GB1580378A - Method of making sintered parts - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 580 378

O O ( 21) Application No 21083/77 ( 22) Filed 19 May 1977 Of > ( 61) Patent of Addition to No 1498359 dated 18 May 1976 ( 19) A.

( 31) Convention Application No 735468 0 ( 32) Filed 26 Oct 1976 in ( 33) United States of America (US) ( 44) Complete Specification published 3 Dec 1980 ( 51) INT CL 3 B 22 F 9/04 ( 52) Index at acceptance C 7 D 8 M 8 Q 8 R Al ( 54) METHOD OF MAKING SINTERED PARTS ( 71) We, FORD MOTOR COMPANY LIMITED, of Eagle Way, Brentwood, Essex CM 13 3 BW, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to the processing of scrap material, such as machine tool 5 swarf, into sintered powder compacts, and is a modification of the inventions described in our earlier British Patents Specifications Nos 1498359, 1494887 and

1508941.

The first two of these prior applications relate to processes in which the waste material is refrigerated, as by liquid nitrogen, to a temperature below its 10 ductile/brittle transition temperature and is impacted to reduce it to powder In both, the powder is then cold-worked by ball milling at ambient temperature and is simultaneously coated with copper In 1494887, copper balls are used as milling media in the ball mill and provide the copper coating by abrasion, while in 1498359 the milling medium comprises iron balls and the copper coating is derived from 15 copper powder also present in the ball mill.

In the case of 1508941, a similar cryogenically comminuted, cold worked and copper coated powder is produced by circulating a mass of liquid nitrogen, scrap, and copper spheres or cylinders within a rotating body such that in one zone a slurry exists in which fragmenting occurs while in another zone the metal pieces are 20 withdrawn from the liquid nitrogen to momentarily increase in temperature allowing cold working to occur, while copper coating occurs in both zones.

In each case the resulting powder is compacted and sintered, the copper coating permitting an improved sintering process.

The present invention is based on the discovery that a similar improvement in 25 sintering can be achieved by providing a coating on the cryogenically comminuted particles which is wholly or partially of nickel or molybdenum.

The present invention accordingly provides a process for producing a powder for sintering from scrap metal, in which scrap metal pieces are refrigerated to a temperature below the ductile/brittle transition temperature thereof and are 30 simultaneously subjected to impacts to reduce them to a cryogenically comminuted powder, and the cryogenically comminuted powder is impacted at a temperature above said ductile/brittle transition temperature with elements laden with nickel or molybdenum.

Embodiments of the invention will now be described, by way of example, with 35 reference to the accompanying drawings, in which:

Figure 1 is a schematic flow diagram of one method embodying the invention; Figure 2 is a graphical illustration of metal-oxide equilibria; Figure 3 is a central elevational sectional view of one type of batch apparatus suitable for use in carrying out the invention; 40 Figure 4 is an end elevation, partly broken away, of the apparatus of Fig 3; and Figure 5 is a central elevational sectional view of an apparatus suitable for a continuous process embodying the invention.

Detailed Description

Cryogenic Embrittlement 45 A preferred mode of carrying out the method of the invention is depicted in Figure 1 and is as follows:

( 1) Scrap metal and particularly machine turnings are selected as the starting material "Machine turnings" is defined herein 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, silicon, chromium, nickel and molybdenum The turnings should be selected to have a 5 surface-to-volume ratio of at least 60: 1, which is characteristic of machine turnings.

The scrap pieces will have a size characterized by a width 1-1 0 ", thickness of 005- 03 ", and a length of 1-100 " 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 10 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 compatible in chemistry as combined in the final product; this is achieved optimally when the scrap is selected from a common machining 15 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 pieces Mere contact of the liquid 20 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 iron ball milling elements 14 are motivated preferably 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 is carried out to apply 25 sufficient fracturing force (defined to mean less than I ft -lb) and a sufficient period of time and rate to reduce said scrap pieces to a powder form The powder 16 will typically have both a coarse and a fine powder proportion The fine particles preferably have a size range from 200 to 325 mesh and constitute no more than 500,, of the total particle volume, whilst the coarse particles have a size range from 60 to 30 mesh Both proportions will be comprised of particles which are flake or layered in configuration; each particle will be highly irregular in shape and dimension, none being spherical in shape A typical screen analysis for the powder 16 after step ( 2) would be as follows (for a 100 gram sample):

