DE102021213888A1 - Method and device for localized nitriding or nitrocarburizing of the surface of a component - Google Patents

Abstract

The invention relates to a method and a device for locally limited nitriding or nitrocarburizing of the surface of a ferrous metal component (1; 1'), wherein in a passivation step (I) at least one partial surface (2; 2') to be kept nitration-free is first produced by in an air or oxygen atmosphere (O2), a temperature-time sequence that forms oxides or mixed oxides acts only on the partial surface (2; 2') in order to form a passivated partial surface (2; 2'), and in a subsequent nitration step (II .) or nitrocarburizing step of heating a surface (3; 3') of the component (1; 1') that encompasses the partial surface (2; 2') with the presence of a nitrogen-emitting process gas in such a way that due to the passivation of only the partial surface (2; 2 ') the surface (3; 3') is nitrated or nitrocarburized essentially, in particular only, outside of the passivated partial surface (2; 2').

Description

The present invention relates to a method and a device for locally limited nitriding or nitrocarburizing of the surface of a ferrous metal component and such a component itself, which is in particular a corrosion-resistant steel component.

The field of application of the present invention extends primarily to those components made of preferably corrosion-resistant steels which, after surface hardening by nitriding or nitrocarburizing, are to be joined by welding during further processing. If nitriding is mentioned below, the same applies to nitrocarburizing. When a nitration step is mentioned below, the same also applies to a nitrocarburizing step. In addition, the solution according to the invention can also be used for low-temperature processes such as Kolsterizing, the so-called expanded austenite process, the S-phase process and the like. The ferrous metal components of interest here, such as fuel injectors, are usually nitrated or nitrocarburized in order to improve the wear behavior and to increase the static strength and vibration stability with the expected low distortion of the components.

State of the art

Nitriding or nitrocarburizing is a well-known hardening process using nitrogen. This creates a surface layer that is resistant to around 500 °C. The process is usually carried out at temperatures of 500 to 520 °C for treatment times of 1 to 100 hours, whereby the core of the material remains ferritic and the formation of near-surface austenite through in-diffusion of nitrogen is also avoided. However, the nitrided layers produced with this method in plasma, in gas, in low pressure or in a salt bath are disadvantageous in terms of weldability, since the nitrogen absorbed by the component leads to increased pore formation and thus to a weakening of a weld seam applied thereto. Especially in critical component areas, such as threads or in the area of press fits, the increased hardness of nitrided layers can lead to layer areas breaking off and thus to increased wear. However, the resulting material particles can also lead to abrasive wear elsewhere in an assembly comprising the component or close narrow bores, such as spray holes in injection systems.

From the DE 10 2015 2013 068 A1 discloses such a method for nitriding a ferrous metal component. The component is sufficiently heated in a first nitriding phase and a nitrogen-releasing process gas is placed on its surface so that nitrogen diffuses through the surface into the component, with a compound layer containing iron nitride being formed in the first nitriding phase. In a subsequent degradation phase, this compound layer is partially dissolved again by heat treatment in order to create a compound layer that is evenly distributed over the surface.

According to the generally known state of the art, a locally limited nitriding or nitrocarburizing of a component surface has hitherto been achieved by using simple mechanical covers, for example during plasma nitriding or plasma nitrocarburizing. This prevents the formation of a so-called plasma fringe. Without the plasma border, no nitriding effect can be achieved with plasma nitriding or plasma nitrocarburizing. The disadvantage, however, is that narrow bores with a diameter of less than 8 to 10 mm or bore intersections can only be nitrided insufficiently or not at all, since the plasma seam forms poorly or not at all in these geometric areas. For this reason, covers usually made of copper materials, such as sleeves or hoods, or cover pastes are used in gas or low-pressure nitriding. The application of covers or cover pastes is quite complex and in the case of cover pastes, residues also stick to the component, which have to be cleaned off again after nitriding. In addition, internal threads or even narrow bores and bore intersections are difficult to automatically protect with a masking paste. For this reason, such special geometric areas are usually provided with a covering paste by hand, which in turn is associated with the risk of an incomplete protective effect as a result of treatment errors.

It is therefore the object of the present invention to create a method and a device for locally limited nitriding or nitrocarburizing of the surface of a ferrous metal component, which can be reproduced in a simple manner in terms of production technology and can be carried out automatically without post-cleaning or post-processing.

Disclosure of Invention

The object is achieved by claim 1 in terms of process technology. Regarding a the procedure performing facility, reference is made to claim 9. A ferrous metal component produced according to the method according to the invention is explicitly specified in claim 7 . The respective dependent claims refer back to advantageous developments of the invention.

