KR20110114689A - 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

Manufacturing method of iron-based parts by high pressure molding {METHOD OF PREPARING IRON-BASED COMPONENTS BY COMPACTION WITH ELEVATED PRESSURES}

The present invention relates to metal powder compositions useful in the powder metallurgy industry. More specifically, the present invention relates to a method for producing a component having a high density by using these compositions.

Compared with the corresponding conventional full dense steel process, several advantages exist by using powder metallurgical methods to manufacture structural parts. Therefore, energy consumption is much smaller and material utilization is much greater. Another important factor when using powder metallurgical routes is that the net or nearly net shaped parts are sintered without expensive molding processes such as turning, milling, boring or grinding. It can be produced immediately. In general, however, fully dense steel materials have good mechanical properties compared to PM parts. This is mainly because voids occur in PM parts. Therefore, efforts have been made to increase the density of PM components in order to reach values as close as possible to the density values of fully dense steels.

Among the methods used to obtain higher density PM parts, the powder forging process has the advantage that a fully dense part may be obtained. However, the process is mainly used for mass production of expensive and heavier parts such as connecting rods. Fully dense material can be obtained by high pressure at high temperatures, such as in hot isostatic molding (HIP), but this method is expensive.

By using warm compaction, a process in which the molding is carried out at high temperatures, generally from 120 to 250 ° C., the density can be increased to about 0.2 g / cm 3, which leads to a significant improvement in mechanical properties. However, a disadvantage is that the warm forming method involves additional investment costs and processes. Other processes such as double pressing, double sintering, sintering at high temperature, etc. may also increase the density. These methods will also incur additional manufacturing costs and reduce the overall cost efficiency.

There is a need for a simple and low cost method of achieving high density compacts with improved static and dynamic mechanical strength in order to expand the marketability of powder metallurgical parts and to take advantage of powder metallurgy techniques.

It has now been found that high density parts can be obtained using high forming pressures with granulated powder. According to the common knowledge that conventionally used powders, ie powders containing fine particles, cannot be formed at high density without the problem of having a damaged or degraded surface of the shaped body, for example, this finding is quite unexpected. In particular, the method according to the invention basically comprises providing an iron-based powder free of particulates; Optionally mixing the powder with graphite and other additives; Molding the powder uniaxially at high pressure in a die and injecting a green body, which may be subsequently sintered.

The term "high density" means a shaped body having a density of at least about 7.3 g / cm 3. Also parts with lower densities can be made but that is not of interest here.

Iron-based powder according to the present invention is pure iron powder, such as atomized iron powder, spongy iron powder, reduced iron powder; Partially diffused alloy steel powder; And fully alloyed steel powders. The partially diffusion alloy steel powder is preferably a steel powder partially alloyed with at least one of Cu, Ni and 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 particle morphology, the particles preferably have an irregular shape as obtained by water atomisation. Also sponge iron having irregularly shaped particles may be of interest.

An important feature of the present invention is that the powder used has coarse particles, ie the powder is essentially free of particulates. The expression “basically free of particles” means that less than about 5% of the powder particles have a size of less than 45 μm as measured by the method described in SS-EN 24 497. The most interesting results so far have been achieved with powders consisting essentially of particles larger than about 106 μm and especially greater than about 212 μm. The expression “consist essentially of” means that at least 50%, preferably at least 60%, and most preferably at least 70% of the particles have a particle size of greater than 106 and 212 μm, respectively. The maximum particle size may be about 2 mm. The particle size distribution in the iron-based powders used for PM production is generally a Gaussian distribution with an average particle diameter in the range of 30 to 100 μm and about 10-30% below 45 μm. Basically iron-based powder free of fine particles may be obtained by removing finer portions of the powder or by preparing a powder having a desired particle size distribution.

The influence of particle size distribution and the shape of particles on the molding properties and the properties of the molded body have been extensively studied. U. S. Patent No. 5,594, 186 discloses a method for producing PM parts having a density of at least 95% of theoretical density by using a substantially linear, acicular metal powder having a triangular cross section. Such particles are suitably produced by a machining or milling process.

Powders with coarse particles are also used to make soft magnetic parts. U. S. Patent No. 6,309, 748 discloses a ferromagnetic powder, the particles having a diameter size in the range of 40 to 600 mu m. In contrast to the iron-based powder particles according to the invention, these particles are provided with a coating.

