US3368890A - Metal powder from cast iron chips - Google Patents

Description

F b- 1 1968 w. SCHROEDER ETAL 3,368,890

METAL POWDER FROM CAST IRON CHIPS Filed Dec. 27, 1966 CAST IRON PARTICLES SCREEN PULVERIZE OVERSIZE CLASSIFY IRON OXIDE ADDITION ANNEAL SEPARATION OF FINES POWDER SUITABLE FOR POWDER METALLURICAL TECHNIQUES INVENTORS ezwiz m TORNEY United States Patent 3,368,890 METAL POWDER FROM CAST IRON CHIPS William L. Schroeder and Peter Vernia, Rochester, Mich, assignors to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Dec. 27, 1966, Ser. No. 605,064 7 Claims. (Cl. 75-211) ABSTRACT OF THE DKSULQSURE Iron powders particularly suitable for compaction by powder metallurgical techniques may be prepared from cast iron particles such as chips, borings, or the like. The particles are first comminuted to a powder of suitable particle size. An iron oxide is then blended with the powder and the mixture annealed at an elevated temperature below the austenite region in a reducing atmosphere. After cooling, any loose graphite or ultrafine iron powder is separated from the mixture.

This invention relates to a method of preparing iron powder for powder metallurgical processing. More particularly the invention relates to the preparation of powdered iron from cast iron chips, borings, turnings, and the like.

Many articles of commerce are prepared by compacting metal powders under high pressures into a compact of desired configuration and subsequently sintering the compact. The physical strength of the article so produced and the shrinkage or expansion of the compact upon sintering are a function of the properties of the starting material. In this regard, sponge iron powders are known to be particularly suitable for ferrous powder metallurgical operations. Ferrous powder in sponge form may be pressed to form a compact having a relatively high green fiber strength and a relatively low distortion upon sintering. To be more specific, sponge iron mixed with 1-25 graphite may be pressed at 30 tons per square inch into a compact having a green fiber strength in excess of 1000 p.s.i. Moreover, when the compact is sintered the change in dimension is very slight, there being an expansion of about 0.1 to 0.2%. While these are very suitable properties in a compact, sponge iron is more expensive than powder produced in other forms. It would be desirable for instance to produce a powder having compaction properties similar to sponge iron which is not as expensive.

Accordingly, it is an object of this invention to provide a method of producing iron powder, suitable for compaction by powder metallurgical techniques, from cast iron particles.

It is a more specific object of this invention to provide a method of converting cast iron chips, borings, and the like to a powder which is suitable to be compacted into an article having high fiber strength before sintering and low shrinkage upon sintering.

There and other objects are accomplished in accordance with our invention by first comminuting cast iron borings, chips, or the like to a powder of suitable particle size, preferably smaller than about 90 mesh. To the iron powder is added oxygen in the form of an iron oxide in a small but effective amount preferably comprising about 0.05 to 0.6% of the weight of the iron powder. The mixture is then heated at an elevated temperature below the critical temperature to react the iron oxide with the uncombined carbon (graphite) which coats the iron powder after comminuting. At the same time that the iron oxide is reacting with graphite on the surface of the iron powder the heat is effecting some changes in the pearlitic cementite within the iron powder. A portion of the pearlitic iron carbide is decomposed into graphite and iron. Much of the rest of the iron carbide is partially spheroidized. The net effect of these changes within the iron powder is to render it more ductile. The heating is accomplished in a reducing atmosphere. While the powder is still in the reducing atmosphere it is cooled to about room temperature and any remaining loose graphite together with ultrafine particles of iron are removed from the mixture to reduce the total carbon content preferably to about 2% by weight. This powder is then ready to be compacted and sintered into a useful article of commerce. In an alternative embodiment of the invention the iron powder so produced may advantageously be blended with up to about 30% by weight sponge iron prior to compaction.

Other objects and advantages of our invention will become apparent from a detailed description of the method which follows. During this description reference will be made to the drawing which is a block diagram of a preferred embodiment of the process illustrating the principles of the invention.

