GB1602582A - Ferrous powder metallurgy - Google Patents

Description

PATENT SPECIFICATION

( 11) 1 602 582 ( 21) ( 62) ( 31) ( 32) ( 33) ( 44) ( 51) Application No 27767/80 ( 22) Filed 25 May 1978 Divided out of No 1602581 Convention Application No 809794 Filed 24 Jung 1977 in United States of America (US)

Complete Specification published 11 Nov 1981

INT CL 3 B 22 F 1/00 ( 19) ( 52) Index at acceptance C 7 D 8 M AI A 3 ( 72) Inventors ALBERT J KLEIN CHIOU-TSE CHEN and LOU KOHL ( 54) IMPROVEMENTS RELATING TO FERROUS POWDER METALLURGY ( 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 generally to improvements in the field of ferrous powder metallurgy.

More particularly, this invention pertains to the conditioning of ferrous particles perparatory to sintering Conditioning inter alia involves removal of surface oxides from such particles.

Ferrous powder which may have its oxide skins reduced by techniques embodying this invention is an as water atomized ferrous metal particle powder.

For clarity in describing the invention, the term "as water atomized ferrous metal particles" refers to particles made by water atomizing and in the condition in which they come out of a water atomizing apparatus without further processing or conditioning except for normal drying That is, the particles have an irregular shape with a martensitic structure, and have a covering of whatever oxides and other impurities are present when they leave the water atomizing apparatus.

Agglomeration is defined herein as any interparticle bonding, visible to the naked eye, which persists through cooling and/or subsequent normal powder handling.

Typical techniques for making as-water atomized ferrous powder are described in United States patents of R A Huseby.

U S Patent 3,325,277 for a "Method of Making Metal Powder" shows the particles have a high density and an irregular shape.

Molten steel is fed by gravity in a downwardly moving stream A plurality of flat sheets or curtains of water are impinged against the stream of molten steel at an angle greater than 50 to atomize the stream of steel into ferrous particles with iron oxide surfaces.

U S Patent 3,309,733 for "Apparatus for Producing Metal Powder" is directed to apparatus, including an improved nozzle assembly, for practicing the process of U S.

Patent 3,325,277.

U S Patent 3,334,408 to M D Ayers for "Production of Powder, Strip and Other Metal from Refined Molten Metal" teaches delivery of powder to compacting rollers.

The compacted strip is then sintered and delivered to another set of compacting rollers.

The shape of powder particles is important particularly in the forming of the particles into a handleable or conveyable sheet or strip The strip compacted from the particles must have good strength and flexibility, whereby the particles from which it is formed should be mostly particles which have an irregular shape.

Water-atomised ferrous particles, such as those made by the Huseby process, are usually irregular in shape, but if the pocess is not properly controlled there are a substantial quantity of rounded or spherical particles The as water atomized ferrous metal particles have an iron core of martensite which is particularly hard, with a skin of iron oxide having a small amount of iron-alloy oxides Martensite is a hard, non-equilibrium structure which is formed by rapid cooling of particles produced from liquid steel For particles of similar chemical composition, we have found that those having a martensitic c P Ca 2 1602582 2 structure are preferred for certain sintering processes.

To provide ferrous metal powder which is suitable for producing e g sheets or strips of steel by sintering, it was formerly thought that the powder needs to be annealed before rolling so that the powder is soft enough that it can easily be formed Oxygen reductions can be effected during the annealing or softening stage of powder manufacturing operations, but such reductions are effected at relatively high temperatures, above 1200 OF and usually between 16000 and 1800 F The high temperatures are disadvantageous because at such temperatures the powder particles stick together or agglomerate The agglomerated particles must then be cooled and tumbled, ground or comminuted, then screened before further processing.

Attention is again requested to the Ayers' U S patent 3,334,408, column 7, lines 5355, wherein a reducing atmosphere is added to the powder in a holding bin According to the patent, the reducing gas, such as hydrogen, is held by the particles for subsequent release during preheat of the powder, a process substantially different than claimed herein See column 9, lines 17-19 of the said patent.

The process of this invention employs as water atomized ferrous metal particles, preferably of irregular shape, which are tumbled in a reducing atmosphere and heated therein long enough to reduce substantially all of the oxygen content of iron oxides on and in the surface layer of oxides and to reduce the total oxygen content of the powder particles while producing non-agglomerated freeflowing ferrous particles whose cores are tempered martensite and whose skin is substantially pure porous iron.

