US2656595A - Chromium-alloyed corrosion-resist - Google Patents

Description

Oct. 27, 1953 G. STERN ET AL -AL.LOYED CORROSION-RESISTANT CHROMIUM METAL POWDERS AND RELATED PRODUCTS Filed March 31, 1950 0 W 5 M L v1 CL mi N E J T T m A Y B Patented ct. 27, 1953 HEB CHROMIUM-ALLOYED CORROSION-RESIST- ANT METAL POWDERS AND RELATED PRODUCTS George Stern, Mamaroneck, Seymour J. Sindeband, Chappaqua,

and Joseph P. Scanlan,

Yonkers, N. Y., assignors to American Electro Metal Corporation, Yonkers, N. Y., a corporation of Maryland Application March 31, 1950, Serial No. 153,286

4 Claims.

invention being of a broader scope.

Among the objects of the invention are powders of metal particles which are in a readily deformable plastic condition, and may be compacted into relatively strong metal bodies with relatively low pressures, at least the surface layer or" the metal powder particles having alloyed therewith chromium deposited thereon from a gaseous or liquid chromium compound at an elevated temperature.

A particular object of the invention is such readily compactible and moldable 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, which particles 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 gaseous 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 from the following description of exemplifications thereof, reference being had to the accompanying drawings, wherel Fig. l. is a greatly enlarged photograph of a 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, have been made of stainless steel powders by powder metallurgy techniques. The stainless steel powders generally used for such bodies have been obtained from previously prepared stainless steel ingots, for instance, by subjecting a stainless steel ingot to grain boundary corrosion followed by disintegration, or by melting a stainless steel ingot and atomizing it in the molten state. The so-produced particles of such stainless steel powders are of high density and hardness, and require high-molding pressure of at least 40 to 45 t. s. i. (tons per square inch), which is much too high for economical production of molded products, and results in rapid wear and frequent break-down of the molding dies. As a result, the production of molded products from stainless steel powders has found only limited use.

' alloys of iron, cobalt and chromium, of iron,

nickel, molybdenum, tungsten 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 foregoing difficulties which stood encountered in the production of corrosion-resistant ferrous metal bodies by powder metallurgy techniques are overcome by the provision of a readily compactible and moldable metal powder of ductile and relatively soft ferrous powder particles, at least the surface layer of the powder particles having combined therewith chromium, which has been deposited thereon from a gaseous or liquid compound at an elevated temperature, the particles being sufficientgreater strength than molded bodies made of metal powders of the invention.

prior stainless steel powder.

Various commercially available soft, plastically readily deformable iron powders are suitable for producing corrosion-resistant soft moldable Among such available and suitable soft iron powders is sponge iron powder having a carbon content of .1 to 2% (unless otherwise specifically stated, all proportions are given herein by weight) electrolytically produced iron powder, carbonyl iron powder, eddy mill iron powder, and atomized molten iron powder of similar carbon 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 difficult to compact fine iron powders containing up to about 2% carbon will, in general, when subjected to the chromium-alloying process of the invention herein described, become softened and ac- 3 quire characteristics which render them readily moldable.

In accordance with a phase of the invention based on the original concept of producing chromium-alloyed soft iron powder-relatively soit 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 diiiused in such powder particles of the difierent metals so as to yield 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 diffusing chromium into metallic powder particles in accordance with the principles of the invention.

Among such known chromium-alloying processes, one that was found particularly suitable for practicing the invention, involves an exchange or displacement reaction of the metal of the treated body with the chromium constituent of a gaseous or vapor phase of a chromium halide, such as chromous chloride gas, at an elevated temperature in the range between about 900 to 1200 C. in accordance with the equation:

wherein M represents the metal of the chromiumalloyed body.

In such process, the chromium of the chromous chloride gas replaces the metal on the surfaces of the treated body causing the chromium deposited on the surface of the body to allow 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 efiectively 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 dil-Iused 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 ferro-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 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, ferro-chromium powder, and a chromium-chloride compound, and enclosed within a sealed envelope which is heated to produce the desired chr-omium-surface-alloying action.