Referring to the drawing, the feed stock comprises cast iron particles, which may be of scrap origin such as chips, borings, and the like so long as they are substantially free of oil and moisture. Depending upon the nature and capacity of the com-minuting equipment employed it may be advantageous to screen or otherwise classify the starting material for efiicient operation. The cast iron particles are comminuted preferably by high impact pulverization means, such as a hammer mill or the like, to a powder of particle size which is suitable for powder metallurgical processing. In general, it is preferred that about 95% of the powder will pass a mesh Tyler standard screen and about 23-24% of the powder will pass through a 325 mesh Tyler standard screen. The comminuting operation may be conveniently performed in commercially available equipment such as a hammer mill. For particularly efficient operation the comminuting equipment can be employed in combination with an internal centrifugal classifier whereby the fine -90 mesh powder is discharged from the classifier and coarser material is recirculated until it has been broken down to a powder of suitable dimensions. Also in connection with the comminution step, a cyclone separator can be operated in combination with the pulverizer to remove excessively fine iron, e.g. 10 to 20 microns in diameter, and free graphite particles from the 90 mesh powdered iron.

In prior art techniques cast iron powders so produced have subsequently been annealed to relieve the stresses induced by 'pulverizatidn and then compacted into articles of commerce. However, upon sintering, these compacts have undergone excessive shrinkage. Moreover, the fiber strength of the unsintered article is too low for many applications. Accordingly, we have developed a process by which the comminuted powder may be treated to produce a powder which is particularly useful for pressing into a compact which has minimum dimension change upon sintering and has a markedly improved fiber strength.

In accordance with the invention, a small amount of oxygen in the form of an iron oxide is blended with the comminuted cast iron powder. Some benefits are obtained from very small additions. However, best results are obtained when an amount of oxygen equivalent to 0.05 to 0.6% of the weight of the iron powder is added. In general, this can be accomplished by adding to 2% of an iron oxide such as FeO, R2 0 or Pe o, based upon the weight of the cast iron powder treated. The iron oxide is employed in ultrafine powder form. Preferably the greatest dimension of an average particle is ten microns or less. Iron oxide particles of this size will coat the particles of iron to more effectively react with the graphite thereon.

Subsequently, this powdered mixture is heated in a suit able furnace containing a neutral or slightly reducing atmosphere to an elevated temperature below the austenitic transformation temperature of the cast iron. Preferably a temperature in the range of 1000 F. to 1400 F. is employed. These temperatures are commonly known as annealing temperatures in the art offerrous metallurgy. The effect and purpose of the heating or anneal is many fold. As in the prior art one effect is to relieve the stresses induced in the powder by the comminuting step. In addition, however, the iron oxides incorporated upon the pulverized cast iron react rapidly with the free graphite which is deposited upon the iron powder in a thin film during the comminution step. Carbon monoxide is evolved and the concentration of graphite on the powder is decreased while the iron oxide is reduced to elemental iron. At the same time during the anneal, the pearlitic carbides within the iron powder, are graphitized or partially spheroidized to render the iron more ductile.

We have found that particularly beneficial results are obtained if the heating or anneal is conducted sequentially at two different temperatures. First, the powders are heated to about 1050 F. At or below this temperature, Fe O or Fe O is believed to be directly converted to iron and carbon monoxide without forming a lower oxide. This reaction path may reduce the required treatment time of the iron oxides if these particular oxides are used. Moreover, it is believed that by treating the powder at a lower temperature before heating further to a higher subcritical temperature, nuclei are formed which enhance the graphitization and spheroidization of cementite. Thus, it is preferred that the powder be heated at 1050 F. for about two hours and then immediately raised to about l400 for an additional two hours. The addition of iron oxide to the comminuted powder has had the effect of markedly increasing the green fiber strength of cast iron powders so treated. For example, a compact formed by compressing milled and annealed cast iron borings under a pressure of 30 tons per square inch was found to have a green fiber strength of 250 p.s.i. However, a compact formed from powders prepared in exactly the same manner, except that an addition of 0.5% by weight Fe O was made prior to annealing, was found to have a green fiber strength of over 400 p.s.i.

Preferably the atmosphere of the furnace, in which the annealing or heating step is carried out, is neutral or slightly reducing. An example of a suitable atmosphere is one comprised of 95% nitrogen and 5% hydrogen. Atmospheres employing up to 98% nitrogen and about 2% hydrogen have been successfully used, but the higher hydrogen content is preferred. The annealing treatment may be prolonged beyond the total of four hours specified above to obtain further spheroidization of pearlitic cementite but little or no additional improvement in the physical properties of the compacted articles is obtained from powder so treated.

