US2381022A - Iron and iron alloy powders - Google Patents

Description

Aug. 7, 1945. J. WULFF 2,331,022

IRON AND IRON ALLOY PQWDER Filed June 4, 1940 FURNACE 2 LADLE I-A CERAMIC SCREEN BALL MILL CARBIfE POWDER snor 10 L. pccanaunnmc.

(FURNACE) CEMNTIT I"- vownm DECARBUmn -D z FURNACE IRON ALLOY POWDER (Low cusou) Joluc Wulff.

Patented Aug. 7, 1945 UNITED STATES PATENT OFFICE 16 Claims.

This invention relates to iron powders, more particularly to an improved method of producing iron and iron alloy powders for powder metallurgy purposes from cheap source materials.

The held of powder metallurgy, as is known, is developing rapidly. This particular phase of the science deals with the production of metallic reguli from discrete solid metal or powder by compression and heating or sintering the compact. From earlier work on special non-ferrous units, such as copper bearings, the art is rapidly developing the field of ferrous compacts for a multitude of purposes. The extension of the uses of iron and iron alloy powders for powder metallurgy, however, is restrictedto no inconsiderable degree by the relatively high cost of suitable iron and iron alloy powder. This high cost in a large degree is due to the expense of the initial starting material, such for example as electrolytic iron. If the cost of iron powder suitable for powder metallurgy could be considerably reduced the tleld of use could quickly and commensurately be expanded. While the potential field of use of metal powders for the production of compacts is very wide, the immediate extension is hampered in a number of particulars. As a broad proposition, where only strictly mechanical factors are involved in the use of a machine element a powder metal compact can compete with a machined casting or forging only when the machining operations are costly. This is due to the fact that per unit or weight, a machine element made up oi a powder compact is considerably more expensive than one produced by conventional methods from the liquid phase metal. The limitations on the extension of powder metallurgy are perhaps most outstanding in those uses where special or alloy steels are specified. It has been suggested heretofore to produce metal compacts by forming a compact in the desired elementshape-from iron powder and modifying the p y ical characteristics of the" surface, such for example, as increased hardness by carburizing or case hardening the surface. It has also been su'ggested to produce fabricated units of alloy steels by mixing predetermined percentages of iron powder andthe powder of the alloying ingredients. Little success however has been achieved here. This is primarily due to thefact that in order to 5 ders from the cheapest source materials and utilizing standard metallurgical equipment. The improved powders are produced by specially correlating certain metallurgical operations to establish phase changes or transformations in the 10 iron-carbon system conducive to the formation of the desired products.

Briefly stated the invention involves and u'tilizes, for a new purpose, the phenomena encountered in fusion metallurgy particularly the char- 15 acteristic phenomenon of the formation f white cast irons. As is known, when 'a cast iron is cooled rapidly from the liquid state the metallurgical structure (with the concomitant diflerential physical properties) depend essentially on the quenching temperature. At a temperature of approximately 1145 C. primary cored grains of austenite, surrounded by a eutectic of austenite.

acteristlc is adviscdly sought and utilized under the present invention so as to insure easy disintegration of the material to the desired particle size.

It will be appreciated at this point that the 35 concept of utilizing a high carbon iron as the starting material for an iron powder (low carbon iron) presents marked commercial advantages. In addition to the possibility of utilizing cheap source materials the invention also insures economic operations. Cast iron, due to its high carbon content has a relatively low melting point -(1150 C.1350 C.) and can therefore readily be melted in a cupola furnace.

After production of the brittle cast iron, the

5 material is disintegrated and the carbon content is reduced by segregation of the carbon-rich (carbide) constituents from the carbon-poor constituents. After separation-of these two phases or fractions the particle size may be further resecure alloying of th a i ion 380MB the mduced if desired and the carbon content of one or acted mass must be heated to a high temperature and held at this temperature for extremely long periods of time in order to permit diiru'sion of the alloying constituents into the hot both fractions reduced to a desired predetermined degree by decarburizing.

The carbon-rich fraction or product may be iron matrix. disposed of directly for use as a blasting powder These requirements of elevated temperatures and u or for other purposes and the reduced low carbon fraction employed for the production of iron powder compacts. In the case of iron alloy powders, the invention comprehends the concept of utilizing a very cheap source of iron, such as relatively high carbon pig iron, melting this down by conventional methods and with standard furnace equipment and adding non-graphltizlng constituents to the melt so as to produce an alloy powder of the desiredultimate analysis. In the new method the high carbon alloy melt is rapidly quenched so as to produce in effect a white iron, i. e. a brittle structure low in austenite and high in cementite, pearlite and martensite. The white irons, and white alloy irons, as is known, are extremely brittle. This brittleness is advisedly sought and utilized under the present invention. After production of the brittle alloy cast iron the material is disintegrated and the carbon content 'of the iron alloy is reduced by segregation of the carbonrich (carbide) constituents from the carbon-poor constituents. After separation of these two phases the particle size may be further reduced if desired and the carbon content further controlled by decarburizing. The carbon-rich byproduct may be sold directly for use as a blasting powder or for use in the production of high carbon products such as the hard metal carbide tools and the like. This by-product may also be utilized in coniunction with ordinary iron powder, e. 3. reduced iron powder, so as to provide a compact having a desired carbon content.

