Iron-based powder composition including a silane lubricant

ES 2 348 522 T3

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to novel metal powder compositions useful for the powder metallurgy industry. The invention also relates to a process for the preparation of high-density metallic components by using these compositions.

There are several advantages to using powder metallurgical processes to produce structural parts compared to the corresponding conventional fully dense steel processes. Thus, the energy consumption is much lower and the material usage is much higher. Another important factor in favor of the powder metallurgical way is that the net-shaped or near-net-shaped components can be produced directly after the sintering process without costly shaping processes such as turning, milling, drilling or grinding. However, normally a fully dense steel material has superior mechanical properties compared to PM components. This is mainly due to the existence of porosity in the PM components. Therefore, an effort has been made to increase the density of the PM components in order to achieve values ​​as close as possible to the density value of a fully dense steel.

Among the procedures used to achieve a higher density of PM components, the powder forging process has the advantage that fully dense components can be obtained. The process is, however, expensive and is used primarily for mass production of heavier components, as can also be achieved by high pressure connecting rods.

Fully dense materials

ES 2 348 522 T3 high temperatures, as in hot isostatic pressing, HIP, but this process is also expensive.

By using hot compaction, a process in which compaction is carried out at elevated temperature, usually 120 to 250 ° C, the density can be increased by approximately 0.2 g / cm³, which leads to a considerable improvement in mechanical properties. A disadvantage is, however, that the hot compaction process involves additional investment and processing. Other processes, such as double pressing, double sintering, high temperature sintering etc., can further increase the density. Also these procedures will add more production costs thereby reducing overall profitability.

In order to expand the market for powder metallurgical components and to use the advantages of the powder metallurgical technique, there is therefore a need for a simple and less expensive process of achieving high density compact objects with improved mechanical properties.

Summary of the invention

It has now been unexpectedly found that high density components can be obtained by using high compaction pressures in combination with a new type of powder compositions.

According to one aspect of the present invention, there is provided a powder composition that includes an iron or iron-based powder and a lubricating amount of an alkyl alkoxy or polyether alkoxy silane, wherein the alkyl group of the alkyl alkoxy silane and the polyether chain of the polyether alkoxy silane includes between 8 and 30 carbon atoms, and the alkoxy group includes 1-3 carbon atoms.

ES 2 348 522 T3

The iron or iron-based powder particles of the composition according to the aforementioned aspect may have a particle size such that less than about 5% of the iron or iron-based powder particles have a size less than 45 pm.

According to another aspect of the present invention, there is provided a process for the preparation of high density non-sintered compact objects comprising the following steps: provision of an iron-based powder composition according to the aspect mentioned above; optional mixing of said composition with graphite and other additives; uniaxic compaction of the powder in a die at a compaction pressure of at least about 800 MPa; and expulsion of the unsintered body.

Detailed description of the invention

The term "high density" is intended to mean compact objects that have a density of at least about 7.3 g / cm³. High Density ”is not an absolute value. A typical density that can be obtained according to the prior art for components with a single press and with a single sinter is approximately 7.1 g / cm³. By using hot compaction an increase of approximately 0.2 g / cm can be achieved³.

In this context, the term "high density" is intended to mean compact objects having a density of approximately 7.35-7.65 g / cm.³ and higher, depending on the type and amount of additives used, and the type of iron-based powder used. Components having lower densities can of course be produced as well but are believed to be of less interest.

The iron-based powder according to the present invention includes pure iron powder, such as gas or water atomized iron powder, iron powder

ES 2 348 522 T3 fluffy, reduced iron powder; steel powder partially alloyed by diffusion; and fully alloyed steel powder. The partially diffusion-alloyed steel powder is preferably a steel powder partially alloyed with one or more of Cu, Ni, Mo. The fully alloyed steel powder is preferably a steel powder alloyed with Mn, Cu, Ni, Cr, Mo , V, Co, W, Nb, Ti, Al, P, S and B. Stainless steel powders are also of interest.

With regard to the shape of the particles, it is preferred that the particles have an irregular shape as obtained by spraying with water. Also fluffy iron powder has irregular shaped particles and can be of interest.

A characteristic of the invention is that the powder used has coarse particles, that is to say the powder is practically free of fine particles. The term "practically fine particle free" is intended to mean that less than about 5% of the iron or iron-based powder particles are less than 45 µm in size as measured by the procedure described in SS-EN- 24-497. Until now, the most interesting results have been achieved with powders consisting essentially of particles above about 106 pm and especially above about 212 pm. The term "consisting essentially of" is intended to mean that at least 40%, preferably at least 60% of the particles have a particle size above 106 and 212 pm, respectively. Until now, the best results have been obtained with powders having a mean particle size above about 212 pm and only less than 5% less than about 212 pm. The maximum particle size can be about 2mm. The size distribution of the

ES 2 348 522 T3 particles in iron-based powders used in the manufacture of PM are normally distributed with a Gaussian distribution with a mean particle diameter of about 30 to 100 pm and approximately 10-30% less than 45 pm. Iron-based powders practically free of fine particles can be obtained by removing the finer fractions of the powder or by manufacturing a powder having the desired particle size distribution.

