US2927015A - Method of producing low density iron powder - Google Patents

Description

March 1, 1960 s. K. WELLMAN METHOD OF PRODUCING LOW DENSITY IRON POWDER Filed March 26, 1956 5 Sheets-Sheet 1 INVENTOR.

I l I l I l I 5 Sheets-Sheet 2 S. K. WELLMAN METHOD OF PRODUCING LOW DENSITY IRON POWDER FIG. 2

March 1, 1960 Filed March 26, 1956 March 1, 1960 s. K. WELLMAN METHOD OF PRODUCING LOW DENSITY IRON POWDER Filed March 26, 1956 5 Sheets-Sheet 3 INVENTOR. SAMUEL K. WELLMAN ATTORNEY March 1, 1960 s. K. WELLMAN METHOD OF PRODUCING LOW DENSITY IRON POWDER 5 Sheets-Sheet 4 Filed March 26, 1956 uvm'vroa. K. WELLMAN SAMUEL abkzy7gwdm FIG. 6

ATTORN EY METHOD or PRODUCING Low nnrrsrrr men rownnn Samuel K. Wellrnan, Cleveland Heights, Gino, assiguor to The S. K. Wellman Company, Redford, Ghio, a corporation of Ohio Application March 26, N56, erial No. 573,708

1 (Ilaim. (Ci. 75.)

This invention relates to improvements in iron powder and articles made therefrom and has particular significance in connection with friction composition products comprising articles of a novel low density iron powder, the powder itself and methods and apparatus for producing the latter.

Substantially pure iron powder has a wide variety of present day uses being, for example, useful for the ultimate production of sintered predominantly metallic friction facing articles for brakes, clutches, automatic transmissions and the like.

Pure iron does not exist in nature. Substantially pure iron having less than several percent of impurities such as oxide and carbon can of course be produced by man and machine, but all substantially pure iron powder that has been known has been characterized by what I consider to be a relatively high density. That is, it is not a soft powder capable of containing a high percentof non-metallic additive as required and of then being pressed at low molding pressure (e.g., at 12.5 t.s.i.) and then sintered to form a structural part strong enough for some applications. As an example, friction facings of sintered predominantly metallic powders have in the past had to be bonded to solid metal backings because the prior art facings were too brittle to stand the force of rivets or otherwise to be regarded as structural parts in and of themselves. This is largely due to poor characteristics of the powders and articles made from them as regards such desiderata as high compression ratio, low molding pressure, good green and sintered strength, high ultimate hardness and proper final density, to adequately protect the article against breakage and chipping both during manufacture and during use.

While it has long been known how to produce iron powder by reducing iron oxides, either natural oxide ore (Fe O or mill oxide known as mill scale (Fe O by heating in the presence of a reducing agent, either gaseous or in the form of a solid such as graphite, charcoal, coke or coal, the practice of such processes have heretofore resulted in a product unsuitable for many applications and such processes have been attended by great expense of materials and of labor and by non-uniformity of results, both as between batches and within any single batch. known as the Chinese method, a mixture of ore, coal and coke is enclosed in small diameter clay tubes which are surrounded by fuel which is set fire and burned for several days to accomplish reduction. vThen the tubes are broken and the reduced iron separated from the clay, an expensive procedure resulting in a relatively high density product, as the words low and relatively high are hereafter explained. In an alternate Swedish process, iron ore is packed in alternate layers with coal and coke in cylindrical containers which are then placed in a furnace where hot gases are passed over them to accomplish the reduction. There still remains the problems of charge density and removing the charge from the containers which are not then usually suitable for re-use, and there According to one method, sometimes.

2,9219% Patented Mar. 1, 1950 2 have heretofore alsobeen additional problems related to lengthy time requirements, inordinate manpower requirements, and to other factors contributing to the expense of the prior art processes and apparatus, to surface oxidation of the reduced charge during cooling, to non-uniformity of results both throughout a single charge and with successive charges, to impurity and brittleness (hardness) of any powder produced, and to consequent difliculties in further processing, eitherduring grinding or during or after mixing, molding and heating powders to produce friction facings.

The metallic product formed by the reduction of iron oxides at temperatures below the fusion point of iron is well known and commonly called sponge iron. That which -I presently propose is a sponge iron product but it has a density lower than that of such products as heretofore known, which prior products may therefore now be referred to as of relatively high density.

