US3419383A - Producing pulverulent iron for powder metallurgy by multistage reduction - Google Patents

Description

nited States Patent 3 419 383 PRODUCING PULVER ULElN T IRON FOR POWDER METALLURGY BY MULTISTAGE REDUCTION Harry R. Hatcher, Crown Point, Ind., and Charles R.

Harr, Johnstown, Pa., assignors, by mesne assignments,

to SCM Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Apr. 19, 1966, Ser. No. 543,545

11 Claims. (Cl. 75-.5)

The present invention relates to powder metallurgy and, more particularly, to a process for producing iron powder adapted for powder metallurgy by subjecting a feed stock or the like to a multistage reduction with intermediate grinding.

Within recent years, the practice of fashioning machine parts or the like from metallic iron powder has undergone significant growth. Articles produced by powder metallurgy commonly replace articles of cast iron or steel formed by the more conventional casting or machining operations. For example, compactible iron powder is used for the fabrication of a wide variety of structural parts, such as sleeves, bushings, gears, etc. For this purpose, the iron powder must possess a wide range of acceptable physical properties, including acceptable values for apparent denstiy, flow, green strength, tensile strength and modulus of rupture after sintering, and still other certain physical characteristics.

In general, to produce an iron powder adapted for use in powder metallurgy a suitable high carbon iron, either alone or mixed with elemental (free) iron, is comminuted to define a powder feed stock which is then decarburized as by chemical oxidation. For instance, a powder feed stock of high carbon iron, and an oxide of iron is treated under such conditions that the carbon in the iron combines with the oxygen of the iron oxide to reduce the latter to iron, while the high carbon iron is simultaneously decarburized. The resulting iron powder is then compacted in a mold into substantially the shape and size desired in a finished product, in this way dispensing with expensive machining operations. After compaction, which may be at a pressure up to 100,000 pounds per square inch, the resulting shape is heated at an exemplary temperature of about 2000 F. to about 2100 F. under nonoxidizing conditions, for a sufiicient time to produce the ultimate sintered article.

It has now been discovered that subjecting a feed stock or the like to a multistage reduction with intermediate grinding, at least one stage subsequent to the grinding step having a reducing atmosphere, results in an improved overall process for producing compactible iron powder and also provides end products of improved sintered strength. In one form, a dual stage or double pass through furnace is employed, the grinding taking place intermediate these two stages. The importance of the intermediate grinding step lies in its effect on controlling particle size distribution, apparent density, and compressibility of the resulting iron product.

The intermediate grinding step of the present invention apparently opens new surfaces of the material being treated and agitates it so as to allow more efficient reduction thereafter. In addition, it has been found that the second or a succeeding stage can be more flexibly controlled as to temperature than would otherwise be the case if only a single pass or stage were employed.

The density of the material fed to a second or successive stage has also been found to be important and preferably should be within a certain range in order to achieve desired results. When the grinding step imparts workhardening to the metal particles, this is also rectified by the treatment afforded by the second stage or pass which not only completes the reduction but anneals the workhardened metal particles.

When the described conditions are followed in processing a ferruginous charge, the resulting compactible iron metal powder flows well and fills a mold well and requires modest compaction pressure for molding. Additionally, the reduced powder of the present invention has a good green strength without sacrificing such desirable pouring and filling properties. Moreover, powders prepared in accordance with the present process provide sintered articles of increased strength.

Generally, the feed stock is a ferruginous material comprising a mixture of iron, iron carbide, and iron oxide. A preferred component furnishing the iron and iron carbide is a hypereutectic composition consisting essentially of coarse grain primary platelets of iron carbide in a matrix of iron carbide-gamma iron uetectic which is preferably free of graphite and contains about 4.1 percent to about 4.6 percent carbon. The instrusions of platelets of iron carbide into the matrix (which can be observed under a microscope) contribute materially to the grindability or frangibility of the eutectic. This is important to easily powderng the composition. The amount of carbon present can also be important. If less than about 4.1 percent carbon is available, an insufiicient number of the platelets is formed which deleteriously affects the desired frangibility. On the other hand, if more than about 4.6 percent carbon is present, graphite is apt to form, as during quenching of the melt which produces the hypereutectic composition. A small quantity of graphite can be tolerated, but for best results none should be present. Graphite tends to nucleate and form still more graphite.