According to the invention, there is provided a method for producing ferrous metal powder suitable for compaction and sintering wherein the powder is as water atomized ferrous powder having a total oxygen content of up to 1 5 weight % and the particles of which comprise martensitic cores and surfaces predominantly of iron oxides, the powder having a particle size distribution such that no more than traces are present of particles of 20 mesh size, no more than 16 weight percent of the powder comprises particles smaller than 230 mesh size and the balance being from 20 to 230 mesh size, and the process includes the steps of:

(a) placing the powder in a substantially horizontal rotary kiln, and (b) conditioning the powder, to reduce its total oxygen content to less than 0 3 weight % and substantially reduce reducible iron oxides on the particle surfaces, by flooding the powder in the kiln with a controlled reducing atmosphere and agitating the powder, by rotating the kiln, for 10 to 30 minutes in the said atmosphere while maintaining the particles at a temperature from 8000 to 12000 F, the powder after conditioning being dry, free-flowing and substantially non-agglomerated, and its particles having tempered martensitic cores with substantially pure, porous ferritic surfaces.

Typical reducing gases which may be used for the reducing atmosphere are hydrogen, dissociated ammonia; forty percent nitrogen, forty percent hydrogen, and twenty percent carbon monoxide, and ninety percent nitrogen, zero to five percent hydrogen, zero to five percent carbon monoxide and zero to ten percent hydrocarbon gases.

The conditioned particles, which preferably have a tempered martensitic core structure, have a surface which causes the particles to adhere together when placed in a press, or otherwise compacted, by e g a compaction roller The resulting green pressing is sufficiently strong to carry its own weight and to be subjected to the pull of compaction rollers.

The production of ferrous strip from conditioned powder produced by the present invention forms the subject of parent application No 22697/78 (Serial No.

1602581).

In the following description which is given by way of example only, the conditioning of as water atomized ferrous metal particles to reduce the oxygen content of their oxide-containing superficial skins is disclosed, so as to render the as water atomized ferrous metal particles suitable for production and forming into structural shapes or parts e g.

by rolling.

The conditioned particles are nonagglomerated, free-flowing, irregularly shaped and have a low total oxygen content.

Conditioned particles are disclosed the metallurgical structure of which exhibit tempered martensitic cores with a porous, substantially pure ferrite surface layer The description following further teaches a powder metallurgical technique employing roll-compaction to produce steel strip from the conditioned ferrous particles.

The invention will now be described in more detail by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic view of apparatus which is typically used to practise the method of this invention; Figure 2 is a picture of a portion of a particle of as water atomized ferrous metal, magnified 200 times; 1,602,582 1,602,582 Figure 3 is a picture of the particle of Figure 2 after conditioning and magnified times; Figure 4 is a picture of conditioned particles showing a broken skin layer of substantially pure iron; Figure 5 is an enlarged view of a portion of a conditioned particle with pieces of the skin broken away; Figure 6 is an enlarged view of a piece of substantially pure iron skin; Figure 7 is a 10,000 x magnified view of the surface of an unconditioned particle; Figure 8 is a 10,000 x magnified view of the skin of a conditioned particle; Figure 9 is a graph of the percentage of oxygen in a particle from a first lot, made by a first manufacturer, plotted against temperature of conditioning in hydrogen; Figure 10 is a graph of the percentage of oxygen in the lot of Figure 9 plotted against time at temperature in hydrogen; Figure 11 is a graph of the percentage of oxygen in a particle from a second lot, made by a second manufacturer, plotted against temperature of conditioning in hydrogen; Figure 12 is a graph of the percentage of oxygen in the lot of Figure 11 plotted against time at temperature in hydrogen; Figure 13 is a graph of the percentage of oxygen in a particle from a third lot, made by a third manufacturer, plotted against temperature of conditioning in hydrogen; and Figure 14 is a graph of the percentage of oxygen in the lot of Figure 13 plotted against time at temperature in hydrogen.

