US2657129A - Aluminum-alloyed corrosion-resistant metal powders and related products and processes - Google Patents

Description

Patented Oct. 27, 1953 ANT METAL POWDER/g AND RELATED PRODUCTS AND peoces'sts George Stern, Mamaroneclq, Seymour J..; Sindeband, Chappaqua,. and Joseph l Seaplan, Xonkers, N; Y., assignors to American Electro Metal corporation, Yonkers, N. Y;, a corners tiofi of Maryland No Drawing. Application March 31-, 1950, Serial No. 153,289

(or. fi -t3) '10 Claims.

This inventionrelates to readily compactible and moldable powders which resist corrosion, shaped bodies of such powders, and methods for producing the same, some aspects of the invention being of a broader scope.

Among the objects of the invention are POW ders of metal particles which are in a readily deformably plastic condition and maybe com; pacted into relatively strong metal bodies with relatively low pressure, at least the surface layer of the metal powder particles having alloyed therewith aluminum from a gaseous or liquid compound at an elevated temperature;

A particular object of the invention is such readily compactible moldable powder containing essentially powder particles of atleast one metal of the group consisting of iron, nickel and cobalt, and of mixtures and alloys of said metals, which particles are in a rcadily deformable, relatively soft plastic condition, at least the surface layer of said particles having alloyed therewith aluminum which has been deposited thereon at an elevated temperature from an aluminum com- Another object of the invention is such alumimim surface=alloyed soft moldable metal powder combined with similar surface alloyed powder particles of at least one metal of the group consisting of moly' bdermm, tungsten, chromium, vanadium, manganese; or mixtures or alloys thereof.

Further objects of the"- invention include com-- pasted bodies made of such surface-alloyed metal powder particles, methods for the econom= ical production of such surface alloyed' soft mold= able metal powder particles, and economical production' or compacted bodies from such powder particles of the invention.

The foregoing and other objects of the invention will be best understood from the following description of specific exemplifications thereof.

For'ye'ars past various corrosion=resistant prod ucts such as gears, valve parts; bearings, filters, and the like, have been made of stainless steel powders by powder metallurgyt'echniques. The stainless steel powders generally used for such bodies have been obtained from previously propared stainless" steel ingots, for instance, by sub jecti-ng a stainless steel ingot to grain boundary corrosion followed by disintegration, or by melting astainless steel ingot and atomiz'ing it in themol= ten state. The SO-Pffidflijd particles of such stainless steel powders are of high density and hardness; and require high-molding pressure of at least- 40 to 45 t. i.- (tons per square" inch) which 2 is much too high for economical production pr hiolded products, and results in rapid wear and frequent break-down of the molding dies. Asa result, the production of molded products from stainless steel powders ha found only limited use.

What has been said above about stainless steel powder, which is an alloy of iron, nickel and chromium, applies also to various other generally s'i i nilar cg'mmmercially available alloys, such as alloys of iron, cobalt and nickel with aluminum.

According to the invention the foregoing diflicultics heretofore encountered in the production of corrosion-resistant ferrous metal bodies by powder metallurgy techniques are overcome by the provision ofa readily compactible and moldable metal powder of relatively soft ferrous powder particles, at least the surface layer of the powder particles having combined therewith aluwhich has been deposited thereon at an elevated temperatu'reso as to render the powder particles corrosion-resistant, the surface alloyed metal powder particles being sufiic iently soft and plastically deformable as to make it possible to compact them with relatively low pressure, for instance, only 25 to 30 t. s, i. into bodies of greater strength than molded bodies made by compactin prior art powder of generally similar composition.

v yarious commercially available soft, plasticall-y readily deformable iron powders are suitable for producing corrosion-resistant soft moldable metalpowders of the invention.- Among such available and suitable soft iron powders is sponge iron powder having a carbon content of .l to 2% (unless otherwise specifically stated,-

all proportions are given herein by weight), elec- .trolytically produced iron powder, carbonyl iron powder, eddy mill iron powder, and atomized molten iron powder of similar iron content. The following impurities present in such available powders,- to Wit, .3% Mo; .-2% Si, and other minor impurities usually present therein, do not impair their usefulness for practicing the invention.

In addition, many relatively hard and'difiicult to compact fine iron powders will, in general, when subjected to the alloying process of the invention herein described, become softened and acquire characteristics which render them readily moldable.

In accordance with a phase of the invention based on the original concept of producing a1umin -rm surface-alloyed ferrous ironpowder-" relatively soft powder particles of difiefefit met= ale; to wit; one or more metals of the group eluding iron, nickel and cobalt, are combined in proper proportion, and a desired proportion of aluminum from an aluminum compound is deposited on and difiused into the interior of such metal powder particles so as to yield a soft powder body of soft powder particles combining aluminum with the other metal or metals in proportions required for producing alloyed bodies of desired characteristics and containing aluminum and such metals in the desired proportions.

Any of the known methods for depositing aluminum from an aluminum compound on the surface of a metal body heated to an elevated temperature may be used for depositing on and diffusing aluminum into metallic powder particles in accordance with the principles of the invention.

