US2657127A

US2657127A - Production of chromium-alloyed corrosion-resistant metal powders and related products

Info

Publication number: US2657127A
Application number: US153287A
Authority: US
Inventors: Seymour J Sindeband; Stern George; Joseph P Scanlan
Original assignee: American Electro Metal Corp
Current assignee: American Electro Metal Corp
Priority date: 1950-03-31
Filing date: 1950-03-31
Publication date: 1953-10-27
Anticipated expiration: 1970-10-27

Description

27, 1953 s. J. SINDEBAND ETAL 2,657,127

PRODUCTION OF CHROMIUM-ALLOYED CORROSION-RESISTANT METAL POWDERS AND RELATED PRODUCTS Filed March 51, 1950 l NVEN TOR S 6. STE/7N & J. smlozemva w Wm,

ATTORNEY Patented Oct. 27, 1953 PRODUCTION OF CHEOlMIUMI-ALLOYED- CORROSION-RESISTANT METAL POW- DERS AND RELATED PRODUCTS Seymour J. Sindeband, Chappaqua, George Stern, Mamaroneck, and Joseph P. Scanlan, Yonkers, N. Y., assignors to American. Electra, Metal; Corporation, Yonkers, N. Y., a corporation of Maryland Application March 31, 1950, Serial No; 153,287

l 8 Claims.

'Ifhis'invention relates. to readily compactible and moldable metal'powders which resist corrosion; shaped bodies of such powders, and methods for producing the same, some aspects of the in' vention being of abroader scope.

Among the objects of-the invention are powders of metal particleswhichare ina readily defor 1-" able plastic condition, and. may be compacted into relatively strong metal bodies with relatively low pressures; at least the surface layer of the metal powder particles having'alloyed therewith chromium deposited: thereon from a gaseous or liquid chromium compound at an elevated temperature.

A particular objectof the'invention is such readily compactible andmoldable metal powder containing essentially powder particles of at least one metal of the group consisting of iron, nickel, and cobalt, and of mixtures and alloys of said metals, whichparticles are in a readily deformable, relatively soft plastic condition, at least the surface layer of said particles having alloyed therewith chromium which has been deposited thereon at an elevated temperature from a gase ous or, liquid chromium compound;

Other objects of the invention include-compacted bodies made of such surface-alloyed powder particles, and economical production of such powder particles, and economical production of compacted bodies from such powder. particles. of the invention.

The foregoing and other objects of the invention will be best understood fromthe following description of exemplifications therof, reference being had to the accompanying drawings, where- 1n:

Fig. 1 is a greatly enlargedphotograph ofa sample of powder particles of the invention made of sponge iron powder; and

Fig. 2 shows a similar photograph of powder particles of the invention made from a mixture of iron oxide and nickel oxide powders.

For years past various corrosion-resistant products such as gears, valve parts, bearings, filters, and the like, havebeen made of stainless steel powders by powder metallurgy techniques. The stainless steel powders generally used for such bodies have been-obtained from previously preparedstainless. steel ingots, for instance,.by subjecting-a stainless steel ingot to grain boundary corrosion: followed by disintegration; or. by melting :a stainless steelingot and atomizing it in themolten state. The so-produced particlesof such-stainless steel powders are of, high density and; hardness, and, require highemolding pres,-

2. sureof at least ill to 45't; s. i; (tons: perxsquare inch) which is muchtoo high for economical production of molded products, and results in rapid wear and 'frequentbrake-down of the molding dies. As aresult; the production-of molded products from stainless steel powders has found only limited use.

What has. been saidxabove ab'out stainless steel powder, whichis an alloy-of: iron, nickel and chromium; applies alsozto. various other general- 1y similar commercially. available alloys; such as alloys of. iron, cobalt-andchromium, of "iron, nickel, molybdenurrntungsten. and chromium, and of generally similar alloys which are used in heat-resisting bodies, electric= heaters, magnetic bodies and in corrosion-resistant: hotstrength bodies.

According to the-basic concept/of the invention',.the foregoingdifliculties which were; en'- counteredsin the production of corrosion-resistant ferrous" metal bodies: by powderxmetallurgy techniques are overcome. by thev provision: of I a readily compactible and moldable metalpowder ofductile andrelatively softferrous powder particles; at least-the surface layer-"of the powder particles :having combined. therewith chromium. which has been: deposited thereomfroma gaseous orliquid compound'at an elevated-temperature, the; particles being. sufficiently. soft and plastic as tomake itpossible to compact'them with .a relatively low pressure, for instance, only 25 to 30 t'. s. i. into bodies of even greater-strength than molded bodies made of prior stainless steel powder.

Various commercially available soft, plasticallyreadily deformableiron powders are suit able for. producing corrosion-resistant softmoldable metal powders of the invention. Amongsuch available and suitable soft iron powdersis. sponge iron powder havingacarbon content of .1 to .2% (unless otherwisespecifically stated, allproportionsare. given hereinby weight) electrolytically produced iron powder, carbonyl iron powder, eddy. mill iron powder, and atomized. molten iron powder ofsimilar carbon content. The-following-impurities present in such available-powders. to wit, .3%-Mo,- 2% Si, and other'minorimpurities usually. presentrtherein; .do not" impair -their usefulness for practicing the invention;

In addition, many relatively *hardand: dilficult to compact fine ironpowders"containingiup to about .2% carbon will, in' general, wherr subjected to the chromiumealloying :process of the invention,.-.herein described; become-softened and ciples of the invention.

acquirecharacteristics which render them read.- ily moldable.

