US 3325277 A
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June 13, 1967 Filed Feb. 1, 1965 R. A. HUSEBY 3,325,277
METHOD OF MAKING METAL POWDER 4 Sheets-Sheet 1 INVENTOR. ROBERTA. HUSEBY BY vndrus Star/Q June 13, A MUSE! METHOD OF MAKING METAL POWDER 4 Sheets-Sheet 12 Filed Feb. 1, 1965 INVENTOR. ROBERT A. Husesv ndr'us \Starkg June 13, 1967 R. A. HUSEEY 3,325,277?
METHOD OF MAKING METAL POWDER Filed Feb. 1, 1965 4 Sheets-Sheet 2 I00 lHAB; NES E 95 RE GTEN T1E= was \12 HARDNE s DEETERM NATIONS Q 5'\%R TE PERATURE 9o 1 I m 85 :1 g 80 a 75 m m I300 I400 I500 i600 1700 e800 TEMPERATURE PF) HAHDNE 8 Vs. 1
l2 HARDN ss DETERMINATIONS AT f E H T E w 75 Z a g 70 m 65 I \\D d 60 a 5 o 53 (I INVENTOR. (HRS) @Mm" A HMSEBV June 13, 1967 R. A. HUSEBY METHOD OF MAKING METAL POWDER 4 Sheets-Sheet 4 Filed Feb. 1, 1965 5 5 E R O U 5 S s W 4 m Q m 5 GOMPACTION PRESSURE TSI INVENTOR. ROBERT fi. Massey BY ffarne United States Patent 3,325,277 METHOD OF li IAKING METAL POWDER Robert A. Husehy, Milwaukee, Wis., assignor to A. 0. Smith Corporation, Milwaukee, Wis., a corporation of New York Filed Feb. 1, 1965, Ser. No. 430,249 2 Claims. (Cl. 75-.5)
This application is a continuation-in-part of application Ser. No. 357,727, filed Apr. 6, 1964, now abandoned.
This invention relates to a method of making metal powder, and more particularly to a method of making steel powder having, when compacted and sintered, a high density and superior physical properties.
There are several basic procedures by which metal powder, to be used in powder metallurgy processes, can be prepared. One method of preparing the metal powder is by an electrolytic process in which the iron or other metal powder is deposited on the cathode. The powder formed by the electrolytic process is rough and crumbly when taking off the cathode and is then crushed and annealed to provide fine, soft particles. When metal powder formed by the electrolytic process has high purity, high density and provides a high green strength, the electrolytic process is expensive, so that it has not been used to a great extent in commercial practice.
A second method of forming metal powder is by an ore reduction process. In this process, magnetite ore is ground to a fine state, mixed with carbon and lime and heated in a tunnel kiln. The carbon reacts with the oxygen in the ore to reduce the oxide so that after the heating period, only about 1% oxide remains. The lime reacts with other impurities in the ore, and the resulting product can be removed. The resulting cake is then ground up and crushed.
Following the grinding, the ore is subjected to a magnetic separation process in which the impurities are separated from the iron powder. Subsequently, the powder is heated at a temperature of about 1800 F. to reduce the residual oxides and soften the powder. Following the anneal, the particles are re-ground to break up the sintered cake and the final product in this process generally consists of about 93 to 95% metallic iron. There is also a somewhat similar process based on hydrogen reduction.
In another conventional process for forming metal powder, scrap iron and carbon are charged in a cupola and melted down to produce iron containing about 4% carbon. The molten iron is then fed by gravity in one or more streams from a tundish, and compressed air is directed against the stream of molten iron to atomize the iron into small particles which fall into a water bath. The resulting particles contain about 4% carbon and have an oxide coating on their outer surface. By raising and lowering the height of the water bath with respect to the tundish, it is possible to vary the amount of oxide film on the particles. The particles are then heated at a temperature of about 1900 F. to react the carbon and oxide film which theoretically eliminates both the carbon and oxygen from the iron particles. The resulting powder is then milled. In commercial practice, it is difiicult to obtain the proper relationship between oxygen and carbon so that both the carbon and oxygen are removed during the annealing treatment. Iron powder made by this cupola process inherently carries impurities normal to cupola iron and, thus, is not of high purity.