Among such known aluminum alloying processes, one that was found particularly suitable for practicing the invention consists in exposing the surface of the metal powder particles to the action of aluminum chloride, in a stream of hydrogen at a temperature between 900 and about 1400 C. which is sufficient to decompose the aluminum chloride compound and to deposit aluminum atoms on the metal particle surfaces and cause these atoms to diffuse into the interior of the particles and to alloy with the metal of the particles. It is believed that the resulting reactions take place either according to equation,

AlC1s+Hz- Al+I-IC1 (1) or according to the equation,

AlC1 +meta1 Al+metal chloride (2) or simultaneously according to both these equations.

The HCl, which is formed according to Equation 1 as well as the metal chloride, which is formed according to Equation 2 and which is gaseous at the temperature of reaction, is carried away by the stream of hydrogen, and the aluminum atoms are deposited on the surfaces of the metal powder particles, and will alloy with the surface layer of the metal particles.

The aluminum chloride may be evaporated and mixed with hydrogen outside the reaction chamher, but it may also be produced in the reaction chamber, e. g. by reaction of hydrochloric acid with either aluminum or ferroaluminum. In a suitable procedure, which will be described in more detail below, the powder body which is to be surface-alloyed is packed in a mixture of ferroaluminum and an inert ceramic material, and a gaseous mixture of HCl and hydrogen is passed through the pack to produce first A1C13 by reaction of H01 with ferroaluminum and then cause the A1013 to decompose at the surface of the material to be surface-alloyed. When such pack containing ferroaluminum is used, the reaction temperature must be kept well below the melting point of the ferroaluminum. In the case of ferroaluminum containing 50% of aluminum, e. g. it is good practice to carry on the treatment at a temperature not higher than 950 C.

In accordance with one phase of the invention, the soft iron particles are first sintered into sinter .cake pieces within a reducing atmosphere, such as pure dry hydrogen, or purified dry cracked ammonia, at an elevated temperature so as to produce a porous sinter cake of low density, not higher than about 4 to 5 g./cc., and of low strength not higher than modulus of rupture of about 6500 to 7000 p. s. i. The powder particles of such porous low density sinter cake pieces prointerior of the powder particles.

duced by the initial sintering treatment, may be readily surface-alloyed with aluminum by known processes for depositing aluminum from an aluminum compound at an elevated temperature at which the deposited metal difiuses into the The porous lowdensit sinter cake pieces do not undergo any material increase in density and strength when submitted to this surface-alloying treatment, and they may be readily comminuted with minimized work hardness into minute soft powder particles of the required size range and high degree of plastic deformability which makes it possible to compact the resulting surface-alloyed ferrous powder particles with a pressure of only 50,000

. p. s. ipinto strong green metal powder compacts having a modulus of rupture of 400 p. s. i. or more, without adding any binder to the powder particles.

In accordance with another phase of the invention, powder particles of the oxides of ferrous metals, such as iron, nickel and cobalt, or mixtures of such oxides, or mixtures of metals and oxideswith or Without additional mixtures of powders of oxides or metals of the group consisting of molybdenum, tungsten, chromium, vanadium and manganese, or powders of these metals, or mixtures of powders of these metals and their oxidesare initially sintered into porous sinter cake pieces at an elevated temperature within a reducing atmosphere so as to produce sinter cake pieces of the reduced metal powder particles having a low density not higher than about 4 /2 to 5 g./cc., and of low strength not higher than a modulus of rupture of about 7000 p. s. i. The metal powder sinter cake pieces obtained by such initial reducing treatment are then surface-alloyed with aluminum by any known process for depositing aluminum from an aluminum compound at an elevated temperature at which the deposited aluminum difiuses into the interior of the metal powder particles. The low-density sinter cake pieces subjected to such surface-alloying treatment do not undergo any material increase in density or strength, and

may be readily comminuted into minute powder particles of the required size range which have only a minimized work hardness, and exhibit a high degree of softness and deformable plasticity which makes it possible to compact the resulting aluminum-surface-alloyed minute metal powder particles with a pressure of only 50,000 p. s. i. into strong green powder compacts having a modulus of rupture of 400 p. s. i or more, without adding any binder to the powder particles. The surface-alloyed metal powders produced in the manner described above from oxides of the difierent metals will yield a powder body, the individual particles of which are alloys of the different metals and substances corresponding to the different metal oxides used for making the initial sinter cake pieces and the proportionof aluminum deposited on the sinter cake particles.

A convenient way for producing such sinter cake pieces for the foregoing treatment is to mix the powders of the metal oxides with a lubricant and binder and pellet the mixture into pellets, the pellets containing enough of the organic lubricant and binder which decomposes and is driven off at the sintering temperature so that the resulting sinter cake pellets or pieces have the required high porosity and the required low density and strength.

Without in any way limiting the scope of the invention, there Will be hereinafter described exsimples ofsati'sfaetcry procedures for producing soft aluminum-surfacealloyed' metal. powders of the invention from commerciallyavailable raw material.-

There willnowbe described by wayof example, a sat'i'sf-actoryprocedure for producing aluminum-surface-alloyed corrosion-resistant soft iron powder." of the inventionfrom commercially available"-sp'onge: iron powder having a carbon oontent-of aboutalfi A typical -commercialiapowder of this type used in the process contained: a mix-ture of minute powder particleshaving following particlesize distribution typical ofium'oldable metal powders. 21% 100, +160: mesh, 30%v -150,. +200 mesh; 13%- -200, +250 mesh, 14%. 250, +325 mesh, and 'the balance. -325 meshvpowder.