One difliculty with most metallic powders produced according to the prior sponge iron art has been that after subsequent compression (with or without sintering) they have not resulted in articles of very great structural rigidity and have lacked other desired properties. For example, friction materials usedfor lining'facings in brakes, clutches, automatic transmissions, and the like must be selected and compounded with many factors in mind if customer acceptance and satisfactory operation are to be assured. These factors include, among others, magnitude of the coefiicient of friction, cost of the materials and of their compounding and assembly, wear of the friction material, wear of the surface which the friction material engages, noise or quietness of the material in operation, change in the magnitude of the coefficient of friction with changes in applied pressure, or in temperature, humidity, or relative speed of the engaging parts. importance is the cost of fabrication and this involves consideration of whether or not flat solid metal backing material (additional to basic curved or flatmembers such as brake shoes and clutch plates) must be provided.

riveted directly to a curved member such as a brake shoe, such organic materials are characterized by a relatively high wear rate and by some ;fading due to a rapid succession of stops, or due to the presenceofoil, grease, moisture, or high temperature. metallic friction facings made by mixing ingredients in powder form, pressing the mixture to form a briquette and heating the briquette to sinter the same have already been used extensively in industry, and in clutches and brakes for military vehicles, large trucks and buses, earth moving machinery and the like, but have had high initial cost, and other disadvantages making them heretofore not always acceptable for pleasure car brake linings.

By way of further explanation it may be stated that While it is quite feasible to make many an article (e.g.,

a door handle) from almost any iron powder used alone and the article will have strength suflicient for its intended purposes, for other applications (e.g., for predominantly metallic friction facings) it becomes necessary to use high amounts of filler (such as graphite) for lubricating or wear or even frictional properties. With prior iron powders as heretofore produced and supplied it has been impossible to use desirably high amounts of filler and still have adequate strength of finished article.

It is an object ofthe present invention to'provide sim ple and inexpensive means for overcoming the above'mentioned difliculties;

Another object of the invention is to provide a novelsoftor' spongy iron powder.

A further object is to-providean'improved low density, high compression'ratio' iron powder comprising predbminantly fern like particles which allow sucliproduct Of particular I While, of course, organic linings, as of asbestos, may be,

Sintered predominantly 3 to be subsequently soft compacted whenever that is desired.

A still further object is to provide a unitary monometallic sintered friction member which is a structural part not required to be bonded to a solid metal backing.

Another object is to provide improved methods of heating, cooling, and removing a charge during the reduction of iron oxide to substantially pure iron.

Another object is to provide an improved process for reducing iron oxide to produce a low density iron powder.

Another object is to provide improved methods for reducing, removing and comminuting a charge of reduced iron with respect to a retort in which it was reduced and with a minimum of processing steps and with minimal damage to the retort.

A further object is to provide improved methods for making softer iron powders, that is, of lower density and with higher compression ratios at lower molding pressures than those heretofore known.

A further object of the invention is to provide improved apparatus suitable for carrying on iron oxide reduction and which is characterized by the ability to hold up during repeated usage without cracking or buckling, with a minimum of maintenance and with a minimum of processing steps, and which will afford uniformity of product at low cost and provide effective sealing against reoxidation during subsequent cooling.

A further object is to provide improved apparatus for iron oxide reduction characterized by substantially uniform heat distribution in every portion of every charge, to provide an improved product comprising predominantly high compression ratio, low density or fern-like particles which allow such product to be subsequently soft compacted whenever that is desired.

A further object is to provide improved apparatus for charging, reducing and removing a charge with minimum time and labor and maximum re-useability of the apparatus employed.

Another object is to provide improved apparatus for removing and pulverizing a charge of reduced iron with respect to a retort in which it was reduced and with a minimum of processing steps and with minimal damage to the retort.

Other objects and advantages will become apparent and the invention may be better understood from consideration of the following description taken in connection with the accompanying drawing, in which:

Fig. 1 is a sectional elevation of a heating pit furnace containing a charge can or retort adapted to be used in accordance with one aspect of the present invention;

Fig. 2 is an enlarged elevation in section and taken at right angles and of a bottom portion of the arrangement of Fig. 1;

Fig. 3 is a plan view of the pit and retort of Fig. 1;

Fig. 4 is a cross-section of the retort can and taken along the line 4-4 of Fig. 1;

Fig. 5 is a plan view taken above the pier blocks on the line 55 of Fig. 1;

Fig. 6 is a sectional portion taken above the burner spider on the line 6-6 of Fig. 1;

Fig. 7 is a schematic representation showing a method of connecting exhaust piping of a can being heated to provide a reducing atmosphere for a can being cooled;

Fig. 8 is a tabular representation of comparative physical properties of prepared, purchased, and ofiered iron powders and characteristics of articles made from some of them, the powder indicated at A being prepared according to the present invention.