A convenient source of the present hypereutectic composition is what is known in the art as white cast iron. This type of iron can be produced by rapidly quenching a melt of a high carbon-iron alloy. A brittle structure results that is low in austenite and high in cementite, pearlite, and martensite. The brittleness also contributes to the desired frangibility of the composition. In particular, Sorelmetal admirably provides a source of the present hypereutectic composition. Sorelmetal, a product of Quebec Iron & Titanium Corporation of Sorel, Canada, is a cast iron obtained from the electric arc furnace smelting of iron-bearing titaniferous ore, ilmenite, in the presence of coal.

An iron oxide-containing ferruginous material is added to and thoroughly mixed with the hypereutectic composition to reduce the iron carbide and simultaneously prO- vide additional free metallic iron from the oxide. Any iron oxide can be used, such as naturally occurring ores like magnetite, hematite, and limonite. Similarly, partially reduced iron oxides having about two percent to about 30 percent hydrogen loss (as determned by a conventional test of the Metal Powder Industries Federation) can also be used. A quite inexpensive and readily available source of an iron oxide is mill scale which comprises mixed iron oxides containing about percent iron. The relative amounts of the iron carbide and iron oxide in the free iron mix are not at all critical as long as at least sufiicient oxide is present stoichiometrically to remove substantially all of the carbon. As previously indicated, complete removal of the carbon is not essential as when the presence of a little (less than about 1.5 percent) can be tolerated. In practice, from about 60 to 85 parts by weight of a Sorelmetal have been mixed with from about 15 to about 40 parts by weight of mill scale. A ratio of about parts of Sorelmetal to about 20 parts of mill scale is preferred.

In the usual procedure, the feed stock is comminuted to powder form by known conventional means. The advantages accruing from grinding the feed stock to a certain particle size form the subject matter of another application, filed of even date herewith in the names of Harry R. Hatcher and William Holtzman, entitled, Iron Powder for Forming Sintered Articles of Improved Strength, and assigned Ser. No. 543,570. Compaction of the powder feed stock may be next carried out by standard machinery, such as a briquetting machine, to form discrete aggregates or briquets, although it is understood that some fines may still remain in the feed stock. The advantages accruing from compacting the feed stock form the subject matter of another application, filed of even date herewith in the name of Harry R. Hatcher, entitled, Producing Pulverulent Iron for Powder Metallurgy by Compacting Feed Stocks, and assigned Ser. No. 543,557. Ordinarily, about 80 percent by weight of the compacted feed stock is coarser than 4 mesh, and about percent by weight is finer than 4 mesh. All references here and in the claims to a mesh size are with respect to the Tyler Scale. The powder feed stock, prior to any reduction treatment, normally has a bulk density up to about three grams per cubic centimeter.

The feed stock is next reduced in accordance with the present invention. Although the multistage reduction can be carried out in several stages or passes (and through the same or different furnaces), two stages usually sufiice. The first stage may be effected at a temperature within the range of about 1600 F. to about 1900 F., and preferably from about 1700 F. to about 1750 F. for efiiciency and economy, for a sufficient time to substantially reduce the feed stock. Temperatures in excess of about 1900 F. tend to produce sintering. The atmosphere of the first stage should be at least neutral (nonoxidizing) with respect to the charge, but it can be chemically reductive if desired. Some agglomeration also occurs in this first stage.

