« This invention relates to the production of metal strip from powder material. It is known to compact metal powder into a

"green" strip by cold pressing using rolls and then

^" to pass the strip so formed through a sintering furnace to increase the bond between individual particles.

The cold compacted green strip typically has a density of the order of 85% of the full density of the metal and thus further processing, normally involving-: a further heating operation, has usually been considered necessary if a dense, non-porous product is to be obtained. The green strip is brittle and will readily fracture under tensile forces. Thus problems arise in handling this strip in order to effect the heat treatment which, in conventional furnaces, requires a long furnace within which the strip must be supported whilst subjected to the minimum of tensile stress.

It has been proposed in Specification No 1488007 to form the green strip by compacting metal powder and then to heat the strip in a non-oxidising atmosphere by heat transfer thereto from a bath of molten metal through which a strip is passed whilst moving it in the direction of its length and applying further compaction or consolidation to the heated strip. The molten metal bavthprovides not only heat transfer to the strip but also serves to support the strip as it passes therethrough. It is an object of the present invention to provide an improved technique for producing a dense metal strip from a strip of compacted metal powder.

According to the present invention a method of manufacturing dense metal strip from a green compacted powder strip of less than full density comprises the steps of, in a chamber containing a gaseous atmosphere at sub-atmospheric pressure suitable for operating a glow discharge electron beam device, passing the strip continuously through heating means comprising one or more glow discharge heating devices, and, as the strip leaves the heating means, passing it into a rolling mill within said chamber to effect further compaction of the strip.

The heating means conveniently comprises a plurality of glow discharge guns which may be, for example, as described in No.7916658 (Publication No.2050044}. ^' It is possible, using such devices

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to have a very short furnace through which the green strip can be fed with negligible tension on the strip, particularly if positive feed means are provided driving the strip into the heating means from a coil of the strip material. In some cases it may be convenient to use radiant heating means in addition to glow .discharge guns. These glow discharge heating means are particularly convenient in enabling the process to be carried out at a very low ambient atmospheric pressure. ' Using the glow discharge gun or guns, the whole chamber is evacuated to a sufficiently low pressure such that the required glow discharge can be produced.

The atmosphere employed in said chamber will be chosen in accordance with the metal being processed.

Commonly a non-oxidising atmosphere will be desired and for example hydrogen may be used in the processing of metals such as stainless steel.

Two or more stages of hot rolling may be employed and, in such a case, it may be preferred to reheat the strip before at least one rolling stage after the first. This may conveniently be done using a further electron discharge heating device or devices.

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After the hot. rolling, the strip may be fed out of said chamber. Commonly however it may be desirable not to expose the strip to an oxidising atmosphere until after it has cooled. Provision may be made for cooling the strip before it is fed out of the chamber but conveniently the strip is coiled in said chamber. The strip may then be allowed to cool in said chamber as a coil before subsequent use or further processing. If a hot strip is to be coiled in said chamber, problems can arise because of successive turns of the coil adhering to one another and/or because of thermal contraction of the strip on cooling. For this reason, if the strip is coiled whilst hot, it is preferred to interleave a layer of refractory flexible material, e.g. a ceramic paper, between successive turns of the coiled metal strip. Conveniently a ceramic fibre paper material, e.g. an alumina fibre material, is employed for this purpose. The invention includes within its scope a metal strip produced by the above-described method.

Furthermore, the invention also includes an apparatus for the manufacture of dense metal strip from a green compacted powder strip of less than full density comprising a chamber containing a gaseous

atmosphere at a sub-atmospheric pressure, glow discharge heating means in the chamber and a rolling mill in the chamber arranged for effecting compaction of the strip as it leaves the heating means. As previously indicated the heating means preferably comprise a plurality of glow discharge guns. The rolling mill may comprise two or more stages and heating means may be provided for reheating the strip before at least one rolling stage ^' after the first. There may be provided, preferably within the chamber, means for carrying the green strip as a coil with drive means for feeding the strip into the heating means. Positive driving to feed the strip into the heating means enables the tension to be kept low as the strip is being heated.

Means may be provided for coiling said strip in said chamber as it leaves the rolling mill. These means may include means for interleaving a layer of refractory flexible material between the successive turns of the coiled metal strip.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a front elevation view, with the front cover removed, of a chamber containing apparatus for making stainless steel strip from a compacted powder strip.

Referring to the drawing, a chamber 10 is provided with means, not shown, for evacuating the chamber to a low pressure suitable for the operation of glow discharge guns. The residual atmosphere is chosen in accordance with the type of metal of which the strip is formed. In this particular example, the strip

*ι is of stainless steel and the residual atmosphere is hydrogen. The pressure is at the required low pressure (typically a fraction of a torr) suitable for maintaining a glow discharge. At this pressure of hydrogen, any residual oxygen will be well below the critical level and there will be no oxidising of the metal on heating.

The green strip is loaded into the chamber as a coil 11, the end of which extends from the coil, as shown at 12, partially around a drive capstan 13 which provides a positive feed for the strip feeding it into a heating system comprising, in this particular embodiment five successive glow discharge guns 14 to 18. Each of these guns is of the kind described in U.K.Patent Application No 7916658 (Publication No.2050044) and reference may be made to that specification for a full description of suitable heating devices.

From the heating system, the strip is fed to a rolling mill containing, in this particular embodiment, two stages formed by rolls 20 and rolls 22

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which effect the required compaction of the heated strip. The compaction is effected immediately after heating so minimising any cooling before the strip enters the mill. In this particular embodiment, a further glow discharge device 21 is provided for reheating of the strip between the two stages. Compaction of powdered material in this way can be effected at a lower temperature, for example, than is required for sintering and it is thus possible to feed the green strip through the heating means and into the rolling mill although the green strip itself is very brittle and fragile. The compaction forms the strip into a dense metal strip which can be fed, as shown at 23, through a strip guide 24 to a take-up spool 25. it may be desirable, in some cases, to interleave a flexible and resilient refractory strip (e.g. a ceramic paper) between the turns of the metal as the metal is fed onto the take-up spool 25. This refractory strip becomes compressed on cooling of the metal but keeps the turns separate and permits of contraction of the metal.

With the apparatus described above, having the. rolling mill in a vacuum chamber, oxidation of the rolls is avoided and it is unnecessary to use a lubricant (such as water) as in conventional hot metal rolling.

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