Referring now to Figs. 1 and 2, there is shown an iron oxide reduction retort and retort receiving pit, the latter comprising a part of an excavation lined with concrete 10. For reasons of economy it is often desirable to have a single excavation contain a plurality such as nine or eighteen retort receiving enclosures 11a, 11b,

4 etc., of which, as shown in the drawing, 11a is an outermost hole. Each such hole is formed as a circular enclosure by insulating wall blocks 12 of refractory material and resting upon base blocks 13 which may be ordinary fire brick.

As most clearly seen in Fig. 2, the base blocks 13 rest upon a concrete foundation 14 which in turn rests upon earth 15a (Fig. 2) or other supporting means. If desired the outer pit walls 10 may also be surrounded by earth 15b which then serves as a good insulating medium. As shown in the drawings, a layer of heat resisting refractory concrete 16 intervenes between the pit wall blocks 12 and the outer, or ordinary, concrete 10. A fuel and aid inlet tunnel or duct 17 lined with sheet metal 18 is provided through a portion of the outermost concrete 10 and through the bottom blocks 13 and at least one pipe 19 extends through this tunnel. While of course other arrangements could be used instead, in Fig. 2 three such pipes 19 are shown and it may be assumed that each pipe serves one retort enclosure location.

A cast refractory cover 29, which is in general square at its outer limits and provided with a central annular opening 21, rests upon the top of the blocks 12 with its central opening 21 being provided for reception of a crucible can or retort indicated generally at 22 (see Fig. 1). In order to provide for heat expansion of each large block 20, adjacent blocks (e.g., 20 and 20b) are separated by a compressible compound 23 which is also provided between each outermost block 20 and the adjacent heat resisting concrete 16.

At the bottom of the furnace at each retort location, one of the pipes 19 is connected to a bottom burner 30 which terminates in a burner screen 31. The burner extends into a burner chamber containing a four armed spider or pedestal 32 which serves to distribute the flame and build up incandescence without melting the burner or screen. Further up, a burner tunnel 33 of refractory material (square at its outer periphery but annular at its inner) rests partly upon the spider 32 and partly upon and within adjacent base blocks 13. Above the burner tunnel 33, and within the furnace opening 11a, four pier blocks 34 of cast refractory support a disc shaped lower clay pedestal tile 35 (which serves to distribute heat to the sides) and an upper clay pedestal tile 36 which serves as a heat resistant support for the retort can 22.

The retort can 22 comprises a long outer cylindrical shell 38 of a heat conducting high temperature resisting metal such as Inconel Ni, 13% Cr, 6.5% Fe) secured as by welding to an Inconel shell bottom 39 which rests upon the pedestal 36 when the retort is in place in the furnace. Inside the retort can thus formed a layer of loose graphite 40 (preferably granulated) is placed upon shell bottom 39, and an Inconel bottom plate 4 1 (having an OD. smaller than the ID. of the shell) is placed within the shell and upon this layer. A cylindrical expansible assembly temporarily held together (that is, until heating) is placed upon bottom plate 41, and, in the arrangement of Figs. 1, 2 and 4, this assembly comprises three arcuate plates 42 which may be of Inconel and which are overlapped at their ends and temporarily held together in cylindrical formation by a combustible tube of cardboard 43, although a combustible adhesive tape, or yieldable metallic clamps or other temporary restraining means could be used instead. The inner diameter of the assembly of plates may be coated with graphite. The outer diameter of the cylindrical formation is considerably less than the inner diameter of the shell 38, and as shown in the drawings the cylindrical formation is centered within the shell and the intervening space between the cardboard and the shell is filled with a loose graphite 44 (preferably ground and granular rather than flake) nearly to the top of the cardboard. Charcoal might be used here instead of graphite, but in any event some loose carbonaceous material is provided so that when the charge expands during reduction it will not jam inside the outer can but, instead, there will be permitted easy extraction of the log (as the charge is called after it is reduced) so that the can may be used again.