As a still further alternative procedure, the sinter cake lumps may be 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 example, a suitable bath composition for the latter treatment may consist of 30% CrClz, 50% BaCaz, and 20% NaCl. By keeping the bath heated to a temperature in the range from about 900 to 1500 C., from about 3 to 10 hours, the desired chromium-surface-alloying action will be produced. The CrClz content of such bath may be replaced by elements which generate CrClz, in which case an equivalent amount of CrCh and chromium metal is included in the bath. In all the foregoing chromium-surfacealloying treatments, the chromium-alloying reaction is represented by the Equation 1 given above.

The chromium-alloying process does not materially increase the density and strength of the sinter cake lumps. As a result, the chromiumalloyed sinter cake lumps may be readily broken up 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 compactingpressure of only 50,000 p. s. i. into strong green compacts having a modulus of rupture of 400 p. s. i., and higher.

In accordance with another phase of the invention, powder particles of the oxides of either iron, nickel and cobalt, or mixtures of such oxides, or of metals and oxides of this metal group, with or without an additional mixture of powders of molybdenum or tungsten or both-either 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 such 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 surfacechromium-alloyed in a manner similar to the chromium-alloying of the sinter cakes of soft iron powder. The resulting 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 surface-chromium-alloyed powders produced in accordance with the invention from a mixture of oxides of the different metals will yield a surface-alloyed powder body, the individual particles of which are alloys of the different metals corresponding to the different metal oxides and the proportion of chromium deposited on the sinter cake pieces.

Without thereby limiting the scope of the invention, in order to enable those skilled in the art to readily practice the invention, there will now be described one example of a satisfactory procedure for producing a corrosion-resistant soft iron powder of the invention from commercially available sponge iron powder having a carbon content of about .15%. A typical commerical powder of this type used in the process contained a mixture of minute powder particles having the following particle-size distribution typical of moldable metal powders: 21% 100, +150 mesh; 30% 150, +200 mesh; 13% 200, +250 mesh; 14% 250, +325 mesh, and the balance +325 mesh powder.

The powder is treated to sinter it into a porous cake mass of low density, and thereby facilitate surface-alloying of the powder particles with chromium. This may be done by depositing a layer of powder of about to A inch thick into sinter boats of suitable metal such as a heat-resistant iron-chromium sheet metal without in any way compacting the powder. The interior surface of each sinter boat is coated with a stick-suppressing or release medium such as a water suspension of magnesium hydroxide to permit ready separation of the sintered powder body from the boat after the sintering op eration. The powder within the boats is then treated within a furnace in a reducing atmosphere of dry hydrogen or purified cracked ammonia at a temperature of about 800 to 1050 C. for one-half to three hours, and then permit ted to cool to room temperature. In general, such treatment in the range of about 900 to 1000 C., such as 950 C., for one hour is sufficient for sintering the powder into a sinter cake having a density of /7 to 4 g./cc. which is readily frangible into lumps or pieces suitable for packing into a chromium-alloying mass. A good way for carrying on the foregoing sintering treatment is to push the powder containing boats through a tunnel furnace from one end to the other, the heat treatment being followed by cooling which is carried out as a part of a continuous treatment as the boats are moved from one end to the other of the furnace.

After completion of the sintering treatment, the sinter cakes are removed from the boat, and broken into fragments of a size suitable for packing in a chromium-alloying pack. The sinter cake pieces or lumps are thereupon packed within a chromous chloride producing pack mass, and placed within baskets of suitable heatresistant metal such as a chromium-nickel-iron alloy. The pack may consist, for instance, of 50% by volume ceramic lumps, such as porcelain pieces, and the balance of chromium-alloy such as ferro-chromium alloy. An alternative chromium-alloying pack which was found highly effective consists of a mixture of titanium oxide with ferro-chromium. Good results are obtained with such pack consisting of about titanium oxide, by volume, the balance consist-- ing of ferrochromium, containing about 70% chromium.

The treating baskets containing the sinter cake lumps packed, for instance, with titanium oxide and ferro-chromium, are placed in a retort and heated to a temperature, in the range between 900 and 1250 C., and a stream of hydrogen and hydrogen halide gas, such as hydrogen chloride gas, is passed through the retort for producing reactions causing chromium atoms to be deposited on the powder particles, and to diffuse into the interior of the particles, and alloy therewith.

