US2352316A - Method of producing shaped bodies from powdery ferrous material - Google Patents

Description

v UNITED STATES Patented June 27, 194.4

METHOD OF PRODUCING SHAPED BODIES FROM! POWDERY FEBROUS MATERIAL Claus G. Goetzel, Yonkers, N. Y., assignor to American Electra Metal Corporation, Yonkers,

GFFECE N. Y., a corporation of Delaware No Drawing. Application August 9, 1941,

Serial N 9. 408,169

9 Claims. (CI. 75-22) obtained consisting of iron and carbon more or less uniformly diiiused'into the iron. The body. however was not of true steel character. The latter implies that, depending upon the carbon content, a certain structure is present which, if the content of carbon is up to about 0.8% comsubstantially pure iron which was used for the entire iron body is quite expensive to manufacture.

It has therefore been attempted to manufacture shaped bodies from steel powder in a sintering process. Scrap steel can be used for this purpose which is available on the market at low cost. contains combined carbon in any desired amount and is, of course, of true steel character. By the use of this material previously considered waste, economical purposes could be served. Scrap or other steel has to be powdered; if it is not of sufiicient brittleness for inexpensive powdering processes, it has first to be heated and quenched. The powder is, however, still brittle and therefore incapable of being pressed into coherent shapes by normal pressures, i. e. pres-' sures which are commercially applicable and range from about to 50 tons p. sq. i. Therefore, such brittle steel powders had to be decarburised at least in surface layers, in order to render them more malleable and capable of bene compacted, under commercial pressures. T is requires much skill and care on the part of the operator and renders the process expensive; in sintering a coherent compact of thus prepared steel powder, again difficulties were encountered as to retaining in the body the desired carbon content and to make the individual steel particles weld together or coalesce completely into a dense and strong body.

In order to overcome the diflicult and expensive steps of superficially decarburizing the steel powder particles, and also to arrive at a more dense and strong body, it was suggested to admix with steel powder obtained in any way described above, powder of pure or ferritic iron which is soft and spongy and therefore usable as a mechanical binder of the steel particles during the pressing and subsequent handling of the pressed compact. By using pure iron powderfor a part only of the mixture the cost of the initial mixture was considerably reduced compared with previously known processes in which this powder was used for the entire body. The relatively high price of such powder was further compensated for by the greater ease with which the mixed powders could be pressed, 1. e., the lower pressure to be applied and the greatly reduced wear of the molds in which the powders were pressed to shape.

The sintered bodies obtained from such compacted powders were quite satisfactory. They exhibited finely laminated pearlitic areas and desirable tensile strength and elongation quite close to that of steel made in usual metallurgical casting processes. Their density was adequate. The steel particles did not change in volume during sintering. However, the carbon content of the body wasstill difilcult to control.

According to the cognisance of the invention, this was due to the difliculty of applying a sintering temperature high enough so as to accomplish complete diffusion of, the carbon contained in the steel particles and sometimes added, into the ferritic material and to attain complete equilibrium. As the reason for this. the inventor found that the protective atmosphere applied changed its character depending on the temperature. While a certain gas, such as carbon dioxide, might stay practically neutral up to temperatures of about 1000 0., it becomes unstable at higher temperatures and converts itself into a decarburising agent easily giving off oxygen to combine with carbon present in the iron-carbon-mass. In the same way natural gas and other hydrocarbons used for this purpose change their behaviour in the high temperature ranges above 1000 C, Hydrogen which is mostly applied in other high temperature sintering processes, is also objectionable at such high temperatures. Mixtures of those gases are conceivable in which the the carburising property of one gas at highly elevated temperatures is compensated for by the decarburising property of another gas admixed to it, and by properly controlling the ratio and kinds of those gases a completely neutral'atmosphere even at the high temperatures mentioned could be obtained.-

However, this requires permanent careful control and adjustment of the mixture of gases. the pressure and proportion of which naturally fluctuates: thus would greatly complicate the sintering process and render it very expensive. Furthermore, the sintering process often cannot be conducted at a single high temperature, but within a certain range of temperatures. It is obvious that a cold or somewhat preheated shaped body is of considerably lower temperature than prevails in the chamber of a furnace into which it is' introduced, and should beheated more or less slowly up to the final sintering temperature.

