EP0029389B1 - Process for producing shaped articles from spherical metal-particle powders - Google Patents

Description

The invention relates to the manufacture of non-fragile shaped parts from powders consisting of spheroidal metal particles.

This invention makes it possible to use these powders in applications where known techniques of mechanical compaction delivered drafts that are too fragile to be handled under industrial production conditions.

If it is desired to obtain sufficient mechanical strength of the part before sintering without it being necessary to make the powder compact by deliberate pressure, the invention can be applied according to Belgian patent BE-A-517440, in which the cohesion is obtained by bonding by mixing before sintering the metal powder with a cellulose-based cement, such as cellulose acetate for example. In this way, a clay mass is obtained, the shaping of which takes place without significant pressure, by simple molding.

On the other hand, to ensure, without cold bonding, the mechanical strength and the crumbling resistance of a tablet, a powder is generally used with an irregular grain shape, characterized by a specific surface related to the large particle size. The mechanical entanglement of grains in the compaction ensures sufficient mechanical strength for all the necessary manipulations of the tablet. These irregularly shaped grain powders are generally obtained by the water atomization process. This process consists of spraying a jet of liquid metal with one or more jets of water under pressure. The powder is sprayed into a water bath where it finishes cooling. After decantation, the powder is dried and undergoes a first deoxidation or reduction. The powder obtained has a particle size mainly below 160 micrometers. The grains have a shape depending on the shade and the atomization conditions, generally quite tormented. In return, this powder must undergo a reduction step, its composition of alloying elements must be limited to those whose oxides are easily reducible. For example for low alloy steels, the typical grade AISI 4600 contains 2% nickel and 0.8% molybdenum. Chromium-manganese grades, which are less expensive, can only be produced at the cost of an expensive carbon reduction treatment at 1200 ° C. followed by grinding of the sintered powder.

The neutral gas atomization process makes it possible to completely overcome this problem of composition. This process involves spraying a jet of liquid metal with several jets of pressurized gas. The solidification of the droplets and the cooling of the powder are carried out in a confined enclosure; the atomizing gas which fills the enclosure is neutral with respect to the atomized metal. Depending on the grade, it can be either argon or nitrogen. The device and the atomization enclosure can advantageously be designed according to the principles of the French breveis 73-43159 of December 4, 1973 or n ° 73-45788 of December 20, 1973. The powder obtained has a particle size between a few microns and 500 microns , with a spheroidal grain shape. The variable oxygen content depending on the grade is typically of the order of 100 to 200 ppm, and the specific surface area related to the particle size is close to the minimum value of a quasi-spherical powder.

For these spheroidal powders, the mechanical strength of the tablets depends on the interparticle contact surface produced during compaction. In practice, it is found that for a Vickers hardness under 500 g greater than 100, the mechanical strength and the resistance to crumbling of the tablets become insufficient to allow normal handling of these tablets, which severely limits the potential applications of these powders for making blanks by cold compaction.

The present invention makes it possible both to overcome this severe limitation and to ensure, from powder made up of spheroidal metallic particles, the production of non-fragile shaped parts having a Vickers hardness under 500 g greater than 500.

The principle used is the incorporation into the metal powder, by dry mixing, of an organic binder in the form of a powder of the methylcellulose type or more generally of cellulose gums which are polymers soluble in water. The progressive incorporation into the mixture of an amount of water equal to that of methylcellulose, that is to say from 0.2 to 2% by weight, allows the solubilization of the methylcellulose and avoids the phenomena of demixing of the different constituents of the mixture, such as graphite powder, solid lubricant powder, etc. The mixture is then ready for use and provides the tablets with mechanical strength and sufficient crumbling resistance for normal handling with compacting pressures of 25 to 75 daN / mm ² . These properties can be increased, if necessary, by a steaming treatment at 120 ° C. The elimination of this organic binder is ensured during sintering by a heat treatment under a neutral or reducing atmosphere comprising a plateau between 300 and 500 ° C., the actual sintering being carried out under the same atmosphere under the conditions of time and temperature required by the application and the shade. The dosages on sintered product indicate an increase in the carbon content of less than 0.2 times the content of the initial mixture in methylcellulose and no variation in the oxygen content compared to the gas atomized powder.

• a) une compaction à froid, éventuellement suivie d'un étuvage, et choisie dans le groupe de modes de compaction constitué par: la compaction unidirectionnelle en matrice, la compaction isostatique, le compactage par laminage.
• b) Un frittage en deux temps, sous atmosphère neutre ou réductrice vis à vis de la poudre compactée, le premier temps s'effectuant entre 300° et 500° C et le deuxième temps s'effectuant à une température notablement plus élevée.