The process of this invention conditions ferrous-metal particles to reduce the oxygen content in their surface layer and forms powder suitable for compaction and sintering Temperatures and times of conditioning are chosen to avoid agglomeration and to enhance free particle flow while still reducing the iron oxides in the skin of each particle.

The particles are conditioned at a temperature of 800 OF to 1200 OF with an optimum temperature of approximately 11000 F to remove substantially all of the iron oxides in the skin of the particles.

The oxides in the core of the particles are not removed and produce small inclusions in the steel which is produced from the particles.

A small quantity of the oxides, estimated at about 1 % of the oxides, particularly on the larger particles, are not pure iron oxides.

That is, manganese, silicon and other impurities form iron alloy oxides on the skin of the particle These oxides can only be reduced, even in a hydrogen atmosphere, at substantially higher temperatures, i e, above 23000 F, than the temperatures of the process of this invention Those alloyed oxides, if not removed, produce inclusions in the steel product which is produced from the powder.

It is informative to examine some photographs of particle surfaces to show the effect of the process of this invention.

Figure 2 shows the surface, magnified 200 x of a typical as water atomized ferrous metal particle before processing.

Only the outer surface of the oxidized skin can be seen Figure 3 shows the surface of a portion of such a particle, hydrogen conditioned and magnified 200 times, showing some of the iron core and a skin which is fragmented, such skin being substantially pure porous iron In the hydrogen conditioned particles of Figure 4, areas of porous iron conditioned skin are shown, together with the tempered martensite iron center of the particles.

Figure 5 shows a more enlarged substantially pure porous iron skin, conditioned by the process of this invention, adjacent the iron core of the particle Figure 6 is a picture of a piece of porous substantially pure iron skin, broken away from the core of a conditioned particle.

It is also instructive to examine the microscopic or fine structure of the skin.

Figur e 7 shows a picture of the skin of a typical particle of as water atomized ferrous metal before conditioning in accordance with this invention Figure 8 shows the porous nature of the substantially pure iron skin conditioned by the process of this invention.

Notice particularly the irregular shape of the skin of Figure 8 That irregular shape assists in the sintering process to produce a relatively strong green sheet or strip of compressed material, for the irregular surfaces readily interlock with each other.

Figure 1 is a schematic view of a powderto-strip apparatus, generally designated 10, wherein ferrous-metal particles 12, in an unannealed state, are delivered to a hopper 14 Delivery may be from a storage bin (not shown) by a bucket conveyor or helical screw (examples only and not shown) The hopper 14 is connected to deliver particles to a conditioning apparatus for conditioning the particles 12 in accordance with the process of this invention.

The conditioning apparatus includes a rotatable tube 16 The tube 16 is sealed off to minimize the escape of gas from the interior thereof, and it is flooded by reducing gas, preferably hydrogen, introduced into tube 16 through pipe 24 and vented through pipe 25 It may be desirable to pre-heat the hydrogen The tube 16 is rotated by a set of gears 18 which are driven by a motor M Tube 16 has in the interior thereof spaced-apart flights or a continuous helical fin (not shown) mounted on its 1,602,582 interior most wall for agitating the powder and assisting in moving the powder axially from left to right through the tube.

The rotatable tube 16 is mounted within a conditioning furnace such as kiln 20, and it is externally heated by heating means such as gas burners 22 mounted on and extending through the outer wall of kiln 20 Kiln 20 and tube 16 need not be, but preferably are pitched at an angle downwardly from the horizontal by typically 2 to 5 degrees, which varies depending upon the length of the tube and the heating time desired for the particles being conditioned.

The particles being conditioned are heated by the furnace jets 22 to and maintained at the desired temperature The speed of rotation of the tube 16, its angle of tilt, the configuration of flights and the length of the tube 16 determine the length of time of heating While the particles are heated, they are subjected to a reducing atmosphere, preferably hydrogen, whereby the skin oxides of iron are reduced leaving a skin of porous iron The temperature maintained inside of the tube 16 by the flames of the burners 22 is within the range of 8000 to 12000 F Contact between the agitated powder and the reducing gas at such temperatures conditions the powder and effects a substantial reduction of the oxygen content in its skin As the heated, conditioned powder 12 ' leaves the tube 16, it typically passes through one or more gravity chutes 26 to a compaction mechanism such as compaction rollers 28.

