CA1061513A - Method for improving the sinterability of cryogenically-produced iron powder (a) - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method is disclosed of making sintered parts with cryogenically-produced powder derived from scrap metal, such as machine turnings. The scrap metal is subjected to two impacting operations (such as by use of a ball milling machine). The first operation is carried out with the use of a refrigerating agent (to lower the temperature of the scrap metal below its ductile-brittle transition temperature) thereby resulting in comminution of the scrap to a cryogenic powder;
The second impacting operation is carried out at ambient temperature conditions utilizing milling elements which impart cold work to at least a portion of the cryogenic powder;
simultaneously, copper is mechanically transferred to sub-stantially each particle of said cryogenic powder to form a continuous copper envelope thereabout. The coated cryogenic powder is then compacted and sintered; the sintered product may be subjected to hardening or tempering treatments if desired.

Description

~6~LS13 The present invention is directed to the production of sintered compacts and to intermediate products.
; Cryogenic powder making is a relatively new mode of providing a powdered raw material which can be put - to use in powder metallurgy techniques and other appli-- cations. Cryogenic powder holds great promise because it can provide powdered material at a significantly lower cost and it may result in more useable physical properties, if not enhanced physical properties~ for a sintered powdered part.
Essentially cryogenic powder making comprises .~ . .
; subjecting scrap metal, or other solid starting metal material, to a temperature below the transition temperature of said metal, such as -(30-40)F for ferrous based material.
The metal becomes so brittle at such deprèssed temperatures that agitation within a conventional ball mill will reduce the scrap or starting metal material to a powder form over a predetermined period of time and s~ress from the ball milling elements. At~the same time, any oil or other organic materials coating the scrap metal, particularly scrap ~metal in the form of machine turnings, will also freeze and be removed during the impaction by the ball milling elements; such frozen debris can be screened and ;~
- , ~ :
~ ~ separated.
.. .
To ensure that-the scrap metal is in the embrittled condition at the point of impaction,~it is .
necessary to direct a supply of liquid nitrogen against the scrap~metal immediately prior to introducing the scxap ~ ~metal into the~mill~itself. The comminuted~particles resuiting from a predetermined amount~of ball milling under ~ 2 -:
., . . . ~., . ... .. ,., , .. . :.. ... .. .. .

~06~5~3 such embrittled conditions, produces metal particle shapes which are flake-like or irregular, certainly not spherical.
The layer-like or flake configuration results from the two facts: (a) the turning was originally ribbon-like, and (b) comminution takes place by fracture.
When such cryogenically produced powder i5 subjected to conventional powder metallurgy techniques, with a compacted quantity of such powder being heated to a sintering temperature, oxidation of ingredients such as manganese and silicon will typically take place prior to diffusion and completion of the sinteriny step. Such oxidation results because these elements require more sintering atmosphere control than is normally possibla in current, more stringent operations.
The present invention is directed to the formation of coated particles, compacts thereof and sintered shapes from scrap metallic turnings by procedures which overcome the prior art problems noted above.
In accordance with one aspect of the present invention, there is provided a method of making an inter-; mediate powder, comprising: (a~ selecting metalli~
turnings which have a surfdce-to-volume ratio of at least 60~ ) simultaneously refrigerating to below the transition temperature of the turnings and impacting the turnings wlth a fracturing force, continuing the freezing and impaction for a perlod of time to com~inute the ~ ~
turninqs, (c~ lmpacting the comminuted particles at ambient ~ -temperature conditions with an abrading force, the ~ ;
~, . . .
~ impacting being carried out b~ the use o~ elements laden with a protective metal having a hardness less than that of~the comminuted particles to promote transfer of the .

~: , ~061513 protective metal to the part;cles upon impact between the elements and particles, the protective metal having a melting temperature below but substantially about theliquidus of the particles, the protective metal being relatively easy to abrade, and (d) continuing to carry out the impaction oE step (c) to pr~vide both coating of the particles with a thin envelope of the protective metal and cold working of substantially each particle having a : size greater than 124 microns to thereby promote at least one defect site therein.
The coated particles resulting from this . - ,. . .
procedure may be formed into a compact by compacting a predete.rmined quantity of the coated particles into a desired shape. A preferred powder compact provided in accordance with another aspect of the invention consists essentially of a uniform and homogeneous mixture oE ferrous-based first paxticles having a size range +200-325 mesh and .~ ferrous-based second particles having a size range of +60 ~-140 mesh, one of the first and second particles having a j~ 20 - protective metal envelope about substantially each of the particles thereof, the protective metal having a hardness less than that of the particles,~a metal temperature below : but sub~tantially about the liquidus of the particles and - being relatively easy~to abrade, and substantially each : ~ ~ of the particles of the first particles having at least ~
one stress defect site therein, the compact having a density : ~: of between 6.6-6.7~g/cc and a volume shrinkage of 7-10%
.~ upon being heated to 20-50F. :
~ - ;
The powder compact may be further treated to 30~ form sintered shapes by~heating the shape to at least the : plastic region for the metal particles in an atmosphere .~:~

. _ '' ~ ' .