The influence of particle size distribution and the influence of particle shape on compaction properties and compacted body properties have been the subject of intensive studies. Thus, US patent 5,594,186 discloses a process for producing PM components with a density greater than 95% of the theoretical density by using substantially linear acicular metal particles having a triangular cross section. Powders having coarse particles are also used in the manufacture of soft magnetic components as disclosed in eg US patents 6-309-748 and 4190-441.

A critical characteristic according to the invention to obtain high density products is the type and quantity of lubricant. It has thus been found that a specific type of lubricants that have not previously been used in connection with metal powders give very promising results. These lubricants belong to the group of alkylalkoxy or polyether silanes and more specifically to alkylalkoxy or polyether silanes in which at least one substituent on the Si atom is an alkyl group having at least 8 carbon atoms, in which the group alkyl can be interrupted by one or more O atoms. Compounds in which the group

ES 2 348 522 T3 alkyl includes one or more oxygen atoms used according to the present invention are referred to as polyether silanes. The chain length of the alkyl or polyether group is an important characteristic of the silanes used according to the present invention and influences the lubricating properties of the silane. So far it has been found that the most interesting results are obtained with alkyl or polyether chains having between 8 and 30, preferably between 10 and 24, carbon atoms. Preferably the silane is selected from the group consisting of octyl-tri-methoxy silane, hexadecyl-tri-methoxy silane, and polyethylene ether-trimethoxy silane with 10 ethylene ether groups.

In this context it can be mentioned that US patents 5766304, 5989304, 6139600, 6235076 and 6451082 disclose that very small amounts, that is, 0.05 or less% by weight of the total composition to be compacted, of organoalkoxysilanes can be use as surface treatment agents for iron powder or iron-based in combination with lubricating agents. The first four US patents discuss the following silane compounds: γ-methacryloxypropyl trimethoxy silane, γ-glycidoxypropyl trimethoxy silane, N-beta- (aminoethyl) -Ytrimethoxy silane, methyl trimethoxy silane, phenyl trimethoxy silane, and diphenoyl dimethoxy silane. In US patent 6451082 the compounds triphenylmethoxysilane, diphenyl dimethoxysilane, phenyltrimethoxysilane, isobutyltrimethoxysilane, and methyltriethoxysilane have been used. The type of organosilanes with lubricating effect used according to the present invention has therefore not been mentioned or discussed.

The organosilane with lubricating effect used according to the present invention is preferably used so that it dissolves or disperses in a suitable solvent, for

ES 2 348 522 T3 for example an organic solvent, such as acetone or ethanol. The obtained solution or dispersion is subsequently added to the iron-based powder during mixing and optional heating. The solvent is finally optionally evaporated in vacuo.

According to a preferred embodiment of the invention and contrary to common practice in powder metallurgy, where conventional PM lubricants are used in the iron powder mixture, or where a lubricant is used in combination with binder and / or surface treatments, as described described in the aforementioned US patents, iron or iron-based powder must not be mixed with a separate (conventional) lubricant before it is transferred to the die. It is also not necessary to use a

<td colspan="2">lubrication</td><td>external (lubrication</td><td>of</td><td>the</td><td>walls</td><td>of the</td>

<td>die)</td><td>in</td><td>the one provided</td><td>a</td><td>lubr</td><td>icante to</td><td>the</td>

<td>walls</td><td>of the</td><td>die before</td><td>that</td><td>I know</td><td>perform</td><td>the</td>

compaction. The invention however does not exclude the possibility of, when it is of interest, using a conventional internal lubrication (in an amount up to 0.5% by weight), an external lubrication or a combination of both.

For some applications it may be necessary to add smaller amounts of graphite to the powder mix to be compacted. Thus graphite in amounts between 0.1-1.0, preferably 0.2-1.0 and more preferably

Or, 3-0.8% by weight of the total mix to be compacted should be added prior to compaction.

Other additives that can be added to the iron-based powder prior to compaction as alloying elements comprising the machinability enhancing compounds Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al,

P, S and B, hard phase material and flow agents.

