WO2009030291A1

WO2009030291A1 - Method of producing a sinter-hardenable powder metal part

Info

Publication number: WO2009030291A1
Application number: PCT/EP2008/004272
Authority: WO
Inventors: Gerold Stetina; Jürgen VOGLHUBER
Original assignee: Miba Sinter Austria Gmbh
Priority date: 2007-09-03
Filing date: 2008-05-29
Publication date: 2009-03-12
Also published as: AT505698A1; AT505698B1; EP2200770A1

Abstract

The invention describes a method of producing a sinter-hardenable powder metal part from a sinter powder containing chromium, comprising the steps of preparing the sinter powder, compacting the sinter powder to form a green compact, pre-sintering the green compact to form a brown compact under a reducing atmosphere followed by slow cooling, calibrating the brown compact and high-temperature sintering of the calibrated brown compact followed by rapid cooling. The pre-sintering process is operated at a temperature selected from a range with a lower limit of 700 °C and an upper limit of 1050 °C.

Description

Method of producing a sinter-hardenable powder metal part

The invention relates to a method of producing a sinter-hardenable powder metal part from a sinter powder containing chromium, comprising the steps of preparing the sinter powder, compacting the sinter powder to form a green compact, pre-sintering the green compact to form a brown compact under a reducing atmosphere followed by slow cooling, calibrating the brown compact and high-temperature sintering of the calibrated brown compact followed by rapid cooling, and further relates to a sinter-hardened powder metal part with an iron base powder containing chromium.

A method of producing sinter-hardened powder metal parts for the automotive industry is known from patent specification DE 101 96 487 Tl, whereby a metallurgical powder of iron containing between 1.0 % by weight and 3.0 % by weight of copper and containing between 0.3 % by weight and 0.8 % by weight of carbon is compressed to form a compressed body at a pressure in the range of between 20 tsi and 70 tsi, the compressed body is then heated to a temperature of 2000° F to 2400° F and maintained at this temperature for at least 15 minutes in order to produce a sintered compressed body. This compressed body is then cooled at a cooling rate of no more than 60° F/min, as a result of which a compressed body with a hardness of no more than RC25 is obtained. The density in at least the surface region of this com- pressed body is then increased to at least 7.6 g/cm³, after which this sintered compressed body is sintered in a final process at a temperature of from 2050° F to 2400° F for at least 20 minutes. After the final sintering process, it is cooled rapidly at a cooling rate of from 120° F/min to 400° F/min, as a result of which the hardness is increased to more than RC25. In addition to the elements mentioned above, the metallurgical powder may also contain nickel, molybdenum, chromium, manganese and vanadium, in which case the molybdenum content may be in the range of between 0 % by weight and 2.0 % by weight, that of the nickel is in the range of between 0 % by weight and 3.0 % by weight, that of the manganese is between 0 % by weight and 0.7 % by weight and that of the chromium is between 0 % by weight and 4.0 % by weight.

Although chromium is mentioned as a possible alloying component in this DE-Tl, it has been found that iron-based powders containing chromium, in particular highly alloyed chromium steels, can not be processed with the method defined in the DE-TI or can be so only with great difficulty, and more especially the high densities of the finished sintered component can not be achieved. On this subject, the DE-Tl also states that a less desirable sinter powder amongst others is one of the type QMP4701, which contains 0.9 % by weight of nickel, 1 % by weight of molybdenum, 0.45 % by weight of manganese and 0.5 % by weight of chromium, with iron making up the rest.

In order to reduce oxide coatings, it has been common practice in the prior art to conduct the pre-sintering process at a temperature in excess of 1000 °C, with a view to obtaining at this stage already a sufficiently good sinter bond due to diffusion in readiness for subsequent processing steps.

The objective of this invention is to propose a sinter hardening method for processing iron- based powders containing chromium as well as corresponding powder metal parts.

This objective is achieved by the invention respectively by the method outlined above where- by the pre-sintering process is conducted at a temperature selected from a range with a lower limit of 700°C and an upper limit of 1050°C and by a sinter-hardened powder metal part based on an iron powder containing chromium, the body of the part having a density of at least 7.2 g/cm³.