Another method of producing metal powder is by a ice Water atomization process in which a stream of molten metal is engaged by a high pressure water sheet which breaks up or particlizes the molten metal. As the atomization is accomplished by a stream of water rather than air, the resulting particles have a lower oxide content than those produced by air atomization. However, iron particles formed by this process generally have a regular spheroidal shape which detracts from the physical properties of the powder and articles made from the powder.
The present invention is directed to an improved water atomization process for forming steel particles having a high density as well as an irregular shape which provides a substantial improvement in the physical properties of the compacted and sintered part. According to the process, molten steel containing less than 3.3% of total alloying elements and having a carbon content less than 1.8% is fed by gravity in the form of a downwardly moving stream. A plurality of fiat sheets or curtains of water are impinged against the stream of molten steel at an angle greater than 5 with respect to the axis of said stream to thereby atomize the stream of molten steel and provide a plurality of chain-like or clump-like agglomerates of spheroidal particles. As atomized, the particles generally have a size such that at least will pass through an 80 mesh screen.
Subsequently, the particles are heated at a temperature of about 1500 F. to 2100 F. in a reducing atmosphere for a period of time suflicient to soften the particles and reduce the carbon content to a value less than 0.05%. The resulting powder has a high density which in the past has only been available in the electrolytic grade. The apparent density of the loose powder is generally in the range of 2.6 to 3.3 grams/cc. and the powder has a pressed density under a compaction pressure of 30 t.s.i. of over 6.5 grams/cc. In addition to the high density, articles fabricated from the steel powder have a green strength of over 3,000 p.s.i., without a lubricant, at a com paction pressure of 30 t.s.i. The elongation of an article made from the powder compacted at a pressure of 50 t.s.i. with lubricant and sintered and without a carbon addition is generally in the range of 8 to 12% measured in a one-inch gauge length.
The process of the invention provides a steel powder which has properties comparable to that previously obtained only by the electrolytic process and yet is produced by a process which is considerably less costly than the electrolytic process.
Other objects and advantages will appear in the course of the following description.
The drawings illustrate the best mode presently contemplated of carrying out the invention.
In the drawings:
FIG. 1 is a schematic representation of the apparatus employed for atomizing the metal particles;
FIG. 2 is a plan view of the apparatus shown in FIG. 1;
FIG. 3 is a microphotograph of the particles taken at a magnification of x;
FIG. 4 is a microphotograph taken at a magnification of 100x and showing the cross section of the steel particles;
FIG. 5 is a graph illustrating the variations in hardness of the metal particles produced at difierent annealing temperatures;
FIG. 6 is a graph illustrating the variations in hardness of the metal particle-s produced by different holding times at the annealing temperatures; and
FIG. 7 is a graph illustrating the variations in green density of the metal powder with compaction pressure.
FIGS. 1 and 2 are schematic representations of the apparatus used for the water atomization of the steel particles. The structure includes a tundish 1 or crucible which contains a quantity of molten steel 2. The bottom of the tundish is provided with a series of outlet nozzles 3, and the molten steel flows downwardly by gravity through the nozzles 3 in the form of a series of streams 4.
The molten steel is particlized by a pair of sheets or curtains of water 5 which are discharged from continuous slots or nozzles 6 located beneath the tundish 1. The water is under a high pressure and impinges against the streams of molten steel 4 to particlize or atomize the stream, and the resulting particles fall downwardly and are collected in the water-filled container 7.
The molten steel 2 produced by one of the conventional steel-making processes, such as open-hearth electric furnace, basic oxygen and the like, contains less than 3.3% by weight, and preferably less than 2%, of total alloying elements and has a carbon content of less than 1.8%, preferably less than 0.20%, and under most conditions in the range of about 0.08% to 0.12%. The steel has a manganese content in the range of 0.01% to 1.5% by weight, and usually in the range of 0.1% to 0.4%.
