US2935402A - Hot rolling of metal powder - Google Patents

Description

May 3, 1960 F, J, TRQTTER ETAL 2,935,402

HOT ROLLING OF METAL POWDER Filed April 15, 1954 INVENTORS Gerhard Naeser' Frederick J. Trotter Richard B. Montgomery Lincoln T. Work BY/ ATTORNEYS p of the equipment.

2,935,402 nor ROLLING or METAL POWDER Frederick J. Trotter, London, England, Richard B. Montgomery, Bethel, C onn., Lincoln T. Work, Maplewood,

N.J., and Gerhard Naeser, Duisburg-Huckingen, Germany, assignors, bymesne assignments, of one-half to Mannesmann Aktiengesellschaft, Dusseldorf, Germany, a German company, and one-half to Chernetals Corporation, New York, N.Y., a corporation of Delaware Application April 15, 1954, Serial No. 423,436

3 Claims. (Cl. 75-221) Thisinvention relates to a process for converting metal powders into wrought metal shapes such as strips, rods, wires, sheets, and the like. More particularly, the invention relates to a process which comprises sintering metal powder to form a sintered bar and hot rolling the sintered bar.

It is well known that metal may be recovered in powder form from scrap or from ores or from concentrates containing them in the form of metal'oxides or sulfides by hydro-metallurgical processes. For example,metal powders such as copper, nickel and cobalt can be recovere'd'i'n substantially pure form by dissolving the metal from the raw material in an aqueous solution and then heating the solution to an elevated temperature and pres- .sure with a reducing gas. The metal is precipitated in substantially pure form, more or less pulverulent inchar- "actor. In its precipitated form, the metal'has at best rather limited fields of use. Generally it mustbe converted into a wrought shape in order to be useful. Such conversion, however, is not a simple matter. Melting the pulverulent precipitate and casting into shapes'for 'rolling'isnot economically desirable because such 'procedure entails large losses of metal due to oxidation and slaggin'g. Melting in a vacuum or in a non-oxidizing or reducing atmosphere greatly increases the cost of oper- 'ation. Hot' extrusion of the pulverulent metal in a refducing'atm'osphere has been proposed, but this procedure also is costly, on account of the excessivewear and tear Fabrication of the metal into wrought or other shapes by conventional techniques ofpowder :metallurgyhas not proved satisfactory, partly because such-techniques are suitable only for making rather'small -articlesg-and partly because the pulverulent metal preeipitate-h'as 1110!. proved itself tobe uniformly suitable for fabrication by the methods of powder metallurgy. :It has also'been proposed to roll the metal powder directly between cold pressure rolls to form a raw product and then subsequently sinter the same and further roll the sintered product to produce wrought metal shapes, .but this technique requires synchronization of 'the forming rolls and the sintering operation, and, for a given vsintering period; long and costly'furn'aces are called, for. When-using this-techniquet-he'metal powder must have *its moisture and, oxide content completely removed before it can be formed into strip or sheet form. The feed of the dry powder into the rolls must also be 'even .in order'to prevent thin spots, and the very powerful pressure rolls themselves must also have a minimum diameter of about 50 to 1,000 times the thickness of v the metal'product being rolled in order to sufliciently consolidate the metal powder.

We have discovered that despite the non-uniform bevhavi'or of the pulverulent precipitates of conventional powder metallurgy fabrication procedures, it is possible to'secure good and consistent results in the consolida- 'tion' of such precipitates into a bar which can be worked further into a wrought metal shape, and also obviate many disadvantages of the conventional consolidation p tes Patent vention. .r noi'sture. advantageous, as a complete drying step takes upcone Patented .May 3, .1960

procedures, both from a cost standpoint and from .the standpoint of producing high quality and uniform'wroug'ht metal products.

Generally, according to the present invention, the metal powder is first fed onto a suitable support in the dimensions desired, and then directly passed to a furnace in which a controlled reducing atmosphere is maintained, to effect at least a partial sintering of the powder to form a fairly self-sustaining sintered bar or slab. The I According to the present invention, thisywrought metal powder, after it has just been precipitated," may' first be suitably washed and passedthrough a dryer. Any conventional type dryer is suitable. The powder discharged from the dryer may contain a small percentage of mois- ,1 two.

