US2284638A - Metallurgy of ferrous metals - Google Patents

Description

June 2, 1942. F. H. CLARK ET AL METALLURGY OF FERROUS METALS Filed Aug. 20, 1938 manna/v50 arr-4 PRODUCL-D FROM S/IVTEFED POWOERS VS/NTERED //?0/v Po'wam [so/mm 70 com mu 0 $7'L 5 ON POWDER AND CA HBO/V PEARL/TE STRUCTURE OF ANNE/MED STEEL P5000650; FROM .Sl/V T15R50 POWDERS SINTEFED MIXTURE 0F INVENTORS Clark ATTORMF) Patented June 2, 1%42 METALLURGY OF FERROUS ME'K ALS Frances K. Clark, New York, and Robert F. Dirkes, Jamaica, N. Y.

Application August 20, 1938, Serial No. 225,844

8 Claims.

This invention relates to the fabrication of hardened or hardenable metals from metal powders, and is a division and continuation in part of our application Ser. No. 205,392.

At the present time articles, such as tools, dies, etc., composed of hardened steel have been rather expensive and it has been difficult to shape accurately such parts in the desired forms, particularly where the forms are complicated, since hardenable steels are diflicult to work by mechanical tools and during such working there is a tendency of the materials to crack, tear, etc, Moreover, it is extremely diificult to obtain identical duplicates by such methods.

It has been proposed heretofore to fabricate metallic products from pressed and sintered powders. Such procedure, however, has not been possible in the production of steel since the commercially available iron powder consists of nearly pure iron and therefore when pressed and sintered the resulting bodies are soft. They may be hardened by carburizing in accordance with known processes, but such carburization requires long and troublesome heating and results only in a relatively thin case or layer of hardened material. The core, however, is of practically pure iron and hence is soft and has little strength as compared with acceptable carburizing steels. Moreover, the penetration of the carbon is not uniform and therefore the thickness of the case is irregular, particularly on articles of complicated shape. Because of the extreme thinness of the hardened portion of the article, and the irregular thickness thereof, extreme care is required in grinding the parts to finished size. This carburization, moreover, destroys the surface smoothness of the parts.

One of the objects of the present invention is to provide a process of producing hardened steel articles in which the disadvantages in the prior methods will be eliminated and more even and uniform hardness obtained.

Another object is to obtain homogeneity in the structure of the parts.

A further object is to eliminate or reduce the amount of mechanical working required on the parts.

A still further object is to provide a method of producing duplicate parts which are substantially identical.

A further object is to facilitate the production of hardened steel parts such as dies, gears, cams, etc.

Another object is to produce hardened steel parts made from pressed metal powders.

A still further object is to provide an improved method of hardening pressed ferrous metals.

Another object is to produce a carburizable steel from pressed ferrous metals. I

Other objects and advantages of the invention will hereinafter appear.

Efforts have been made heretofore to provide a steel by pressing and sintering mixtures of ferrous powders and carbon. Such methods have been unsuccessful, however, since at elevated temperatures, carbon in the solid state does not readily diffuse into iron, in the absence of oxygen. Such parts cannot be heated directly in oxygen since the oxygen produces very heavy oxidation and scaling of the parts and consequently it is necessary to use a reducing gas such as carbon monoxide. This is expensive, however, and results only in a surface or case hardening of the parts since carbon monoxide does not penetrate to any substantial depth into the compressed material. We have found, however, that when carbon is once combined with the iron in the form of iron carbide, the carbon is mobile at elevated temperatures, when mixed with pure iron powder. By mixing iron powder and iron carbide powder and pressing the mixture in a die at high pressure, we are able to attain very close mechanical contact of the two constituents. If this compact is then heated above the critical range where the microstructure is austenitic, the carbon in the combined form readily diffuses through the iron powder and a steel results which has the carbon distributed in a uniform or zero concentration gradient, i. e., the microstructure is the same throughout the whole piece.-