The invention includes the procedural teaching that for locally limited nitriding or nitrocarburizing of the surface of a ferrous metal component, in a passivation step, at least one partial surface that is to be kept nitration-free is first produced in that an oxide or mixed oxide-forming temperature is maintained in an air or oxygen atmosphere for a sufficiently long period of time Time acts on the part surface. Local heating of the component surface is therefore carried out until an oxide layer has formed on it. In a subsequent nitration step, the surface is heated in the presence of a nitrogen-releasing process gas in such a way that the surface outside of the passivated partial surface is nitrated.

In other words, the solution according to the invention allows passivation to be effected there against subsequent nitration by means of a targeted formation of oxides or mixed oxides with the aid of a local temperature-time sequence carried out on the component. In the area of the passivated partial surface, the absorption of nitrogen during the nitriding is reduced at least to such an extent that the passivated partial surface remains weldable, for example. Depending on the process parameters, nitrogen absorption in the area of passivated part surfaces can even be completely prevented. In addition, a passivated partial surface of a ferrous metal component also has improved corrosion resistance compared to the base material.

A layer thickness of oxides or mixed oxides of >500 nm, preferably >100 nm, is preferably produced by the temperature-time sequence in the passivation step. This latter relatively small layer thickness is already sufficient to ensure the advantages associated with the solution according to the invention. As part of the temperature-time sequence of the passivation step, an oxidation temperature of between 500° C. and 680° C., preferably between 520° C. and 600° C., is applied to the partial surface. In particular in the preferred range, a sufficiently thick layer, as indicated above, can be achieved within a relatively short process time. If the temperature is further increased beyond the upper limit of the range, controlled oxidation to produce a desired layer thickness cannot be produced. On the other hand, oxidation does not start beyond the lower limit of the range, or only over a long period of time. The optimal period of time for the passivation step is >90 s, preferably >25 s, within the context of the temperature-time sequence. Shorter periods of time at correspondingly higher temperatures lead, as already mentioned above, to uncontrolled oxidation, whereas longer periods of time result in a longer process time that is disadvantageous in terms of production technology would cause.

According to a measure further improving the invention, it is proposed to mask the partial surface to be passivated before the passivation step in such a way that only the surface outside of the partial surface of the component to be passivated is covered. As a result, passivated sub-surfaces that are sharply delimited from the rest of the surface of the component can be created. The covering means also guarantee high-quality reproducibility. This measure serves to reduce or prevent unwanted heating of an area to be nitrided later or to improve the selectivity between the passivated and active part surface.

In the area of bores to be passivated, a pin-shaped inductor can be used in particular, which after insertion heats the inner surface of the bore. In addition, it is also conceivable to introduce the heat required for the passivation by contacting the partial surface to be passivated. However, care must be taken here to ensure that no air is trapped so as not to impede the oxidation of the component surface. A local application of heat can also be carried out by means of electrode emitters, laser emitters and/or infrared emitters with or without additional masking.

According to a measure that improves the masking of only the partial surface to be passivated, it is proposed that the covering means provided for this purpose be additionally provided with cooling channels. For example, a flat cover plate can be provided with cooling channels on the rear or integrated. In addition, it is proposed that the covering means for masking should be made of a material with high thermal conductivity that is temperature-resistant for the passivation step, for example metal masks made of steel or copper. In addition to a cover plate with or without additional cooling channels, a hollow body through which a coolant flows can also be used as the covering means.

During the passivation step, the oxygen already present on the surface in the form of oxygen compounds or the oxygen from a surrounding gas atmosphere is removed together with the alloying elements involved converted into oxides and/or mixed oxides. Depending on the layer thickness to be achieved, the temperature and the holding time can be controlled or regulated according to the predefined temperature-time sequence.

According to a further measure improving the invention, it is proposed to abruptly terminate the passivation step after completion by cooling the component. Simple airflow cooling or inert gas cooling is suitable for this. In the latter case, for example, nitrogen, argon or helium can be used. In addition, it is also conceivable to bring about cooling by thermal conduction within the component as a result of self-cooling if there are correspondingly sufficient temperature differences in the component after the passivation step. In principle, the cooling can be carried out under air or oxygen-containing atmospheres or preferably under the aforementioned inert gas conditions or also under nitration conditions, for example with ammonia, carbon dioxide, carbon monoxide, nitrous oxide, nitrogen, cracked gas and/or hydrogen and mixtures thereof or under a low pressure up to a vacuum . The ferrous metal component or several components pretreated in this way in the passivation step can then be subjected to a normal nitriding process.

A suitable ferrous metal component for carrying out the method according to the invention is a component that can be assigned to the component group of alloyed case-hardening steels, alloyed working steels or corrosion-resistant steels and contains an alloy composition that is selected from a group of elements comprising: aluminum, calcium, chromium, cobalt, copper , magnesium, manganese, molybdenum, nickel, silicon, titanium, zinc and mixtures thereof, which have more than 3% by mass of the alloy.

