GB2234527A - Methods of producing metallic powders and metallic powders produced by such methods - Google Patents

Abstract

A process for producing a metallic powder comprises the steps of forming an aqueous slurry of a metallic powder and a film-forming cellulose derivative, applying a coating of this slurry onto a support surface, and heating the slurry coating to a temperature sufficient to induce gelling of the cellulose derivative thereby to produce a self-supporting strip comprising essentially the metallic powder. This self-supporting strip is fed into and through a heat treatment furnace in a reducing atmosphere at a temperature and for a period of time sufficient to achieve reduction in the oxygen and/or nitrogen and/or sulphur content of the metallic powder. The strip is withdrawn from the furnace and reduced to particulate form for subsequent use.

Description

Methods of Producing Metallic Powders and Metallic Powders produced by such Methods This invention relates to methods of producing metallic powders and to metallic powders produced thereby More especially, but not exclusively, the invention relates to methods of producing metallic powders having impurity contents less than those normally present in such powders. The invention also relates to methods of effecting changes in the chemical composition of metallic powders.

Metallic powders are conventionally produced by several processes, these including atomisation of freely falling liquid metal streams by water or gas jets directed at the liquid metal streams and chemical processes in which a melt is subject to a sequence of chemical reactions and processes Where water atomisation is employed, the resulting metal powder has a relatively high oxygen content inconsistent with the minimum level acceptable for many products. Metal powders produced by gas atomisation processes in which nitrogen is employed as 'the atomising gas predictably have nitrogen contents which are inconsistant with the minimum level acceptable for certain products. Metallic powders produced by chemical processes can also have relatively high contents of oxygen, nitrogen and sulphur. Additionally, powder produced by chemical processes is generally expensive to purchase.

In order to reduce excessive oxygen, nitrogen and/or sulphur levels present in metallic powders or compacts produced from such powders, it is known to subject bulk quantities of the powders or the compacts to a heat treatment process. To heat treat bulk powders or compacts produced from such powders is both time consuming and expensive.

The present invention inter alia sets out to produce from metallic powders having oxygen, nitrogen and/or sulphur contents higher than required, metallic powders having significantly reduced oxygen, nitrogen and/or sulphur contents.

The invention also sets out to provide a process by which the chemical composition of metallic powders can be varied during heat treatment.

According to the present invention in one aspect there is provided a process for producing a metallic powder which comprises the steps of forming an aqueous slurry of a metallic powder and a film-forming cellulose derivative, applying a coating of this slurry onto a support surface, heating the slurry coating to a temperature sufficient to induce gelling of the cellulose derivative thereby to produce a self-supporting strip comprising essentially of the metallic powder, and feeding this self-supporting strip into and through a heat treatment furnace in a reducing atmosphere at a temperature and for a period of time sufficient to achieve reduction in the oxygen and/or nitrogen and/or sulphur content of the metallic powder, withdrawing the strip from the furnace and reducing the strip to particulate form for subsequent use.

The reducing atmosphere may comprise hydrogen or nitrogen. Particulate graphite may be added to the powder or to the slurry to inhibit sintering of the powder particles during heat treatment.

The carbon content of the initial powder particles may be increased to a level in excess of that required in the final powder particles to assist in the removal of oxygen during the heat treatment.

The strip is preferably supported on a moving support surface as it travels through the heat treatment furnace.

According to the present invention in another aspect there is provided metallic powder produced by the process defined in the preceding four paragraphs.

According to the present invention in a still further aspect, there is provided a method of producing metallic powder which comprises the steps of forming an aqueous slurry of a metallic powder and a film-forming cellulose derivative, applying a coating of this slurry onto a support surface, heating the slurry coating to a temperature sufficient to induce gelling of the cellulose derivative thereby the produce a self-supporting strip comprising essentially of the metallic powder, and feeding this self-supporting strip into and through a heat treatment furnace in a controlled atmosphere at a temperature and for a period of time sufficient to effect a chemical reaction between the gaseous content of the atmosphere and the composition of the powder content of the strip, withdrawing the strip from the furnace and reducing the strip to particulate form for subsequent use.

The atmosphere present in the furnace may comprise a hydrocarbon gas such as methane.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawing in which the sole Figure is a schematic side view of apparatus for operating a process in accordance with the invention.

In the apparatus shown, a slurry is retained in a vessel 1. The slurry conveniently is based upon multiples of 300 grammes of methyl cellulose treated with glyoxyl as a solubility inhibiter together with 12 litres of water optionally containing suitable slurrying and wetting agents. Incorporated in the aqueous methyl cellulose is approximately 35 kg of a metallic powder produced by a gas or water atomising powder typically of below 80 BS mesh.

Water atomised metallic particles typically have oxygen contents of 5000 ppm and gas atomised powders typically have nitrogen contents of 25 ppm. The concentration of the metal powder in the aqueous slurry is approximately 75% by weight although lower or higher concentrations may be used according to the mechanical and/or thermal properties which are required for the final product.