During the period of heating up the gases controlled to be neutral for the highest sintering temperature would behave differently at the lower temperatures prevailing in the surroundings of the body Just undergoing heating up, and a carburising or decarburising action would resuit. Moreover, depending upon the mixture of the body, it should be heated up according to a certain law, so as to allow the carbon of the steel, or additional carbon admixed, to-diffusegradually into the initially ferritic phase of the shaped mass, so as to reduce the sintering temperature of that phase and raise the sintering temperature'of the steel phase simultaneously undergoing decarburisation by giving up carbon to the initially ferritic phase until a state of the mass is attained for which the final sintering temperature is appropriate. In order to overcome these dimculties, it is conceivable to use a furnace or kiln which is subdivided into several heating zones in which different temperatures are maintained, the first zone into which the compacted shape is introduced being operated at lowest temperature andthe last zone at highest final sintering temperature, while in the intermediate zone or zones the temperature is gradually raised.

Furnaces or kilns of this type are known per se for other purposes and could be usefully applied in the present instant. However, this solution of the problem indicated above does not pendent, or almost independent from the atmosphere used in the sintering process.

I It is still a further object of the invention to produce from ferrous powdery initial material shaped bodies of a true, or almost true steel structure and of controllable average carbon content. I

It is another object of the invention to use in the production of shaped bodies resembling steel from ferrous powdery initial material steel parti- 7 ales, and in particular scrap steel particles. and

to substantially retain in the completed body the structure of true steel.

It is still a further object of the invention to prevent substantially an undesired change of the chemical composition of ferrous compacts undergoing sinterlng. I

It is still another object ohthe invention to make possible in a sintering process of shaped ferrous bodies obtained from powdery initial material, the use of an atmosphere which otherwise would detriment'ally or undesirably affect the chemical composition of the body under treatment.

These and other objects of the' invention will be more clearly understood when the speciiication proceeds. v

According to the present invention, a compacted shape obtained from initial ferrous powder containing a pretermined amount of carbon, is first provided with a protecting cover which does not react with the material of the shape,

is refractory and preferably porous to some extent. The cover may be'obtained from a pasty mass substantially consisting of a refractory powdery material, such as powdery aluminum oxide, magnesium oxide, silica, lime, beryllium oxide, zirconium oxide, sillimanite, porcelain, chamotte, kaolin, clay, or any suitable mixture of any two or more of them. I The addition of a filler, suchas quartz or quartz sand, e. g., up to about 30% by weight and/or of a cement, such as alundum cement, blast furnace cement, etc. is sometimes desirable. The powder ofrefractory material, filler and/or cement, may be ad-,

mixed with water or any other suitable liquid to form a paste, the compacted shape dipped into powder and filler. They permit occluded and other gases or vapors which evolve or develop from the coated shape when heated up, to escape without cracking the coating. They also give the coating some resiliency to avoid its being cracked or destroyed by slight expansion, if any, of the shape when heated up. It should be understood, however, in this connection that usually during sintering some, though small shrinking of the shape occurs so that destruction of the coating due to the larger expansion coeflicient of the metal used in the shape com-' pared with that of the ceramic coating, is not to be expected.

After all the gases or vapors developed from the shape undergoing sintering have escaped in the way described, it was surprisingly found that the protective gases present in the furnace,

or even air, are incapable of penetrating in any. detrimental amount through the pores of the coating to the shape contained therein, and thus the coating satisfactorily protects the shape undergoing final sintering against carburising, decarburising or even oxidising efiects of the gas or air present in the furnace. This may be due to the fact that the pressures prevailing in the furnace are insufflcient to force gas or air through the pores. A

The invention prefers 'to use a pasty material for-the coating which is prepared by admixing water'or another inexpensive liquid to the refractory powder so that the coating can be rethereafter sintered in a protective atmosphere within a temperature range of about 1000" to about 1300 C., and even. for 1 to 2 hoursf it.