15~3 at least non-oxidizing to the envelope of protective metal surroundiny each particle to permit atomic diffusion to take place between particles in adjacent contact within the shape. The envelope, about each particle prohibits oxidation of the ingredients of the particles during the heating and the defect sites provide increased diffusion.
The invention is described further, by way of illustration, with reference to the accompanying drawings, in which:
Figure 1 is a schematic flow diagram of the comprehensive method of this invention;
Figure 2 is a photograph of two sintered shapes, ~ -one comprised of cryogenically produced powder and the other comprising conventionally produced atomized ferrous based powder; and --~
Flgure 3 is a photograph of the fracture surface along one end of each of the specimens illustrated in :
- Figure 2.
A preferred mode for carrying out the method aspects of this invention is depicted in Figure 1 and is as follo~s: ~
(1) Scrap metal and particularly machine turnlngs 10 are sel~ected as the starting material. "Machine turnings" ie defined llere~in to mean segments of ribbons of low alloy steel.~ They typically are shavings cut from alloy bar. But machine~turnings, preferably ferrous based, include alloying ingredients such as manganese, 5ilicon chromium, nickel and molybdenum. The turnings~should be 1 30 ~ selected to have a surface-to~volume ratio of at least 60:1, whLch~is characteristic o~machin~ turnings. The 4A~ -- .

~61S~L3 - scrap pieces wlll have a size characterized by a width 0.1-1.0 inch, thickness of 0.005-.03 inches, and a length of 1-100 inches. Machine turnings are usually not suitable for melting in an electric furnace because they prevent efficient melt down due to such surface-to-volume ratio.
This process can be performed with other types or larger pieces of scrap metal, although capital investment costs may increase due to the difficulty of impacting scrap metal sized in particle pieces beyond .03" thick. The scrap pieces should be selected to be generally compatiblë in chemistry when in the final product; this is achieved optimally when the scrap is selected from a common machining operation where the same metal stock was utilized in forming the turnings.

(2) The selected scrap pieces 10 are then put into a suitable charging passage 11 leading to a ball milling machine 12 or equivalent impacting device. Within the passage, means 13 for freezing such metal pieces is introduced, such as liquid nitrogen; it is sprayed directly onto the metal piecesO
Mere contact of the li~uid nitrogen with the scrap pieces will freeze them instantly. The application of the liquid nitrogen should be applied uniformly~throughout its path to the point of impaction. The ball milling elements 14 are motivated pre-*erably by rotation of the housing 17, to contact and impact .: .
~the frozen pieces 15 of scrap metal causing them to fracture and be comminuted~. Such impaction lS carried out to apply sufficient~fracturlng force ~deflned to mean less than 1 ft.
lb.) and for suEficient period of time and rate to reduce ~ ~ sai~ scrap pieces~to a powder form. The powder 16 will ;~ 30 typicall~ have;~both a~coarse~and a~ine pow~er proportion.
Both propoxtions will~be comprlsed of partio~les which are flake " ~3615~L3 or layered in configuration; each particle will be highly irregular in shape and dimension, none being spherical in shape. A typical screen analysis for copper coated powder - - would be as follows (for a l00 gram sample):

Mesh No Milling After 72 Hrs (in ~rams) (in ~rams?
60.0 31.5 ~
100 19.5 ,11.0 140 5.5 7.5 200 6.5 18~0 325 4.5 22.5 -325 4.0 9.5

(3) The comminuted CryOgeniG powder 16 is then subjected to another impacting step, but this time at ambient temperature conditions. The powder is placed preferably in another ball milling machine, the machine having copper laden elements l9 preferably in the form of solid coppèr balls of ~' about .5 inch in diameter. In trLals performed herein, the interior chamber was a 3" x 6" cylinder, powder charge was 10 in. 3, and the milling time was about 48 hours. Milling ~time and rate depend on mill volume, mill diameter size o~
copper balls, and the speed of rotation. The function of this second impacting step is to transfer, by impact, a pox-tion of the copper ingredient carried-by the ball millin~
elements l9 so as to form a copper shell about substantially each particle o~ the powder 16. The finer powder will obtain i a copper coating by true abrasion of scratching with the - ~
~surface of the ball milling elements~l9~. Ball milling elements l9 should have a~diameter at least 50 times the lar-gest dimension of any of t~e~particle shapes of the cryogenic ~-~ 30 powder~16~ Seoondly, the ba1l milling operation must gener- -- ~ ate defect sites in substantially~all powder~particles~above 124 microns; the ball milling-;operation herein should be . ~ .
~ - 6 '-6:~LS~3 carried out so that substantially each coarse particle has at least one de~ect si-te therein. Th:is can be accomp]ished by rotating the housing 20 to impart a predetermined abrading force from the balls l9.
When this step is completed, the particles will be in a condition where they will all substantially have a con-tinuous copper envelope (coating or shell) and be stressed sufficiently so as to have a high degree of cold work. The term "de~ect site" is defined herein to mean a defect in local atomic arr~ngement. The term "copper shell" is defined herein to mean a substantially continuous thin envelope intimately formed on the surface of the particle. Although the shell should preferably be an impervious continuous envelope about each particle, it is not critical that it be absolutely im-pervious. It has been shown, by the test examples performed in connection with reducing t~is invention to practiee, that cold working of the partieles is predominantly influential in inereasing diffusion kineties of this invention, the copper eoating or shell operating to predominantly form an~anti-20 ~ oxidation barrier.

(4) ~A predetermined quantity of powder condition from step (3~ is eompacted by a conventional press 20 to a predetermined density, preferably about 6.6 g./ccO This is brought about by the applieat1on of forces in the range of 3~0-35 ts1. The presence of the eopper envelope about the powder particles improves compressibility. With prior un-eoated powders,~ a~density of about~6.4 g.jce. is typically obtained using~a compre~ssive force of 85,000 psi; with the powder herein, densities~of about 6.6 g./cc. are now obtained at the~same~force level.
The shape;21 into whioh suc~ powder is compaoted lS

~06~3 designed to have an outer configuration larger than that desired for the final part. A significant and highly im-proved shrinkage takes place as a result o the next step (5~; the shrinkage can be a predetermined known factor and allowance can be made in the compacted shape 21 of this step.
Shrinkage will be in the controlled limits of 15-15.

