EP3167094B1 - Method of nitriding a component of a fuel injection system - Google Patents

Description

The invention relates to a method for nitriding a high pressure loaded, made of an alloy steel component of a fuel injection system.

State of the art

From the publication DE 102 56 590 A1 It is known that an injector of a fuel injection system becomes very resistant when the injector has a nitrided state. Above all, the corrosion resistance and the wear resistance increase. However, the nitriding method itself is not dealt with in this document.

Furthermore, from the published patent application WO 2001/042528 A1 a method for nitriding an injection nozzle is known. The known nitriding method comprises, in a first step, a nitrocarburizing process in a salt bath and then in a second step a gas nitriding process at a temperature between 520 ° C and 580 ° C at low nitriding potential or low nitriding coefficient (in the range between 0.08 and 0, 5), ie in the so-called α-range of the teacher diagram.

DE 10 2009 035288 A discloses thermochemical diffusion nitration (ammonia, 24h, 480 ° C) of a nozzle body and / or a longitudinally movable nozzle needle, each of alloyed steel as parts of a fuel injection system.

The loads on the components of a fuel injection system under very high pressure fuel - especially in the field of throttle bodies - lead to a very high cavitation load of these components.

Even with the components treated with the nitriding process described above, this can lead to greater cavitation damage.

Disclosure of the invention

On the other hand, the nitriding method of the present invention minimizes the cavitation damage caused by the high pressure load by further increasing the ductility (toughness) under the material surface of the components by the nitriding method. In addition, the nitriding process has a positive effect on the swelling resistance. As a result, the life or fatigue strength of the components is increased.

• Aktivieren des Bauteils in anorganischer Säure,
• Voroxidieren des Bauteils in sauerstoffhaltiger Atmosphäre zwischen 380°C und 420°C,
• Nitrieren des Bauteils zwischen 520°C und 570°C bei einer hohen ersten Nitrierkennzahl K_N,1 im ε-Nitrid-Bereich,
• Nitrieren des Bauteils zwischen 520°C und 570°C bei einer niedrigen zweiten Nitrierkennzahl K_N,2 im γ'-Nitrid-Bereich, wobei die Nitrierkennzahl definiert ist als K_N = p(NH₃)/p(H₂)^3/2.

For this purpose, the method for nitriding a high pressure-loaded, made of an alloy steel component of a fuel injection system comprises the following method steps:

• Activating the component in inorganic acid,
• Preoxidizing the component in an oxygen-containing atmosphere between 380 ° C and 420 ° C,
• Nitriding of the component between 520 ° C. and 570 ° C. at a high first nitriding index K _{N, 1} in the ε-nitride range,
• Nitriding of the component between 520 ° C. and 570 ° C. at a low second nitriding index K _{N, 2} in the γ'-nitride range, the nitriding index being defined as K _N = p (NH ₃ ) / p (H ₂ ) ^{3 / 2} .

Activating reduces the resistance of the component to the diffusion of nitrogen. This step thus increases the nitridability of the component. The subsequent pre-oxidation leads to the component having a higher corrosion resistance during operation.

• Ein erster Nitrierschritt mit einer ersten Nitrierkennzahl K_N,1 im ε-Nitrid-Bereich dient der Stickstoffaufnahme des Bauteils und damit der Erhöhung der Härte des Bauteils, sowohl in der sogenannten Verbindungsschicht an der Oberfläche des Bauteils als auch in der darunter liegenden Diffusionsschicht.
• Ein zweiter Nitrierschritt mit einer zweiten Nitrierkennzahl K_N,2 im γ'-Nitrid-Bereich führt dazu, dass die Verbindungsschicht nicht zu dick wird. Die Verbindungsschicht besitzt zwar eine hohe Härte, ist aber gleichzeitig sehr spröde und damit auch sehr anfällig gegenüber Kavitationsbelastungen.

The actual nitriding is divided into two steps using ammonia-containing gas:

• A first nitriding step with a first nitriding characteristic K _{N, 1} in the ε-nitride region serves for nitrogen uptake of the component and thus for increasing the hardness of the component, both in the so-called bonding layer on the surface of the component and in the underlying diffusion layer.
• A second nitriding step with a second nitriding index K _{N, 2} in the γ'-nitride region results in the bonding layer not becoming too thick. Although the bonding layer has a high hardness, but at the same time is very brittle and thus also very susceptible to cavitation.

In addition to the reduction in the thickness of the brittle connecting layer, the nitriding process according to the invention reduces, above all, the nitride inclusions along the grain boundaries in the diffusion layer compared with the known nitriding processes. As a result, the grain boundaries are less susceptible to breakage, which increases the toughness and thus the robustness against cavitation attack, as well as the swelling resistance of the component.

Advantageously, the first nitriding characteristic K _{N, 1 is} between 1 and 10, preferably between 2 and 8. The first nitriding characteristic K _{N, 1} is thus comparatively high. This results in the teacher diagram at temperatures between 520 ° C and 570 ° C substantially in the ε-nitride region, which ensures a high nitrogen uptake of the activated and flowed around by the nitriding gas component.