The cylindrical formation of overlapped arcuate liner plates or segments 42 forms an inner can which is filled with a charge 45 to be reduced and which, as in general practice in the past, may comprise a mixture of powdered red iron ore (E2 0 and charcoal, or mill scale (Fe O and charcoal, and in proportions such that the oxide will be reduced and the carbonaceous material will be consumed in the process. As an example, I have had good results using for the charge (45) 83% Fe O and 17% (by weight) of ground charcoal.

When it is desired to commence operation with. mill scale, it is preferable to obtain dry mill scale and scalp the same to remove any foreign material such as gravel, sticks or paper, and then mix it with the powdered charcoal. The mill scale is preferably in a dried form to prevent balling up of the material (Wetting of the charcoal, and subsequent non-uniformity of distribution throughout the entire reduction mix. Dryness also insures close packing of the reduction mix during loading which is important in order to obtain reduced iron logs of maximum weight. Of course with the charcoal, and with the mill scale as Well, it is desirable to have a minimum variance in chemical and physical properties but adequate limits in this regard are generally maintained by the commercial suppliers of such materials.

The materials are well mixed together, as for example in a ball roll mixer or by using a conical tumbler or double cone blender for something like 60 minutes with additional mixing time being required if a laboratory analysis then indicates an unequal distribution of charcoal throughout the blend.

From Figs. 1, 2 and 4, it will be seen that when the charge is in place, it is separated from the outer shell 38 of the can first by the segmented sheet metal cylinder made up of liner plates 42, second by the cardboard cylinder 43, and finally by the layer of loose graphite 44.

A top sealing cover is pushed down on the charge and as shown this cover comprises an inner annular vent pipe 48 secured to a top cover bottom plate 49 which in turn is secured to a top cover outer cylindrical shell 59-. These pieces may be made of Inconel and they are rigidly secured together as by welding with the outer shell 50 extending above the bottom plate to form with the bottom plate and the inner vent pipe an annular opening. The shell 50 also extends below the bottom plate to form a depending skirt portion 50d which is pushed down into the loose graphite to lengthen the path which any gases would have to traverse to get into or out of the charge from around the outside. The vent pipe is provided in order that gases resulting from reduction of the charge may escape, and the annular opening between the vent pipe and the outer wall is filled with a castable insulating concrete such as Firecrete or other refractory of insulating material 51, such Sil-O-Cel which is a proprietary name for a diatomaceous earth composition, exact contents of which are unknown to me but the composition is readily obtainable by this name and is characterized by good heat insulating properties and light weight. The cover as above described effectively seals against normal gas pressure from within the retort while allowing means for pressure relief in the event of blockage of exhaust lines, and it is characterized by easy attachment and removal. The refractory 51 may be cast as it is placed within the annular opening and then covered by a top plate 52 which may be of asbestos and provided in order to prevent dusting.

The outer shell 38 of the retort can 22 is provided near its top with lifting means which in the illustrated embodiment (Fig. 1) take the form of a metallic ring 53, and the assembly of retort, its: charge, and its cover just described, is lifted thereby, and preferably with a the can and the length of the enclosure being so selected anddesigned that a top portion-of the can' (including the lifting means) is well out of the enclosure and therefore not apt to be weakened by the heating.

Assuming, for purposes of description, that a solid log can 22 (Fig. l) is first to be lowered into pit enclosure lla, and then filled while in place therein, the outer can (shell 38 and bottom 39) is lowered into the pit by a crane and special tongs designed to catch and hold beneath the rim 53 welded on the topouter diameter of the retort can, and after the can has been lowered into the pit a few scoopfuls of graphite are poured into the can to form a layer of graphite 40 approximately /2" thick on top of which is then'placed a A thick circular bottom liner plate 41. Plate 41 may be graphite coated to prevent sticking. Next, three rolled 5 thick steel arcuate liner plates 42 whose inner wall surfaces have been coated with graphite may be overlapped around a steel mandrel (not shown) and the overlapping edges clamped as by the cardboard cylinder 43. The thus formed hollow cylinder is removed from the mandrel and lowered into' the retort can until it rests on the plate 41 and is centered. Some graphite 44 is then poured into the open space between the cylinder '43 and the outer :wall 38 of the retort can to keep the liner plates and cardboard cylinder centered during the addition of the reduction mix- Reduction mix 45 is slowly pouredinside the cylinder formed inside the plates 42 until the level of the charge comes within an inch or so of the top of this cylinder. Thetop or seal'can' (48-52) is then lifted, by means of a crane hoist for example, and lowered into the retort can until its bottom 49 rests on the top of the liner plates, the bottom skirt of the can sliding ino the open area between the liner plates and the inner walls of the retort can. Additional. graphite 44 is then poured into the open space between theouter wall 50 of the top can and the outer wall 38' of the retort can until the graphite comes within approximate- 1y one inch of the top of the can.