It is intended that the pressurized expression of

ES 2 348 522 T3 high compaction means at pressures of at least about 800 MPa. More interesting results are obtained with higher pressures such as pressures above 900, preferably above 1000, more preferably above 1100 MPa. Conventional compaction at high pressures, i.e. pressures above about 800 MPa with conventionally used powders that include finer particles, is generally considered inadequate due to the high forces required to eject compact objects from the die, the high supplemental wear die and the fact that component surfaces tend to be less shiny or deteriorated. By using the powders according to the present invention it has been unexpectedly found that the expulsion force is reduced at high pressures, about 1000 MPa, and that components having acceptable or even perfect surfaces can be obtained.

Compaction can be performed with standard equipment, which means that the new procedure can be performed without costly investments. Compaction is carried out uniaxically and preferably in a single stage at room or elevated temperature. Alternatively, compaction can be carried out with the aid of a percussion machine (Hydropulsor Model HYP 35-4) as described in patent publication WO-02/38315.

Sintering can be carried out at normally used temperatures within the PM range, for example at low temperatures like 1100-1140 ° C or higher temperatures like 1200-1300 ° C and in conventionally used atmospheres or vacuum.

Other treatments can also be applied to the raw or non-sintered component, such as machining the

ES 2 348 522 T3 not sintered, box carburizing, surface densification, steam treatment.

In summary the advantages obtained using the process according to the present invention are that high density non-sintered compacts can be produced economically. The new procedure also allows the production of higher components that are difficult to produce using conventional technique. Additionally, standard compaction equipment can be used to produce high-density compacts that have an acceptable or even perfect surface finish.

Examples of products, which can suitably be manufactured by the new process, high-performance structural parts such as connecting rods, cam lobes, gears and other structural components subjected to high loads. Due to the use of stainless steel powders the flanges are of special interest.

Since a main object of the present invention is to achieve high density products, silanes having a lubricating effect have been described especially in connection with coarse powders. However, it has also been found that these silanes can also be used in combination with powders that include higher amounts of fine particles, ie the type of powders that are conventionally used in the PM industry today. Example 4 cited below illustrates the effect of the silanes according to the present invention both in conventional powders and in coarse powders. As can be seen, very high densities are also obtained with a conventional powder that includes higher amounts of fine particles. Compositions that include iron-based or iron powders with the size distributions of the

ES 2 348 522 T3 customary particles and the silanes according to the present invention may be of particular interest for certain applications and are also within the scope of the invention.

The invention is further illustrated by the following examples.

Example 1

The iron-based powder composition prepared from AstaloyMo, which is a pre-alloyed iron-based powder alloyed with 1.5% by weight of molybdenum available from Hoganas AB, Sweden, and where particles less than 212 pm have been removed, it was mixed with 0.1 and 0.15%, respectively, of hexadecyl trimethoxy silane. The mixing process was carried out as follows: the hexadecyl trimethoxy silane was diluted in ethanol to a 20% solution, by weight, and the solution was stirred for 60 minutes. An amount of this solution corresponding to 0.1 and 0.15% by weight, respectively, was added during mixing to the iron-based powder mixtures, which had been previously heated to

75 ° C in the mixer. Intensive mixing was carried out in the same mixer for 3 minutes followed by mixing at a lower speed for 30 minutes and under vacuum in order to evaporate the

<td colspan="2">solvent. Mix</td><td>obtained</td><td>was sifted</td><td>with</td><td>a</td><td colspan="2">sieve</td>

<td>from 500 pm.</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Rings with</td><td>a</td><td>diameter</td><td>internal of</td><td> 35</td><td>mm</td><td>^Y</td><td>a</td>

<td>external diameter</td><td>of</td><td>14 mm and</td><td>a height</td><td>of</td><td> 10</td><td>mm</td><td>I know</td>

compacted in a uniaxic way in a single stage at different compaction pressures. As can be seen from figure 1-1, crude densities of 7.67 g / cm were obtained.³ at a pressure of 1100 MPa for both compositions. The total energy required for ejection is somewhat lower for compacts prepared from the 0.15% silane composition than

ES 2 348 522 T3 for the expulsion of the compacts prepared from the powder that had been treated with 0.1% by weight of silane, see figure 1-2.

Example 2

The same powder and procedure was used as in Example 1 except that the powder was mixed with 0.2% by weight of hexadecyl trimethoxy silane. Two compositions were prepared, one with 0.2% by weight of graphite and the other with 0.6% by weight of graphite. Green density and green strength were measured.

As can be seen from figure 2-2, a crude density above 7.65 g / cm was obtained.³ for a sintered component containing 0.2% graphite compacted at 1200 MPa. For a non-sintered component containing 0.6% graphite, a green density of 7.58 g / cm was obtained.³.

Figure 2-1 shows that green strength increases with increasing compaction pressure and that green strength is high enough to allow handling of unsintered components.

