SINTERED METAL PARTS AND METHOD FOR THE MANUFACTURING THEREOF

Description

FIELD OF THE INVENTION

[0001] The invention relates to powder metal parts. Specifically the invention concerns sintered metal parts which have a densified surface and which are suitable for demanding applications. The invention also includes a method of preparing these metal parts.

BACKGROUND OF THE INVENTION

[0002] There are several advantages by using powder metallurgical methods for producing structural parts compared with conventional matching processes of full dense steel. Thus the energy consumption is much lower and the material utilisation is much higher. Another important factor in favour of the powder metallurgical route is that components with net shape or near net shape can be produced directly after the sintering process without costly shaping such as turning, milling, boring or grinding. However, normally a full dense steel material has superior mechanical properties compared with PM components. Therefore, the strive has been to increase the density of PM components in order to reach values as close as possible to the density value of a full dense steel.

[0003] One area of future growth in the utilization of powder metal parts having high density is in the automotive industry. Of special interest within this field is the use of powder metal parts in more demanding applications, such as power transmission applications, for example, gear wheels. Problems with gear wheels formed by the powder metal process are that powder metal gear wheels have reduced bending fatigue strength in the tooth root region of the gear wheel, and low contact fatigue strength on the tooth flank compared with gears machined from bar stock or forgings. These problems may be reduced or even eliminated by plastic deformation of the surface of the tooth root and flank region through a process commonly known as surface densification. Products which can be used for these demanding applications are described in e.g. the US patents 5 711 187, 5 540 883, 5 552 109, 5 729 822 and 6 171 546.

[0004] The US 5 711 187 (1990) is particularly concerned with the degree of surface hardness, which is necessary in order to produce gear wheels which are sufficiently wear resistant for use in heavy duty applications. According to this patent the surface hardness or densification should be in the range of 90 to 100 percent of full theoretical density to a depth of at least 380 microns and up to 1,000 microns. No specific details are disclosed concerning the production process but it is stated that admixed powders are preferred as they have the advantage of being more compressible, enabling higher densities to be reached at the compaction stage. Furthermore it is stated that the admixed powders should include in addition to iron and 0.2% by weight of graphite, 0.5% by weight of molybdenum, chromium and manganese, respectively.

[0005] A method similar to that described in the US patent 5 711 187 is disclosed in the US 5 540 883 (1994).

[0006] According to the US patent 5 540 883 bearing surfaces from powder metal blanks are produced by blending carbon and ferro alloys and lubricant with compressible elemental iron powder, pressing the blending mixture to form the powder metal blank, high temperature Sintering the blank in a reducing atmosphere, compressing the powder metal blanks so as to produce a densified layer having a bearing surface, and then heat treating the densified layer. The sintered powder metal article should have a composition, by weight percent, of 0.5 to 2.0% chromium, 0 and 1.0% molybdenum; 0.1 and 0.6% carbon, with a balance of iron and trace impurities. Broad ranges as regards compaction pressures are mentioned. Thus it is stated that the compaction may be performed at pressures between 25 and 50 ton per square inch (about 390-770 MPa).

[0007] The US 5 552 109 (1995) patent concerns a process of forming a sintered article having high density. The patent is particularly concerned with the production of connecting rods. As in the US patent 5 711 187 no specific details concerning the production process are disclosed in the US patent 5 552 109 but it is stated that the powder should be a pre-alloyed iron based powder, that the compacting should be performed in a single step, that the compaction pressures may vary between 25 and 50 ton per square inch (390-770 MPa) to green densities between 6.8 and 7.1 g/cm³ and that the sintering should be performed at high temperature, particularly between 1270 and 1350°C. It is stated that sintered products having a density greater than 7.4 g/cm³ are obtained and it is thus obvious that the high sintered density is a result of the high temperature sintering.

[0008] In the US 5 729 822 (1996) a powder metal gear wheel having a core density of at least 7.3 g/cm³ and a hardened carburized surface is disclosed. The powders recommended are the same as in the US patents 5 711 187 and 5 540 883 i.e. mixtures obtained by blending carbon, ferro alloys and lubricant with compressible a powder of elemental iron. In order to obtain high sintered core density the patent mentions warm pressing; double pressing, double sintering; high density forming as disclosed in the US patent 5 754 937; the use of die wall lubrication, instead of admixed lubricants during powder compaction and rotary forming after sintering. Compacting pressures of around 40 tons per square inch (620 MPa) are typically employed.

