GB1590953A

GB1590953A - Making articles from metallic powder

Info

Publication number: GB1590953A
Application number: GB4121876A
Authority: GB
Original assignee: Powdrex Ltd
Current assignee: Hoganas Great Britian Ltd
Priority date: 1977-10-04
Filing date: 1977-10-04
Publication date: 1981-06-10

Description

(54) MAKING ARTICLES FROM METALLIC POWDER (71) We, POWDREX LIMITED, a British Company, whose registered address is 20 Copthall Avenue, London ER2R 7JN, 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 the making of articles from metallic powder, particularly tool steel, alloy and stainless steel powders, and powders of cobalt or nickel base alloys. By "metallic powder" as used in this specification is meant metal and metal alloys in particulate or powder form. The metallic powder may optionally be blended with minor additions of metal oxides powders or other non-metallic powders.

Conventionally such articles are made by first cold compacting the metal powder in a press to form a green compact in which the powder particles are mechanically locked together and in which the density of the compact is significantly less than the true density of the solid material from which it is made. After forming, the green compact is sintered in a controlled atmosphere to bond the particles together chemically and metallurgically and increase the relative density.

The powder, particularly when it has been made by atomisation with water quenching, is manufactured with relatively high oxygen content and therefore needs to be deoxidised before it is sintered. Conventionally the powder is deoxidised solely in loose form by heating, for example, in a tumbler furnace in an inert atmosphere. This produces insufficient deoxidation with consequent loss of strength in the finished article.

A further problem with existing practice is that for certain alloy materials, in particular high speed tool steels, the sintering temperature necessary to achieve a reasonable chemical bond and densification of a compact is so high that excessive growth of metal grain size and carbides may occur. The enlargement of metal grain size and the growth and change in form of the carbides in such alloy materials can lead to low impact strength of the resulting metal article.

One object of the present invention is to achieve more efficient deoxidation by deoxidising at higher temperatues. This is achieved by following deoxidation of the powder in the loose form by further deoxidising of the powder in the compacted form. The final deoxidation is better achieved in compacted form since higher temperatures can be used than with powder, since with heating the powder at high temperatures it agglomerates to a state such that treatment in a hammer mill or other apparatus to break up lumps significantly affects the characteristics of the metal particle hardness, particle shape or the like.

However, using a first deoxidisation in the powder stage allows one to have a closer control of the final carbon content in the powder to be sintered and thus have more accurate selection of the most advantageous sintering temperature since the solidus temperature of the powder is dependent on carbon content.

Another object, at least of the preferred form of the invention, is to achieve substantially fully dense articles made from tool steel, alloy steel and stainless steel and cobalt and nickel based alloy powders by substantially filling all the voids in the compact during sintering without coarsening the steel structure in such a way as to reduce its mechanical properties.

This is achieved by close control of the sintering temperature.

We have found that while powder metallurgy techniques may produce a satisfactory product on a laboratory scale (below five Kilograms) inconsistencies in quality appear when a process, which is apparently the same, is carried out on a production scale. A further object of the invention, at least in its preferred forms, is to provide a process which can be operated reliably and consistently on a production scale: that is 20-500, preferably 100-300, Kilograms of powder treated formed into compacts and sintered together in a furnace with a common heating control.

In one aspect the present invention provides a method of making a metallic article which comprises the steps of: a. Making powder of predominantly tool steel, alloy steel, stainless steel or nickel or cobalt based alloy by atomisation and water quenching of a melt of metal alloy, b. heating the powder in a sub-atmospheric pressure to reduce the oxygen content below 1500 ppm and cooling the powder at a controlled rate to anneal it, c. analysing the resultant powder for carbon and oxygen content and adding more carbon if necessary, d. compacting the powder at a pressure of at least 25,000 psi to form a compact having a relative density in the range 60 to 92neo, e. heating the compact in a sub-atmospheric pressure below the solidus temperature of the alloy for at least half an hour in order to further reduce the oxygen content to below 700 ppm, f. raising the temperature of the compact to a selected sinter temperature while maintaining a vacuum or controlled atmosphere and maintaining the temperature within plus or minus 10 C of that selected temperature for at least half an hour to sinter and cause densification of the compact to at least 93% relative density.

Preferably the sintering is at or above the solidus temperature of the powder composition or at least one constituent thereof and the temperature is selected to densify the product without coarsening the structure to significantly reduce its mechanical properties.