After the first stage treatment and after the resulting intermediate product has cooled, it is ground to produce a charge preferably having an apparent density within the range of about 2.2 to about 2.5 grams per cubic centimeter to obtain a final product having an apparent density in the range of about 2.4 to about 2.6 grams per cubic centimeter. If the intermediate product has a density greater than about 2.5, this specification is diflicult to meet, and there is an undesirable loss of green strength in the tiltimate powder produced. On the other hand, if the density is less than about 2.2 grams per cubic centimeter, the resulting powder does not flow as well. The grinding of the material intermediate the stages is usually to an average particle size coarser than that of the ultimate resulting iron product.

If the density of the powder at this intermediate point in the process falls without the stated range, it is within the contemplation of the invention to adjust the density prior to a second stage treatment. The density of the powder can be effected by the length of time of the first stage treatment and/or by the extent of the grinding step. Normally, this grinding is sufficient to pass the powder through a mesh size of about to 60, the coarser material being recycled back to a first stage operation. However, it is also within the contemplation of the invention to adjust the density by adding to the ground powder of the first stage fresh ingredients of the materials constituting the raw feed stock. For example, Sorelmetal or mill scale can be added prior to the second stage treatment. The addition of mill scale lowers the density, while the addition of Sorelmetal raises the density.

The powder, treated as just described and with an adjusted density if necessary or desirable, now passes to the second stage. The furnace used for the first stage may be used for the second or succeeding stages, but usually different furnaces are used for each stage for simplicity of operation. The atmosphere of the second stage should be reductive to complete substantially the reduction of the charge thereto and the heating therein anneals the charge to remove any work-hardening efiects imparted by the intermediate grinding step. The reductive atmosphere can comprise pure hydrogen, hydrogen containing small proportions of nitrogen, or dissociated ammonia, or other common reducing gas mixtures containing a high proportion of hydrogen, optionally with some carbon monoxide. The reduction preferably is carried out in the substantial absence of molecular oxygen, carbon dioxide, water vapor, or other gases capable of oxidizing iron under normal operating conditions. A typical gas feed which supplies a good reductive atmosphere is 75 volume percent hydrogen and 25 volume percent nitrogen having a dewpoint of 39 F. It has been found that the operating conditions of the second stage can be more flexible than those of the first. The temperatures can be somewhat higher or lower than those indicated for the first stage, to provide a greater degree of freedom in operation. For example, the second stage treatment can be effective at temperatures as low as about 1350 F. and as high as about 2000 F. and higher where the described hypereutectic composition has now been decarburized, although the most efiicient temperature range is still about 1700 F. to about 1750 F.

The preferred apparatus for reduction is a thin layer continuous belt furnace, although the reduction can be erforrned batch-wise or in trays or saggers passed through a tunnel kiln. A slight positive pressure generally is maintained during reduction to prevent air from seeping into the apparatus. If the apparatus can be made tight, a high vacuum can be used, the rarefied atmosphere thus provided being suitable in itself as a neutral atmosphere when venting of gaseous byproducts is practiced.

The following examples only illustrate the invention and should not be construed in any manner to limit the claims. All percentages indicated are by weight. In the case of screen analysis, the percentage listed represents the amount by weight passing through the indicated screen size.

Example 1 The following is a working example illustrating the technique followed in exemplary operation. A Sorelmetal and mill scale were used having the chemical and screen analyses as given by Table A.

TABLE A Sorelmetal Mill scale Chemical analysis:

1l 0.30 max.

a )on.... Sulfun- Copper. Silicon....

Thru 28 mesh 5-25 -.II. On so mesh-310.

Thru 60 mesh-1030.

accordance with M.P.I.F. Std. 2-64, issued 1948,

accordance with M.P.I.F. Std. 6-64, issued 1954 Dctermined in revised 1964.

