PL208668B1 - Method of preparing iron-based components by compaction with elevated pressures - Google Patents

Abstract

The present invention concerns a process for the preparation of high density green compacts comprising the steps of providing an iron-based powder essentially free from fine particles; optionally mixing said powder with graphite and other additives; uniaxially compacting the powder in a die at a compaction pressure of at least about 800 MPa and ejecting the green body. The invention also concerns the powder used in the method.

Description

Description of the invention

The present invention relates to iron-based green compacts useful in the powder metallurgy (PM) industry. More particularly, the invention relates to a method for the production of high-density iron-based green compacts.

In the production of structural members, many advantages come from the powder metallurgical methods as compared to the conventional methods of producing solid steel. And yes, there is much less energy consumption and a much better use of materials. Another important factor in favor of the powder metallurgy method is that the finished shapes or near-finished shapes can be produced immediately after the sintering process without costly shaping processes such as turning, milling, boring or grinding. Typically, however, the solid steel material has better mechanical properties compared to PM components produced by powder metallurgy methods.

This is mainly due to the porosity phenomenon in the PM elements. Therefore, attempts have been made to increase the density of the PM elements in order to obtain values as close as possible to the density values of the solid steel.

Among the methods used to obtain a higher density of PM elements, the method of forging sintered forgings has the advantage that completely solid elements can be obtained. The method, however, is costly and is mainly used in mass production of heavier components such as tie rods. Solid materials can also be obtained using elevated pressures at high temperatures, such as in hot isostatic pressing, HIP, but also this method is expensive.

In a method in which pressing is carried out at elevated temperature, typically 120 to 250 ° C, by using hot pressing, the density can be increased by about 0.2 g / cm ³ , resulting in a significant improvement in the mechanical properties of the material. A disadvantage, however, is that the hot pressing method requires additional investment and processing. Other methods, such as double pressing, double sintering, sintering at elevated temperatures etc., can further increase the density. Also, these methods will result in increased production costs and hence a reduction in overall economic efficiency.

In order to expand the market for powder metallurgical components and to take advantage of the advantages of this technique, there is a need for a simple, inexpensive method of obtaining high density compacts with improved static and dynamic strength properties.

It has been found that high-density elements can be obtained by using high compaction pressures in combination with coarse powders. This conclusion is quite unexpected since it is generally known that the powders commonly used, i.e. those containing fine grains, cannot be compressed to high densities without problems, e.g. in the form of damaged surfaces in compacts or which have deteriorated. More particularly, the method of the present invention comprises the steps of providing an iron-based powder substantially free of fine grains; optionally mixing the above-mentioned powder with graphite and other additives; uniaxial pressing of the powder in a die under high pressure and ejection of the compact, which can then be sintered.

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The term "high density" is intended to mean compacts having a density of about at least 7.3 g / cm ³ . Of course, parts having lower densities can also be manufactured, but are considered less interesting.

The iron-based powder of the present invention includes pure iron powder, such as sputtering iron powder, sponge iron powder, reduced iron powder, partially dispersed molten steel powder, and fully molten steel powder. The partially dispersed molten steel powder is preferably a partially melted steel powder with one or more of the following elements: Cu, Ni, Mo. The fully melted steel powder is preferably a steel fused with Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S and B. Stainless steel powders are also of interest.

As to the grain shape, it is preferable that the grains have an irregular shape, such as is obtained by spraying with water. Also, sponge iron powders having irregularly shaped grains may be of interest.

It is a feature of the invention that the powder used is coarse grains, i.e. the powder is substantially free of fine grains. The term "substantially free of fine grains" is intended to mean that less than 5% of the powder grains are less than 45 μm, as measured by the method described in SS-EN 24 497. So far, the most interesting results have been obtained with powders

Consisting essentially of grains greater than 106 µm and more particularly greater than 212 µm. The term "essentially consisting" is intended to mean that at least 50%, preferably at least 60% and most preferably at least 70% of the grains have a grain size greater than 106 and 212 µm, respectively. The maximum grain size may be 2 mm. The grain size distribution for iron-based powders used in PM production is typically a Gaussian distribution with an average grain diameter in the region of 30 to 100 µτη and about 10-30% of the grains have an average grain size less than 45 µ ^ ι. Iron-based powders substantially free of fine grains can be obtained by removing the finer fractions of the powder or by producing a powder having a desired grain size distribution.

The influence of the grain size distribution and the influence of the grain shape on the pressing parameters and the properties of the pressed mass were thoroughly investigated. Thus, US Patent 5,594,186 describes a method of producing PM (powder metallurgy) elements having a density greater than 95% of the theoretical density using virtually linear, acicular metal grains having a triangular cross-section. Such grains are suitably produced by machining or milling.