After annealing the iron powder is cooled, preferably w1thin the reducing atmosphere, to prevent oxidation. Upon cooling to a suitable temperature at or near room temperature, remaining loose graphite and ultrafine iron particles (l020 microns in greatest dimension) are separated from the mixture. This may be readily accomplished, for example, in a counter flow type air separator wherein the powders are directed downwardly against an upwardly flowing stream of air. The light graphite particles and the fine iron particles are carried upwardly with the air out of the separator while iron powder of desired dimension drops to the bottom. The product withdrawn from the bottom of the separator is a ductile iron powder which contains carbon preferably in u'ncornbined form which has been at least partially spheroidized so as to increase the toughness of the material. Powder at this stage of the processing typically contains only about l.8%2% by weight carbon. It may be directly compacted in a suitable press under a pressure of about 30 ton p.s.i. into a desired configuration. The green fiber strength of the compact so produced is typically about 400 p.s.i. However, the compact is then sintered for 30- 50 minutes at 2040-2090 F. whereupon an article having a minimum fiber strength of 60,000 pounds p.s.i. is obtained. It has been observed that the linear shrinkage upon sintering of the compact so produced is about 1.3% or less.

If linear shrinkage of about 1.3% upon sintering cannot be tolerated it has been found that up to about 30% by weight sponge iron may advantageously be mixed with the powdered cast iron prior to compacting. Sponge iron powders are known to effect an increase in the size of powder compacts upon sintering. Thus, the addition of the more expensive sponge iron will increase the linear shrinkage upon sintering. It will also tend to increase the green fiber strength of the compact. We have found that a mixture of 70 parts of cast iron powder produced in accordance with our invention, 30 parts sponge iron, and about 0.3 part graphite (to compensate for the absence of carbon in the sponge iron) may be compressed at 30 tons per square inch to yield a compact having a green fiber strength of about 700 p.s.i. Upon sintering at 2040 F.2090 F. for 30 to 50 minutes a minimum fiber strength of 60,000 p.s.i. is obtained and the linear shrinkage is only about 0.3%.

A specific example of the invention would be useful for purposes of illustration. Cast iron borings comprising by weight about 3.4% carbon, 2.3% silicon, 0.2% phosphorus, 0.08% sulfur, and the balance iron were pulverized in a commercially available hammer mill which contained an internal centrifugal classifier. The classifier was adapted to pass particles mesh and smaller, the oversize particles being immediately recycled. A cyclone separator (air classifier) was employed on the -90 mesh powder to remove excessively fine iron and free graphite particles. In this manner sufficient graphite was removed to lower the carbon content from 3.4% to about 2.6%.

A small amount of Fe O powder, comprising about 0.5% by weight of the cast iron powder, was mixed with the powdered iron. The mixture was annealed two hours at 1050 F. and then two hours at 1400 F. The atmosphere in the furnace comprised nitrogen and 5% hydrogen. The annealed powder was cooled to about normal room temperature.

The ultra fine iron, 10 to 20 microns in greatest dimension, and the loose graphite were removed from iron powder by counter flow air separation. This was accomplished by charging the powder at the top of a vertically disposed tube downwardly against an upwardly flowing stream of air. The fine iron and graphite were carried out with the air. About 45% by weight of the material was lost and carbon content was reduced from 2.6% by weight to about 2.0% by weight.

A portion of the powdered product of the separation process was pressed under a pressure of 30 tons per square inch. The resulting compacts had a green fiber strength of 400 p.s.i. These compacts were sintered at 2080 F. and 40 minutes. The fiber stress of the sintered compacts was in excess of 60,000 p.s.i. and the linear shrinkage upon sintering was 1.3%.

A second portion of the powdered iron as prepared above was mixed with 30% of its weight of sponge iron and 0.3% of its weight of graphite. This mixture was compressed under 30 tons per square inch to yield a compact having a green fiber strength of 700 p.s.i. This compact was sintered as above to yield a structure having a fiber strength in excess of 60,000 p.s.i. with a shrinkage of 0.6%.

While our invention has been described in terms of a preferred embodiment, it is, of course, realized that other forms may be readily adapted by those skilled in the art. Therefore, our invention is to be considered limited only by the scope of the appended claims.

