IRON BASED POWDER

Abstract

A diffusion-bonded powder having an iron powder having 1-5%, preferably 1.5-4% and most preferably 1.5-3.5% by weight of copper particles diffusion bonded to the surfaces of the iron powder particles. The diffusion bonded powder is suitable for producing components having high sintered density and minimum variation in copper content. The iron powder may be produced by providing an atomized iron powder with an oxygen content of 0.3-1.2% by weight and with a carbon content of 0.1-0.5% by weight, and subjecting the atomized iron powder and a copper containing powder to a reduction annealing process in a reducing atmosphere to obtain the iron based powder.

Claims

1. An iron based powder consisting of particles of reduced copper oxide diffusion bonded to the surface of an atomized iron powder, wherein the content of copper is 1-5% by weight of the iron based powder, wherein the iron based powder is produced by providing an atomized iron powder with an oxygen content of 0.3-1.2% by weight and with a carbon content of 0.1-0.5% by weight, and subjecting the atomized iron powder and a copper containing powder to a reduction annealing process in a reducing atmosphere to obtain the iron based powder.

2. The iron based powder according to claim 1, wherein the iron based powder has a maximum particle size of 250 μm, at least 75% is below 150 μm and at most 30% is below 45 μm, wherein the iron based powder has an apparent density of at least 2.70g/cm.sup.3 and an oxygen content of at most 0.16% by weight, and wherein other impurities are at most 1% by weight of the iron based powder.

3. The iron based powder according to claim 2 having an SSF-factor of at most 2.0, wherein the SSF-factor is defined as the quotient between the Cu content in weight % in the fraction of the iron based powder which passes a 45 μm sieve and the Cu content in weight % in the fraction of the iron based powder which does not pass a 45 μm sieve.

4. The iron based powder according to claim 1, wherein the maximum copper content in a cross section of a sintered component made from said iron based powder is at most 100% higher than the nominal copper content, wherein the sintered component is produced by mixing said iron-based powder with 0.5% of graphite, having a particle size, X90, of at most 15 μm measured with laser diffraction according to ISO 13320:1999, and 0.9% of lubricant and the obtained mixture is transferred into a compaction die for production of tensile strength samples (TS-bars) according to ISO 2740:2009 and subjected to a compaction pressure of 600 MPa and the compacted sample is thereafter ejected from the compaction die and subjected to a sintering process at 1120° C. for a period of time of 30 minutes in an atmosphere of 90% nitrogen/10% hydrogen at atmospheric pressure and the maximum copper content is determined through lines scanning in a Scanning Electron Microscope (SEM) equipped with a system for Energy Dispersive Spectroscopy (EDS), wherein the magnification is 130×, working distance is 10 mm and the scanning time is 1 minute.

5. The iron based powder according to claim 1, wherein the largest pore area in a cross section of a sintered component made from said iron based powder is at most 4,000 μm.sup.2, wherein the sintered component is produced by mixing said iron-based powder with 0.5% of graphite, having a particle size, X90, of at most 15 μm measured with laser diffraction according to ISO 13320:1999, and 0.9% of lubricant and the obtained mixture is transferred into a compaction die for production of tensile strength samples (TS-bars) according to ISO 2740:2009 and subjected to a compaction pressure of 600 MPa and the compacted sample is thereafter ejected from the compaction die and subjected to a sintering process at 1120° C. for a period of time of 30 minutes in an atmosphere of 90% nitrogen/10% hydrogen at atmospheric pressure and the largest pore area is determined in a Light Optical Microscope (LOM) at a magnification of 100×0 with the aid of a digital video camera and a computer based software and the total measured area is 26.7 mm.sup.2.

6. An iron-based powder composition comprising: 10 to 99.8 weight % of the iron based powder according to claim 1, optionally graphite up to 1.5% weight %, and when graphite is present, lubricant is present at 0.3-1.5 weight %, and up to 1.0 weight % of machinability enhancing additives.

7. An iron-based powder composition comprising: 50 to 99.8 weight % of the iron based powder according to claim 1, optionally graphite up to 1.5% weight %, and when graphite is present, lubricant is present at 0.3-1.5 weight %, and up to 1.0 weight % of machinability enhancing additives.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0017] FIG. 1 shows variation in copper content for sample ac.