The iron powder is subjected to. an initial treatment. which sinters: it into a porous iron powder "cake-of low density and high porosity so as to: permitready surface-alloying of the cake powder particles witlnsilic'on. deposited. thereon. Such porous low-density sinter cakes may be. produced as follows: A layer of the iron powder about /8 to /4" thick. is deposited into. sinter boatsof suitablev heat. resistant metal such as iron-chromium. sheet metal,.-without in any way compactingthe deposited iron powder. The interior surf-ace or. each sinter boat is coated with a*--stick -suppnessing or. release medium such a a water' suspension of magnesium hydroxide to permit ready" separation of the sintered powder body from the boat after-the sintering operation. The. powder within the boat is then treated within. afurnace'in a. reducing atmosphere, such as purified. dry hydrogen or purified cracked ammonia, atra temperature of about 800 to 1050 C. for "one-half to three hours, and then permitted to cool within thesame atmosphere to room temperature. In general, such treatment at a temperature'between about900 to 1000' C., such as 950 C-.,tf-or one hour, is sufficient for sintering commercial sponge. iron. powder into a sinter cake having a density-of .1.57 to A g./cc., which is readily frangible-into lumps or sinter cake pieces suitable for undergoing: the desired surface-alloying treatment.

A good way for carrying onthe foregoing sin.- tering treatment-is to push the powder containing-boats through a tunnel "furnace within which is'maintained the desired reducing atmosphere, the "heat treatment being followed by cooling within the same atmosphere as a part of a continuous process, in the course of which the boats are moved from oneend of the furnace to the other.

After completion of the initial sintering treatment, the iron powder sinter cakes are removed from the boats and are broken into sinter cake fragments or pieces of asize suitable for the subsequent aluminizing treatment. sinter cake pieces having cross-sectional dimensions in the range between about inch to about 1% or inch have beenfound suitable for the subsequent surface-alloying. treatment.

example of. a satisfactory procedure for surface-alloying. such iron powder sinter cake pieces with aluminum. will now be described: The porousiron powder sinter cake pieces are packed withinia packmass which :is eifective in depositing aluminum on the sinter cake powder particles. The pack may consist, for instance, of 50% by volume ceramic lumps, such as porcelain pieces, the balance an aluminum alloy such as ferroa-luminum. containing. (by weightl about? to60% aluminum.

The sinter cake pieces packed; withxporcelaim and ferroaluminum are placed within treatmentbaskets of suitable heat-resistant metal,.'suclr as a chromium-nickel-iron alloy, and; the packed treatment baskets which are placed within' arretort. The contents of the retortrarewheatedtm an. elevated temperature in the range :be'tweem about 900 and 1100 C butwell'belowthe melti-ng temperature of the packing materiaLandian atmosphere containing pure. dry hydrogen: -and v dry hydrogen chloride gas, is passed. through the retort'for producing reactions caus'ingsal'umiimm; atoms to be deposited on themetalpowder ticles of the sinter cake lumps, and to difiusedntm the interior of the metal particles and alloy therewith. In such surface-alloying treatment" the hydrogen chloride gas passing through. the; retort interacts with the ferroaluminum; to; form: aluminumchloride gas. The AlCla gas isdecomiposed and the aluminum atoms depositedon the. surfaces of the powder particles. diffuse into-the interior of the particles and alloy withthe: metal: of the particles.

By way of example, the following; surfaces alloying treatment gave satisfactory resultsi. The: broken up sinter cake lumps arepacked in acircular treatment basket havingga. diameter of 22" and a height of 10". As packing 'materialvthere was. used a mixture of equal parts of porcelain and ferroaluminum lumps, the ferroalumi-num containing equal proportionsv of aluminum and iron. Four packed baskets are stacked upon each other in a closely fitting retort of 60" height. .A mixture of 3 parts of gaseou HCl and 20 parts of E2 was passed, at 940 0., through the retort, first for 2 hours at a rate of approximately 40 cu. ft. per hour, and then for 10 additional hours at a rate of approximately 20 cu. ft. per hour. After being subjected to the foregoing surfacealloying treatment, the resulting sinter cakes have about the same density and the same strength or modulus of rupture: which they exhibited prior to the surface-alloying treatment. Because of the low density and strength of the surface-alloyed sinter cake pieces, theymay be readily comminuted with minimized work hardness into soft metal powder particles of. a. size range and high degree of plastic deformability required for compacting the powder particles into strong pellets with relatively low compacting pressure of only 50,000 p. s. i. into green com.- pacts having a modulus of rupture of at least 200 and higher.

By way of example, surface-alloyed iron powder sinter cakes prepared in the manner described above, were disintegrated by a disc crusher into surface-alloyed powder particles, of which passed through a mesh screen, the powder containing about 8% 325 mesh particles, about 50% 100, +200 mesh particles, balance 200, +325 mesh particles. Green tests of such surface-alloyed powder particles compactedwithout any binderunder apr'essure of 25 t. s. 1. exhibited an average modulusof rupture of about 850 p. s. i. (pounds per square inch);

The surface-alloyed iron powder particles :of the invention, produced by the. treatment of, the invention, have a spongy character, tentacle-like shape. Green compacted bodies produced .from such powders exhibit excellent corrosion-resistance at elevated temperatures.