Alternatively, for example, the powder 12 ' may be delivered by a helical screw (not shown) The powder is allowed to cool only a few hundreds of degrees, in chute 26, to a compaction temperature That temperature is typically maintained by heating means such as, for example, gas burners or electrical strip heaters 27 Alternatively, the temperature may be maintained by using heavy insulation on the duct or chute 26 through which the powder passes Chute 26 or helical screw (not shown) also contains reducing gas to prevent re-oxidizing of the particles.

The times of heating at the desired conditioning temperature is at least 10 minutes At the optimum temperature of 11000 F, most of the oxides have been removed after 10 minutes of heating at temperature, and further conditioning contributes only a small amount of removal of additional oxygen from the skin oxides It is estimated that it typically takes five minutes in tube 16 to bring the particles up to conditioning temperature, whereby the time of travel of the particles through tube 16 is five minutes plus the conditioning time.

The conditioned powder may be pressed into molds and sintered to form conventional powder metallurgy parts.

Alternatively, the powder can be compacted to form a metal strip on a continuous production basis.

The conditioned powder is fed by a feeding means shown generally at 29 from chute 26 into the nip of compaction rollers 28 which roll-compact the powder into a compacted strip of high density The compacted strip 30, referred to as a green strip, is processed in an enclosure 32 and into and through furnace 34 Furnace 34 has a controlled reducing atmosphere which is introduced through conduit 36 and exhausted through conduit 31 The compacted strip 30 is heated and sintered in furnace 34, for example, to a hot rolling temperature between 18000 and 24001 F to prepare the compacted strip for hot rolling.

As the strip passes through furnace 34 it is supported by suitable means such as rollers 38.

Upon leaving the furnace the heated strip is delivered immediately to hot rollers generally designated at 40 to obtain a hot roll strip 30 ' of full density and reduced thickness Note that the rolls 40 are dry, and it is undesirable to cool them conventionally with water because the water would oxidize the hot strip The strip 30 ' is then cooled by means such as jets of nitrogen introduced through pipes 42 between plates 44.

Preferably, the cooling means has three cooling zones (not shown), and each zone is separately servoed to control the flow of the cooling nitrogen Other neutral or reducing gases may be used to cool the strip.

The aforementioned processing steps are usually effected in a controlled reducing atmosphere, preferably hydrogen, which is introduced through respective tubes 31, 36 and 39 and maintained within the respective enclosure 32, furnace 34, and enclosure 41.

The cooled strip can be optionally passed through a pair of pinch rolls 46 to further working apparatus such as cold rollers 48 for cold rolling to desired thicknesses and to coilers and slitters (not shown) for further handling Looper rolls or dancer rolls 49 and are important to the continuous nature of the process Whenever the continuous strip is processed through two rolling mills, such as 28 and 40, an intermediate device, such as dancer roll 49, moves up or down to take up slack in the strip The upward and downward motion of 49 or 50 is sensed by electrical sensors (not shown) and the signal is used to servo the speed of the rollers.

A typical distribution of particles sizes in weight percent is: a trace of + 20 mesh, 18 % maximum between -20 and + 40 mesh, 22 % 3 % between -40 and + 60 mesh, 17 % 2 % between -60 and + 80 mesh, % 2 % between -80 and + 100 mesh, 11 % 2 % between -100 and + 140 mesh, 1,602,582 13 % O 2 %/ between -140 and + 230 mesh, and 16 % maximum of -230 mesh.

The particles are preferably substantially martensite of primarily irregular shape, although they are occasionally of a mixed martensite-ferrite structure with a primarily irregular shape ^ A typical chemical composition of a powder as-received and before processing is:

Element 0 C si Mn p S Weight Percent 1.5 % maximum 0.006 % to 0 20 % 0.02 % maximum 0.05 % to 0 30 % 0.015 % maximum 0.02 % maximum, the balance being iron.

Figs 9 to 14 are graphs showing oxygen reduction by the process of this invention for three different lots of ferrous metal particles received from three different manufacturers.

The data for the curves was obtained by placing a 70 gm sample of particles into a closed, 36 inches x 64 mm tube, then flowing hydrogen through the tube while heating it in a furnace The tube was rotated, while heating, about its major axis at four RPM Five minutes was allowed to bring the particles up to furnace temperature In larger tubes with larger amounts of particles, additional time or heat, or both, would be needed to bring the particles up to temperature.