In older to more clearly explain the invention a flow sheet of a typical method of production of the improved product is shown in the accompanyin drawing.

In carrying out the process cheap raw material, such as a pig iron low in phosphorus, sulphur, silicon, and manganese, is charged to a suitable furnace I, such furnace may be of any suitable type such as an electric furnace, as shown, or

-6. The product discharged from the stamp mill a cupola furnace. Where iron powder is to be produceda cupola furnace may be employed. Where a plant is to produce both iron powder and iron alloy powder, an electric furnace is we ferred.

In the preparation of iron powders the charge in the furnace l is heated to a desired elevated temperature to form a high carbon heat. The cast iron thus made is poured into ladle 2' and the content of the ladle is discharged onto a refractory screen 3 constructed of graphite or a suitable ceramic material. Screen 3 serves to subdivide the molten metal into a series of streamlets, and, as shown, these are immediately impinged by jets of a quenching medium,'such as water ejected from the jets 4. This quenching and shotting 'divides the cast metal into shot 01' the order of it to 1 of an inch or finer. The shotted material is allowed to fall into 'thewatcr quenching tank 5.

As previously. explained, the material thus produced is essentially a white cast iron and because of its particular metallurgical structure is characterized by an extreme brittleness. If the silicon and other. graphitising agents are kept low in the melt there will be a minimum or free graphite or temper carbon in the shot and this shot material will consequently be extremely hard and brittle. Such brittleness is advisediy sought for the purpose of disintegratingthe shot in' cheap standard emiip a su dividin i into a carbon-rich fraction and a carbon-poor fraction.

The disintegration of the shotted material may be effected inanydesiredmanner. Asshownin is charged to a seconddisintegrator, such as a ball mill. in which the particle size is further reduced. If desired the shotted material may be recycled a number of times through the stamp mill in order to reduce it to a predetermined particle size before passage to the ball mill. It

will be understood that the brittle cementite in the material will crush very readily and will also assist in crushing or disintegrating the more metallic pearlite, martensite, troosite or undecomposed austenlte which may be present.

The material discharged from the ball mill 1 is preferably classified by passing over the screen I. By utilizing such classification a fine powder of from 100 to 200 mesh or more average size is separated out and conveyed to the magnetic separator the larger particles discharged from the screen 8, as shown, may be returned to the ball mill for further treatment. The material discharged from the ball mill, which is finer than 100 mesh, will be comprised largely of cementite and will contain of the order of 75% cementite.

The magnetic separator serves to divide the magnetic from the non-magnetic material, l e. the carbon-poor from the carbon-rich particles. The pearlitic constituent, containing of the order of 1.8 carbon being more magnetic than the carbide constituent (about 6.6% of carbon) is readily separated in separator 9. The carbide powder fractionated in the magnetic separator lmay be recovered-and marketed directly, without further treatment as a fine metal blasting shot or for any other desired purpose. If desirable this carbon-rich fraction may be treated in a decarburizing furnace to reduce the carbon content to any predetermined low value. This carbide materialeither with or without decarburization may be mixed with pure iron powder so as to produce iron powder compacts of any predetermined analysis.

The carbon-poor or pearlite-rich fraction, separated out magnetically in the separator 9, may be charged to any suitable decarburizing furnace ID. This powder may be decarburized, as is known, at a temperature of from 400" to C. ina hydrogen-steam or other decarburizing atmosphere so as to reduce the carbon content to the desired extent. With such a treatment'the pearlite-rich powder may be reduced from a carbon contentof the order of from 1.8 to 0.8 or lower. Such decarburization treatment at elevated temperatures also advantageously modifies the powder for its intended use in powder compacts. During such decarburization treatment, it will be appreciated, the pearlitic surface 01 the particles is decomposed to form the more plastic or softer ferrite. It will be appreciated that by properly controlling the temperature and time of this 'decarburizing treatment'the depth of the ferritic surface or case may-be controlled.

It will be understood that prior to treatment in the decarburizing furnace I. the-magnetic concentrate may be re-milled and classified so as to procure the optimum particle size distribution and may be passed through the magnetic separator to further concentrate the material with respect to the c'ementlte-rlch traction. In this manner a cementite powder of -the order of or more of cementite may be produced.