Coarse grained powders are also used in the manufacture of soft magnetic parts. Thus, US Patent 6,309,748 discloses a ferromagnetic powder, the grains of which have a diameter size in the range of 40 and 600 µm. In contrast to the iron-based powder grains of the present invention, these powder grains are provided with a coating.

US 4,190,441 discloses a powder composition for producing sintered soft magnetic parts. In accordance with this description, the iron powder comprises grains less than 5% of which exceed 417 µ ^ ι and less than about 20% of the powder grains have a size less than 147 µηι. The description states that, due to the very low content of grains smaller than 147 μ ^ ι, the mechanical properties of elements made of this coarse-grained, very pure powder are very poor. Moreover, the specification teaches that if greater strength is desired, it is not possible to increase the content of grains having a size less than 147 µm without compromising the properties of soft magnetic materials. Therefore, this powder is mixed with specific amounts of ferrophosphorus. Graphite that can be used in the present invention is not mentioned in this patent, and the presence of graphite would deteriorate the magnetic properties.

Powder mixtures containing coarse grains are also disclosed in US 5,225,459 (EP 554 009), which also relates to powder mixtures for the production of soft magnetic elements. Also, these powder mixtures do not contain graphite.

It is further known in the field of forging sintered forgings that coarse grained pre-melted iron powders can be used. US Patent 3,901,661 discloses such powders. The description states that a lubricant may be included, and more particularly that the amount of lubricant should be 1% by weight (example 1). If the powders of the present invention were mixed with such a large amount of lubricant, it would be impossible to obtain high densities.

In order to obtain compacts having satisfactory mechanical properties with respect to the sintered part of the present invention, it is necessary to add some graphite to the compacted powder mixture. Thus, graphite may be added before the pressing in amounts between 0.1-1.0, preferably 0.2-1.0 and most preferably 0.2-0.8% by weight, based on the total mixture which is pressed. .

Other additives may be added to the iron-based powder prior to pressing, such as alloying elements including Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S and B. These alloying elements may be added in amounts up to 10% by weight. Further additives are compounds improving the machinability, hard phase material and flow improvers.

The iron-based powder can also be combined with the lubricant before being transferred to the die (internal lubrication). The lubricant is added to minimize friction between the grains of the metal powder and between the grains and the die during the pressing or squeezing step. Examples of suitable lubricants are, for example, stearates, waxes, fatty acids and their derivatives, oligomers, polymers and other organic lubricants. The lubricants are preferably added in the form of grains, but may also be combined and / or applied to the grains. 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.

The method according to the invention can also be carried out using external lubrication (die wall lubrication), where before the pressing is carried out on the die wall

A lubricant is provided. A combination of external and internal lubrication can also be used.

The term "high pressure pressing" means pressing at pressures of at least 800 MPa. More interesting results are obtained at higher pressures such as pressures above 90 MPa, preferably above 1000 MPa, more preferably above 1100 MPa.

Conventional pressing at high pressures, e.g. pressures above 800 MPa, with the commonly used powders containing finer grains, in admixture with small amounts of lubricants (less than 0.6% by weight), is generally considered unsuitable due to the high forces. required for pushing out the moldings from the die, attendant high die wear and the fact that the surfaces of the moldings tend to be less shiny or deteriorate. By using the powders in accordance with the present invention, it has surprisingly been found that the ejection force is lower at high pressures, around 1000 MPa, and that components having acceptable or even perfect surfaces can be obtained, also when no die wall lubrication is used.

The pressing can be performed with standard equipment, which means that the new method according to the present invention can be carried out without expensive investment. Pressing is performed uniaxially, in one step, at ambient or higher temperature. Alternatively, the pressing can be performed with an impact device (Model HYP 35-4 from Hydropulsor) disclosed in WO 02/38315.

Sintering can be performed at temperatures conventionally used in the field of PM-powder metallurgy, e.g. at standard temperatures between 1080 and 1160 ° C or at higher temperatures above 1160 ° C and in commonly used atmospheres.

Other treatments for the green or sintered part can also be used, such as machining, surface hardening, surface densification or other methods used in PM technology.

Briefly, the advantages obtained by using the method of the present invention are that high-density green compacts can be produced economically. The new method also makes it possible to produce better parts which are difficult to produce with the conventional technique. In addition, standard pressing equipment can be used to produce high-density compacts having an acceptable or even excellent surface finish.