In such surface-alloying treatment, the hydrogen chloride gas passing through the retort interacts with the ferro-chromium to form chromous chloride gas. The chromium of chromous chloride replaces the metal on the surfaces of the treated powder particles primarily by the exchange reaction. Part of the chromous chloride gas is absorbed by the packing material, such as the ceramic material or the titanium oxide in the pack.

In the particular example herein described, the pack-treatment baskets were of circular shape, and had an outside diameter of 22", and a height of 10". Four such packed baskets were stacked within a closely fitting retort about high. The packing material consisted of TiOz, 20% by volume, the balance ferro-chromium containing chromium. During the initial part of the chromiumsurface-alloying treatment, purified dry hydrogen was caused to flow through the enclosed retort space at a rate of 40 cubic feet per hour, while the temperature was raised to about 950 C. The flow of pure hydrogen was thereafter continued at a rate of 20 cubic feet per hour, for four hours at the same temperature of about 950 C. Thereafter the baskets within the retort were subjected to a succession of five treatment sequences, at about 950 0., each treatment sequence lasting 'six hours, and consisting of: (a) passing through the retort at a rate of 20 cubic feet per hour a mixture of 20 parts of hydrogen and 3 parts of H01 gas for one hour, followed by (b) passing pure hydrogen at the same rate for one hour,

followed by (c) passing the same mixture of hydrogen and HCl gas as in (a) for one hour, followed by (b) passing pure hydrogen at the same rate for three hours. After a succession of six such treatment sequences of six hours duration each, the treatment was ended by turning off the heat and permitting the retort with its contents to cool while continuing the flow of hydrogen through the retort space until the temperature of its contents was brought down to about room temperature.

Upon completing the foregoing chromiumsurface-alloying treatment, the treated metal powder pieces are removed from the baskets, and comminuted by crushing to powder, for instance, in a disc crusher.

Without in any way limiting the scope of the invention, and in order to enable those skilled in the art to readily practice the invention,

there are given below the results obtained by subjecting packed iron powder particles to the specific 30-hour chromium-alloying treatment described above.

The resulting chromium-alloyed powder conmesh; 35.5% -100, mesh; 28% 150, +200 mesh; 10% 200, +250 mesh; 9% 250, +325 mesh; and 9% +325 mesh powder. The apparent density of the powder was 2.56 g./cc., and the weight loss due to the 1 hydrogen treatment (hydrogen loss) was .053

carbon content 045%, titanium content .2%, and chromium content 15%, balance iron. The chromium-alloyed powder was then compacted into test bars and tested for "green strengt Figure 1 is a photograph of a sample of such i bon content.

7 powder particles of the invention greatly enlarged: on a scale to 1.

"The table below gives the modulus of rupture (also called transverse rupture strength") cal-v culated from breaking tests, of chromium-alloyed I powders, compacted into test bars underpr'essures of '25, and is t s. i, designated in the tablea's C. A. P; The table also shows the comp'arative modulus of ruptureof compacted green cars 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 1B%-8% and 13% 10%, respectively.

-Modulus of Tuptureoj three green test bars pressed at25, 35 arid t. s; z.

The foregoing, test results show the characteristlo. distinction and the marked superiority in green strength of bodies produced from surface diffusion chromium-alloyed iron powder of the invention as compared with green bodies produced from available stainless steel powdersi processing operations, the. powder should not be subjected to any material work The chromium-alloyed powder particles of the invention retain their spongy character, have a tentacle-like. shape, and they have a lowered car- 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 deform'able 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 technique.

The results of rupture tests 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 A" 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 differ 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 -alt least 2 to 4 times greater than similar green powders of similar composition and corrosion-resistance.

A distinguishing characteristic of the corrosion resistant chromium 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 2st. 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 pres-' sure. In particular, such green test bar cornpacts'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 00 pounds per-square inch,

and higher.

