CS216185B2 - Method of making the metal objects - Google Patents

Abstract

1457661 Production of dense sintered alloy articles CABOT CORP 15 Jan 1974 [15 Jan 1973] 01801/74 Heading C7D Sintered alloy articles are made by mixing the alloy powder with not more than 5 wt. per cent dry organic binder, than adding a solvent for the binder, consolidating the mixture and then drying and crushing the consolidated mixture to form agglomerates which are pressed in a die and then sintered. The alloy powders used are preferably atomized Fe, Co or Ni based alloys, the dry binders may be camphor, polyvinyl alcohol, methyl alcohol, paradichlorobenzene, chloroacetic acid, naphthalene, benzoic acid, phthalic anhydride, glycerine, metal stearates, acrylamides, ACROWAX (Trade Mark) and CARBOWAX (Trade Mark). Water may be used as solvent for water soluble binders and the mixture may be consolidated by extrusion.

Description

The purpose of the invention is to propose a method for producing metal articles which economically utilizes atomized powders, extends the range of annealing temperatures, allows the use of larger particles than prior art processes, and allows for high quality alloys that cannot be sintered by prior art powder metallurgy processes.

For this purpose, a metal powder is prepared by mixing a high-alloy alloy with a dry, finely divided organic binder together with a suitable solvent in an amount not exceeding mass. which is uniformly dispersed in the powder to form a plastic mixture which is solidified to the bulk density between the metal powder and the casting, dried after solidification to evaporate the solvent and crushed into agglomerates which are placed in a mold of the selected shape and pressed at least 50% of the casting density per cohesive green compact which, after removal from the mold, is sintered at a temperature between the solidus temperature and the liquidus temperature of the alloy.

The invention relates to a process for the production of high alloy alloy metal articles by powder metallurgy.

The alloys to which the invention relates are high-quality, chromium-based alloys based on cobalt, nickel and iron, resistant to wear, heat and corrosion. These alloys are either difficult to process or difficult to produce, and are commonly used to produce castings that can be grinded or turned when needed. A range of small articles of high quality alloys, such as thread guides for textile machines, valve seat inserts and the like, can be manufactured in a complicated and uneconomical manner by casting in such large quantities as the market requires.

Attempts have therefore been made to produce such articles by a powder metallurgy process, for example by pouring slurry or by pressing the articles from fine powders, and then sintering them. However, it has been shown that these processes, which are satisfactory and economical for some alloys, are difficult and too expensive to apply to hard alloys such as the high-quality alloys of the present application.

One difficulty is to achieve the desired high density in the finished article. In general, it has been believed that in order to achieve optimum specific gravity and thus facilitate subsequent densification, the dust particles should have a spherical shape and random size distribution over a wide range of dimensions. U.S. Pat. No. 3,639,179, a size range of 150 to about 10 microns is recommended. However, according to the invention, it has been found that many high-quality powder alloys can be sintered in a very narrow temperature range when pressed in this manner, and in some cases not at all.

Experiments have shown that sintering of metal powders in general can be improved by reducing the particle size of the powder to 325 mesh or less. This is done by sieving the powder through a fine sieve, but using only part of the powder, which is uneconomical. In the powder atomized from the molten alloy, which is the type of powder most commonly used in powder metallurgy, only 25-35% of the powder has a 325 mesh size. Attempts have been made to utilize oversize grinding to a finer powder, and it has been found that a sinterable powder of high quality alloys can be obtained. In many cases, however, this powder has unsatisfactory properties and has low cohesion under pressure unless it is ground to a much smaller grain size than that required for sintering.

In the manufacture of powdered iron or ordinary alloy powders, it is customary to compress the powder into so-called raw moldings in the shape of the article to be manufactured, and then place the moldings in a furnace where they are sintered. The moldings must retain their shape until the particles are joined by sintering. - The stresses that the moldings must withstand depend inter alia on the shape of the molding and its dimensional tolerances.

The bulk density of the moldings ranges from about 50% of the density of the casting to about 70% when high compression pressures are used. Since it is generally desired that the density of the sintered article corresponds to at least 95% of the weight of the casting, all moldings shrink by 25 to 40 ° / s during sintering. In the case where the sintered compact must have exact dimensional tolerances, the compacts must be applied during sintering, for example by pressure. In the manufacture of valve seat inserts, which must be made up to slight internal diameter tolerances, the raw compacts are slid onto the core and sintered in this position. If the cohesion between the powder grains is insufficient, the compact will burst.

