CA1041324A - Process for the production of high apparent density water atomized steel powders - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The apparent density of molding-grade, water atomized steel powder can be significantly increased by employing the following prescribed treatment. Coarse particles are removed in order that at least 80% of the initial powders are finer than ??
mesh. The size distribution of the powders is then determined.
The powders are then annealed to both reduce the carbon and oxygen contents and soften the particles. The annealed and agglomerated particles are then ground in a disk mill at specified speeds and gap spacings, depending on the size distribution of the initial powders. Apparent densities in excess of 3.2 may be achieved by (a) employing powders with a coarser particle size distribution, (b) increasing the rotational speed of the disks and (c) decreasing the mill gap.

Description

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15 ,. This lnvention is directed to an economical method for the production of water atomized steel powders with a high apparent density, and more particularly to a method for increasing the - apparent density of such water atomized particles.

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Various methods are now employed for the production of ` metal powders. Thus, metal powders may be produced by (a) electrolytic deposition, (b) direct reduction o~ metal oxides, ~c) reduction of metal halidesJ and by (d) atomization with high pressure flulds, e.g. water and inert gases. For the production of molding grade-steel powders in large quantities, metal oxide reduction and water atomization are considerably more economical.
Of the latter two methods, steel powders that are water atomized ~have a generally lower metalloid content. Water atomized powders also exhibit better flow rates, i.e. better press-feeding efficiency and therefore permit higher production rates in the production of powder-metallurgy parts. U. S. Patent 3,325,277 ,-exemplifies such a water-atomization process. Although such a ij . , .
'Iproce~s offers a number of commercial advantages, it ls somewhat j,limlted in the range of mechanical properties of powders which can t~ be produced thereby. Thus, the apparenk densities of` commercially '~ available water atomized st~el powders is generally within the s range of 2.8 to 3.2 gms/cc. Apparent density is determined by i measuring the weight of powder in a calibrated cup. Since the density can be effected by the mode of packing, this measurement has generally been standardized (ASTM B212-48), i.e. by flowing '! ~
the powder through a 0.1 inch diameter by 0.125 inch long orlfice located one inch above the top surface of a 25 cc cup. i ., ~ ~ - .
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A method for the prodl~ction of high apparent density cast iron shot is disclosed in U.S. Patent 3,597,188. However, this method is limited to the use of coarse powders which are brittle, by reason of their extremely high carbon contents. Since the bulk of powders produced by water atomization are finer than 80 mesh (with a major portion finer than 200 mesh), this latter process is uneconomical because it requires the discarding of more than half of the initially produced powders. Equally important, the disc~osed process is not appropriate for the production of steel powders, i.e.
those with carbon contents below about 1.7%.
This invention provides a method for producing water-atomized steel powders with an apparent density greater than 3.2 gm/cc, and preferably greater than 3.4 gm/cc.
The method produces water-atomized steel powders, with ;
an apparent density equal to or greater than 3.2 gm/cc, wherein a major portion of the as-atomized powders can be utilized therein.
The resulting molding grade steel powders exhibit a combination of high apparent density and a green strength suffic-ient to permit normal handling before sintering.
Thus the present invention provides a method for the production of molding grade steel powders with high apparent densities, which comprises, ~a) providing as-water-atomized ste~l particles with a prescribed size distribution, wherein at least 80% of said particles are finer than 80 mesh and said distribution exhibits a PSC value of between 2.0 to 4.0, (b) annealing said particles at a temperature within the range of 1400 -2100 F for a time at least sufficient to (i) -effect the desired softening thereof, and (ii) reduce the oxygen 3~ content thereof to a value below about 0.2 weight percent, said annealing causing said particles to sinter together, ~ ~3~
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(c) feeding t:he annealed, sintered particles to a disk mill operated at a speed of between about 200 to 5000 revolutions per minute and a mill gap of between about 0.01 to 0.10 inches, wherein the linear speed _ of said disks is sufficiently high and the mill gap G is sufficiently close to grind said cake to molding grade powders with an apparent density in excess of 3.2 ym/cc, substantially all of which are finer than 80 mesh.

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Figure 1 is a graphical representation showing the effect of particle size distribution, disk speed and a mill gap of 1/16-~inch on apparent denslty.
Figure 2 is a graphical representation showing ~he effect5 of particle size distribution, disk speed and a mill-gap of 1/64-inch on apparent density.
The method of this invention is applicable to water-atomized steel powders from virtually any source. Water-atomized steel powders generally contain impurities, primarily ln the form of oxides, that must be removed before the powder has commercial value for the production of powder-metallurgical parts. In order to produce steel powders with maximum compressibility, it is also desirable that the final powders have a carbon content below about ` 0.10~, and preferably below 0.01~. However, it is generally ~ impractical to provide an initial steel melt with such a low ; carbon content. Therefore, such steel melts may contain up to , o.8~0 C, but preferably less than about 0.15% C, and the carbon content of the atomized powders is thereafter lowered by annealing i in a decarburizing atmosphere. Atomization by high pressure water jets results in rapid quenching of the liquid metal droplets during the early stage of the atomization process. Therefore, even if a relatively low carbon steel were employed (i.e. eliminating the need for decarburization) in the atomization process, it would still be necessary to anneal the powders to effect both softening and lowering the oxygen content thereof (to a value below about 0.2~o). The initial oxygen contents of the as-water-atomized particles is generally far in excess of 0.2~, generally about l.O~o.