[0018] FIG. 2 shows variation in copper content for sample bc.

[0019] FIG. 3 shows variation in copper content for sample bd.

[0020] FIG. 4 shows variation in copper content for sample be.

[0021] FIG. 5 shows variation in copper content for sample ad.

DETAILED DESCRIPTION

Iron Powder

[0022] The iron powder used to produce the diffusion bonded powder is an atomized iron powder, and in a preferred embodiment having an oxygen content of 0.3-1.2%, preferably 0.5-1.1% by weight, and a content of carbon of 0.1-0.5% by weight. In one embodiment the content of oxygen is 0.5-1.1% by weight, and the content of carbon is above 0.3% by weight and up to 0.5% by weight. When water atomizing an iron melt it is more economical to allow higher contents of oxygen and carbon why this embodiment is preferred from a production economical point of view.

[0023] In an alternative embodiment the oxygen content is at most 0.15% by weight and the carbon content is at most 0.02% by weight.

[0024] By using an iron powder having a defined oxygen content, it has surprisingly been shown that the adhesion of the copper particles to the iron powder after the diffusion bonding-, reduction heat treatment-, process is significantly improved.

[0025] The maximum particle size of the iron powder is typically 250 μm and at least 75% by weight is below 150 μm. At most 30% by weight is below 45 μm. The particle size measured according to ISO4497 1983.

[0026] The total content of other unavoidable impurities, such as Mn, P, S, Ni and Cr is at most 1.5% by weight.

Copper Containing Powder

[0027] The copper containing powder used to produce the diffusion bonded powder is cuprous oxide, (Cu.sub.2O) or cupric oxide (CuO), preferably cuprous oxide is used. The copper containing powder has a maximum particle size, X.sub.90, of 22 μm, here defined as at least 90% of the particles are below the maximum particle size, and a weight average particle size, X.sub.50, of at most 15 μm, preferably at most 11 μm, determined with laser diffractometry according to ISO 13320:2003.

Diffusion-Bonded Powder

[0028] The iron powder is mixed with copper containing powder in proportions to obtain the final content of copper in the diffusion-bonded powder. After thoroughly mixing the powders, the mixture is subjected to a reduction-annealing process in a reducing atmosphere containing hydrogen at atmospheric pressure and at a time and temperature sufficient to reduce the copper containing powder into metallic copper and simultaneously allow copper to partially diffuse into the iron powder. Typically, the holding temperature is 800-980° C. for a period of 20 minutes to 2 hours. The obtained material after the reduction-annealing process is in form of a loosely bonded cake which after a cooling step is subjected to crushing or gentle grinding followed by classifying yielding the final powder. The maximum particle size of the obtained diffusion-bonded powder is 250 μm and at least 75 by weight is below 150 μm. At most 30% by weight is below 45 μm. The particle size measured according to ISO4497 1983.

[0029] The oxygen content in the new powder is at most 0.16% by weight and the amount of other inevitable impurities is at most 1% by weight.

[0030] The apparent density of the new powder, AD, as measured according to ISO 3923:2008 is at least 2.70 g/cm.sup.3 in order to obtain sufficiently high green density and consequently sintered density at production of components.

[0031] The diffusion bonded powder is characterized by having a degree of bonding of copper to the iron-based powder with an SSF-factor of at most 2, as measured by the SSF method. It has also surprisingly been shown that when the oxygen content of the iron powder used for production of the new powder is between 0.3-1.2% by weight, the SSF-factor is at most 1.7.

[0032] The SSF method is here defined as a method for determine the degree of bonding of copper to the iron or iron-based powder by separating the diffusion bonded powder into two fractions, one fraction having a particle size below 45 μm and another fraction having a particle size of 45 μm and above. This separation may be performed with a 45 μm standard sieve (325 mesh). The procedure according to ISO 4497:1986 may be followed with the proviso that only one sieve, 45 μm, is used. The quotation between the copper content in the finer fraction which passes the 45 μm sieve, and the copper content in the coarser fraction which do not passes the 45 μm sieve, gives a value, degree of bonding or SSF-factor.