No Milling After 72 Hrs 3 r Mesh (in grams) (in grams) 60 0 31 5 19 5 11 0 5 5 7 5 200 6 5 18 0 40 325 4 5 22 5 325 4 0 9 5 ( 3) The comminuted cryogenic 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 elements 19 45 laden with a coatable agent described later The elements are preferably in the form of solid balls of about 5 inch in diameter The coatable protective agent should be characterized by (a) a hardness less than that of the coated powder to promote transfer to said particles upon impact between said elements and particles, (b) being completely soluble m F the metal of the particles, (c) easy to abrade, and (d) 50 acts as an oxidation barrier.

In trials 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 of the balls and the speed of rotation.

The function of this second impacting step is two-fold: to provide an oxidation 55 barrier on each powder particle and to cold work each coarse particle The balls transfer, by impact, a portion of the ingredient carried by the ball milling elements 19 so as to form a shell about substantially each particle of the powder 16 The finer powder will obtain a coating by true abrasion or scratching with the surface of the ball milling elements 19 Ball milling elements 19 should have a diameter at least 50 60 times the largest dimension of any of the particle shapes of the cryogenic powder 16 The ball milling operation also must generate non-natural defect sites (resulting from cold working) in substantially all powder particles above 124 microns; the ball I 1,580,378 3 1,580,378 3 milling operation herein should be carried out so that substantially each coarse particle has at least one defect site therein This can be accomplished by rotating the housing to impart a predetermined abrading force from the balls 19 Assurance of cold work in substantially all coarse particles is obtained by following ball milling techniques described in the Handbook for Chemical Engineers, Ball milling 5 section, incorporated herein by reference; physical stressful contact can be made between all coarse particles by observing the relationship between time, mass and container shape.

When this step is completed, the particles will be in a condition where substantially all will have a continuous envelope (coating or shell) and be stressed 10 sufficiently so as to have a high degree of cold work The term "defect site" is defined herein to mean a defect in local atomic arrangement The term "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 15 absolutely impervious It has been shown, by the test examples performed in connection with reducing this invention to practice, that cold working of the particles is predominantly influential in increasing diffusion kinetics of this invention the coating or shell operating to predominantly form an antioxidation barrier 20 ( 4) A predetermined quantity of powder conditioned from step ( 3) is compacted by a conventional press 20 to a predetermined density, preferably about 6.6 g /cc This is brought about by the application of forces in the range of 30-35 tsi The presence of the envelope about the powder particles improves compressibility With prior uncoated powders, a density of about 6 4 g /cc is 25 typically obtained using a compressive 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 which such powder is compacted is designed to have an outer configuration larger than that desired for the final part A significant and highly improved shrinkage takes place as a result of the next step ( 5); the shrinkage 30 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 01500.

( 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 35 21000 F, for ferrous based cryogenic powder The temperature to which the compact is heated should be at least the plastic region (typically sintering temperature) for the metal constituting the powder A controlled or protective atmosphere is maintained in the furnace, preferably consisting of inert or reducing gases 40 At the sintering temperatures, atomic diffusion takes place between particles of the powder particularly at solid contact points therebetween, certain atoms of one particle are supplied to fill the defect sites or absence of certain atoms in the crystal structure of the contacting particle, said defect sites being present as a result of cold working in step ( 3) Diffusion is accelerated to such an extent, that an 45 increase of more than 100 times is obtained It is theorized that at least 60 , of the improvement in physical properties of the resulting sintered shape is due to the controlled cold working of the coarse powder particles The increased diffusion is responsible for the increase in shrinkage.

The envelope on the particles serves to essentially prevent oxidation of certain 50 elements or ingredients within the powder particles, particularly manganese and silicon With typical ball milling parameters (physical size of mill, speed change and ball size) sufficient to the job, it can be statistically calculated that substantially each particle of the cryogenic powder will possess an impervious copper or iron shell However, a totally impervious shell is not absolutely essential to obtaining an 55 improvement of some of the properties herein.