Thus, the subject of the present invention is a process for manufacturing shaped parts from a mixture of powders of metal particles spheroidal liquids, of stearate type lubricant and organic binder of cellulose gum type, characterized in that said mixture of dry powders is moistened with water, so that the powder mixture obtained contains, by weight, 0 , 2% to 2% of stearate type lubricant, from 0.2% to 2% of organic binder of water-soluble cellulosic gum type, and between 0.2% and 2% of water, and in that said powder mixture successively in the following two operations:

• a) cold compaction, possibly followed by steaming, and chosen from the group of compaction modes consisting of: unidirectional matrix compaction, isostatic compaction, compaction by rolling.
• b) Sintering in two stages, under a neutral or reducing atmosphere with respect to the compacted powder, the first stage being carried out between 300 ° and 500 ° C and the second stage being carried out at a significantly higher temperature.

According to the invention, the powder consisting of spheroidal metal particles used can advantageously be obtained by atomization of liquid metal by means of gaseous jets.

According to the invention, the cellulose gum used can advantageously be methylcellulose.

In order to clearly understand the invention, four embodiments of the method according to the invention will be described below, by way of nonlimiting examples.

Example 1

Realization, from a low alloy steel grade, of specimens for the determination of the mechanical strength "green", in the raw state, of unidirectional cold compaction, without the final sintering phase, in order to demonstrate only the improvement, due to the invention, of the "green" resistance of the cold-compacted blanks for sintering

a) Known technique

• - 500 grammes de poudre atomisée par gaz, nuance d'acier 5 CD4 (C: au plus égal à 0,05%, Cr: 1%, Mo: 0,25%, Mn: 0,8%), granulométrie <500 micromètres dureté Vickers sous 500 g HV 0,5 = 206 ± 20,
• - 1,5 gramme de poudre de graphite,
• - 7,5 grammes de poudre de lubrifiant solid du type stéarate.

From a gas atomized powder, the following mixture was prepared

• - 500 grams of gas atomized powder, steel grade 5 CD4 (C: at most equal to 0.05%, Cr: 1%, Mo: 0.25%, Mn: 0.8%), particle size <500 Vickers hardness micrometers under 500 g HV 0.5 = 206 ± 20,
• - 1.5 grams of graphite powder,
• - 7.5 grams of solid lubricant powder of the stearate type.

Parallelepipedic specimens of 33 x 12 x 6 mm were compacted from this mixture under 75 daN / mm ² . The flexural strength of these test pieces, determined according to AFNOR A 95-206 standard, is 0.100 daN / mm ² . This value is barely sufficient for handling the tablets. In addition, the resistance to shattering of the edges and surfaces of the tablets is low.

b) Technique according to the invention

• - 500 grammes de poudre atomisée par gaz, nuance d'acier 5 CD4, granulométrie <500 µm, dureté Vickers HV 0,5 = 206 ± 20,
• - 0,5 gramme de poudre de graphite,
• - 2,5 grammes de poudre de lubrifiant solide du type stéarate,
• - 5 grammes de poudre de méthylcellulose.

By comparison, a mixture was prepared from the following constituents:

• - 500 grams of gas atomized powder, steel grade 5 CD4, particle size <500 µm, Vickers HV hardness 0.5 = 206 ± 20,
• - 0.5 gram of graphite powder,
• - 2.5 grams of solid lubricant powder of the stearate type,
• - 5 grams of methylcellulose powder.

5 cm ³ of water were incorporated to form a homogeneous moist mixture which is kept in a closed container.

The test pieces compacted under 75 daN / mm ² have a flexural breaking strength of 0.200 daN / mm ² . After baking at 120 ° C, the flexural strength increases to 0.400 daN / mm ² . These values allow handling of the tablets without special precautions. In addition, the use of hydrolyzed methylcellulose as a compaction binder brings a spectacular improvement in the resistance to the erosion of the edges and the sufaces of the tablets.

Example 2 Production according to the invention, from a grade of low-alloy steel, of sintering-forging blanks of sliding gear in steel grade 35 CD4 by unidirectional cold compaction

• - 0,1% de poudre de graphite,
• - 0,5% de poudre de lubrifiant solide du type stéarate,
• - 1% de poudre de méthylcellulose,

on a préparé un mélange humide dans un mélangeur industriel de capacité 200 kg par incorporation de 1% d'eau.From a powder atomized by gas of grade 5 CD4, particle size <500 _μm , comprising the following additives by weight:

• - 0.1% graphite powder,
• - 0.5% solid lubricant powder of the stearate type,
• - 1% methylcellulose powder,

a wet mixture was prepared in an industrial mixer with a capacity of 200 kg by incorporating 1% of water.