2 Determined in revised 1964.

Each component was separately dried and then separately ground in a ball mill. In each case, grinding was continued until about 72 percent to about percent passed 325 mesh, and about 0.2 percent to about 1.0 percent 'was retained on mesh. The Sorelmetal and mill scale were next thoroughly and intimately mixed together in a weight ratio of 4:1, respectively, and then briquetted by a conventional briquetting machine at a pressure of about 2400- p.s.i. to about 2600 p.s.i.

The briquets were reduced in a thin layer continuous belt furnace at a temperature of 1750 F. in an atmosphere of dissociated ammonia. The bed of the furnace was a moving belt traveling at 5% inches per minute to yield a reduction time at temperature stated of about minutes. The approximate feed rate in pounds per hour was 1325. The bed depth of briquets being reduced was about 1 /2 inches. The intermediate product emerged continuously from the furnace at an approximate rate of 1165 pounds per hour, having a hydrogen loss of 1.50 maximum and carbon content of 0.30 maximum. After cooling, the intermediate product was ground in an attrition mill in an atmosphere of inert gas to particles passing 60 mesh and having a density between about 2.2 and 2.4 grams per cubic centimeter.

The intermediate product then was sent through a second thin layer furnace maintained at 1750 F. The belt speed in the second furnace was about six inches per minute and the bed depth was about 1 /8 inches to yield a holding time at temperature stated of about -25 minutes. Particle feed was continuous at the rate of about 1200 pounds per hour. The product output came off the furnace continuously at about 1160 pounds per hour. The product was ground in inert atmosphere so that it passed an 80 mesh screen. The product had a carbon content of 0.08 percent maximum, a hydrogen loss of 0.50 percent maximum, an apparent density of 2.25 to 2.40 grams per cubic centimeter, and 35 percent maximum of the particles were finer than 325 mesh. This resultant iron powder product could then be used to form shaped articles by standard pressing and sintering operations.

Example 2 This example illustrates the increased sintered strength obtained from articles produced with iron powder prepared by the multistage operation of the present invention as compared with articles produced with iron powder produced by a single stage operation. Table B lists the compositions of powder blends.

TAB LE B Composition Parts of of prepared No. of stages Parts of prestandard Blend powder (Sorelfor prepared pared powder iron powder metal/mill powder in blend in blend scale) In Table B, the term prepared powder designates powder prepared in accordance with the present invention. The term standard iron powder designates an available, commercial grade of iron powder known for its good sintered strength.

All of the feed stocks comprised Sorelmetal and mill scale in the various ratios. All of the feed stocks were also processed in accordance with the process of Example 1, except that the prepared powders of Blends 1 through 4 had only one pass through the furnace with no intermediate grinding. After reduction, some of the prepared powders (both from the single and double passes) were mixed with the standard commercial grade of iron powder known for its good sintered strength.

Four different sintering compositions were prepared for each of the blends of Table B, namely, (1) 100 percent of the blend; (2) 93 percent of the blend and 7 percent copper; (3) 99 percent of the blend and 1 percent carbon; and (4) 94 percent of the blend, 5 percent of copper, and 1 percent of carbon. In addition, 1 percent zinc stearate Was added to each sintering composition merely as a mold lubricant. Three sintered test bars were prepared for each composition and tested to failure in transverse rupture. To judge the overall result of sintered strength (rather than the strength of any particular formula) an index was devised which represented the sum of the moduli of top ture corrected to 6.5 sintered density for three bars of each of the four compositions. Thus, in Table C which shows TABLE C.SINTERED STRENGTHS Blend Stages Strength index KONNNNNHHHD- Those powders reduced in a two-stage treatment show significantly increased sintered strengths over powders reduced in a one-stage treatment. For example Blend 5 had better strength than Blend 1, although Blend 1, otherwise having the same composition as Blend 5, contained 1.2 parts of the commercial grade of an iron powder having known good sintering properties and Blend 5 contained none of such powder. Blend 6 had better strength than Blend 2, although Blend 2, otherwise having the same composition, contained 12 parts of the commercial iron powder while Blend 6 contained none. Blend 7 had better strength than Blend 3, the former having slightly more of the commercial powder than the latter. Each of Blends 9 and 10 had better strength than Blend 4, these three last mentioned blends having exactly the same composition.