As a result of the temperature control during the process of producing the powder metal part, organic binding agents and lubricants which are still adhered to the green compact from the initial processes are burnt away, in the same way as in the prior art, thereby enabling a higher density of the finished powder metal parts to be obtained than if there were no pre-sintering. What this also does, however, is reduces to a minimum diffusion processes within the alloy powder, which are usually desired when pre-sintering without a proportion of liquid phase, even through a minimum degree of pre-sintering takes place, but the occurrence of brittle phases such as FeCr (σ-phase), for example, is avoided, which means that these do not have a negative effect on the subsequent compaction during the calibration step, thereby enabling higher densities to be produced, even in the case of sintered components containing chro- mium. Furthermore, because of the reducing atmosphere, the formation of oxides is also avoided, in particular the occurrence of chromium oxides, which also form hard phases and thus have a negative effect on the subsequent compaction of the brown compact. The method proposed by the invention therefore enables powder metal parts containing chromium to be produced from an iron-based powder, which have a density almost corresponding to the full density but at least 7.2 g/cm³. This method also results in a very homogeneous structure of the powder metal parts.

The temperature during pre-sintering may also be selected from a range with a lower limit of 750 °C and an upper limit of 1000°C, in particular from a range with a lower limit of 775 °C and an upper limit of 950 ⁰C.

In particular, the body of the part has a density with a lower limit of 7.2 g/cm³ and an upper limit of 7.5 g/cm³.

In one embodiment of the method, the reducing atmosphere during pre-sintering and optionally during high-temperature sintering may be a nitrogen-hydrogen mixture, which contains a proportion of hydrogen selected from a range with a lower limit of 0 % by volume, in particular 5 % by volume, and an upper limit of 30 % by volume. With such unusually high proportions of hydrogen in the reducing atmosphere, it is possible to conduct the pre-sintering and optionally the high-temperature sintering at higher temperatures without the risk of a massive oxide formation, as a result of which the overall processing time can be shortened. Over and above 30 % by volume of hydrogen, no further improvement in the method has been observed.

The proportion of hydrogen in the atmosphere may also be selected from a range with a lower limit of 7 % by volume and an upper limit of 25 % by volume, in particular from a range with a lower limit of 10 % by volume and an upper limit of 15 % by volume.

In another embodiment, a carburizing gas is added to the reducing atmosphere during pre- sintering and optionally high-temperature sintering or a carburizing gas is used as a reducing atmosphere. At the same time as pre-sintering and optionally high-temperature sintering, this enables the carbon content in at least the surface regions of the finished powder metal part to be increased, which is conducive to the formation of martensite during the rapid cooling proc- ess which follows the high-temperature sintering. Another advantage of this is that pre-sintering can be conducted at a lower temperature than described in the prior art because higher carbon contents can be achieved without the risk of the hardness of the brown compact being too high for the subsequent calibration. - A -

After pre-sintering, the brown compact may also be cooled at a rate selected from a range with a lower limit of 0.1 K/s and an upper limit of 2 K/s in order to avoid premature hardening of the brown compact as far as possible or reduce the degree of hardening.

In this respect, the cooling rate may be selected from a temperature range with a lower limit of 0.7 K/s, in particular 0.5 K/s. By increasing the lower limit, premature hardening can be even more effectively avoided.

With regard to the desired mechanical properties of the powder metal part, the brown compact is preferably calibrated to a density of at least 7. 2 g/cm³ prior to high-temperature sintering. In particular, the calibration is effected to a density selected from a range with a lower limit of 7.2 g/cm³ and an upper limit of 7.5 g/cm³, in other words a density which corresponds to almost the full density of the material.

As already mentioned, the sinter powder used is preferably an iron-based powder.

This iron-based powder may contain chromium in a proportion selected from a range with a lower limit of 1 % by weight and an upper limit of 5 % by weight. This proportion may be in pre-alloyed form or may be added by some other means used in sintering technology (such as in the form of ferro-alloys for example). In other words, highly alloyed chromium steels may be produced using the method proposed by the invention, which - in a known manner — have an excellent resistance to corrosion and very high hardness levels. In particular, the hardness may be between 250 HV5 and 600 HV5.

