Method of Producing a Tantalum Stock Material of High Ductility

The invention relates to a method for the production of a tantalum stock material of high ductility for use in the field of high-speed deformation.

In the field of high-speed deformation such as, for example, in the case of projectiles, high-ductility iron or copper is usually used. The depth of penetration of the projectile is a function both of the density of the high-ductility material and of the density of the material on which the projectile impacts. Efforts have long been made to produce a highly ductile material of very high density.

The object of the inventive method is the production of a tantalum stock material of high ductility which will be suitable for use in the field of high-speed deformation, especially for projectiles.

This object is achieved by producing a bar which is pressed from directly reduced tantalum powder containing less than 100 micrograms of niobium, tungsten and possibly molybdenum per gram of powder. The bar is melted down as a consumable electrode in an electron beam furnace which has a pressure maintained at less than 5.times.10.sup.-4, mbar, the molten metal is collected in a cooled mold, and an ingot is formed. The ingot is remelted at least twice in the electron beam furnace while maintaining a pressure of less than 5.times.10.sup.-4 mbar and the ingot obtained in the final remelting cycle is shaped into a slab.

The slab is machined to a smooth surface on all sides to a maximum depth of roughness of 25 microns. From the smoothed slab, stock materials are made by a conventional shaping process and in this manufacturing process at least one heat treatment is included in either an inductively heated or a resistance heated furnace while maintaining a pressure of less than 5.times.10.sup.-4 mbar. Both furnaces may be used together for the heating treatment.

Sodium-reduced tantalum powder is preferably used in the inventive method. The ingot which is obtained by melting down the pressed tantalum powdered body is preferably melted in the electron beam furnace at a higher melting rate (kg/h) than the pressed body. It is preferable to make the melting rate of the ingot at least twice as great as the melting rate of the pressed body.

In making the slab, the ingot obtained from the final remelting cycle is first cold-forged to stock which is divided into individual blocks. Then each individual block is heated in a slightly oxidizing atmosphere to a temperature of about 650.degree. C. and after it is removed from the furnace, when the piece is still at a temperature in the range of from 450 to 600.degree. C., it is upset. After cooling completely to room temperature the upset piece is cold-forged to a slab.

In the process of producing the stock material it is preferable to include a cold rolling step with shaping degrees of .rho..gtoreq.1.2 (.epsilon.70%), which includes not only rolling in the direction of the axis of the ingot last obtained but also rolling in a transverse direction. It is preferable to perform a tension-relieving annealing before the cold rolling step at a temperature of about 650.degree. C. and after the cold rolling step to perform a recrystallization heat treatment in the range of about 900.degree. C.

As the product of the process according to the invention, highly ductile tantalum stock materials are obtained which are isotropic in regard to their mechanical properties and their structure. The tantalum stock materials prepared according to the invention are texture-free and have a grain size that is finer than 30 microns (according to ASTM E 112). Their tensile strength is less than 200 N/mm.sup.2, their elongation is greater than 60%. The overall purity of the materials according to the invention is computed by determining the residual resistance ratio (electrical resistance at a temperature of 273 K: electrical resistance at the temperature of 4.2 K). Materials obtained by the inventive method have a residual electrical resistance ratio of at least 200.

A method for preparing a highly ductile tantalum stock material will be described with the aid of the following example.

To produce the pressed bar, sodium-reduced tantalum powder was used whose content of impurities (micrograms per gram) is to be seen in the following table:

From this tantalum powder a bar was prepared by cold isostatic pressing. This bar was used as a consumable electrode in an electron beam furnace and melted at a rate in the range of 25 to 35 Kg/h. During the melting process a pressure of 2.times.20.sup.-4 mbar was maintained. The melt was gathered in a water-cooled mold and an ingot was formed having a diameter of 150 mm. This ingot was then twice remelted in the electron beam furnace, the ingot produced being used again as the consumable electrode.

During the first remelting a pressure of 8.times.10.sup.-5 mbar was maintained in the electron beam furnace, and the melting rate was in the range of 70 to 100 Kg/h. The corresponding figures for the second remelting were 6.times.10.sup.-5 mbar, and the melting rate was again in the range of 70 to 100 Kg/h, while in the final remelting the pressure in the electron beam furnace had been lowered to 3.times.10.sup.-5 mbar and the melting rate was 120 Kg/h. The diameter of the ingot obtained in the final remelting cycle was 175 mm.

The ingot finally obtained was then shaped by thermomechanical methods to a slab. First the ingot was cold formed to a 150 mm octagon shape, and then it was divided into single blocks of 350 mm length. Each piece was then heated in a gas-fired hearth furnace in a slightly oxidizing atmosphere at 650.degree. C. and held at this temperature for a period of about 1 to 3 hours. After removal from the hearth furnace the individual pieces were upset on a forging hammer at a temperature of about 550.degree. C. After it had completely cooled to room temperature the upset individual piece was cold-formed to a slab, down to a size of about 160.times.65.times.800 mm. This was followed by a milling process to a rough depth of 20 microns to smooth the slab. The smoothed slab was degreased and pickled first in aqua regia and then in an acid mixture consisting of two parts by volume of concentrated nitric acid and two parts by volume of water. This was followed by a cold rolling at a high rate of thickness reduction per pass, the rolling being performed both in the direction of the axis of the last-obtained ingot and in the direction across it. The deformation amounted to .rho..gtoreq.1.3 (.epsilon..gtoreq.75%). After this cold rolling process the cold-rolled piece was degreased and pickled. Then an annealing to reduce tensions was performed in an inductively heated oven in which a pressure of 2.times.10.sup.-4 mbar was maintained during the treatment. After this annealing the annealed piece was subjected to several cold rollings, the rolling being performed again in directions across and parallel to the axis of the last-obtained ingot. This cold rolling was also performed at a high rate of thickness reduction. The degree of deformation was .rho..gtoreq.1.9 (.epsilon..gtoreq.85%). After this cold rolling the workpiece was, as already described above, ground, degreased and pickled and then subjected to a recrystallization treatment at 875.degree. C. in a resistance-heated vacuum furnace.

The highly ductile tantalum product thus obtained was texture-free and had a grain size finer than 30 microns according to ASTM E 112). Its tensile strength was 192 N/mm.sup.2, its elongation 65%, and the residual resistance ratio was found to be 220.