We have found that a moisture content of from about 0.5 percent to about 1.0 percent does not interfere with the subsequent consolidation of the powder. The moisture-containing powder may be directly fed from the dryer onto a suitable supportand directly passed into a furnace to, effect sintering thereofland otherwise consolidated according to the process of the present in- The sintering operation readily eliminates This feature of the present invention is highly siderably. more time and is costly. Furthermore, when completely drying the metal powder, the danger of oxidation, and of incipientaggregation at the comparatively high temperatures required, both of which interfere with subsequent operations, is always present, and in order to avoid this it is often necessary to dry in a reducing atmosphere. It is thus possible to maintain the temperature of the dryer sufliciently low so as to prevent any oxidation of the particular metal powder being dried and consolidated. Using such a temperature eliminates any necessity for using a protective gas atmosphere to prevent oxidation, and such a process makes it possible to utilize such a drying procedure and maintain the process on a sound and commercial economical basis, while producing good and uniform wrought metal products The temperature used which will elfect a sufficient drying of the metal powder and yet prevent oxidation without the use of a protective atmosphere, will, of course, vary depending upon the particular product being dried and consolidated. Such temperature will be readily apparent to those skilled in the art. When drying and consolidating copper powder, for example, the temperature of the dryer should not exceed about 110 C. At this temperature the copper powder may be efliciently dried for the purposes of this invention without adversely affecting the powder.

After the powder has been sufiiciently washed and dried, it is spread upon a suitable support, such as a continuous metal belt, in the desired dimensions. The powder emerging from the dryer is advantageously fed onto the continuous belt while it is still hot to save heat and prevent oxidation thereof. The thickness and width of the metal powder mass will be determined in part by the particle size of the powder being consolidated, and the'dimensions of the powder mass will also determine to a certain extent the temperature used to efiect the heatingof the powder and the time necessary to sufficiently heat the same. I It is obvious that since the metal powder mass is very porous, it. isa poor heat conductor; and the larger the powder mass is in thickness and width, the longerit will take to completely and sufficiently heat the powder mass throughout its cross-sectional area. a We have found that a powder mass of about 2 inches in thickness and about 2 inches 'inwidth may be satisfactorily heated, for any particular metal, at fairly low temperatures in two to three minutes. Smaller powder masses may of course be heated in less time. Due to the fact that when the metal powder is first deposited on a continuously-moving belt, it is at times unevenly distributed thereover, it is also advantageous to smooth the metal powder mass by some suitable means such as low-pressure rolls, or even a doctor blade.

Upon heating the meal powder mass, any remaining moisture is first readily driven from the still porous mass. To prevent oxidation the heating should take place in a reducing atmosphere. Hydrogen gas is an example of a suitable reducing atmosphere and may be supplied to a furnace by any suitable and conventional means. The heating of the powder mass causes the powder particles to cohere and form a fairly rigid and strong coherent porous bar. This bar may be further heated to a higher temperature, at or about the sintering temperature of the particular metal being consolidated, and directly fed while still hot between a pair of powerful pressure rolls to consolidate the same to almost complete densification. This powerful rolling action, however, should be so regulated and applied as to prevent any substantial elongation of the bar being compressed. If the pressure of this rolling action is sufliciently high to cause substantial elongation of the bar, cracking often occurs in the bar rendering it useless for many purposes. After the powerful rolling action, the bar may again be heated to a temperature at or above the sintering temperature of the metal being consolidated, and again compressed between powerful pressure rolls to completely densify the bar. In this latter compressing operation, the pressure may be, and advantageously is, sutficient to cause substantial elongation of the bar. During this additional rolling step there is no serious tendency for the bar to crack when applying sufficient pressure to cause elongation thereof.