In producing surface-alloyed soft, plastically readily deformable ferrous powders of the invention, it is important thatat all stagesof the particles hardening forces. Thus, for instance, it is essential that in initially sintering the soft iron powder particles into porous sinter cakefragments suitable for packing into the chromium-alloying pack, the powder which is to be subjected 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; exarm 'ple organic lubricant and/or binder material.

If a substantial compacting pressure, even as'low as E- 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 chromiuinalloyed sinter cake fragments will acquire a rela 'tively great density and strength, and large forces will be required for crushing. them, and the re sulti'ng 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 pressure-- the crushing energy required for pulverizing 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 surfacechroniiur'n 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 lumps-produced 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 treat meht is carried on.

As explained above, the proper range of the density of the sinter cake lumps produced in 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 with much higher pressure.

9 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 4.0 g./ cc.

Furthermore, as long as the sinter cake lumps, produced in preparation for the chromiumalloying treatment-are of low density and have a low modulus of rupture in the range set forth abovethey 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 chromium-alloyed powders of the invention, it is sufficient to control the density and/or modulus of rupture of the sinter cakes or sinter cake lumps produced in preparation for the chromium-alloying treatment. As long as the density of such sinter cake lumps is not more than about 4.0 g./cc., and their modulus of rupture is not more than about 6500 p. s. i. they will, after the chromium-alloying treatment, remain of sumciently 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 as to make it possible to compact such powders into green compacts having a modulus of rupture of 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 stainless steel powders of similar composition which have been compacted Good results are obtained by sintering green compacted bodies of such chromium-alloyed powder at a tempera ture in the range of 1200 to 1400 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 low dew point atmosphere may be obtained by a suitable getter such as pure chromium powder or ferro-chrome powder (70% Cr) mixed with aluminum oxide A1203.

By way of example, a green test bar of the specific chromium-alloyed iron powder of the invention, having properties set forth above, was compacted under pressure of t. s. i. and then sintered at 1300 C. for one hour within a protective atmosphere in the manner described above. The so-sintered bar exhibited a shrinkage of 8.23%, and had a density of 6.68 g./cc. It had Rockwell hardness F 37.5, yield point 21,600 p. s. i., ultimate tensile strength 26,450 p. s. i., elongation 8.5%, reduction in area 4.06%.

The test bar, sintered in the above manner, repressed or coined at a pressure of 35 t. s. i., and resintered for another hour under the same sintering conditions, exhibited a shrinkage of 1.73%. The resintered bar had Rockwell hardness F 25.5, yield point 26,900 p. s. i., ultimate tensile strength 45,000 p. s. i., elongation 20.5%, reduction in area 19.0%. Each such sintered body exhibited excellent resistance to corrosion under prolonged exposure to salt-spray as well as to hot and cold HNOs.

Green compacted bars made from soft chromium-alloyed iron powder of the invention may be utilized as a very efiective electric contact material, as bearing material or the like, by infiltrating the green compacted bar with silver, copper and alloys thereof. By way of example, a green bar produced by compacting the specific chromium-alloyed iron powder of the invention having the specific properties set forth above, and compacted under a pressure of 25 t. s. i. and having a density of 5.47 g./cc., was infiltrated with pure fine silver at 1220 C. for one hour in a protective atmosphere of dry hydrogen. Such silver-infiltrated bar exhibited an infiltration shrinkage of 4.42%, a density of 7.59 g./cc., a resistance of 17.9 microhms-cm. It had Rockwell hardness F 63, yield point 22,800 p. s. i., ultimate tensile strength 37,000 p. s. i., elongation 9.3% and reduction in area 10.5%.

A similar body compacted under a pressure of 35 t. s. i., and infiltrated with silver in the same manner, had a Rockwell hardness F 75.5, infiltration shrinkage 7.9%, yield point 27,000 p. s. i., ultimate tensile strength 39,600 p. s. i., elongation 10.5%, reduction in area 9.9%. It is thus seen that such silver-infiltrated green compacted powder bodies of the invention exhibit excellent physical properties, and the infiltration may be efiected at relatively low temperature in generally available furnaces and atmospheres. Such silver-infiltrated bodies have also the desired dimensional stability and exhibit excellent 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 for applications requiring corrosion-resistance at elevated oxidizing 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 elevated temperature, such as 1100-1250 C. with an infiltrant of copper, or a suitable copper alloy.