. neither expands nor shrinks to any extent and moved from the sintered and completed body by simply dipping it into the same liquid which plasticises again and eventually removes the coating from the completed body. The coating could also be scrapped or brushed ofi, or broken away in tumbling mills, and last traces removed by sand blasting if its removal by means of a liquid iseventually moved through the furnace, if it be of the belt type, or placed in it and heated to final sintering temperature and kept at it, according to any law desired.

During such heating a desired diffusion and reaction with alloying substances if any admixed in amounts up to a few per cent, such as additional carbon, chromium, cobalt, molybdenum,

etc.. can be accomplished, independent from the surrounding atmosphere, and the sintered body will be exactly or within permissible tolerances of the desired composition and structure.

In preparing a shape particularly suited for the purposes of the invention, one may use an initial mixture consisting of steel powder or alloy steel powder of any origin, composition and structure,

admixed with substantially ferritic spongy iron.

In preparing the latter, preferably mill scale is used which is mainly iron oxide: It is porous'and light, can easily be comminuted, dried and cleaned, and subjected to separation. The porous iron oxide powder is then reduced in any suitable manner, such as in push-type furnace in which the iron oxide powder is subjected to reduction by hydrogen, or any other reducing gas at a temperature preferably of 1100 to 1150 C. as a minimum and preferably not exceeding about 1250" C. Complete reduction can be obtained at a temperature of or above 1150 C. within about 20 minutes.

If hydrogen is used as reducing medium, it should be of higher grade, purified and desiccated. Carbon monoxide, formic acid, etc., can be admixed to the hydrogen. Sometimes it is advisable to admix to the iron oxide some solid carbon in finely divided state, such as lamp black. That admixture should not exceed about 2% by weight in order to prevent carburisation.

The pure iron powder so obtained is thereafter compressed in order to reduce its apparent density (loading weight). It may be briquetted in presses, or small pills be made therefrom. Most I surprisingly it has been found that by such compression and densification the porous structure is not removed. The briquettes or pills are thereafter powdered in a crusher which is preferred to ball milling; the latter results in a flaking-out and overstraining of the particles which is not desirable for their further handling.

The pure iron powder so obtained is of remark able behaviour. When pressed to shape and the magnitude of those changes, if any, normally does not exceed 1%. It can be compressed in the cold to perfectly coherent shapes under normal pressures of about to -tons p. sq. 1.

.Although the powder thus appears quite dead, it is nevertheless still porous and spongy, soft and malleable. If admixed in amounts from about 15% to (or with steel or alloy steel powder, such as obtained from scrap steel, compacts can be formed under normal pressures,

, from about 15 to 50 tons p. sq. in., the lower pressure applicable to the higher content on pure iron.

Upon heating to sintering temperature and slowly cooling, a body results which is of true steel character and in which pearlitic areas of extremeflneness and uniformity can be found almost the same as in the initially admixed steel powder. This structure adds to the high tensile strength, toughness and ductility of the resulting product. The areas are practically free from carbides and other imperfections.

Besides, ferritic areas can be found completely coalesced with the particles cementing them.

The initial grain boundaries disappeared to a, great extent, and some diffusion of carbon from the steel particles into portions of the ferritic ones can be established. The steel particles having been obtained in a complete metallurgical process and therefore being substantially free from pores, occluded gases or other admixtures which could evaporate at sintering temperature, do not change their volume due to the sintering treatment. The iron powder produced in the way described above and having very little tendency to change its volume neither gives rise to any significant expansion or shrinkage.

Therefore, the final body obtained is of the exact size and shape imparted to it in the preceding compacting or in a subsequent repressing process.