This alloy, on cooling, has a pearlite structure and can be treated like an ordinary steel. It can be reheated at normalizing temperatures and quenched in water or oil to produce a hardened structure, like tool steel and tempered subseordinarily considered as impurities in steels, The

carbon in the combined form of iron carbide may be added to the iron powder in the form of a powdered -ferro alloy, high in combined carbon, such as ferro-chrome, ferro-nickel, ferro-sillcon,

etc. It is desirable, however, to remove the free carbon from such powdered ferro alloys, as, in

the case of magnetic materials, by magnetic separation.

The percentage of iron carbide added to the powdered iron may be varieddn accordance with the particular characteristics desired in the ultimate product and when ferro-alloys, such as mentioned above, are employed, the amount of such powders used depends on the percentage of iron carbide in such alloys. For the production of tool steels we prefer to add from about 5 to 20% iron carbide to pure iron powder but of course the amount is not critical and depends entirely upon the desired characteristics of the finished product. For example, we may mix iron powder in the portion of 84% to 16% of an iron carbide containing approximately 6.67% of combined carbon and press the powders into coherent form as set forth hereinafter to produce a .carbon steel containing approximately 1 of carbon.

In the case of steel which does not require the hardness of tool steel the carbon content may be as low as one-tenth of a per cent, the precise amount of combined carbon depending upon the degree of hardness desired. For instance, steel which is extremely tough and shock resistant has v been produced with percentages of combined ened steel with an extremely hard surface portion.

The iron powder andpowdered carbide or car-e bide containing alloy may bemixed in the proper proportions in a ball or baflie mill, preferably the latter, since it results in a more general and intimate mixing without changing the shape or size of the individual metal or carbide particles. After intimate mixing thereof,the powders may ture employed. After the pressing operation the assesses use of a mild steel core in parts such as gears, cams, etc., relieves the strains set up in the hardened outer shell.

In order that the invention may be more fully understood, reference will be had to the accompanying drawing wherein: 4

Fig. 1 is a photo-micrograph, at 1000K, of a sample of steel before hardening, produced in accordance with the present invention;

Fig. 2 is a photo-micrograph, at 1000K, of a similar sample, after hardening;

Fig. 3 is a photo-micrograph, a't 500x, of a sintered mixture of iron powder and carbon; and

Fig. 4 is a photo-micrograph, at 500x, of the bond between a composite body produced by pressing ferrous powder in contact with a low carbon steel and sintering the resulting compact.

For fabrication of an article the powdered material is placed in suitable container which is filled to a depth of approximately three times the depth of the finished article. In accordance with the present invention such mixture may consist of powdered iron and iron carbide either in the forms of pure iron carbide or as a ferroalloy rich in iron carbide, such as ferro-chrome, etc. The plunger is thereafter forcedinto the container under the pressure of approximately one hundred thousand pounds per square inch, which pressure compacts the powder to approximately one-third of its original bulk. The exact reduction in volume of the powder, upon compression, is dependent upon the particular mixcompressed powders are coherent and may be readily removed from the mold, by means of an ejector of the usual type. The compact may then be sintered at a suitable temperature in an inert atmosphere, such as hydrogen, for a sumcient period to coalesce the constituents into a uniform mass. The sintering temperature should beabove' the critical range of the particular composition so that the carbide will be dissolved in the austenite, as in the ordinary heat treating be pressed into the desired shape by hydraulic or other forms of pressesunder apressure of the magnitude of 100,000 lbs. per squareinch.-, The Y I pressed parts are then sintered in a non-oxidizing atmosphere ata temperature above the criti- 1 cal range of the metal, for a sufiicient period to obtain a thorough diffusion of the carbide through the mass, whereby a homogeneous structure is obtained. The time of heating is dependent upon the sizeof the article, the percentage ofcarbide therein and the nature of I the, ferro-alloy' employed and may vary from a few minutes to an hour or more. During this period the powdered particles coalesce .and a fine, uniform and homogeneous structure is produced.