In accordance with a phase of the invention based on the original concept of producing chromium-alloyed soft iron powder'-relatively soft powder particles of different metals, to wit, one or more metals of the. group including iron, nickel and cobalt, are combined in a proper proportion, and a proper proportion of chromium from a gaseous or liquid chromium compound is deposited on and diffused in such powder particles of the difierent metals so as toyield a soft powder body with chromium in the proportions required for producing alloy bodies of the desired characteristics.

- Any of the known methods for, depositing chromium from a gaseous or liquid compound on the surface of a metal body heated to an elevated temperature below the melting temperature of the body, and of chromium, may be used for depositing on and diifusing chromium into metallic powder particles in accordance with, the prin- Among such known chromium-alloying processes, one that was found particularly suitable wherein M represents the metal of the chromiumalloyed body.

partially omitted from the gaseous stream and only hydrogen gas passed through the pack. As another alternative procedure, the sinter cake lumps are packed in a pack of porcelain powder, term-chromium powder, and a chromium chloride compound, and enclosed within a sealed envelope which is heated to produce the desired .chromium-surface-alloying action.

As a still further alternative procedure, the

sinter cake lumps maybe immersed in a molten salt bath containing chromous chloride and subjected therein to a chromium-surface-alloying.

treatment at an elevated temperature. By way of examplaa suitable bath composition for the latter treatment may consist of 30% CrCla 50% BaCaz, and NaCL' By keeping the bath-heated to a temperature in the range from sinter cake lumps. alloyed sinter cake lumps maybe readily broken:

In such process, the chromium of the chromous I I chloride gas'replaces the metal on the surface of the treated body causing the chromium deposited on the surface of the body to alloy therewith and to diffuse into its interior while the gaseous chloride compound of the displaced metal escapes.

Instead of chromous chloride, other chromium halides may be used for effectively carrying out the chromium-alloying process in accordance with the principles of the invention.

In accordance with one phase of the invention, the soft iron particles are first sintered into sinter cake lumps within a reducing atmosphere, such as dry hydrogen, or dry cracked ammonia at an elevated temperature so as to produce a porous sinter cake of low density not higher than about 4 g./cc., and of low strength, not higher than a modulus of rupture of about 6500 p. s. i. The powder particles of such sinter cake may be readily surface-alloyed with chromium. To this end, the sinter cake may be broken up into lumps which are then packed into a chromium-alloying pack and heated to elevated temperature, so as to cause chromium from chromous chloride gas to be deposited on the surface of the powder particles, and diffused into the interior of the particles.

Any of a variety of known chromium-surfacealloying processes may be used for this purpose. By way of example, the sinter cake lumps may be packed in a pack containing term-chromium and. a stream of a gaseous mixture of hydrogen and HCl is passed through the pack in which case the chromizing agent, that is chromous chloride gas, is produced within the pack. Alternatively, the pack may be impregnated with separately prepared chromous chloride, in which case the hydrogen chloride gas may be at least about 900 to 1500 C., from about 3 to 10 hours, the desired chromium-surface-alloying action will be produced. The CIC12 content of such. bath may bereplaced by elements which generally are CrClz, in which case an equivalent amount of CrCh and chromium metal is included in the bath. In all the foregoing chromiumsurface-alloying treatments, the chromium-alloyin'g'reaction is represented by the Equation 1 given above.

The chromium-alloying process does not materially increase the density and strength, of the I As a result, the chromiumup into powder particles of the required size range, and high degree of softness which makes it possible to compact such chromium-surfacealloyed-powders under a compacting pressure of only 50,000 p.'s. i., into strong green compacts having a modulus of rupture 017400 p. s. i., and,

higher. I

In accordance vention, powder particles of the oxides of either iron, nickel and cobalt, or mixtures of such 0:;- ides, or of metals and oxides of this metal group, with or without an additional mixture of powders of molybdenum or tungsten or botheither as free metals or in the form of oxides-are sintered into porous sinter cake lumps within a reducing atmosphere such as dry hydrogen or dry cracked ammonia at an elevated temperature so as to reduce the oxides and produce a sinter cake of low density not higher than about 5 g./cc., and low strength not higher than a modulus of rupture of about 7,000 p. s. i. A convenient way for producing such sinter cake lumps is to mix the powders of the metal oxides with a lubricant and binder and pellet the mixture into pellets which are then reduced and sintered to produce sinter cake pellets or lumps of the required low density and low strength. The sinter cake pellets or lumps so obtained are then packed into a chromium-alloying pack, and their powder particles are surface-chromiumalloyed in a manner similar to the chromium-alloying of the sinter cakes of soft iron powder. The r sulting chromium-alloyed sinter cake pellets have the required low degree of density and strength. As a result, they may be broken up into minute surface-chromium-alloyed powder particles of the required degree of range of sizes and low degree of softness for making it possible to compact them into strong green bodies having a modulus of rupture of at least 400 p. s. i. at a pressure of only 50,000 p. s. i. The surfacechromium-alloyed powders produced in accordance with the invention from a mixture of oxides with another phase of the in-' :of;thedifierentmetalszwill yieldaazsurface zelloyed powder body,:the z'individual :particles izo'f which are fialloys :oftheiidifierentunetals:corresnondin :to-. the; di'fierentmetalaoxidesxandzthep proportion IOf-ZChIOIIIiUlTi: depositedcng-thersinter cake pieces.

