GB1593029A

GB1593029A - Powder metallurgical processes

Info

Publication number: GB1593029A
Application number: GB22699/78A
Authority: GB
Original assignee: American Can Co
Current assignee: Primerica Inc
Priority date: 1977-06-27
Filing date: 1978-05-25
Publication date: 1981-07-15
Also published as: DE2827694A1; JPS5411008A; FR2395800A1; FR2395800B3

Description

(54) IMPROVEMENTS RELATING TO POWDER METALLURGICAL PROCESSES (71) We, AMERICAN CAN COMPANY, a corporation organised and existing under the laws of the State of New Jersey, United States of America, residing at American Lane, Greenwich, Connecticut 06830, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to improvements in powder metallurgical processes, for example such processes as make use of ferrous particles.

Rapid sintering of metal powders is an ideal which has long been sought in the powder metallurgy art. Speed is important in production lines and in reducing costs.

Another desire of powder metallurgy parts producers is to manufacture parts for high performance applications which require stronger particle to particle bonds and fewer inter-metallic inclusions than obtainable with presently available commercial powders.

Typically, powder metallurgy particles are produced with a minimum of surface oxides.

If oxides occur, they may be removed, for example, by placing the particles in a reducing atmosphere. Such a technique allows good quality parts to be made, but requires substantial sintering times which may last from thirty minutes upward.

Thirty minutes is a typical minimum time for sintering of the best prepared as-wateratomized ferrous metal particles where the surface oxides have been removed by chemical reduction.

As-water-atomized ferrous metal particles are particles produced by intercepting a downwardly moving stream of molten steel by jets of water. The stream is broken and the resulting metallic particles drop into a pool of water where they are cooled. The core material of the particles is mostly martensite covered with a skin of mostly iron oxides with some alloy oxides and a small residue of other materials such as silica. As-water atomized ferrous metal particles are suitable for treatment by the process according to this invention.

When as-water-atomized ferrous metal particles are made, some oxides are produced by reaction of the cooling water on the skin of the particles. These oxides are easily reducible by heating in a reducing atmo sphere.

Other ferrous and ferrous alloy oxides in the skin, however, are suspended in the melt from which the particles are made. These ferrous alloy oxides historically are difficult to reduce by normal reduction annealing.

The oxides appear along with molten steel in -the molten stream, from which particles are made. The steel melting temperature is sub stantially higher than the oxides' melting temperature, so that as the particles solidify, the steel solidifies before the oxides solidify.

Although some of the oxides are trapped in the ferrous metal cores, most of the oxides, which are sticky when hot, adhere to the outside of the ferrous metal cores and harden to form a brittle skin.

In the conventional production of ferrous metal powders, as-water-atomized particles are subjected to a reduction anneal in a reducing atmosphere but, while this treat ment reduces iron oxides to metallic iron, ferrous alloy oxides remain essentially unaf fected, and these constitute diffusion barriers and give rise to nonmetallic inclusions in powder metal products. It is an objective of the present invention to remove the iron oxides and ferrous alloy oxides from ferrous powder particle surfaces.

By this invention improved sinterability of metal particles is accomplished by cracking and shattering the oxide skin of the particles, by confining them in a whirling high velocity gas stream to cause them to strike each other.

The cracked skins. constituted predominantly by oxides which are very brittle, are shattered into a fine dust-like powder while the underlying cores of metal powder are burnished. The burnishing removes sharp points and this enhances Ilow characteristics of the powder. The surfaces of the cores are mechanically strained in the process which promotes and enhances sintering.

The invention accordingly provides a process of improving metal particles prepared tor sintering in a powder metallurgical process, wherein oxide skins on the particles are detached, to expose the underlying cores, and the surfaces of the latter are strained, by subjecting the particles to impacts in a turbulent gas whirlwind under conditions which (i) cause cracking and shattering of the oxide skins and burnishing of the cores and (ii) avoid pulverising or subdividing the cores.

The invention also provides a powder metallurgical process wherein the metal particles to be sintered are subjected to impacts in a turbulent gas whirlwind under velocity conditions which cause cracking and shattering of oxide skins on the particles, to detach the skins and expose the underlying particle cores, and cause burnishing of the cores without pulverising or subdividing them, to store strain energy in the surfaces of the cores, the strained cores then being compacted and sintered.