Although Blends 1 through 4 are characterized as having a single stage reduction and Blends 5 through 10 are characterized as having a two stage reduction, it is understood that the extent of reduction of all blends was substantially the same so as to put the resulting data on a comparative basis.

While the foregoing described several embodiments of the present invention, it is understood that the invention may be practiced in still other forms within the scope of the following claims.

What is claimed is:

1. In a process for producing pulverulent iron adapted for compaction by powder metallurgy wherein a particulate iron oxide-containing ferruginous material is reduced with a reducing agent in a reaction zone having a nonoxidizing atmosphere, some agglomeration of the resulting iron product takes place, and said ultimate resulting iron product is ground to a fine state, the improvement which comprises: reducing said material in a plurality of stages, and grinding said material intermediate two of said stages, at least one stage subsequent to the grinding having a reducing atmosphere.

2. The process of claim 1 wherein the grinding of said material intermediate two of said stages is to an average particle size coarser than that of the ultimate resulting iron product.

3. The process of claim 1 wherein said material is partially reduced in one of said stages prior to grinding, and thereafter is substantially completely reduced and annealed by succeeding stages.

4. The process of claim 1 wherein the density of said material is within the range of about 2.2 to about 2.5 grams per cubic centimeter after said grinding intermediate two of said stages.

5. The process of claim 1 wherein said ferruginous iron oxide-containing material comprises a mixture of iron, iron carbide, and iron oxide.

6. The process of claim 5 wherein the density of said material after said grinding intermediate two of said stages is adjusted to a value within the range of about 2.2 to about 2.5 grams per cubic centimeter by adding to said material one or more of the ingredients of said oxidecontaining material.

7. The process of claim 1 wherein the nonoxidizing atmosphere of said reaction zone is neutral-to-reductive in one of said stages prior to the intermediate grinding, and reductive and annealing with respect to said material in one of said stages subsequent to said intermediate grindmg.

8. The process of claim 5 wherein said iron carbide is a hypereutectic composition consisting essentially of primary platelets of iron carbide in a matrix of iron carbide-gamma iron eutectic.

9. The process of claim 5 wherein said hypereutectic composition is substantially free of graphite and contains from about 4.1 percent to about 4.6 percent by weight of carbon.

10. The process of claim 5 wherein said iron oxidecontaining material includes Sorelmetal.

11. The process of claim 5 wherein said iron oxide comprises mill scale.

References Cited UNITED STATES PATENTS 1/1963 Silbereisen et a1. 75211 7/ 1965 Storcheim 752l1 10/1965 Von Bogdandy et al. 75-211 6/1967 Riibel et a1. 75201 2/1968 Schroeder et a1. 75-5 US. Cl. X.R.

Claims (1)

1. IN A PROCESS FOR PRODUCING PULVERULENT IRON ADAPTED FOR COMPACTION BY POWER METALLURGY WHEREIN A PARTICULATE IRON OXIDE-CONTAINING FERRUGINOUS MATERIAL IS REDUCED WITH A REDUCING AGENT IN A REACTION ZONE HAVING A NONOXIDIZING ATMOSPHERE, SOME AGGLOMERATION OF THE RESULTING IRON PRODUCT TAKES PLACE, AND SAID ULTIMATE RESULTING IRON PRODUCT IS GROUND TO A FINE STATE, THE IMPROVEMENT WHICH COMPRISES: REDUCING SAID MATERIAL IN A PLURALITY OF STAGES, AND GRINDING SAID MATERIAL INTERMEDIATE TWO OF SAID STAGES, AT LEAST ONE STAGE SUBSEQUENT TO THE GRINDING HAVING A REDUCING ATMOSPHERE.