The steel in the tundish 1 is at a temperature of about 3100 F. and flows by gravity through the outlet slots or nozzles 3. The nozzles 3 generally have an internal diameter in the range of 7 to of an inch. As the nozzle size is reduced below /8 inch there is an increased tendency for freezing of the steel in the nozzle unless the nozzle is heated. Similarly, as the nozzle size is increased beyond inch there is an increased tendency to form larger size particles during the atomization.
The water is directed against the stream 4 of molten steel at an angle greater than 5 with respect to the axis of the stream and generally at an angle of 15 to 55 from the vertical is preferred. This angle is shown as A in FIG. 1. If the angle A is increased beyond 55", molten steel and atomized particles may be deflected back upwardly and bridge across the nozzles to freeze the same, depending on the spacing of the nozzles and the geometry of the overall apparatus. The water sheets 5 may engage the steel stream 4 at opposite, horizontally aligned positions, or the sheets 5 can engage the molten stream at vertically spaced positions. In addition, the oppositely directed sheets 5 can each be at different angles to the vertical as long as both sheets are within the previously mentioned angular range.
While the drawings show a pair of flat water sheets 5 it is contemplated that a series of flat sheets can be used, or the sheet can be in conical form surrounding the stream of molten steel.
The temperature of the water employed in the atomization process is not critical and is generally less than 160 F.
The water which is used for the atomization is under substantial pressure above 500 p.s.i. and for most operations above 1,000 p.s.i. There is no maximum pressure limit for the water and normally, the maximum pressure is based on the pumping equipment used.
The water pressure must be correlated with the angle A, at which the water sheet is directed against the molten metal stream. When the angle A is decreased and approaches the lower limit of 5 the water pressure must be increased. Conversely, when the angle A is increased, the water pressure can be reduced toward the lower limit of 500 p.s.i. It has been found that in all cases the horizontal component of water velocity should be above 105 feet/ second to produce the desired agglomerated type of spheroidal particles.
It is desirable that the sheets 5 of water have a minimum thickness when they engage the molten steel stream 4. For most operations, the sheet 5 of water should have a thickness less than 0.075 inch, and preferably less 4 than 0.05 inch, at the point of discharge from the nozzle 6. The nozzle should be designed so that the sheets 5 do not flare out to any appreciable extent, but maintain this thickness when impinging against the molten steel stream 4.
The thin sheets 5 of water strike the molten steel stream 4 and atomize or particlize the steel to produce chainlike or clump-like agglomerates of generally spheroidal particles, as shown in FIGS. 3 and 4. The particles formed are substantially different from those formed by conventional processes in that the agglomerated nature of the spheroidal particles, which is developed in the atomization process, provides an irregular structure which results in a higher green strength for the articles subsequently made from the powder. The irregular shape of the agglomerated particles can be readily seen in FIG. 3. The steel powder, as atomized, has a particle size such that at least will pass through an 80 mesh sieve and at least 75% will normally pass through a mesh sieve.
Following the atomization, the steel powder is subjected to an annealing treatment which serves to soften the particles, reduce the oxide film and substantially decrease the carbon content. The powder is heated to a temperature in the range of 1500 F. to 2100 F. and preferably 1650 F. to 1800 F., in a reducing atmosphere, such as dissociated ammonia, hydrogen or other conventional decarburizing reducing gases. FIG. 5 is a graph illustrating the hardness of the annealed metal particles at various annealing temperatures. The hardness decreases quite rapidly at annealing temperatures of 1500 F. to about 1675 F., then increases slightly to a temperature of about l750 F. and again decreases in the annealing range of 1750 F. to 1800 F. From the graph in FIG. 5 it can be seen that the optimum annealing temperature is about 1675 F. for annealing at this temperature provides an unexpected decrease in hardness which falls considerably outside of the normally expected straight line function.
The annealing treatment serves to soften the steel particles, as well as reducing the carbon content to a value below 0.05%, and generally to a value in the range of 0.001% to 0.020%. To obtain the optimum ductility and subsequently obtain the maximum density for a given compaction pressure and the increased physical properties in the sintered product, the powder should be held at the annealing temperature for a period of at least 1.5 hours and preferably 2 hours.