[0009] The surface densification of sintered PM steels is discussed in e.g. the Technical Paper Series 820234, (International Congress & Exposition, Detroit, Michigan, February 22-26, 1982). In this paper a study of surface roll= ing of sintered gears is reported. Fe-Cu-C and Ni-Mo alloyed materials were used for the study. The paper reveals the results from basic research on the surface rolling of sintered parts at a density of 6.6 and 7.1 g/cm³ and the application of it to sintered gears. The basic studies includes surface rolling with different diameters of the rolls, best results in terms of strength were achieved with smaller roll diameter, lesser reduction per pass and large total reduction. As an example for a Fe-Cu-C material a densification of 90% of theoretical density was achieved with a roll of 30 mm diameter to a depth of 1,1 mm. The same level of densification was achieved to a depth of about 0.65 mm for a 7.5 mm diameter roll. The small diameter roll however was able to increase the densification to about full density at the surface whereas the large diameter roll increased the density to about 96% at the surface. The surface rolling technique was applied to sintered oil-pumps gears and sintered crankshaft gears. In an article in Modern Developments in Powder Metallurgy, Volume 16, p. 33-48 1984 (from the International PM Conference June 17-22, 1984, Toronto Canada,) the authors have investigated the influence of shot-peening, carbonitriding and combinations thereof on the endurance limit of sintered Fe+1.5% Cu and Fe+2% Cu+2.5% Ni alloys. The density reported of these alloys were 7.1 and 7.4 g/cm³. Both a theoretical evaluation of the surface rolling process and a bending fatigue testing of surface rolled parts is published in an article in Horizon of Powder Metallurgy part I, p.403-406. Proceedings of the 1986 (International Powder Metallurgy Conference and Exhibition, Dusseldorf, 7-11 July 1986).

[0010] According to the prior art many different routes have been suggested in order to reach high sintered density of a powder metallurgical component. However, the suggested processes all include steps adding additional costs. Thus warm compaction and die wall lubrication promote high green density. Double pressing and double sintering result in high sintered density and shrinkage as a result of high temperature sintering also results in high sintered density.

[0011] Furthermore, for high load applications such as gear wheels, special precautions has to be taken in account regarding the pore size and pore morphology in order to achieve sufficient fatigue properties. A simple and cost effective method for the preparation of gear wheels and similar products with a high sintered density and mechanical strength, regardless the pore size and morphology, would thus be attractive and the main object of the present invention.

[0012] Dizdar et al. "Process, Quality and Properties of High-Density P/M Gears", PM²TEC2003, Las Vegas, discloses a P/M component compacted to 7.6 g/cm² produced by a high velocity compaction single pressing and single sintering operation. It is also disclosed that P/M gears may be surface densified.

SUMMARY OF THE INVENTION

[0013] In brief it has now been found that powder metal parts is more demanding applications, such as power transmission applications, for example, gear wheels, can be obtained by subjecting an iron or iron-based powder to unaxially compaction at a pressure above 700 MPa to a density above 7.35 g/cm³, sintering the obtained green product and subjecting the sintered product to a densification prccess. A characteristic feature of the core of the metal part according to the invention is the pore structure, which is distinguished by comparatively large pores.

[0014] According to an aspect of the present invention, there is provided a sintered metal part which has a densified surface, a sintered density of at least 7.35 g/cm³ and a core structure distinguished by the pore structure obtained by single pressing to at least 7.35 g/cm³ and single sintering of a mixture of a coarse iron or iron-based powder and optional additives; characterized in that the core of the metal part has a pore structure wherein at least 50% of the pore area in a cross section consists of pores having a pore area of at least 100 µm², and that the iron or iron-based powder has a particle size such that at most 10% of the particles are less than 45 µm, and that the surface densified part is densified to a depth of at least 0.1 mm.

[0015] According to another aspect of the present invention, there is provided a method for producing powder metal parts having a densified surface, comprising the steps of: uniaxially compacting an iron or iron-based powder having coarse particles to a density above 7.35 g/cm³ in a single compaction step at a compaction pressure of at least 700 MPa, wherein the iron or iron-based powder has a particle size such that at most 10% of the particles are less than 45 µm; subjecting the parts to sintering in a single step at a temperature of at least 1100°C to a density of at least 7.35 g/m³, whereby the core of the sintered parts has a pore structure wherein at least 50% of the pore area in a cross section consists of pores having a pore area of at least 100 µm²; and subjecting the parts to a surface densifying process wherein the surface densified parts are densified to a depth of at least 0.1 mm.

[0016] The density levels above concerns products based on pure or low=alloyed iron powder.

DETAILED DESCRIPTION OF THE INVENTION

Powder types

[0017] Suitable metal powders which can be used as starting materials for the compaction process are powders prepared from metals such as iron. Alloying elements such as carbon, chromium, manganese, molybdenum, copper, nickel, phosphorous, sulphur etc can be added as particles, prealloyed or diffusion alloyed in order to modify the properties of the final sintering product. The iron-based powders can be selected from the group consisting of substantially pure iron powders, pre-alloyed iron-based particles, diffusion alloyed iron-based iron particles and mixture of iron particles or iron-based particles and alloying elements. As regards the particle shape it is preferred that the particles have an irregular form as is obtained by water atomisation. Also sponge iron powders having irregularly shaped particles may be of interest.

