Patent Publication Number: US-4836850-A

Title: Iron group based and chromium based fine spherical particles

Description:
This application is a division, of application Ser. No. 905,015 U.S. Pat. No. 4,778,515 issued Oct. 18, 1988filed Sept. 8, 1986. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This invention is related to the following applications: &#34;Fine Spherical Particles and Process For Producing Same,&#34; now U.S. Pat. No. 4,756,746, &#34;Spherical Refractory Metal Based Powder Particles and Process For Producing Same,&#34; now U.S. Pat. No. 4,783,218, &#34;Spherical Copper Based Powder Particles And Process For Producing Same,&#34; now U.S. Pat. No. 4,711,661, &#34;Spherical Precious Metal Based Powder Particles And Process For Producing Same,&#34; now U.S. Pat. No. 4,711,660, &#34;Spherical Light Metal Based Powder Particles And Process For Producing Same,&#34; now U.S. Pat. No. 4,780,131, &#34;Spherical Titanium Based Powder Particles And Process For Producing Same,&#34; now U.S. Pat. No. 4,783,216, all of which are filed concurrently herewith and all of which are by the same inventors and assigned to the same assignee as the present application. 
     BACKGROUND OF THE INVENTION 
     This invention relates to fine spherical powder particles and to the process for producing the particles which involves mechanically reducing the size of a starting material followed by high temperature processing to produce fine spherical particles. More particularly the high temperature process is a plasma process. 
     U.S. Pat. No. 3,909,241 to Cheney et al relates to free flowing powders which are produced by feeding agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles and collecting the particles in a cooling chamber containing a protective gaseous atmosphere where the particles are solidified. 
     Fine spherical metal particles such as iron, cobalt, nickel, chromium, and alloys thereof are useful in applications such as filters, precision press and sinter parts, and injection molded parts. Typical alloys include but are not limited to low alloy steels, stainless steels, tool steel powders, nickel and cobalt based superalloys. In such applications the powders are consolidated by standard methods such as hot or warm extrusion, PM forging and metal injection molding, or pressing and sintering. 
     Some of the better commercial processes for producing such metal powder particles are by gas or water atomization. Only a small percentage of the powder produced by atomization is less than about 20 micrometers. Therefore, yields are low and metal powder costs are high as a result and in the case of water atomization, the powder is often not spherical. 
     In European Patent Application No. WO8402864 published Aug. 2, 1984, there is disclosed a process for making ultra-fine powder by directing a stream of molten droplets at a repellent surface whereby the droplets are broken up and repelled and thereafter solidified as described therein. While there is a tendency for spherical particles to be formed after rebounding, it is stated that the molten portion may form elliptical shaped or elongated particles with rounded ends. 
     A process for efficiently producing fine spherical metal particles would be an advancement in the art. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of this invention there is provided a powdered material which consists essentially of iron group and chromium based spherical particles. The particles are essentially free of elliptical shaped material and elongated particles having rounded ends. The material has a particle size of less than about 20 micrometers. 
     In accordance with another aspect of this invention, there is provided a process for producing the above described powdered material. The process involves mechanically reducing the size of a starting material to produce a finer powder the major portion of which has a particle size of less than about 20 micrometers. The finer powder is entrained in a carrier gas and passed through a high temperature zone at a temperature above the melting point of the powder to melt at least about 50% by weight of the powder and form the spherical particles of the melted portion. The powder is then directly solidified. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of some of the aspects of the invention. 
     The starting material of this invention can be iron group based materials or chromium based materials. The term &#34;based materials&#34; as used in this invention means the metal or any of its alloys, with or without additions of compounds selected from the group consisting of oxides, nitrides, borides, carbides, silicides, as well as complex compounds such as carbonitrides. The iron group based materials as used in this invention can be iron, cobalt and nickel. The especially preferred materials are stainless steels, low alloy steels, tool steels, maraging steels, and high speed steels, alloys of iron and nickel with varying amounts of carbon ranging from about 0.00% to about 1.5% by weight, nickel and cobalt-based wear resistant alloys, and alloys of iron containing an additional element selected from the group consisting of aluminum, cobalt, and mixtures thereof. 
     The size of the starting material is first mechanically reduced to produce a finer powder material. The starting material can be of any size or diameter initially, since one of the objects of this invention is to reduce the diameter size of the material from the initial size. The size of the major portion of the material is reduced to less than about 20 micrometers in diameter. 
     The mechanical size reduction can be accomplished by techniques such as by crushing, jet milling, attritor, rotary, or vibratory milling with attritor ball milling being the preferred technique for materials having a starting size of less than about 1000 micrometers. 