It was found that the average particle size for efficient compression was in the worst case less than 5 microns, and the grinding time required to produce such a fine powder was measured for days. This of course greatly increases the cost of the powder. In addition, the greatly increased surface area of the fine-grained powder and the long time required to grind it favored the oxidation of the powder, so that despite all precautions its oxygen content was much greater than in the powder. High oxygen content is undesirable for many reasons, the main reason being that it reduces the sintering range of the powder. As a result, for many alloys, the sinterable powders were not pratically compressible, while the powders suitable for compression were not sinterable.

It is therefore an object of the present invention to provide a process for producing high-grade steel articles by powder metallurgy so that it economically utilizes atomized powders, extends the sintering temperature range, and allows larger particles to be used than prior art processes. Another object of the invention is to provide a method applicable to those high-quality alloys that cannot be sintered by the prior art.

This purpose is achieved according to the invention by preparing a metal powder from the superalloy, mixing it with a dry, finely divided organic binder together with a suitable solvent in an amount not exceeding 5% by weight. % - which is uniformly dispersed in the powder to form a plastic mixture which is solidified to the bulk density between the metal powder and the casting, dried after solidification to evaporate the solvent and crushed into agglomerates which are - placed in a mold of selected shape - and compress to at least 50% of the density of the - casting - to - a cohesive raw compact which - after removal - is sintered at a temperature between the solidus temperature and the liquidus temperature of the alloy.

A further feature of the invention is that the solvent is a solvent that prevents oxidation of the alloy powder.

According to another feature of the process according to the invention, the alloy powder is mixed with an organic binder and powdered oxide of ko216185

6, which is reduced by carbon at the sintering temperature.

Further, the method of the invention is characterized in that a chemically stable compound, such as powdered tungsten carbide, is dispersed in the powder with the metal powder.

According to a further feature of the invention, a raw metal powder molding having a weight content of 20 to 36% chromium, 2 to 22% tungsten, 0.3 to 3.8% carbon, 0.01 to 13 why, nickel, 0.01 up to 5.2% vanadium, 0.01 to

1.5% of silicon, 0.01 to 1.7% of boron, the rest of cobalt and random admixtures such as iron to 5%, manganese to 1%, molybdenum to 1.5%, phosphorus to 0.4% and sulfur to 0.04%, sintered at a temperature between 1171 and 1274 ° C.

Another feature of the invention is that a raw metal powder molding having a content of 9-11% cobalt, 11.5-13.5% iron, 25-27% chromium, 9-11% molybdenum, 9-11% tungsten, 1.3 to 1.5% carbon, the rest of the nickel and random admixtures such as silicon up to 1 manganese up to 0.75%, boron up to 1%, phosphorus up to 0.04% and sulfur up to 0.03%, sintered at temperature between 1210 and 1221 ° C.

Finally, the process according to the invention is characterized in that a raw metal powder molding having a content of 0.5 to 1.5 why silicon, 15.5 to 18.5% chromium, 14.5 to 17.5% molybdenum, 2.9 to 3.4% carbon,

1.65 to 2.10% vanadium, traces up to 0.5% manganese, the remainder iron and random admixture, sintered at a temperature between 1171 and 1188 ° C.

The process according to the invention allows the use of the full range of atomized alloy powder dimensions if the article to be manufactured does not have to be of maximum weight. The process according to the invention is applicable to high-quality alloys which cannot be sintered or surface-treated by conventional methods. The method of the invention comprises grinding a relatively coarse fraction of the atomized melt or the entire product to reduce particles to a size not accompanied by unacceptable oxidation, dry blending the powder with a binder and mixing the mixture with a binder solvent to form a plastic, solidifying the mass into discrete bodies intermediate density, drying and crushing said bodies and sieving the resulting agglomerates to a size of about 100 mesh, by compressing the agglomerates into raw moldings that retain their shape, placing the moldings in a furnace and sintering them.

The composition of a large number of alloys for which the process according to the invention is well suited is shown in the table.