[0018] As regards PM parts for high demanding applications, especially promising results have been obtained with pre alloyed water atomised powders including low amounts such as up to 5% of one or more of the alloying elements Mo and Cr. Examples of such powders are powders having a chemical composition corresponding to the chemical composition of Astaloy Mo (1.5% Mo and Astaloy 85 Mo (0.85% Mo) as well as Astaloy CrM (3 Cr,0.5 Mo)and Astaloy CrL (1.5 Cr,0.2 Mo) from Höganäs AB, Sweden.

[0019] A critical feature of the invention is that the powder used have coarse particles i.e. the powder is essentially without fine particles. The term "essentially without fine particles" is intended to mean that less than about 10%, preferably less than 5% of the powder particles have a size below 45 µm as measured by the method described in SS-EN 24 497. The average particle diameter is typically between 75 and 300 µm. The amount of particles above 212 µm is typically above 20%. The maximum particle size may be about 2 mm.

[0020] The size of the iron-based particles normally used within the PM industry is distributed according to a gaussian distribution curve with a average particle diameter in the region of 30 to 100 µm and about 10-30% of the particles are less than 45 µm. Thus the powders used according to the present invention have a particle size distribution deviating from that normally used. These powders may be obtained by removing the finer fractions of the powder or by manufacturing a powder having the desired particle size distribution.

[0021] Thus for the powders mentioned above a suitable particle size distribution for a powder having a chemical composition corresponding to the chemical composition of Astaloy 85 Mo could be that at most 5% of the particles should be less than 45 µm and the average particle diameter is typically between 106 and 300 µm. The corresponding values for a powder having a chemical composition corresponding to Astaloy CrL are suitably that less than 5% should be less than 45 µm and the average particle diameter is typically between 106 and 212 µm.

[0022] In order to obtain sintered metal parts having satisfactory mechanical sintered properties according to the present invention it may be necessary to add graphite to the powder mixture to be compacted. Thus, graphite in amounts between 0.1-1, preferably 0.2-1.0, more preferably 0.2-0.7% and most preferably 0.2-0.5% by weight of the total mixture to be compacted could be added before the compaction. However, for certain applications graphite addition is not necessary.

[0023] The iron-base powder may also be combined with a lubricant before it is transferred to the die (internal lubrication). The lubricant is added in order to minimize friction between the metal power particles and between the particles and the die during a compaction, or pressing, step. Examples of suitable lubricants are e.g. stearates, waxes, fatty acids and derivatives thereof, oligomers, polymers and other organic substances with lubricating effect. The lubricants may be added in the form of particles but may also be bonded and/or coated to the particles.

[0024] Preferably a lubricating coating of a silane compound of the type disclosed in WO 2004/037467 is included in the powder mixture. Specifically the silane compound may be an alkylakoxy or polyetheralkoxy silane, wherein the alkyl group of the alkylalkoxy silane and the polyether chain of the polyetheralkoxy silane include between 8 and 30 carbon atoms, and the alkoxi group includes 1-3 carbon atoms. Examples of such compounds are octyl-tri-metoxy silane, hexadecyl-tri-metoxy silane and polyethyleneether-trimetoxy silane with 10 ethylene ether groups.

[0025] According to the present invention the amount of lubricant added to the iron-based powder may vary between 0.05 and 0.6%, preferably between 0.1-0.5% by weight of the mixture.

[0026] As optional additives hard phases, binding agents, machinability enhancing agents and flow enhancing agents may be added.

Compaction

[0027] Conventional compaction at high pressures, i.e. pressures above 600 MPa with conventionally used powders including finer particles, in admixture with low amounts of lubricants (less than 0.6% by weight) is generally considered unsuitable due to the high forces required in order to eject the compacts from the die, the accompanying high wear of the die and the fact that the surfaces of the components tend to be less shiny or deteriorated. By using the powders according to the present invention it has unexpectedly been found that the ejection force is reduced at high pressures and that components having acceptable or even perfect surfaces may be obtained also when die wall lubrication is not used.

[0028] The compaction may be performed with standard equipment, which means that the new method may be performed without expensive investments. The compaction is performed uniaxially in a single step at ambient or elevated temperature. Preferably the compaction pressures are above about 700, more preferably above 800 and most preferably above 900 or even 1000 MPa. In order to reach the advantages with the present invention the compaction should preferably be performed to densities above 7.45 g/cm³.

Sintering

[0029] Any conventional sintering furnace may be used and the sintering times may vary between about 15 and 60 minutes. The atmosphere of the sintering furnace may be an endogas atmosphere, a mixture between hydrogen and nitrogen or in vacuum. The sintering temperatures may vary between 1100 and 1350C. With sintering temperatures above about 1250°C the best results are obtained. In comparison with methods involving double pressing and double sintering the method according to the present invention has the advantage that one pressing step and one sintering step are eliminated and still sintered densities above 7.64 g/cm³ can be obtained.