"Relative density" is the ratio of the actual density of the compact to the density of the solid material from which it is made, expressed as a percentage.

The sintering temperature for steel powders and particularly high speed tool steel powders should be at or within 20"C, preferably within 10 C below, that temperature at which carbide networks form. Normally this will be at or within 10 C of the solidus temperature of the steel powder. With alloys such as cobalt based hard metal powders a temperature at which carbide networks form may be desirable. However, once the desirable temperature has been selected the sintering temperature must be closely controlled to control the amount of liquid phase present in the compact during sintering.

For high speed tool steels for example it may be maintained within I 10 C of the selected temperature, preferably within + 3"C and with advantage within + 11/2 C of that temperature. Sintering will be effected in a half hour to four hours. With advantage the average carbide size in the resultant sintered product will not exceed 10 preferably 6 microns and preferably the maximum carbide size will not exceed 10 microns.

In an alternative treatment the above sintering temperature is lowered by 20"C and after sintering for 1/2 to 3 hours at this first temperature the compact is raised in temperature rapidly to 10 C to 400C above the first temperature, at which higher temperature some liquid forms. The material is held under vacuum for from five seconds to fifteen minutes at this higher temperature. Provided the time and temperature for this further sintering are held below that necessary to cause grain growth, the resulting metal will remain of fine grain structure and fine even carbide distribution and yet be sufficiently dense since at this higher temperature densification is relatively rapid. However this process does require tight temperature control.

The tight control of sinter temperature in both forms of sintering over a charge of many parts is achieved by using high reflectivity radiation shields in close proximity to the charge and low temperature ceramic fibre insulation behind these radiant shields. By this means it is possible to achieve + 1"C. temperature variation over a batch at temperatures in excess of 1150"C. Preferably the compacts are stood on alumina pads small enough to resist thermal shock, the alumina pads being supported on carbon trays which are stacked one above another in the furnace. Preferably the furnace uses graphite or molybdenum heating elements. The particular temperature selected for sintering is critically affected by final carbon content. For consistency, final carbon content over the whole charge Is with advantage in the band f 0.02%, preferably 1 0.01%. This is possible to achieve if the carbon and oxygen contents of the annealed powder are determined prior to compaction, and a measured quantity of carbon added in powder form in blending to compensate for the further reaction of oxygen and carbon at higher temperatures.

The final carbon content is difficult to control if the oxygen content before compaction is greater than 1500 ppm and impossible with contents as high as 3000 ppm. Preferably the oxygen content is reduced below 1000 ppm and with advantage in the range 600 - 200 ppm during the powder heating before compaction.

In a preferred form the invention provides a method of making metallic articles on a production scale which comprises heating a powder of predominently tool steel, alloy steel, stainless steel or cobalt or nickel based alloy, made by atomisation and water quenching a melt of the alloy to give irregular shaped particles in a vacuum better than 1.0 Torr for at least half an hour at a temperature above 900"C (preferably above 1000"C) to reduce the oxygen content below 1500 ppm (preferably below 1000" ppm) and cooling at a controlled rate to anneal the powder, analysing the resultant powder for carbon and oxygen content, adding more carbon if necessary, compacting the powder of known oxygen and carbon content to form green compacts of at least 60% full density, loading at least 20 Kilograms of compacts into a furnace so that the compacts are stacked close together and surrounded by radiation shields, heating the compacts in a controlled atmosphere in at least two stages; in a first stage the compacts are heated in a vacuum at 75 to 200"C below the solidus temperature of the powdered composition for at least half an hour to reduce the oxygen content below 500 ppm andin a second stage the batch of compacts is raised to a selected sinter temperature at or within 20"C above and preferably within 10 C above the lowest solidus temperature of the deoxidised powder composition and maintained within + 10 C preferably within +3 C and with advantage within +1 C of the selected temperature for half an hour to three hours to produce a compact of at least 93% preferably 95-98% relative density.

Methods of making articles from metallic powder will now be described in greater detail.

The metallic powder is made by atomising a falling stream of molten steel by gas, water or steam jets directed at the stream and water quenching the resultant hot steel droplets in such a manner as to give irregular shaped particles.

