IRON BASED POWDER FIELD OF THE INVENTION The present invention relates to an iron based powder intended for the powder metallurgical manufacturing of components. The invention further relates to a method of manufacturing the iron based powder and a method for manufacturing a component from said iron based powder and an accordingly produced component.

BACKGROUND

In industry the use of metal products manufactured by compacting and sintering iron-based powder compositions is becoming increasingly widespread. The quality requirements of these metal products are continuously raised, and as a consequence, new powder compositions having improved properties are developed. Beside density, one of the most important properties of the final, sintered products is the dimensional change, which above all have to be consistent. Problems with size variations in the final product often originates from inhomogenities in the powder mixture to be compacted. Such inhomogenities may also lead to variations in mechanical properties of the final components. These problems are especially pronounced with powder mixtures including pulverulent components, which differ in size, density and shape, a reason why segregation occurs during handling of the powder composition. This

segregation implies that the powder composition will be non-uniformly composed, which in turn means that parts made of the powder composition exhibits varying dimensional change during its production and the final product will have varying properties. A further problem is that fine particles, particularly those of lower density such as graphite, cause dusting in the handling of the powder mixture.

Differences in particle size also create problems with the flow properties of the powder, i.e. the capacity of the powder to behave as a free-flowing powder. An impaired flow manifests itself in increased time for filling dies with powder, which means lower productivity and an increased risk of variations in density and composition of the compacted component, which may lead to unacceptable deformations after sintering.

Attempts have been made at solving the problems described above by adding various binding agents and lubricants to the powder composition. The purpose of the binder is to bind firmly and effectively the small size particles of additives, such as alloying components, to the surface of the base metal particles and, consequently, reduce the problems of segregation and dusting. The purpose of the lubricant is to reduce the internal and external friction during compaction of the powder composition and also reduce the ejection force, i.e. the force required to eject the finally compacted product from the die.

The most commonly employed powder compositions for manufacturing of components by compaction and sintering contains iron, copper and carbon, as graphite, in powder form. In addition, a powdered lubricant is also normally added. The content of copper is normally between 1 -5% by weight of the composition, the content of graphite between 0.3-1 .2% by weight and the content of lubricant is normally below 1 % by weight.

The alloying element carbon, as graphite, is normally present as discrete particles in the powder which particles may be bonded to the surface of the coarser, low carbon containing, iron- or iron based powder in order to avoid segregation and dusting. The option of adding carbon as a pre-alloyed element in the iron or iron based powder, i.e. added in the melt before atomization, is not an alternative as such high carbon containing iron or iron- based powder would be too hard and extremely difficult to compact.

The alloying element copper may be added in elemental form as a powder and optionally bonded to the iron or iron based powder by means of a binder. A more efficient alternative to avoid e.g. copper segregation and copper dusting is however to diffusion bond, partially alloy, copper particles to the surface of the iron or iron based powders. By this method an unacceptable increase of the hardness of the iron or iron-based powder is avoided which otherwise would be a consequence if copper was allowed to be totally alloyed, pre-alloyed, to the iron or iron- based powder.

Diffusion bonded powders where copper is diffusion bonded to the surface of the iron or iron- based powder have been known for decades. In the GB patent GB1 1 62702, 1 965, (Stosuy) a process for preparing a powder is disclosed. In this process alloying elements are diffusion-bonded, partially alloyed, to the iron powder particles. An unalloyed iron powder is heated together with alloying elements, such as copper and molybdenum, in a reducing atmosphere at a temperature below the melting point to cause partially alloying and agglomeration of the particles. The heating is discontinued before complete alloying and the obtained agglomerate is ground to a desired size. Also the GB patent GB1 595346, 1976, (Gustavsson), discloses a diffusion-bonded powder. The powder is prepared from a mixture of an iron powder and a powder of copper or easily reducible copper compounds. The patent application discloses an iron-copper powder having a content of 1 0% by weight of diffusion bonded copper. This master powder is diluted with plain iron powder and the resulting copper content in the powder composition is 2% respective 3% by weight of the powder composition.

Examples of other patent documents disclosing various copper containing diffusion bonded iron or iron - based powders are JP3918236B2 (Kawasaki), JP63-1 14903A (Toyota), JP8-092604 (Dowa), JP1 -290702

(Sumitomo).

The Kawasaki patent document describes a manufacturing method for manufacturing a diffusion bonded powder where atomized iron powder having an oxygen content of 0.3-0.9% and a carbon content less than 0.3% is mixed with a coarse metal copper powder having an average particle size of 20-1 00 μηι.

The Toyota patent application discloses a highly compressible metal powder consisting of a pre-alloyed iron powder having particles of copper diffusion bonded to its surfaces. The pre-alloyed iron powder is composed of 0.2-1 .4% Mo, 0.05-0.25% Mn and less than 0.1 % C, all percentage by weight of the pre-alloyed iron powder. The pre-alloyed iron powder is mixed with copper powder or copper oxide powder having a weight average particle size of at most 1 /5 of the weight average particle size of pre-alloyed iron powder, the mixture is heated whereby the copper particles are diffusion bonded to the pre-alloyed iron powder. The copper content of the resulting diffusion bonded powder is 0.5-5% by weight.

In the Dowa patent application, it is described a manufacturing method for producing a diffusion bonded copper containing iron powder wherein fin particulate copper oxide powder having a particle size of at most 5μηι and a specific surface area of at least 1 0m ² /g, is mixed with an iron containing powder. The mixture between the copper oxide powder and the iron containing powder is further subjected to a reducing atmosphere at a temperature between 700-950°C to reduce and deposit metallic copper on the iron powder surface at a content of 10-50% by weight of the resulting diffusion bonded powder.