     A preferred attritor mill is manufactured by Union Process under the trade name of &#34;The Szegvari Attritor&#34;. This mill is a stirred media ball mill. It is comprised of a water jacketed stationary cylindrical tank filled with small ball type milling media and a stirrer which consists of a vertical shaft with horizontal bars. As the stirrer rotates, balls impact and shear against one another. If metal powder is introduced into the mill, energy is transferred through impact and shear from the media to the powder particles, causing cold work and fracture fragmentation of the powder particles. This leads to particle size reduction. The milling process may either wet or dry, with wet milling being the preferred technique. During the milling operation the powder can be sampled and the particle size measured. When the desired particle size is attained the milling operation is considered to be complete. The particle size measurement is done by conventional methods as sedigraph, micromerograph, and microtrac with micromerograph being the preferred method. 
     The resulting reduced size material or finer powder is then dried if it has been wet such as by a wet milling technique. 
     If necessary, the reduced size material is exposed to high temperature and controlled environment to remove carbon and oxygen, etc. 
     The reduced size material is then entrained in a carrier gas such as argon and passed through a high temperature zone at a temperature above the melting point of the finer powder for a sufficient time to melt at least about 50% by weight of the finer powder and form essentially fine particles of the melted portion. Some additional particles can be partially melted or melted on the surface and these can be spherical particles in addition to the melted portion. The preferred high temperature zone is a plasma. 
     Details of the principles and operation of plasma reactors are well known. The plasma has a high temperature zone, but in cross section the temperature can vary typically from about 5500° C. to about 17,000° C. The outer edges are at low temperatures and the inner part is at a higher temperature. The retention time depends upon where the particles entrained in the carrier gas are injected into the nozzle of the plasma gun. Thus, if the particles are injected into the outer edge, the retention time must be longer, and if they are injected into the inner portion, the retention time is shorter. The residence time in the plasma flame can be controlled by choosing the point at which the particles are injected into the plasma. Resicence time in the plasma is a function of the physical properties of the plasma gas and the powder material itself for a given set of plasma operating conditions and powder particles. Larger particles are more easily injected into the plasma while smaller particles tend to remain at the outer edge of the plasma jet or are deflected away from the plasma jet. 
     As the material passes through the plasma and cools, it is rapidly solidified. Generally the major weight portion of the material is converted to spherical particles. Generally greater than about 75% and most typically greater than about 85% of the material is converted to spherical particles by the high temperature treatment. Nearly 100% conversion to spherical particles can be attained. The major portion of the spherical particles are less than about 20 micrometers in diameter. The particle size of the plasma treated particles is largely dependent of the size of the material obtained in the mechanical size reduction step. As much as about 100% of the spherical particles can be less than about 20 micrometers. 
     More preferred particle sizes are less than about 15 micrometers in diameter and most preferably less than about 10 micrometers in diameter, and it is preferred that the particles be greater than about 3 micrometers in diameter. Such powders are used in applications such as metal powder injection molding, powder forging, press and sinter, and other precision and conventional powder consolidation techniques. 
     The spherical particles of the present invention are different from those of the gas atomization process because the latter have caps on the particles whereas those of the present invention do not have such caps. Caps are the result of particle-particle collision in the molten or semi-molten state during the gas atomization event. 
     After cooling and resolidification, the resulting high temperature treated material can be classified to remove the major spheroidized particle portion from the essentially non-spheroidized minor portion of particles and to obtain the desired particle size. The classification can be done by standard techniques such as screening or air clasification. The unmelted minor portion can then be reprocessed according to the invention to convert it to fine spherical particles. 
     The powdered materials of this invention are essentially spherical particles which are essentially free of elliptical shaped material and essentially free of elongated particles having rounded ends. These characteristics can be present in the particles made by the process described in European Patent Application No. WO8402864 as peviously mentioned. 
     Spherical particles have an advantage over non-spherical particles in injection molding and pressing and sintering operations. The lower surface area of spherical particles as opposed to non-spherical particles of comparable size, and the flowability of spherical particles makes spherical particles easier to mix with binders and easier to dewax. 
     To more fully illustrate this invention, the following non-limiting example is presented. Example 
     About 2.5 kilograms of coarse gas atomized iron alloy is milled in a Union Process 1-S laboratory attritor mill. Tungsten carbide 1/4&#34; balls are used as media with n-hexane as a milling fluid. The powder is milled for about 4 hours at about 155 rpm agitator speed. The speed is reduced to about 140 rpm and milling continues for about an additional 10 hours. The powder slurry is heated to evaporate the n-hexane, yielding dry powder. This size reduced powder is fed to a plasma heat source with argon as a carrier gas at a flow rate of about 3 liters per minute. The plasma torch is run at the following conditions: 
     Gas flow: 
     Argon--about 28 liters per minute 
     Helium--about 25 liters per minute 
     Power: 10.5 kw. 
     The powder is collected after plasma melting. It is then screened and air classified to obtain the desired particle size, as well as to remove most of the minor portion of non-spherical particles. 
     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.