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As a result of thls high surface oxygen content and the particle configuration thereof, the as-atomized particles will pack to a high apparent density, i.e. well in excess of 3.2 gm/cc. However, after the requisite annealing and reduction of the oxygen content thereby~
the apparent density will be within the range of 2 8 to 3.2 gm/cc.
Annealing is conducted at temperatures of 1400F to 2loooF~ in a reducing atmosphere such as hydrogen or dissociated ammonia for a time sufficient to effect the desired softening and reduction of impurities. This annealing treatment not only purifies the steel po~lder, but causes the particles to stick together in the form of a sintered cake~ thereby necessitating a breaking up of the cake to ,return it to powder consistency. In thé process of U. S. Patent !3,325,277~ this requisite break-up is accomplished in a hammermill;
employing impact shattering to return ~he particles to their orl~lnal as-atom.ized size. The instant invention departs from this process .by performing a true grinding operation in a disk mill; employing a shear mechanism for comminution. It has now been found t,hat by regulation of such a grinding operation, the final apparent density can be tailored to specific requirements, depending on the size distribution of the original as-atomized particles.
The size distribution of the as-atomized powder may be determined by conventional screen analysis. This screen analysis is then employed to develop the particle size characteristic (PSC) of the powder. It has been found that unduly coarse, as-atomized particles cannot be ground to achieve the desired objects of this invention. Thus, to achieve the requisite grindingJ it is necessary that at least 80%, and preferably greater than 95~, of ~the as-atomized particles be finer than 80 mesh (U. S. Series).

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While a number of dif~erent methods are available for deflning PSC value, for purposes o~ this invention, this value ls determined in the following manner. A cumulative weight percenkage is first determined of the particles that are retalned on U. S. Standard 100-, 140-, 200-~ 230- and 325- mesh screen and the pan fractlon.
Thereafter, the so-determined cumulative percentages are totalled and divided by 100. Thus, utilizing this definition, an increase in PSC reflects a coarser particle size distribution and a low PSC is indicative of a fine particle size distribution.
For example, the PSC of the following po~der would be calculated as follows:
,U. S. Standard ~ Cumulative Mesh Retained ~ Retained oo 2,1~ . 2.
l~ o : 5.3 7.7 j,200 16.1 23.8 '~230 28,7 325 12.9 41.6 Pan 58.4 100.0 100.0 204 .2 ThereforeJ the PSC of this powder would be 204 2/100, or 2.o4.
Water atomized particles with PSC values of about 2 0 to 4.0 may be effectively employed in the instant process. Once the PSC is known, and the powder has been annealed, a grinding cycle can be established to tailor the properties to specific requirements.
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A Disk Attrltion Mill is then employed to effect the requisite grinding. As a result of annealing, the particles sinter together in the form of a calce. If necessary~ the resultant sinter cake is first broken in pieces small enough, generally less than about one inch, to be fed into the Disk Attrition Mill. In such a mill, grindlng occurs between disks, which generally rotate in either a vertical or horizontal plane. The feed enters near the center of the disk, travels by centrifugal force to the peripheral, grinding-plate portion thereof, and is then discharged. While in certain disk mills, spike tooth plates have been employed, it should be understood that such plates are not applicable to the instant invention, which is limited to the use of conventional, friction grinding plates. The mill gap referred to hereln, is the distance , between the grinding plates. The disk mill is partlcularly sui~ed for the purposes o~ this invention since it has been found that su~
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a mill is capable of yielding a controlled and predictable degree of grinding which is basically a function of (a) the mill gap, and (b) the linear speed o~ a point on medial radius rm, of the 'grinding plates. In a disk mill, the locus of the grinding plates ~orm a ring, (i.e. two concentric circles); where the distance from the center to the grinding plate, i.e. from the center to inner circle is rl. The distance from the center to the . .
peripheral portion of the grinding plate, i.e. from the center to the outer circle, is r2. Therefore, the medial radius rm is then rl+r2/2. Since linear speed, v, is equal to the angular speed (w) times the radius, the linear speed of a point on the i~ !
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32~ 1 medial radius may easily be determined from the revolutions per minute Or the grinding plates. Thus, for example, if grinding plates with a rm Or 12 inches are rotated at 3000 rpm's, the linear speed (v) will be:
_ = ~r or v = 3000 x 2~r x 12 = 72,000tr inches/min.
Through the use of statistical regression and ~;
engineering interpretation analysis, the effect of the above variables on the apparent density o~ the final product powder was found to be described by the equation:
Apparent Density (g/cc) _ 2,16 -~ 0.30 PSC
-1.28 10 5v t-2.87 10 LG -~1.93 10 v PSC
+4.00 10-11 v2 -3.96 1o-6 v- LG
where PSC is the particle size characteristic of the as-water atomized po~ider~ prior to annealing v is the linear speed of a point on the medial radius of the grinding plates, in inches per minute, and _ is the Log of (mill gap in inches).
Throu~h the use of the above equation, a grinding cycle can therefore be established to tailor the properties of the final product powder to specific requirements. To provide a better understanding of the use thereof, the process equation was employed to develop the graphs of Figures 1 and 2, for a laboratory sized disk mill with a 13-inch diameter dis~, having a rm of 5.31 inches.