Structure

[0030] A distinguishing feature of the core of the high density green and sintered metal part is the presence of large pores. Thus, as an example, in a cross section of the core of a sintered metal part according to the invention, at least about 50% of the pore area consists of pores having a pore area of at least 100 µm², whereas, in a cross section of a core prepared from a corresponding normal powder (i.e. a powder including normal amounts of fine particles which has to be double pressed and double sintered in order to reach the same density), at least about 50% of the pore area consists of pores having a pore area of about 65 µm².

Surface densification

[0031] The surface densification may be performed by radial or axial rolling, shoot peening, sizing etc. A preferred method is radial rolling as this method provides short cycle times in combination with great densification depth. The powder metal parts will obtain better mechanical properties with increasing densifying depth. The densification depth is preferably at least 0.1 mm, preferably at least 0.2 mm and most preferably at least 0.3 mm.

[0032] In this context is should be recalled that normally the presence of large pores in sintered parts is regarded as a drawback and different measures are taken in order to make the pores smaller and rounder. According to the present invention, however, it has surprisingly been found that the negative effect of the comparatively high amount of larger pores can be totally eliminated by a surface densification process. Thus, when comparing the effect of surface densification on the bending fatigue strength of sintered samples containing larger pores in the core with the effect on samples containing smaller pores, it has been found that the surface densification process increases the bending fatigue strength to a much higher extent when the samples are produced from metal powder with the particle size distribution discussed above. After the surface densification process, the bending fatigue strength of samples produced of these powders will surprisingly reach the same level as that of surface densified samples which are produced from powders having a normal particle size distribution(given the same chemical composition and the same sintered density level). Accordingly, as high sintered density can be reached in a single pressing, single sintering process, costly processes, such as double pressing= double sintering, warm compaction, can be avoided by utilising the method according to the present invention for production of for example gear wheels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] 

Fig 1 shows the bending fatigue strength before and after the surface densification process of samples produced from the mixes 1A and 1B according to example 1.

Fig 2 is a light optical micrograph of a cross section of a surface densified sample prepared from mix 1A.

Fig 3 is a light optical micrograph of a cross section of a surface densified sample prepared from mix 1B.

Fig 4 shows the bending fatigue strength before and after surface densification process of samples produced from the mixes 2C and 2D according to example 2.

Fig 5 is a light optical micrograph of a cross section of a surface densified sample prepared from mix 2C.

Fig 6 is a light optical micrograph of a cross section of a surface densified sample prepared from mix 2D.

The invention is further illustrated by the following non-limiting examples.

[0034] The following iron-based powders were used;
Powder A;
Astaloy 85 Mo, an atomised pre=alloyed iron base powder with a Mo content of 0.80-0.95%, a carbon content of at most 0.02% and an oxygen content of at most 0.20%.
The particle sized distribution of powder A is similar to the particle size distribution for powder normally used in powder metallurgy; about 0% greater than 250 µm, about 15-25% between 150 and 250 µm and about 15 to 30% less than 45 µm.
Powder B;
The same chemical composition as powder A but with a coarser particle size distribution according to the table below;

Particle size µm	% by weight
>500	0
425=500	1.9
300-425	20.6
212-300	27.2
150-212	20.2
106-150	13.8
75-106	6.2
45-75	.9
<45	4.2

Powder C;
Astaloy CrL, an atomised Mo-, Cr- prealloyed iron based powder with a Cr content of 1.35-1.65%, a Mo content of 0.17-0.27%, a carbon content of at most 0.010% and an oxygen content of at most 0.25%.
The particle sized distribution of powder C is similar to the particle size distribution for powder normally used in powder Metallurgy; about 0% greater than 250 µm, about 15-25% between 150 and 212 µm and about 10 to 25% less than 45 µm.
Powder D;
The same chemical composition as powder C but with a coarser particle size distribution according to the table below;

Particle size µm	% by weight
>500	0
425-500	0.2
300-425	7.4
212-300	21.9
150-212	25.1
106-150	23.4
75-106	11.2
45-75	7.1
<45	3.7

Example 1

[0035] Two mixes, Mix 1A and Mix 1B were prepared by thoroughly mixing before compaction.

[0036] Mix 1A was based on powder A with an addition of 0.2% by weight of graphite and 0.8% by weight of H wax.

[0037] Mix 1B was based on powder B with an addition of 0,2% by weight of graphite and 0.2% by weight of hexadecyl trimetoxy silane.

[0038] FS-strength test bars according to ISO 3928 were compacted.