By way of example, a green bar of the specific chromium-alloy iron powder of the invention set forth above, compacted under pressure of 35 t. s. i., and having a green density of 5.98 g./cc., was sintered for one hour at a temperature of 1300 C. in a protective atmosphere in a manner specifically described in the previous example. The resulting sintered body having a density of 6.68 g./cc. was infiltrated with a copper base alloy containing about copper, 2% iron, 8% manganese, /2% titanium. The infiltration was carried on at a temperature of about 1200 C. for one and one-half hours in an atmosphere of purified cracked ammonia, with a pure chromium getter present to keep the dew point of the atmosphere at the desired low temperature. The resulting infiltrated body had a density of 7.9 g./cc., yield point 87,700 p. s. i., ultimate tensile strength 105,250 p. s. i., elongation 6.3%, reduction in area 8.3%. The so-obtained body thus exhibited excellent dimensional stability, and excellent resistance to corrosion under prolonged exposure to salt-spray and relatively hot oxidizing gases.

In other words, sintered compacted powder bodies of the invention, after infiltration with the copper alloy, have high strength, and dimens1ona1 stability, and they also have excellent resistance to corrosion.

According to a further phase of the invention, surface-difiusion chromium-alloyed soft, readily 11 plastically. deformable ir n, QWdQrs are prgduged me; very economical Way. from iron c ides such as black mill scale containing, principally iron oxides (Fe Oi). and (F69) Whichis fo l d when rolling and forging iron and: steel,

The. re: oxide mill scale. is treatedby subjecting it irra pulverized. state to a reduction, process ancl then sintering, the resultingiron p0wderparticles into porous pieces, such as. lumps suitable for sub; je'cting them to the subsequent. chromium alloyr ing'processes in the manner described. above if it" is desired. to combine, with. such iron powder additional elements such. asnickel, and/0r cobalt, the proper proportionsof. powdersof the. oxides of such additional, metal a'remixed with the iron oxide powder before subjecting thepOWder mixtime to the initial. reduction and sintering treatment.

By way of example, there will nowbe. described asatisfa'c'to'ry process for producingachromium alloyed powder containing softv moldable. iron and nickel alloy powder-partially chromium al killed by surface-diiiusion or. chromium in accordance 'with the process' of the. invention. Powder particles of. irony oxide mill. scale. contaming essentially EezOz 'andFeOare mixed with p'owd'ere'dnickel'oxide and-lamp black, and ball milled into a powder or -1o0 mesh. As an example, 200 'parts ofthe milLscale'are mixed with 22 .6" parts nickel oxide and one part of. lamp black. The powder mixture is then ball. milled t'o'powder of- -100'me'sh, and plac'edin treatment boats of heat-resistant metal coated. on the. in.- tenet with" a stick suppressing medium, and treated me 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. Sat.- isfactor'y results are obtained. by such treatment carried on 'at'a' temperature of about 9.50? C. for one'hour. Thepowder mix may be pelleted. into pellets before subjecting it to the foregoing treat 'ment, or it' may be placed. into the treatment boatas a thin powder layer about. A; to. /4..de.p, in which'ca's'e the 'resulting sinter cake produced by the reducing heat treatment 'is broken upinto lumps; The "powder is formed into pellets by mixing it'with a lubricant and. binder so that the resultantmixed.- powder mass may be readily made up. into small porous pellets, foninstan'ce, of' cylindrical. shape, having a diameter ofabout /4 to /27, and. the same height, 'with. a pellet density of. 3Jt'o 4. grams per cubic centimeter. Any"'suitable lubricant: and binder which. decomposes and/or volatiliz'es at elevatedtemperatures ofJabout 8.00? to. 900?. C. and above, maybe used .as alubricantandbinder inmakin such pellets. For instance. h h fat y-a d s ch. as st ar acid, and salts of stearic, acidsuch aszinc stearate and..tlie like are. suitableforwusaas a lubri- Qan i, carbohxdratessuch as dextrosedissolved in water, or camphordissolved in alcoholmaybe used as. abincleixin. making such pellets.