To ensure control on the reduction process, the tube is first purged of air by using nitrogen gas The nitrogen is then purged by hydrogen During cooling, the hydrogen atmosphere was maintained The hydrogen was then flushed out by nitrogen.

Each of the lots of particles was conditioned at temperatures of 8000 F, 9000 F, 10001 F, 11001 F, 1200 'F, and 13000 F It was found that the powder just began to agglomerate at 12000 F, and a temperature of 1300 OF is not useful The data above 12000 F is not plotted in Figures 9 to 14 The powder was at room temperature when placed into the conditioning process Five minutes was allowed to bring the particles to temperature After being agitated and heated at temperature in a flood of hydrogen reducing atmosphere for 5 minutes, 10 minutes, 15 minutes, 20 minutes and 30 minutes, the conditioned powder was analyzed, and the amount of remaining oxygen in the sample was determined.

Analysis revealed the total oxygen content after conditioning could be reduced to less than 0 30 weight %.

Figures 9, 11 and 13 are graphs of oxygen content for various times at temperatures, plotted against the conditioning temperature, for lots 1, 2 and 3 of particles, respectively.

Figures 10, 12 and 14 are graphs of oxygen content for various conditioning temperatures, plotted against the time at temperature, for lots 1, 2 and 3 of particles, respectively.

In Figures 9 to 14, where the curves flatten out, substantially all of the reducible oxygen for the particular temperature or time at temperature has occurred.

In Figures 9, 11 and 13 the five minute curve does not flatten out within the, temperature range of 800 to 12000 F The oxygen content is reduced It may be reduced enough for a particular steel product, but a higher temperature would be required before the curve would flatten.

Such higher temperature would cause the particles to agglomerate-which is not acceptable.

Figures 9, 11 and 13 show that after ten minutes at temperature (plus five minutes for heating to temperature), the curves flatten above 10000 F In the second lot, for Fig 11, the curve flattens above 9000 F It may safely be said that substantially all of the reducible oxides which may be reduced in ten minutes at temperature (plus five minutes for heating to temperature) are reduced with processing temperatures between 1000 and 12000 F.

After fifteen minutes at temperature (plus five minutes for heating to temperature), the curves flatten above 9000 F In a second lot, for Fig 11, the curve is reasonably flat above 8000 F It may safely be said that substantially all of the reducible oxides which may be reduced in fifteen minutes at temperature (plus five minutes for heating to temperature) are reduced with processing temperatures between 900 and 12000 F.

After twenty minutes at temperature (plus five minutes for heating to temperature), the curves flatten above 90001 F In the second lot, for Fig 11, the curve is reasonably flat above 8000 F It may safely be said that substantially all of the reducible oxides which may be reduced in fifteen minutes at temperature (plus five minutes for heating to temperature) are reduced with processing temperatures between 900 and 12000 F.

After thirtv minutes at temperature (plus five minutes for heating to temperature), the curves flatten above 8000 F It may safely be said that substantially all of the reducible oxides which may be reduced in thirty minutes at temperature plus five minutes for heating to temperature) are reduced with processing temperatures between -800 and 12000 F.

The optimum temperature is chosen at 1100 OF because the curves have substantially flattened at that temperature, 1,602,582 and it is substantially below the 12000 F temperature where agglomeration starts.

In Figures 10, 12 and 14, the 800 'F curve starts to flatten after thirty minutes of conditioning at temperature In Figure 10 the curve starts to flatten after twenty minutes at temperature In Figure 12 the curve starts to flatten after fifteen minutes at temperature That is, substantially all of the reducible oxides which may be reduced at 8000 F are reduced after at least thirty minutes conditioning at temperature, plus five minutes to bring the particles up to temperature.

The 900 F curve flattens after fifteen minutes of conditioning at temperature In Figure 12 the curve starts to flatten after ten minutes of conditioning at temperature.

Substantially all of the reducible oxides which may be reduced at 900 O are reduced after at least fifteen minutes of conditioning at temperature, plus five minutes to bring the particles up to temperature.

The 10000 F curve flattens after fifteen minutes of conditioning at temperature In Figures 10 and 12 the curve flattens after ten minutes of conditioning at temperature.