[0039] Test bars based on Mix 1A was compacted to a green density of 7.1 g/cm³ and pre sintered at 780°C for 30 minutes in an atmosphere of 90% nitrogen and 10% hydrogen. After sintering the samples were subjected to a second compaction at a pressure of 1100 MPa and finally sintered at 1280°C for 30 minutes in an atmosphere of 90% nitrogen and 10% of hydrogen. The sintered density was measured to 7.61 g/cm³.

[0040] The sample prepared from mix 1B was compacted in a single compaction process at 1100 MPa was subsequently sintered at 1280°C for 30 minutes in an atmosphere of 90% nitrogen and 10% of hydrogen. The sintered density was 7.67 g/cm³.

[0041] The results are summarized in table 1 below.

Table 1

MIX	POWDER	Pressure	Sintering	Pressure	Sintering	SD
		MPa/GD	°C	MPa	°C	g/cm3
1A	Astaloy 85 0.80-0.95 Mo standard 0.2 graphite	7.1	780	1100	1280	7.61
1B	Astaloy 85 0.80-0.95 Mo coarse 0.2 graphite			1100	1280	7.67

[0042] Half of the number of the obtained sintered bodies was subjected to a surface densifaction process by shot peening at 6 bars air pressure with steel spheres with a diameter of 0.4 mm.

[0043] Both the surface densified samples and the samples not subjected to a surface densification process were case hardened at 920°C for 75 minutes at a carbon potential of 0.8% followed by a tempering operation at 200°C for 120 minutes.

[0044] Bending fatigue limit (BFL) was determined for all of the samples.

[0045] Figure 1 shows the bending fatigue limit for both the surface densified samples and the samples which were not subjected to surface densification.

[0046] From figure 1 it can be concluded that surface densification of the samples produced with the coarser powder contributes to a much higher increase in BFL compared with the increase in BFL which was obtained by surface densification of the samples produced with a powder having a conventional particle size distribution.

[0047] Figure 2 is a light optical micrograph showing a cross section of a surface densified sample prepared from mix 1A and figure 3 is a similar micrograph from a surface densified sample prepared from mix 1B.

[0048] Image analysis according to ASTM E 1245 of cross section of surface densified samples produced from sample 1A shows that about 50% of the total cross section pore area consists of pores having a surface area of 65 µm² or more, whereas the same measuring of surface densified samples produced from mix 1B shows that about 50% of the total cross section area consists of pores having a surface area of 200 µm² or more.

Example 2

[0049] Two mixes, Mix 2C and Mix 2D were prepared by thoroughly mixing before compaction.

[0050] Mix 2C was based on powder C with an addition of 0.7% of nickel powder, 0.2% by weight of graphite and 0.8% by weight of H wax,

[0051] Mix 2D was based on powder D with an addition of 0.7% of nickel powder 0.2% of graphite and 0.2% of hexadecyl trimetoxy silane.

[0052] FS-strength test bars according to ISO 3928 were prepared.

[0053] Test bars based on mix 2C, was compacted to a green density of 7.1 g/cm³ and pre sintered at 780°C for 30 minutes in an atmosphere of 90% nitrogen and 10% hydrogen. After sintering the samples were subjected to a second compaction at a pressure of 1100 MPa and finally sintered at 1280°C for 30 minutes in an atmosphere of 90% nitrogen and 10% of hydrogen. The sintered density was measured to 7.63 g/cm³.

[0054] Test bars prepared from mix 2D was compacted in a single compaction process at 1100 MPa followed by sintering 1280°C for 30 minutes in an atmosphere of 90% nitrogen and 10% of hydrogen. The sintered density was measured to 7.64 g/cm³.

[0055] The results are summarized in table 3 below.

Table 3

MIX	POWDER	Pressure	Sintering	Pressure	Sintering	SD
		MPa/GD	°C	MPa	°C	g/cm3
2C	CRL Standard 1.35-1.65 Cr 0.17-0.27 Mo+0.7% Ni	7.1	780	1100	1280	7.63
2D	CRL Coarse 1.35-1.65 Cr 0.17-0.27 Mo+0.7% Ni			1200	1280	7.64

[0056] Half of the number of the obtained sintered bodies were subjected to a surface densification process by shot peening at 6 bars air pressure with steel spheres with a diameter of 0.4 mm.

[0057] Both the surface densified samples and the samples not subjected to a surface densifaction process were case hardened at 920°C for 75 minutes at a carbon potential of 0.8% followed by a tempering operation at 200°C for 120 minutes.

[0058] Bending fatigue limit (BFL) were determined for all of the samples.

[0059] Figure 5 shows the bending fatigue limit for both the surface densified samples and the samples which were not subjected to surface densification.

[0060] From figure 5 it can be concluded that surface densification of the samples produced with the coarser powder con= tributes to a much higher increase in BFL compared with the increase in BFL which was obtained by surface densification of the samples produced with a powder having a conventional particle size distribution.

