Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2002-02-22
2003-01-28
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C419S019000, C419S028000, C419S029000
Reexamination Certificate
active
06511523
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an oxide-dispersion strengthened platinum material in which zirconium oxide is finely dispersed in platinum. In particular, it relates to an oxide-dispersion strengthened platinum material that consists of coarse platinum grains and to its production process.
BACKGROUND ART
A platinum material exhibiting good high-temperature strength has been used mainly as a structural material for glass melting for a long time. High-temperature strength required to the platinum material is so-called creep strength. In particular, the most important objective in developing a platinum material is considered to be how long a durable time until creep rupture will be extended.
For improving creep strength, there has been conventionally used a technique that a particular oxide is finely dispersed in platinum. As such an oxide-dispersion strengthened platinum material, a material in which zirconium oxide is dispersed in platinum is known.
However, zirconium-oxide dispersion strengthened platinum material that has been conventionally known can ensure a certain level of creep strength, but is in the present state that the creep strength is required to be improved further.
DISCLOSURE OF THE INVENTION
Accordingly, the present invention is aimed at providing a platinum material in which creep strength is more improved than that in conventional materials by improving a metal grain shape in a conventionally known oxide-dispersion strengthened platinum material in which zirconium oxide is dispersed, and providing a process for producing the platinum material.
In order to solve the above problems, the inventors of the present invention paid their attentions to the fact that creep strength depends on the metal grain of a platinum material, that is, the size of a platinum grain and have completed a technique for further improving the creep strength by making crystal grains in a platinum material to be finished as a final product more coarse than those in the conventional product.
That is, the present invention is characterized in that in an oxide-dispersion strengthened platinum material in which zirconium oxide is dispersed in platinum and which is obtained through rolling and thermal recrystallization, platinum grains that constitute the platinum material have an average grain size in a rolling direction ranging from 200 to 1500 &mgr;m and have an average grain aspect ratio of 20 or more.
Because a platinum material in the present invention is an oxide-dispersion strengthened material in which zirconium oxide is dispersed in platinum and which is obtained through thermal recrystallization after rolling, if being considered liken to a plate material, platinum grains in the platinum material texture are in a state of being stretched in a direction of a plate surface, that is, in a state of being extended in a longitudinal direction. That is, the platinum grains constituting a platinum material of the present invention are those in which the average grain size in the rolling direction, that is, in the plate thickness direction is in the range of 200 to 1500 &mgr;m and the crystal grain aspect ratio, that is, the ratio of the crystal size in the plate surface direction to the crystal size in the plate thickness direction is 20 or more. As long as the inventors of the present invention. know, there is no platinum material that is composed of-such coarse platinum grains in conventional zirconium-oxide dispersion strengthened platinum materials.
According to the platinum material of the present invention, the creep strength is further improved compared to that in conventional platinum materials, and even when the material is used as a structural material for glass melting, the material can decrease the amount of platinum eluted into molten glass. Generally, because spots where creep rupture and an elution phenomenon of a platinum material occur are considered to be caused mainly by grain boundaries, the reason why the platinum material of the present invention can achieve the improvement of creep strength and the decrease in the elution phenomenon of platinum is considered that the number of grain boundaries themselves is extremely few in the material because platinum grains constituting the platinum material are coarse.
An oxide-dispersion strengthened platinum material according to the present invention can be obtained by the following production process. That is a process for producing an oxide-dispersion strengthened platinum material where zirconium oxide is finely dispersed in platinum, comprising the steps of pouring powdered platinum into water to prepare a platinum suspension; adding a zirconium nitrate solution and a pH adjusting liquid in the platinum suspension for adjusting the suspension to a given pH to precipitate zirconium hydroxide and thus to form zirconium hydroxide carrying platinum; collecting the zirconium hydroxide carrying platinum, which is then cold isostatic pressed into a molding; sintering and forging the molding under the conditions in which secondary recrystallization growth in a platinum grain is restrained, to form a platinum ingot; and cold-rolling the platinum ingot in a reduction ratio of 70% or more and then thermally recrystallizing the product.
A production process according to the present invention is characterized in that first, a given powdered platinum is prepared and then a zirconium-hydroxide carrying platinum in which zirconium hydroxide is supported is formed with the use of a chemical precipitation reaction. And in the process, using the powder of this zirconium hydroxide carrying platinum, forming, sintering, forging, cold rolling and thermally recrystallizing are sequentially conducted, but the process is characterized in that sintering and forging among these processes are conducted under the conditions whereby the secondary recrystallization growth of platinum is restrained to the utmost. In the following, a production process according to the present invention will be detailed one by one.
First, in contrast to so-called coprecipitation (a coprecipitation process), in a production process of the present invention, platinum is first processed into given powders; the powdered platinum is used to prepare a platinum suspension; a zirconium nitrate solution and a pH adjusting liquid are added for adjusting the suspension to a given pH to precipitate zirconium hydroxide and thus to form zirconium-hydroxide carrying platinum; and the zirconium-hydroxide carrying platinum is collected and is then cold isostatic pressed into a molding.
When a zirconium-hydroxide carrying platinum is formed by such a process, platinum alone is powdered in advance. Thus, platinum powders may be appropriately prepared as those having a particle size suitable for subsequent molding and sintering steps. In general, powdered platinum exhibits quite higher gas adsorption ability. However, according to the production process of the present invention, gas adsorption on a platinum surface may be reduced due to the presence of zirconium hydroxide supported on the platinum surface, so that the formation of unwanted pores due to adsorbed gas, which becomes an issue during molding and sintering, i.e., the formation of internal defects in the platinum material to be finally obtained, can be effectively prevented.
Further, it is preferable in the production process of the present invention to use heated powdery platinum when forming a zirconium-hydroxide carrying platinum. The heating process is conducted at temperatures of 400° C. or higher. Such heating may considerably inhibit pore formation due to adsorbed gas during the subsequent molding and sintering processes. And, after the heating process, the surface of the powdered platinum becomes smooth, so that zirconium hydroxide can be homogeneously and finely supported by each platinum surface and thus zirconium oxide can be quite homogeneously and finely dispersed in a platinum material. This heating process may be conducted during or after the powdering process.
And when a zirconium-hydroxide carryin
Hitomi Soichi
Shoji Toru
Takagi Yoshikazu
Watanabe Yoshinobu
Arent Fox Kintner & Plotkin & Kahn, PLLC
Mai Ngoclan
Tanaka Kikinzoku Kogyo K.K.
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