Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/14—Treatment of metallic powder
  • B22F1/148—Agglomerating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2301/00—Metallic composition of the powder or its coating
  • B22F2301/35—Iron

Definitions

• the metal powder which is the subject of the invention relates to the production of parts by compression and sintering from stainless steels, other stainless or refractory metals or alloys and alloy steels, intended for the production of quality.
• powders with angular particles obtained by spraying liquid metal by jet of water under pressure are most commonly used according to various methods known to those skilled in the art.
• metal powders by spraying a metal or liquid alloy by means of a gas jet which may be, for example, a neutral gas such as argon or nitrogen or any suitable gas. Powders are thus obtained having a much lower oxide content than the powders obtained by spraying with water and which thus have substantially the same purity as the starting metal.
• a gas jet which may be, for example, a neutral gas such as argon or nitrogen or any suitable gas. Powders are thus obtained having a much lower oxide content than the powders obtained by spraying with water and which thus have substantially the same purity as the starting metal.
• the elementary particles of these powders have a substantially spherical shape but in the state are not easily shaped.
• these powders make it possible, after compression and sintering, to obtain by compression parts having an apparent density at least equal to that which allow the usual angular powders to be obtained.
• the green part obtained from spherical particles has a mechanical strength often insufficient to allow its manipulation and in particular its ejection from the mold, then the transfer towards the sintering furnace, without chipping or cracking.
• Patent application GB 2 228 744 describes a similar method for joining elements of alloys such as graphite, phosphorus or the like with a base metal. According to this method, a mixture of an acrylic acid ester with a methacrylic acid ester and an unsaturated polymerizable acid is used as binder.
• the aim is to avoid the segregation of finer or lighter particles such as graphite or others compared to a base metal, such as iron, by binding these particles together.
• these binders after implementation, must be removed before or during sintering and contamination of the metal powders is often observed with certain components of the binders.
• the metal powder based on spherical particles, suitable for cold forming by compression followed by sintering, which is the subject of the invention as well as the process for the preparation of this powder which is also the subject of the invention allow to solve all the problems thus posed.
• the metal powder according to the invention consists of a set of granules each comprising a group of elementary metal particles of spherical shape agglomerated by gelatin at a content of at least 0.5% of the weight of the metal powder.
• the spherical particles are advantageously obtained by a spraying process, by means of a gas which can be air or a neutral or reducing gas such as N2, H2, NH3, Ar, or other, of a metal or liquid alloy .
• a gas which can be air or a neutral or reducing gas such as N2, H2, NH3, Ar, or other, of a metal or liquid alloy .
• the main metals or alloys which can be used are, for example, stainless steels, stainless or refractory metals or alloys or even alloy steels with high mechanical characteristics.
• the dimensions of these elementary particles and those of the granules are chosen mainly as a function of the dimensions and, in particular, density characteristics of the molded parts which it is a question of producing.
• each granule in order to obtain, by cold pressure forming, a part having sufficient mechanical strength when raw, it is preferable for each granule to contain a sufficient number of spherical particles agglomerated by the gelatin. It is also necessary that these granules are capable of correctly filling the mold in every corner. However, a certain number of granules can be formed of isolated elementary particles coated with gelatin without affecting the quality of the final product obtained.
• a maximum diameter "d1”, elementary spherical particles and a maximum width "d2" of the granules obtained are determined. It is found that the powder according to the invention must preferably have a ratio of d2 / d1 ⁇ 3 so that the cold-pressure molded parts reach sufficient mechanical strength.
• this d2 / d1 ratio can reach at least 4 or more.
• a maximum width "d2" of the granules of 300 microns constitutes a lower limit.
• the gelatin content must be determined according to the average size of the elementary spherical particles which are agglomerated by the gelatin. This content also depends on the jelly strength of the gelatin. This strength in jelly is expressed in Bloom (standardized unit) and can vary between 50 and 250 blooms depending on the gelatin used.
• gelatin with a stronger Bloom can make it possible to reduce the percentage of gelatin in the granules and therefore bring to a minimum value the duration of elimination of the gelatin before reaching the sintering phase proper at high temperature.
• the granules the elementary spherical particles of which are agglomerated by means of gelatin, do not stick to the walls of the molds, even when the latter heat up to temperatures reaching 100 ° C. or more, lubrication by a small amount of Zn stearate or another suitable lubricant can be used.
• the invention also relates to the process for the preparation of a metal powder based on elementary spherical particles suitable for cold forming by compression and then for sintering.
• an agglomeration is carried out in granules of the elementary spherical particles.
• gelatin is added to the elementary spherical starting particles in the form of an aqueous solution, the amount of water used being of the order of two to five times the amount of gelatin and the temperature of the water being 40 to 80 ° C.
• the amount of gelatin to be used depends on the size of the elementary spherical particles and also on the gel strength of the gelatin.
• the mixture of elementary particles and gelatin solution is triturated for the time necessary to obtain the wetting of the metal particles and during cooling the gradual formation of a gel.
• partial drying is carried out, for example, by blowing a gas stream which makes it possible to give the mixture a pasty consistency, then a fragmentation of this paste is carried out, for example by pressing it on a sieve whose width mesh size is determined by taking into account the diameter of the elementary spherical particles.