[0061] Figure 6 is a light optical micrograph showing a cross section of a surface densified sample prepared from mix 2C and figure 7 is a similar micrograph from a surface densified sample prepared from mixture 2D.

[0062] Image analysis according to ASTM E 1245 of cross section of surface densified samples produced from sample 2C shows that about 50% of the total cross section pore area consists of pores having a surface area of 50 µm² or more, whereas the same measuring of surface densfied samples produced from mix 2D shows that about 50% of the total cross section area consists of pores having a surface area of 110 µm² or more.

Claims

1. A sintered metal part which has a densified surface, a sintered density of at least 7.35 g/cm³ and a core structure distinguished by the pore structure obtained by single pressing to at least 7.35 g/cm³ and single sintering of a mixture of a coarse iron or iron-based powder and optional additives; characterized in that the core of the metal part has a pore structure wherein at least 50% of the pore area in a cross section consists of pores having a pore area of at least 100 µm², that the iron or iron-based powder has a particle size such that at most 10% of the particles are less than 45 µm, and that the surface densified part is densified to a depth of at least 0.1 mm.

2. Metal part according to claim 1 wherein the green and the sintered densities are at least 7.45, preferably at least 7.5 g/cm³.

3. Method for producing powder metal parts having a densified surface, comprising the steps of

- uniaxially compacting an iron or iron-based powder having coarse particles to a density above 7.35 g/cm³ in a single compaction step at a compaction pressure of at least 700 MPa, wherein the iron or iron-based powder has a particle size such that at most 10% of the particles are less than 45 µm;

- subjecting the parts to sintering in a single step at a temperature of at least 1100°C to a density of at least 7.35 g/m³, whereby the core of the sintered parts has a pore structure wherein at least 50% of the pore area in a cross section consists of pores having a pore area of at least 100 µm²; and

- subjecting the parts to a surface densifying process wherein the surface densified parts are densified to a depth of at least 0.1 mm.

4. Method according to claim 3, wherein the powder includes alloying additives in an amount up to 5% by weight.

5. Method according to claim 4, wherein the alloying additives are selected from the group consisting of at least one element selected from the group consisting of graphite, chromium, molybdenum, manganese, nickel and copper.

6. Method according to any one of the claims 3-5, wherein the powder includes a lubricant.

7. The method according to claim 6, wherein the lubricant is an organosilane selected from the group consisting of alkylakoxy or polyetheralkoxy silane, wherein the alkyl group of the alkylalkoxy silane and the polyether chain of the polyetheralkoxy silane include between 8 and 30 carbon atoms, and the alkoxi group includes 1-3 carbon atoms.

8. The method according to claim 7, wherein the organosilane is selected from the group consisting of octyl-tri-metoxy silane, hexadecyl-tri-metoxy silane and polyethylene ether-trimetoxy silane with 10 ethylene ether groups.

9. Method according to any one of the claims 3-8, wherein the iron-based powder is a pre-alloyed, water atomised powder.

10. Method according to any one of the claims 3-9, wherein the iron-based powder has a particles size such that at most 5% of the particles are less than 45 µm.

11. Method according to any one of the claims 3-10, wherein the compaction is performed at a pressure of at least 800 MPa, more preferably at least 900 MPa and most preferably at least 1000 MPa.

12. Method according to any one of the claims 3-11, wherein the sintering is performed at a temperature of at least 1200°C, preferably at least 1250°C.

13. Method according to any one of the claims 3-12, wherein the compacted parts are sintered for a time of 15 to 60 minutes.

14. Method according to any one of the claims 3-13, wherein the compacted parts are sintered in an endogas atmosphere, a mixture between hydrogen and nitrogen or in vacuum.

15. Method according to any one of the claims 3-14, wherein surface densifying is performed by rolling.

16. Method according to any one of the claims 3-15, wherein the surface densified parts are densified to a depth of at least 0.2 mm, preferably at least 0.3 mm.

17. Method according to any one of the claims 3-16, wherein the produced powder metal parts are gears, bearings, rolls, sprockets, shafts.

Ansprüche

1. Gesintertes Metallteil, das eine verdichtete Oberfläche, eine gesinterte Dichte von mindestens 7,35 g/cm³ und eine Kernstruktur hat, die sich durch eine Porenstruktur auszeichnet, welches durch eine einmalige Pressung auf mindestens 7,35 g/cm³ und einmalige Sinterung eines Gemisches eines groben Eisen- oder eisenbasierten Pulvers und optionaler Zusatzstoffe erhalten ist; dadurch gekennzeichnet, dass der Kern des Metallteils eine Porenstruktur hat, wobei mindestens 50 % der Porenfläche in einem Querschnitt aus Poren besteht, die eine Porenfläche von mindestens 100 µm² haben, dass das Eisen- oder eisenbasierte Pulver eine solche Teilchengröße hat, dass höchstens 10 % der Teilchen kleiner als 45 µm sind, und dass der Teil mit verdichteter Oberfläche bis zu einer Tiefe von mindestens 0,1 mm verdichtet ist.

