DE102018222387A1 - Process for carbonitriding a metal part and metal part - Google Patents

Abstract

The invention relates to a method for carbonitriding a metal part, in which the metal part is heated and embroidered with a nitrogen donor gas in at least one nitrification phase (D, F) and is preferably carburized with a carbon donor gas in at least one carburization phase (H), the at least one Carburizing phase (H) is preferably carried out after the completion of at least one nitriding phase (D, F).

Description

Technical field

The invention relates to a method for carbonitriding a metal part, as is used in steels, in order in particular to achieve good material properties, for example with regard to rolling fatigue and particle robustness. Carbonitriding is understood to mean an additional introduction of carbon and nitrogen in the edge region of the metal part. The invention further relates to a metal part which is produced by means of the method according to the invention.

State of the art

A method for carbonitriding a metal part with the features of the preamble of claim 1 is known from the DE 10 2009 002 985 A1 known to the applicant. The method known from the cited document is characterized in that after a heating phase of the metal part to a treatment temperature of approximately 950 ° C., a first nitriding phase and then a first carburizing phase are carried out. The metal part is arranged in a nitrogen donor gas atmosphere or coal donor gas atmosphere, the action of the gases on the metal part in relation to the ambient pressure being able to take place under reduced pressure. The first nitriding phase and the first carburizing phase can be followed by further nitriding phases and carburizing phases in which the metal part is in each case exposed to the treatment temperature mentioned.

Disclosure of the invention

The method according to the invention for carbonitriding a metal part with the features of claim 1 has the advantage that nitrogen can be introduced particularly efficiently into the edge region of the metal part. It has been found that in the case of steels which are already high in carbon (meaning carbon contents of more than 0.4% by mass), such as Quenched and tempered steel, it is difficult to introduce additional nitrogen during the austenitizing of the metal part (which starts at treatment temperatures of more than 780 ° C) in the range of several tenths of a millimeter, since both carbon and nitrogen occupy the same places within the iron grid, and the carbon has already largely occupied it. The method according to the invention thus makes it possible, particularly in the case of metal parts with a relatively high carbon content, to introduce additional nitrogen in a particularly simple and effective manner in order to influence or improve the material properties in the edge regions of the metal part.

The invention is based on the idea that the initial state of the metal part, which is advantageous for soft machining, is subjected to a nitriding process (i.e. a nitriding process) before the actual hardening process. This takes advantage of the fact that the carbon in this initial state is predominantly in solid compound as carbide. These nitrides are not dissolved during nitriding by lower treatment temperatures (in particular treatment temperatures of less than 600 ° C.), which means that the diffusion of nitrogen into the iron lattice is only minimally hindered by the carbon. The edge region of the metal part that is more enriched with nitrogen then, together with the carbon that dissolves during austenitizing, gives the desired carbonitrided state of the metal part.

Against this background, the teaching of the invention suggests in the most general form that, in order to carry out the at least one nitriding phase, the metal part is heated to a first temperature which is lower than a second temperature during at least one carburizing phase and / or during a hardening phase. A method for carbonitriding a metal part is particularly advantageous, in which the metal part is heated and embroidered with a nitrogen donor gas in at least one nitrification phase and carburized with a carbon donor gas in at least one carburization phase, the at least one carburization phase being carried out after the completion of at least one nitrification phase , whereby to carry out the at least one embroidery phase ( D ) the metal part is heated to a first temperature which is lower than a second temperature during the at least one carburizing phase.

Advantageous developments of the method according to the invention for carbonitriding a metal part are listed in the subclaims.

In a first advantageous development of the general inventive concept described so far, it is provided that before the first nitrogen phase is carried out, the metal part is heated to a temperature which is lower than the first temperature (during the first nitrogen phase), and preferably after the ( lower) temperature, the metal part is pre-oxidized in an oxygen-containing atmosphere. This (lower) temperature is, for example, approximately 380 ° C.

In a further advantageous embodiment of the invention, it can be provided that before the at least one carburizing phase is carried out, a nitriding phase is carried out in which the metal part already has the second temperature (for the first carburizing phase). In other words, this means that at least two nitriding phases have already been carried out before the first carburizing phase takes place. This method is particularly advantageous for reducing or avoiding nitrogen effusion. For this purpose, a nitrogen-releasing process gas, such as ammonia, is offered.