FIG. 6 illustrates the hardness of the particles at various holding times at the annealing temperature of 1675 F. Hardness decreases rapidly as the time is increased and then flattens out rather abruptly at a time of 2 hours. Thereafter the hardness does not decrease appreciably with an increase in holding time. It is not necessary to maintain a substantially anhydrous annealing atmosphere. In practice, it has been found that dew points as high as 40 F. in the cold zone produce satisfactory results.
After the anneal, the particles are caked together and are broken apart by a hammermill process. The hammermilling, which is an impact process, breaks up the sintered cake while not breaking up the irregular agglomerated nature of the particles, and restores the original atomized particle size. In carrying out the hammermilling, a hammermill screen having M1" holes is initially used and this relatively large hole size insures that the particles will not be unduly subject to cold working while being broken up by the hammermilling. Following this, the particles are subjected to a second hammermilling operation using screens with about 0.040" diameter openings and this two-phase hammermilling serves to restore the atomized particle size without destroying the agglomerated nature of the particles and without adversely affecting the physical properties of the particles.
The resulting steel powder has an apparent density, which is the non-compacted density as defined by test procedure ASTM B2l248, in the range of 2.6 to 3.3
grams/cc. FIG. 4, a cross section of the particles taken at the magnification of 100x, shows the dense nature of the agglomerated particles which can be seen to be substantially free of voids, rifts, fissures or non-metallic inclusions.
In addition, the steel powder has a pressed density of over 6.5 grams/cc. and generally in the range of 6.6 to 6.9 grams/cc. The pressed density is based on a compaction pressure of 30 tons/sq. in. as defined in the test procedure ASTM B-331-58T, except that 0.5% dry zinc stearate lubricant was mixed with the powder. The powder has a high purity, at least 98% metallic iron.
In addition to the high density, the compacted steel powder also has a high green strength, over 3,000 p.s.i. without a lubricant, under a compaction pressure of 30 tons/sq. in. as measured by the ASTM test procedure B-312-58T.
The resulting steel powder can be used to form any desired machine parts or combination of parts by conventional powder metallurgy procedures. For example, a conventional lubricant, such as zinc stearate, and additional carbon, if desired, can be blended with the steel powder by suitable blending equipment. The blended powder is then compacted into the desired shape by a compaction pressure generally above 15 tons/sq. in. and preferably at 30 tons/ sq. in. or more.
The compacted powder is then sintered in a reducing atmosphere at a temperature in the range of 2000 F. to 2300 F. for a period of minutes to 1 hour depending on the composition and the final density desired.
Because of the higher density of the steel powder of the invention, better mechanical properties are provided in the final product with a given compacting pressure. Conversely, a lesser compacting pressure can be employed to obtain the same density.
While many metal parts can be made from powder having a green density of less than 6.8 grams/co, these parts, without special processing such as coining, resintering, or the addition of copper or other alloy additions, are normally not used in load transmitting applications nor in applications where the parts are subjected in service to shock loads or repeated loading. Furthermore, parts made from metal powder having a green density less than about 6.8 grams/ cc. cannot be satisfactorily surface hardened by carburizing, cyaniding, plating or the like, because of the porous nature of the part. Thus, many conventional metal powders have limited application because these powders, even though compacted at high pressures, have a green density below about 6.8 grams/ cc. In contrast to this, a green density of over 6.8 grams/cc. can be readily achieved with the metal powder of the invention with a single pressing.
FIG. 7 shows the relationship between the green compacted density of the powder and the compaction pressure. From FIG. 7, it can be seen that the green density increases with the compaction pressure and reaches a value of about 6.8 grams/cc. at a compaction pressure of about 32 t.s.i., and thereafter the green density continues to increase, reaching a value of about 7.4 grams/ cc. at a compaction pressure of 60 t.s.i. The high green density which is obtainable with the metal powder of the invention enables the powder to be used for the fabrication of parts such as gears, shafts, etc., which are subjected to repeated loading and shock in service. Moreover, due to the high sintered density the metal parts can be surface hardened by carburizing, cyaniding, and can be directely plated.