The melt is atomised by jets directed at a falling stream of the steel at between 12" and 18 to the vertical for steam, or 12" - 30 for gas or water, and at a pressure of between 20 and 200 psi for gas or steam and 800 - 4,000 psi for water. The atomised droplets are water quenched by contacting with free flowing water, and the quenched droplets fall, in the form of powder, into a water bath from which it is removed by a pump or by an electromagnet.

The total free fall of the drops prior to quenching should be no greater than 18" and preferably 6 - 9" to give irregular shaped particles. Powder made by such gas, water or steam atomisation with water quenching has the advantage of good compactibility which facilitates the production of a high relative density compact. The quench water is preferably treated with a rust inhibiting inoculant, e.g. an amine based, water soluble corrosion inhibitor, to reduce formation of metal oxides and to assist in the annealing process by reducing agglomeration.

If such inoculants are used an anti-foaming reagent is further added to the quench water and optionally to the primary atomising jets where water atomisation is used. This invention is the subject of our British Patent No. 1547866. The presence of foam reduces the irregularity of the particles and hence their compactibility. In the deoxidation treatment process of the metal powder before compaction it is treated by placing it in shallow trays in a vacuum furnace heating to between 900"C and 1,300"C for a minimum period of half an hour. The temperature selected is the highest possible which will not so agglomerate the metal powder that treatment in a hammer mill or other apparatus to break up lumps significantly affects the characteristics of metal particle hardness, particle shape etc.

Typically for tool steels a temperature of 1,000 - 1,050"C is used. In practise, however, it has been found that providing the vacuum level is held below 1 Torr. and preferably below 0.3 Torr and providing the depth of the powder in the trays does not exceed 3 centimetres, and providing the starting carbon content of the powder is in excess of 0.85% and preferably 1% high speed tool steel containing levels of carbon between 0.75% and 1.5% and with an initial typical oxygen content of 1500 - 3000 ppm may be deoxidised to a level of 200 - 700 ppm by this treatment. Part of the carbon in the steel serves to combine with the oxygen to release gaseous oxides of carbon which are removed in the vacuum pumping system. In practise it has been found that if the depth of powder in the trays exceeds 3 centimetres, full deoxidation throughout the powder may not be achieved. It is however recognised that means of ventilating a deeper bed could readily be engineered to achieve the same result. The powder is allowed to cool to 600"C to 700"C at a rate of 25 - 50"C per hour to anneal it. Thereafter it is furnace cooled to ambient temperature.

The carbon and oxygen content of the powder is analysed. Taking as an example a high speed tool steel, the annealed powder is milled, sieved through a 60 mesh sieve, and then mixed with a lubricant for example 0.25% to 1.0% magnesium stearate, up to 0.4% carbon in graphite form, and, if required, fine cobalt powder. The carbon addition gives close control of the final carbon content, an important feature in obtaining accurate temperatures of sintering. The stearate acts as a lubricant in the mechanical pressing of the compact, while the cobalt powder may be added to correct the metallurgical composition, or to act as a grain refiner. The addition of lubricants, however, can reduce the pressure required to obtain satisfactory relative densities. For example, the addition of metallic stearates in the range 0.5% to 1.0% by weight can reduce the desirable pressure range to 60,000 to 100,000 psi.

The annealing produces "soft" powder which has compactability, and which therefore produces a compact having a high relative density.

The compact may be made in any of a number of differing ways, dependent on the final article. Thus, where a complex shape; such as a tool, is to be produced, the powder may be packed into a mould of stiffly deformable material having a shape approximating to the shape required of the article, and the powder-filled mould subjected to isostatic compression to form the compact. The addition of a volatile lubricant to the powder may be used to achieve higher relative density for a given isostatic pressure. Alternatively, and for the same purpose, the powder may be preformed uni-directionally in a die mounted in a ram press under relatively low pressure to the required shape, the preform given a protective coating sealing the pores of the preform, and the coated preform then subjected to a isostatic compression at a relatively high pressure. In the latter case, the preform is preferably made in a ram compaction press, the die of which has the required shape; the loosely compacted preform, after removal from the die, is coated with a rubber or a plastics material, as by spraying or dipping, and is then subjected to the high isostatic compression.

When sufficiently high compaction pressures are used it is possible to profile the compact by normal metal cutting operations after compaction and before sintering. By this method a much higher rate of removal of material is possible than on sintered or conventionally made material. For most applications it is possible to effect all the necessary compaction in a ram press without subsequent isostatic compaction, using a suitable volatile lubricant blended into the powder mass prior to compaction or merely sprayed onto the compaction press die to reduce die wall friction and hence die wear.

