DE20023102U1 - Nitrided alloy steel workpiece - Google Patents

Description

PCT/ATOO/ 00329 STIEYR DAIMLER PUCH FAHRZEUGTECHNIK AG & CO KG

29 July 2002 DEGD-79032.9

" REGISTRATION DOCUMENTS"

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PCT/ATOO/ 00329 STEiYR DAIMLER PUCH FAHRZEUGTECHNIK AG & CO KG

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NITRIDED WORKPIECE MADE OF ALLOYED STEEL

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The invention relates to a nitrided workpiece made of alloyed steel. This particularly, but not exclusively, applies to finished workpieces of complex shape with hard-to-reach areas that are exposed to very high alternating loads at high temperatures. Such workpieces are, for example, parts of injection nozzles for internal combustion engines. In the following, only nitriding is referred to, although nitrocarburizing is also included, unless expressly excluded.

Two types of nitriding processes are known: those in a gas atmosphere and those in a salt bath. In gas nitriding, the workpiece is exposed to an atmosphere of a gas mixture containing ammonia at temperatures between about 500 and 600 ⁰ C for a longer period of time (up to 100 hours); in salt bath nitriding, the workpiece is exposed to a liquid bath consisting of various salts at similar temperatures.

These processes are described in the brochure "Gas nitriding and gas nitrocarburizing" by AGA Gas GmbH. Gas nitriding has mainly

and generally the disadvantage of the long duration. This is longer the lower the temperature. However, low temperatures are desirable in order to remain safely in the α zone of the LEHRER diagram (see below). In this zone the material remains tough, fatigue-resistant and heat-resistant even when it is very hard. In alloyed steels, nitriding is generally hindered by the passivation of the surface of the workpiece, which is more pronounced the more highly alloyed the steel is, especially with chromium. Passivation is the increased resistance that the surface offers to the penetration of nitrogen and/or carbon atoms into the workpiece.

The salt bath process has the disadvantage that a relatively thick and very brittle bonding layer can form on the surface of the workpiece. This is described as the γ' zone of the LEHRER diagram. Although this is particularly hard and abrasion-resistant, it is disadvantageous for workpieces that are exposed to less mechanical wear, but should be tough and heat-resistant with high strength.

Since the aforementioned passivation can be particularly intense in certain areas, harmful hindrances to the nitrogen transfer, known as nitriding stops, occur there. Nitriding stops are surface zones with reduced hardening and thus lower strength, which impair the process reliability of the hardening.

In the case of workpieces with a complex shape, such as the nozzle body of an injection nozzle with its fine spray holes and a blind hole at the lower end, there is also the problem that the nitrogen uptake depends on the local nitriding potential and thus on the local concentration of the nitriding gas. Since this is close to the surface of the workpiece,

If the water content of the piece is fundamentally depleted, the stray gas exchange must be supported by additional circulation. Otherwise, insufficient hardening can easily occur in dead water areas, which also affects process reliability.

The aim of the invention is therefore to provide a nitrided workpiece made of alloyed steel with high heat and fatigue strength that can be produced quickly and reliably.

&iacgr;&ogr; This object is achieved according to the invention in that the workpiece, after suitable pre-cleaning, is first subjected to a nitrocarburizing process in a salt bath with a cyanate content of between 30% and 40% until the compound layer which forms reaches a thickness in the range of 0.5 to 3 μm; and the workpiece is then subjected to a gas nitriding process at a temperature between 520 and 580° Celsius, the composition of the nitriding gas being adjusted to a low nitriding potential.

During nitrocarburizing, the very brittle compound layer forms first, which is not what you want, and underneath it, as a side effect, the diffusion layer begins to form. The latter ultimately has the advantageous properties desired here. However, the compound layer should remain as thin as possible. This is why this treatment is only carried out briefly. The diffusion layer that is created at the same time is inevitably too thin. This is why the nitriding is completed in the next step. During gas nitriding, this compound layer is re-formed by the low nitriding potential, with nitrogen atoms being released into the nitriding gas.

What remains, however, is an enlarged geometric surface with optimally formed crystal surfaces, thereby overcoming the passivity of the surface, the entire surface is safely activated throughout, and nitriding stops can no longer occur anywhere.

Above all, the surface can now easily and quickly absorb nitrogen atoms to form the diffusion layer. This means that the nitriding time is significantly shortened and that a low nitriding potential is sufficient, which excludes the formation of nitrides (compound layer, nitride cells) and thus the diffusion layer remains in the α-zone throughout, i.e. it is tough, heat-resistant and fatigue-resistant. But these are exactly the properties required and they are achieved in a much shorter time.

The low nitriding potential in the second step has several effects:

• The bonding layer disintegrates again, leaving behind a porous surface that accelerates the transfer of nitrogen into the workpiece,

• Local concentration fluctuations of the nitriding medium (reduced gas supply) have less of an effect, so that hardening is also good in inaccessible areas of the workpiece,

• The diffusion layer remains safely in the α-zone. This corresponds to a very tough and hard structure,

• The risk of an accumulation of nitrogen atoms on the

Grain boundaries (nitride lines), which would also lead to embrittlement, are lower.

This means that the three main objectives have been achieved: maximum heat resistance, high process reliability and short nitriding time. It is surprising that, thanks to the invention, a detour leads to the goal better and faster.

Gas nitriding is preferably carried out with a nitriding potential in the range between 0.08 and 0.5. This ensures that the nitriding temperature remains between 500 and 600° Celsius in the α-zone.

In a particularly advantageous process, the workpiece is treated with an organic acid, especially citric acid, after salt bath nitriding and before gas nitriding. Organic acids form metal salts that decompose particularly easily in the subsequent gas nitriding and leave behind a particularly porous and also particularly reactive surface, which helps to further shorten the nitriding time.