The problem of prevention of oxidation of alloying elements dissolved in the iron powder is a kinetic one involving diffusion through the coating material to the particle 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 60 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 65 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 free.

Selection of a coating that will not oxidize in endothermic gas atmospheres, or 5 at least not have a stable form at sintering temperatures, may be made from equilibrium thermodynamic data for metal-oxide reactions The Diagram shown in Figure 2 is a plot of this type of data for metals of interest here With the natural logarithm of the equilibrium oxygen pressure of the atmosphere gas plotted versus temperature, each of the equilibrium lines shown describes the temperature and 10 oxygen pressure conditions required to cause dissociation of the metal oxide into metal+oxygen gas The atmospheres with greater oxygen pressures or potentials than the dissociation pressure of an oxide lie above the equilibrium line and are in the region of oxide stability Atmospheres with oxygen pressures less than the dissociation pressure lie below the equilibrium line and favor reduction of the oxide 15 to pure metal The cross-hatched areas shown in Figure 5 represent the operational ranges of endothermic gas generators and dissociated ammonia atmospheres.

Note that the data shown here indicates why the common alloying elements in steel, other than copper, nickel and molybdenum, are not particularly well suited for iron sintering furnace atmospheres Because of this the choice of coating 20 materials (for ferrous scrap) from the elements shown reduces to copper, nickel, molybdenum or iron itself The use of iron and copper is already disclosed and claimed in our earlier patent specifications.

A number of additional powders were examined Some of these powders, their nominal chemistry and screen analyses, are listed in Table 1 The first three 25 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 30 chips to high impact loads at temperatures below their ductile-to-ductile fracture transition The combination of high impact loading and low temperature seemed to reduce the powder shape characteristics associated with mechanical comminution of ductile materials considerably These materials, after comminution, were given a decarburizing anneal to reduce carbon levels below 0 1 / by weight 35 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 319 differed 40 inadvertently in silicon concentration as well as in the intended particle size distribution.

The cast iron swarf powder (Iron 139) 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; 45 probably carried over from the grinding operation The cast iron powder was not given a decarburising anneal Decarburization was accomplished during sintering by additions of Fe 2 03 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, 50 processing of the coated powders was the same as for uncoated powders Powders were blended with 1 zinc stearate and sufficient graphite to achieve a final combined carbon concentration of 0 6-0 8 % In the case of the cast iron, 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 55 rupture bars or tensile bars Pressures used were kept constant at 414 M Pa ( 30 tsi) and green densities were recorded as a measure of compressibility Sintering was accomplished in endothermic gas atmospheres using a thermal cycle of 30 minutes at 788 VC ( 14500 F) for burnoff of lubricant and 20 minutes at 11210 C ( 205001 F) for sintering, followed by a controlled cool to room temperature 60 The sintered transverse rupture strengths (T R S) of each powder were used as measures 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-0 7 , carbon sintered alloy at a density of 6 7 g /cc 65 I 1,580,378 1,580,378 5 will have a T R S of the order of 550 M Pa ( 80 ksi), while the same alloy with 1 5 / copper, admixed can attain a T R S of 786 M Pa ( 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 Although copper was used as coating material, the results of the 5 experiments are reproduced below to illustrate the effect of the process The results are summarised in Table II The first five entries represent attempts at sintering without prior coating The data demonstrates the difficulty encountered in attaining acceptable property levels with ordinary wrought steel chemistry The best situation, sintering at 1 1501 C for 40 minutes in dissociated ammonia produces 10 transverse rupture strengths only 73 O/ of 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 rupture strengths after sintering The data presented for coated samples in Table II describes only the effects of coating variables The code indicated in the table has coating treatment 15 indicated by 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 highest strength Although it is not evident from the sintered densities shown, the B 4 sample differed from the B 3 in the amount of shrinkage which occurred during sintering; the B 4 having shrunk significantly more 20 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 plating and the adherence of the coating From the data in Table II it is evident that B 4, C 2, and D 5 treatments all provide properties equivalent to or better than the best 25 commercial iron powder with 1 5 % 4 copper admixed Analysis of the Cryo138 powder indicates about 1 % coating material present in samples with the thicker coatings.