Where a composite article is to be made, a powder of a first composition may be introduced into a compressible mould around a metallic insert of a second composition, and the mould subjected to isostatic compression; the subsequent sintering of the resulting compact bonds the surrounding powder metallurgically to the insert. The insert may be a solid metal member formed previously from powder or by conventional means, or the insert itself may consist of previously compacted powder or of uncompacted powder having a composition differing from that of the surrounding powder. The insert need not be of tool steel, alloy steel, stainless steel or cobalt or nickel based alloy.

Where the powder is compacted into contact with a mandrel, by choosing a mandrel which does not bond with the powder metalurgically in sintering, it is possible to retain the mandrel shape through the sintering step, only removing the mandrel after sintering. Using this method complex section female dies or similar components with good mechanical properties can be formed to very close tolerances. Alternatively the mandrel may be withdrawn after pressing and before sintering.

We have found it advantageous and believe it to be novel per se to use an insert of a material of significantly higher thermal expansion than the powder to be sintered, where the insert is to be retained during the sintering step to maintain dimension and then withdrawn. For example using an AISI 304 Grade austenitic stainless steel insert and an AISI M35 high speed sintered part the superior thermal contraction of the stainless steel on cooling from the sintering temperature allows the insert to be easily withdrawn.

Where, for example, the article is to be a composite billet to be subsequently shaped, the powder resulting from the water, gas or steam atomisation is preferably sieved to separate the powder into a fine component and a coarse component. Then a mould may be filled with the coarse and fine powders, so that a core of the former is covered by an outer layer of the latter over at least a major part of its surface. The mould is thereafter subjected to pressure to compact the powder. This divided filling may be used to obtain good surface quality but is not an essential part of the process for the production of homogeneous steel.

Mechanical compaction is preferably performed in a shaped die and at a pressure in the range 25,000 to 200,000 psi and preferably in the range 50,000 to 100,000 psi. Where the fully prealloyed powder has poor compactability it may be advantageous to prealloy part of the powder by melting and atomisation, and blend with a more compactible constituent to obtain the final chemical composition. Examples of this are the addition of cobalt powder to cobalt base alloys or cobalt powder to high speed steel powders.

A compact formed by mechanically compressing high speed steel powder at a pressure of 70,000 to 85,000 psi results in a body having a relative density of about 75%. Where die wear is to be reduced, lower pressures may be employed. Where dimensional control is critical high pressures may be employed.

The compacts are heat treated firstly to remove the stearate lubricants, secondly for deoxidation and thirdly to sinter and densify the compacts. For that purpose the compacts are loaded into a vacuum furnace maintained at a vacuum of at least 1.0 Torr and preferably 10-4 Torr, and the temperature raised to a value between 200 and 600"C, preferably 200C to 400"C where the lubricant is magnesium stearate, and held for 1/2 hour to 2 hours until all included lubricant is outgassed. The temperature is then raised to a value 75 to 200"C and preferably about 100"C below that employed for sintering the particular metallurgical composition in use to full density and the temperature maintained for 1/2 to 2 hours with the aim of removing substantially all carbon oxides to give an oxygen content less than 500 ppm for alloy steels and preferably less than 200 ppm for tool steels and stainless steels. Finally the temperature is again raised to the sintering temperature which is held accurately for a period of sufficient time ( hour to 4 hours) to cause sintering throughout each compact to obtain .substantially complete densification ( > 93%, preferably > 98%). Deodixation is normally in the temperature range 1000 - 1200"C preferably 1070 - 1150"C, and sintering in the temperature range 1180"C - 12800C preferably 1200"C - 12500C. Metallic volatiles may be suppressed by worsening the vacuum somewhat above the temperature at which such volatiles are emitted. Typically, a vacuum of 10-4 torr. employed up to 11000C may be worsened to 1.0 torr for higher temperatures by the injection of an inert or reducing gas, e.g. nitrogen, hydrogen, argon or helium. Different gases may be used at different temperature levels. We may supply such gases and alternately raise and lower the pressure to scavenge carbon monoxide and carbon dioxide from the interior of the compact. For example, the furnace chamber may be alternately back-filled with inert gas to 0.2 - 1.0 torr and re-evacuated to a pressure of 0.5 - 0.1 torr, this cycle being repeated as often as required. However we have not found this to be necessary on high speed steel.