The. reducing. and. sintering ftreatment. carried out atv an. elevated. temp r ture. inthe. mann r described above causes the different metal constituents of thedifierent. powderparticles, i. e. oi nickel. and iron. powder, to. mutually diffuse with. eachother, andthe individual powder particles. become. actually a yed.

In the particular example referred toherein, the, reducin and sintering. treatment... was performed. at. a? temperature of, about.;10503.,0. for one hour, resulting in sinter cake pellets having v mwn allqxsqe a, density about, 2.3, g/EEJ'I, with a Weight loss of about. The reduced sintered powder pellets were then packed in'baskets within. a, chromou's' chloride produingfi'nass, and subjected to a chiiomium alloyiiigfdiffusion treatment in a man; similar to that. described in. connection with the fraginented' slinter, cakes of sponge iron powden above, except, that theTconterits of each basket was subjected ltoa continuous chromiurni alloying treatmenthfl'an overal duration. oi about 52 hours; as renews The same initial treatment inludingfthe four hours during hydrogenonly was, passed. through the retort space, followed by eight treatment Seqlienc'esv of p awl-s e cn r i each sequence being identical with, the treatment, sequence, applied.v to the sponge iron powder. as described, above. The foregoing chromium-alloying.treatment was carried on at a temperature of 900 0., andfth'e baskets were then cooled in an atmosphere, of owin h ro e The so -ohr.orniu1n,- al-loyed sinter cake fragrnents. were then 0 she'd into powder which had the following physical properties: Apparent density 1 .9Q g./ cc. flow 43,6;seconds'per 59 grains, hydrogen loss .ql2;%, ir oncontent 74.85%, nickel 8.6%, chromium 1,65%, titanium 2%, carbon Q.5 The so-gl romium alloye'd, powders I when compacted into green test bars underia; ressure 0 325.15, i., had: a calculated, modulus brine,- ture of 960 p. s. i.; when compacted underfa pressure of- 35; t s, i a modulus of. r pture of 50 .1? s. and. when. comp e Lieder a sureof 45 t. s. i., arnodulus of rupture? of p. s. 1.

The iron niclrel alloyed pqwder particles which have been subjected to the chromiuin allo'yin'g r atment ntha ma eer sc b abdv; 'w q mpaq s niq. aped; bQd a ha i d ans/Q 1. afili aiea. exhi e subs ab characteristics in a mannerfa nalogou's to the tered and/or infiltrated compacted bodies fro ense rminqwder dsse ibe Bodies similar in. .pr nerties .tjof those exhibited h p ng onei o 'r morefloithe tale of the -g ro1 1 p noliidirig,iron, 'nicliel,' coelt with. it i tl cu iiy n f ia Y suh a mplrb enum. wa a i vmbifiaoh w ch mi m may e r sdfi =am i ii a ogous to that described in qonnctio'n with tlie .mi Fid liq r l: ni l ow rs T T d,

settle s. Q? t e. @ifi snt" "eta m ls he than. hrqmwm; ar mixe nob fi b d abs? hamixedz ewqe sare-su i c 1 h v e uc n and. hr m m-e iti t a nenis. in ear er. m r; the r fil described above. as. applied to the. niixtur' t war'dfthj interior "dft e t h 4 U 1 ls to. be substantially .un'iforrn ly alloyed, w th} chrom eria e; inte marria eanneal 13 duce in an economical way from readily 'moldable powders of the different 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 diiferent metals in a molten state. In contrast therewith, in order to make similar shaped bodies by prior-art practice, the alloy metal ingot combining the different 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 difiicult 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 different metals by subjecting the mixtures of the diiierent oxide powders to a combined reducing and sintering treatment which also causes alloying of their dilferent 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 l 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 appli d 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 and 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 similar heat treatments are applied; however, both these treatment 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 par-- ticles 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 difierent metals in proportions corresponding to the desired alloy. Such chromiumalloyed powder particles may be readily compacted or coined into bodies of desired shape, whereupon the so-compacted bodies may be subjected to a sintering operation with or without an infiltrating treatment, whereby the shaped body is given the desired great strength. The sintering operation may be carried on for a suflicient length of time to cause the different metal constituents of the different powder particles to alloy with each other, and with chromium to any desired extent. It is thus possible to produce in a highly economical way strong bodies exhibiting great hot strength and corrosion-resistance out of oxide particles of the different metals which heretofore had to be alloyed in molten state at high temperatures in order to obtain bodies of comparable strength.