Without. thereby limiting :the. SCOPBnOfathB invention, and inaorder 110561133316 those skilled, in :the :art 5130 readily; practice the invention, there v:willrnow-;be:describedpne practice. of the: invention, therewilknow be described,onerexainple-of "a satisfactorygproceclure.for producing a-;corro .sion-resistant: soft iron;.po.wder of the invention from commercially available; spongeironjpowder havinga carbon content .of;about,1 5;%. A= typical commercial powder-cf this :typenused in :the process contained a ;mixture f minute powder particles having the 1 following 1 particleesize: distribution typical :.of rmoldable metal ,powders: 21%--..100, ,+150;mesh;,.30% .l'50,- l.200 mesh: 13% 200,--+O mesh;, 14% -..250,-l:325. mesh, ,and..the balance -325. meshpowder.

The powder is treated to sinter' itiinto. a porous cake, mass of low density, and thereby facilitate surface-alloying of the powder particles "with chromium. This'may bedone by depositing-a layer of-powderof about, to 4 inch thick into sinter "boats of suitable metal such as "a heatresistant iron-chromium sheet metal without I in any 'way'compacting the powder. The l interior surface of each-sinter boat is 'coated withza stick- :suppressing or release medium suchas as. water suspension of magnesiummy'droxide to :permit readyv separation of athe isintered powder srbody :from'thehoat:aftenthe sinteringpperation. The powder within the boats 115 then treated -;within a furnace 1115.2. reducingeatmospherewf ,dry.hy- .drogen or purifiedtcrackedcammoniaq'at; a ttemperature of. about-80mm.1050JC. .for oneehalfsto three hours, andthen;perm-itted.to:.cool to room temperature. In;;general,.-such. treatment in-tlie range of-about 900 to -1000-- C..,;such as 950\.C.., for- ,one hour is-sumcient. for sintering the powder into ,asinter cake-,havingla.densitynf 1.7 to #4 ;g-.,/cc., which is readily frangible into dumps or pieces suitable .for packing into achrom-i-umalloying. mass. ;-A,,goodway forcarryinghon .the foregoing sintcring .treatment -is ato push --the= powder containing :boats through Fa ..tunnel rfur- .nace-from. one, end. to the Hother, .the heattreat- :ment being followed :by cooling'which-is carried out as a .part .of continuous treatment as the hoats are moved from-.one. end to the other-end of itheifurnace.

After completion of the ,sintering treatment, the sinten cakesoareiremoved from-the boat,,.and broken into fragments of a size-suitable forpack- .ing-ina chromiumal.1oying. .pack. .The sinter cake pieces or. lumps arelthereupon packed with- .in .a. chromous chloride .producingpaekwmass; and placed withinbaskets of ,suitableheateresistant metal such .as achromiumenickel-iron alloy. .The pack; may. consist,1for instance, of 5 0% by volume ceramic. lumps, such. as porcelampieces,sandtthe balance .a chromium alloy such as ferro=chr.o- .mium .alloy. ,Analternative. chromium-alloying pack which [was .found highly effective. consists of a. mixture of .titanium .oxide with ferro -chr.o mium. Good results-arecbtained.with such. pack consisting. of about 20% =titaniumloxicle, 1. by vo1- ume, .the balance consisting of ferroechromium, containing .-.about 70 chromium.

-The treatingibaskets containingthe-sinter cake lumps-.packedior instance,-=withtitanium; oxide and erro=chromium,-are placed-inna retortrand heated to a temperature in the range between 6 i900 andl2501G.c-and:a istream-of hydrogen and hydrogenrhalide rgas,;rsucha.as hydrogen chloride gas; is: passed :through the :retort, for producing reactionsrcausing chromium-atoms to be deposited on the powder particles, andto-fliiT-useinto the interior; of ,thel'particles, and. :alloy therewith.

. Insuchsurface-alloying treatment, thehydroigerrchloridegas;passing. through theretort interzactszwith-itheiferroechromium .to .form ,chromous chloride gas. The; chromium of :chromous chloridereplaces.the-metal ,on the-surfaces of the .treatedipowder particles primarily by the ex- ;changerreaction. :Rart .of ;the chromous chloride :gas is-absorbed: by the gpacking. material, such .as the :ceramic material :or :the titanium :oxide in the-pack.