-Aften sintering, the parts may be.

process of steels. After sintering the part may be quenched from the sintering temperature, in the accepted manner ordinary to steel treatment,

but if it is desired to machine or lap the die, in

cases where extreme accuracy is necessary, it may, after sintering, be-cooled slowly to keep it in an annealed condition so that the microstructure will be sorbitic-pearlite. In this annealed condimild steel body with a layer of any desired thickquenched and hardened like ordinary steel or if it is desiredto machine the partsthey may-"be cooled slowly so' as to produce-the 'sottrstructure of annealed-"steel and after'macl inhig they may be reheatedand' hardened by quehching and then tempered'in accordance withi'the practice employedin the ordinary metallurgy-of c'arb'onnnd ing formed by using powders of different composition in-diiferent parts of the pressed article. The

ness of hardened'tool steel thereon. This may be effected by. placing. the' powders in the container so that the central portion will consist of plain iron powder on a mixture low in coma bined carbon, surrounded by a layer of a material alloyv steels. Ifdesired the'en icle may. 7 be composed of hardened steel or it in be comrich in combined carbon, After compression the resulting compact is removed from the container and sintered to the critical rangeof the mixture of the pure iron and iron carbide, during which the particles coalesce-as before stated and form an outer steel jacket with-a less hardened core.

The mild steel core absorbs the strains set up in the outerdacket during the hardening process.

centage of such materials employed depends upon the degree of porosity desired. Such a mixture may then be pressed and sintered as described above, to insure the uniform concentration of the iron carbide therein and cohesion of the particles so as to give the physical properties, such as strength and ductility, of a hardenable steel. The porous structure is then hardened as set forth above and quenched in either oil or water depending upon the composition thereof and the hardness desired. Such a bearing is designed to withstand heavy loads and to have an increased resistance to seizure in comparison with the usual bronzeoilless bearings. During the sintering the additive materials decompose, leaving voids which may be filled with oil, as by soaking in hot lubricating oil.

The term iron carbide as used in the specification and appended claims includes not only pure iron carbide but the various complex carbides which occur in ferro alloys, such as iron chromium carbide, iron vanadium carbide, etc., and mixtures thereof, or any material containing iron and combined carbon.

It is obvious, of course, that many changes may be made in the method of compacting the powders and uniting powders of different compositions, those shown being by way of illustration only. Therefore we do not desire to be limited to the specific processes disclosed, but contemplate all variations thereof as coming within the v steel which comprises producing a mixture of powdered iron and powdered iron carbide, the

'mixture being substantially free of uncombined carbon and the iron carbide being present in the mixture in a sumcient quantity to impart hardening characteristics to the steel produced, pressing saidmixture to render it coherent, heating the mixture in an atmosphere which is substantially non-oxidizin to above the critical temperature for the particular composition and continuing the heating for a period sufiicient to cause sintering of the powdered materials and substantial diffusion of the iron carbide'through the same.

2. The method of making a hardenable carbon steel which comprises producing a mixture of powdered iron containing carbon in the form of iron carbide, said mixture being substantially free of uncombined carbon, the iron carbide being present in suflicientquantity to impart a carbon content of at least 0.1% to the steel produced, pressing said mixturetorender it coherent, heating the mixture in an atmosphere which is substantially non-oxidizing, to above the critical temperature for the particular composition and continuing the heating for a period sufficient to cause sintering of the powdered materials and substantial diiiusion of the iron car'- bide through the mass.

3. The method of making a hardenable'carbon ing iroiicarbida the mixture being j'substant an z; free of uncombined carbon and the iron carbide .fbeingfpresent in the re. insllfiicient'quan -j" tity tofimpart a carbon contentofat least 0.1%

to the'steel produced, pressing'said mixture to render it coherent, heating the mixture in an atmosphere which is substantially non-oxidizing, to above the critical temperature for the particular composition and continuing the heating for a period suflicient to cause sintering of the powdered materials and substantial diffusion of the iron carbide through the mass.