The gas is then turned on andlighted, the charge brought up to temperature and held there for a sufficient time (as may have been determined experimentally from prior runs), then the gas turnedolf, the retort can re moved, allowed to cool (preferably in a controlled non oxidizing atmosphere), the cover removed, and the charge removed, then cut up and ground to produce the desired iron powder.

Of course, the process steps just described can be al tered. For example, it may be found easier to fill the retort crucible 22 outside the retort enclosure, and it is not necessary to use a mandrel for the arcuate plates if a cardboard cylinder is used as their temporary binding means because after the bottom layer 40 of graphite is placed within the outer shell and the sheet metal plate 41 disposed thereon, the cardboard cylinder 43 may be" I put in next, then the graphite 44 poured in between card board and outer shell, and then the segments 42 placed within the cardboard, and then the charge dumped into the inner 'shell thus formed by the segments. Butin any event, it is contemplated that the segmented shell of pieces 42 will merely rest loosely upon the bottom plate 41 so that it is free to move with respect thereto as the charge expands during reduction. 1

Alternatively the gas may be turned on and the furnace kept hot all the time, and, whenever it is desired to commence heating a particular charge, the charge in a completely assembled retort can may then be lowered by means of a crane-hoist into .the heated hole until approximately three quarters of. its length is lowered into the hole. It may then be held suspendednin this position for five minutes to. permit a slow overall heat ing upperiod for the can and charge, preventing a cold surface from coming into intimate contact with the hot bottom refractory plate 36 thereby to minimize danger of cracking and to increase the life of the parts, as well as to prevent a too rapid evolution of gases within the charge since the entrapped gases if evolved too rapidly would tend to cause a blowing out of the graphite packing filler. After such a five minutes suspension and preheating period, the can is lowered into position and comes to rest on the bottom refractory furnace plate. In this position the can is allowed to remain at a temperature which is suitable, for example 1810" F., and for a time which is suitable, e.g. for 72 hours or until there is a substantially complete reduction.

The retort can may then be removed by crane hoist and hooks and set in an area reserved for the cooling of cans and at this time a freshly charged can may be inserted into the emptied hole. When can and charge have cooled to room temperature and top seal can then removed, the charge may be dumped, or, if it sticks within the liner plates, a long screw hook (not shown) may be turned into the iron log and the iron log thus drawn out of the can, the liner plates pulled away, and as a final step the completed log is ground to a powder.

Preferably the retort is heated during operation not only by the bottom burner 30 already described, but also by a ring of circumferentially spaced burners 54 (Fig. 1) located somewhat near the top of the can. For the retort location 11a there are six of these peripheral or ring burners 54 each fed by a different conduit 55, with the six conduits 55 tapped off of -a top ring burner manifold 56 which in turn is fed with the proper mixture of fuel and air from a ring burner manifold feed conduit 57. Each top block 20 is, preferably at the time of its casting, perforated by six conduit receiving holes or slots 58 to accommodate the downwardly extending burner conduits 55. The burners 54, themselves, may be directed downwardly as shown and in order to avoid local overheating, I prefer to provide small diameter holes 59 in each burner pipe somewhat back from the extreme end of the burner and in such manner that these will allow gas to be forced out in several directions, where it is ignited to give very good heat distribution within the pit and eliminate local hot spots.

In the arrangement of Fig. l a grating 60 is provided over the top end of the ventilating duct 17 and through this grating air may flow (or be forced) to follow, for example, the direction indicated by the arrows 61 to provide pit ventilation.