In the particular examplehereindescribed,fthe pack-treatment baskets were of circular shape, and had :anroutside diameter of 22", and a; height of 10". .Four suchipackedxbaskets.were stacked within a closely fitting retort about high. The packing material consisted of T102, 20% -by volume, the balaneeferro-chromium a closely fit- ..ting. retort-about .60 high. The. packing. material consisted of T102, 20% by volume, balance Iferro-chromium containing chromium. During the; initial part ofthe chromium-surface- .alloying treatment, purified dry hydrogen was :caused to flow through the enclosed retort space --a.t-a' rate of 40 cubic feet per hour, while the'temperature was rraised to about 950 C. The flow of pure hydrogen was thereafter continued at a rate of'20 cubic feet perhour, for four hours at the same temperature of about 950 C. Thereafter,- the .basketswithin the retort'weresubj ected .to a.:succession:.of :fiveztreatment sequences, at about :950 C., each treatment. sequence lasting :six hours,, andconsistingrof (a) passing-through the; retort :rat ;a:. rate of 20 vcubic feet per. hourl'a mixture of.20; partszof hydrogen and 3 parts of HCLgas for one1hour,"followed by (19) passing Pure.hydrogentatthegsame rate for one hour,:fo1- lowedzby' ('c) spassinggthe same mixture .of hydrogen; and-HUI gas as in (a) for. one hour, followed ;by (17) :passing .pure. hydrogen .at the same rate forfthreet hours. After :a. succession of six such treatmentysequences of. :six hours :duration each, .theatreatmentawasended: by, turning off the heat and permitting the retort with its contents to cool while .zcontimu'ng I the zflow of hydrogen through the retortispa-ce'until. theitemperature of its. con- :ents.wasr broughtudown.to about room temperal 1U pon completing. the foregoing chromium-surfaceea'lloyingtreatment, :the treated metal pow- :der pieces are removed from the baskets, and comminute'd bycrushing to powder, for instance, in a disc crusher.

JVitl-iout-inany way limiting the scope of the inventiornand in order to enable those skilled in theart to-readi1y: practice the invention; there are given below the results obtained by subjecting packed ironpowder particles to the rspecificsohour chromium-alloying treatment described above.

The resulting .chromiumralloyed powder contained 8.5% .mesh; 35.5% -10o, mesh;..28.% .150, +200' mesh; 10% 200, +250 mesh; ,.9%- 250, +325 mesh; and 9% 325 mesh powder. :Theapparent density oi the powder was -2i5fi;; ./cc.,wand'the'weightloss.due to1the hydroen treatment (hydrogen 5 loss) :was ".053 ;,car bon :content .:.0.45.%, titanium. content .2 and :chromium content 15 balance iron. The-chromium-alloyed powder was then compacted into test bars and tested for green strength.

Figure 1 is a photograph of a sample of such powder particles of the invention greatly enlarged on a scale 30 to 1.

Ihe table below gives the modulus of rupture (also called transverse rupture strength) calculated from breaking tests, of chromium-alloyed powders, compacted into test bars under pressures of 25, 35 and 45 t. s. i. designated in the table as C. A. P. The table also shows the comparative modulus of rupture of compacted green bars made from two commercially available stainless steel powders designated in the table as SH 18-8, and SV 18-10 representing stainless steel powders with a chromium and nickel content of 18%-8% and l8%-l0%, respectively.

Modulus of rupture of three green test bars pressed at 25, 35 and 45 t. s. 2'.

The foregoing test results show the characteristic distinction and the marked superiority in green strength of bodies produced from. surfacedifiusion chromium-alloyed iron powders of the invention as compared with green bodies produced from available stainless steel powders. The chromium-alloyed powder particles of the invention retain their spongy character, have a tentacle-like shape, and they have a lowered carbon content. Such compacted green bodies produced from soft, mold-able chromium-alloyed iron powder of the invention, as well as the powder itself, exhibit excellent corrosion-resistance under prolonged salt-spray tests, and they also resist attacks by 30% HNOs, cold or hot.

It should be noted that similar results are obtained with other types of soft, plastically deformable iron powders. Furthermore, the invention is not limited to metal powders having the particular size distribution given in the example described above, but may be practiced with soft powders of any particle size distribution of the type used for producing bodies by powder metallurgy techniques.

The results of rupture test given in the foregoing table were obtained by test equipment and test methods described by J. P. Scanlan and R. P. Seelig in Powder Metallurgy Bulletin, vol. 4, p. 128 (1949) with each test bar 1 long, wide, and 24;, thick.

The tests shown in the table are representative of the characteristic distinguishing chromiumsurface-alloyed ferrous powders of the invention over corrosion-resistant prior art powders of similar composition. Thus, soft chromium-surfacealloyed and corrosion-resistant ferrous powders of the invention difier from heretofore available corrosion-resistant powders of generally similar composition by the fact that green compacts made from powder of the invention have a materially greater rupture strength, at least 2 to 4 times greater than similar green powders of similar composition and corrosion-resistance.

A distinguishing characteristic of the corrosion resistant-chromium-surfaceal1oyed 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, such compacted green body exhibits a modulus of rupture several times greater than a similar body produced by compacting prior art corrosion-resistant powders of the same composition, under the same pressure. In particular, such green test bar compacts made of powders of the invention have a modulus of rupture of at least 200 pounds per square inch, the modulus of rupture being in fact in all cases at least 400 pounds per square inch, and higher.

In producing surface-alloyed soft, plastically readily deformable ferrous powders of the inven.. tion, 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 chromium-- alloying pack, the powder which is to be subject-ed to the initial sintering action should not be compacted under any substantial pressure unless special precautions are taken, such as the addition of substantial amounts of volatile material, for example organic lubricant and/or binder material. If a substantial compacting pressure, even as low as 5 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 chromium-alloying treatment, such chromium-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 chromium-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 substantial pressurethe crushing energy required for pulverizlng the chromium-alloyed sinter cake fragments would be so large that the resulting chromium-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 surfacechromium-alloyed soft ferrous powders of the invention, it is essential that the sintered powder cake fragments or lumps produced in preparation for the chromium-alloying treatmentshould 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 chromium-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 chromium-alloying treatment should have high porosity so that they are permeable to the chromium halide gases, by means of which the chromium-alloying treatment is carried on.