Table III contains the data obtained from powder produced from essentially a plain carbon steel swarf Cryo-314 is the finest particle size distribution examined 30 from this material, Cryo 319, the coarsest Once again the effect of coating is quite marked although the properties are not as high as for the Cryo-138 The Cryo-314 is, however, within the range of commercial iron powders without copper additions The Cryo-319 is obviously too coarse and thus has too few interparticle contacts to provide adequate strength The contacts existing did, however, sinter 35 satisfactorily as indicated by the data The abnormally high silicon in Cryo-314 did not appear to have influenced the sintering process.

Table IV 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 green densities in all of these materials of the order of 40 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 A 4 a represents the same coating conditions as A 4 except that Fe 2 03 was not added to the initial powder mixture for A 4 a samples The higher combined carbon appears to have been responsible for the lower density 45 Finally, the results obtained from coated and uncoated commercial iron powders, atomized and sponge, are shown in Table V With no alloying elements present to cause oxidation problems during sintering, no effect would be expected.

The unusual results obtained prompted examination of tensile properties as well.

These too are shown in Table V A definite improvement, albeit small compared to 50 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 thin coatings, to be sure, and coatings may not have been continuous but the 55 dramatic difference in effect between atomized and sponge is real Other atomized powders and regularly shaped powders, like carbonyl iron, were coated and sintered also with similar results i e, a small but perceptible effect on sintered properties.

One Step or Continuous Embrittlement 60 (I) Scrap metal is selected to have the proper size and desirable chemistry as outlined in the preferred embodiment Other types or larger pieces of scrap metal can be used, but capital investment costs will increase due to the difficulty of impacting scrap metal sized in pieces beyond 03 inches thick The scrap pieces 6 1,580,3786 should be selected to be generally compatable in chemistry as desired 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) Although not critically necessary, it is preferable to degrease the ferrous 5 based pieces by conventional modes which may include vapor degreasing or dipping the pieces in a solvent bath usually containing benzene or methylethylketone However, with the type of cryogenic processing taught herein, it is now possible to remove oil and other organic materials without any separate cleaning.

This occurs as a result of allowing the organic material to freeze upon being 10 subjected to cryogenic temperature levels The frozen material can then be removed during or after impaction by ball milling elements; the frozen organic debris can be screened and separated as an inherent result of this process.

( 3) A circulating mass of said ferrous based pieces 31 in a predetermined path 32 is defined This is conveniently provided by introducing the scrap pieces to a ball 15 milling drum 34 having an insulating body 34 a encased between metal walls 34 b and 34 c, as specifically shown in Figures 3 and 4 The scrap pieces (here shown as comminuted in Figure 3) and milling elements 35 are loaded into the drum 34 prior to closure of cover 34 d; liquid nitrogen is added later through conduit 36 The circulating mass is stimulated by the rotary movement (such as in direction 38) of the drum of the ball 20 milling machine 34 The circulatory path 32 can be selected by the uniform or nonuniform rotary speed of the drum Liquid nitrogen is introduced to the drum in a predetermined quantity to provide a liquid level 37 slightly below the top surface 39 of the slurry created by the composite of the pieces and liquid nitrogen As the drum is rotated at a predetermined speed, the liquid nitrogen will be generally 25 retained in a zone A, often assuming a crescent shape silhouette as shown in Figure 4 The ferrous based scrap pieces will be influenced somewhat differently and will undergo a circulatory movement as indicated in path 32 which rises above the liquid nitrogen in zone B Thus, for a portion of the circulation along path 32, the ferrous spaced pieces will not be exposed to the liquid nitrogen As a result of both 30 the heating (experienced by collision between the ferrous based pieces and the ball milling elements 35 causing a release of energy and the divorce from the liquid nitrogen, the ferrous based particles will experience an increase in temperature in zone B such that they will be momentarily above the ductile-brittle transition temperature of the particles Upon return to zone A, of course, the ferrous based 35 particles will again be contacted by liquid nitrogen to be reduced below the ductilebrittle transition temperature.