The proportion of chromium may also be selected from a range with a lower limit of 1.2 % by weight and an upper limit of 4 % by weight, in particular from a range with a lower limit of 1.5 % by weight and an upper limit of 3 % by weight.

It is also possible for up to 10 % by weight of at least one non-iron alloying element and/or graphite to be added to the iron-based powder in a range with a lower limit of 0.1 % by weight, in particular 1 % by weight, and an upper limit of 5 % by weight. The unusually high proportion of graphite assists the formation of an at least almost completely martensitic struc- ture. It is not usual because such high proportions of graphite do not normally allow subsequent pressing due to tearing.

By adding other non-iron alloying elements such as copper, nickel, manganese, silicon, mo- lybdenum and vanadium, the properties of such iron-based powder metal components can be improved accordingly, as is already the case in the prior art with steels. For example, by adding molybdenum to the alloy, the initial brittleness of such chromium steels can be prevented. This improves hardenability and toughness. The creep resistance at higher temperatures can also be increased. Nickel enables the cold forming capacity to be improved. Manganese im- proves tensile strength and yield strength. During tempering, silicon prevents the precipitation of cementite from the martensite.

Since the primary effect of these alloying elements is known per se from the prior art, there is no need for further explanation at this point.

The proportion of non-iron alloying elements may also be selected from a range with a lower limit of 0.2 % by weight and an upper limit of 8 % by weight, in particular from a range with a lower limit of 1 % by weight and an upper limit of 6 % by weight.

In this respect, copper may be used in a proportion selected from a range with a lower limit of 0 % by weight and an upper limit of 6 % by weight, in particular from a range with a lower limit of 0.1 % by weight and an upper limit of 4 % by weight, preferably from a range with a lower limit of 0.2 % by weight and an upper limit of 2 % by weight.

The proportion of nickel may be selected from a range with a lower limit of 0 % by weight and an upper limit of 8 % by weight, in particular from a range with a lower limit of 0.1 % by weight and an upper limit of 4 % by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 2 % by weight.

The proportion of manganese may be selected from a range with a lower limit of 0 % by weight and an upper limit of 10 % by weight, in particular from a range with a lower limit of 0.1 % by weight and an upper limit of 5 % by weight, preferably from a range with a lower limit of 0.2 % by weight and an upper limit of 2 % by weight. The proportion of molybdenum may be selected from a range with a lower limit of 0 % by weight and an upper limit of 3 % by weight, in particular from a range with a lower limit of 0.1 % by weight and an upper limit of 1.5 % by weight, preferably from a range with a lower limit of 0.2 % by weight und an upper limit of 0.85 % by weight.

The proportion of silicon may be selected from a range with a lower limit of 0 % by weight and an upper limit of 5 % by weight, in particular from a range with a lower limit of 0.1 % by weight and an upper limit of 2 % by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 0.5 % by weight.

The proportion of vanadium may be selected from a range with a lower limit of 0 % by weight and an upper limit of 8 % by weight, in particular from a range with a lower limit of 0.1 % by weight and an upper limit of 2 % by weight, preferably from a range with a lower limit of 0.2 % by weight and an upper limit of 0.5 % by weight.

The proportion of graphite may also be selected from a range with a lower limit of 0.2 % by weight and an upper limit of 1.9 % by weight, in particular from a range with a lower limit of 0.4 % by weight and an upper limit of 0.8 % by weight.

In order to achieve higher densities in the green compact already, it is of advantage if a compacting agent is added to the iron-based powder in a quantity of up to 3 % by weight and/or an organic binding agent in particular in a quantity of up to 1 % by weight. This also makes it possible to create a porosity due to these agents burning out during pre-sintering, which makes subsequent compaction during the calibration easier, which in turn brings the advan- tage of being able to use a lower temperature during pre-sintering and avoids or reduces diffusion processes. In particular, it also simplifies the operation of compressing sinter powders which are difficult to compress, such as sinter powders containing chromium. Above a total of 4 % by weight of agents, the porosity may be too high, which can lead to lower final densities of the finished sintered component under certain circumstances.