As explained'above, the proper-range'ofrthe density of the sin-ter' cakelumps-producdin preparation for chromium-alloying treatment-is about 1.5 to 4.0 g./cc. The-modulus of rupture of such sinter cake lumps is correlated to their density, being about 50 p. s..i. iorsinter :cake lumps having a density of 1.7 f./-cc. and increasing to about 6500 p. s.'i. for sinter cake lumps having a density of 4.0 g./cc.

Furthermore, as long as the sinter cake lumps, produced inpreparation for the chromium-alloying treatment-are of low density'and have :a low modulus of rupture in the ranges set forth above--they will also have the same density and the same modulus of rupture after being subjected to the chromium-alloying treatment whereby chromium is surface alloyed with the individual .powder particles of such sinter cakes.

Accordingly, to obtain chromiumaalloyed powders of the invention, it is .sufiicient to-control the density and/or modulus of rupture .of the sinter cakes or sinter cake lumps produced preparation for the chromium-alloyed treatment. As long as the density of such sinter cakev lumps is not more than about 4.0 g./cc.,- and their modulus of rupture isnot more thanabout 6500- p. s. i. they will, after the chromium-alloying treatment, remain of sufficiently small density and-strength as to permit their pulverization without material work hardening of'the powder particles. As a result, the chromium-alloyed powder obtained from such sinter cakes will have-the desired. high degree of softness and plastic deformability-esto make it possible to compact such powdersuinto green compacts having a modulusof'ruptnreof at least 200 p. s. i. and higher.

By sintering such green compacted bodies. of chromium-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 compacting-and sintering prior-art stainlesssteel powderspfsimh lar composition which have been compacted with much higher pressure. Good results are obtained by sintering' green compacted bodies of such chromiumalloyedpowder at a temperature in the range of -1200 to'l l00"C. 'withinaprotective atmosphere, suchas 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'low dew point asmosphere may be obtained by a suitable getter such as pure chromium powder'or ferrochrome powder ("70% chrome) "mixed with aluminum oxide (A1203).

By way of example, a-green testbar of the specific chromium-alloyed iron powder of the invention, having properties set forth above, was compacted under pressure'of '25 .t. s. i. and then sintered at 1300 C. for one hour within a protective atmosphere in the manne described above. The so-sintered barexhibited a shrinkage of 8.23%, and had a density of'6i68 g-./cc. "It had Rockwell hardness F 3725, yield point "21,600 p. s. i.,ultimate tensile strength 26,450 "1). 's; i., elongation 8.5 70,- reduction'in area 4.06%.

The test bar, sintered in the above manner, repressed or coined ata pressure of 35 t.'s. i., and resintered for another hour under the same sintering conditions, exhibited a shrinkage of 1.73%. The resinteredbar had Rockwell hardness F 25.5, yield point 26,900 p. vs. i., ultimate tensile strength 45,000 p. Sui. elongation 20.5%. reduction in area 19. Each such sintered body exhibited excellent 'resistance to' corrosion ll "l" 1'0 under-prolonged exposure'to-salmspray as well as to hot and 00101 30 HNO3.

Green compacted bars made from' 'soft chrominim-alloyed iron powder of the invention may be utilized as a very effective electric contact material, aslbearing-material or the like, by infiltrating the green-compacted bar withsilver, copper and alloys'thereof. By way of example, a greenbar produced compacting the specific chromium-alloyed iron powder-of the invention having the specific properties "set forth abovepand compacted under a pressure of 25 t. s'i., and ha.v-ing-=a density of 5.47g./cc., was infiltrated with pure fine silver at 1220 C. for one hour 'in a protective "atmosphere of dry hydrogen. Such silver infiltrated ibat exhibited an infiltration shrinkage'oi 4.42%, a density of 7.59 g./cc., -a resistance of 17:9 microhms-cm. It had-'Rockwell'hardness F: -63,y-ield point-22,800 p.---s. i.,ultimate tensiie strength ;-3'7, 000 p. s. i., elongation 9 .3%and -reductionin area 10.5%.

-A similarbod-y compacted under a-pressure of 35 t. s. i., and infiltratedwith-silver inthe samemannerfha'd a Rockwell hardness=F 75.5, infiltration shrinkage 7.9%, yield point 2'7,- pss. i.,-ultimate tensiie'strength 39,600 '10. s.i., elongation 10.5%, reduction area 9.-9-%. It is thus seen that such silver-infiltrated green compacted powder bodies of the invention exhibit excellent physical properties, and tha infiltration maybe-effected at relatively-low temperature in 'generaliy available furnaces and atmospheres. Such silver-infiltrated bodies have also -the desired dimensional stabilityand exhibit -=-exce'llent -corrosion-resistance :under prolonged'salt-spray.