Substantially all of the reducible oxides which may be reduced at 1000 F are reduced after at least fifteen minutes of conditioning at temperature, plus five minutes to bring the particles up to temperature.

The 11000 F curve flattens after ten minutes of conditioning at temperature.

That is, substantially all of the reducible oxides which may be reduced at 11000 F are reduced after at least ten minutes of conditioning at temperature, plus five minutes to bring the particles up to temperature.

The 12001 F curve flattens after ten minutes of conditioning at temperature In Figure 12 the curve flattens after five minutes of conditioning at temperature.

Substantially all of the reducible oxides which may be reduced at 1200 OF are reduced after at least ten minutes of conditioning at temperature, plus ten minutes to bring the particles up to temperature.

The optimum time of conditioning at temperature appears from the curves to be at least twenty minutes.

The metallurgical structure of the core of the as-received particles is martensite, a hard, non-equilibrium structure formed by rapid cooling of powder particles which are produced from liquid steel Given powder particles of similar chemical composition, those having a martensite structure have higher hardness than annealed powders In the past it has been considered necessary to anneal the powders to make the cores sufficiently soft to form a steel strip The martensite structure of the particle subjected to the temperature conditioning parameters of this invention, from 8000 to 12000 F does change to a tempered martensitic structure softened about 50 %, in comparison to the as-received particles.

The powder to which Figs 13 and 14 relate is not pure martensite, but part ferrite The curves demonstrate that the process of this invention is effective not only on particles which consist essentially only of martensite, but also particles which exhibit a martensitic structure in part.

When part-ferritic particles are annealed to a temperature in the region of 18000 F, they remain ferritic until they reach a temperature of about 14000 F at which temperature the ferrite begins to transform into an austenitic structure.

The process of this invention, however, never exceeds 12000 F because the particles tend to agglomerate at that and higher temperatures, so part-martensite/partferrite particles will not undergo the austenite transformation in the present process.

The particle size distribution range of the powder used in the process of this invention is one in which the particles are substantially free flowing while they are being agitated and heated in a substantially pure hydrogen reducing atmosphere at a temperature from 800 F to 1200 OF for at least 10 minutes.

"Substantially free flowing" means that with agitation, provided for example bymeans such as rotatable tube 16, the powder particles neither agglomerate nor temporarily stick to each other or to the interior of the tube, and their movement relative to each other is not significantly restricted The free flowing condition of the powder during agitation allows maximum exposure of powder particles to the reducing atmosphere and maximum or substantial reduction of the total oxygen content of the reducible oxides on and in the surface or skin of the powder particles.

"Substantially free flowing" also means that upon leaving tube 16, or after conditioning the conditioned powder can be efficiently and effectively fed in a controlled, unsaturated manner directly to a compaction mechanism such as rollers 28 without any need for special prefeeding or pre-communition which would otherwise be needed to separate or break up aggregated, agglomerated, stuck together or adhered particles.

The conditioning steps of this invention not only condition the powder, but also vaporize volatile contaminants on the powder particles such as hydrocarbons:

minor amounts of oil and grease.

It is also important that substantially all 1,602,582 water be removed by the flowing hydrogen after release from the particles Note that the powder particles, during conditioning, use at least 0 17 cubic feet of hydrogen per pound of powder The flow rate in a rotary kiln whose tube is 30 feet long and has an inside diameter of 4 feet typically uses a hydrogen flow rate of from 2 to 8, and preferably 2 to 5 cubic feet per hour per pound of powder The flow is adequate to sweep out all of the produced water vapor.

The hydrogen ratio required is proportional to the quantity of oxide to be reduced.

Other typical reducing gases which may be used are dissociated ammonia; forty percent nitrogen, forty percent hydrogen, and twenty percent carbon monoxide; and ninety percent nitrogen, zero to five percent hydrogen, zero to five percent carbon monoxide and zero to ten percent hydrocarbon gases These gases have an advantage that they produce less water vapor, and they are cheaper than hydrogen.

Thus, the process of this invention conditions ferrous-metal particles which have a surface layer of iron oxide thereon to substantially reduce the oxygen content of the iron oxide in or on the surface layer and to obtain a free flowing, non-agglomerated conditioned ferrous powder whose total oxygen content is substantially reduced.