[0033] SSF-factor=weight % Cu in the finer fraction, (−45 μm)/weight % Cu in the coarser fraction, (45 μm and above).

[0034] The copper content in the fractions is determined by standard chemical methods with at least an accuracy of two figures.

[0035] Another distinguishing characterization of the new powder is that it enables production of sintered component characterized by having a minimum of variation of the nominal copper content, within each individual component as well as between the components. This can be expressed as that the maximum copper content in a cross section of a sintered component, produced at specified production conditions, should be at most 100% higher than the nominal copper content.

[0036] The samples for measuring variations in the copper content, maximum and minimum copper content, pore sizes and pore area are prepared according to the following;

[0037] A copper containing diffusion bonded powder according to the present invention is mixed with 0.5% of graphite, having a particle size, X90, of at most 15 μm measured with laser diffraction according to ISO 13320:1999, and 0.9% of the lubricant described in the patent publication WO 2010/062250, wherein the description of the lubricant described in the patent publication WO 2010/062250 is incorporated herein by reference. The obtained mixture is transferred into a compaction die for production of tensile strength samples (TS-bars) according to ISO 2740:2009 and subjected to a compaction pressure of 600 MPa. The compacted sample is thereafter ejected from the compaction die and subjected to a sintering process at 1120° C. for a period of time of 30 minutes in an atmosphere of 90% nitrogen/10% hydrogen at atmospheric pressure.

[0038] A particulate composite lubricant particle may comprise a core of 10-60% by weight of at least one primary fatty acid amide having more than 18 and not more than 24 carbon atoms and 40-90% by weight of at least one fatty acid bisamide, said lubricant particle also comprising nanoparticles of at least one metal oxide adhered on the core.

[0039] The maximum copper content is measured in a cross section of the sintered component, i.e., a cross section perpendicular to the longest extension of the sintered TS-bar, through line scanning in a Scanning Electron Microscope (SEM) equipped with a system for Energy Dispersive Spectroscopy (EDS). The magnification is 130×, working distance is 10 mm and the scanning time is 1 minute.

[0040] The maximum copper content, measured by the above-mentioned method, is at any point along the line at most 100% higher than the nominal copper content. It has also surprisingly been shown that when the oxygen content of the iron powder used for production of the new powder is between 0.3-1.2% by weight, the maximum copper content, measured by the above-mentioned method, is at any point along the line at most 80% higher than the nominal copper content and no measurements show 0% copper.

[0041] Alternatively, or in addition to the above-mentioned variation of copper content, a distinguishing characterization of the new powder is that it enables production of sintered component characterized by exhibiting a maximum size of the largest pore. This can be 15 expressed as that the maximum pore area in a cross section of a sintered component, produced at the specified production conditions as described earlier, is at most 4000 μm.sup.2.

[0042] The pore size analysis is carried out on a Light Optical Microscope (LOM) at a magnification of 100× with the aid of a digital video camera and a computer based software. The total measured area is 26.7 mm.sup.2. The software is operating in black and white mode and detects pores using “detection of black area in measured area”, where black area is equal to pores.

[0043] The following definitions is applied:

[0044] Largest pore length: The largest length of all pores in the fields

[0045] Largest pore area: The area of the largest pore from those measured in the fields.

Manufacture of Sintered Component

[0046] Before compaction, the diffusion-bonded powder is mixed with various additives such as lubricants, graphite, and machinability enhancing additives.

[0047] Thus, an iron-based powder composition according to the invention contains or consists of 10 to 99.8 weight % of the diffusion bonded powder according to the invention, optionally graphite up to 1.5% weight % and when graphite is present the content is 0.3-1.5 weight %, preferably 0.15-1.2 weight %, 0.2 to 1.0 weight % of lubricant and up to 1.0 weight % of machinability enhancing additives, balanced with iron powder.

[0048] In one embodiment, an iron-based powder composition according to the invention contains or consists of 50 to 99.8 weight % of the diffusion bonded powder according to the invention, optionally graphite up to 1.5% weight % and when graphite is present the content is 0.3-1.5 weigh %, preferably 0.15-1.2 weight %, 0.2 to 1.0 weight % of lubricant, up to 1.0 weight % of machinability enhancing additives, balanced with iron powder.