A device carrying out the method for locally limited nitriding or nitrocarburizing of the surface of such a ferrous metal component comprises a heat input unit that is structurally and functionally adapted to the partial surface to be passivated, for example an inductor for borehole areas or an infrared radiator for flat surface areas, which is connected to a control unit in the passivation step corresponding partial surface is heated according to a predetermined temperature-time sequence in order to create an oxide or mixed oxide layer on the partial surface to be kept nitration-free from the oxygen present in the environment. The area surrounding the component is then filled with a nitrogen-releasing process gas in order to nitride the surface outside of the passivated partial surface in a manner known per se using the same or another heat input unit, for example an infrared radiator. In the process, nitrogen penetrates very little or not at all into the previously passivated partial surface of the component.

Further measures improving the invention are presented in more detail below together with the description of preferred exemplary embodiments of the invention with reference to the figures.

exemplary embodiments

• 1: einen schematischen Längsschnitt eines mit Abdeckmitteln versehenen Bauteils zur Erzeugung einer auf einer flachen Teiloberfläche lokal zu erzeugenden Passivierungsschicht,
• 2 einen schematischen Längsschnitt durch ein eisenmetallisches Bauteil mit einer in einem Bohrungsbereich zu passivierenden Teiloberfläche;
• 3 eine graphische Darstellung zur Illustration des Verfahrens unter Verwendung einer exemplarischen Temperatur-Zeit-Folge zum Passivieren und anschließendem Nitrieren eines Bauteils, und
• 4 eine vergleichende graphische Darstellung des Stickstoffgehalts einer nitrierten Oberfläche mit und ohne lokaler Passivierung.

It shows:

• 1 : a schematic longitudinal section of a component provided with covering means for producing a passivation layer to be produced locally on a flat partial surface,
• 2 a schematic longitudinal section through a ferrous metal component with a partial surface to be passivated in a bore area;
• 3 a graphic representation to illustrate the method using an exemplary temperature-time sequence for passivation and subsequent nitriding of a component, and
• 4 a comparative graphic representation of the nitrogen content of a nitrided surface with and without local passivation.

According to 1 If, in the case of a ferrous metal component 1, a partial surface 2 of a surface 3 which is flat in this exemplary embodiment is to remain free of a nitriding layer produced in a nitriding process which is known per se. For this purpose, the partial surface 2 is first heated with a local heat input unit 4 in the form of an infrared radiator as part of a passivation step I. in an air or oxygen atmosphere O ₂ in accordance with an electronic control unit 5, which is carried out on the basis of a predetermined temperature-time - explained in more detail below - Follow the heating in such a way that a layer of oxides or mixed oxides is formed on the partial surface 2.

For the additional masking of the partial surface 2 in the passivation step I., covering means are provided in this exemplary embodiment, which are designed here as a covering plate 6 with integrated cooling channels 7 .

After completion of the passivation step I. and removal of the masking, the entire flat surface 3 of the component 1 is removed under nitrogen by the heat input unit 4 as part of the subsequent nitration step II Bdem process gas is heated in such a way that a nitrided layer is formed over the surface 3 with the exception of the passivated partial surface 2 in a manner known per se.

At the in 2 Illustrated alternative exemplary embodiment is to be kept free of a nitriding layer in a ferrous metal component 1', the upper region of a bore 1a' as a partial surface 2', whereas the outer flat surface 3' is to be provided with a nitriding layer. For this purpose, as part of the passivation step I., a heat input unit 4' designed as an inductor is introduced into the area of the bore 1a' and the desired local oxide or mixed oxide layer is formed in the area of the cylindrical partial surface 2' under an air or oxygen atmosphere O ₂ . The entire area of the bore 1a' can also be correspondingly passivated by a lifting movement of this heat introduction unit 4'.

The subsequent nitration step II is as above 1 described carried out so that on the outer surface 3 'with the exception of the passivated partial surface 2' in the bore 1a 'forms a nitriding layer under the nitrogen-emitting process gas.

The 3 shows a graphical representation of an exemplary temperature-time sequence, which is specified by the above-mentioned control unit as part of the initial passivation step I and the subsequent nitration step II to the heat input unit 4 or 4'. The temperature-time sequence is related to the above exemplary embodiment as an example.

Accordingly, as part of passivation step I., heating up to 600° C. is initially specified, which is maintained here for a period of around 25 s in order to form an oxide or mixed oxide layer of around 100 nm on the corresponding partial surface for the purpose of passivation. The component temperature is then increased in stages and, as part of the subsequent nitration step II., held at around 500° C. for such a period of time until a desired thick nitrided layer has formed on the component surface outside the passivated partial surface. Accordingly, this nitration step II can last for several hours.