Meanwhile a mixture of fuel and air is forced in the direction of arrows 62 to supply the bottom burner 30 and after ignition by some conventional means (not shown} heat therefrom follows in the direction indicated by arrows 63, that is upward and out between the can outer wall 38 and the inner circumference 21 of the top block 20. Meanwhile a similar mixture of fuel and air is forced in the direction of arrows 64 through the feed conduit 57 to the ring conduit 56 and down to the ring burners 54 where, after ignition by some conventional means (not shown) heat is produced and the exhausted products of combustion eventually turn upward and also discharge out between the can outer wall 33 and the inner circumference 21 of the top block.

Thus, and while electric or some other heating means might be used instead, as above described the mill scale and ground charcoal are gas-fire heated in a closed (and graphite sealed) metal cylinder and this will produce carbon dioxide (CO and carbon monoxide (CO) gases and a porous log of sponge iron. The by-products of reduction, that is the carbon monoxide and carbon dioxide gases, are carefully carried away as through hose or conduit 66 coupled-to exhaust conduit 48. They may thus be carried to a common manifold (not shown) or, if desired, the arrangement may be as in Fig. 7 where the exhaust conduit 48 of one can 22a which is being heated is shown connected through piping 67 to the exhaust conduit 48 of another can 22b which is being cooled with the result that the can 22b during cooling is kept from absorbing any atmospheric air which might tend to reoxidize the log contained therein. Since the cold can will not absorbxas much of these gases as will be evolved from the can being heated, I have also shown a by-pass take-off through a T fitting 68 and through a hand valve 69 to a nozzle 70 where excess gases may be discharged to and burned in the open air. Any products of burner fuel combustion in the pit enclosure are readily discharged to the open air, or to an exhaust hood (not shown), and if they contain any dangerous CO gases it is a simple matter to either increase the air supply to the burners or else burn such gases at the opening between the heated can outer wall 38 and the associated top cover 20.

Using the arrangement of Figs. 1-4, and after considerable experimentation to determine optimum physical dimensions with due regard for heat distribution and penetration and chemical analysis of the finished product, it was found that a 310 pound charge could be effectively reduced to a solid 208 pound log of desired content in seventy-two hours, with test results as follows:

lrVgt. of VVgt. Ultimate Oxide Carbon Chg, Reduced Temp., Time, Content, Con tent, Pounds Log, F. Hrs. Final Final Pounds applied to the bottom'and outside of the can, with the heating continued for a time sufiicient to reduce the charge. Then the sealed can is removed from the pit and may be placed on an open floor where it is allowed to cool while the charge is subjected to a controlled atmosphere, for example of city gas unless it is desired to use the waste gases from another can being heated (as in Fig. 7). Next the top seal can is removed and the charge is thereafter removed without destroying the can, for example by the prior employment of the liners described in connection with Figs. 1, 2 and 4. The removed charge is then broken up as with an axe, if that is necessary, and next the material is ground as in a motor driven grinder, and if desired it may be further processed in a motorized sifter with coarse rejects being returned to the grinder, but in any event as a final product satisfactorily pure and very desirably soft iron powder is obtained.

During operation of apparatus as above described the outer burners help assure a uniform heat distribution from the top to the bottom of the associated hole, and also facilitate attainment of relatively even temperature distribution as between burners, and the latter gives a long life expectancy for both the burners and the pit construction and also for the furnace cans by eliminating burn-outs in bottom areas as a result of overfiring at bottom in order to sufiiciently heat top portions.

The top can serves as an effective seal, requires little or no maintenance, holds up well with repeated usage without cracking or buckling.

The retort can and furnace above described provide for easy removal of the retort can from the furnace, and includes means for removing the charge without destruction of the can. If necessary the arcuate liner plates (42) may be cleaned and rerolled to shape but I have found that the liner plates are capable of being re-used on an average of five to six times before requiring reworking. When liners are used the double construction of retort can helps assure that there will be no buckling, distortion or warpage, and with all the arrangements disclosed the can extends above the pit for cooling of the top or lifting portion to prevent weakness at this point (thus enabling the top to retain sufiicient strengthito enable it to bear the can total weight when it is to be lifted from the furnace).

With the apparatus and processes of the invention, compared to the prior art, a relatively large weight of charge may be processed in a greatly reduced processing time, and without the prior art disadvantages of destruction of a part of the processing apparatus with each run.