[0049] After addition and admixing of additives the obtained mixture is subjected to a compaction process at a compaction pressure of at least 400 MPa, the subsequently ejected green component is sintered in a neutral or reducing atmosphere at a temperature of about 1050-1300° C. for a period of time of 10 to 75 minutes. The sintering step may be followed by a hardening step, such as case hardening, through hardening, induction hardening, or a hardening process including gas or oil quenching.

EXAMPLES

Example 1

[0050] Various diffusion-bonded powders were produced by mixing iron powders according to table 1 with copper containing powders according to table 2 in an amount sufficient to yield a content of 3% of copper in the subsequently obtained diffusion-bonded powder. The obtained mixtures were subjected to a reduction-annealing process at a temperature of 900° C. in a reducing atmosphere for a period of time 60 minutes. After the reduction-annealing process the obtained loosely sintered cake was gently crushed to a powder having a maximum particle size of 250 μm.

[0051] The following tables show raw materials used.

TABLE-US-00001 TABLE 1 Iron powder Iron powder O [%] C [%] D.sub.50 [μm] a) 1.02 0.41 98 b) 0.08 0.004 107

TABLE-US-00002 TABLE 2 Copper containing powder Copper containing powder Cu [%] O [%] D.sub.50 [μm] D.sub.95 [μm] c) Cu.sub.2O 88.1 Not measured 15 22 d) Cu 100 99.5 0.18 85 160 e) Cu 200 99.6 0.15 60 100

[0052] The obtained diffusion bonded powders were designated ac, bc, bd, be, ad and ae according to type of raw materials used.

[0053] Determination of SSF-factors for the diffusion bonded powders according to the invention were performed according to the method described in the detailed description. The following results according to table 3 were obtained.

TABLE-US-00003 TABLE 3 SSF-factor Sample SSF-factor ac 1.56 bc 1.97

[0054] Samples for measuring maximum pore size, maximum pore area and copper variation were prepared according to the procedure in the detailed description.

[0055] The maximum copper content was measured with the aid of a FEG-SEM, type Hitachi SU6600. The EDS system was manufactured by Bruker AXS.

[0056] After inserting the specimen in the vacuum chamber and having adjusted the working distance to 10 mm, the electron ray was aligned to use the lowest possible magnification, 130×. The strait scanning line was chosen with as few pores as possible (deep pores could be capturing photons of importance). The scanning time was set to 1 min.

[0057] The results are presented in FIGS. 1-6 and in table 4.

[0058] The pore size analysis was carried out on a Light Optical Microscope (LOM) at a magnification of 100× with the aid of a digital video camera and a computer based software, Leica QWin. The module in the software called “Largest Pore Measurement” was used. The total measured area is 26.7 mm.sup.2 corresponding to 24 measure fields.

[0059] All specimens were measured with a horizontal press orientation and a side way stepping of the cross section.

[0060] The software was operating in black and white mode and detected pores using “detection of black area in measured area”, where black area is equal to pores.

[0061] The following table 4 shows the results from the measurements.

TABLE-US-00004 Mini- Largest Largest Maxi- % of mum Diffusion pore pore mum Cu nominal Cu bonded length area content Cu content powders [μm] [μm.sup.2] [%] content [%] ac Invention 144 3196 5.5 183 0.7 bc Invention 142 3130 5.9 197 0.0 bd Comparative 199 9034 8.1 270 0.0 be Comparative 160 5128 7.5 250 0.0 ad Comparative 178 8515 7.3 243 0.0 ae Comparative 162 5070

[0062] From table 4 it can be concluded that components made from the diffusion bonded powders according to the invention show smaller largest pore areas and less variation in copper content compared to the comparative examples. It can further be concluded that when iron powder having higher oxygen content is used for producing the diffusion bonded powder according to the invention, the variation of copper content is less compared to when using iron powder having low oxygen content (ac-bc)

Example 2

[0063] Four different iron-based powder compositions were prepared by mixing four different copper containing powders at an addition corresponding to 2 weight % copper in the metal powder composition with the atomized iron powder ASC100.29, available from Höganäs AB, Sweden, 0.5% of synthetic graphite F10 from Imerys Graphite & Carbon, and 0.9% of the lubricant described in the patent publication WO2010-062250.