In the following, the invention is described and explained using figures and exemplary embodiments. They show:

Fig. 1: The LEHRER diagram,

Fig. 2: An example of a workpiece to be treated according to the method according to the invention,

Fig.3: The measured hardness curve for the two embodiments of the method according to the invention.

Fig. 1 shows the LEHRER diagram, which represents the different phases of the iron-nitrogen system depending on the nitriding temperature (ordinate) and the nitriding potential (abscissa). The nitriding potential

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potential, also called nitrogen activity of the atmosphere, is proportional to the ammonia concentration and inversely proportional to an expression of the hydrogen concentration.

The α zone corresponds essentially to that of the iron-carbon diagram, the nitrogen atoms are deposited in the α lattice and increase its strength and toughness without embrittlement, as long as no nitride lines form at the grain boundaries. The zone marked γ' corresponds to the brittle compound layer during nitriding and the ε zone corresponds to that during carbonitriding and nitrocarburizing. A trapezoid A is drawn in the diagram, which indicates the temperature range and the range of the nitriding potential for the second step of the process according to the invention. The nitriding temperature is between 500 and 600° Celsius, the nitriding potential between 0.04 and 0.4.

Fig. 2 shows the hatched body 1 of an injection nozzle with the nozzle needle 2 that can be moved within it. The nozzle body 1 ends at the bottom in a nozzle tip 3, which has a conical needle seat surface 4 on the inside, a central blind hole 5 and spray holes 6 distributed around the circumference. The nozzle needle 2 is guided in guides 7, 8 with the highest precision. Between the two guides 7, 8 and between the guide 7 and the nozzle tip 3 there is an annular space 9, 10, to the upper of which fuel is supplied through a channel 11 at a pressure of 2000 bar and more from an injection pump (not shown). The nozzle needle 2 is loaded downwards by a very strong spring (not shown) so that it rests on the needle seat surface 4.

The stresses to which this nozzle body is subjected are extremely high and varied. The entire nozzle body is radially expanded by the internal pressure acting in the chambers 9, 10 and is thus subjected to pulsating tensile stress. The guides 7, 8 must be extremely precise and hard in order to guide the nozzle needle 2 cleanly. The needle seat surface 4 must withstand the internal pressure and the impacts of the flowing valve needle 2. All surfaces where the flow is diverted or a cross-section change takes place are also at risk of cavitation, this is particularly the area around the spray holes 6 and the blind hole 5. It can be seen that even if gas flows through with the nozzle needle removed, only a very slow movement of the nitriding gas is possible inside the nozzle tip 3 and especially in the spray holes 6, corresponding to gas nitriding. This is where the risk of inadequate nitriding exists.

The hardening of this workpiece was carried out using two different process variants:

Example I:

The nozzle body according to Fig. 2 was first thoroughly cleaned in the usual way and then nitrocarburized in a salt bath for 20 minutes at a temperature of 580 ⁰ C. A suitable salt bath contains 32 to 38% of a potassium and/or sodium cyanate (CNO' - ion). In this case, the TENIFER® process was used, and the composition of the salt bath was also the same. TENIFER® is a process and trademark protected by DEGUSSA. As soon as the thickness of the bonding layer had reached one μm, the

The workpiece is removed from the salt bath, cooled in cold salted water and thoroughly cleaned in the usual way.

Next, the workpiece was gas nitrided, the composition of the ammonia-containing nitriding gas being selected to give a nitriding potential of 0.1. This nitriding potential corresponds to a residual ammonia content of the nitriding gas of 8%. In this atmosphere, the workpiece was left in the nitriding furnace at a temperature of 550 ⁰ C for 82 hours, then removed, slowly cooled and finally subjected to a test which gave the value shown in Fig. 3 (curve I).

In Fig.3, the hardness curve for the needle seat is shown on the ordinate as Vickers hardness (HV 0.5). The distance from the edge of the needle seat is shown on the abscissa in millimeters.

Example II:

The procedure was the same as in Example I, but between the first and second steps the workpiece was treated in 8% citric acid (citric acid dihydrate). Formic acid, acetic acid or oxalic acid would also be suitable. This operation only took a few minutes and was carried out with moderate heating and good flow. Due to this bath, the time spent in the nitriding furnace during gas nitriding could be reduced to 41 hours, i.e. by about half. The finished workpiece was subjected to another test. The measured Vickers hardness is again shown in Fig. 3, the curve is marked II. It can be seen that, despite the half-time period, even higher values were achieved in some cases.

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Furthermore, the workpiece was also tested on all external and internal surfaces (bores, injection holes 6, blind hole 5 in Fig. 2). All surface layers showed well-matched nitriding hardness depths and hardness values.

Claims (4)

1. Nitrided workpiece made of alloy steel, in particular nozzle body of an injection nozzle, manufactured by the following steps: a) The workpiece is subjected to a nitrocarburising process in a salt bath with a cyanate content between 30% and 80% until the resulting compound layer reaches a thickness in the range of 0.5 to 3 µ-meter; b) the workpiece is then subjected to a gas nitriding process at a temperature between 500 and 600°Celsius, whereby the composition of the nitriding gas is adjusted to a low nitriding potential so that the compound layer decomposes while increasing the geometric surface. 2. Workpiece according to claim 1, characterized in that the nitriding potential in the gas nitriding process according to step b) is in the range between 0.04 and 0.4. 3. Workpiece according to claim 1, characterized in that between steps a) and b) the workpiece is treated with an organic acid. 4. Workpiece according to claim 3, characterized in that the organic acid is citric acid.