In a development of the last method, it is provided that a diffusion phase is carried out between the nitriding phase (at the second temperature) and the at least one carburizing phase, in which the metal part is exposed to the inert gas or the atmosphere around the metal part is evacuated. The inert inert gas can e.g. is molecular nitrogen.

A further advantageous embodiment of the method according to the invention provides that after the last carburization phase has ended and before a quenching phase of the metal part, in which the temperature of the metal part is preferably reduced from the second temperature to the ambient temperature, a diffusion phase is carried out, in which the metal part exposed to inert gas or the atmosphere around the metal part is evacuated. As a result, undesired carbides that are formed during the last carburizing phase are dissolved again, which improves the durability of the metal part, for example in the event of changing stress.

The nitriding and carburizing phases can basically be carried out in different plants. In this case, it is not necessary to keep the metal part at an elevated temperature or at the first temperature during the transfer between the two systems. Rather, for handling reasons, it may be desirable or advantageous to cool the metal part down after the first embroidery phase for transfer to a second system. It is therefore provided in this case that the at least one nitriding phase and the at least one carburizing phase are carried out in different plants and that the metal part is cooled down, for example to an ambient temperature, before the at least one carburizing phase is carried out.

Alternatively, it is of course also possible to carry out the at least one nitriding phase and the at least one carburizing phase in a single installation for the most effective production and to avoid additional handling processes, the metal part for carrying out the at least one carburizing phase from the first temperature the at least one nitriding phase is heated to the second temperature without reducing the temperature in order to carry out the at least one carburizing phase.

As already explained at the beginning, it is essential that the first nitriding phase is carried out at a temperature which is below the austenitizing temperature of the metal part. Against this background, it has proven to be advantageous if a steel part is used as the metal part, and if the first temperature is between 550 ° C. and 600 ° C., preferably 580 ° C. and the second temperature is more than 800 ° C., preferably 850 ° C. is.

With regard to the pre-oxidation in a process step preceding the first nitriding phase, (third) temperatures which are less than 400 ° C., preferably 380 ° C., have proven to be advantageous.

With regard to the atmosphere in which the at least one carburization phase takes place at the second temperature, process gases which have acetylene as carbon donor gas and ammonia for the at least one nitrogenation phase have proven to be advantageous.

The invention also relates to a metal part which is produced by a method according to the invention described so far. It is also important for a high quality of the metal part that the surface of the metal part is designed to be non-porous after the at least one nitriding phase has ended.

The method according to the invention is particularly advantageously used in steels whose carbon content is more than 0.4% by weight.

Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments and from the drawing.

Figure list

• The single figure shows a diagram of the temperature response over the treatment time for the schematic illustration of different embodiments of the method according to the invention.

Embodiments of the invention

In the single figure, the temperature profile T is shown schematically over time t during the treatment of a metal part (not shown) in the form of a steel part.

It can be seen that the metal part during a heating step A1 for example from the ambient temperature first to a first treatment temperature B1 is heated from 380 ° C. Once the metal part reaches the first treatment temperature B1 has been achieved by means of an oxygen-containing atmosphere C. pre-oxidized.

Then there is a second heating step A2 at which the metal part from the first treatment temperature B1 to a second treatment temperature B2 is heated from 580 ° C. It is essential that the second treatment temperature B2 is below the austenitizing temperature of the metal part, which is typically more than 780 ° C for steels. After reaching the second treatment temperature B2 the metal part is exposed to a nitrogen-releasing atmosphere during a first nitriding phase D (for example, through an ammonia-nitrogen atmosphere slightly above the ambient pressure or with plasma nitriding or also low-pressure nitriding below the ambient pressure).

This is followed in the event that the first embroidery phase D and a first carburizing phase H is carried out in different plants, a cooling phase E at which the metal part of the second treatment temperature B2 for example, is cooled to the ambient temperature.