Proper heat distribution is assured to all portions of the charge, and, due to the overall design, without danger of leakage (as of pressure outwards or air inwards), and the final product has a relatively low-oxide, low-carbon content as observed, for example, in the critical top and bottom areas of each log sampled and analyzed, and as also indicated by dynamometer tests after completed logs have been ground, molded, sintered and properly assembled to determine frictional wear properties of varying batches from logs so produced. For such powders, moldgrowth, green strength, sintered strength, workability and friction and wear properties were all found to be excellent and it was also found that iron reduction methods and apparatus of the type described produced a fernlike powder which, compared with any heretofore known, was softer and more compressible, thus givinglower costs for fabrication, longer die life, and longer wearing friction material having a higher coefiicient of friction and having other advantages apparent to those skilled in the art.

Those skilled in the art will recognize that the ground powders have many uses. They may, for example, be utilized in the production of friction articles, such as brake and clutch linings made by pressing and sintering mixtures of metallic and non-metallic powders to provide numerous advantages over friction linings of asbestos or other non-metallic materials in that the sintered articles are less susceptible to changes. in temperature and atmospheric conditions, are less affected by extraneous oil, grease and foreign matter, and show less wear with the same use.

To have the best iron powder for such friction applications it is preferable that the powder have a low carbon content (to avoid the formation of hard carbides) but any carbon not exceeding 1% seems quite tolerable and any oxide content below 3.0% is quite permissible so far as friction and wear and other properties of the ultimate material are concerned.

It is not exactly clear why a powder which is superior to anything known to the prior art results from the use of apparatus and methods as above described. Form and purity of the initial materials is of some importance, but it is not absolutely essential that they be pure. Graphite instead of charcoal would probably not be suitable because its use would tend to increase reduction time cycles for same weight of charges.

Time and temperature are probably of great importance but no definite limits can be set up because each depends upon the other and because together they depend in turn on the dimensions of the apparatus and of the charge, location of burners, and many other factors. Those skilled in the art will quite easily recognize, of course, that too little heat will result in an insufiiciently reduced product, too much heat may cause sinterlng of the log and cause it to be too hard. I have found that in many cases reducing temperatures of 1700 F. took too long for the process to be economical; 1800 F. (with a certain arrangement of burners) is just right; 1850 F. produced a powder which was too hard; and 1900 F. produced a compact log resembling ordinary iron or steel.

With the arrangements above described, it is easier than ever before to provide controlled conditions (e.g., of heating time and temperature and pressure and even 10 particle," size). which will result in a. desired. degree of sponginess of the iron product making it well suited for the ultimate production of powder metallurgy friction articles comprising so-called iron base mixes, and they will then have characteristics quite comparable to that of more conventional (but more expensive) copper base mixes, while being far superior to copper base mixes. in many respects.

That is, the retort can and furnace, designs disclosed not only provide for easy removal of the retort. can from the furnace, but still allow the buildup of gas pressure within the charge during reduction (which may account for the novel product) while means are also. provided for removing the charge without destruction of the. can;

I have found that it lies well within the province of the mechanic after brief experience with apparatus and processes such as those described to produce a novel product having desired characteristics and characterized by an apparent density in the range of 1 to 1.5 (gms./cc.).

Possibly the novel product is due to the use of a closed and sealed can and consequent production of some gas pressure therein, though even in the Swedish'process there is undoubtedly some gas pressure produced between the alternate layers of coal and coke. However, neither the products of producers of Swedish type nor the producers of other so-called low density powders seem able to achieve an apparent density less than 1.59 and throughout the world all producers of. the so-called low ratus and processes described in the present application.

Referring to Fig. 8 there is there shown comparative physical properties of prepared, purchased and oifered iron powders, where the properties listed in column A are those of powders, compacts and sintered articles made from such powders prepared by applicant according to his processes, those under column B are for powders from supplier B, those under column C for powders of supplier C, and so on, there being two columns for: each of E,'F, and G to show comparison of suppliers data and local tests made upon powders from each of these sources. In some cases suppliers data was found not to be available to me, and in other cases data furnished by supplier quickly indicated that local tests would be quite unnecessary.