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For ease of interpretation, (i.e. the avoidance of hlghly cumbersome numbers) the linear speed v, was converted to the rpm _ ;of this disk mill. It should be understood, however, that these graphs are only applicable to a mill with a rm Of 5.31 inches. In commercial practice, a larger diameter disk mill would generally be employed. The curves of Figures 1 and 2 would then be shifted to lower rpm values as the size of the mill is increased, since the linear speed v (at any given rpm) would be correspondingly higher.
In general, such mills will be operated at speeds of about 200-5000 rpm, with mill gaps ranging from about 0.01 to 0.10 inches.
The utilization Or the graphs of Figures 1 and 2 will be ~.
described for powders exhibiting the f~llowing exemplary screen analyses:
Powder-~100 ~ 0 ,~200 ,-230 l-325 Pan PSC
l ~10.4 18.0 26.0 6.2 15.3 2L~.1 3.29 B4.4 9-5 21.0 6.o 17 41.6 2.52 If as-atomized powder A were to be employed, and the mill were to be operated at a gap of 1/16 inch (Figure 1), it may be seen, for example, that apparent densities of 3.2 gm/cc and 3.45 gm/cc could be achieved by employing speeds of 2500 rpm and 3650 rpm respectively.
The effect of reduced mill gap may be seen by ~comparison with Figure 2. If the same powder (A) were to be employed, similar densities could be achieved at materially lower disk speeds. Thus, a speed of only 1600 rpm would produce a ~- product with a density of 3.2 gm/cc, while a density of 3.45 gm/cc would be achieved with disks operated at a speed of 2775 rpm.

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1~34~4 Powder B, belng inherently finer, cannot be as readily used (in a small diameter mill) to produce high apparent densities.
~Nevertheless, if the mill gap is reduced to 1/64 inch, as in F~gure 2, the equivalent apparent densitles can be produced at disk speeds Or 3225 rpm and 4050 rpm respectively.
From the illustrative examples above (or from the process equation itself) it may therefore be seen that apparent density ; -increases as:
(a) the PSC value of the as-atomized particles is increased, (b) the disl~ speed of the mill is lncreased, and il i (c) the mill gap ls decreased.
, It was also found, ~llthln khe specl~ied temperature range Or ; 1400-2100F, that apparent denslty could be slightly increased by 1 decreasing the annealing temperature. However, the use of lower ' temperatures would necessitate the employment of longer annealing periods. As a practical compromise of these competing effects, i.e., the achievement of high apparent density within a reasonably short annealing time, it is therefore preferable to anneal within the range o~ about 1700 to 1~00F.

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Claims (6)

We claim:

1. A method for the production of molding grade steel powders with high apparent densities, which comprises, (a) providing as-water-atomized steel particles with a prescribed size distribution, wherein at least 80% of said particles are finer than 80 mesh and said distribution exhibits a PSC value of between 2.0 to 4.0, (b) annealing said particles at a temperature within the range of 1400°-2100°F for a time at least sufficient to (i) effect the desired softening thereof, and (ii) reduce the oxygen content thereof to a value below about 0.2 weight percent, said annealing causing said particles to sinter together, (c) feeding the annealed, sintered particles to a disk mill operated at a speed of between about 200 to 5000 revolutions per minute and a mill gap of between about 0.01 to 0.10 inches, wherein the linear speed v of said disks is sufficiently high and the mill gap G is sufficiently close to grind said cake to molding grade powders with an apparent density in excess of 3.2 gm/cc, substantially all of which are finer than 80 mesh.

2. The method of claim 1, wherein said linear speed v and said mill gap G are correlated with the PSC value of said particles in accord with the following equation:
+0.30 PSC -1.28 ?10-5v +2.87 ?10-2 LG +1.93 ?10-6?v PSC
+4.00 ?10-11 ?v2 -3.96 ?10-6?v? LG > 1.04

3. The method of claim 2, wherein at least 95% of said as-water-atomized particles are finer than 80 mesh, with a major portion finer than 200 mesh.

4. The method of claim 2, wherein the carbon content of said as-atomized particles is less than 0.15% and said annealing temperature is between about 1700°-1900°F.

5. The method of claim 4, wherein the PSC value of said atomized particles is greater than about 2.5 and the speed v and mill gap G are correlated with said PSC value in accord with the following equation:
+0.30 PSC -1.28?10-5v +2.87?10-2 LG?+1.93 ?10-6?v?PSC
+4.00 ?10-11 v2 -3.96 ?10-6 ?v? LG > 1.24 to yield a molding grade powder product with an apparent density greater than 3.4 gm/cc.

6. The method of claim 5, wherein at least 95% of said as-water-atomized particles are finer than 80 mesh, with a major portion finer than 200 mesh.