(5) The compacted shape 21 is subjected to a sintering treatment within a furnace 22 wherein it is heated to a temperature preferably in the range of 2000-3100F, for ferrous based cryogenic powder. The temperature -to which the compàct is heated should be at least the plastic region for the metal constituting the powder. A controlled or protec-tive atmosphere is maintained in the furnace, preferably con-sisting of inert or reducing gases.
At the sintering temperatures, atomic diffusion takes place between particles of the powder particularly at , solid contact points therebetween; certain atoms of one par ticle are supplied to fill the de~ect sites or absence of certain atoms in the crystal structure of the contacting particle, said defect sites~belng present as a result of cold~
workin~ in step (3). Diffusion is accelerated to such an extent, that an increase of~more than 100 times is obtained.
- It is theorized that at ~east 60~ of the improvement in physical properties of the resulting~sintered shape is due to the controlled cold working of the powder. The increased diffusion is responsible for the increase in shrinkage.
The copper envelope on the~particles servès to essen- ;
tially prevent~oxldation of certain elements or ingredients ~- ~ within~the~powder partlcles~, partlcularly manganese and slllcon.
30- ~ With~ ypical ba~l milling~parameters, (such as physical siz~
of mill speed chan~e and ball~ size) sufficient to the job, it ~ ~ :: ~ , . . .

. ~ :
.:
: . .
-~ ... .

l5~3 can be expected that substantially each particle of the cryo-genic powder will po~sess an impervious copper shell. However, a totally impervious shell is not absolutely essential to obtaining an improvement of some of the properties herein.
As a basis for comparison, several as~sintered test samples were prepared. The procedure for preparing the test samples was varied to investigate aspects such as the effect of cold working, the influence of a copper coating wi-thout cold working, the manner in which the copper coating is applied, and the influence of particle size. All of the test samples were prepared according to the following fabrication and thermal treatment except as noted. A cryogenically produced powder quantity was admixed with 1% zinc stearate (useful as die wall lubricant) and 0.7-0.8~ gràphite. The admixture was ~
compacted at a pressure of 25 tons/sq.in. into standard `
M.P.I.F. transverse rupture strength bars. The bars were pre-heated at 1450F for 20 minutes to burn off the lubricants, the heating was carried out in an endothermic type atmosphere at a 45F dew point. Sintering was carried out at a higher ~ temperature in the same endothermic atmosphere for an addi-tional 20 minutes.
The~first three samples are considered represen-*ative of the prior art as~a reference base since no separate cold working or copper coating was employed.

Sample No. Sintering Transverse Rupture Hardness As-Sintered Temp. ~F3 Strength ~psi) (RB~ Density 2050~ 16,000 62 6.5 2 ~ 2075 ~ 20,000 68 6.~
3 ~ 2100;~ 2~2,~000 73 6.~ --~ 30~ To investigate~the~e~fect o~ cold working, the p~der ball~mllled was in~a mill employi~g steel balls; the

6:~LS~3 ball milling time was varied for each of the three samples in the following sequence: 20 hours, 44 hours and 96 hours.

Sample No. ~intering Transverse Rupture Hardness As-Sintered Temp.(F) Strength (psi) (RB) Density 42100 28,000 - 6.6 52100 46,000 - 6.6 62100 59,000 - 6.3 ; To further separate or analyze the effect of fine particle sizes, the starting material was not milled but rather it was screened so as tv pass fine particles through a - 10 100 mesh screen. The screened fine particles were than sub-jected to the treatment outlined above. The results showed Sample No. Sintering Transverse Rupturé Hardness As-Sintered Temp.(F) Strength (psi) (~) Density

7 2100 25,000 20-25 5.~8 An investigation of the lnfluence of copper coating, ~ -by 1tse1f, without cold working from ball milling elements, was pursued. A copper coating was applied chemicall~ to the particles of the cryogenic powder; for~the following irst three samples, the coating was applied electro~ytically ~ using~
~;a copper sulphate~(CuSO4) salt in the electroplatin~ bath and 20 ~ the;next two~samples were~prepared utilizing a copper nitrat~
(CuN~3) saIt. ~ ~

Sample Nou~Sintering ~sverse~By~h~e Hardness As-Si~tered Temp.( F~ Strength (psi) (~) Density

8~- 2100 3,000 - 5~5(no special handling)

9~ ~ ~2100 ; ~~5,000 ~ ~ ~ - 6.0(the p~der was pretreated in ~Cl beore plating~

10 ~ `2100 ~ ~ 15,000~ - 605(an alcohol~
; rinse~was applied after plating)~
2100~ 18,000~ 6.5tno special hand~ g) ~' '.' 12~ 2100 ~ 12,~000~ - 6.4tan alcohol rinse was~applied after plating) ;: :

~06~5~3 An investigation was made as to whether fine parti~
cles, simply copper coated, would provide an improvement. The copper coating was again applied electrolytically utilizing a copper nitrate (cuNQ3) salt, the powder particles were restricted to ~100 mesh.