Similar results are obtained with other types of soft, plastically deformable iron powders subjected to the aluminum-surface-alloying treatment of the invention. In fact, as explained above, many relatively hard and difficult to compact iron powders will, in general, when subjected to the aluminum-surface-alloying treatment of the invention of the type herein described, become. softened and acquire the high degree of plastic deformability which renders them readily moldable into strong green compacts of high strength of at least 200 p. s. i.

The foregoing rupture test data were obtained by test equipment and test methods described in the article of J. P. Scanlan and R. P. Seelig in Powder Metallurgy Bulletin, 44, p. 128 (1949), using test bars 1 /2" long, {'g" wide, and /4" thick.

The test results given above are representative of. the characteristic distinguishing aluminum surface-alloyed ferrous powders of the invention over prior art of corresponding compositions. Thus, soft aluminum-surface-alloyed and corrosion-resistant ferrous powders of the invention difier from heretofore available corrosion-resistant ferrous powders by the fact that green com pacts made from powders of the invention have a materially greater rupture strength, at least two to four times greater than prior art powders of similar composition.

A distinguishing characteristic of the corrosion-resistant surface-alloyed soft ferrous powders of the invention is the fact that when compacted into a green test bar body of the dimensions given above under pressure of 25 t. s. i. with no lubricant or binder, such compacted green body exhibits a modulus of rupture several times greater than a similar body produced by compacting corresponding prior art powders, under the same pressure. In particular, such green test bar compacts made from powders of the invention have a modulus of rupture of at least 400 p. s. i.

In producing surface alloyed soft, plastically readily deformable ferrous powders of the invention, it is important that at all stages of the processing operations, the powder particles should not be subjected to any material work hardening forces. Thus, for instance, it is essential that in initially sintering the soft iron powder particles into porous sinter cake fragments suitable for packing into the alloying pack, the powder which is to be subjected to the initial sintering action should not be compacted under any substantial pressure. If a substantial compacting pressure, even as low as t. s. i. is initially applied to such soft ferrous metal powder '(Fe, Ni, Co) in preparation for the initial sintering process, which precedes the aluminumalloying treatment, such surface-alloyed sinter fragments will acquire a relatively great density and strength, and large forces will be required for crushing them, and the resulting powder particles will be severely work hardened. Unless the sinter cake fragments which are to be subjected to the surface-alloying treatment are of low density and strength-obtainable if the powder particles subjected to the preliminary sintering action have not been initially compacted under pressure-the crushing energy required for pulverizing the alloyed sinter cake fragments would be so large that the resulting surface-alloyed powder particles would be distorted in shape and work hardened, making it necessary to apply undesirably large pressure for compacting them into bodies of the required final shape.

In other words, when producing aluminumsurface-alloyed soft ferrous powders of the invention, it is essential that the sintered powder cake fragments or lumps-produced in preparation for the surface-alloying treatment-should have a low density, and correspondingly low strength, so as to minimize any work hardening imparted to the powder particles when pulverizing the sintered powder cake fragments or lumps following the surface-alloying treatment, thus resulting in powder particles exhibiting only minimized work hardening. Furthermore, it is also essential that the sintered powder cake fragments or lumpsproduced in preparation for the surface-alloying treatment-should have high porosity so that they are permeable to the gaseous metal compound by means of which thesur face-alloying treatment is carried on.

By controlling the density and strength of the sinter cake lumps-produced in preparation for the surface-alloying treatmentit is possible to control the preparation of soft, plastically deformable aluminum surface-alloyed powders of the invention.

As explained above, the proper range of the density of the sinter cake lumpsproduced in preparation for the surface-alloying treatmentis about 1.5 to 4 g./cc. The modulus of rupture of such sinter cake lumps is correlated to their density, being about 50 p. s. i. for sinter cake lumps having a density of 1.7 g./cc. and increasing to about 6500 p. s. i. for sinter cake lumps having a density of 4 g./cc.

Furthermore, as long as the sinter cake lumps, produced in preparation for the surface-alloying treatment are of low density and have a low modulus of rupture after being subjected to the surface-alloying treatment, aluminum is surfacealloyed with the individual powder particles of such sinter cakes.

Accordingly, to obtain aluminum-surfacealloyed powders of the invention, it is sufficient to control the density and/or modulus of rupture of the sinter cake lumps produced in preparation for the surface-alloying treatment. As long as the density of such sinter cake lumps is not more than about 4 g./cc., and their modulus of rupture is not more than about 6500 p. s. i. they will, after the surface-alloying treatment, remain of sufficiently small density andstrength as to permit their pulverization without material work hardening of the powder particles. As a result, the surface-alloyed powder obtained from such sinter cakes will have the desired high degree of softness and plastic deformability as to make it possible to compact such powders into green compacts having a modulus of rupture of at least 400 p. s. i. and higher.

By sintering such green compacted bodies of aluminum-surface-alloyed soft iron powder of the invention, compacted with only a relatively small pressure, there are obtained bodies having the same strength as those produced by prior art powders of similar composition which have been compacted with much higher pressure. Good results are obtained by sintering green compacted bodies of such aluminum-alloyed powder at a temperature in the range of l200 to 1350 C. within a protective atmosphere such as dry hydrogen, or of purified cracked ammonia. It is also desirable to maintain the protective atmosphere at a dew point of about .50 C. or below. Such 10w dew point atmosphere may be obtained by a suitable getter such as pure chromium powder or ferro-silicon powder (70% Gr),

mixed with aluminum oxide.