Because of the high particle density, high purity and high pressed density of the iron powder of this invention, sintered parts show good magnetic properties for soft magnetic applications. At a magnetizing force of 10 oersteds, the sintered and compacted parts with about a 93% theoretical density have a magnetic saturation in excess of 10,000 gauss.
Comparative D.C. magnetic measurements on solid ingot iron, which is one of the most commonly used soft magnetic materials, and pressed and sintered electrolytic grade iron powder and pressed and sintered iron powder of this invention are approximately equal. The two iron powders having a 93% theoretical density as sintered, which is a sintered density of 7.32 grams/cc, are within 7 to 10% of being equal to the full density ingot iron. Sponge iron pressed and sintered parts made under similar compacting pressures have densities less than 6.8 grams/ cc. and have a magnetic saturation and permeability of only one-fourth to one-tenth that of the pressed and sintered powder of the invention.
The particle size of the steel powder is determined by the atomization step and no grinding, crushing or milling operation is required to obtain a small particle size, as is necessary in most conventional processes. The grinding, crushing or milling, as used in the conventional process, not only increases the overall cost of the process, but also serves to work the particles, with the result that the physical properties are reduced. The step of hammermilling after annealing is merely to break up the sintered cake and is not a true grinding operation for the size of the individual particles is not reduced but the particle size is merely restored to the atomized condition.
Specific examples of the process of the invention are as follows:
Example 1 Molten steel having the following composition in weight percent was supplied to a tundish:
Percent Carbon 0.08 Manganese 0.27 Phosphorus 0.014 Sul-fur 0.024 Silicon 0.030 Iron Balance The temperature of the molten steel was 3150 F. and the steel flowed downwardly by gravity through an outlet nozzle having an internal diameter of inch. Two oppositely directed streams or curtains of water, positioned at a lownward angle of 20 with respect to the axis of the molten steel stream, were impinged against the metal to atomize the molten steel. The temperature of the water was initially 91 F. and had a final temperature of 125 F. The water was under a pressure of 1,110 p.s.i. and had a flow rate of 275 gallons per minute. The water streams were discharged through slots 3 inches long and 0.04 inch wide.
The resulting atomized steel powder had the following chemical analysis in weight percent:
Percent Carbon 0.08 Manganese 0.27 Phosphorus 0.012 Sulfur 0.023 Silicon 0.028 Hydrogen loss 1.20 Iron Balance The steel powder had the following screen analysis:
Percent retained Tyler sieve size: on sieve 28 0.5 35 0.4 48 0.9 3.5 7.0 200 38.5 325 27.4 Pan 21 8 The steel powder was then annealed in dissociated ammonia at a temperature of 1675 F. for 2 hours, subsequently cooled in the dissociated ammonia atmosphere to F. and then air-cooled to room temperature.
The annealed steel powder had the following analysis in weight percent:
Percent Carbon 0.014 Manganese 0.25 Phosphorus 0.008 Sulfur 0.019
Silicon 0.020 Hydrogen loss 0.11 Iron Balance The steel powder had an apparent density of 3.07 grams/cc, a pressed density at a compaction pressure of 30 tons/ sq. in. of 6.65 grams/cc. and a green strength of 3300 p.s.i., without lubricant, after pressing at 30 tons/ sq. in.
Example 2 Molten steel having the following composition in weight percent was supplied to a tundish:
Percent Carbon .09 Manganese a- .33 Phosphorus .003 Sulfur .017 Silicon .014 Iron Balance The temperature of the molten steel was 3150 F. and the steel flowed downwardly by gravity through three outlet nozzles having an internal diameter of inch. Two oppositely directed streams or curtains of water, positioned at a downward angle of 37 /2" with respect to the axis of the molten steel stream, were impinged against the metal to atomize the molten steel. The temperature of the water was initially 61 F. and had a final temperature of 125 F. The water was under a pressure of 1,110 p.s.i. and had a flow rate of 860 gallons per minute. The water streams were discharged through slots 10 inches long and 0.04 inch wide.