[0064] The copper containing powders used were: [0065] The diffusion bonded powder ac according to Example 1. [0066] Distaloy® ACu, available from Höganäs AB Sweden. Distaoy® ACu is an iron powder having 10% of copper diffusion bonded on the surfaces if the iron powder. [0067] Cu-200, the elementary Cu powder described in table 2. [0068] Cu-100, the elementary Cu powder described in table 2.

[0069] The following table 5 shows the copper containing powders used and the content of the ingredients in the metal powder compositions.

TABLE-US-00005 TABLE 5 Copper Iron-based con- powder taining Graph- Lubri- composi- Copper con- powder ASC100.29 ite cant tion No. taining powder [%] [%] [%] [%] 1 ac 66.7 31.9 0.5 0.9 2 Distaloy ®ACu 20 78.6 0.5 0.9 3 Cu-200 2 96.6 0.5 0.9 4 Cu-100 2 96.6 0.5 0.9

[0070] The iron-based powder compositions were compacted into test bars at 700 MPa according to IDO3928. After compaction the ejected green test bars were sintered in an atmosphere of 90/10 N.sub.2/H.sub.2 at a temperature of 1120° C. during 30 minutes and cooled to ambient temperature. Thereafter the test bars were subjected to through hardening at 860° C. for 30 minutes at an atmosphere with a carbon potential of 0.5%, followed by quenching in oil.

[0071] The heat treated test bars were tested for fatigue strength at R=−1 with a run out limit of 2×106 cycles according to MPIF standard 56. The endurance limit was determined at 50% probability of survival.

[0072] The following table 6 shows he results from the fatigue test.

TABLE-US-00006 TABLE 6 Test bars made from Iron-based Fatigue strength 50% powder composition No. probability [MPa] 1 352 2 328 3 327 4 320

[0073] Table 6 shows that samples made from an iron-based powder mixture containing the diffusion alloyed powder according to the invention exhibits increased fatigue strength compared to samples made from iron-based powder mixtures containing elemental copper powders or known copper containing diffusion bonded powders.

EMBODIMENTS

[0074] 1. An iron based powder consists of particles of reduced copper oxide diffusion bonded to the surface of an atomized iron powder wherein the content of copper is 1-5%, preferably 1.5-4% and most preferably 1.5-3.5% by weight of the iron based powder.

[0075] 2. An iron based powder according to embodiment 1 wherein the maximum particle size is 250 μm, at least 75% is below 150 μm and at most 30% is below 45 μm, the apparent density is at least 2.70 g/cm.sup.3 and the oxygen content is at most 0.16% by weight and other inevitable impurities at most 1% by weight.

[0076] 3. An iron based powder according to embodiment 2 having an SSF-factor of at most 2.0, preferably at most 1.7 wherein the SSF-factor is defined as the quotation between the Cu content in weight % in the fraction of the iron based powder which passes a 45 μm sieve and the Cu content in in weight % in the fraction of the iron based powder which do not passes a 45 μm sieve.

[0077] 4. An iron based powder according to any of embodiments 1-3 characterized in that the maximum copper content in a cross section of a sintered component made from said iron based powder is at most 100% higher than the nominal copper content, preferably at most 80% higher than the nominal copper content wherein the sintered component is produced by mixing said iron-based powder with 0.5% of graphite, having a particle size, X90, of at most 15 μm measured with laser diffraction according to ISO 13320:1999, and 0.9% of the lubricant described in the patent publication WO 2010/062250 and the obtained mixture is transferred into a compaction die for production of tensile strength samples (TS-bars) according to ISO 2740:2009 and subjected to a compaction pressure of 600 MPa and the compacted sample is thereafter ejected from the compaction die and subjected to a sintering process at 1120° C. for a period of time of 30 minutes in an atmosphere of 90% nitrogen/10% hydrogen at atmospheric pressure and the maximum copper content is determined through lines scanning in a Scanning Electron Microscope (SEM) equipped with a system for Energy Dispersive Spectroscopy (EDS) wherein the magnification is 130×, working distance is 10 mm and the scanning time is 1 minute.