Green compacted bodies made from soft surface-alloyed iron powder of the invention may be infiltrated with other metals such as copper and copper alloys for producing composite bodies having the desired combined characteristics. Such infiltrated bodies may be subjected to heat treatments, eitherat the time of the infiltration, or subsequent thereto for causing the infiltrant and the metal particles and substances of the powder compact skeleton to diffuse into each other to any degree as desired.

According to a further phase of the invention, aluminum-surface alloyed soft plastically readily deformable powders of either iron, nickel or cobalt, or alloys or mixtures thereof, are produced in a very economical way from the oxides, or mixtures of the oxides of these metals, or mixtures of these oxides and metals. Among the oxides suitable for' this purpose is black mill-scale containing principally iron oxides- (F6304 and FeO') which is formed when rolling and forging iron: and steel.

For producing aluminum-surface-alloyed' powders of the invention from oxides of iron, nickel -or cobalt, or' mixtures of the different oxides,

the desired metal oxide or oxides may be subjected to a combined reducing and sintering treatment.

By way of example, there will now be described a satisfactory process for producing aluminum-alloyed. powder containing soft moldable iron and nickel alloy powder particles partially aluminum-alloyed in accordance with theprocess of the invention. Powder particles of iron oxide mill-scale containing: essentially F8203 and FeO are mixed with nickel oxide powder and lamp black, andball milled into a powder' of -100 mesh. As an example, 200 parts of'the mill scale are mixed with 226 parts nickel oxide and one part of: lamp black. The powder mixture is then ball milled to powder of 100 mesh, and placed in treatment boats of heat-resistant metal coated on the interior with a stick-suppressing medium, and treated ina. furnace under reducing. atmosphere such as dry hydrogen; or cracked ammonia at a. temperature inv the range between 800- and 11100:? C.. for one-half to three hours; and" then permitted to coo11in;tl"ie. same atmosphere: Satisfactory results are obtained: by suchv treatment carriedon at: a temperature of about: 950 C, for one hour. The. powder mix'may bepelleted in.- to pellets before subjecting it. ill)? the foregoing treatment, or it maybe: placeddnto theitreatment boat as a thin powder layer about A;.- to A deep, in-whichcase the:resulting-sinten'cake produced by? the reducing heat: treatment is broken up intolumps. The powder is. formed into pellets by mixing: it? with a: lubricant and binder so that'the resultantimixed powderrmass may be readily made upinto smalLporousapelilets, for instance, of cylindricalishape;havingza diameter of about A; to inch, and th'e same height, with a pellet-density of '3ito' 4 grams per cubic centimeter-1 Any suitable organic lubricant and binder which decomposes at elevated tern:- peratures of about 800 to' 900 C. and above, may be used asa lubricant and binder inlmak ing such pellets; For'instance, high'fatty 'a'cids, such as stearic acid, and salts of'st'earic acid, such as zinc stearateand=the2 like; are suitable for'use asa= lubricant. Carbohydrates; suchias dextrose dissolved in water; or" camphor' disi- 10 solved in alcohol, may be used as a binder in making such pellets.

The reducing and sintering treatment carried out at an elevated temperature in the manner described above reduces the oxides and causes the different metal constituents of the different powder particles, i. e. of nickel and iron powder to mutually dii fuse with each other, and the individual powder particles become actually alloyed.

In the particular example referred to herein, the reducing and sintering treatment was performed at a temperature of about 1050" C. for one hour, resulting in sinter cake pellets having a density of about 2.3 g./cc'., with a weight loss of about .5%.

The reduced sintered powder pellets or sinter cake pieces produced in the manner described above are then subjected to an aluminum-surface-alloying treatment similar to that applied to sinter cake pieces made from spong'e iron powder as described above. The surface-alloyed sinter cake pellets obtained by such treatment have about the same density as they had before i being subjected to the surface-alloying treatment, and they may be comminuted with minimized work hardness into s'oft surface-alloyed powder particles having the desired high degree" of plastic deformability. Byway of example surface-alloyed sinter cakepellets produced' in the manner described above, yield upon comminution, a surface-alloyed iron powder of high softness, and plastic deformability, so that a test bar of such powder compacted under a pressure of 25 t. s. i without any lubricant or binder, exhibits amodulus of rupture of about 900 p. s. i.

By prolonging the' surf a'ce-alloying treatment, or by subjectingthe surfa'ce-alloyed sinter cake pieces, such as the sinter'cakefragm'ents or sinter cake pellets, to a suitably long additional heat or si'nt'ering' treatment, the aluminum content of the" o'uter'layers' of the powder particles of such bodies may be caused to diffuse into the interior of the individual powder particles, and thus cause suchpowder particles to be substantially uniformly alloyed. Furthermore, Suchprolonged diffusion treatment will cause the: difie'rent metals of the individual powder particles, as well as their aluminum content, to mutually diftu'se, thereby giving the resulting body. the characteristic desired alloy composition.