After changing the system, the metal part is again heated, for example, from the ambient temperature in a third heating step A3 to a third treatment temperature B3 heated from 850 ° C. After reaching the third treatment temperature B3 there is optionally a further nitriding phase F, in which the metal part is exposed to a nitrogen-releasing process gas, for example ammonia, in order to reduce or avoid nitrogen emission. Depending on the application of the metal part, this allows even higher nitrogen concentrations to be achieved in the outer edge area.

After a subsequent diffusion phase G1 , in which the treatment chamber of the system is flushed with an inert gas or evacuated with a vacuum pump, there is a first carburizing phase H with a carbon-releasing process gas, e.g. acetylene. Depending on the use of the metal part, even higher carbon concentrations than the basic carbon content can be set in the outer edge region. If no further carburization is desired, the carburizing phase is carried out H not done. Before a quenching phase I, the treatment chamber of the system is again during another diffusion phase G2 flushed with an inert gas, for example containing molecular nitrogen, or evacuated with a vacuum pump.

As an alternative to the method described so far, it can be provided that the austenitizing of the metal part, ie the treatment of the metal part during the second treatment temperature B3 using a process gas P is carried out, which has both an acetylene supply and an ammonia supply. This process gas P is also able to keep the carbon and nitrogen concentration constant in the edge area of the metal part or even to increase it further in the outer edge area. Carburization takes place during the entire treatment temperature B3 instead, if necessary also with a separate embroidery phase F .

The method described so far can be modified or modified in a variety of ways without deviating from the inventive concept.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included 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 102009002985 A1 [0002]

Claims (12)

Method for carbonitriding a metal part, in which the metal part is heated and embroidered with a nitrogen donor gas in at least one nitrogenation phase (D, F), characterized in that, in order to carry out the at least one nitrogenization phase (D), the metal part is heated to a first temperature (B2) is heated, which is lower than a second temperature (B3) during at least one carburizing phase (H) and / or during a hardening phase. Procedure according to Claim 1 , characterized in that before the first nitrogen phase (D) is carried out, the metal part is heated to a temperature (B1) which is lower than the first temperature (B2), and that, preferably after reaching the temperature (B1), the metal part in a oxygen-containing atmosphere (C) is pre-oxidized. Procedure according to Claim 1 or 2nd , characterized in that before the at least one carburizing phase (H) is carried out, a nitriding phase (F) is carried out, in which the metal part already has the second temperature (B3). Procedure according to Claim 3 , characterized in that a diffusion phase (G1) is carried out between the nitriding phase (F) and the at least one carburizing phase (H), in which the metal part is exposed to an inert gas or the atmosphere around the metal part is evacuated. Procedure according to one of the Claims 1 to 4th , characterized in that after the end of the last carburizing phase (H) and before a quenching phase (I), in which the temperature of the metal part is preferably reduced from the second temperature (B3) to an ambient temperature, a diffusion phase (G2) is carried out at which exposes the metal part to an inert gas or evacuates the atmosphere around the metal part. Procedure according to one of the Claims 1 to 5 , characterized in that the carrying out of the at least one nitriding phase (D) and the at least one carburizing phase (H) is carried out in different plants, and that the metal part is cooled down before carrying out the at least one carburizing phase (H). Procedure according to one of the Claims 1 to 5 , characterized in that the at least one nitriding phase (D, F) and the at least one carburizing phase (H) are carried out in a single installation, and in that the metal part for carrying out the at least one carburizing phase (H) from the first temperature (B2 ) is heated to the second temperature (B3) without reducing the temperature. Procedure according to one of the Claims 1 to 7 , characterized in that a steel part is used as the metal part, and that the first temperature (B2) between 550 ° C and 600 ° C, preferably 580 ° C and the second temperature (B3) more than 800 ° C, preferably 850 ° C is. Procedure according to one of the Claims 2 to 8th , characterized in that the third temperature (B1) is less than 400 ° C, preferably 380 ° C. Procedure according to one of the Claims 1 to 9 , characterized in that the at least one carburizing phase (H) is carried out at the second temperature (B3) by means of a process gas (P) which has acetylene and ammonia. Metal part produced by a method according to one of the Claims 1 to 10 . Metal part after Claim 11 , characterized in that the surface of the metal part is designed to be non-porous after completion of the at least one nitriding phase (D, F).

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