Sample No. Sintering Transverse Rupture Hardness As-Sintered Temp.(F) Streng~h (psi) (~) Density 13 2100 72,000 - 6.2 Finally, the combined e~fect o~ (a) cold working through a ball milling operation and (b) the application o~
a copper envelope or coating on each of the particles at the same time the ball milling is carried out, was investigated.
It is important to point out that the copper coating was applied mechanically by an abrading action between copper balls and the cryogenic powder within the milling machine.
Fine particles below 120 mesh probably obtained a copper coating merely by abrading of the soft copper thereonto, while the coarser particle achieved a copper envelope much more by abrading action along with receiving cold work. The ball ~ milling was carried out for a perlod of 96 hours. The results were as follows:

Sample No. Sintering Transverse Rupt ~ Hardness As-Sintered Temp.(F) Strength (psi) - (~) Density 14 ~ 210Q ~ 90,000 84 6.7 8y ball milling for extended~periods of time or at an increased~stress rate, a transverse rup-ture strength of at least 95,000 can be obtained. Accordingly, it is concluded ; that~not only is the transverse-rupture strength improved by the combination effect herein but such improvement lS beyond that obtainable by utilizing conventional atom~zed powder under~the~same processin~condltions but without cold work or ~; 30 copper coating. Typically,~atomized powder will obtain at best~a transverse rupture~strength o~85,000 psi~with a density 15~3 - of around 6.7g-/cc. when processed under the most favor~ble conditions known to the prior art. Accordingly, with the decrease in cost by use of scrap materials reduced to a powder cryogenically along with the improvement in physical characteristics herein, important advantages have been ob~ained.
Other conclusions which can be drawn from the above data include- (a) the general effect of cold working by ball milling increases the sinterability of the cryogenically pro-duced powder, (b) decreasing the average particle size of thepowder has little effect by itself on the final physical pro-.
perties, (c) copper coating, by itself, appQars only to im-prove sinterability of fine powders, and (dj the combination of cold working and copper coating by use of copper balls, increases the sintered strength 4-5 fold.
Turning now to Figures 2 and 3, there ~s illustra~
ted comparative examples of an as-sintered shape. The sample '! in the left hand portion of Figures 2 and 3 represents a shape -produced in accordance with this invention utilizing cryo genically produced powder and prooessed with a second ball - milling operation where cold working and copper coating lS:
obtained. The sample in the right hand portion in each of the photographs represents~an as-sintered shape obtained by conventional powder metallurgy techniques utilizing ordinary atomised iron powder. Such ordinary atomized powder typically consists of primarily g9.L~iron, the remainder may consist of:
carbon .01~.Q45%; silicon .005-.015~;~sulphur .00~-.016;
phosphorus .007_027;~ Mn .04-.26%; residual oxides - weight loss~in H2 is~.2~-.6%. The atomised powder was merely sub-~;; 30; jeoted to a~compactinq step~achieving a green density of ; about 6.4, and was-~subjected to~heating at a sintering 12 -~

~6~5~3 temperature of 2050F.
In Figure 2, the right-hand sample of this invention has a particularly evident smooth outer surface as opposed to the relative rough heterogeneously shaded outer surface for the sample on the left. Figure 3 shows the end face where ~racture took place as a result of destructive testing. The sample on the left has a typical fracture, rough and highly porous surface. The sample on the right has a fibrous appear-ance. The as-sintered shape of this invention is particularly comprised of ferrous particles which are randomly irregular in con~iguration, none of which are spherical; the particles are bound together by molecular diffusion at contact points therebetween, said shape having no apparent porosity and has a fractured surface as a result of destructive testing which - appears as glassy. It is further characterized by a weight to volume ratio of 6.6-6.7, a typical transv~rse rupture`
strength of 95,000 psi with the compact at a density of 6.6-6.7 g./cc. ~resulting from compression forces of 25-30 tsi). The hardness of such as-sintered shape is a~ Least 84 RB~
-A new powder compact has been achieved as a result of practicing a portion of the disclosure herein. Such powder .
compact uniquely consists essentially of uniformly and homo-geneously mixed ferrous based particles having a proportion -` ~ of fine particles in the size range of +200-325 and a coarse particle proportion~in the size range of +60-140, the ~ine particles being present in~the ratio of 1:1 to the coarse ` ~particles, the flne~and coarse particles each have a copper envelope about substantially each of the coarse particles ~ thereof, and zubstantially each of the coarse particles have at least one defect slte therein, said compact havlng a den-sity~of at leazt 6.6~g./cc. and a volume shrinkage of~about lQ~ upon being heated to 2050F.