The slurry is transferred by way of a train of rollers 3 onto a coating roller arranged to deposit a slurry coating of a selected thickness and width onto an endless moving belt 4 looped around drums. The belt is preferably constructed of an inert metal such as stainless steel.

Other means of slurry deposition may be employed these including curtain coating or extrusion.

Drive applied to at least one of the drums feeds the belt through a drying oven 5 initially to raise the temperature of the deposited slurry layer to about 45tC to induce gelling of the methyl cellulose and drive water from the gelled slurry. The slurry film emerges from the oven as a flexible and self-supporting strip 6 which can readily be removed from the surface of the belt. The belt may conveniently be pre-treated to ensure easy release.

The flexible self-supporting strip is generally referred to as "flexi strip The flexi strip is admitted to a heat treatment furnace 7. As the strip enters the furnace it is supported on the upper surface of a moving endless belt 8 and is transported through the furnace with its lower surface in contact with the upper surface of the belt. The endless belt is typically produced from an inert material such as stainless steel. As the strip passes through the furnace, at a temperature typically of 11800 C, the nitrogen and/or oxygen and/or sulphur impurity content of the powder particles is reduced by reaction with the gaseous content of the furnace e.g. hydrogen or nitrogen. The reducing gas is continuously pumped into and through the furnace during operation of the process to ensure a consistent gaseous presence free of contaminants.

Typically, the thickness of the flexi strip is of the order of 1.5 mm. Thus, heat treatment of the flexi strip is both relatively rapid and consistent along the length of the flexi strip. Typically, the residence time of the strip as it travels through the furnace is of the order of 2 minutes to achieve a reduction in oxygen from 5000 ppm to 300 ppm and a reduction in nitrogen of from 25 ppm to less than 5 ppm.

Flexi strip removed from the furnace is subsequently reduced once again to a particulate form by, for example, a grinding process. Due to the heat treatment process, the final powder product of the process has relatively low oxygen and/or nitrogen contents. Typically these contents are 300 ppm and less than 5 ppm.

Graphite powder, typically of particle size of between 1 and 10 microns, may be admixed with the powder or with the slurry to provide a physical barrier between powder particles during the heat treatment process to inhibit sintering thereof. The graphite enables higher temperatures to be employed during the heat treatment process.

To enhance the removal of oxygen from the powder, the carbon content of the liquid metal from which the initial powder is produced may be increased to a level in excess of that required in the final powder product.

In the embodiment described above, a reducing atmosphere is present within the heat treatment furnace.

Alternatively, a gas which chemically reacts with the powder content of the strip may be employed. Thus, a hydrocarbon gas such as methane may be employed to increase the carbon or nitrogen content of the powder.

It is to be understood that the foregoing is merely exemplary of a process in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention.

Claims (10)

1 A process for producing a metallic powder which comprises the steps of forming an aqueous slurry of a metallic powder and a film-forming cellulose derivative, applying a coating of this slurry onto a support surface, heating the slurry coating to a temperature sufficient to induce gelling of the cellulose derivative thereby to produce a self-supporting strip comprising essentially the metallic powder, and feeding this self-supporting strip into and through a heat treatment furnace in a reducing atmosphere at a temperature and for a period of time sufficient to achieve reduction in the oxygen and/or nitrogen and/or sulphur content of the metallic powder, withdrawing the strip from the furnace and reducing the strip to particulate form for subsequent use.

2 A process as claimed in claim 1 wherein the reducing atmosphere comprises hydrogen or nitrogen.

3 A process as claimed in claim 1 or claim 2 wherein particulate graphite is added to the powder or to the slurry to inhibit sintering of the powder particles during heat treatment.

4 A process as claimed in any one of claims 1 to 3 wherein the carbon content of the initial powder particles is increased to a level in excess of that required in the final powder particles to assist in the removal of oxygen during the heat treatment.

5 A process as claimed in any one of claims 1 to 4 wherein the strip is supported on a moving support surface as it travels through the heat treatment furnace

6 Metallic powder produced by a process as claimed in any one of claims 1 to 5.

7 A method of producing metallic powder which comprises the steps of forming an aqueous slurry of a metallic powder and a film-forming cellulose derivative, applying a coating of this slurry onto a support surface, heating the slurry coating to a temperature sufficient to induce gelling of the cellulose derivative thereby the produce a self-supporting strip comprising essentially the metallic powder, and feeding this self-supporting strip into and through a heat treatment furnace in a controlled atmosphere at a temperature and for a period of time sufficient to effect a chemical reaction between the gaseous content of the atmosphere and the composition of the powder content of the strip, withdrawing the strip from the furnace and reducing the strip to particulate form for subsequent use.

8 A method as claimed in claim 7 wherein the atmosphere present in the furnace comprises a hydrocarbon gas.

9 A method as claimed in claim 7 wherein the atmosphere present in the furnace comprises methane.

10 A method of producing metallic powder substantially as herein described and as described with reference to the sole Figure of the accompanying drawing.