Chromium-alloyed iron powders *of the invention of the type "described above, are 'also "very useful for producing corrosion-resistant bodies of relatively "high hot-strength suitable f or applications requiring corrosion-resistance i atelevatedoxidizing temperatures, as well-as high strength. To this end; the shaped sintered compacts of chromium-alloyed readily moldable powders of the invention 'of the type described herein are infiltrated at an 1 elevated "temperature,- such*as -1l-00--1:250 Cxwith' an infiltrant of copper, or asuitable copper-alloy.

By wayof-example, a green bar of the-spec-ific chromium-alloy iron powder oi the invention'set forth above, compacted under pressure of -35 -t.--s.- i., and having-a green density of 5.98 g./cc. 'was sinteredfor one hour ata 'temperature of =1300 C. in a protective atmosphere-in a-mannerspecifically described in the previous example. The resulting sintered body *having a density 6:68'g./cc. was infiltratedwith a copperbase alloy containing about copper, 2% iron, 18% manganese, /2'% titanium. The infiltration was -carried .onata temperature of about .1200 ,C. .Ioroneandone-half hours in an atmosphere of .purified cracked ammonia, with v a pure chromium getter present to keep the-dew:point:.ofzthe atmosphere at the desired lowtemperature. 'Thez-resulting. infiltrated bod-y had;a:;density i7;.-95rg./cc.;yield .zpoint 87,700.11. s. i., ultimate tensile-;strengtht;250 p..-.s.. i., elongation 16.3%, reduction .in area 118.3%. :The soobtained body thus exhibitedwexcellent dimensional stability, .and excellent :resistancento -.corrosion under prolongedaexposure .to :salt spray and melativelyhot oxidizing gases.

in other-l words, sintered compacted bodies .of the invention, :after: infiltration .with v:the .rzcpper 11 alloy, have high strength, and dimensional stability, and they also have excellent resistance to corrosion.

According to a further phase of the invention, surface-diffusion, chromium-alloyed, soft, readily plastically deformable iron powders are produced in a very economical way from iron oxides, such as black mill scale containing principally iron oxides (F6304 and FeO) which is formed when rolling and forging iron and steel. The iron-oxide mill scale is treated by subjecting it in a pulverized state to a reduction process, and then sintering the resulting iron powder particles into porous pieces, such as lumps suitable for subjecting them to the subsequent chromium-alloying processes in the manner described above. If it is desired to combine with such iron powder additional elements such as nickel, and/r cobalt, the proper proportions of powders of the oxides of such additional metal are mixed with the iron oxide powder before subjecting the powder mixture to the initial reduction and sintering treatment.

By way of example, there will now be described a satisfactory process for producing a chromium-alloyed powder containing soft moldable iron and nickel alloy powder partially chromiumalloyed by surface-diffusion of chromium in accordance with the process of the invention. Powder particles of iron oxide mill scale containing essentially F8203 and FeO are mixed with powdered nickel oxide and lamp black, and ball milled into a powder of -l00 mesh. As an example, 200 parts of the mill scale are mixed with 22.6 parts nickel oxide and one part of lamp black. The powder mixture is then ball milled to powder of -l00 mesh, and placed in treatment boats of heat-resistant metal coated on the interior with a stick-suppressing medium, and treated in a furnace under reducing atmosphere such as dry hydrogen or cracked ammonia at a temperature in the range between 800 and 1100 C. for one-half to three hours, and then permitted to cool in the same atmosphere. Satisfactory results are obtained by such treatment carried on at a, temperature of about 950 C. for one hour. The powder mix may be pelleted into pellets before subjecting it to the foregoing treatment, or it may be placed into the treatment boat as a thin powder layer about A; to deep, in which case the resulting sinter cake produced by the reducing heat treatment is broken up into lumps. The powder is formed into pellets by mixing it with a lubricant and binder so that the resultant mixed powder may be readily made up into small porous pellets, for instance, of cylindrical shape, having a diameter of about to inch, and the same height, with a pellet density of 3 to 4 grams per cubic centimeter. Any suitable lubricant and binder which decomposes and/or volatilizes at elevated temperatures of about 800 to 900 and above, may be used as a lubricant and binder in making such pellets. For instance, high fatty acids such as stearic acid, and salts of stearic acid such as zinc stearate and the like are suitable for use as a lubricant. Carbohydrates such as dextrose dissolved in water, or camphor dissolved 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 causes the different metal constituents of the different powder particles, 1. e. of nickel and iron powder to mutually diffuse 12 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 about 2.3 g./cc., with a weight loss of about .5%. The reduced sintered powder pellets were then packed in baskets within a chromous chloride producing mass, and subjected to a chromium-alloying diffusion treatment in a manner similar to that described in connection with the fragmented sinter cakes of sponge iron powder above, except that the contents of each basket was subjected to a continuous chromium-alloying treatment of an overall duration of about 52 hours as follows: The same initial treatment including the four hours during which hydrogen onl was passed through the retort space followed by eight treatment sequences of six hours each, and each sequence being identical with the treatment sequences applied to the sponge iron powder as described above. The foregoing chromiumalloying treatment was carried on at a temperature of 900 C., and the baskets were then cooled in an atmosphere of flowing hydrogen.