The proportion of compacting agent may also be up to 2.5 % by weight, in particular 2 % by weight, and the proportion of binding agent may be up to 0.75 % by weight, in particular 0.5% by weight. The rapid cooling after high-temperature sintering may be run at a cooling rate selected from a range with a lower limit of 2 K/sec and an upper limit of 16 K/sec. This can be conducive to creating the martensitic hardness pattern, especially if the high-temperature sintering process is conducted in a carburizing medium, and internal stress curves can be achieved which are favourable in terms of mechanical properties, in particular fatigue properties.

The cooling rate may also be selected from a range with a lower limit of 2.5 K/sec and an upper limit of 15 K/sec, in particular from a range with a lower limit of 3 K/sec and an upper limit of 10 K/sec.

Another option is to temper the high-temperature sintered brown compact after sintering and after cooling in order to impart a high degree of toughness to the finished powder metal part in addition to high strength values. This being the case, the tempering temperature is prefera- bly not in excess of 250° C so as to avoid reducing the high martensite hardness. In particular, the tempering temperature is a max. of 200 ° C, preferably max. 180° C. The upward rate during tempering is optimally in the range of between 2 K/min and 30 K/min.

In one embodiment of the powder metal part, the proportion of carbon decreases from an ex- ternal surface in the direction towards a core of the body of the formed part following a gradient, as a result of which this powder metal part will have a high surface hardness whilst simultaneously exhibiting a good toughness at the core. This being the case, the proportion of carbon may decrease from 0.5 % by weight at the surface to 0.35 % by weight in a coating thickness of 0.1 mm, for example, following a linear or an exponential curve.

The formed body of the powder metal part may be a sliding sleeve or a coupling element.

In order to provide a clearer understanding of the invention, it will be explained in more detail below with reference to examples of embodiments.

Firstly, it should be pointed out that individual features or combinations of features from the different embodiments described may be construed as independent inventive solutions or solutions proposed by the invention in their own right. AIl the figures relating to ranges of values in the description should be construed as meaning that they include any and all part-ranges, in which case, for example, the range of 1 to 10 should be understood as including all part-ranges starting from the lower limit of 1 to the up- per limit of 10, i.e. all part-ranges starting with a lower limit of 1 or more and ending with an upper limit of 10 or less, e.g. 1 to 1.7, or 3.2 to 8.1 or 5.5 to 10.

General description of the process:

As explained above, the invention relates to a method of producing sintered components for automotive applications with densities of >7.2 g/cm³ from powders which are not so easy or difficult to compress. The components are subjected to powder pressing processes in order to obtain densities of >7.1 g/cm³, released and pre-sintered to a density of >7.35, calibrated in the pre-sintered state and then hardened by sintering.

As a rule, the powders which are medium to difficult to compact comprise hybrid, pre-alloyed or alloyed iron powders which serve as base powders and cause an alloy to form due to diffusion during sintering as a result of adding other alloy powders (in commercially available states or degrees of purity) such as Cu, Ni, Mn, Cr, Si, Mo, V, etc., as well as graphite and pressing agents. The alloy systems are formulated so that a cooling process run after sintering at cooling rates of approximately 2 to 16 K/s will cause the component to harden. The advantage of hybrid alloys is that these hardening alloy elements are already distributed accordingly in the micro-structure.

1^*) Mixing the powders

Medium to difficult to press iron powder mixtures are prepared containing chromium in a proportion of up to 4 % by weight and with a total of up to 10 % by weight of metallic non-iron alloy elements, up to 5 % by weight of graphite, up to 3 % by weight of pressing agents and up to 1 % by weight of organic binding agents. These mixtures are either pre-mixed in a so- called master mixture in a highly concentrated form, optionally also using temperature and solvents, and are then admixed with iron powder in the relevant quantities or are added directly to the iron powder by adding the individual elements. The binding agents used may be resins, silanes, oils, polymers or adhesives. Pressing agents include stearates, silanes, amides and polymers, amongst others.