~16:~5~3 SUPPLEMENTARY DISCLOSURE
In the principal disclosure, there are described a method of making sintered shapes from metallic particles, a method for the formation of an intermediate powder and a method forming a powder compact which include various combinations of the following operations~
Metallic turnings having a surface-to-volume ratio of at least 60:1 are impacted with a fracturing force at a temperature below the ductile-brittle transition - 10 temperature to comminute the turnings. The comminuted particles then are impacted at ambient temperature conditions with an abrading force to provide a thin envelope metal coating on each of the particles r cold working of each of the particles and at least one defect site in substantially each of the particles having a size above 124 microns~ The impacted particles are compacted to a desired shape and , . . .
the shape is heated to at least the plastic region for the metal particles to permit atomic diffusion to take place between particles in ad~acent contact within the shape ~ while the metal envelope prohibi~s oxidation of the ingredients of the particles during the heating and the defect sites promote increased diffusion.
The abrading force may be provided b~ elements having a transverse dimension at least 50 times the longest dlmensi~on of any of the particles, such as, spheres hsvlng a diameter of~at least 0.1 inch, for example, about 0.5 inches.
The powder compact formed from~the impacted particles prs~erably~aonsists~sssentially of a D iform and 30~ ~: homogenso~us~mixture of ferrous-based~first particles having a ~ize range of ~200-325~mesh and ferrous-based second~
partlclss having~a~size of~+60-140 mesh. One of ths ~i st -L5~3 and second particles has a metal envelope about substantially each of the particles and substantially each of the first particles has at least one stress defect site therein.
The compact has a density between 6.5 to 6.7 g/cc and a volume shrinkage of 7 to 10% on being heated to 2050F.
In the principal disclosure, the metal providing the envelope is a protective metal having a hardness less than that of the coated powder to promote transfer to the particles upon impact between elements laden with the protective metal and the particles. The protective metal has a melting temperature below but substantially about the - liquidus of the particles and lS completely soluble in the metal of the particles. The protective metal i5 relatively easy to abrade.
The as-sintered shape preferably consists essentially of ferrous-based particles having a protective metal content of no less than 0.05 and being randomly irregular in configuration and non-spherical. The particles are bonded together by molecular diffusion between and at contact points of the particles in the shape. The shape has no apparent porosity and an outer surface appearing as a~melted glassy surface.~
The principal disclosure specifically describes copper as the protective metal. ~In accordance with this upplementary~isclosure, the protective metal may be iron.
This Supplementary Disclosure also provides additional sintering test~data supplementing that of the ; principal disclosure.
me problem~of~prevention of oxidation of alloying `~
elements dissolved in the iro~ powder is a kin~tic one nvolvlng diffu6ion through the coating material to thè particle ~i ll513 surface. If diffusion is slow, chemical potentials of the metallics can be kept low enough at the outer surface of the coated particle, where oxygen potentials are the highest, to avoid oxidation. For the times and temperatures involved in formation of initial sinter bonds this is probably the case; at least for most substitutional elements soluble in iron. However, the fact that the diffusion process, as well - as the process of solution of the coating material, is going on continuously during sintering implies that oxidation will occur to some extent after formation of the initial sinter bonds. However, once these bonds are formed they can continue to grow independently of the state of oxidation of the free particle surfaces. Further oxidation, therefore, is important only if subsequent operations require that inner pore surfaces be oxide freeO
, ~ . .
Selection of a coating that will not oxidize in endothermic gas atmospheres, or at least not have a stable form at sintering temperatures, may be made from equilibrium thermodynamic data for metal-oxide reactions. Figure 4 accompanying this Supplementary Disclosure is a plot of this type of data for metals of interest here. With the natural logarithm of the~e~uilibrium oxygen pressure of the a~mosphere gàs plotted versus temperature, each of the equilibrium lines shown descri~es the temperature and oxygen pressure conditions ;
required to cause dissociation of the metal oxide into metal +
oxygen gas. Thus atmospheres with greater oxygen pressures or potentials tha~ ths dissociation pressure of an oxide lie above the;equilibrium line and are in the region of oxide~
. . .
~ stability. Atmospheres-with oxygen pressures le$s than the ..
~ `30 - dissociation pressure lie~below the equilibrium line and ~
.::
~ favour reduction~of the~o~ide~to pure~metalO The cross-hatched .
~ 16 -:
:

~061513 areas shown in Figure 4 represent the operational ranges of endothermic c3as generators and dissociated ammonia atmospheres.
Note that the data shown here indicates why the common alloying elements in steel, other than c~pper, nickel and molybdenum, are not particularly well suited for iron sintering furnace atmospheres. Because of this the choice of coating materials from the elements shown reduces to copper, nickel, molybdenum or iron itself. Copper was chosen for initial trials because of the variety of coating methods available~ both chemical and mechanical. Since copper melts at 1083C (1981F), it offers the additional advantages of being liquid at sintering temperatures and -- thus potentially promoting the rate of sintering via the action of capillary forces.
A number of additional powders were examined. ~ -- Some of these powders, their nominal chemistry and screen ~-analyses are listed in Table I appended to and forming part of this Supplementary Disclosure. The first three powders originated from steel machining swarf and the fourth from cast iron machining swarf. The steels, SAE 1050 (Cryo 314, Cryo 319) and 8620, ~Cryo 138~ were comminuted at cryogenic temperatures to avoid excessive plastic deformation. This was accomplished after preliminary cleaning and shredding by immersion in liquid nitrogen followed by hammer milling, a process subjecting the chips to high impact loads at .
temperatures below their ductile-to-du~tile fracture transi-tion. The combination of high impact loading and low temperature s~emed ~o reduce the powder shape characteristics assooiated~with mechanical comminution of ductile materials ~ ~considerably. ~These materials, after comminution, wére given a decarburizing anneal to reduce carbon levels below ~ 17 -: ~ ; :

5~3 0.1% by weight.
The 314 powder, supposedly from the same scrap source as the 319, illustrates a problem which must be contended with in dealing with the processing of scrap of this sort. The combination of abnormally high carbon and silicon of the 314, compared to the 319, suggested that the apparently "segregated" scrap did contain some cast iron swarf also. Thus, after decarburization, the 314 and 31~
differed inadvertently in silicon concentration as well as in the intended particle size dlstribution~
The cast iron swarf powder (Iron 1393 was comminuted by ordinary grinding procedures since it was inherently brittle. It was used in the ferritized condition to enhance compressibility properties. Some silicon carbide -was noted in the powder; probably carried over from the ~ .
grinding operation. The cast iron powder was not given a ~i decarburizing anneal. Decarburiæation was accomplished during sintering by additions of Fe2O3 powder to the powder mix.
All powders were coated for varying lengths of time to determine the effect of thickness and continuity of the coating on the sintered properties. Once coated, processing o~ the coated powders was the same as for uncoated powders. Powders were blended with l~ zinc stearate and sufficient gr~aphite to achieve a~final co~bined carbon concen-tration of 0.6-0~8%. In the case of the cast ir~n, the stoichiometric amount~of iron oxide required to reduce carbon to this level was substituted for the graphite.
Powders were compacted into M.P.I.F. transverse rupture bars or tensile bars~ Pressures used wexe kept constant at 414 MPa (30 tsi) and green densities were recorded as a measure of compressibility. Sintering was accomplished in endothermic - :
~ ~ - 18 - ~ ~