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Composition of typical alloys in% by weight

Alloying elements

Alloy Co Ni Si Fe Mn Cr Mo W C

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C \ T θ rH θ 'θ'- со СО * СМ * СМ r-Γ СМ * гЧ r-Γ СчГ со *

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^H й tH rH rf ⁿ rf -Ι- Ο + O OD D m 1 I I I CO II tT ^{1 1} ' ¹ a O r4 t < W Η H

A A. A A A A A A A A A A0 ρ co * r ~? r- co * rfT ooT r-T t oo * cm t o o t o * C) CCCJCCCCC) C1C) NCJHH

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CQ

CM + pLD O A * cd PQ

CO r-l co

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The alloy powder used according to the invention is preferably produced by spraying a melt of the desired composition. The melt is heated to about 92 ° C above the hearth melting point. Preferably, the melting is performed under vacuum or under protection of an inert gas such as argon. The melt is then poured into a preheated refractory tundish having a small diameter hole at the bottom through which molten metal flows into the spray chamber. The effluent stream is divided into fine particles by a high pressure flow of inert gas or water that meets the melt stream just below the tundish outlet. The particles or droplets are cooled almost immediately by the spray fluid and fall into the tank at the bottom of the spray chamber. Of these particles, only the portion that passes the 30 mesh screen is used. The particles are approximately spherical in shape and about 25-35% of them are below 325 mesh. The 325 mesh sieve permits particles whose largest dimension is 44 microns.

According to the invention, polyvinyl alcohol is preferred as a powder binder, but other solid known binders may also be used. Examples thereof are camphor, methanol, paradichlorobenzene, chloroethic acid, naphthalene, benzoic acid, phthalic anhydride, glycerin, polymers of ethylene oxide, synthetic rubbers such as acrylamide and metal stearic salts: The binder solvent must be suitably selected. Water is suitable for water-soluble binders.

The mixing of the powder and the binder particles is carried out in any suitable mixing device. The amount of binder is not critical and a few weight percent is sufficient. Extrusion of the plastic or clay mixture of particles, binder and solvent is the most advantageous way to form agglomerates from the plastic mixture, but of course other methods can also be used, for example briquetting between rolls.

The extruded agglomerates are then dried, crushed in a roller crusher, hammer mill or the like, and screened. The crushed powder binder fraction below 100 mesh is largely a fine fraction. 60 to 80% of the particles have a size below 325 mesh, corresponding to a density of 2.0 to 80 mesh

3.3 g / cm ³ . The percentage of fines and the specific gravity of the material are, of course, less than that of the milled material. It can be assumed that each powder particle in the material is coated with a binder film as a result of mixing and extrusion, and that in the raw moldings of the material, the powder particles are held together by the film binder.

The powder and binder agglomerates are compressed in molds or presses to the desired shape at a pressure of about 760 MPa. The pressing pressure may be 300 to 1060 MPa, the density of the blank being higher at the higher pressures used ¹ . At a pressure of 300 MPa, the density of the compact is about 56 to 58% by weight of the casting, and at a compression pressure of 1060 MPa, about 70 to 72%.

The desired density of the finished article can be achieved by sintering the compact in a vacuum or reducing atmosphere at temperatures between the solidus and liquidus temperatures of the alloy. Sintering can be completed in about an hour, but if its duration is extended to 2 or at most 3 hours, the temperature can be reduced somewhat without degrading the properties of the finished article. Properly sintered compacts have a density of 98% or higher of the density of the casting.

According to the invention, the powder particles resulting from the melt spraying can be ground or milled as required. The relatively coarse spray powder, for example 30 mesh, is ground in ball mills, impact mills, spreading, vibratory milling or other processes to be comminuted to particles having at least 98% size below 325 mesh, whereupon the particles processed into sintered products of excellent properties. Methanol is preferably used as the carrier medium for grinding, the mill is preferably evacuated to reduce oxidation of the feed, and in the case of a ball mill, the ball is selected from a wear-resistant alloy whose composition is compatible with the product being processed. The grinding time lasts 8 to 36 hours, and the average particle size of the product below 325 mesh is between 30 to 9 microns, depending on the milling conditions. After grinding, the batch is discharged from the mill and the powder allowed to settle. Decant the alcohol and filter the slurry in vacuo. The filter cake is dried under vacuum or in air and then crushed to bond agglomerates below 60 mesh.