Pre-alloying elements may be Cr, Mo, V, Si, Mn.

Hybrid alloy powders may be Fe powder pre-alloyed with Cr-Mo containing diffusion- alloyed Ni and/or Cu, Fe powder pre-alloyed with Mo containing Ferro-Cr and Ferro-Mn, even if these are diffusion-alloyed.

2) Pressing

The iron powder mixtures above, pre-treated by the method described above, are compacted by coaxial pressing methods and shaped. In this respect, care must be taken to ensure that allowance is already made for the changes in shape and structure that will occur during the subsequent process steps when manufacturing the pressing tools. It is helpful to use appropriate lubricants and binding agents when compacting to densities of >7.1g/cm³ with powders that are medium to difficult to press. Depending on the bulk density and theoretical density of the powder mixtures, pressing pressures of 600 to 1200 MPa are used for this purpose.

The mouldings (also referred to as green compacts) obtained in this manner are the starting point for the subsequent process steps.

Instead of coaxial pressing methods, other pressing methods of the type employed as standard in sintering technology may also be used, including isostatic pressing methods, etc., for ex- ample.

3) De-waxing + pre-sintering

By contrast with the standard process route for formed parts produced by powder metallurgy, the mouldings are pre-sintered by a thermal treatment under the effect of gases creating a reducing atmosphere. To this end, reducing atmospheres can be created using nitrogen-hydrogen mixtures with a proportion of up to 30 % by volume of hydrogen. It is preferable to use mixtures with a proportion of hydrogen of between 5 % by volume and 30% by volume, al- though it would also be possible within the scope of the invention to use mixtures with less than 5 % by weight. Optionally, it would also be possible to use carburizing gases (endogas, methane, propane and such like) or add them to the nitrogen-hydrogen mixture. This being the case, the proportion may be selected from a range with a lower limit of 0.01 % by volume and an upper limit of 2.55 % by volume by reference to the total mixture.

The temperatures during pre-sintering are between 740 and 1050°C and the sintering time may be between 10 minutes and 2 hours.

In any case, the objective of the pre-sintering process is to burn out the organic binding agents and lubricants and produce a light sintered bond between the particles. The incomplete dissolution of individual alloy elements is deliberately obtained, thereby resulting in a lower hardness level. The hardness of the sintered component is preferably set so that high degrees of formability with an extra factor of up to 30 % are possible during the subsequent compaction process (calibration). In particular, with a hardness of less than 140 HB 2.5/62.5, a surprisingly high formability was observed.

Oxygen-refined alloy elements in particular, such as Cr for example, are difficult to handle during pre-sintering. By selecting the process parameters from within the specified limits, a massive oxide formation during pre-sintering can be avoided, at least for the most part, so that this does not have a negative effect on formability.

In the pre-sintered state, pre-alloyed base powders containing Cr-Mo can also be calibrated because, due to the inadequate diffusion of the alloy elements and low cooling rates during pre-sintering, the structure is not able to harden or is so to only an inadequate degree. Cooling preferably takes place at a rate of a maximum of 2 K/min in the critical region of the material until the M_f (Martensite Finish Temperature) is reached.

During pre-sintering, the powder grains are sintered to only a limited degree, which causes a weak sinter bonding.

By opting for a pre-sintering temperature below 1 100°C, the graphite is only incompletely diffused into the iron matrix material. The pre-sintered green compacts produced in this manner are referred to as "brown compacts".

4) Subsequent pressing/ calibration

The brown compacts are then compacted by means of coaxial pressing methods. Depending on the demands placed on the component, height changes of up to approximately 30% of the total component height can be re-shaped. At pressing pressures of up to 1400 MPa - even higher die pressures are possible locally - a density increase to >7.35 g/cm³ is achieved (lo- cally, higher densities are possible).

The lubricants which may optionally be used may be applied to the component either by conventional immersion methods or preferably by means of a spraying process before or during pressing.