The resulting mixture of relatively large particles of metal, together with a fine particulate of oxide materials, may be conditioned, and then press-molded, after which the shaped part is sintered. The conditioned particles may be compacted into a sheet or strip and sintered; instead it may be compacted or press-molded in a die before sintering. If desired, a compacted shape may be pre-sintered and forged to a near-net shape.

When ferromagnetic particles are to be processed, the skins removed from the cores of the particles by impaction and reduction to powder can be separated by a magnetic separator. The e.g. oxide powder and dust is nonferromagnetic, whilst the cores are ferromagnetic.

The metallic particles may be delivered to a double screen sifter wherein the first screen removes all particles which exceed a predetermined size. The second screen removes all particles smaller than a second predetermined size. Sifting is suitable for condition- ing not only a non-ferromagnetic metal powder, but also ferromagnetic material.

Then, sifting can be used alone or as an adjunct to magnetic separation. The particles remaining after separation by sifting may optionally then be delivered to a blender to produce a powder having a random distribution of particle sizes within the predetermined size range governed by the two sifter screens.

The resulting particles will have substantially no loose oxides, for the oxides will have crumbled into fine grain powder and then have been removed by the double screen sifter.

The surfaces of the remaining particles will be metallic and shiny, and sharp spikes and hooks will be blunted whereby the particles may flow easily, for example, into a compacting mill roll gap or into a compacting die.

Further, the highly burnished surfaces contain a substantial amount of energy of deformation in the form of induced stresses caused by the burnishing. The high surface energy of the particle surfaces is conducive to rapid sintering and tougher particle to particle bonds. For example, the particles may be sintered into strip at a temperature of 2300F in a time of less than two minutes. The toughness, defined as the area under a stressstrain curve, of sintered bars of particles, and the speed of sintering are increased compared to sintered bars of prior art powder.

The present invention will now be described by way of example only with reference to the accompanying drawings, in which: Fig. I is a block diagram of a first embodiment of a powder metallurgical process of this invention; Fig. 2 is a block diagram of a second embodiment of a powder metallurgical process of this invention; and Fig. 3 is a schematic diagram of a typical apparatus used to practice the process of this invention.

The process now to be described in detail modifies metal powder particles, which may be ferrous or not ferrous and here are exemplified particularly as-water-atomized ferrous metal particles, by removing the outer, oxide skin which shatters into a fine powder. The cores of the particles are burnished to remove sharp points and to strain the surface thereof to promote and enhance the speed of sintering and the toughness of the sintered product. Sintered mill products and engineering components prepared from the treated metal particles exhibit low oxide contents and porosities.

The bright metal burnished steel cores are initially mixed with the finely shattered oxide material which can be separated from the cores if desired. That mixture can then be pressed, e.g. in a die and sintered, and the sintering occurs extremely rapidly to produce a metal sinter having tough core-to-core bonds with only fine oxide inclusions due to the finely divided shattered oxides. Pores are substantially eliminated during the rapid sintering.

If preferred, the finely divided oxides may be removed, and the bright metal cores remain to produce lower oxide-containing metal.

The non-treated particles, stored in a storage bin 10 are delivered through a conduit 12 to an air impact machine 14. The particles are typically delivered into the air impact machine where they collide with each other, bounce off of the walls of the machine, and occasionally hit the rotor which creates a high velocity highly turbulent whirling gas stream to crack and shatter the skins of the particles, leaving substantially solid metal cores which then are burnished by continued striking together of the cores. The burnishing removes sharp points allowing the bright metal cores to flow more easily and places a strain in the surface of the bright metal cores thereby promoting and enhancing the ability of the cores to sinter.