• Granules are thus formed, the drying of which is continued until elimination, preferably as complete as possible, of the water.
• the amount of gelatin contained in the granules is at least 0.5% of the weight of the metal powder obtained.
• the fines can be separated by a suitable opening sieve. It is noted that if the maximum diameter "d1" of the elementary spherical particles and the width "d2" of the granules obtained after drying are in a ratio d2 / d1 at least equal to 3 and, preferably, equal or greater than 4, it is possible to obtain by cold compression in a mold parts whose raw mechanical strength is much higher than in the case of the same powder with spherical particles not agglomerated into granules.
• this lubricant and the gelatin are removed by preheating the compressed green part at a temperature generally between 300 and 500 ° C.
• Preheating can be carried out in air or in the presence of a neutral or reducing gas such as Ar, H2, NH3 or other.
• a neutral or reducing gas such as Ar, H2, NH3 or other.
• the examples below describe, without limitation, the characteristics of the metal powder agglomerated into granules based on elementary spherical particles suitable for forming by cold compression and then for sintering according to the invention.
• the examples also describe, without limitation also, an embodiment of such an agglomerated powder and an embodiment of this powder for the production of parts by compression in a mold then sintering.
• This example relates to the process according to the invention for preparing a metallic powder agglomerated in granules from elementary spherical particles which has the properties according to the invention of aptitude for cold compression forming and sintering.
• Spherical particles obtained in known manner are used by spraying with a neutral gas a bath of stainless steel, the composition of which corresponds to grade 316 defined by the ASTM standard.
• a batch of these particles is prepared by sieving, the particle diameter of which is not more than 106 microns.
• An aqueous solution based on deionized water is prepared containing by weight 30% of a gelatin whose jelly strength is 50 blooms. The solution is heated to 50 to 70 ° C to completely dissolve the gelatin.
• a mixture is produced containing 95% of 316 steel particles of not more than 106 microns in diameter and 5% of aqueous solution, ie 1.5% by weight of gelatin.
• An intimate mixture must be produced in order to wet the entire surface of the elementary particles with the solution.
• Granules are thus obtained.
• the drying of these by cold or hot air is continued then a second sieving is carried out in order to separate the granules from each other and to calibrate them by passage through a sieve with a mesh of 500 microns.
• Granules are thus obtained whose dimensional ratio is at least 4.7 compared to the maximum diameter of the metal particles.
• These dried granules are made up of agglomerated spherical metallic particles, firmly bound to one another by gelatin films, certain granules however being able to be constituted by isolated elementary particles coated with gelatin.
• the powder thus agglomerated into granules is capable of forming, by cold compression in a mold, parts having a raw mechanical strength much greater than that obtained with the starting metal particles.
• Adding a small amount of a lubricant such as zinc stearate to the agglomerated granular powder further facilitates forming. Demolding is also facilitated by the fact that the solidified gelatin does not stick to the walls of the molds when they are heated.
• a series of tensile test pieces by cold compression in a mold according to ASTM B312 standard is thus produced by means of the agglomerated powder in granules according to the invention, added with approximately 0.75% by weight of zinc stearate.
• two different compression ratios of 314 and 422 MPa are also produced.
• test pieces compressed under a load of 422 MPa, are preheated in air to around 500 ° C to remove the gelatin and zinc stearate and then sintered by heating to about 1280 ° C.
• Tensile tests carried out on the specimens thus sintered give the following average results: Elastic limit 137.9 MPa (20 ksi) Breaking load 344.7 MPa (50 ksi) Elongation at break 25%.
• a comparative test is made on the same grade of stainless steel 316 using spherical particles having a maximum diameter not greater than 150 microns.
• Agglomeration with gelatin is carried out with the same gelatin concentrations and the same sieving conditions as above, the diameter of the granules obtained being not greater than 500 microns.
• the dimensional ratio between granules and spherical particles is therefore reduced to 3.3.
• the dimensional ratio of 3 between granules and spherical elementary particles is in the immediate vicinity of the acceptable limit and that in practice it is advantageous to choose a dimensional ratio at least equal to 4.
• a metallic powder agglomerated in granules is produced by the method described in Example 1 from a stainless steel type 904 L which contains in% by weight: Cr 20; Ni 25; Mo 4.5; Cu 2: remains Fe.
• the starting material contains spherical particles with a diameter of not more than 106 microns.
• the agglomeration is carried out in the same way as in the case of the steel 316 described in example 1, the final calibration of the granules being made by passing through a mesh screen with a mesh of 500 microns per side, the dimensional ratio between granules and spherical particles therefore being 4.7.

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• Powder Metallurgy (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)

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• 1993
  • 1993-07-06 FR FR9308535A patent/FR2707191B1/fr not_active Expired - Fee Related
• 1994
  • 1994-06-29 JP JP17023794A patent/JP3325390B2/ja not_active Expired - Fee Related
  • 1994-07-04 DK DK94420188T patent/DK0633083T3/da active
  • 1994-07-04 EP EP94420188A patent/EP0633083B1/de not_active Expired - Lifetime
  • 1994-07-04 CA CA002127344A patent/CA2127344A1/fr not_active Abandoned
  • 1994-07-04 ES ES94420188T patent/ES2162849T3/es not_active Expired - Lifetime
  • 1994-07-04 AT AT94420188T patent/ATE205430T1/de not_active IP Right Cessation
  • 1994-07-04 DE DE69428236T patent/DE69428236T2/de not_active Expired - Lifetime
  • 1994-07-06 US US08/268,117 patent/US5460641A/en not_active Expired - Lifetime
  • 1994-07-06 KR KR1019940016267A patent/KR960013531A/ko not_active Withdrawn

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