In this way sintered steel containing up to about 0.8% carbon can be produced in a throughout repeatable manner. ZThe over-all content on carbon of the body can'be controlled by proper proportioning the initial iron-steel mixture, and the overt-all composition is not changed during the subsequent sintering process. If the latter is desired, carbon in corresponding amounts should be added to the initial mixture or admixed to the coating or cover.

The initial powers can easily be compacted even at room temperature. 1

The final sintering temperatures are chosen preferably at or above about 1225 C. in order to secure complete coalescence of the mass under treatment. sintering at that temperature should be performed for about two hours or more.

If the temperature be raised to 1300 C., complete homogenisation, of the structure can be ,obtained, l.-e., coalescense and extended diffusion of the carbon and other alloying substances, if any present, in a shorter period of time, such as with n about one hour.

If the sintering process be performed at lower tempratures, particularly below 1200 C., it has been found that the steel particles retain substantially their original boundaries and higher carbon concentrations. On the other hand, by exceeding l300 C.,the period of sintering can still be shortened; limits have been observed at the "NJ E line in the iron-carbon constitution diagram) because grain growth became excessive. .At sintering temperatures close to about. 1325 C. the grain size of the final ferrite-pearlite structure' of the ,sintered product was substantially normal. No undesirable difference in fineness of the pearlitic laminae in the original steel areas and those resulting from difiusions in portions of the ferritic matrix or binder can be observed.

The initial steel powder is to be quenched in order to render it brittle for crushing. Its structure is therewith made substantially martensitic'. In-order to facilitate compacting, annealing of the powder is sometimes advisable up to below 700 C., where the martensitic structure of .the

eutectoid or hypo-eutectold composition is not yet transformed into the austenitic one, but the cementite is remarkably spheroidized. This phase is known as very stable in carbon content and therefore resists decarburisation, i. e., gives up carbon to adjacent ferrite. by diffusion, to ,a considerable extent during. subsequent sintering.

' Steel powders thus annealed are of suflicient softness and plasticity so as to be compacted with a very small amount of ferritic binding material.

13y addition of the latter however the density of the compact pressed under normal pressures at room or slightly elevated temperatures could be raised from 75% to 90%- and even higher... As it has been stated above, the final structure of the sintered body resembles steel to a great extent and the initial over-all carbon content of temperatures of about 1375" to 1425 C. (close to ingredients admixed and theirpre-treatment, if an! It should be understood that instead of a ferriticbinder, any other binder can be used, such as avolatile binder, e. g., water, and the pasty mass obtained by this mixture pressed .0 shape and thereafter sintered in a protective coating or casing according to the invention. It is preferable to drive off the volatile constituents of the binder by heating to low temperature of a few hundred degrees C., before the coating is applied to the shaped body or the latter is-placed into the protective casing. In such event, annealing of the crushed steel powder in the way described above, has, proven advantageous.

It should be further understood that according to the invention not only shaped bodies resembling low or high-carbon steel can be obtained, but also alloy steels containing any other alloy element or elements besides carbon in desired amount. and contained either in the initial steel particles or. admixed in their metallic or suitably compounded form to the initial mixture,

or placed in the-easing if any be used, or admixed to the coating applied. a

The invention is not confined to any of the exemplifications given hereinbefore, but to be derived in its broadest aspect from the appended claims.

What I claim is: v

1. In a method of producing from powdery ferrous metal material shaped bodies resembling the mass has not been changed. Diffusion of car bon, of course, takes place at leastinto adjacent portions of the ferritic binder, but results upon slow cooling in formationof pearlitic and ferritic areas. The areas resulting from steel particles initially admixed, though they exhibit a fine pearlitic structure, are thus reduced in their carbon content.