The fractions produced according to the invention, as will bc understood, have separate utility in the art. The fine low carbon iron powder produced in the decarburizing furnace in may be blended or diluted with carbon-rich fines so as to produce a compact of any desired carbon analysis. The high carbon material separated out in the magnetic separator may be utilized directly as an abrasive material. If desired this carbide powder may be admixed with pure iron powder, such as reduced iron powder, for powder metallurgy purposes or it may be homogeneously mixed with tungsten and iron, tungsten and cobalt, chromium, vanadium and other such powdered metals to produce improved types of carbide tools. In carrying out the process for the preparation of alloy iron powders, a cheap raw material, such as pig iron low in phosphorus, sulphur and silicon, is charged to a suitable furnace I. This furnace may be of any suitable type such as a cupola or an electric furnace. Depending upon the particular ultimate iron alloy powder desired, alloy addition agents, such as chromium, tungsten, manganese, vanadium, molybdenum and the like in the form of pure metals, rerroalloys or alloy scrap are added to the charge in furnace I. By the proper proportioning of the constituents of the melt in furnace I this melt may thus be made to simulate any desired alloy steel analysis and will differ from these analysis only in carbon content.

The charge in furnacel may be heated to a desired elevated temperature of the order of from approximately 2600 F. to 2750 F. The alloy cast iron thus made is poured into ladle 2 and the ladle is discharged on to a refractory screen 3 constructed of graphite or a suitable ceramic. The screen serves to divide the mass of molten metal into a series of streamlets and these are immediately-quenched and shotted by means of water or other suitable quenching medium forced through the appropriately positioned jets 4. This quenching and shotting serves to divide the cast metal into shot of the order of about 1% to A of an inch or flner. The shotted material is discharged into a water container 5.

The material thus produced is essentially a white alloy cast iron and because of its high percenta e of cementite is characterized by an extreme brittleness. If the silicon and other graphitizing agent are kept low in the melt there will be a minimum of free graphite or temper carbon and the shot material will be extremely hard. This brittleness is advisedly sought and emplayed for the purpose of disintegrating the shot in standard equipment and subdividing the material into a carbon rich fraction and an alloy rich fraction relatively low in carbon. The subsequent treatments of stamping, crushing and magnetic separation, together with decarburization will be carried out in the same manner as described above in the treatments of iron powders.

It will be appreciated that the several products produced in the process are eminently useful. The fine iron alloy powder of low carbon content may be diluted and homogeneously admixed with the carbon-rich fines to produce a compact of the desired carbon analysis. As indicated previously the carbide powder has immediate utility as an abrasive or metal cleaning compound. Also if desired it may be admixed with pure iron powder for powder metallurgy or it may be mixed with tungsten and iron, tungsten and cobalt; chromium and vanadium and the like, to produce improved carbide tools which closelyapproximate the composition of high speed tool teels.

In manufacturing compacts from the improved iron powder it will be understood that the flow factor of the powder may be improved and controlled by coating the powder with a suitable dry lubricant such as stearic acid.

It will be appreciated that the present invention provides a very simple and economical method for producing iron powder of substantially any desired carbon analysis. The process operates upon the cheapest types of raw material and by the utilization of standardized equipment enables the production of an improved iron powder as well as a valuable by-product such as the carbide powder. It will be appreciated further that the present invention also provides an exceptionally economical method of p oducing iron alloy powders. of any desired composition or analysis. The process involves the use of the cheapest type of raw material and, as explained insures the production of a. valuable marketable by-product. The wide permissive range of alloy powders which is producible under the invention establishes new and hitherto commercially unfeasible uses for metal compacts.

It will be understood that while preferred modiflcations of the invention have been described these are given to explain the underlying principles involved and not as limiting the useful scope of the invention to the particular source materials or to the end products described.

Iclaim:

1. A method of producing iron powder for powder metallurgy which comprises forming a brittle iron of the white cast iron type, distintegrating the material to form carbon-rich and carbonpoor components, and separating and recovering the components.

2. A method of producing iron powder for powder metallurgy which comprises melting a high carbon iron containing Ill-5.0% carbon quickly quenching the high carbon iron heat, distintegrating the quenched iron to form carbon-rich and carbon-poor particles and separately recovering the carbon-rich and carbon-poor particles.

3. A method of producing improved iron powder of low carbon content from high carbon starting material containing ill-5.0% carbon which comprises melting, then atomizing and quenching a high carbon iron heat to produce shotted material of an essentially brittle structure, disintegrating the material to produce carbon-rich and carbon-poor particles and separating and recovering the carbon-poor particles.