Particles conditioned by the process according to the invention are not exclusively suitable only for use in powder metallurgical processes involving compaction by rolling The conditioned particles can be used to make ferrous dieformed articles, and the invention embraces any powder metallurgical process which uses the conditioned particles as the source material.

Claims (1)

1. WHAT WE CLAIM IS:-

   1 A method for producing ferrous metal powder suitable for compaction and sintering wherein the powder is as water atomized ferrous powder having a total oxygen content of up to 1 5 weight % and the particles of which comprise martensitic cores and surfaces predominantly of iron oxides, the powder having a particle size distribution such that no more than traces are present of particles of 20 mesh size, no more than 16 weight percent of the powder comprises particles smaller than 230 mesh size and the balance being from 20 to 230 mesh size, and the process includes the steps of:

   (a) placing the powder in a substantially horizontal rotary kiln, and (b) conditioning the powder, to reduce its total oxygen content to less than 0 3 weight % and substantially reduce reducible iron oxides on the particle surfaces, by flooding the powder in the kiln with a controlled reducing atmosphere and agitating the 65 powder, by rotating the kiln, for 10 to 30 minutes in the said atmosphere while maintaining the particles at a temperature from 8000 to 1200 'F, the powder after conditioning being dry, free-flowing and 70 substantially non-agglomerated, and its particles having tempered martensitic cores with substantially pure, porous ferritic surfaces.

   2 The method according to claim 1, 75 wherein the reducing atmosphere is substantially pure hydrogen.

   3 The method according to claim 1, wherein the reducing atmosphere is dissociated ammonia 80 4 The method according to claim 1, wherein the reducing atmosphere is forty percent nitrogen, forty percent hydrogen, and twenty percent carbon monoxide.

   The method according to claim 1, 85 wherein the reducing atmosphere is ninety percent nitrogen, zero to five percent hydrogen, zero to five percent carbon monoxide and zero to ten percent hydrocarbon gases 90 6 The method according to any of claims 1 to 5, wherein the particles are maintained at a temperature from 1000 to 1200 'F.

   7 The method according to any of claims 1 to 5, wherein the time is at least 15 95 minutes, and the particles are maintained at a temperature from 900 to 12000 F.

   8 The method according to claim 7, wherein the time is at least 20 minutes.

   9 The method according to any of claims 100 1 to 5, wherein the time is at least 30 minutes and the particles are maintained at a temperature from 800 to 12000 F.

   The method according to any of claims 1 to 9, wherein the particles are held 105 at 11000 F.

   11 The method according to any of claims 1 to 5, wherein -the temperature of said particles is 9000 F, and the time period at this temperature is at least 15 minutes 110 12 The method according to any of claims 1 to 5, wherein the processing temperature of said particles is 10000 F and the time period at this temperature is at least 15 minutes 115 13 The method according to any of claims 1 to 5, wherein the processing temperature of said particles is 11000 F, and the time period at this temperature is at least 10 minutes 120 14 The method according to any of claims 1 to 5, wherein the processing temperature of said particles is substantially 12000 F, and the time period at this temperature is at least 10 minutes 125 The method according to any of claims I to 5, wherein the time period at temperature of said particles is at least 20 minutes.

   1,602,582 16 The method according to any preceding claim, wherein the size distribution of said particles is as follows:

   % By Weight Trace 18 % maximum 22 %+ 3 % 17 % 2 % % 2 % 11 % 2 % 13 '/ 2 % 16 % maximum Mesh + 20 mesh -20 and + 40 mesh -40 and + 60 mesh -60 and + 80 mesh -80 and + 100 mesh -100 and + 140 mesh -140 and+ 230 mesh -230 mesh 17 The method according to any preceding claim, wherein the chemical composition of said particles is as follows:

   Element 0 C Si Mn P S Weight Percent 1.50 % maximum 0.006 % to 0 20 ,0 0.02 % maximum 0.05 % to 0 30 % 0.015 % maximum 0.02 % maximum, and the balance is Fe.

   18 Conditioned ferrous particles produced by the process claimed in any of claims I to 17.

   For the Applicants, GRAHAM WATT & CO, Chartered Patent Agents, 3, Gray's Inn Square, London, WCIR 5 AH.

   Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1981 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.