( 4) Impaction of said ferrous based pieces or particles is inherently carried out by the circulatory movement which collides one particle against the other For the purposes of fragmenting the pieces at the sub-brittle temperatures, milling 40 elements 35 are employed in the form of balls having a diameter of at least 0 1 inches and preferably about 5 inches Cylindrical rods or segments can also be employed Such balls are constituted of nickel or molybdenum which have melting temperatures below, but substantially close to the liquidus of said ferrous based particles Said protective metals are completely soluble in the material of which 45 said particles are constituted, are relatively easy to abrade and can be abraded at the sub-brittle temperatures of this method Thus, in zone A, the impacting elements, preferably in the form of balls, will impart a fracturing force to the particles causing them to separate into a classifiable comminuted condition At the same time, each of the fragmented particles will receive an infinitesimal portion of 50 the ball upon each collision, which when multiplied by a large number of repeated collisions will form a partial envelope or layer on the outer surface of each particle.

Upon the occurrence of a predetermined number of circulatory revolutions, it has been determined that a complete envelope or protective metal envelope is formed upon each particle The time necessary to achieve such complete envelope is a 55 function of milling time and rate which in turn is dependent upon mill volume, mill diameter, size of balls, and the speed of drum rotation.

A temperature of -40 Celsius or less is achieved by introducing the liquid nitrogen through said feed conduit 36 extending through one end, such as the cover 34 d, and substantially coincident with the axis of rotation Liquified nitrogen is 60 employed, although other mediums which may be used include dry ice with acetone or other organic liquids A vent is provided in one location of the milling chamber to exhaust gaseous nitrogen as it evaporates.

In zone B of the circulatory movement, the ball milling elements will impart sufficient cold work to the comminuted particles to generate defect sites in 65 1,580,378 substantially all particles of about 1-4 microns; the ball milling operation herein should be carried out for a sufficient time so that substantially each coarse particle has at least one defect site therein. When these steps are completed, the particles (in powder form) are

separated from the slurry, the resulting powder will have all particles coated with a 5 continuous envelope (shell) and each particle ( 1-4 microns) will be sufficiently stressed so as to have a high degree of compactability The term "defect site" is defined herein to mean a defect in local atomic arrangement The term "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 impervious 10 and continuous about each particle, it is not critical and that it be absolutely impervious.

It has been demonstrated by test examples, performed in connection with reducing this invention to practice, that cold working of the particles predominantly influences diffusion kinetics when sintering powder of this 15 invention, the coating or shell operating to predominantly perform an antioxidation barrier during sintering of the powder herein.

( 5) The intermediate or resulting product from the above steps can then be subjected to powder metallurgy techniques A predetermined quantity of conditioned powder is compacted by a conventional press to a predetermined 20 density, such as preferably 6 6 g /cc This is brought about by the application of forces in the range of 30-35 psi The presence of the envelope about the powder particles improves compressability With prior uncoated powders, a density of about 6 4 g /cc is typically obtained using a compressive force of 85,000 psi; with the powder herein, densities of about 6 6 g /cc are now obtained at the same force 25 level.

The shape into which such powder is compacted should have an outer configuration slightly larger than that desired for the final part A significant and highly improved shrinkage takes place as the result of the sintering step The shrinkage is a predetermined factor and allowance can be made for it in the 30 compacted shape Shrinkage will be in the controlled limits of 002 inches/inch.

( 6) The compacted shape is subjected to a sintering treatment within a furnace wherein it is heated to a temperature preferably in the range of 20002100 'F for ferrous based cryogenic powder The temperature to which each compact is heated should be at least to the plastic or sintering region for the metal constituting the 35 powder A controlled or protective atmosphere is maintained in the furnace, which may be inert or reducing Typically, sintering of ferrous material is effected at 20500 F for 20 minutes.

At the sintering temperature, atomic diffusion takes place between particles of the powder, particularly at solid contact points Certain atoms of one particle are 40 supplied to fill the defect sites (absence of certain atoms in the crystal structure of the contacted particle) said defect sites being present as a result of cold working.

Diffusion is accelerated to such an extent, that an increase of more than 100 times is obtained It is theorized that at least 60 , of the improvement in physical properties of the resulting sintered shape is due to the controlled cold working of 45 the powder The increased diffusion is responsible for the increase in shrinkage.