When the loose powder mass is first heated and the particles cohere, the metal powder shrinks to a considerable extent throughout its thickness, but the shrinkage in a lateral direction, or along the width of the powder mass, is very slight. For example, we have found that a powder mass of about two inches in thickness and about two inches in width shrinks when heated to about an inch and a quarter in thickness and the shrinkage along the width (and length) is negligible. We have also found that due to the shrinkage of the loose metal powder, the resulting sintered bar may contain irregularities, and when further heated and directly fed between a pair of powerful pressure rolls, thin spots or areas might develop in the resulting bar which are difficult, if not impossible, to remove. We have found that if the bar, after it has been heated sufficiently to cause the metal particles to cohere and before it is heated to a temperature above the sintering temperature, is first slightly or partially compressed or consolidated to a modest extent sulficiently to remove the irregularities or unevenness, that the bar may be further heated and completely consolidated into a uniform density without the presence of thin areas.

Another advantage of this slight consolidation resides in the fact that the even bar, partially consolidated, is more rapidly heated throughout to higher temperatures due to the elimination of some of the porosity of the bar. In all cases where the hot bar is to be rolled, it is advantageous to heat the rolls to prevent chilling of the hot bar. We have found that if the rolls are heated to a temperature of about 400 C., that is sufiicient to prevent chilling of the hot bar for the purposes of this invention.

For a more detailed description of the invention, reference may be had to the drawing in which:

Figure 1 schematically depicts an apparatus for carrying out the process of the present invention, and

Figure 2 schematically depicts an alternative apparatus for practicing the process of the present invention.

Although any metal powder may be processed into a wrought metal shape, copper powder is used as illustrative in the following detailed description, in order to more particularly point out the details of the process and set forth a specific illustrative procedure.

Referring to Figure 1 in the drawing, the wet copper powder, after it has been precipitated, is placed in a centrifuge 1 to wash the powder free of any contaminants. The powder, when taken from the centrifuge usually contains about 5%-10% moisture. The moist powder is then passed to a suitable drying apparatus such as the rotary dryer 2 shown in the drawing. The copper powder emerging from the dryer contains about 0.5% to 1.0% moisture. The copper powder 3 is then transferred, advantageously while still hot, from the dryer onto a suitable conveying apparatus such as the continuous conveyor belt 4. The conveyor belt may be composed of any suitable material such as steel, and it is only necessary that the material of which the conveyor is composed be able to withstand the temperature to which the metal powder is to be subjected. For example, when consolidating copper powder, it may be subjected to temperatures of about 1050 C. and the conveyor belt must be able to withstand such temperatures. The belt may either be flat or fitted with skirts extending upward to aid in retaining the powder. The advisability of using skirts on the belt will of course depend upon the thickness of copper powder desired deposited on the belt. The belt may be driven by any suitable means. In the apparatus as shown in Figure 1, the belt is driven and guided by the rollers 5 and 6.

As it is at times diflicult to deposit the powder onto the conveyor with the uniformity desired for subsequent operations especially when strip of considerable width is to be produced, the powder may be smoothed over or flattened by suitable means such as the rolls 7.

It has also been found that if the powder mass is subjected to tensile forces prior to the sintering and rolling thereof, that it has a tendency to crack. Any cracking in the powder mass may subsequently show up in the completely consolidated product as thin or weak spots. It is thus highly advantageous to prevent any cracking of the powder mass prior to the subsequent sintering and rolling operations. We have found that by utilizing the rolls 7, or other suitable means, to subject the powder mass to a very mild compressive force, any tendency for the powder mass to crack is greatly minimized. Thus the rolls 7 not only even out any irregularities in the thickness of the powder being fed, but also serve to prevent cracking by making sure that the metal is properly distributed when fed through the apparatus.

The rolls may be composed of any suitable material; however, rubber-faced rolls are preferable. After the rolls 7 have performed their smoothing operation on the powder, the powder is continuously fed into a suitable furnace 8. The copper powder is subjected to a temperature of about 300-600 C. in the furnace 8. A temperature of about 600 C. is preferable, and at this temperature the heating period need not exceed 2 to 3 minutes. This heating of the copper powder causes the particles to cohere or sinter together into a bar and considerable shrinkage of the copper powder occurs. For example, copper powder measuring about two inches square deposited on the belt 4 will shrink in thickness to about one and one-quarter inches while shrinkage in a lateral direction will be very slight. The apparent density of the strip after it leaves furnace 8 has been found to be about 1.5. This shrinkage of the metal powder may cause some irregularities to form in the bar, in which case it is advantageous at this point to remove the unevenness in the bar by some suitable means such as by the rolls 9 as shown in the drawings. The rolls 9 are preferably made of cast iron or steel, but any other suitable materials may be used. These rolls merely serve the purpose of consolidating to, a modest extent "(suflicient to remove the unevenness) the still hot porous bar, and are preferably placed at a point in the heat treatment of the powder where the temperature thereof has not risen above about 700 C.'