By proceeding. in a similar manner with a mixture of 'oxides of one'or more of the metals iron, nickel andc'obalt, with one or more ofthe metals molybdenum, tungsten, chormium, vanadiuni, manganese; or'the'o'xides of these metals, there may be producedaluminum-surface-alloyed soft, plastica-lly readily" deformable powder mixtures-of the desired different metals, to wit, iron, nickel, cobalt, molybdenum, tungsten, chromium, vanadium, and manganese, exhibi i'ng th'e' combined characteristics of the different combined constituents;

It should be noted" that when producing surface-alloyedsoft metal powders of the invention from oxides of the desired metals, such as oxides of iron, nickel, cobalt, molybdenumand tungsten, and mixtures thereof, the oxidepowders may be pelleted" into pellets for the preliminary reducing and sintering treatment; When such pellets are subjected to the combined reducing and sintering treatment of the type described above, the admixed'lubricant and binder is decomposed" and driven off. As a result, the reduced sinter cake pellets have the required low density and small strength, comparable to the strength of the sinter cake fragments produced by sintering sponge iron powder deposited in a layer of about /4" within the treatment boats in the preliminary treatment of sponge iron powder described hereinbefore. As long as the sinter cake pellets resulting from the preliminary reducing and sintering treatment have a density not exceeding about 5 g./cc.. and a modulus of rupture not exceeding about 6500 p. s. i. the sinter cake pellets will yield aluminum-surface-alloyed powders having the desired high degree of softness and plastic deformability as to make it possible to compact such powders into green compact bodies having a modulus of rupture of at least 400 p. s. i. and higher with a pressure of only 50,000 p. s. i.

As used in the specification and claims, the expression sinter cake lumps includes both sinter cake lumps and sinter cake pellets of sufficiently low density and strength that upon comminution of such silicon-alloyed sinter cake body into minute silicon-alloyed powder particles having the desired high degree of softness and plastic deformability which makes it possible to compact such powders into green compacted bodies having a modulus of rupture of at least 400 p. s. i. and higher with a pressure of only 50,000 p. s. i.

Our copending application, Ser. No. 153.286 filed March 31, 1950, discloses and claims novel, readily compactible and moldable soft metal powders of loose powder particles composed of at least one metal of the group consisting of iron. nickel, and cobalt, with or without at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of said metals, the surface layer of the particles having alloyed therewith chromium deposited on the surfaces of the particles from a chromium compound at an elevated temperature so as to give them effective resistance to oxidation, the particles having sufficiently great softness so that when compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch, the resulting green body will exhibit a modulus of rupture of at least about 200 pounds per square inchand also novel methods of producing such oxidation-resistant soft metal powder.

Our co-pending application, Ser. No. 153,288, filed March 31, 1950, discloses and claims novel, readily compactible and moldable soft metal powders of loose powder particles composed of at least one metal of the group consisting of iron, nickel, and cobalt, with or without at least one metal of the group consisting of molybdenum and tungsten. and of mixtures and alloys of said metals, the surface layer of the particles having alloyed therewith silicon deposited on the surfaces of the particles from a silicon compound at an elevated temperature, so as to give them effective resistance to oxidation, the particles having sufliciently great softness so that when compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch, the resulting green body will exhibit a modulus of rupture of at least about 200 pounds per square inchand also novel methods of producing such oxidation-resistant soft metal powder.

According to another phase of the in ention, soft. readily compactible moldable metal powders of loose powder particles composed of at least one metal of the group consisting of iron, nickel, and cobalt, with or without at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of said metals, are surface-alloyed with at least two substances of the group consisting of chromium, silicon, aluminum, by depositing on the surfaces of the metal powder particles the different alloying substances from compounds of the different alloying substances at an elevated temperature so as to cause the deposited atoms of the alloying substances to difiuSe into at least the surface layers of the metal powder particles.

Such soft metal powders of the invention having surface-alloyed with the powder particles at least two of the substances of the group consisting of chromium, silicon, aluminum, may be produced by subjecting the powder particles in sequence to the different surface-alloying procedures disclosed in our said applications and the present application, for instance, by first subjecting the metal powder particles to a surfacealloying action with chromium deposited thereon, followed by subjecting the metal powder particles to a surface-alloying action with silicon deposited thereon, followed by subjecting the metal powder particles to surface-alloying action with aluminum deposited thereon, or by proceeding in a different sequence with such successive different surface-alloying treatments.

However, the sinter cake pieces may also be subjected to simultaneous surface-alloying action of several or all the desired surface-alloying substances. Thus, by way of example, the following procedure may be used for producing oxidationresistant metal powders of the invention by subjecting sinter cake pieces formed of the soft metal powder particles to the surface-alloying action with chromium, silicon and aluminum deposited from compounds of these substances. sintered cake pieces of the metal powder which is to be surface-alloyed in accordance with the invention is packed in a pack mass consisting of equal parts of ferrochromium, ferrosilicon, ferroaluminum, and a corresponding volume of porcelain pieces. A gaseous stream of hydrogen and H01 is then passed through a retort within which the sopacked sinter cake pieces have been placed to cause chromium, silicon and aluminum of the compounds of the substances produced within the retort to be deposited on the powder particles of the sinter cake pieces in the same manner as described in our applications referred to hereinabove until the powder particles are surface-alloyed with the desired content of the surfacealloying substances. The balance of the procedure is generally similar to the other surfacealloying procedures described in our aforesaid applications.