Preferably sintering produces a 10 o% difference in volume of the compact before and after heating.

The process of this invention can be carried out continuously, such as by an apparatus shown in Figure 5 The cylindrical apparatus 40 has rifling or ribs 41 50 spirally located about the interior 42 a of the continuous processing drum 42 Liquid nitrogen is introduced at one end 42 b of the drum through a conduit 43 preferably aligned with the axis of the drum Another conduit 44 is also arranged along the axis to introduce scrap material in the form of machine turnings or comparable ferrous scrap material The drum is inclined at an angle to the horizontal preferably 55 in the range of 1 to 20 so that the slurry 45 comprised of liquid nitrogen 46, ball milling elements 47 and the ferrous scrap material 48 will undergo a transcillatory as well as a rotary movement along the length of the drum 42 and about the diametrical interior of the drum At the opposite end 42 c of the processing drum, an exit opening 49 is provided which is covered by a sieve 50 effective to allow exit 60 of processed particles only of a certain size which rise to the surface of the slurry mixture Here again, the ferrous particles will undergo a circulatory movement along a path 51 which includes a zone C when the particles are immersed in the liquid nitrogen (with their temperature below the ductile-brittle transition point) and a zone D when the particles are lifted momentarily out of the slurry as a result 65 I 1,580,378 of the rotary action During this momentary exposure, the ferrous based particles will experience an increase in temperature to above the ductile-brittle transition temperature (but below ambient temperature conditions) to permit imparting cold work to the particles The exit opening 49 permits discharge of gasified nitrogen as well as a small portion of the liquid nitrogen It has been found that regulation of 5 the opening shape between wires of the screen is important as well as the mesh size of the screen To this end, -30 mesh has been found preferably to achieve a type of powder which has optimum sinterability and compaction characteristics The sieve opening shape should be preferably square shaped.

TABLE I 10

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 0 44 0 58 0 14 0 26 Cryo 314 1 12 0 74 0 86 0 02 0 01 0 01 0 03 0 05 0 04 15 Cryo 319 0 53 0 80 0 25 0 02 0 02 0 01 0 03 0 05 0 01 Iron 139 1 57 0 62 2 30 0 03 0 15 0 03 0 04 0 05 0 31 Sponge 0 11 0 74 0 86 0 01 0 01 0 03 0 04 0 04 Atomized 0 01 0 27 0 04 0 01 0 03 0 04 0 04 0 04 0 14 B Sieve Analyses 20 (weight percent) Mesh Size Powder + 60 + 100 + 140 + 200 + 325 -325 Cryo 138 60 5 17 0 5 4 7 2 5 0 5 0 Cryo 314 59 0 17 3 10 2 8 6 4 4 0 6 25 Cryo 319 80 4 10 6 4 0 4 5 0 5 Iron 139 26 8 31 1 21 5 14 6 6 0 Sponge 6 2 25 5 39 2 21 0 8 1 Atomized 7 3 27 0 37 4 20 4 7 9 TABLE II: Cryo 138 Results 30 Sintering Conditions Final Type Time Temp Density T R S.

Coating (min) ( C) Atm (g/cc) (M Pa) No Coating 40 1150 D A 6 9 399 2 No Coating 20 1121 Endo 6 8 121 4 35 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 A 4 20 1121 Endo 6 7 648 1 B 3 20 1121 Endo 6 5 655 0 40 B 4 20 1121 Endo 6 4 931 8 C 2 20 1121 Endo 6 8 742 3 D 5 20 1121 Endo 6 4 813 6 Dissociated Ammonia Endothermic Generator Gas (Dew Poi t= 7 C) 45 TABLE III: CRYO 314 and CRYO 319 Results Sintered 20 min at 1121 C in Endothermic Gas (Dew Point 7 C) Final Trans Rupture Density Strength 50 Coating (g/cc) (M Pa) None 6 8 144 8 314 D 2 6 7 586 1 None 6 7 17 2 55 319 D 2 6 5 103 4 1,580,378 TABLE IV: Iron 139 Results Sintered 20 min at 11210 C in Endothermic Gas (Dew Point 7 IC) Final Trans Rupture Density Strength 5 Coating (g/cc) (M Pa) None 4 7 50 3 A 3 5 8 562 7 A 4 6 5 666 7 A 4 a 6 3 647 4 10 TABLE V: Atomized and Sponge Iron Results Sintered 20 min at 11210 C in Endothermic Gas (Dew Point 7 IF) Trans Rupture Tensile Density Strength Strength 15 Coating (g/cc) (M Pa) (M Pa) None 6 6 557 8 248 2 Atomized Dl 6 4 587 4 303 4 None 6 3 724 0 243 4 20 Sponge Dl 6 2 558 5 262 0