The graphical representation according to 4 describes the course of the layer thickness of a nitrided layer in the area of a locally passivated partial surface (dashed line) compared to an untreated surface (solid line) of a corrosion-resistant steel with already 0.3% by mass of nitrogen in the base material. The ordinate of the coordinate system indicates the nitrogen content in % by mass after the nitration step has been carried out. In this example, the nitrogen content in the area of the untreated surface is initially 6 times higher than on the locally passivated partial surface, whose nitrogen content remains at approx. 0.3% by mass due to the material. Due to the constantly low nitrogen content that can be achieved via the passivation step according to the invention on the partial surface, welding of the otherwise nitrated component can be carried out here, for example.

The invention is not limited to the preferred embodiments given above. Rather, modifications of this are also conceivable, which are included in the scope of protection of the following claims. For example, it is also possible to use masking covers of other shapes in order to delimit differently designed partial areas on material surfaces for passivation of other surface areas. This usually does not apply to partial surfaces that are delimited from other surfaces by the component geometry, such as inner bore areas and the like. The selection of a suitable heat input unit also depends on the shape and size of the surface areas to be treated. All of the contents described above and in the claims for nitriding also apply to nitrocarburizing.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 1020152013068 A1 [0004]

Claims (11)

Method for locally limited nitriding or nitrocarburizing of the surface of a ferrous metal component (1; 1'), in which, in an initial passivation step (I.), at least one partial surface (2) to be kept low in nitration is first produced in that under an air or oxygen atmosphere ( O ₂ ) a temperature-time sequence that forms oxides or mixed oxides acts only on the partial surface (2; 2') in order to form a passivated partial surface (2; 2'), and in a subsequent nitration step (II.) or nitrocarburization step to heat the surface (3; 3') of the component (1; 1') that encompasses the partial surface (2; 2') with the presence of a nitrogen-releasing process gas in such a way that due to the passivation of only the partial surface (2; 2'), the surface ( 3; 3') is nitrated or nitrocarburized essentially, in particular only, outside of the passivated partial surface (2; 2'). procedure after claim 1 , characterized in that the temperature-time sequence in the passivation step (I.) produces a layer thickness of oxides or mixed oxides of less than 500 nm, preferably less than 100 nm. procedure after claim 1 , characterized in that the surface (3; 3') is covered for masking only outside of the partial surface (2; 2') to be passivated prior to the passivation step (I.). procedure after claim 1 , characterized in that an oxidation temperature between 500 °C and 680 °C, preferably between 520 °C and 600 °C, is applied to the partial surface (2; 2') as part of the temperature-time sequence of the passivation step (I.). becomes. procedure after claim 1 , characterized in that as part of the temperature-time sequence of the passivation step (I.), the oxidation temperature is applied to the partial surface (2; 2') for a period of less than 90 seconds, preferably less than 25 seconds. procedure after claim 1 , characterized in that the passivation step (I.) is terminated after the predetermined period of time by cooling the component (1; 1'), comprising air flow cooling or inert gas cooling. Method according to one of the preceding claims, characterized in that an alloyed case-hardened steel, alloyed work steel or corrosion-resistant steel is used as the ferrous metal component (1; 1') which contains an alloy composition which is selected from a group of elements comprising: aluminium, calcium, Chromium, cobalt, copper, magnesium, manganese, molybdenum, nickel, silicon, titanium, zinc and mixtures thereof, which have more than 3% by mass of the alloy. Ferrous metal component (1; 1'), in particular corrosion-resistant steel component, the surface (3; 3') of which is partially nitrated or nitrocarburized using a method according to one of the preceding claims. Device for locally limited nitriding or nitrocarburizing of the surface (3; 3') of a ferrous metal component (1; 1') with a method according to one of Claims 1 until 7 , comprising a heat input unit (4; 4') for local heat input only in the partial surface (2; 2') of the component (1; 1') under an air or oxygen atmosphere (O ₂ ) according to an electronic control unit (5) in the frame the passivation step (I.) to form the passivated partial surface (2; 2') in the form of a local oxide or mixed oxide layer, the heat input unit (4) after a passivation step (I.) covering the entire surface (3; 3') of the component (1; 1') heated under process gas giving off nitrogen as part of the nitration step (II.) or nitrocarburizing step. setup after claim 9 , characterized in that the heat input unit (4; 4') is designed as an infrared radiator, inductor or electrode/laser radiator. setup after claim 9 , characterized in that covering means serving to mask the surface (3; 3') only outside the partial surface (2; 2') to be passivated are designed as at least one covering plate (6) provided with cooling channels (7).

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