Powder castings of alloy No. 7 of the table, produced by spraying of melt whose particles are below 30 mesh, cannot be sintered. ^{On the other hand, the} fraction of this powder having a size below 325 mesh having an average particle size of about 31 microns can be sintered, although the temperature range for the 95% density is very narrow. However, as already mentioned, the 325 mesh fraction of the atomized powder constitutes only about 25 to 35% of the powder. The 30 mesh pulverized powder, ground to an average particle size of about 15 microns, can be sintered to a density of 95% or better within a temperature range of 25 to 30 °. This range is large enough for operation. The oxygen content of the milled powder is about 0.44%.

Interestingly, the addition of a relatively small amount of the fine fraction of atomized particles to the ground powder greatly impaired its sinterability. In another experimental procedure, a batch of Alloy No. 7 atomized powder having a particle size of from 30 to 270 mesh was ground in a ball mill for 25 hours to an average particle size of about 10 microns.

This material was admixed with an atomized 270 mesh powder of 30 wt%, agglomerate. The average particle size of this mixture was 23.5 microns. The moldings of this mixture could not be sintered as well as the 30 to 270 mesh spray powder moldings milled in a ball mill for 18 hours to an average particle size of 15 microns. The first powder had to be sintered at 1260 ° C for more than an hour to reach a density of 95%. Sintering at 1265 ° C for 1 hour gave an object with a density of 98.25%. The second powder had a density of 95% after 1 hour of sintering at 1250 ° C, and 98% after sintering for 1 hour at 1255 ° C.

Example 1

The 325 mesh powder spray fraction of alloy No. 3 of the table was dry blended with a binder, preferably 100 mesh polyvinyl alcohol, in a blender, in an amount of 2-3 wt.

The powder particles used had an average dimension of about 30 microns. Then enough water was added to form a plastic mixture of powder and binder. The mixture was then extruded into rolls or rolls of about 50 mm in length and about 12.5 mm in diameter under such pressure that they were compressed to a density of 60% / casting density. The discs were dried, then crushed in a roller crusher, hammer mill or the like, and the crushed material was sieved to 100 mesh. The 100 mesh agglomerates of the powder and binder particles were compressed under a pressure of about 760 MPa to crude moldings of the desired shape having sufficient strength to withstand further processing. The crude compacts were then sintered for 1 to 3 hours at a temperature of 1240 to 1275 ° C. The binder was evaporated during sintering and the sintered articles had a density of 97-99% of the casting density.

Example 2

The powder produced by spraying molten nickel-based alloy 7 in an inert gas was passed through a 30 mesh screen. 45.35 kg of screened powder was placed in a 9 m &lt; -1 &gt; spherical mill together with 50 l of methanol and about 362.87 kg of alloy # 3 balls. The mill was evacuated and run for 10 hours with a speed corresponding to approximately 80% critical. speed (54 mi ^^ ¹ ). The average particle size of the resulting powder was about 17.5 microns. About 98% of the powder was 325 mesh. The powder was discharged from the mill, filtered, dried and dry blended with 2 wt., Why, 100 mesh polyvinyl alcohol particles of approx. Another binder was then mixed with water to form a paste, which was then extruded into discs, dried, crushed, placed in a press, pressed and removed from the press. The coherent raw compacts were placed in a sintering furnace and sintered at a temperature between 1210 and 1220 ° C for 1 to 3 hours. The articles produced had a casting density of 98-99% and a Rockwell hardness of 41-44.

Example 3

The powder produced by the spraying of molten cobalt-based alloy 6 in an inert gas was ground according to Example 2 except that grinding lasted 36 hours to a powder with an average particle size of 11.5 microns. This powder was then further processed as described above, except that the binder consisted of 3% polyvinyl alcohol plus 1% of the other binder composition. The powder was compressed into coherent moldings which were placed in a sintering furnace and sintered at a temperature between 1170 and 1185 ° C. The finished products had a density of 96-98% of the casting density.

Example 4

Sprayed particles in the inert melt gas of the No. 8 iron-based alloy were passed through a 325 mesh screen. The sieved particles were then mixed with the binder as described in Example 1 except that the binder was 3% ^! polyvinyl alcohol, and further processed according to Example 1 to crude moldings. These moldings maintained their shape and were placed in a sintering furnace and sintered at a temperature between 1178 and 1188 ° C to articles having a density of 97% of the density of the casting.

Example 5

The particles formed by spraying Alloy No. 8 in an inert 30 mesh gas were milled in a ball mill for 24 hours to particles having an average size of about 9 microns. These particles were then mixed with 3 wt. % polyvinyl alcohol particles and 1 wt. % of the particles of another binder preparation and further processed according to Example 1 to coherent moldings. The moldings were sintered at temperatures between 1170 and 1188 ° C to articles having a density of 97% of the density of the casting.