4. The method of making a hardenable carbon steel which comprises producing a mixture of powdered iron and a powdered ferro-alloy of the group \consisting of .ferro-chromium, ferro-silicon, ferro-vanadium and ferro-nickel, containing chemically combined carbon, the mixture being substantially free of uncombined carbon, and the combined carbon being present in the mixture in suflicient quantity to impart hardening characteristics to the steel produced, pressing said mixture to render it coherent, heating the mixture in an atmosphere which is substantially non-oxidizing, toabove the critical temperature for the particular composition and continuing the heating for a period suflicient to cause sintering of the powdered materials and substantial diffusion of the iron carbide through the mass.

5. The method of making a hardenable carbon steel which comprises producing a mixture of powdered iron and powdered iron carbide, the

mixture being substantially free of uncombined carbon and the iron carbide being present in the mixture in the proportion of 5 to 20% thereof, pressing said mixture to render it coherent, heating the mixture in an atmosphere which is substantially non-oxidizing, to above the critical temperature for the particular composition and continuing the heating for a period sufiicient to cause sintering of .the powdered materials and substantial-diffusion oi the iron carbide through the mass.

6. The method of producing a hardenable carbon steel which comprises pressing into a coherent form a mass of powdered iron and substantially free from uncombined carbon and containing carbon substantially solely in the chemically combined form in such quantity as to produce a carbon content of at least 0.1% in the mass, heating the mass under substantially nonoxidizing conditions at a temperature below the melting point of any of the constituents thereof and above the temperature at which the microstructure of the mass is austenitic and continuing the. heating for a period sufiicient to cause sintering of the mass and substantial diffusion of the iron carbide through the mass.

7. A composition for the production of a hardenable carbon steel comprising powdered iron containing chemically combined iron carbide in suflicient amount to impart a carbon content of at least 0.1% thereto and substantially free ofuncombined carbon.

8. A composition for the production of a hardamount to impart a carbon content of at least 0.1% to the mixture, said mixture being substantially free of uncombined carbon-.1.

FRANCESQ'IH. g ROBERT F. DIRKES.

n addition it acts as a shock absorbent for the art in operation.

It is not essential, however, that the mild steel me be formed from the compressed powder, but i desireda preformed core of the desired shape, ize and composition may be utilized and the nixture of pure iron and iron carbide compressed |.round the same. It has been found in such :ases that during the sintering, the mixture of )owders forms a bond with the solid iron core ;o the same extent as it does with one formed of :ompressed powder.

Of course, if desired the transition from hardaned to mild steel may be gradual, as by progresiively decreasing the iron carbide content of the nixture as the mild steel portion is approached.

Fig. l is a photomicrograph taken at magnifization of one thousand diameters of a sample of steel produced by pressing iron and iron carbide with a small amount of ferrochrome, at a pres sure of about one hundred thousand pounds per square inch and sintering in a hydrogen atmosphere at a temperature of about 1500 R, which was above the critical range for the particular mixture employed. The sample was permitted to cool slowly and shows the typical pearlite structure of steel in the annealed state. This structure is uniform throughout the mass and corresponds in all respects to annealed steel as produced by melting and casting.

Fig. 2 shows the micro-structure, at one thousand diameters, of the steel produced in accordance with the above process after it has been hardened by heating above the critical range and quenching in accordance with the usual practice of treating steels. It shows the typical martensite structure of hardened steel.