The partly compacted porous bar is then passed continuously through a second furnace in which the bar is rapidly heated to a temperature of about 700 C. to about 1050 C. Due to the partial consolidation by the rolls 9,,this second heat treatment may be very rapidly accomplished due to the higherheat conductivity of the slightly compacted bar issuing from the rolls 9.

It is not necessary to sinter the bar; once its mass has reached the required temperature, the bar can be passed immediatelythrough the rolls 11. The rolls 11 are preferably made of cast iron and may be heated to avoid -*chilling of the bar. The bar compacted in this manner has been found to attain a high degree of densification j du'ring a single pass'through the pressure rolls. In fact, a porous jc'opper bar having a density of about 1.5 after sintering in furnace 8 has been by the present process compacted to an apparent density of about 8.35. The degree of compaction by the rolls his so adjusted that no substantial elongation of the bar takes place.

' After thebfar leaves the rolls 11, it is continuously passed through a cooling chamber 12 and may then be further worked if desired by conventional rolling procedures.

Further densification and elongation may be effected by utilization of an apparatus as shown in Figure 2. In this figure, the pressure rolls 13 are identical to the pressure rolls ll'shown in Figure 1 and receive the hot porous bar from the furnace 10 as shown in Figure l to effect the first consolidation of the bar. The compacted bar is then the bar reduces the same to finaldensity and the degree of compaction by the rolls 15 is so regulated as'to effect substantial elongation and complete densification. The bar after it leaves the pressure rolls 15 is then continuously passed through a cooling chamber 16 to cool the 'co'pper bar. The resulting copper bar may then be further worked by conventional rolling practice as desired.

We have also found that it is advantageous to coat the wet powder as it emerges from the processing plant with a small quantity of a material which will, when subjected to heat in the sintering furnace, decompose and yield a protective reducing gaseous atmosphere. We have found that a volatile liquid hydrocarbon such as transformer oil is quite suitable for this purpose and good results may be accomplished by incorporating about 0.5% to 1% of the material with the metal powder. This, of course, at least partly eliminates the necessity of supplying a protective atmosphere from an external source and eliminates the necessity of extra equipment and specially designed furnaces.

The furnaces may be heated in any manner desired, but it is preferable to heat the low temperature furnaces (up to 600 C.) by gas and the higher temperature furnaces by electricity for economic reasons.

The furnaces 8 and 10 as shown in Figure 1 are separate; however, they may be joined together or be interconnected so as to make one single long furnace'and the rolls 9' may be placed inside the furnace. In this embodiment the furnace is gradually heated from a temperature of between 300 C. and 600 C. in the first part of the furnace and the latter end kept at a temperature of about 700 C. to about 1050 C. in which case the rolls 9 should be placed at a position in the furnace at which the temperature is about below the sin- ,tering temperature of the metal, which the. case of copper is about 700 C. It is also advantageous to enclosethe sintering and rolling operations by a jacket or any other suitable means such as the jacket 17 shown in Fig. 1, in order t'o rnairi-i tain the sintering and rolling steps, under an inert atm'os phere. As a practical matter, however, itlisadvantageous to enclose the apparatus by nothing more 'than suitable hoods extending from the outlets of each'piece of treating.

apparatus to the inlet of the next. 7 For example, ai hood extending between the furnaces 8 and 10 .and ove'rfthe As hereinbefore pointed out, any metal powder such as copper, cobalt, nickel, iron, lead and the like may be converted into wrought metal shapes by sintering and rolling according to-the present invention. Wrought alloy products may also be produced by employing. mixtures of metal powders. The temperatures and rolling pressures to be utilized for the conversion of any particular metal powder into a wrought metal shape will, in view i of the above description and common knowledgepossessed by one skilled in the art, be readily apparent. ample, nickel and iron powder may be processed accord ing to the present invention by subjecting them to a temperature of about 800 C.-l000 C. and then subsequently raising this temperature to about 1150 C. before effecting the densification thereof.