The carrier used in ball mill grinding has some influence on sintering. Although it would be advantageous to use water, it has been found that its use results in a measurable increase in the oxygen content of the sintered article and a narrowing of the sintering temperature range. Where the oxygen content of the alloy is critical or where the sintering temperature range is limited, it is preferable to use a solvent other than water.

For example, in the case of Alloy No. 7, made from a powder with an average size of 18 microns, the increase in oxygen content of the alloy due to the use of water was about 0.43%.

For this reason, methane218185u is preferred as a vehicle which causes an increase in the oxygen content of only about 0.12%. Other useful solvents that may serve as the vehicle are ketones, aromatic hydrocarbons and methane compounds.

On the other hand, the decomposition of the organic solvent increases the carbon content of the sintered article by about 0.1 to 0.2%. In Alloy 3 and other low carbon steels, this increase in carbon levels may be significant; in such cases, it is convenient to add a small amount of metal oxide to the powder, which is reduced by carbon at the sintering temperature. For example, cobalt oxide is suitable for alloy No. 3. For other alloys, oxide can be used

Claims (7)

1. A process for the production of metal alloys of high-alloy alloys by powder metallurgy, characterized in that a metal powder is prepared from a high-alloy alloy, mixed with a dry, finely divided organic binder together with a suitable solvent in an amount not exceeding 5% by weight. %, which is uniformly dispersed in the powder to form a plastic mixture which is solidified to the bulk density between the metal powder and the casting, dried after solidification to evaporate the solvent and crushed into agglomerates which are placed in a mold of the selected shape and pressed at least 50% of the casting density per cohesive green compact which, after removal from the mold, is sintered at a temperature between the solidus temperature and the liquidus temperature of the alloy. 2. The process of claim 1 wherein the solvent is a solvent to prevent oxidation of the alloy powder. 3. The process of claim 1 wherein the alloy powder is mixed with an organic binder and a powdered metal oxide which is reduced by carbon at the sintering temperature. 4. A process according to claim 1, wherein a chemically stable compound, such as powdered nickel or oxides of other metals compatible with the alloy composition, is mixed with the metal powder. The process according to the invention is advantageous in the manufacture of powdered alloys containing the dispersed phase. According to the process of the present invention, alloys comprising a matrix of Alloy No. 2 were prepared in which tungsten carbide particles were dispersed in an amount of 25 to about 60 wt. ° / o. Tungsten carbide in powder form is added to the alloy powder and mixed mechanically with it. The powder mixture is then mixed with a suitable binder and, from this point on, processed as described in the Examples. Preferred embodiments of the invention have been described in the foregoing description, but are not limited thereto. BACKGROUND OF THE INVENTION that the tungsten bid is dispersed in the powder. 5. A process according to claim 1, characterized in that a raw metal powder molding having a weight content of 20 to 36% chromium, 2 to 22% tungsten, 0.3 to 3.8% carbon, 0.01 to 13% nickel, 0.01 to 5.2% vanadium, 0.01 to 1.5 θ / ο ¹ silicon, 0.01 to 1.7% boron, the rest of cobalt and incidental impurities such as iron to 5%, manganese to 1%, molybdenum to 1.5%, phosphorus to 0.04% and sulfur to 0.04% are sintered at a temperature between 1171 and 1274 ° C. 6. A process according to claim 1, wherein the raw metal powder molding has a cobalt content of 9 to 11% by weight, 13.5 θ / d of iron, 25 to 27% chromium, 9 to 11 why, molybdenum, 9 to 11% tungsten, 1.3 to 1.5% carbon, nickel residue and incidental impurities such as silicon up to 1%, manganese to 0.75 ^'O / OI, boron, up to 1%, phosphorus up to 0.04 ° / o to 0.03% sulfur, is sintered at temperatures between 1210 and 1221 ° C. 7. A process according to claim 1, characterized in that a raw metal powder molding having a content of 0.5 to 1.5% silicon, 15.5 to 18.5% chromium, 14.5 to 17.5% molybdenum, 2 9 to 3.4% of carbon, 1.65 to 2.10% of vanadium, traces of up to 0.5% of manganese, the remainder iron and random impurities are sintered at a temperature between 1171 and 1188 ° C.