We claim:

1. A method of producing particulate iron suitable for powder metallurgical processing from cast iron particles comprising the steps of comminuting clean, dry, cast iron particles to a fine iron powder, the particles of said iron powder having on the surface thereof loose graphite as a consequence of said comminuting step; mixing oxygen in the form of an iron oxide with said iron powder, said iron oxide being in the form of particles which are substantially smaller than said cast iron powder particles so as to coat the surface thereof and the amount of said oxide being small but sufficient to react with said loose graphite coating on said powdered iron particles; heating said mixture to an elevated temperature below the austenitic transformation temperature of the cast iron powder for a time whereby said iron oxide is substantially fully reacted with said loose graphite coating and where- 'by the iron carbides which are inherently present as pearlitic cementite in said cast iron are in part graphitizer and the rest partially spheroidized; cooling said reacted mixture and separating therefrom loose graphite and ultrafine iron to yield said particulate iron for powder metallurgical processing.

2. A method as in claim 1 wherein the amount of said iron oxide which is blended with said iron powder is sufiicient to add 0.05%0.6% by weight of oxygen based upon the weight of said iron powder.

3. A method of producing particulate iron suitable for powder metallurgical processing from cast iron particles comprising the steps of comminuting clean, dry, cast iron particles until -at least about 95% by weight of said particles will pass through a 90 mesh screen, the particles of said comminuted cast iron having on the surface thereof loose graphite as a consequence of said comminuting step; mixing small amounts up to about 2% by weight of an iron oxide with said iron powder, said iron oxide being in the form of particles which are substantially smaller than the comminuted cast iron particles so as to coat the surface thereof; heating said mixture in a reducing atmosphere to an elevated temperature below the austenitic transformation temperature of said mixture for a time whereby said iron oxide is substantially fully reacted with said loose graphite coating and whereby the iron carbides which 'are inherently present as pearlitic cementite in said cast iron are in part graphitized and the rest partially spheroidized to render said comminuted iron more ductile; cooling said reacted mixture in a re ducing atmosphere to about room temperature and separating loose graphite and iron particles smaller than -20 microns in largest dimension from said reacted mixture to yield said particulate iron for powder metallurgical processing.

4. A method as in claim 3 wherein said blended mixture of said cast iron and iron oxide is heated at a tema 6 perature in the range of about 1000 F. to 1400 F. for a period of up to about four hours.

5. A method of producing particulate iron suitable for powder metallurgical processing from cast iron particles comprising the steps of comminuting clean, dry, cast iron particles to a fine iron powder, the particles of said iron powder having on the surface thereof loose graphite as a consequence of said comminuting step; mixing oxygen in the form of an iron oxide with said iron powder, the amount of oxygen being (MOS-06% by weight of said iron powder and said iron oxide being in the form of particles which are substantially smaller than said cast iron powder particles so as to coat the surface thereof; heating said mixture to an elevated temperature below the austenitic transformation temperaure of said mixture for a time suflicient to react substantially all of said iron oxide with said loose graphite and whereby the iron carbides which are inherently present as pearlitic cementite in said cast iron are in part graphitized and the rest partially spheroidized'; cooling said reacted mixture; separating loose graphite and ultrafine iron from said reacted mixture; and mixing up to 30% by weight of sponge iron powder with the remaining iron powder to yield a particulate iron suitable for powder metallurgical processing.

6. A method of producing an article of manufacture wherein the iron powder produced in accordance with the method of claim 1 is compressed under pressure of at least 25 tons p.s.i. into a compact of predetermined configuration and said compact is sintered at a temperature of 20402090 F. for 3050 minutes.

7. A method of producing an article of manufacture comprising the steps of compacting the powdered iron and sponge iron mixture produced in accordance with claim 5 into a compact of predetermined configuration under a pressure of at least 25 tons per square inch and subsequently sintering said compact at a temperature in the range of 20402090 F. for a period of 30-50 minutes.

References Cited UNITED STATES PATENTS 2,541,153 2/1951 Chadwick .55 X 2,775,516 12/1956 Shafer 75.54 2,784,073 3/1957 Michalke 75-.55 3,073,695 1/1963 Silbereisen 75-.55 3,194,658 7/1965 Storchheim 75-211 X 3,326,676 6/1967 Riibel 75-201 CARL D. QUARFORTH, Primary Examiner. BENJAMIN R. PADGETT, Examiner. A. J. STEINER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,368,890 February 13, 1968 William L. Schroeder et a1.

pat-

d that error appears in the above numbered It is hereby certifie and that the said Letters Patent should read as ent requiring correction corrected below.

Column 1 line 58 for "There" read These column 4 line 13, for "increase" read decrease column 5, line 17, for "graphitizer" read graphitized Signed and sealed this 15th day of April 1969.

(SEAL) Attest:

EDWARD J BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

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