The so-chromium-alloyed sinter cake fragments were then crushed into powder which had the following physical properties: Apparent density 1.90 g./cc., flow e35 second per 50 grams, hydrogen loss 012%, iron content 74.85%, nickel 8.6%, chromium 16.5%, titanium 2%, carbon .05%. The so-chromium-alloyed powders when compacted into green test bars under a pressure of 25 t. s. i. had a calculated modulus of rupture of 960 p. s. i.; when compacted under a pressure of 35 t. s. i., a modulus of rupture of 1800 p. s. i.; and when compacted under a pressure of 4.5 t. s. i., a modulus of rupture of 2580 p. s. i.

The iron nickel alloyed powder particles which have been subjected to the chromium-alloying treatment in the manner described above, when compacted into shaped bodies and then sintered and/or infiltrated, exhibited superior physical characteristics in a manner analogous to the sintered and/or infiltrated compacted bodies from sponge iron powder described above.

Bodies similar in properties to those exhibited by known alloys containing one or more of the metals of the group including iron, nickel, cobalt, with or without modifying additions, such as molybdenum, tungsten, in combination with chromium, may be produced in a manner analogous to that described in connection with the mixed iron and nickel powders. To this end, powders of the different selected metals other than chromium, are mixed in proper proportion and the mixed powders are subjected successively to the reducing and chromium-alloying treatment in a manner similar to the treatments described above as applied to the mixture of powders of the oxides and nickel.

By prolonging the chromium-alloying treatment, or by subjecting the chromium-alloyed sinter cake pieces, such as pellets or cake frag ments, to a suitably long additional heat or sintering treatment, the chromium content of the outer layers of the powder particles of such bodies may be caused to diiluse to any desired degree toward the interior of the individual powder particles, and thus cause such powder particles to be substantially uniformly alloyed with chromium. Furthermore, such prolonged 13 diffusion treatment causes the difierentmetals of the individual powder particles, .aswell as their chromium content, to mutuall diffuse, thereby giving the resulting body the characteristics of the desired metal alloy composition.

The invention thus makes it possible topro duce in an economical way from readily moldable powders of the difierent metals, such as iron, nickel, cobalt, molybdenum, tungsten, and the like, bodies of desired complicated shape having properties heretofore obtainable only by making such shaped bodies out of ingots produced by alloying the different metals in a molten state. In contrast therewith, in order to make similar shaped bodies by prior-art practice, the alloy metal ingot combining'thediiferent metals had to be shaped'by machining operations, or in the alternative, the'alloy ingot had to be first pulverized and the hard powder particles of such alloy ingot used for making the desired shaped body by a more difficult powder metallurgy process.

There will now be described by way of example, one satisfactory procedure for producing a chromium-alloy powder body containing in desired proportions iron, nickel, tungsten, molybdenum, from the powder oxides of these diiferent metals by subjecting the mixtures of the different oxide powders to a combined reducing and sintering treatment which also causes alloying of their different metallic constituents, followed by a chromium-alloying treatment in a manner analogous to the treatment described above as applied to powder mixtures of iron and nickel oxide powders.

Powders of nickel oxide, iron oxide (mill scale), molybdenum oxide (M003) are mixed in proper proportions so that after the reducing and chromium-alloying treatments, the resulting chromium-alloy powder will contain the different metals in proportions corresponding to the desired end product. The powders of the different metal oxides are ball milled to bring them down to'a proper size, such as 100 mesh, and then subjected to a reducing and sintering treatment in a manner similar to that described in connection with the reducing and sintering treatment applied to the mixture of mill scale powder with nickel oxide powder. The ballmilled mixture of the powders of iron oxide, nickel oxide, molybdenum oxide, and lamp black is thereupon first subjected to a reducing treatment at about 600 C. for one hour in a protective atmosphere of dry hydrogen for partially reducing the iron oxide, nickel oxide, and for reducing M003 to M002. The treatment temperature is then raised to about 1000-1150 .C., and the treatment continued for another hour to reduce all remaining oxides to a metallic state and at least partially alloy them. Where tungsten content is desired, the proper proportion of tungsten oxide powder is added to the original oxide powder mix, and simila'r'heat treatments are applied; however, both these treat= ment temperatures are raised by about 50 to 70 C.

The broken sinter cake lumps or pellets of the reduced metal powder alloy are then subjected to a chromium-alloying treatment carried on in a manner similar to that described above in connection with the chromium-alloying treatments applied to the alloy powders of iron and nickel powder, resulting in alloy powder particles containing the proper proportions of the metals iron, nickel, molybdenum, and/ or tungsten, combined with'the' proper-proportions of chromium :so that the-resulting" powder contains the different'metals'in'proportions "corresponding to the desired alloy. Such chromium alloyed powder'particles may be readily 'compacte'dor coinedv into bodies of desired shape, whereupon the so-compacted bodies-may beisubjected to a sintering operation with orwithout an infiltrating treatment, whereby the shaped body is rgiven the "desired great strength. The sintering'operation may be carried on fora sufiicient length oftimeitocause the 'diiferent metal'constituents of the diiferent powder particles to-alloy with eachother, and with chromium to any desired extent. It is thus possible to produce in a highlyseconomical way strongbodies exhibiting 'hot strength and corrosion-resistance out of oxide particles or the di-fEerentmetals'wh-ich heretoforehad to healloyed in molten state at high temperatures-in order to obtain bodies of comparable. strength.