[0078] 5. An iron based powder according to any of embodiments 1-4 characterized in that the largest pore area in a cross section of a sintered component made from said iron based powder is at most 4 000 μm.sup.2 wherein the sintered component is produced by mixing said iron-based powder with 0.5% of graphite, having a particle size, X90, of at most 15 μmeasured with laser diffraction according to ISO 13320:1999, and 0.9% of the lubricant described in the patent publication WO2010-062250 and the obtained mixture is transferred into a compaction die for production of tensile strength samples (TS-bars) according to ISO 2740:2009 and subjected to a compaction pressure of 600 MPa and the compacted sample is thereafter ejected from the compaction die and subjected to a sintering process at 1120° C. for a period of time of 30 minutes in an atmosphere of 90% nitrogen/10% hydrogen at atmospheric pressure and the largest pore area is determined in a Light Optical Microscope (LOM) at a magnification of 100× with the aid of a digital video camera and a computer based software and the total measured area is 26.7 mm.sup.2.

[0079] 6. An iron-based powder composition containing or consisting of 10 to 99.8 weight % of the iron based powder according to any of embodiments 1-5, optionally graphite up to 1.5% weight % and when graphite is present the content is 0.3-1.5 weight %, preferably 0.15-1.2 weight %, 0.2 to 1.0 weight % of lubricant and up to 1.0 weight % of machinability enhancing additives, balanced with iron powder.

[0080] 7. An iron-based powder composition containing or consisting of 50 to 99.8 weight % of the iron based powder according to any of embodiments 1-5, optionally graphite up to 1.5% weight % and when graphite is present the content is 0.3-1.5 weight %, preferably 0.15-1.2 weight %, 0.2 to 1.0 weight % of lubricant and up to 1.0 weight % of machinability enhancing additives, balanced with iron powder.

[0081] 8. A process for producing an iron based powder comprising the following steps;

[0082] providing an iron powder having a content of oxygen of 0.3-1.2% by weight, a content of carbon of 0.1-0.5% by weight, a maximum particle size of at most 250 μm and at most 30% by weight below 45 μm and providing a copper containing powder having a maximum particle size, X90 of at most 22 μm and a weight average particle size, X50, of at most 15 μm, preferably at most 11 μm,

[0083] mixing said iron powder and said copper containing powder,

[0084] subjecting said mixture to a reduction annealing process in a reducing atmosphere at 800-980° C. for a period of 20 minutes to 2 hours,

[0085] and crushing the obtained cake and classifying into desired particle size.

[0086] 9. A process for making a sintered component comprising the steps of

[0087] providing an iron based powder composition according to any of embodiments 6-7,

[0088] subjecting the iron based powder composition to a compaction process at a compaction pressure of at least 400 MPa and ejecting the obtained green component,

[0089] sintering said green component in a neutral or reducing atmosphere at a temperature of about 1050-1300° C. for a period of time of 10 to 75 minutes,

[0090] optionally hardening the sintered component in a hardening process such as case hardening, through hardening, induction hardening, or a hardening process including gas or oil quenching.

[0091] 10. A sintered component made according to embodiment 9.

[0092] 11. A sintered component according to embodiment 10 characterized in that the maximum copper content in a cross section is at most 100% higher than the nominal copper content, preferably at most 80% higher than the nominal copper content wherein the maximum copper content is determined through lines scanning in a Scanning Electron Microscope (SEM) equipped with a system for Energy Dispersive Spectroscopy (EDS) wherein the magnification is 130×, working distance is 10 mm and the scanning time is 1 minute.

[0093] 12. A sintered component according to embodiment 10 or 11 characterized in that the largest pore area is at most 4,000 μm.sup.2 wherein the largest pore area is determined in a Light Optical Microscope (LOM) at a magnification of 100× with the aid of a digital video camera and a computer based software and the total measured area is 26.7 mm.sup.2.