The Sumitomo document discloses a diffusion alloyed iron powder having good compressibility suitable to be used for manufacturing compacted and sintered components having high strength, high toughness and excellent dimensional stability, without the need of using nickel as an alloying element. The diffusion alloyed powder is produced by mixing atomized iron powder with iron oxide powder, at a content of 2-35% by weight of the iron powder, and copper powder and optionally molybdenum powder. The mixture is subjected to a reduction heat treatment process whereby the alloying elements and the reduced iron oxide is diffusion bonded to the surface of the atomized iron powder. The amount of copper in the resulting diffusion bonded powder is 0.5- 4% by weight.

Although many attempts have been made in order to find a cost-effective diffusion- bonded copper containing iron powder for manufacturing pressed and sintered components, there is still a need for improving such powder with respect of cost and performance.

SUMMARY

The present invitation discloses a new diffusion- bonded powder consisting of an iron powder having 1 -5%, preferably 1 .5-4% and most preferably 1 .5-3.5% by weight of copper particles diffusion bonded the surfaces of the iron powder particles. The present invention also discloses a method for producing the diffusion-bonded powder as well as a method for manufacture of a component from the new diffusion-bonded powder and the produced component. DETAILED DESCRIPTION

Iron powder

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.

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.

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.

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

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

The copper containing powder used to produce the diffusion bonded powder is cuprous oxide, (CU2O) or cupric oxide (CuO), preferably cuprous oxide is used. The copper containing powder has a maximum particle size, X90, of 22 μηι, here defined as at least 90% of the particles are below the maximum particle size, and a weight average particle size, X50, of at most 15 μηπ, preferably at most 1 1 μηπ, determined with laser diffractometry according to ISO 13320 : 2003.

Diffusion- bonded powder

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 μηι and at least 75 by weight is below 150 μηι. At most 30% by weight is below 45 μηι. The particle size measured according to ISO4497 1 983.

The oxygen content in the new powder is at most 0.1 6% by weight and the amount of other inevitable impurities is at most 1 % by weight.

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

The diffusion bonded powder is characterized by having a degree of bonding of copper to the iron -based powder with a 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.

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 μηι and another fraction having a particle size of 45 μηι and above. This separation may be performed with a 45 μηι standard sieve (325 mesh). The procedure according to ISO 4497:1986 may be followed with the proviso that only one sieve, 45 μηπ, is used. The quotation between the copper content in the finer fraction which passes the 45 μηπ sieve, and the copper content in the coarser fraction which do not passes the 45 μηπ sieve, gives a value, degree of bonding or SSF-factor. SSF-factor=weight% Cu in the finer fraction, (-45 μηπ) / weight% Cu in the coarser fraction, (45 μηπ and above).

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

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.

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;

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 15μηι measured with laser diffraction according to ISO 13320:1999, and 0.9% of the lubricant described in the patent publication WO2010-062250.. 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 600MPa. The compacted sample is thereafter ejected from the compaction die and subjected to a sintering process at 1 120°C for a period of time of 30 minutes in an atmosphere of 90%nitrogen/10%hydrogen at atmospheric pressure.

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 130X, working distance is 10 mm and the scanning time is 1 minute.

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.

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 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 μηπ ² .

The pore size analysis is carried out on a Light Optical Microscope (LOM) at a magnification of 100X with the aid of a digital video camera and a computer based software. The total measured area is 26.7 mm 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. The following definitions is applied:

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

Largest pore area: The area of the largest pore from those measured in the fields. Manufacture of sintered component

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

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.

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 0.1 5-1 .2 weight%, 0.2 to 1 .0 weight% of lubricant, up to 1 .0 weight% of machinability enhancing additives, balanced with iron powder.

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 1050-1 300°C for a period of time of 1 0 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.

FIGURE LEGENDS

Figure 1 shows variation in copper content for sample ac.

Figure 2 shows variation in copper content for sample be.

Figure 3 shows variation in copper content for sample bd.

Figure 4 shows variation in copper content for sample be

Figure 5 shows variation in copper content for sample ad.

EXAMPLES

Example 1

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 μηι.

The following tables show raw materials used.

Table 1

Iron powder

Table 2

Copper containing powder

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

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 3

SSF-f actor Samples for measuring maximum pore size, maximum pore area and copper variation were prepared according to the procedure in the detailed description.

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

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, 130X. 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.

The results are presented in Figures 1 -6 and in table 4.

The pore size analysis was carried out on a Light Optical Microscope (LOM) at a magnification of 100X 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 26.7mm ² corresponding to 24 measure fields.

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

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.

The following table 4 shows the results from the measurements.

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

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 Hoganas 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.

The copper containing powders used were:

The diffusion bonded powder ac according to Example 1

Distaloy®ACu, available from Hoganas AB Sweden. Distaoy®ACu is an iron powder having 10% of copper diffusion bonded on the surfaces if the iron powder.

Cu- 200, the elementary Cu powder described in table 2. Cu- 100, the elementary Cu powder described in table 2.

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

The iron-based powder compositions were compacted into test bars at 700 MPa according to IS03928. After compaction the ejected green test bars were sintered in an atmosphere of 90/10 N2/H2 at a temperature of 1 120°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. The heat treated test bars were tested for fatigue strength at R=-1 with a run out limit of 2x10 ⁶ cycles according to MPIF standard 56. The endurance limit was determined at 50% probability of survival.

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

Table 6

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.