The temperature at which sintering is effected at least for some powder compositions is critical and is dependent on the composition of the compacts being treated particularly the final carbon content. The following table shows variations of sinter temperature against final carbon content.

S = Furnace temperature measured as 12300C.

Sintered Carbon Content Furnace Temperature OC 0.775 S+6 0.85 S 0.925 S-6 1.0 S - 14 1.075 S - 19 1.15 S - 24 Too high a temperature leads to carbide growth, grain growth and segregation and consequential embrittlement of the final products, while too low a temperature results in insufficient densification. Because sintering temperature is critical, it must be held within close limits during the period of sintering, and has been found in practise to require to be held to an accuracy of + 10 deg. C and preferably + 11/2 deg. C. A tool steel containing more than 2% cobalt has been found advantageous in the process of this invention.

The sintering temperature selected is at or slightly above the lowest solidus temperature of the steel or alloy composition of the compacts. At that temperature the lower melting temperature compositions of the steel or alloy are brought into the liquid phase while the other compositions remain in the solid phase. By so doing, the sintering process of volume diffusion, internal mass flow and formation and solid solutions and other chemical compounds are accelerated, and, at the same time, surface tension pressures are generated sufficient to collapse the majority of the pores within the compact and to cause voids to diffuse to the surface of the compact so as to achieve a density which approximates to the full density of the steel from which the compact is formed.

After sintering, the compacts are cooled and annealed in conventional manner.

The invention will be more readily understood by the following description of examples of making tool steel articles and a cobalt based alloy article in accordance therewith.

Example I The tool steel melt had the following composition besides iron: carbon 1.2% vanadium 2% tungsten 6% manganese 0.2% molybdenum 5% sulphur 0.03% chromium 4% phosphorous 0.03% All percentages are given by weight.

Steel powder was made from the melt, dried, initially deoxidised and annealed, milled, sieved, analysed for carbon content and oxygen content. These were found to be 0.84% and 600 ppm respectively. The powder was then mixed with lubricant and 0.08% by weight of graphite as described above. Compacts were passed from the powder in a mechanical press at a pressure of 35 tons. per sq. in. The compacts of 2" diameter and 2" length had a relative density of between 75% and 80% and the sintered final composition was 0.85%C, 6.0% W, 5.0% Mo., 4.0% Cr., 2.0% V.

The reduction in carbon occurred during deoxidising and annealing.

The compacts were loaded onto small alumina pads and placed on carbon discs in a cylindrically heated vacuum furnace, the discs being stacked one above another. The close spacing of the discs, together with the use of high reflectively radiation shields above and below and around the charge, enabled close control of furnace temperature to be achieved.

The pressure within the furnace was lowered to, and maintained at, 10-4 torr. After removal of the lubricant and deoxidation as described, the compacts were sintered at a measured temperature of 1217"C which was held for a period of three hours, with an accuracy of + 11/2 deg. C. The sintered compacts were found to have a porosity of less than 2% (relative density greater than 98%) and a maximum carbide size of 10 microns. The oxygen content was below 150 ppm.

Example II Compacts were made as in Example I, except that the steel melt had a composition besides iron of: carbon 1.2% vanadium 2% tungsten 6% cobalt 5% molybdenum 5% chromium 4% The annealed powder had a carbon content of 0.84% and an oxygen content of 600 ppm.

0.08% graphite was added prior to compaction. The final sintered part composition contained 0.85% carbon, and the other components unchanged. The measured sinter temperature in this case was 1215"C and sinter duration three hours.

Example III The steel melt contained besides iron: carbon 1.2% chromium 4% tungsten 1.5% vanadium 1.2% molybdenum 9.5% cobalt 5% the composition of the compacts being the same, except that the carbon content was 0.80%.

The measured sinter temperature employed was 1212"C and the sinter duration was one hour.

Example IV A cobalt based alloy melt had the following composition besides cobalt: B 1% W 5% Cr 26% C 1.05% after deoxidation and annealing of the powder as above the powder had a carbon content of 1% and an oxygen content of 500 ppm.

Compacts were made, deoxidised and sintered as described above, the measured sintering temperature being 1225"C for a period of 21/2 hours and the final carbon content being 1% C. The relative density was greater than 93% and the oxygen content below 500 ppm.