U. S. Patent No. 4,190, 441 discloses powder compositions for the production of sintered soft magnetic parts. In this patent the iron powder comprises less than 5% of particles greater than 417 μm, and less than about 20% of the powder particles have a size of 147 μm or less. According to the patent, due to the very low content of particles less than 147 μm, the mechanical properties of the parts made from the very high purity powder of the assembly are very low. Moreover, according to this patent, when greater strength is required, it is not possible to increase the particle content having a size of less than 147 μm without simultaneously deteriorating the soft magnetic properties. The powder is therefore mixed with a certain amount of ferrophosphorus. Graphite, which may be used in the composition according to the invention, is not mentioned in the above patent and will degrade magnetic properties in addition to the presence of graphite.

Powder mixtures comprising coarse particles are also disclosed in US Pat. No. 5,225,459 (EP 554 009) on powder mixtures for the production of soft magnetic parts. These powder mixtures also do not contain graphite.

It is further known in the field of powder forging that prealloyed iron-based powders with coarse particles can be used. U. S. Patent No. 3,901, 661 discloses such powders. According to this patent a lubricant may be included and in particular the amount of lubricant should be 1% by weight (Example 1). If the powder according to the invention is mixed with such a large amount of lubricant it may not be possible to achieve high density.

The sintered part according to the invention needs to be added to the powder mixture in which a certain amount of graphite is molded in order to obtain a shaped body having satisfactory mechanical sintering properties. Therefore, graphite in the range of 0.1-1, preferably 0.2-1.0 and most preferably 0.2-0.8% by weight of the total mixture to be molded may be added before molding.

Other additives such as alloying elements including Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S, and B may be added to the iron-based powder before molding. These alloying elements may be added up to 10% by weight. Still other additives are machinability enhancing agents, hard phase materials and flow agents.

The iron-based powder may be combined with a lubricant before being delivered to the die (internal lubrication). Lubricants are added to minimize friction between the metal powder particles and between the particles and the die during the molding or pressing step. Examples of suitable lubricants are, for example, stearic acid, waxes, fatty acids and their derivatives, oligomers, polymers and other organic materials with lubricating effects. The lubricant is preferably added in the form of particles but may also be bound and / or coated on the particles. The amount of lubricant added to the iron-based powder according to the invention may range from 0.05 to 0.6% by weight, preferably 0.1 to 0.5% by weight of the mixture.

The method according to the invention may also be carried out using external lubrication (die wall lubrication) where the wall of the die is provided with lubricant before the molding is carried out. Combinations of external and internal lubrication may also be used.

The expression "high molding pressure" means a pressure of about 800 MPa or more. More interesting results are obtained with higher pressures, such as at least 900, preferably at least 1000, more preferably at least 1100 MPa.

In the state of mixing with a small amount of lubricant (less than 0.6% by weight), it is generally considered inappropriate to mold conventionally used powders containing particulates at high pressure, i. This is due to the large force required to inject, the enormous wear of the die and the fact that the surface of the part tends to be shiny or degraded. By using the powders according to the invention unexpectedly the injection force is reduced at high pressure, about 1000 MPa, and parts with acceptable or even perfect surfaces may be obtained even when die wall lubrication is not used.

Molding may be carried out with standard equipment, which means that the new process may be carried out without expensive costs. Molding is performed uniaxially in a single step at room temperature or higher. Alternatively molding may be carried out with the aid of a percussion machine (model HYP 35-4 made from Hydropulsor) disclosed in patent publication WO 02/38315.

Sintering may be performed at temperatures generally used in the PM art, for example at standard temperatures in the range from 1080 to 1160 ° C or higher temperatures above 1160 ° C and conventionally used atmospheres.

Other treatments of green or sintered parts may also be applied, such as machining, surface treatment, surface densification or other methods used in PM technology.

In short, the advantage obtained by using the process according to the invention is that a high density green molded body can be produced at low cost. The novel method also allows the manufacture of larger parts that are difficult to manufacture by using conventional techniques. In addition, standard molding equipment can be used to produce high density molded articles having acceptable or even perfect surface finish.

Examples of products that can be properly produced by the novel method are connecting rods, gears and other structural parts subjected to high loads. By using stainless steel powder, the flange is of particular interest.

The invention is illustrated in more detail by the following examples.

1A is a graph depicting green density versus forming pressure in three different iron based powder compositions,
1B is a graph showing injection force versus molding pressure in three different iron based powder compositions,
FIG. 2A is a graph depicting green density versus molding pressure in two different iron based powder compositions
2B is a graph showing injection force versus molding pressure in two different iron based powder compositions.

Example  One

Two different iron based powder compositions according to the invention were compared with standard iron based powder compositions. All three of these compositions were made from Asalloy Mo, manufactured by Hoganas Avesa, Sweden, and 0.2 wt% graphite and 0.4 wt% lubricant (Kenolube ^™ ) were added to the compositions. In one of the iron-based powder compositions according to the present invention, the atalloy Mo particles having a diameter of less than 45 μm were removed and less than 212 μm of the atalloy Mo particles were removed from the other composition according to the present invention. Molding was performed at room temperature and standard equipment. As can be seen in FIG. 1A, a clear increase in density at all molding pressures is obtained with a powder having a particle size of greater than 212 μm.