Possibly the most important figures in the table of Fig. 8 are those for apparent density. Such density was measured by a Scott-Hall flowmeter in a manner well known to those skilled in the art. Of course this factor of apparent density must be considered only in connection with consideration of particle size distribution. In general a fine powder is desired but finer powders can be obtained only by grinding, or selective screening. Selective screening is not usually economically feasible without returning coarse rejects for grinding but thetmore the grinding the more the cold work, so that the finer the powders are ground the harder the material becomes. It will be seen from Fig. 8, however, that the A powder (that is, powder according to the present invention) had an apparent density not more than 1.5 grams per cubic centimeter even with a particle size such that substantially all (namely, 95.5% or more) passed through an mesh screen (Tyler standard).

The full significance of apparent density becomes 3P. parent when subsequent molding is considered. Low apparent density gives low pressed density at low molding pressure. Of course particle shape has to be considered for even a hard powder might be used to produce an article of low pressed density if the pressed article was provided with many pores, but I am talking about a homogeneous, imperforate, substantially solid pressed article, i.e., one substantially free from noticeable voids or one whose apparent porosity is less than 30% of total volume. Low apparent density also gives a high compression ratio, and while the compression ratio seems to depend to some extent on actual molding pressure, height of fill and size of compact, it should be apparent from the table that with applicants powder there is a high compression ratio even at the very moderate (and therefore preferred) molding pressure of 12 /2 (tons per sq. in.), a pressure lower than that recommended by any of the suppliers (even though resorted to by one of them when proceeding at applicants direction).

Considering the properties listed under horizontal subtitle 2 (for green molded compacts) although the green hardnesses are comparable for the various powders, the green (pressed) density of the A powder was the least and, of more importance, the green strengths of compacts made from this powder was, because of the higher compression ratio (i.e., better compressibility) by far the highest though this last mentioned factorcould be found only by handling compacts and noting breakage and could not very well be tabulated in numbers.

Attainment of high compression ratio with the desired low molding pressure, permits maximum strength of green compacts with minimal pressing equipment.

Through experience at applicants company it has been found that only the A powder can be molded automatically because only the A powder lends itself for automatic operation at low pressures. Using such a low density, high compression ratio powder, increases life and reduces wear of dies, die walls and press itself. Whereas at the present day it is prohibitively expensive to order a press big enough to process other powders, with the A powder existing press equipment (for example that already built for copper base mixes) can be used for making iron base friction materials, because applicant has found that with powder made according to the invention he can mold excellent iron base sintered friction facings at the very low pressures of 12.5 tons per sq. in. as contrasted with the 25, 30, 50, or even 100 tons per sq. in. as heretofore used.

Of great importance is the fact that the new iron powder, because it is so much softer, greatly facilitates the formation of a friction article substantially entirely a pressed and sintered mix of iron and graphite, or carbon. As mentioned in the beginning, while many old articles (ash trays, etc.) could be made with iron or other metal and little filler, wherever substantial amounts of a filter such as graphite is required (as for pressed and sintered metallic friction facings) the new apparatus, processes, powder and articles of the invention permit use of high amounts of such filler while permitting processing equipment of minimum cost and finished articles of requisite strength.

While I have described particular embodiments, various modifications may obviously be made without departing from the true spirit and scope of my invention which I intend to define in the appended claim.

A method of making substantially pure iron powder which method comprises mixing dry mill scale and ground charcoal, obtaining a retort can having an outer shell and spaced inwardly therefrom a hollow cylindrical expansible inner liner, filling the space between inner liner and outer shell with loose graphite, filling the space defined within the inner liner with a charge of the mixture of mill scale and graphite, placing a top cover over the charge, heating the charge in the can to a temperature of from 1700 to 1850 F. and for a time suflicient to reduce the charge while allowing for escape of the gaseous products of reduction, applying an inert gas to the charge while cooling the same, removing the reduced and cooled charge from the can, and pulverizing the reduced charge to provide a substantially pure low density iron powder.

References Cited in the file of this patent UNITED STATES PATENTS 390,964 Hay Oct. 9, 1888 2,094,727 Stella Oct. 5, 1937 2,332,737 Marvin et al. Oct. 26, 1943 2,369,502 Walker Feb. 13, 1945 2,606,110 Berge Aug. 5, 1952 2,665,981 Marquaire Jan. 12, 1954 2,678,879 Nuesch et a1 May 18, 1954 2,739,807 Stuart Mar. 27, 1956 2,757,078 Edstrom July 31, 1956 OTHER REFERENCES Chemical Engineers Handbook, 3rd ed., 1950, page 963. Published by McGraw-Hill Book C0,, 1110., New York, N.Y.

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