~06~L5~L3 gas atmosphere using a thermal cycle of 30 minutes at 788C
(1450F) for burnoff of lubricant and 20 minutes at 1121C
(2050F) for sintering, followed by a controlled cool to room temperature.
The sintered transverse rupture strengths (T.R.S.) ; of each ~owder were used as m~lsures of the "quality" of sintering. The problems associated with the oxidation of silicon, manganese, and other alloying elements are reflected in significantly lower strengths than normally obtained with commercial iron powder mixes. As a reference, an iron ~ O.7%
carbon sintered alloy at a density of 6.7 g/cc will have a T.R.S. of the order of 550 MPa (80 ksi), while the same alloy with 1.5% copper, admixed can attain a T.R.S. of 786 MPa ~110 ksi).
Most of the initial test work was performed on Cryo-138 because of its high alloy concentration and because a plentiful supply of comminuted powder was available.
These results are summarized in Table II appended to and forming part of this Supplementary Disclosure. The first 20~ five entries represent attempts at sintering without prior coating. The data aemonstrates the difficulty encountered in attaining acceptable property levels with ordinary wrought steel chemietry. The-best situation, sintering at 1150~C
- for 40 minutes in dissociated ammonia produces transverse rupture strengths only 73% vf that possible with commercial iron powder at lower sintering temperatures and shorter times. The coated samples on the other hand, all possessed significantly higher transverse xupture stxengths after :
- ~ ; sintering. ~he data presented for coated samples in Table 3a ~ describes only the effects of coating variables. The code ndicated in the~Table~has~coating treatment indicated~by . . , . ~ , ~ : :

~L~6~5SL3 the letter prefix and coating time or thickness by the number. With the exception of the B treatment, the values selected for the Table were optimum treatments for highes~
strength. Although it is not evident from the sintered densities shown, the B4 sample differed from the B3 in the amount of shrinkage which occurred during sintering; the B4 having shrunk significantly more.
The marked effect of coating was observed to be -highly reproducible and relatively independent of the nature of the starting material itself. The coating treatment variations examined were primarily designed to vary the rate of copper plating and the adherence of the coating. From the data in Table II it is evident that B4, -C2, and D5 treatments all provide properties equivalent to .
- ~ or better than the best commercial iron powder with 1.5 copper admixed. Copper analysis of the Cryo-138 powder indicates about 1% Cu pres nt in samples with the thicker . : : . :
coatings.
Table III appended to and forming part of this Supplementary Disclosure contains the data obtained from powder produced from essentially a plain carbon steel swarf~
~Cryo-314 is the flnest particle size distribution examined ; irom this material,~Cryo-319, the coarsest. Once again the effect of~coating is quite~marked although the properties are not~as~high as or the Cryo-138. The-Cryo-314 iS9 , however, within the~range~of commeroial iron~powders without ; , . -copper additions. The;Cryo-319 is obviously too coarse and hus~ has too few -interparticle contacts to provide adequate strength~ The contacts existlng did, however, sinter ~ sati~sfactorily as indicated~by the data. The abnormally high silicon~ln Cr~o-314 did not~appear~to have~influence~ the 2a-~ ~7~; :: :

~LO~;~5~L3 sintering process.
Table IV appended to and forming part of this Supplementary Disclosure lists the results obtained from Iron 139 coating experiments. The coating procedure not only improved the sintered strength but increased the sintered density also. With g:reen densities in all of these materials of the order of 5.0 g/cc, the large shrinkage ~;- during sintering is probably associated primarily with the high concentration of "fines" in the comminuted scrap. The coating schedule ~4a represents the same coating conditions as A4 except that Fe2Oj was not added to the initial powder mixture for A4a samples. The higher combined carbon appears to have been responsible for the lower density.
Finally, the results obtained from coated and -uncoated commercial iron powders, atomized and sponge, are ~ -,~ . . .
- shown in Table V appended to and forming part of this ; Supplementary Disclosure. With no alloying elements present to ~ause oxidation problems during sintering, no effect would be expected. ~The unusual results obtained prompted examina-~ tion o tensile properties as well. These too are shown in Table V. ~ definite improvement, albeit small compared to the mechanically comminuted powders, is present in both , :
transverse rupture and tensile strengths for atomized iron.
Sponge iron, however, shows a definite deterioration of transverse strength-and little or no effect on tensile .
strength comparing coated~to uncoated forms.~ ;The coating treatments were short time~or t~in coatings, ~o be sure, - ~ . .
~ and coatings~ma~not have~been continuous but the dramatic ~ : . . ~ . : .
di~f~erence~in~effect between atomized and sponge is real.
3~ Other~atomized powders~and~regularly =haped powders, like carbonyl iron,~were coated~and~sintered also with simllar A ~ ~ ~ 21 ~ ~
~ ' :