The sintering temperatures quated for examples I to III apply to high speed tool steels whose final sintered carbon content is in the range 0.82% - 0.88%. Lower temperatures should be used for higher carbon contents and a non-linear relationship applies between carbon content and sintering temperature from 0.7% to 1.2% carbon. The temperature should be in the range 1200"C to 12500C. For M35 high speed tool steels of 0.85% carbon content a temperature of approximately 1235"C is required.

According to the required final product and the method of making the compact, the sintered compact may require little further work on it other than surface finishing, or may be hotworked to improve its properties. It can then be machined to form the final tool. This hotworking may take the form of forging, open or closed pass rolling or rotary swaging.

Alternatively, the process of surface finishing may be combined with a densification step by th

Claims

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

content a temperature of approximately 1235"C is required.

According to the required final product and the method of making the compact, the sintered compact may require little further work on it other than surface finishing, or may be hotworked to improve its properties. It can then be machined to form the final tool. This hotworking may take the form of forging, open or closed pass rolling or rotary swaging.

Alternatively, the process of surface finishing may be combined with a densification step by the use of cold swaging, cold forging, or by cold drawing through rolls, which should be of a hard material, such as tungsten carbide.

Attention is directed to our Patent No. 1495705 which claims a method of making a metallic article from a powder which is predominantly of tool steel, alloy steel or stainless steel which comprises: a) making powder by atomisation and water quenching of a melt of alloy, b) heating the powder in sub atmospheric pressure and cooling the powder at a controlled rate to anneal it,

c) compacting the powder to form a compact, d) heating the compact in a vacuum at a temperature above 900"C but below the sintering temperature to reduce the oxygen content to not more than 400 p.p.m., e) vacuum and/or controlled atmosphere sintering the deoxidised compact to densify the compact by sintering at a temperature above the solidus temperature of the lower melting temperature constituents of the steel or by infusion of a secondary metal or metal alloy into the voids.

WHAT WE CLAIM IS: 1. A method of making a metallic article which comprises the steps of: a. Making powder of predominantly tool steel, alloy steel, stainless steel or cobalt or nickel based alloy by atomisation and water quenching of a melt of metal alloy, b. Heating the powder in a sub-atmospheric pressure to reduce the oxygen content below 1500 ppm and cooling the powder at a controlled rate to anneal it, c. Analysing the resultant powder for carbon and oxygen content and adding more carbon if necessary d. Compacting the powder at a pressure of at least 25,000 psi to form a compact having a relative density in the range 60 to 92%, e. Heating the compact in a sub-atmospheric pressure below the solidus temperature of the alloy for at least half an hour in order to further reduce the oxygen content to below 700 ppm, f. Raising the temperature of the compact to a selected sinter temperature while maintaining a vacuum or controlled atmosphere and maintaining the temperature within plus or minus 10 C of that selected temperature for at least half an hour to sinter and cause densification of the compact to at least 93% relative density.
2. A method according to Claim 1 in which during the heating of the powder the oxygen content is reduced below 1000 ppm.
3. A method according to Claim 1 or Claim 2 in which the powder is heated above 900"C for at least half an hour in a vacuum better than 1.0 Torr.
4. A method according to any of Claims 1 to 3 in which before compaction a lubricant is added to the powder and the compact is heated to 200 - 6000C for at least half an hour before said further deoxidation until the lubricant is substantially all outgassed.
5. A method according to Claim 4 in which the lubricant is magnesium stearate added in excess of 0.25% by weight and the compact is heated in the range 200"C to 400"C to outgas the lubricant.
6. A method according to any of Claims 1 to 3 in which the powder is packed into a mould of deformable material having a shape approximating to the shape required of the article, and the powder filled mould is subjected to isostatic compression to form the compact.
7. A method according to Claim 4 in which the powder is preformed uni-directionally in a die mounted in a ram press under relatively low pressure to the required shape, the preform given a protective coating sealing the pores of the preform, and the coated preform then subjected to isostatic compression at a relatively high pressure.
8. A method according to any of Claims 1 to 6 in which the isostatically formed compact is machined prior to sintering.
9. A method according to Claim 4 in which all the necessary compaction is performed in a ram press without subsequent isostatic compaction.
10. A method according to any of Claims 1 to 9 in which a composite article is formed.
11. A method according to any of Claims 1 to 10 in which a mandrel is used to retain the shape of the compact during sintering.
12. A method according to Claim 11 in which the mandrel material is of significantly higher thermal expansion than the thermal expansion of the material to be sintered.
13. A method according to Claim 12 in which the mandrel material is austenitic stainless