The resulting atomized steel powder had the following chemical analysis in weight percent:
Percent Carbon .09 Manganese .33 Phosphorus .003 Sulfur .017 Silicon .014 Hydrogen loss 1.50 Iron Balance The steel powder had the following screen analysis:
Percent retained Tyler sieve size: on sieve 28 1.0 35 0.7 48 1.9 80 6.4 100 4.3 200 37.4 325 24.4 Pan 23.9
The steel powder was then annealed in dissociated ammonia at a temperature of 1675 F. for two hours, subsequently cooled in the dissociated ammonia atmosphere to 140 F. and then air-cooled to room temperature.
The annealed steel powder had the following analysis in weight percent:
Percent Carbon .01 Manganese .33 Phosphorus .003 Sulfur .017 Silicon .014 Hydrogen loss .15 Iron Balance The steel powder had an apparent density of 2.8 6 grams/cc, a pressed density at a compaction pressure of 30 tons/sq. in. of 6.74 grams/cc. and a green strength of 3430 p.s.i., without lubricant, after pressing at 30 tons/ sq. in.
Various modes of carrying out the invention are contempla-ted as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.
1. A method of forming steel powder, comprising the steps of discharging a stream of molten steel from a source of supply with the molten steel having a chemical analysis corresponding to low carbon steel whereby said steel contains less than 3.3% by weight of alloying elements and up to about 0.20% by weight of carbon, impinging a flat sheet of water under a pressure greater than 500 p.s.i. against the stream at an angle greater than 5 with respect to the longitudinal axis of said stream to atomize the stream of molten steel and provide a plurality of agglomerates of solid sphere-like particles of a high density corresponding to low carbon steel and substantially free of voids, rifts, fissures and non-metallic inclusions, and said agglomerated particles as-atomized having a size sufficiently small to thereby eliminate any grinding operations to process the agglomerated particles to a smaller size and shape desirable for pressing the agglomerated particles into an article, heating the agglomerated particles in a decarburizing reducing gas to a temperature of r1500 F. to 2100 F. for a period of time suificient to soften the agglomerated particles and reduce the carbon content to less than 0.05% by weight and reduce the residual oxygen content to a value less than 0. 15% by weight and provide a cake-like structure, and thereafter breaking up the cakelike structure to substantially restore the as-atomized particle size of the agglomerates.
2. A method of formin steel powder, comprising the steps of atomizing a stream of low carbon molten steel having a tot-a1 alloy content of less than 2% by weight and having a carbon content in the range of 0.08% to 0.12% by weight and a manganese content in the range of 0.1% to 0.4% by weight by engaging the stream of molten steel with oppositely directed fiat sheets of water under a pressure of at least 1,000 p.s.i. and having a thickness less than 0.075 inch, said sheets engaging said stream at an angle of 15 to 55 with respect to the longitudinal axis of the stream to thereby provide a plurality of agglomerates of sphere-like particles having a size such that at least 85% of the agglomerated particles will pass through an mesh screen, thereafter directly annealing the agglomerated particles in a decarbur-izing reducing gas at a temperature of about 1 675" F. for a period of at least 1.5 hours to soften the agglomerated particles and provide a cake-like structure having a carbon content in the range of 0.001% to 0.020% by weight and an oxygen content of a value less than 0.15% by weight, said agglomerated particles being substantially free of voids, rifts, fissures and non-metallic inclusions, and subsequently hammermilling the cakelike structure to restore the as-atomized particle size of the agglomerates and provide a steel powder having a purity of at least 98% metallic iron, an apparent density greater than 2.6 grams/cc. and a pressed density at a compaction pressure of 30 tons/ sq. in. greater than 6.5 grams/ cc. with lubricant.
References Cited UNITED STATES PATENTS (Other references on following page) UNITED Silbereisen et a1. 750.5
10 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, v01. 1, pages 231-235.
5 DAVID L. REOK, Primary Examiner.
HYLAND BIZOT, Examiner.
N. F. MARKVA, W. W. STALLARD,