It should be noted that when producing chromium-surface-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 powders 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 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 A, of an inch 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 5.0 g./co., 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 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 body includes both sinter cake lumps and sinter cake pellets of sufiiciently 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 and plastic deformability which make it possible to com- .pact 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.

Ihe principles of the invention described above in connection with specific exemplificationsv 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 powder particles which is readily comminutable into soft 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 a chromium-alloying treatment causing at least the surface layer of said particles to become alloyed with chromium deposited on the particles from a chromium compound at elevated temperatures, said cake body containing at least 3% chromium and having at goodies i5 most a densityof about 5 rams per cubic eenti meter.

2- Asintered cake. body of met n wd r art? cleswhich is readily comminutable into oft metal powder particles, Said, cake body bein com-p s d essentially of at least one metal of: the roup. consisting of iron containing at. most about .2 carbon, of nickel and of cobalt, to ether with. at least. one metal of the group consisting ofmo1bdenum and tungsten, and of mixtures: and a1- loys of said metals; said cake. body having been subjected to a chromiumealloying treatment causing at, least the surface. layer of. said particles to become alloyed with chromium deposited on the particles from. a. chromium compound. atelevated temperatures said cake. body containing at least 3%. chromium and having: at most a density of about 5 grams per cubic centimeter.

6.. A sintered cake. body of. metal powder particles which. is readily comminutable into soft metal powder particles, the particles; of said amtered body being composed essenti'allyof 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 ofsaid metals, said cake body having been subjected to a chromium alloyinet treatment causing, at least the suriace layer of said particles to become alloyed with. chromium deposited on the particles from. a

chromium comnound at elevated temperatures so thatsaidhody contains at least 3 chromium. said cake. body having at most a. density oi about 5 grams per cubic centimeter, and a modulus. of rupture which is at most about 650Q pounds per square. inch, whereby said body may be. pulverized into minute chromium-alloyed powder particles which exhibit minimized work hardness and stiflioiently great softness. so that when the. chromium-alloyed particles are compacted into a green body underpressure of about 50,QQO pounds per square inch, said green body exhibits a modu:

16 111s of rupture of at. l ast 20.0. pounds. nor squar inch.

4. A sintered cake bodyof metal powder particles whichv is readily oomininutable 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, andof c.0- balt, together with atleast one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of said metals; said cake body havingbeen subjected to a. chromiumra11oyi-ng treatment causing at least the surface. layer of said particles to become alloyed with chromium deposited on the particles from a, gaseous chromium compound at elevated temperatures, said cake body having at; most a density of about grams per cubic centimeter, and a modulus, of rupture which is at. most about 6500. pounds per square, inch, whereby said body may be, pulverized into minute chromium-alloyed powder particles which exhibitminimized work hardness. and suiii- ,ciently' great softness so that when the chro- References Cited in the, file. of this patent FOREIGN PATENTS Country Date Great Britain Feb. 12, 1948 OTHER REFERENCES Treatise on Powder Metallurgy, vol. 1, pages 71 and 74. Edited by Goetzel. Published in 19.49. by Intel-science Publishers, Inc., New York.

Number

Claims (1)

1. A SINTERED CAKE BODY OF POWDER PARTICLES WHICH IS READILY COMMINUTABLE INTO SOFT 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 A CHROMIUM-ALLOYING TREATMENT CAUSING AT LEAST THE SURFACE LAYER OF SAID PARTICLES TO BECOME ALLOYED WITH CHROMIUM DEPOSITED ON THE PARTICLES FROM A CHROMIUM COMPOUND AT ELEVATED TEMPERATURES, SAID CAKE BODY CONTAINING AT LEAST 3% CHROMIUM AND HAVING AT MOST A DENSITY OF ABOUT 5 GRAMS PER CUBIC CENTIMETER.

Images