steel and the sintered material is high speed steel.
14. A method according to any of Claims 1 to 13 in which during the heating of the compact to achieve further deoxidation the oxygen content is reduced to below 500 ppm.
15. A method according to any of Claims 1 to 14 in which the powder is of tool steel or stainless steel and during the heating of the compact the oxygen content is reduced to below 200 ppm.
16. A method according to any of Claims 1 to 15 in which the heating of the compact to achieve the further deoxidation is at a temperature in the range 75 - 200"C below the sinter temperature.
17. A method according to any of Claims 1 to 16 in which during sintering the temperature is maintained within + 3"C of the selected temperature.
18. A method according to any of Claims 1 to 17 in which during the sintering the temperature is maintained within + 1M"C of the selected temperature.
19. A method according to any of Claims 1 to 18 applied to tool steels in which during sintering the temperature is within 20"C below that temperature at which carbide networks form.
20. A method according to any of Claims 1 to 19 in which the sintering temperature is at or within 20"C above the lowest solidus temperature of the steel or alloy composition.
21. A method according to any of Claims 1 to 19 in which the sintering temperature is below the lowest solidus temperature of the steel or alloy composition and the compacts are held at this temperature for a half to three hours and the temperature of the compacts is then raised rapidly to 10 C to 400C above sintering temperature and to a temperature above that solidus temperature and held at this temperature under vacuum for from 5 seconds to 15 minutes.
22. A method according to any of Claims 1 to 21 in which during sintering a sub-atmospheric pressure is maintained.
23. A method according to any of Claims 1 to 22 in which a gas is injected intermittently into the chamber during the deoxidation and/or sintering steps to scavenge gaseous carbon oxides from the interior of the compact.
24. A method according to any of Claims 1 to 23 in which the final articles after sintering have a density of at least 98% relative density.
25. A method according to any of Claims 1 to 24 in which the metal is a tool steel containing more than 2% cobalt.
26. A method according to any of Claims 1 to 25 in which a charge of many compacts are sintered simultaneously in a furnace having a common heating control.
27. A method according to Claim 26 in which the compacts are stacked closely together in the furnace with high reflectivity radiation shields above, below and around the charge, and low temperature insulation is located behind these radiant shields.
28. A method according to Claim 26 or Claim 27 in which the variation in the carbon content over the whole charge is within the band + 0.02% preferably + 0.01%.
29. A method according to any of Claims 26 to 28 in which the compacts are stood on alumina pads small enough to resist thermal shock.
30. A method according to any of Claims 26 to 29 in which the furnace uses graphite or molybdenum heating elements.
31. A method according to any of Claims 26 to 30 in which the charge has a weight above 20 kilograms, preferably above 100 kilograms.
32. A method of making metallic articles on a production scale which comprises heating a powder of predominantly tool steel, alloy steel, stainless steel or cobalt or nickel based alloy, made by atomisation and water quenching a melt of the alloy to give irregular shaped particles in a vacuum better than 1.0 torr for at least half an hour at a temperature above 900"C (preferably above 1000"C) to reduce the oxygen content below 1500 ppm (preferably below 1000" ppm) and cooling at a controlled rate to anneal the powder, analysing the resultant powder for carbon and oxygen content, adding more carbon if necessary, compacting the powder of known oxygen and carbon content to form green compacts of at least 60% full density, loading at least 20 kilograms of compacts into a furnace so that the compacts are stacked close together and surrounded by radiation shields, heating the compacts in a controlled atmosphere in at least two stages; in a first stage the compacts are heated in a vacuum at 75 to 200"C below the solidus temperature of the powdered composition for at least half an hour to reduce the oxygen content below 500 ppm and in a second stage the batch of compacts is raised to a selected sinter temperature at or within 20"C above and preferably within 10 C above the lowest solidus temperature of the deoxidised powder composition and maintained within + 10 C, preferably within + 3"C, and with advantage within + 1"C of the selected temperature for half an hour to three hours to produce a compact of at least 93% preferably 95 - 98% relative density.
33. A method of making metallic articles as claimed in claim 1 or 32 and substantially as described herein.
34. A metallic article made by the method of any of Claims 1 to 33.