~ L~6~5~3 ':
results i.e., a small but perceptible effect on sinte~ed properties.
The principal disclosure discloses the application of the treatment process to ferrous based metal particles, mainly machine scrap turnings. In accordance with this Supplementary Disclosure, the invention may be used in connection with hypoeutectoid steel or hypoeutectoid alloy steel which is normally ductile. Such material is firs~ subjected to heat treatment embrittlement procedures, as desc~ibed in detail and claimed in our copendi'ng application Serial No. a ~o, 1~ filed concurrently with this Supplementary Disclosure. The embrittlement procedure involves providing particles having a substantially martensite structure, preferably at least 80% SO, and according to this Supplementary Disclosura, the in~ention is also applicable to particles having a predominantly ~;
martensite structure.
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~6~5~3 TABLE I
A. Nominal Chemical Analyses (weight percent) Powder C Mn Si P S Ni Cr Mo Cu Cryo 138 0.20 0.42 0.33 0.03 0.03 O.d~4 0.58 0.14 0.26 Cryo 314 1.1~ 0.74 0.860.02 0O01 0.01 0.03 0.05 0.04 Cryo 319 0.53 0.80 0.25 Q. 02 0.02 0.01 0.03 0.05 0.01 Iron 139 1.57 0.62 2.300.03 0.15 0.03 0.04 0.05 0.31 Sponge0.11 0. 74 Q.86 0.010.01 - 0.03 0.04 0.04 Atomized 0.01 0. 270.040.01 0.03 0.04 0.04 0.04 0.14 .'. ~ . B. Sieve Analyses .:
- (weight percent) PowdarMesh Size .
~60 +100 +140 +200 ~325 -325 Cryo 138 60.5 17.05.4 7.2 5.0 5.0 Cryo 314 59.0 17.310.2 8.6 4.4 0.6 Cryo 319 80.4 10.64.0 4.5 0.5 Iron 139 -- 26.831.1 21.5 14.6 6.0 `: :
Sponge -- 6.225.5 39.2 21.0 8.1 Atomized -- 7.327.0 37.4 20.4 7.9 i .
.' ',.

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.

:

1C~6~3 TABLE II: CRYO 138 RESULTS

Sintering Conditions FinaI
Type Time Temp. Density T.R.S
Coating (min.) (C) Atm.(g/cc) (MPa) No Coating 40 1150 D.A.*6.9 399.2 No Coating 20 1121 Endo**6.8 121.4 No Coating 20 1121 Endo**6.5 113.8 No Coating 20 1135 Endo**6.5 141.3 No Coating 20 1149 Endo**6.6 151.7 A4 20 1121 Endo**6.7 648.1 B3 20 1121 Endo**6.5 655.0 B4 20 1121 Endo**6.4 931.8 C2 20 1121 Endo**6.8 742.3 D5 20 1121 Endo**6.4 813.6 * Dissociated Ammonia ** Endothermic Generator Gas (Dew Point = 7C) , '~
:
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~L~61~3 TABLE III: CRYO 314 AND CRYO 319 RESULTS
Sintered 20 min. at 112rUrC in Endothermlc Gas ~Dew Point 7C) FinalTrans. Rupture Density Strength Coating (g/cc)_ _(MPa) None 6.8 144.8 D2 6.7 586.1 None 6.7 17.2 D2 6.5 103.4 .- :
TABLE IV: IRON 139 RESULTS
Sintered 20 min. at 1121~C in Endothermic Gas (Dew Point 7C) - FinalTrans. Rupture Density Strength Coating (g/cc? (MPa) - None 4.7 50.3 A3 5.8 562.7 /~ A4 6.5 666.7 - 20 A4a 6.3 647.4 .

TABLE V: ATOMIZED AND SPONGE IRON RESULTS
Sintered 20 min. at 1121C in Endothermic Gas (Dew Point 7F) Trans. Rupture Tensile Density Strength Strength Coating (g/cc) (MPa? (MPa) ,~
None 6.6557.8 248.2 Atomized ~; Dl 6.4 587.4 303.4 None 6.3 724.0 243.4 Sponge Dl 6.2558.5 262.0 '~: : , ~: ~
: .

~ 25 -~ .

Claims (26)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making an intermediate powder, comprising:
(a) selecting metallic turnings which have a surface-to-volume ratio of at least 60:1, (b) simultaneously refrigerating to below the transition temperature of said turnings and impacting said turnings with a fracturing force, continuing said freezing and impaction for a period of time to comminute said turnings, (c) impacting said comminuted particles at ambient temperature conditions with an abrading force, said impacting being carried out by the use of elements laden with a protective metal having a hardness less than that of the comminuted particles to promote transfer of said protective metal to said particles upon impact between said elements and particles, said protective metal having a melting temperature below but substantially about the liquidus of said particles, said protective metal being relatively easy to abrade, and (d) continuing to carry out the impaction of step (c) to provide both coating of said particles with a thin envelope of said protective metal and cold working of substantially each particle having a size greater than 124 microns to thereby promote at least one defect site therein.

2. The method of claim 1, wherein the step of refrigeration is carried out by the use of liquid nitrogen, said impaction being carried out by the use of a ball mill having ball elements to impact with said particles.

3. The method of claim 1, wherein said ball elements of said ball mill are comprised of solid copper.

4. A method of making a powder compact, comprising:
(a) selecting metallic machine turnings comprised substantially of ferrous based material, said turnings having a surface-to-volume ratio of at least 60:1, (b) simultaneously refrigerating to below the transition temperature of said turnings and impacting said turnings with a fracturing force, continuing said freezing and impaction for a period of time to comminute said turnings to a powder, (c) repeatedly impacting a charge of said powder at ambient temperature with an abrading force, said impact being carried out using a plurality of protective metal laden elements having a transverse dimension at least 50 times the largest dimension of any particles of said powder, said protective metal havlng a hardness less than that of the powder to promote transfer of said protective metal to said particles upon impact between said elements and powder particles, said protective metal having a melting temperature below but substantially about the liquidus of said particles, said protective metal being relatively easy to abrade, (d) continuing to carry out the impaction of step (c) to simultaneously coat substantially each of said particles with a thin protective metal shell and to stress substantially each of said particles above 124 microns to effect cold working therein and to establish at least one defect site in each of said particles above 124 microns, and (e) compacting a predetermined quantity of said coated particles into a desired shape.