With this method, it is possible to obtain an increase in density to >7.35g/cm³, even with powders whose theoretical density is <7.4 g/cm³, because during pre-sintering all the non- metal components of the mixture burn or diffuse into the iron matrix, thereby increasing the theoretical density which can be achieved.

5) Sintering / sinter hardening

The compacted brown compacts are sintered with a reducing atmosphere (nitrogen, hydrogen, as explained above) with the optional introduction of carburizing gases (endogas, methane, propane and such like), preferably in continuously operated sintering ovens. The temperatures during this high-temperature sintering may be between 1 100 °C and 1350°C. The powder metal parts may be maintained at this temperature for between 10 minutes and 65 minutes.

They are then cooled using appropriate cooling aggregates at the outlet of the oven at a cool- ing rate of between 2 K/s and 16 K/s from the sintering heat to below the M_f and thus hardened. The abrupt cooling and optionally the carburizing media during sintering result in the martensitic hardness structure and internal stresses, which are conducive to the mechanical properties and in particular to the fatigue properties. Afiter the sintering process, in addition to hardening out of the sintering heat, the hardened parts may also be tempered.

During this process sequence, it is possible that the green compact, the brown compact or the finished sintered component may be subjected to mechanical processing of the type known from the prior art.

This generally preferred process sequence culminates in high-strength, highly compacted pre- cision powder metal parts, such as sliding sleeves for automotive applications or a coupling body for example, based on the examples below. However, a number of other applications would also be feasible.

Example 1 : sliding sleeve for an automotive transmission:

Composition of the sinter powder: Astaloy CrM (Cr-Mo-pre-alloyed Fe powder) + 0.4%C + 0.5% high-performance pressing agent

Pressing pressure to produce the green compact: 950 MPa (7.25 g/cm³ density)

Temperature during pre-sintering: 920 °C

Pre-sintering time: 35 minutes

Pressing pressure during calibration: 900 MPa (7.4 g/cm³)

Temperature of high-temperature sintering: 1280 °C

High-temperature sintering time: 40 minutes

Composition of the reducing atmosphere: N2/H2/C3H8 (90/9.95/0.05)

Proportion of carburizing gas in the high-temperature sintering atmosphere: 0.05 %. The finished sintered sleeve had a density of 7.4 g/cm³ and compared with conventional sliding sleeves known from the prior art exhibited better resistance to wear during permanent service operation but also in the abuse test.

Example 2: sliding sleeve for an automotive transmission:

Pressing pressure to produce the green compact: 650 MPa (7.0 g/cm³)

Temperature during pre-sintering: 915 °C

Pre-sintering time: 35 minutes

Pressing pressure during calibration: 920 MPa (7.35 g/cm³)

Temperature of high- temperature sintering: 1250 °C

High-temperature sintering time: 40 minutes

Composition of the reducing atmosphere: N2/H2/C3H8 (90/9.9/0.1)

Proportion of carburizing gas in the high-temperature sintering atmosphere: 0.1 %

The finished sintered sleeve had a density of 7.35 g/cm³ and compared with conventional sliding sleeves known from the prior art exhibited a better resistance to wear during permanent service operation for the same abuse behaviour.

Example 3: coupling body for an automotive transmission:

Composition of the sinter powder: Astaloy CrM (Cr-Mo-pre-alloyed Fe powder) + 0.5%C + 0.5% high-performance pressing agent Pressing pressure to produce the green compact: 700 MPa (7.1 g/cm³ density)

Temperature during pre-sintering: 900 °C

Pre-sintering time: 40 minutes

Pressing pressure during calibration: 900 MPa (7.38 g/cm³ overall, locally in the region of the top toothing >7.45)

Temperature of high-temperature sintering: 1270 ⁰C

High-temperature sintering time: 40 minutes

Composition of the reducing atmosphere: N2/H2/C3H8 (95/4.9/0.1)

The finished coupling body had a density of 7.38 g/cm³ (with local densities in the region of the toothing >7.45g/cm³) and compared with conventional coupling bodies known from the prior art exhibited a better resistance to wear.