2. Metallteil nach Anspruch 1, wobei die ungesinterten und die gesinterten Dichten mindestens 7,45, vorzugsweise mindestens 7,5 g/cm³ betragen.

3. Verfahren zum Herstellen von Metallpulverteilen, die eine verdichtete Oberfläche haben, welches die folgenden Schritte umfasst:

- einachsige Verdichtung eines Eisen- oder eisenbasierten Pulvers, das grobe Teilchen hat, bis zu einer Dichte von mehr als 7,35 g/cm³ in einem einzigen Verdichtungsschritt bei einem Verdichtungsdruck von mindestens 700 MPa, wobei das Eisen- oder eisenbasierte Pulver eine solche Teilchengröße hat, dass höchstens 10 % der Teilchen kleiner als 45 µm sind;

- Sinterung der Teile in einem einzigen Schritt bei einer Temperatur von mindestens 1100 °C bis zu einer Dichte von mindestens 7,35 g/cm³, wobei der Kern der gesinterten Teile eine Porenstruktur hat, wobei mindestens 50 % der Porenfläche in einem Querschnitt aus Poren bestehen, die eine Porenfläche von mindestens 100 µm² haben; und

- Ausführen eines Oberflächenverdichtungsprozesses an den Teilen, wobei die oberflächenverdichteten Teile bis zu einer Tiefe von mindestens 0,1 mm verdichtet sind.

4. Verfahren nach Anspruch 3, wobei das Pulver Legierungszusätze in einer Menge von bis zu 5 Gewichts-% umfasst.

5. Verfahren nach Anspruch 4, wobei die Legierungszusätze aus der Gruppe, die mindestens ein Element umfasst, bestehend aus Graphit, Chrom, Molybdän, Mangan, Nickel und Kupfer, ausgewählt sind.

6. Verfahren nach einem der Ansprüche 3-5, wobei das Pulver ein Gleitmittel umfasst.

7. Verfahren nach Anspruch 6, wobei das Gleitmittel ein organisches Silan ist, das aus der Gruppe bestehend aus Alkylalkoxy- oder Polyetheralkoxy-Silan ausgewählt ist, wobei die Alkylgruppe des Alkylalkoxy-Silans und die Polyetherkette des Polyetheralkoxy-Silans 8 bis 30 Kohlenstoffatome umfasst und die Alkoxygruppe 1-3 Kohlenstoffatome umfasst.

8. Verfahren nach Anspruch 7, wobei das organische Silan aus der Gruppe bestehend aus Octyltrimetoxy-Silan, Hexadecyltrimetoxy-Silan und Polyethylenethertrimetoxy-Silan mit 10 Ethylenethergruppen ausgewählt ist.

9. Verfahren nach einem der Ansprüche 3-8, wobei das eisenbasierte Pulver ein vorlegiertes, mit Wasser zerstäubtes Pulver ist.

10. Verfahren nach einem der Ansprüche 3-9, wobei das eisenbasierte Pulver eine solche Teilchengröße hat, dass höchstens 5 % der Teilchen kleiner als 45 µm sind.

11. Verfahren nach einem der Ansprüche 3-10, wobei die Verdichtung bei einem Druck von mindestens 800 MPa, vorzugsweise mindestens 900 MPa und am besten mindestens 1000 MPa ausgeführt wird.

12. Verfahren nach einem der Ansprüche 3-11, wobei das Sintern bei einer Temperatur von mindestens 1200 °C, vorzugsweise mindestens 1250 °C ausgeführt wird.

13. Verfahren nach einem der Ansprüche 3-12, wobei die verdichteten Teile über eine Zeit von 15 bis 60 Minuten verdichtet werden.

14. Verfahren nach einem der Ansprüche 3-13, wobei die verdichteten Teile in einer Endogas-Atmosphäre, einer Mischung aus Wasserstoff und Stickstoff oder in einem Vakuum gesintert werden.

15. Verfahren nach einem der Ansprüche 3-14, wobei die Oberflächenverdichtung durch Walzen ausgeführt wird.

16. Verfahren nach einem der Ansprüche 3-15, wobei die oberflächenverdichteten Teile bis zu einer Tiefe von mindestens 0,2 mm, vorzugsweise mindestens 0,3 mm verdichtet werden.

17. Verfahren nach einem der Ansprüche 3-16, wobei die erzeugten Metallpulverteile Zahnräder, Lager, Walzen, Kettenräder, Wellen sind.