5. The method of claim 4, wherein said protective metal laden elements consist of solid copper balls having a diameter substantially about 0.5 inches, said copper balls operating within a revolving housing of a ball mill, said housing being rotated so as to impact said copper balls with said comminuted particles at a predetermined rate and stress frequency so as to produce said protective metal coated particles and defect sites therein.

6. A method of making sintered shapes, comprising:
(a) selecting metallic turnings which have a surface-to-volume ratio of at least 60:1, (b) simultaneously refrigerating to below the transition temperature of said turnings and impacting said turnings with a fracturing force, continuing said freezing and impaction for a period of time to comminute said turn-ings to a powder, (c) repeatedly impacting a charge of said powder particles with a plurality of elements laden with a protective metal having a hardness less than that of the coated powder to promote transfer of said protective metal to said particles upon impact between said elements and particles, said protective metal having a melting temperature below but substantially about the liquidus of said particles, said protective metal being relatively easy to abrade, each of said elements having a transverse dimension at least 50 times the longest dimension of any of said particles, (d) continuing to carry out the impaction of step (c) to simultaneously coat substantially each of said particles with a thin envelope of said protective metal and to stress substantially each of said particles above 124 microns to effect cold working therein and to provide at least one defect site in substantially each of said particles having a size above 124 microns, (e) compacting a predetermined quantity of said impacted particles into a desired shape, and (f) heating said shape to at least the plastic region for said metal particles in an atmosphere at least non-oxidizing to said envelope to permit atomic diffusion to take place between particles in adjacent contact within said shape, said envelope about each of said particles prohibiting oxidation of the ingredients of said particles during said heating, and said defect sites promoting increased diffusion.

7. The method of claim 6, wherein said protective metal laden elements comprise solid copper spheres having a diameter of at least 0.1 inch.

8. The method of claim 6, wherein said charge of metallic particles is comprised of both coarse and fine particles, each of said particles having an irregular flake configuration prior to impaction, the fine particles constituting no more than 50% of said total particle volume, said fine particles being substantially devoid of defect sites and cold work after said impacting step while said coarse particles each having at least one defect site after impacting.

9. The method of claim 6, wherein said charge of metallic particles is comprised of ferrous based particles some of which contain manganese and/or silicon in solid solution, said thin envelope about each of said particles operating to prevent oxidation of said manganese and silicon during said heating step.

10. The method of claim 6, wherein said impacting step is carried out with the use of a ball mill, the rate and frequency of contact between said metal particles and the elements of said ball mill being adjusted to achieve a rate of stress over a period of time to achieve said at least one defect site in the particle sizes above 124 microns.

11. The method of claim 6, wherein said compaction step is carried out to produce a green density in said shape compact of at least 6.4 g/cc by the use of 30 tsi.

12. The method of claim 6, wherein said shape has a first volume as a result of said compacting step and has a second volume as a result of said heating step, the difference between said first and second volumes being at least 10%.

13. The method of claim 6, wherein said charge of metal particles is comprised of randomly irregular ferrous particles each of which are non-spherical, said shape being heated to a sintering temperature of 2050°F and held at said sintering temperature for a period of at least 20 minutes, whereby shrinkage between the cold compacted shape and said sintered shape is at least 7.0%.

14. The method of claim 6 wherein said elements impacting said metallic particles cause a local cleansing of the metallic particle surface at the area of impact while simultaneously transferring some of the protective material.

15. A powder compact consisting essentially of a uniform and homogeneous mixture of ferrous-based first particles having a size range +200-325 mesh and ferrous-based second particles having a size range of +60-140 mesh, one of said first and second particles having a protective metal envelope about substantially each of the particles thereof, said protective metal having a hardness less than that of the particles, a metal temperature below but substantially about the liquidus of said particles and being relatively easy to abrade, and substantially each of the particles of said first particles having at least one stress defect site therein, said compact having a density of between 6.6-6.7 g/cc and a volume shrinkage of 7-10% upon being heated to 20-50°F.

16. The product of claim 15 wherein said protective metal is copper.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

17. The method claimed in claim 1, 4 or 6 wherein said protective metal is iron.

18. The method of claim 6 wherein said protective metal laden elements comprise solid iron spheres having a diameter of at least 0.1 inch.

19. The method of claim 2 wherein said ball elements of said ball mill are comprised of solid iron.

20. The method of claim 4 wherein said protective metal laden elements consist of solid iron balls having a diameter of substantially about 0.5 inches, said iron balls operating within a revolving housing of a ball mill, said housing being rotated so as to impact said iron balls with said comminuted particles at a predetermined rate and stress frequency to produce said protective metal coated particles and defect sites therein.

21. The product of claim 15 wherein said protective metal is iron.

22. The method of claim 6 wherein the metal particle charge is comprised of ferrous based particles some of which contain at least one of manganese, silicon, chromium or vanadium in solid solution, the thin envelope about each of the particles operating to prevent oxidation of the manganese and silicon during the heating step.

23. The method of claim 1, 4 or 6 wherein said metallic particles or turnings have a substantially martensitic structure.

24. The product of claim 15 wherein said ferrous particles are martensitic ferrous-based particles.

25. The method of claim 6 wherein said metallic particles initially had a hypoeutectoid composition which was heat treated to provide a substantially martensitic structure.

26. The method of claim 1 or 4 wherein said metallic turnings have a hypoeutectoid composition and, prior to said refrigeration and impaction step, said turnings are heat treated to provide a martensitic structure therein of at least 80%.