Further advantageously, the second Nitrierkennzahl K _{N, 2 is} between 0.2 and 0.4. The second Nitrierkennzahl K _{N, 2} is therefore relatively low. This obstructs a deep in-diffusion of a high nitrogen content into the component. It mainly increases the nitrogen content in the connecting layer; in the base material, the nitrogen content increases to not more than about 6%. The toughness of the component is thus largely retained.

According to the invention, a component which has been nitrided by the process according to the invention has on its surface a mass fraction of the nitrogen of between 11% and 25%. This provides a very hard, cavitation, wear and corrosion resistant surface of the component.

In a further advantageous embodiment, a component which has been nitrided by the method according to the invention, at a first depth t ₁ of 10 microns to the surface of the component, a mass fraction of nitrogen between 3% and 8% up. The comparatively large drop in the mass fraction of nitrogen already in 10 μm component depth leads to a comparatively high toughness of the component despite the high surface hardness. The transition from the connecting layer to the diffusion layer is also approximately at this component depth.

In a further advantageous embodiment, a component which has been nitrided by the method according to the invention, in a second depth t ₂ of 15 microns to the surface of the component to a mass fraction of nitrogen between 2% and 7%. This leads to a further increase in the toughness of the component compared to known nitriding.

In a further advantageous embodiment, a component which has been nitrided by the method according to the invention, in a third depth t ₃ of 20 microns to the surface of the component to a mass fraction of nitrogen between 2% and 6%. This leads to a further increase in the toughness of the component compared to known nitriding.

From this component depth, the nitrogen content runs asymptotically to the end of the diffusion zone, and then drops relatively abruptly towards the end of the diffusion zone to the nitrogen content already contained in the base material. Typically, the diffusion zone extends to about 500 microns into the component interior. The nitrogen content is lowered so far from the third depth t ₃ that only a few nitride intercalations form. The necessary toughness of the material is thus given from this component depth.

In one embodiment of the invention, the component is a nozzle body of a fuel injector for injecting fuel into a combustion chamber of an internal combustion engine, wherein the fuel injector has a nozzle needle which is longitudinally movably guided in the nozzle body. Due to the high pressure and the high flow rate of the fuel in the fuel injector and there especially in the nozzle body just just the nozzle body is suitable for a nitriding process according to the invention. For example, at the injection openings of the nozzle body, which open into the combustion chamber of the internal combustion engine, there may be a very high cavitation load. Due to the increased swelling resistance of the Nozzle body can be minimized by the nitriding process according to the invention caused by cavitation or even avoided altogether.

drawings

• Fig.1 shows a teacher diagram in which the nitriding index K _N is plotted against the nitriding temperature T, wherein an area for a step of the method according to the invention with a second nitriding index K _{N, 2 is} characterized.
• Fig.2 shows a diagram in which the mass fraction of nitrogen of a nitrided with the inventive method component depending on the component depth.
• Figure 3 schematically shows a part of a fuel injector, wherein only the essential areas are shown.

description

Fig.1 shows a teacher diagram: The different state phases of the system iron-nitrogen of a component as a function of the temperature T and the nitriding coefficient K _{N are} shown. The nitriding index K _N is plotted logarithmically above the nitriding temperature T. The nitration time is not indicated in the teacher diagram, but usually ranges from 1 hour to 100 hours.

The nitriding index K _N is defined as $$K_N=\frac{p\left({\mathit{NH}}_3\right)}{\mathrm{<figure-callout\; id="2"\; label="p\; H"\; filenames="imgf0003.png"\; state="\{\{state\}\}">p\; </figure-callout>}{\left(H_{}\right)}^{}{\left({}_2\right)}^{3/2}}$$

Here, p (NH ₃ ) is the partial pressure of the ammonia and p (H ₂ ) is the partial pressure of the hydrogen. The partial pressure is in each case the pressure in an ideal gas mixture which is assigned to a single gas component. That is, the partial pressure corresponds to the pressure that would be exerted by the single gas component in the presence of the respective volume. The Partial pressure is usually used instead of the mass concentration when considering the diffusion behavior of the dissolved gas.

The state phases of the iron-nitrogen system are divided into an ε-nitride region, a γ-nitride region, a γ'-nitride region, and an α-nitride region. ε-nitrides have very high proportions of nitrogen and are generally found on the surface of the nitrided component, the so-called connection layer or the underlying diffusion layer. The y'-nitride region also has a high nitrogen content, but with more order of nitrogen atoms than in the ε-nitride region. The y'-nitride region is also found in the bonding and diffusion layer. Both the ε-nitride region and the y'-nitride region are comparatively hard and brittle. At very high temperatures, but outside the nitriding process according to the invention, γ-nitrides also occur, which have very high nitrogen concentrations. The α-nitride region has a comparatively low nitrogen concentration and is comparatively tough. α-nitride regions are usually found in the diffusion layer and in the base material.