Revendications

1. Pièce métallique frittée qui présente une surface densifiée, une densité frittée d'au moins 7,35 g/cm³ et une structure de noyau qui se distingue par la structure de pores obtenue par pression unique jusqu'à au moins 7,35 g/cm³ et frittage unique d'un mélange d'un fer ou d'une poudre à base de fer à gros grains et d'additifs optionnels ; caractérisée en ce que le noyau de la pièce métallique présente une structure de pores dans laquelle au moins 50 % de la surface de pores dans une section transversale consiste en des pores présentant une surface de pores d'au moins 100 µm², en ce que le fer ou la poudre à base de fer présente une taille de particules telle qu'au plus 10 % des particules sont inférieures à 45 µm, et en ce que la pièce à surface densifiée est densifiée jusqu'à une profondeur d'au moins 0,1 mm.

2. Pièce métallique selon la revendication 1, dans laquelle les densités à cru et frittée sont au moins de 7,45, de préférence au moins 7,5 g/cm³.

3. Procédé pour la production de pièces de poudre métallique présentant une surface densifiée, comprenant les étapes consistant à :

- compacter de manière uniaxiale un fer ou une poudre à base de fer présentant des particules à gros grains jusqu'à une densité supérieure à 7,35 g/cm³ en une étape unique de compactage à une pression de compactage d'au moins 700 MPa, dans lequel le fer ou la poudre à base de fer présente une taille de particules telle qu'au plus 10 % des particules sont inférieures à 45 µm ;

- soumettre les pièces au frittage en une étape unique à une température d'au moins 1100° C à une densité d'au moins 7,35 g/cm³, ce par quoi le noyau des pièces frittées présente une structure de pores dans laquelle au moins 50 % de la surface de pores dans une section transversale consiste en des pores présentant une surface de pores d'au moins 100 µm² ; et

- soumettre les pièces à un processus de densification de surface dans lequel les pièces à surface densifiée sont densifiées jusqu'à une profondeur d'au moins 0,1 mm.

4. Procédé selon la revendication 3, dans lequel la poudre comprend des alliages d'addition dans une quantité jusqu'à 5 % en poids.

5. Procédé selon la revendication 4, dans lequel les alliages d'addition sont sélectionnés parmi le groupe constitué d'au moins un élément sélectionné parmi le groupe constitué de graphite, de chrome, de molybdène, de manganèse, de nickel et de cuivre.

6. Procédé selon l'une quelconque des revendications 3 à 5, dans lequel la poudre comprend un lubrifiant.

7. Procédé selon la revendication 6, dans lequel le lubrifiant est un organosilane sélectionné parmi le groupe constitué d'alkylalkoxy ou de polyétheralkoxysilane, dans lequel le groupe alkyle de l'alkylalkoxysilane et la chaîne polyéther du polyétheralkoxysilane comprennent entre 8 et 30 atomes de carbone, et le groupe alkoxy comprend de 1 à 3 atomes de carbone.

8. Procédé selon la revendication 7, dans lequel l'organosilane est sélectionné parmi le groupe constitué d'octyltrimétoxysilane, hexadécyltrimétoxysilane et éthertrimétoxysilane de polyéthylène avec 10 groupes d'éther d'éthylène.

9. Procédé selon l'une quelconque des revendications 3 à 8, dans lequel la poudre à base de fer est une poudre atomisée par eau, préalliée.

10. Procédé selon l'une quelconque des revendications 3 à 9, dans lequel la poudre à base de fer présente une taille de particules telle qu'au moins 5 % des particules sont inférieures à 45 µm.

11. Procédé selon l'une quelconque des revendications 3 à 10, dans lequel le compactage est réalisé à une pression d'au moins 800 MPa, plus préférablement d'au moins 900 MPa et le plus préférablement au moins 1000 MPa.

12. Procédé selon l'une quelconque des revendications 3 à 11, dans lequel le frittage est réalisé à une température d'au moins 1200° C, de préférence au moins 1250° C.

13. Procédé selon l'une quelconque des revendications 3 à 12, dans lequel les pièces compactées sont frittées pendant une durée de 15 à 60 minutes.

14. Procédé selon l'une quelconque des revendications 3 à 13, dans lequel les pièces compactées sont frittées sous une atmosphère d'endogaz, un mélange entre de l'hydrogène et de l'azote, ou sous vide.

15. Procédé selon l'une quelconque des revendications 3 à 14, dans lequel la densification de surface est réalisée par laminage.

16. Procédé selon l'une quelconque des revendications 3 à 15, dans lequel les pièces à surface densifiée sont densifiées jusqu'à une profondeur d'au moins 0,2 mm, de préférence au moins 0,3 mm.

17. Procédé selon l'une quelconque des revendications 3 à 16, dans lequel les pièces de poudre métallique produites sont des engrenages, des roulements, des rouleaux, des pignons, des axes.

Drawing