The annular compaction was carried out at 40 daN / mm ² with an industrial mechanical press. The density obtained is 6.4 g / cm ³ . The filling of the powder and the ejection of the blank were carried out automatically at rhythm of 400 pieces / hour.

These blanks could be shaped by the sintering-forging technique, a thermal reheating cycle in a protective atmosphere comprising a plateau between 300 and 500 ° C before the sintering proper at 1150 ° C. The sintered blanks were densified by closed die forging according to the technique known as sintering-forging, well known to specialists. The parts obtained have full density. The residual carbon content after forging conforms to grade 35 CD4, as does the response to the surface hardening treatment by carbonitriding.

Example 3 Production according to the invention of hard alloy shaped parts by unidirectional cold compaction

• - 500 grammes de poudre atomisée par gaz, nuance Stellite 6, dureté Vickers HV 0,5 = 490 ± 20,
• . 2,5 grammes de poudre de lubrifiant solide du type stéarate,
• - 5 grammes de poudre de méthylcellulose.

A mixture was prepared from the following constituents:

• - 500 grams of gas atomized powder, Stellite 6 grade, Vickers HV hardness 0.5 = 490 ± 20,
• . 2.5 grams of solid lubricant powder of the stearate type,
• - 5 grams of methylcellulose powder.

5 cm ³ of water were incorporated to form a homogeneous moist mixture which is kept in a closed container.

Samples in the form of plates of 33 x 12 x 4 mm and rings of outside diameter 20 mm, inside diameter 12 mm, height 12 mm, were compacted under pressures varying from 25 daN / mm ² to 75 daN / mm ² . Although the relative density of these samples is little different from the relative density of the packed powder, their handling can be carried out without special precautions.

By comparison, the tablets produced from mixtures without hydrolyzed methylcellulose are very friable and cannot be handled, even after compression under 150 daN / mm ² . A practically complete densification of these tablets could be obtained by a heat treatment under vacuum comprising a plateau between 300 and 500 ° C. and a sintering at high temperature depending on the structure to be obtained, this temperature being able in some cases to be between 1250 ° C and 1350 ° C, for example.

Example 4 Production according to the invention of hard alloy shaped parts by cold isostatic compaction

From the same mixture as that indicated in Example 3, spherical blanks were produced by cold isostatic compaction. The powder is placed in latex molds and the molds placed in an enclosure where a hydraulic pressure of 2,000 to 3,500 bans is applied. Beads 8 to 30 mm in diameter were thus obtained which were sintered under the conditions indicated in Example 3.

Example 5 Production according to the invention of porous stainless steel strips by compaction-rolling

From a gas atomized powder, steel grade 316 L, particle size 100 to 250 µm, comprising 1% by weight of methylcellulose, 1% of water was incorporated by passage through a continuous mixer. The wet mixture is fed continuously into the inter-cylinder of a duo rolling mill provided with cylinders of 370 mm in diameter and 300 mm in table width with their respective axes of rotation arranged in the same horizontal plane. A coherent strip 300 mm wide, 1 mm thick, density 6.50 g / cm ³ , was produced at 0.5 m / minute. After compaction by rolling, the strip passes at the same speed in a passage oven under a cracked ammonia atmosphere comprising a zone at 300-500 ° C where the organic binder is eliminated and a zone at 1150 ° C where is carried out the actual sintering. This strip retains a porosity of 25% by volume and can be used, for example, as a filter medium.

Claims (3)

1. A process for manufacturing shaped parts from a mixture of powders of spherical metallic particles, a lubricant of the stearate type and an organic binder of the cellulose gum type, characterised in that the said mixture of dry powders is moistened with water so that the pulverulent mixture obtained contains, by weight, from 0.2% to 2% of lubricant of the stearate type, from 0.2% to 2% of organic binder of the water-soluble cellulose gum type, and between 0.2% and 2% of water, and in that the said pulverulent mixture is subjected in turn to the two following operations:

a) A cold compaction followed, if appropriate, by baking, and chosen from the group of methods of compaction consisting of: unidirectional die compaction, isostatic compaction and compaction by lamination.

b) A sintering in two stages, in an atmosphere which is neutral or reducing towards the compacted powder, the first stage being carried out between 300° and 500°C, and the second stage being carried out at a temperature which ist markedly higher.

2. A process for manufacturing shaped parts according to Claim 1, characterised in that the powder consisting of spherical metallic particles which is employed is obtained by atomising the liquid metal by means of gas jets.

3. A process for manufacturing shaped parts according to either of Claims 1 and 2, characterised in that the cellulose gum employed is methyl cellulose.