Fig.1 shows a hatched region 12, which is located substantially in the y'-nitride region, with a temperature T in the range between about 520 ° C and 570 ° C and with a nitriding index K _N in the range between about 0.2 and 0 ; 4. In the nitriding process according to the invention, this hatched region identifies the process step with the low second nitriding characteristic K _{N, 2} .

Fig.2 shows a diagram in which the mass fraction of the nitrogen "mass% of N" of a nitrided with the inventive method component over the component depth "t [μm]" is plotted. In this case, the component depth t is perpendicular to the surface and the mass fraction of the nitrogen is specified for a range which is at least 1 mm from the next edge or the next contour transition. The curve "MAX" represents the maximum and the curve "MIN" the minimum mass fraction of the nitrogen of the treated component.

In Fig.2 It can be seen that the nitrogen-containing compound layer of a component treated with the method according to the invention is only about 5 μm to 10 μm thick and then the diffusion layer begins. The diffusion layer can extend to over 500 microns in the component depth, but for reasons of representation in the Fig.2 not shown.

Figure 3 schematically shows a part of a fuel injector 1, wherein only the essential areas are shown. The fuel injector 1 has a nozzle body 4, in which a pressure chamber 2 is formed. The pressure chamber 2 is filled with high-pressure fuel and is fed for example by a common rail, not shown, or a high-pressure pump, not shown, of a fuel injection system. In the pressure chamber 2, a nozzle needle 3 is arranged longitudinally movable. The nozzle needle 3 opens and closes by their longitudinal movement in the nozzle body 4 formed injection openings 5 for injecting fuel into a combustion chamber of an internal combustion engine, not shown. The nozzle body 4 is exposed to cavitation risks, especially in the area of the injection openings 5. In order to increase the cavitation resistance of the nozzle body 4, the nitriding method according to the invention is used.

1. 1) Aktivieren des Bauteils in anorganischer Säure.
2. 2) Voroxidieren des Bauteils in sauerstoffhaltiger Atmosphäre zwischen 380°C und 420°C.
3. 3) Nitrieren des Bauteils zwischen 520°C und 570°C bei einer hohen ersten Nitrierkennzahl K_N,1 im ε-Nitrid-Bereich, vorzugsweise mit 1 ≤ K_N,1 ≤ 10.
4. 4) Nitrieren des Bauteils zwischen 520°C und 570°C bei einer niedrigen zweiten Nitrierkennzahl K_N,2 im γ'-Nitrid-Bereich, vorzugsweise mit 0,2 ≤ K_N,2 ≤ 0,4,

wobei die Nitrierkennzahl definiert ist als K_N = p(NH₃)/_P(H₂)^3/2.The method according to the invention for nitriding a high pressure-loaded, alloyed steel component of a fuel injection system, for example the nozzle body 4, consists of the following method steps:

1. 1) Activation of the component in inorganic acid.
2. 2) pre-oxidation of the component in an oxygen-containing atmosphere between 380 ° C and 420 ° C.
3. 3) nitriding of the component between 520 ° C and 570 ° C at a high first nitriding index K _{N, 1} in ε-nitride range, preferably with 1 ≤ K _{N, 1} ≤ 10.
4. 4) nitriding the part between 520 ° C and 570 ° C at a low second nitriding potential K _{N, 2} in the γ'-nitride region, preferably with 0.2 ≤ K _{N, 2} ≤ 0.4,

wherein the nitriding index is defined as K _N = p (NH ₃ ) / _P (H ₂ ) ^3/2 .

As a result, a mass fraction of the nitrogen as a function of the component depth t, as shown in FIG . 2, arises for the component.

Claims (5)

1. Method for nitriding a component of a fuel injection system, said component being subject to high pressure and being composed of an alloyed steel,
   characterized by the following method steps:

   - activating the component in inorganic acid,

   - pre-oxidizing the component in an oxygen-containing atmosphere between 380°C and 420°C,

   - nitriding the component between 520°C and 570°C at a high first nitriding potential K_N,1 in the ε nitride range,

   - nitriding the component between 520°C and 570°C at a low second nitriding potential K_N,2 in the γ' nitride range,

   wherein the nitriding potential is defined as $$K_N=\frac{p\left({\mathit{NH}}_3\right)}{p{\left(H_2\right)}^{3/2}}$$

   
2. Method according to Claim 1, characterized in that the first nitriding potential K_N,1 is between 1 and 10.
3. Method according to Claim 1 or 2, characterized in that the second nitriding potential K_N,2 is between 0.2 and 0.4.
4. Component nitrided by a method in Claims 1-3, characterized in that the percentage of nitrogen by mass at the surface of the component is between 11% and 25%.
5. Fuel injector (1) for injecting fuel into a combustion chamber of an internal combustion engine, having a nozzle needle (3) which is guided for longitudinal movement in a nozzle body (4), characterized in that the nozzle body (4) is a component according to Claim 4.

Images