As heretofore stated, it has not been found possible to produce these structures by adding solid carbon'alone to the iron powder, since the carbon in a solid condition does not diffuse into the iron when treated in accordance with any of the commercial or practical methods available today. In Fig. 3 is shown a photomicrograph taken at 500 diameters, of a sample produced by heating such a compact of pure iron powder and carbon under the same conditions as the sample shown in Fig. 1. It will be noted from this photomicrograph that the carbon is in the uncombined state as indicated by the lack of pearlite structure and by the large masses of free carbon, such as shown by the dark areas (A) throughout the photograph.

Fig. 4 is a photomicrograph taken at a magnification of five hundred diameters to show the bond obtained between sintered powdered iron when compressed in contact with a piece of cold rolled steel. It will be noted that the bond between the sintered iron portion C and the cold rolled steel portion B is extremely intimate, with the crystal boundaries so oriented that they cross the original junction line D of the two parts. Attention is directed to grain marked E which shows how upon sintering of the mass, the grains of the original cold rolled steel portion B have been elongated and grown into the sintered portion 0. Consequently the weld or juncture has a strength equal to that of any other portion of the body.

The iron carbide used in the mixture may be produced by carburizing powdered sponge iron, as by heating the same in a suitable container of iron, alundum, etc., in a gas carburizing furnace, such as a rotary gas carburizing furnace or a reverberatory furnace of the type used in roasting ores. Agitation of the powder during carburlzation decreases the time required to effect the desired result. We prefer to employ a soft iron which is comparatively free of impurities. The length of heating depends mainly upon the degree of carburization desired, the nature and rate .of flow of the carburizing gas employed, the agitation of the material and the particle size of the powder. The carburizing processmay be substantiallyv the same as that employed in the carburizing of solid iron or steel and may be conducted in such gases as propane, natural gas, carbon-monoxide or other well known commercial carburizing fluids by which carbon is introduced into iron by the gaseous method. It is necessary of course to exclude oxygen. In one instance carburizing in propane gas flowing at therate of 1.3 per cubic foot per hour at 1700 degrees F. for a period of one hour resulted in a 1 percent combined carbon in the product. However, by extending the period to 2 or 3 hours it is possible to carburize up to two or three percent combined carbon. In some cases it may be desirable to conduct the treatment at a lower temperature for a longer period in order to reduce the balling up of the powder. After carburizing the powder should be ground to break the lumps that may have formed. The carburizing may be conducted to a point just sufficient to produce the required amount of combined carbon in the finished part in which case the powders may be pressed directly into the desired form following the carburizing process. Or the carburizing may be sufiicient to produce a mixture rich in combined carbon, requiring dilution of the same with iron powder so as to obtain the desired carbon content in the steel. Other materials containing combined carbon, such as ferro alloys, may also be added if desired either in addition to the iron carbide or in place thereof. In some instances nickel added to'the powder appears to facilitate the diffusion of the carbides through the mass during the sintering process. During the gas carburizing described above each particle of iron powder forms its allotment of carbide, thereby facilitating, the diffusion of the carbides homogenously through the sintered mass. As a specific example of the process, a mixture comprising from 1 to 10 percent of ferrochrome, which contains 65 percent chromium and 9 percent combined carbon, 3 percent nickel and the remainder iron powder substantially free from impurities was pressed at a pressure of approximately 50 tons per square inch and sintered in a hydrogen atmsophere at a temperature of 1100 degrees C. for a period of four hours and thereafter cooled slowly in the hydrogen atmosphere. The resulting metal was subsequently hardened, by heating at a temperature of from 1600 to 1800 degrees F. in accordance with the usual hardening practice and oil quenched. The structure was typical of a chrome nickel tool steel of the particular composition produced and had a hardness commensurate with the amount of combined carbon employed. The micro-constituents were typical of tool steel, as for instance martensite.

For such purposes as oilless bearings the sintered body may be rendered porous by the addition of a porosity forming material such as stearic acid dissolved in a solvent such as ether or such substance as petroleum oil, salicylic acid,

ammonium chloride, mica, talc or carbonates or bicarbonates. If desired graphite may be added to improve the bearing characteristics. The per-

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