A typical machine 14 is an air impact pulverizer. The machine has a plurality of radially directed vanes on its rotor. The unprocessed particles are introduced along the axis of the rotor, and the high velocity gas stream causes them to whirl rapidly around the rotor axis. The inertia of the particles causes them to move radially outward until they strike a plurality of bars which are positioned around the periphery of the drum or housing and which are stationary. These bars deflect the particles in random directions toward the center of the rotor. The specific direction of motion of the particles after striking the outer bars or striking other particles is not precisely known, but it is known that a substantial gas turbulence is produced to increase the probability of one particle striking another particle. One circumferential zone of the machine 14 has an opening and a screen.The openings in the screen are substantially larger than the particles, and the opening size determines the amount of average dwell time of the cores of the particles within the air impact machine 14. The shattered oxides, however, being very small are drawn off more rapidly and shortly after being shattered. The cores of the particles remain for a time within the air impact machine where continued striking against each other burnishes the cores, removes sharp spikes, and places a strain in the surface of the burnished cores. Each of the burnished cores eventually leaves the air impact machine via the opening 16. The cores and shattered skins are delivered through a conduit 18 into a filter 20 where the particulates are collected from the gas or air which is pumped out of the air impact machine 14.The burnished cores together with the shattered finely divided oxides may be delivered, for example, into a hopper 22.

It has been found that the highly burnished cores with strained surfaces sinter rapidly. For example, typical conventional powder metallurgy sintering may take fortyfive minutes at 2050 F. Sintered products made by the process of this invention have equivalent strength and toughness when sintered from one-sixth ro one-third the time at 2050"F. That is, 15 minutes at 2050"F produces strength and toughness properties equal to that obtained by forty-five minutes of sintering of conventional particles.

Further, if the sintering temperature is increased to 2300 F, compacted particles produced by the process of this invention reach a toughness in two minutes of sintering time, equal to the touchness obtained in 15 minutes at 2050 F. To obtain the same levels in compacts of conventionally produced powder would require a sintering treatment of about forty-five minutes at 2050"F.

Metallurgical inspection of sintered products reveals that the pores of the rapidly sintered material of this invention are substantially fewer and smaller than the pores in slower sintered materials.

Three samples of the same lot of as-wateratomized ferrous particles were prepared.

Samples I and II were not treated according to the process of this invention. Sample III was treated according to the invention.

Samples I and III were sintered for five minutes, and Sample II was sintered for thirty minutes at 2050 F. Identically shaped and sizes samples of the sintered product were tensile tested. Sample I had an ultimate strength of 18,300 psi and an elongation of 4%. Samples II and III had almost identical ultimate strengths of 22,000 and 22,200 psi.

Sample III, however, had an elongation of 6% compared to the elongation of 5% for Sample II. Thus, the toughness of Sample III was greater than that of Sample II and over three times greater than Sample I.

When a low oxide steel is desired, the magnetic separator 24 may be used to separate the ferromagnetic particles from the nonferromagnetic particles. All the particles in the separator 24 are delivered onto a belt 26. A plurality of electromagnets 28 moves with and adjacent to the belt 26. The electromagnets 28 are cycled so that they first attract magnetic particles from the belt 26 then drop the attracted particles onto the top of the layer of particles on the belt 26. That process is repeated as the mixture proceeds along the belt 26. As it comes to the end of the belt 26, the magnets 28 pick up the ferromagnetic particles and deliver them to the hopper 30. The nonferromagnetic materials are delivered to the hopper 32 and then into a discard bin (not shown).

The ferromagnetic particles are delivered to a pair of sieves or wire strainers 36 and 38 which are preferably agitated (not shown).

The mesh of the sieve 36 is large so that only very large particles are blocked. The remain der of the material goes through the sieve 36 onto the sieve 38 which passes the finely shattered oxides. The very large particles are removed through hopper 35; the very fine particles are removed through hopper 37.

The metallic cores within a predetermined range sizes, are then pumped by pump 39.

and delivered through conduit 40 to the blender 42.

Alternatively, if the desired particles are nonferromagnetic, the magnetic separator 24 may be bypassed.

The blender 42 receives particles from the conduit 40, and air introduced through the conduits 44 is directed to cause the particles in the blender 42 to swirl to mix the various sizes into a homogenous mixture. The resulting mix is delivered to the hopper 46.

The mix in the hopper 46 may, if desired, be further processed by compacting and sintering, then further cold and/or hot rolling it to produce a strip of low oxide metal.