Claims (1)

1. WHAT WE CLAIM IS:-

   1 A process for producing a powder for sintering from scrap metal, in which scrap metal pieces are refrigerated to a temperature below the ductile/brittle 25 transition temperature thereof and are simultaneously subjected to impacts to reduce them to a cryogenically comminuted powder, and the cryogenically comminuted powder is impacted at a temperature above said ductile/brittle transition temperature with elements laden with nickel or molybdenum.

   2 A process according to claim 1, in which the scrap metal is ferrous 30 3 A process according to claim I or claim 2, in which the scrap metal pieces are turnings having a surface-to-volume ratio of at least 60:1.

   4 A process according to any preceding claim, in which the scrap is refrigerated by contacting it with liquid nitrogen.

   5 A process according to any preceding claim, in which the scrap metal is 35 cryogenically comminuted in a ball mill, and is thereafter treated at ambient temperature in a further ball mill using nickel or molybdenum balls as milling media.

   6 A process according to claim 5, in which the balls have a diameter at least 50 times the largest dimensions of any particles of said cryogenically comminuted 40 powder, and the cold working stresses each of said particles above 124 microns to establish at least one non-natural defect site therein.

   7 A process according to claim 5 or claim 6, in which said balls have a diameter of at least 0 1 inch.

   8 A process according to any one of claims I to 4, in which the comminuting 45 of the scrap and the impacting of the comminuted powder with the metalladen elements is carried out by circulating said scrap and said elements and a refrigerant to establish a first zone below said ductile/brittle transition temperature in which comminution of the scrap occurs and a second zone above said transition temperature in which cold working of the comminuted particles occurs 50 9 A process according to claim 8, in which said circulation is carried out continuously in an inclined cylinder undergoing movement to translate and agitate said particles, the refrigerant comprising liquid nitrogen introduced into the high end of the cylinder to form a slurry in said first zone.

   10 A process according to claim 8 or claim 9, in which said elements are balls 55 having a diameter of at least 0 1 inch.

   11 A process according to any of claims 8 to 10, in which the cold work carried out in said second zone is effective to impart a defect site in each of the powder particles having a size above 124 microns.

   12 A process according to claim 9, in which the comminuted and cold worked 60 particles are separated from the slurry by the use of a sieve having a mesh size of 30.

   1,580,378 13 A process according to any preceding claim, including the further steps of compacting the coated powder to a desired shape, and heating the compact to at least the plastic region and below the melting temperature of the scrap metal material to sinter the compact.

   14 A process according to claim 13, in which the compaction step is carried 5 out on ferrous material to produce a green density in said compact of at least 6 4 gm/cc by a compacting pressure of 30 tsi.

   A process according to claim 13 or claim 14, in which the volume difference of said compact before and after heating is at least 1000.

   16 A process according to any of claims 13 to 15, in which the scrap material is 10 ferrous and the compact is sintered at 2050 OF for at least 20 minutes.

   17 A process according to any of claims 13 to 16, in which the powder compacted and heated comprises fine particles having a size range -200 + 325 mesh and coarse particles having a size range -60 + 140 mesh, the fine particles constituting no more than 5000 of the total particle volume 15 18 A process according to any of claims 13 to 17, in which said powder is decarburised prior to compaction.

   19 A sintered powder compact produced by the process of any of claims 13 to 18.

   PETER ORTON, Chartered Patent Agent.

   Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa 1980 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.

   1.580 378 In