The embodiments given as examples represent possible design variants of the sintered component and it should be pointed out at this stage that the invention is not specifically limited to the design variants specifically illustrated, and instead the individual design variants may be used in different combinations with one another and these possible variations lie within the reach of the person skilled in this technical field given the disclosed technical teaching. Accordingly, all conceivable design variants which can be obtained by combining individual details of the design variants described and illustrated are possible and fall within the scope of the invention.

The objective underlying the independent inventive solutions may be found in the description.

Claims

C l a i m s

1. Method of producing a sinter-hardenable powder metal part from a sinter powder containing chromium, comprising the steps of preparing the sinter powder, compacting the sinter powder to form a green compact, pre-sintering the green compact to form a brown compact under a reducing atmosphere followed by slow cooling, calibrating the brown compact and high-temperature sintering of the calibrated brown compact followed by rapid cooling, characterised in that the pre-sintering process is operated at a temperature selected from a range with a lower limit of 700 ⁰C and an upper limit of 1050 °C.

2. Method as claimed in claim 1, characterised in that the reducing atmosphere is a nitrogen-hydrogen mixture, with a proportion of hydrogen selected from a range with a lower limit of 0 % by volume, in particular 5 % by volume, and an upper limit of 30 % by volume.

3. Method as claimed in claim 1 or 2, characterised in that the reducing atmosphere during pre-sintering has a carburizing gas added to it or a carburizing gas is used as the reducing atmosphere.

4. Method as claimed in one of the preceding claims, characterised in that, after pre- sintering, the brown compact is cooled at a rate selected from a range with a lower limit of

0.1 K/s and an upper limit of 2 K/s.

5. Method as claimed in one of the preceding claims, characterised in that the brown compact is calibrated to a density of at least 7.2 g/cm³.

6. Method as claimed in one of the preceding claims, characterised in that an iron base powder is used as the sinter powder.

7. Method as claimed in claim 6, characterised in that chromium is added to the iron base powder in a proportion selected from a range with a lower limit of 1 % by weight and an upper limit of 5 % by weight.

8. Method as claimed in claim 6 or 7, characterised in that at least one other non-iron alloy element is added to the iron base powder in a quantity of up to 10 % by weight and/or graphite in a range with a lower limit of 0.1 % by weight and an upper limit of 5 % by weight.

9. Method as claimed in one of claims 6 to 8, characterised in that pressing agent is added to the iron base powder in a quantity of up to 3 % by weight and/or an organic binding agent in particular in a quantity of up to 1 % by weight.

10. Method as claimed in one of the preceding claims, characterised in that, after sintering, cooling of the high-temperature sintered brown compact takes place at a cooling rate selected from a range with a lower limit of 2 K/s and an upper limit of 16 K/s.

11. Method as claimed in one of the preceding claims, characterised in that the high- temperature sintered brown compact is annealed after sintering or after cooling.

12. Powder metal part based on an iron powder containing chromium, with a formed body, characterised in that the formed body has a density of at least 7.2 g/cm³.

13. Powder metal part as claimed in claim 12, characterised in that it contains chromium in a proportion selected from a range with a lower limit of 1 % by weight and an upper limit of 5 % by weight.

14. Powder metal part as claimed in claim 12 or 13, characterised in that at least one other non-iron alloy element is added to the iron base powder in a quantity of up to 10 % by weight.

15. Powder metal part as claimed in claim 14, characterised in that at least one other non- iron alloy element is selected from a group comprising copper, nickel, manganese, silicon, molybdenum, vanadium.

16. Powder metal part as claimed in one of claims 12 to 15, characterised in that the iron powder contains carbon in a proportion selected from a range with a lower limit of 0.1 % by weight and an upper limit of 1 % by weight.

17. Powder metal part as claimed in one of claims 12 to 16, characterised in that the pro- portion of carbon decreases from an external surface in the direction towards a core of the formed body following a gradient.

18. Powder metal part as claimed in one of claims 12 to 17, characterised in that the formed body is a sliding sleeve or coupling body.