Thus, the process of this invention substantially reduces the oxides in the powder particles to produce low oxide metal and particularly low oxide steel. Such low oxide steel is preferably made from as-wateratomized ferrous metal particles.

Just as importantly the invention produces metallurgical powder which sinters very rapidly into tough low pore metal shapes. It particularly produces compacted and sintered as-water-atomized particles for very tough steel products.

WIIAT WE CLAIM IS: 1. A process of improving metal particles prepared for sintering in a powder metallurgical process, wherein oxide skins on the particles are detached, to expose the underlying cores, and the surfaces of the latter are strained, by subjecting the particles to impacts in a turbulent gas whirlwind under conditions which (i) cause cracking and shattering of the oxide skins and burnishing of the cores and (ii) avoid pulverising or subdividing the cores.

2. A process according to claim 1, wherein said particles include ferrous metal, the process further comprising magnetically separating ferromagnetic particles from nonferromagnetic particles.

3. A process according to claim 1 or claim 2, further comprising separating and discarding particles which are larger or smaller than a predetermined size range.

4. A process according to claim 3, further comprising blending the particles which are within the said predetermined size range.

5. A process according to any of the claims I to 4, wherein the particles are randomly impacted.

6. A process according to claim 5, wherein the particles are as-water-atomizediron.

7. A process of preparing metal powder for sintering in a powder metallurgical process, substantially as herein described with reference to the accompanying drawings.

8. A process according to any of claims 1 to 7, further comprising compacting the ferromagnetic particles and sintering and compacted particles.

9. A powder metallurgical process wherein the metal particles to be sintered are subjected to impacts in a turbulent gas whirlwind under velocity conditions which cause cracking and shattering of oxide skins on the particles, to detach the skins and expose the underlying particle cores, and cause burnishing of the cores without pulverising or subdividing them, to store strain energy in the surfaces of the cores, the strained cores then being compacted and sintered.

10. A process according to claim 9, wherein the particles are randomly impacted.

I I. A process according to claim 9 or 10, wherein the particles are of ferrous metal.

12. A process according to claim 11, wherein the particles are as-water-atomized iron.

13. A powder metallurgical process substantially as herein described with reference to and as shown in the accompanying drawings.

14. Powder for use in a powder metallurgy process when prepared by the process as claimed in any of claims 1 to 7.

15. A sintered produced prepared by the powder metallurgical process as claimed in any of claims 8 to 13.

16. Plant for use in practising the process claimed in any of claims 1 to 8 constructed and arranged to operate substantially as herein described with reference to and as shown in Fig. 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. der of the material goes through the sieve 36 onto the sieve 38 which passes the finely shattered oxides. The very large particles are removed through hopper 35; the very fine particles are removed through hopper 37. The metallic cores within a predetermined range sizes, are then pumped by pump 39. and delivered through conduit 40 to the blender 42. Alternatively, if the desired particles are nonferromagnetic, the magnetic separator 24 may be bypassed. The blender 42 receives particles from the conduit 40, and air introduced through the conduits 44 is directed to cause the particles in the blender 42 to swirl to mix the various sizes into a homogenous mixture. The resulting mix is delivered to the hopper 46. The mix in the hopper 46 may, if desired, be further processed by compacting and sintering, then further cold and/or hot rolling it to produce a strip of low oxide metal. Thus, the process of this invention substantially reduces the oxides in the powder particles to produce low oxide metal and particularly low oxide steel. Such low oxide steel is preferably made from as-wateratomized ferrous metal particles. Just as importantly the invention produces metallurgical powder which sinters very rapidly into tough low pore metal shapes. It particularly produces compacted and sintered as-water-atomized particles for very tough steel products. WIIAT WE CLAIM IS:

1. A process of improving metal particles prepared for sintering in a powder metallurgical process, wherein oxide skins on the particles are detached, to expose the underlying cores, and the surfaces of the latter are strained, by subjecting the particles to impacts in a turbulent gas whirlwind under conditions which (i) cause cracking and shattering of the oxide skins and burnishing of the cores and (ii) avoid pulverising or subdividing the cores.