Example 3

This example shows the effect of removing different fractions of the iron-based powder. Four different iron-based powder compositions were tested. Three of the iron-based powder compositions contained Astaloy Mo which included 0.2% hexadecyl trimethoxy silane and the mixing procedure of Example 1 was used. The first composition contained Astaloy Mo thicker than 45 pm, the second composition contained Astaloy Mo thicker than 106 pm, and the third composition contained Astaloy Mo thicker than 212 pm. The fourth composition contained Astaloy Mo which had particles coarser than 212 pm. The particles of this composition were mixed with 0.1% by weight of hexadecyl

ES 2 348 522 T3 trimethoxysilane. Furthermore, all compositions contained 0.2% graphite. All compositions were uniaxically compacted in a single stage in a die forming rings with an external diameter of 35 mm, an internal diameter of 14 mm and a height of 10 mm.

Figure 3-1 shows that green densities increased and ejection forces decreased with increasing particle sizes.

Figure 3-2 shows that the ejection forces decrease when the amount of silane is increased from 0.1 to 0.2% by weight.

Example 4

This example demonstrates the effect of the chain length of the alkyl or polyether group, the particle size distribution, and the added amount of silanes on the lubrication properties on ejection after high pressure compaction. Two types of powder were used, namely a standard 100 mesh iron-based powder, Astaloy 85 Mo with about 20% of the particles less than 45 pm (S powder) and a powder having the same chemical composition without particles. fine and a weight average particle size of about 212 µm (powder C).

Five different types of silanes were used according to table a)

A Methyl-tri-methoxy silane

B Propyl-tri-methoxy silane

C Octyl-tri-methoxy silane

D Hexadecyl-tri-methoxy silane

E Polyethylene ether-trimethoxy silane with 10 ethylene ether groups

Different contents of silanes were added to the iron-based powder and the obtained mixtures were compacted at 1100 MPa in a uniaxic pressing movement into ingots with a diameter of 25 mm and a

ES 2 348 522 T3 height 12 mm. During ejection the dynamic ejection force was measured and after ejection the green surface finish was evaluated and the density was measured as shown in the table.

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<td>Dust S</td><td> 0,5%</td><td></td><td>29 kN Compliant 7.56 g / cm³</td><td>97 kN * Compliant 7.53 g / cm³</td><td></td><td></td>

<td>Dust S</td><td>o \ o or</td><td></td><td></td><td>66 kN Compliant 7.60 g / cm³</td><td></td><td></td>

<td>Dust C</td><td>o \ o or</td><td></td><td></td><td>37 kN Compliant 7.60 g / cm³</td><td></td><td></td>

<td>Dust S</td><td> 0,3%</td><td>3 8 kN Compliant 7.63 g / cm³</td><td>34 kN Compliant 7.62 g / cm³</td><td></td><td></td><td></td>

<td>Dust C</td><td> 0,3%</td><td>33 kN Compliant 7.66 g / cm³</td><td>36 kN Compliant 7.64 g / cm³</td><td>jamming</td><td>jamming</td><td>jamming</td>

<td>Dust S</td><td> 0,2%</td><td>65 kN Compliant 7.61 g / cm³</td><td>47 kN Compliant 7.63 g / cm³</td><td></td><td></td><td></td>

<td>Dust C</td><td> 0,2%</td><td>3 9 kN Compliant 7.67 g / cm³</td><td>46 kN Compliant 7.66 g / cm³</td><td></td><td></td><td></td>

<td>Dust C</td><td> 0,1%</td><td>39 kN Compliant 7.66 g / cm³</td><td>48 kN Conforming 7.65 g / cm³</td><td></td><td></td><td></td>

<td>Dust C</td><td> 0,05%</td><td>62 kN Compliant 7.67 g / cm³</td><td></td><td></td><td></td><td></td>

<td>Dust C</td><td> 0,03%</td><td>jamming</td><td></td><td></td><td></td><td></td>

<td></td><td>Silane</td><td>ω</td><td>Q</td><td>or</td><td>m</td><td>YES</td>

* Unstable value (Compliant = good / satisfactory surface finish;

binding - stuck component surface with rating marks)

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As can be seen from the table a chain length of at least 8 atoms in the alkylene chain is needed in order to successfully expel the component for an added amount of silanes of 0.050.5%. Amounts added above 0.5% are believed to be of less interest as the density of the unsintered component will be negatively influenced. The table also shows that when the silane content is less than 0.05% ejection without damaging the component and the die surface is not possible for silanes with a chain length of 30 atoms.

From the table below it can also be concluded that also powder with a standard particle size distribution can be compacted at high densities of 7.60 g / cm³ and higher, and successfully expelled, provided that the amount of silane added is less than 0.5% and the length of the aforementioned alkylene or polyethylene ether chain is more than 8 atoms.