It should benotedthat-when producing chromium-suriace-alloyed soft metal powders .of the invention from oxides of the desired metals, such .as oxides of iron, nickel, cobalt, molybdenum, and

tungsten, and mixtures thereof, the oxide pow- 'ders may be pelletediintopellets for the preliminary reducing and sintering treatment. When such pelletsare subjected to the combined 'reducing and sintering treatment of the type :described above, the admixed lubricant and. binder is decomposed, and driven'off. Asaresult, the reduced :pellets have the required low density and small strength, comparable tothe strength of the sinter cake fragments produced by sintering sponge iron ipowder deposited in a layer of about 4 of an inch within the treatment boats in the preliminary treatment of sponge iron powder described hereinbefore. As longas the sinter cake pellets resultingfromthe 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 chromium-alloyed powders having the desired high degree of softness and plastic deformabi'lity 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. 1.

'As'used in the specification and claims, the expression sinter cake body include both sinter cake lumps and sinter cake pellets of sufliciently low density and strength that upon comminution of such chromium-alloyed sinter cake body into minute chromium-alloyed powder particles having the desired high degree of softness andplastic deformability which make'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.

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. 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'o'f iron containing 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 a chromium-alloying treatment in which chromium from a chromium 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% chromium, and thereafter comminuting said cake pieces into minute plastically readily deformable metal powder particles having the deposited chromium alloyed therewith so that when said chromium-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 about 200 pounds per square inch.

2. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake piece of sintered metal 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 at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of said metals: so that said sinter cake pieces have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a chromium-alloying treatment in which chromium from a chromium 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% chromium, and thereafter comminuting said cake pieces into minute plastically readily deformable metal powder particles having the deposited chromium alloyed therewith so that when said chromiumalloyed 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.

3. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake ieces composed essentially of sintered metal powder particles of 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 a chromium-alloying treatment in which chromium from a gaseous chromium compound is deposited on the powder particles of said cake pieces and cause to difiuse into said powder particles so that said powder particles contain at least about 3 chromium, and thereafter comminuting said cake pieces into minute plastically readily deformable metal powder particles having the deposited chromium alloyed therewith so that when said chromium-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 about 200 pounds per square inch.

l. 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 said metals so that said sinter cake pieces have at most a densityof about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a chromium-alloying treatment in which chromium from a gaseous chromium compound is deposited on the powder particles of said cake pieces and cause to diffuse into said powder particles so that the said powder particles contain at least about 3% chromium, and. thereafter comminuting said cake pieces into minute plastically deformable metal powder particles having the deposited chromium alloy therewith so that when said chromium-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 about 200 pounds per square inch.

5. The method of providing a readily compactible and moldable metal powder comprising 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, producing readily frangible porous metallic sinter cake pieces by heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. so that said cakes have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a chromium-alloying treatment in which chromium from a chromium 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 chromium, and thereafter comminuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited chromium alloyed therewith so that when said chromium-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.

6. 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 ofmolybdenum and of tungsten, and of mixtures and alloys of said metals, at least some of said powder particles containing the metal in the form of an oxide, producing readily frangible porous metallic sinter cake pieces by heating the powder mass in a reducing atmosphere at a temperature of at least about 700% C. so that said cakes have at most a density of about 4 grams per cubic centimeter, and there after subjecting said cake pieces to a chromiumalloying treatment in which chromium from a chromium 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% chromium, and thereafter comminuting said cake pieces into minut plastically readily deformable powder particles of said metals having the deposited chromium alloyed therewith so that when said chromium-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.

7. 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 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, producing readily fragible porous metallic sinter cake pieces by heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. so that said cakes have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a chromium-alloying treatment in which chromium from a gaseous chromium compound is deposited on the powder particles of said cake pieces and caused to difiuse into said powder particles so that the said powder particles contain at least about 3% of chromium, and thereafter comminuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited chromium alloyed therewith so that when said chromium-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.

8. 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 of 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, producing readily frangible porous metallic sinter cake pieces by heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. so that said cakes have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a chromium-alloying treatment in which chromium from a gaseous chromium 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 chromium, and thereafter com minuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited chromium alloyed therewith so that when said chromium-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.

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

References Cited in the file of this patent FOREIGN PATENTS Country Date Great Britain Feb. 12, 1948 OTHER REFERENCES Number

Claims

1. THE METHOD OF PROVIDING READILY COMPACTIBLE AND MOLDABLE METAL POWDER COMPRISING FIRST PRODUCING READILY FRANGIBLE POROUS SINTER CAKE PIECES COMPOSED ESSENTIALLY OF SINTERED METAL POWDER PARTICLES ESSENTIALLY OF SINTERED METAL 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 SO THAT SAID CAKE PIECES HAVE AT MOST A DENSITY OF ABOUT 4 GRAMS PER CUBIC CENTIMETER, AND THEREAFTER SUBJECTING SAID CAKE PIECES TO A CHRONIUM-ALLOYING TREATMENT IN WHICH CHROMIUM FROM A CHROMIUM 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 BAOUT 3% CHROMIUM, AND THEREAFTER COMMINUTING SAID CAKE PIECES INTO MINUTE PLASTICALLY READILY DEFORMABLE METAL POWDER PARTICLES HAVING THE DEPOSITED CHROMIUM-ALLOYED THEREWITH SO THAT WHEN SAID CHROMIUM-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 ABOUT 200 POUNDS PER SQUARE INCH.