1-2 show that the most important factor for obtaining a deteriorated surfaceless part is the reduction or removal of the smallest particles, ie, particles less than 45 μm. Furthermore, the injection force required for the molded article produced from the standard iron-based powder composition having about 20% of the particles having a particle size of less than 45 µm is about 20% of the particles produced by the iron-based powder composition having no particles less than 212 µm from FIGS. It can be seen that the comparison is considerably reduced. The injection force required for shaped bodies made from the iron-based powder compositions according to the invention without particles smaller than 45 μm is also reduced when compared to standard powders.

A notable phenomenon is that the injection force on the shaped bodies produced according to the invention decreases with increasing injection pressure, but the opposite applies for standard compositions.

It has been observed that when the standard powder is molded at a pressure above 700 MPa, the resulting body has a deteriorated surface and thus is unacceptable. Basically, the molded article obtained when the powder free of particles of less than 45 μm is molded at a pressure of more than 700 MPa has a less shiny surface, which is acceptable at least in some cases.

Example  2

Example 1 is repeated but 0.5% of EBS (ethylene bisstearamide) is used as a lubricant and molding is carried out with the help of a percussion machine (model HYP 35-4 made from Hydropulsor, Sweden). Was performed.

It can be seen from FIGS. 2-1 and 2-2, respectively, that greater green density and smaller injection force were obtained in the powder composition according to the invention as compared to the powder composition with standard powder. It can also be seen that parts made from standard powders have a degraded surface at all molding pressures.

Claims (17)

As a manufacturing method of a high density green molded object,
Providing iron or iron based powders, wherein less than 5% of the iron based powder particles have a size of less than 45 μm,
Mixing the powder with 0.1 to 1.0 wt% graphite and 0.05 to 0.6 wt% lubricant;
Molding the powder monoaxially in the die at a molding pressure of at least 800 MPa; And
Injecting a green body from the die,
Method for producing a high density green molded body. The method of claim 1,
Wherein the forming step is carried out in a single step,
Method for producing a high density green molded body. The method according to claim 1 or 2,
At least 50%, preferably at least 60%, and most preferably at least 70% of the iron-based powder consists of particles having a particle size of greater than 106 μm,
Method for producing a high density green molded body. The method of claim 3, wherein
At least 50%, preferably at least 60%, and most preferably at least 70% of the iron-based powder consists of particles having a particle size of greater than 212 μm,
Method for producing a high density green molded body. The method of claim 4, wherein
The maximum size of the particles is 2 mm,
Method for producing a high density green molded body. The method according to claim 1 or 2,
Wherein the forming step is performed in a lubricated die,
Method for producing a high density green molded body. The method according to claim 1 or 2,
From the group consisting of alloying elements comprising at least one of Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S, and B, machinability enhancers, hard phase materials and flow agents Mixing at least one selected additive with the powder prior to the forming step,
Method for producing a high density green molded body. The method according to claim 1 or 2,
Wherein the forming step is carried out at a pressure of at least 900 MPa, more preferably at least 1000 MPa, most preferably at least 1100 MPa,
Method for producing a high density green molded body. The method according to claim 1 or 2,
The forming step is carried out at room temperature,
Method for producing a high density green molded body. The method according to claim 1 or 2,
Wherein the forming step is carried out at a temperature higher than room temperature,
Method for producing a high density green molded body. The method according to claim 1 or 2,
Further comprising a single sintering step at a temperature above 1100 ° C. after the injection of the green body to produce a sintered product,
Method for producing a high density green molded body. The method according to claim 1 or 2,
At least 60% of the iron-based powder consists of particles having a particle size of greater than 106 μm,
Method for producing a high density green molded body. The method according to claim 1 or 2,
70% or more of the iron-based powder is composed of particles having a particle size of more than 106 μm,
Method for producing a high density green molded body. The method of claim 3, wherein
At least 60% of the iron-based powder consists of particles having a particle size of greater than 212 μm,
Method for producing a high density green molded body. The method of claim 3, wherein
70% or more of the iron-based powder is composed of particles having a particle size of more than 212 μm,
Method for producing a high density green molded body. The method according to claim 1 or 2,
Wherein the forming step is carried out at a pressure of 1000 MPa or more,
Method for producing a high density green molded body. The method according to claim 1 or 2,
Wherein the forming step is carried out at a pressure of 1100 MPa or more,
Method for producing a high density green molded body.

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