Itis sometimes of advantage to presinter the pressed compacts at temperatures close to 700 (2., but below 710 C. have been found best. The presintered compact is comparatively soft and malleable and can be shaped by machining or densified by forgingor pressing, preferably in the heat below about 710 C. By such densification the subsequent sintering process is greatly facilitated, the uniformity of the grain structure in-' creased and diffusion of carbon, and other alloying elements, if any present, assisted. Also the time of final sintering can thus be shortened whereby the additional expehse for presintei'ing is compensated for at least in part.

r In proportioning the initial mixture, it should beQclearly differentiated between the'average or 7' over-all content on carbon of the final body and the carbon content of the steel-areas.

should not contain above 0.8% carbon in case a The latter eutectoid or liypo-eutectoid steel phase is desired.

By diffusion of carbon into the ferritic binder the carbon content of the initial steel particles is re- "duced, .and if therefore the latter is 'to be retained, addition of solid c-arbon for carburising part of the fer'ritic binder is advisable, and/or hyper-eutectoid steel containing up to about 1.5%

' 0., should be used. Upon sintering at sufficiently high temperatures for a sufficient period and subsequent slow cooling, as stated above, these carburised areas will almost completely result in a'steel-like and particularly pearlitic structure.

If the body is sintered in a protective coating or casing, the initial all-over chemical composition of the mass will be retained almost completely. Its structure however depends upon the steel of substantially predetermined chemicalcomposition, the steps of forming an intimate mixture of finely divided particles of substantially pure iron in amounts of about 5% to 60% and of a balance substantially of steel or alloy steel of predetermined carbon content, compacting the mixture under commercial pressures of about 5 to tons p. sq. i. into desired shape, covering the shaped compact with a coherent refractory mass exhibiting pores of controlled fine size, and

' sintering said compact within said cover.

-composition,'the steps of shaping a powdery mixturecontaining in substantial amount finely divided particles of steel or alloy steel of predetermined carbon content, covering the shaped compact with a coherent refractory mass exhibiting pores 'of controlled fine size, and sintering said compact within said cover at temperatures from about 1225 C. to about 1375 C.

4. In a method of producing from powdery ferrous metal material shaped bodies resembling 'steel of substantially predetermined chemical composition, the steps of shaping a-powdery mixture containing in substantial amount finely divided particles of steel or alloy steel of predetermined carbon content, covering the shaped body with a pasty mass of refractory material and a volatile binder, said material melting considerably above the sintering temperature of said body, and heating the covered body so that said mass solidifies but retains pores of controlled fine size and said body sinters and upon cooling attains'substantially steel-like structure. 5. In a method as described in claim 4, said paste substantially consisting of refractory inorganic oxide.

6. In a method as described in claim 4, the step of solidifying said pasty mass by heat treatment before the covered body is subjected to sintering.

7. In a method of producing from powdery ferrous metal material shaped bodies resembling steel of substantially predetermined chemical composition, the steps of annealing finely divided steel or alloy steel powder at a temperature close to but below about 710 C., forming and shaping a powdery mixture containing in steel of predetermined carbon content, covering the shaped body with a pasty mass of refractory material and a volatile binder, said material melting considerably above the slntei'ing temperature or said body, and heating said covered body, so that said binder evaporates and said mass solidifies but retains pores of controlled fine size, and thereafter to a higher temperature within the range from about 1225 to about 1375 C. and in an atmosphere which otherwise would detrimentally aflect the composition or structure oijhe shaped body so that the latter sinters and, upon cooling, attains a substantially steel-like structure.

- 9. In a method of producing from powdery ferrous metal material shaped bodies resembling steel of substantially predetermined chemical composition, the steps of shaping under pressure a powdery mixture containing in substantial amount finely divided particles of steel or alloy steel of predetermined carbon content, covering the shaped body with a pasty mass of refractory material and a volatile binder-I said material melting considerably above the sintering temperature of said body, heating said covered body to a temperature at which said mass solidifies to form a brittle casing with pores of controlled fine size. and thereafter to a higher temperature within about 1375 C.-so said casing and, upon cooling, v stantially steel-like structure, and removing subsequently said casing from said sintered shaped body.

CLAUS G. GOETZEI.

that said body sinters within the range from about 1225 to attains a sub-