The present process provides a continuous'economic method for converting metal powders into wrought metal shapes of good and consistent quality. By the process of the present invention it is possible to continuously and completely process metal powder from the moment of its production into a consolidated wrought metal product without once cooling the metal powder.

The resulting wrought metal product producedaccording to the present invention also possesses highly advantageous properties. It may be cold rolled by conventional rolling procedures and to any desired thickness without annealing and generally possesses the properties of completely annealed metal. I

We claim:

l. The process of producing fabricated wrought metal products from metal powder which comprises heating the metal powder to form a shaped coherent sinteredporous bar, compressing the bar while it is still .hot between a" pair of pressure rolls sufficiently to cause nearly complete 2. The process of producing fabricated wrought metal' products from a metal powder obtained in a hot, wet state by a hydro-metallurgical process which comprises drying the hot metal powder to remove the major amount of moisture contained therein and at a temperature which is insufficient to cause oxidation of the metal powder, distributing the still hot metal powder after it has been dried on a continuously moving support in the desired dimensions to form a smooth powder mass, continuously moving the powder mass through a heating zoneto heat the powder mass and cause coherence of themetal powder particles to form a shaped coherent sintered porous bar,

slightly compressing the bar while it is still hotto re-, 7

For exmove any unevenness in the bar and to partially consolidate the same, continuously moving the bar through another heating zone to heat the bar to a temperature substantially above the sintering temperature of the metal bar removing the hot bar from the continuously moving support and continuously moving the hot bar between a pair of powerful heated pressure rolls to compress the metal bar sufficiently to cause nearly complete densification but insuffigfiently to cause any substantial elongation of the bar, continuously moving the hot bar through another heating zone to efiect a further heating of the bar, and continuously moving the hot bar between a pair of powerful heated pressure rolls to compress the bar sufliciently to cause complete densification and also substantial elongation of the bar.

3. The process for producing fabricated wrought copper products from copper powder which comprises heating the metal powder to a temperature of about 600 C. to cause coherence of the metal powder particles to form a shaped coherent sintered porous bar, slightly compressing the bar while it is still hot to remove any unevenness in the bar and partially consolidate the same, continuing heating the bar to a temperature between about 700 C. and 1050 (3., compressing the bar while it is still hot between a pair of pressure rolls sufiiciently to cause nearly complete densification but insufficiently to cause any 8 substantialy elongation of the bar, again heating the bar to a temperature of about 700 C. to about 1050 C., andfag'ain compressing the bar while it is still hot between a pair of pressure rolls sufiiciently to cause complete densification and also substantial elongation of the'bar.

References Cited in'the file of this patent UNITED .STATES PATENTS 2,027,532 Hardy Jan. 14, 1936 2,134,366 Hardy Oct. 25, 1938 2,252,697 Brassert Aug. 19, 1941 2,287,663 Brassert June 23, 1942 2,290,734 Brassert July 21, 1942 2,300,048 Koehring Oct. 27, 1942 2,332,746 Olt Oct. 26, 1943 2,341,732 Marvin Feb. 15, 1944 2,350,179 Marvin May 30, 1944 2,362,007 Hensel Nov. 7, 1944 2,362,701 Koehring Nov. 14, 1944 2,747,256 Wyatt et a1.v May 29, 1956 2,771,637 Silvasy Nov. 27, 1956 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, pages 440- 451, vol. 2, 1950.

Claims (1)

1. THE PROCESS OF PRODUCING FABRICATED WROUGHT METAL PRODUCTS FROM METAL POWDER WHICH COMPRISES HEATING THE METAL POWDER TO FORM A SHAPED COHERENT SINTERED POROUS BAR, COMPRESSING THE BAR WHILE IT IS STILL HOT BETWEEN A PAIR OF PRESSURE ROLLS SUFFICIENTLY TO CAUSE NEARLY COMPLETE DENSIFICATION THEREOF BUT INSUFFICIENTLY TO CAUSE ANY SUBSTANTIAL ELONGATION THEREOF, AGAIN HEATING THE BAR TO A

Images