to 34, characterized in that if the coil springs have been stored for> 24 hours, they are blasted with glass beads or beads of equivalent materials before nitriding.

36. The method according to any one of claims 11 to 35, characterized in that the coil springs are subjected to a sputtering step in the plasma reactor before nitriding.

37. The method according to any one of claims 10 to 36, thereby the coil springs are briefly tempered at about 440 ° C after winding.

38. The method according to claim 37, characterized in that the coil springs are left on for 10 minutes.