Examples of products that can suitably be produced using the new method are connecting rods, gears and other structural elements subjected to high loads. Of particular interest is the use of stainless steel powder flanges.

The following examples further illustrate the invention.

P r z k ł a d 1

Two different iron-based powder compositions prepared according to the present invention were compared with the standard iron-based powder compositions. All three compositions were made with Astaloy Mo available from Hoganas AB, Sweden. 0.2% by weight of graphite and 0.4% by weight of a lubricant (Kenolube ™) were added to the composition. In one of the iron-based powder compositions according to the invention, Astaloy Mo grains smaller than 45 µm in diameter were removed, and in the other composition according to the invention, Astaloy Mo grains smaller than 212 µm were removed. Ironing was performed at ambient temperature using standard equipment. As can be seen from diagram 1-1 for a powder having a grain size greater than 212 µm, a marked increase in density is obtained at all compaction pressures.

Figure 1-2 shows that if the goal is to obtain components without deteriorated surfaces, the most important factor is the reduction or elimination of the smallest grains, e.g. grains below 45 µ ^ ι. In addition, it can be seen from this diagram that the force needed to eject the compacts produced using the iron-based powder composition without grains smaller than 212 μ ^ ι was significantly lower compared to the ejection force needed for the compacts made of the standard iron-based powder composition. having about 20% of grains smaller than 45 μ ^ ι. The ejection force needed for compacts made from the iron-based powder composition of the invention without grains smaller than 45 µm is also lower compared to the standard powder. In the graphs, the standard powder is marked as (- ▲ -) Std., Or (- • -) according to the invention (- ♦ -) + 2T27 (——) +212.

PL 208 668 B1

A noteworthy phenomenon is that the ejection force for compacts made in accordance with the present invention (using powder with grains greater than 212 μm and powder with grains greater than 45 μm) decreases with increasing ejection pressure, while with the standard formulation the opposite is true.

It was also found that compacts obtained when standard powder was compressed at pressures above 700 MPa had surfaces that deteriorated and therefore were not acceptable. The compacts which were obtained when a powder substantially free of grains smaller than 45 μm was compressed at a pressure above 700 MPa had a less shiny surface which is at least acceptable under some circumstances.

P r z k ł a d 2

Example 1 was repeated, but 0.5% EBS (Ethylene Bisstearamide) was used as the lubricant, and compression was performed with an impact device (Model HYP 35-4 from Hydropulsor, Sweden).

From graphs 2-1 and 2-2, respectively, it can be seen that in the case of the powder compositions according to the invention, higher green densities and lower ejection forces were obtained compared to the powder composition with the standard powder. It can also be seen that items made of standard powder had surfaces that deteriorated at all pressing pressures.

Claims (10)

A method for producing high-density green compact parts comprising the following steps: - providing a sputtering iron powder optionally containing elements selected from the group consisting of; Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S and B where less than 5% of the iron-based powder grains are less than 45 µm; - mixing said powder with 0.1-1.0% by weight graphite and with a lubricant in an amount between 0.05 and 0.6% by weight, and optionally with machinability enhancers, hard phase materials and flow improvers; uniaxially compressing the powder in the die at a compaction pressure of at least 800 MPa and - pushing out the molded part from the die. 2. The method according to p. The process of claim 1, wherein pressing is performed in one step. 3. The method according to p. The iron-based powder as claimed in claim 1 or 2, characterized in that at least 50%, preferably at least 60%, most preferably at least 70% of the iron-based powder consists of grains having a grain size greater than 106 µm. 4. The method according to p. The iron-based powder as claimed in any one of the preceding claims, characterized in that at least 50%, preferably at least 60% and most preferably at least 70% of the iron-based powder consists of grains having a grain size greater than 212 µm. 5. The method according to p. The process of claim 4, wherein the maximum grain size is 2 mm. 6. The method according to any of claims A process as claimed in any of the claims 1 to 5, characterized in that the pressing is carried out in a die coated with a lubricant. 7. The method according to any of claims A process as claimed in any of the claims 1 to 6, characterized in that pressing is carried out at a pressure of at least 900 MPa, more preferably at least 1000 MPa, most preferably above 1100 MPa. 8. The method according to any of claims A process as claimed in any of the claims 1 to 7, characterized in that ironing is carried out at ambient temperature. 9. The method according to any one of claims 1 to 9 A process as claimed in any of the claims 1 to 8, characterized in that pressing is carried out at elevated temperature. The method according to any of claims 1 to 10 according to any of the preceding claims, further comprising one sintering step at a temperature above 1100 ° C for producing sintered products.