4. A method of producing iron powder of low carbon content for powder metallurgy which comprises atomizing and quenching a high carbon iron melt to produce shotted material of the white cast iron type, disintegrating the material in a suitable mill, passing the disintegrated material through a magnetic separator and recovering from the separator a carbon-rich and a carbonpoor fraction.

5. A method of producing iron powder for powder metallurgy which comprises forming a heat of high carbon pig iron low in graphitizing elements, quenching the heat to form shotted material of the white cast iron type, disintegrating and classifying the material to produce a particulate product of a predetermined particle size, subjecting the material to magnetic separation to separate the low-carbon from the high-carbon particles and decarburizing the low carbon particles to produce a low carbon iron powder.

6. A method of producing an iron powder for powder metallurgy which comprises forming a heat of a high carbon iron, quenching the heat to form shotted material or the white cast iron t pe. disintegrating the material to the extent that carbide-rich particles separate from carbidepoor particles classifying the material to separately recover the carbide-poor traction and decarburizing such carbide-poor fraction to a desired low carbon analysis.

7. A method of producing a low carbon iron powder for powder metallurgy which comprises utilizing cheap iron source material of the p18 or scrap iron type, forming a high-carbon iron heat of such material, quenching the heat to form shotted material of the white cast iron type, disintegrating the material in a suitable mill to form carbon-rich and carbon-poor particles, separating the carbon-poor from the carbon-rich particles, decarburizing the carbon-poor particles and blending such decarburized particles with other iron powder, such as reduced iron powder, to form an iron powder compact oi predetermined carbon analysis.

8. A method of producing iron alloy powder which comprises forming a brittle iron alloy of the white cast iron type, disintegrating the alloy to form carbon-rich and carbon-poor components and separating and recovering the components.

9. A method of producing iron alloy powder which comprises quickly quenching a high carbon iron alloy heat, to produce a material low in aus-. tenite and high in cementite, pearlite and martensite, disintegrating the quenched alloy to form carbon-rich and carbon-poor particles and separately recovering the carbon-rich and carbonpoor particles.

10. A method of producing improved iron alloy powder which comprises atomizing and quenching an alloy iron melt of high carbon content to produce shotted material of an essentially brittle structure, low in austenite and high in cementite, pearlite and martensite, disintegrating the material to produce carbon-rich and carbonpoor particles and separating and recovering the particles.

11. A method of producing an iron alloy powder of low carbon content and a hard abrasive material which comprises producing high carbon iron alloy heat by melting pig iron and alloying metal components in a furnace, quenching the heat to produce shotted material of an essentially brittle structure, low in austenite and high in cementite, pearlite and martensite, disintegrating the material to produce fine material of difierential carbon content, separating the low carbon material from the high carbon material, classifying the high carbon material to produce an abrasive shot. and decarburizing the low carbon material to produce an iron alloy powder.

12. That method of producing low carbon alloy iron powder for powder metallurgy i'rom cheap source material which comprises utilizing a pig iron together with alloy steel scrap to form a high carbon melt or a predetermined analysis oi alloying constituents, quenching the heat to produce brittle shot material, low in austenite and high in cementite, pearlite and martensite, disintegrating the shot to the extent where a carbon-rich phase separates from a carbon-poor phase and separating and recovering the phases.

13. That method of producing low carbon alloy iron powder for powder metallurgy from cheap source material which comprises utilizing a pig iron together with alloy steel scrap to form a high carbon melt of a predetermined analys s of alloying constituents, quenching the heat to produce brittle shot material, low in austenite and high in cementite, pearlite and martensite, distintegrating the shot to the extent where a carbon-poor phase and a carbon-rich phase is formed and magnetically separating and recovering the phases. a

14. A method of producing alloy steel powder for powder metallurgy which comprises adding desired alloy constituents to a high carbon iron melt, quenching and atomizing the melt, to produce particles relatively low in austenite and relatively high in cementite, pearlite and martensite, disintegrating the particles thus produced and passing the particles through a separating system to separate the relatively magnetic from the relatively non-magnetic material.

15. That method of producing alloy steel powder for powder metallurgy which comprises forming an iron alloy 0! predetermined analysis of alloying constituents, and characterized by high,

carbon and low carbon compounds, disintegrating the alloy to liberat relatively high carbon and relatively low carbon particles, separating the particles and decarburizing the low carbon particles to a predetermined extent.

16. That method of producing low carbon alloy iron powder for powder metallurgy from cheap source material which comprises utilizing an alloy scrap to form a high carbon melt of a predetermined analysis of alloying constituents, quenching the heat to produce brittle shot material, low in austenite and high in cementite, pearlite and martensite, disintegrating the shot to the extent where a carbon-rich phaseseparates from a carbon-poor phase and separating and recovering the phases.

JOHN WULFF.

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