The principles of the invention described above in connection with specific exemplifications thereof, will suggest various other modifications and applications of the same. It is accordingly desired that the present invention shall not be limited to the specific exemplifications shown or described therein.

We claim:

1. A sintered cake body of metal powder particles which is readily comminutable into soft metal powder particles, the particles of said cake body being composed essentially of at least one metal of the group consisting of iron containing at most about 2% carbon, of nickel, and of cobalt, and mixtures and alloys of said metals, said cake body having been subjected to an aluminum-alloying treatment causing at least the surface layer of said particles to become alloyed with aduminum: depositedi. on: the particles from: an

aluminum. compound at elevated temperatnresl'in which; at: least."- 3%? of the. :metal Off; said? b'ody-i'is -displacedibyaluminum, said cake:.body having at most a density of about 5 gramsrperr-cubic centimeter; whereby. SBiCl-ibOdYf may-be pulverized into: minute aluminumsalloyed 1 powder: particles which exhibit:minlmized'iworlr"hardness: and :suf'; fi'ciently' greatsoftness s'.='.tha:ti.wh'enr;thee'alumn num a'lloyedi-particlesiarercompacteddnto a=green body: unders-pressure of about 150,000." poundsper facezlayer. of saidi'particlesztobecome: alloyedzwith aluminum: deposited. on. the particles, froman aluminum". compound-- at elevated w temperatures; said: cake body containing at: least .3 aluminum and having at most: a density of: aboutgrams per cubiccentimeter:

3: .A sinteredcakerbodyi of: metal powder particleszwhich iswreadilyg'comminutable-- into soft metallpowderparticles,;.theiparticles of saidcake body beingcomposed-ressentially: of: at-1east one metal of: the group-consisting of= iron-containing at-rmost 'ab.out":.2.%.' carbon, of nickel, and of. cobaltytogether'with" atzleast :onecmetaliof :the 1 group consisting of molybdenum and tungsten; andof mixturep and; alloys of said metals; said cake body; having; been subjected to an aluminumalloyingtreatment causing at least the surface layer. of. said particles. to. become alloyed with aluminum deposited on the particles from a gaseous aluminumcompound-at elevated temperatures, said cake body-having at most a density of: about 5 grams'pen-cubic; centimeter. and a modulus ofirupturetoflat.most-6500 pounds per square inch, whereby saidbody may be pulveriz'ed into minute a'luminum alloyed powder particles whichexhibit minimized work hardness and suinciently great" softness so that whom the aluminum-alloyed particles are compacted into a green body under pressure of about 50,000 pounds per square inch, said green body exhibits a modulus of rupture of at least 200 pounds per square inch.

4. A sintered cake body of metal powder particles which is readily comminutable into soft metal powder particles, the particles of said cake body being composed essentially of at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and of cobalt, together with at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of said metals; said cake body having been subjected to an aluminumalloying treatment causing at least the surface layer of said particles to become alloyed with aluminum deposited on the particles from an aluminum compound at elevated temperatures, said cake body containing at least 3% aluminum and having at most a density of about 5 grams per cubic centimeter.

5. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake pieces composed essentially of sintered metal powder" particles: of at least one metali off. the group consisting of ironiconta-iningratimost:about .2% carbon, of-x'nickel; and of cobaltyandrmixatures and: alloys. of: at? least two; of: said; metals sozthat: said" cake.=pieces haveiat Lmost: aidensity of l'abouti4 grams -.per' cubiczcentimeter; anditherer' after:subjecting:saidicake:pieces to: an aluminium, alloying: treatment in .which; aluminumafrom: an aluminum .compoundeiss deposit'edwzoni: thezpowder particlesiof saidz'cake pieces; and icausedzto-diffinse into: said; powder." particles so that. said. powder particles contain: at: least about: 3%: aluminum; andithereaftencomminuting:saidicakezpieces-sinto minute *pl-astically readilyrdeformablemetalpow;- der particles having, the? deposited :aluminum; a'l loyedi therewith: sa': that when. said alum-mums alloyed particles are; compacted in'. the absence offa -binder into agreentbody under; aipressureof about: 50,000. pounds-apers squares inch;said green body exhibits; a; modulus. ofv rupture; of: at: least about:20'0:-pounds*per'squareinch".

6: The: method" of providing: as readily? come pa'ctib'le: and": moldable: metal -.powder comprising first: producing? readily frangible; porous: sinter cake P160655: of; sintered; metal powder: particles composed essentially of E at. least gonee metaliiofl the group :consisting =of:iron containing ainmostsabo'ut .2%. carbon, of: nickel;. and: of: cobalt; together witli'aup' to atuleastr one.:-metal of; thegroupzcona sistingioffmolybdenumrand:tungsten; andtofrmixe tures and'ialloys of saidametals'g: sothatzsinter cake pieces-have atamostj a: density: of; about- 4 grams perscubic centimeters. and: thereafter :snb, jectin'g'rsaidicake pieces; 1203311: aluminumealloying treatment in whichzaluminumfromnnaluminum compound is depositedi on; the: powder;- particles of'isa'idcake pieceszand :caused :to: diffuseintoesaid powderparticlesso zth'attthe said-:powdenparticles contain'atueast about's3l% aluminum; andithere: after'comminuting saidtcak'e piecessintmminute plastically; readilyi-deformabde meta-hpowdenparticles: having: the deposited aluminum alloyed therewitht so that; when. sanl a'luminumealloyed particle'srare;compacteddni-the; absencezof a binder into a'sgreenr. body: underya pressure: of: about 50;0005: pcundsmcnsquare: inch, send green body exhibitsaas. modulus of. ruptureoffat; leasti about 200Ipoundsper.squareinch:

7. The methodrofiprovidingazreadilwcompact? ible and moldable metal powder comprising first producing readily frangible porous sinter cake pieces composed essentially of sintered metal powder particles of the group consisting of iron having at most about .2% carbon, of nickel, and of cobalt, and mixtures and alloys of at least two of said metals so that said cake pieces have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to an aluminum-alloying treatment in which aluminum from a gaseous aluminum compound is deposited on the powder particles of said cake pieces and caused to diffuse into said powder particles so that said powder particles contain at least about 3% aluminum, and thereafter comminuting said cake pieces into minute plastically readily deformable metal powder particles having the deposited aluminum alloyed therewith so that when said aluminum-alloyed particles are compacted in the absence of a binder into a, green body under a pressure of about 50,000 pounds per square inch, said green body exhibits a modulus of rupture of at least 200 pounds per square inch.

8. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake pieces composed essentially of sintered metal powder particles of at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and of cobalt, together with up to about 30% of at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of saidmetals so that said sinter cake pieces have at most a density of about 4 grams per cubic centimeter, and theresaid green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

9. The method of providing a readily compactible and moldable metal powder comprising first preparing a powder mass of powder particles composed essentially of at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and of cobalt, and mixtures and alloys of at least two of said metals, at least some of said powder particles containing the metal in the form of an oxide, heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. to produce porous sinter cakes having at most a density of about five grams per cubic centimeter, and thereafter subjectin said cake pieces to an aluminumalloying treatment in which aluminum from a gaseous aluminum compound is deposited on the powder particles of said cake pieces and caused to diffuse into said powder particles so that the said powder particles contain at least about 3% of aluminum, and thereafter comminuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited aluminum alloyed therewith so that when said aluminum-alloyed metal particles are compacted in the absence of a binder into a green body having a pressure of about 50,000 pounds per square inch, said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

10. The method of providing a readily compactible and moldable metal powder comprising first preparing a powder mass of powder particles composed essentially of at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and of cobalt, together with up to about 30% of at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and of alloys of said metals; at least some of said powder particles containing the metal in the form of an oxide, heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. to produce porous sinter cakes having at most a density .01 about five grams per cubic centimeter, and thereafter subjecting said cake pieces to an aluminumalloying treatment in which aluminum from a gaseous aluminum compound is deposited on the powder particles of said cake pieces and caused to diffuse into said powder particles so that the said powder particles contain at least about 3% of aluminum, and thereafter comminuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited aluminum alloyed therewith so that when said aluminum-alloyed metal particles are compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch, said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

GEORGE STERN. S. J. SINDEBAND. J. P. SCANLAN.

References Cited in the file of this patent FOREIGN PATENTS Number Country Date 598,181 Great Britain Feb. 12, 1948 OTHER REFERENCES Treatise on Powder Metallurgy, vol. I, pages 71 and 74. Edited by Goetzel. Published in 1949 by Interscience Publishers, Inc., New York.

Claims (1)

1. A SINTERED CAKE BODY OF METAL POWDER PARTICLES WHICH IS READILY COMMINUTABLE INTO SOFT METAL POWDER PARTICLES, THE PARTICLES OF SAID CAKE BODY BEING COMPOSED ESSENTIALLY OF AT LEAST ONE METAL OF THE GROUP CONSISTING OF IRON CONTAINING AT MOST ABOUT .2% CARBON, OF NICKLE, AND OF COBALT, AND MIXTURES AND ALLOYS OF SAID METALS, SAID CAKE BODY HAVING BEEN SUBJECTED TO AN ALUMINUM-ALLOYING TREATMENT CAUSING AT LEAST THE SURFACE LAYER OF SAID PARTICLES TO BECOME ALLOYED WITH ALUMINUM DEPOSITED ON THE PARTICLES FROM AN ALUMINUM COMPOUND AT ELEVATED TEMPERATURES IN WHICH AT LEAST 3% OF THE METAL OF SAID BODY IS DISPLACED BY ALUMINUM-ALLOYED POWDER PARTICLES AT MOST A DENSITY OF ABOUT 5 GRAMS PER CUBIC CENTIMETER, WHEREBY SAID BODY MAY BE PULVERIZED INTO MINUTE ALUMINUM-ALLOYED POWDER PARTICLES WHICH EXHIBIT MINIMIZED WORK HARDNESS AND SUFFICIENTLY GREAT SOFTNESS SO THAT WHEN THE ALUMINUM-ALLOYED PARTICLES ARE COMPACTED INTO A GREEN BODY UNDER PRESSURE OF ABOUT 50,000 POUNDS PER SQUARE INCH, SAID GREEN BODY EXHIBIT A MODULUS OF RUPTURE OF AT LEAST 200 POUNDS PER SQUARE INCH