Patent Application
20110052441
The subject matter disclosed herein relates to a method and container for forming billets using hot isostatic pressing and, more particularly, to a method and container for preventing diffusion of metals between a high value powder alloy and the container used for hot isostatic pressing.
Metallurgical techniques have been developed for the manufacture of a metal billet or other object from metal powders created in a predetermined particle size by e.g., microcasting or atomization. Usually highly alloyed with Ni (nickel), Cr (chromium), Co (cobalt), and Fe (iron), these powders are consolidated into a dense mass approaching 100 percent theoretical density. The resulting billets have a uniform composition and dense microstructure providing for the manufacture of components having improved toughness, strength, fracture resistance, and thermal expansion coefficients. Such improved properties can be particularly valuable in the fabrication of e.g., rotary components for a turbine where high temperatures and/or high stress conditions exist.
The consolidation of these metal powders into a dense mass typically occurs under high pressures and temperatures in a process referred to as hot isostatic pressing (HIP). Typically, the powders are placed into a container (sometimes referred to as a “can”) that has been sealed and its contents placed under a vacuum. The container is also subjected to an elevated temperature and pressurized on the outside using an inert gas such as e.g., argon to avoid chemical reaction. For example, temperatures as high as 480° C. to 1315° C. and pressures from 51 MPa to 310 MPa or even higher may be applied to process the metal powder. By pressurizing the container that is enclosing the powder, the selected fluid medium (e.g., an inert gas) applies pressure to the powder at all sides and in all directions. Under the extreme temperatures and pressures of the HIP process, the container is substantially deformed or crushed as the volume of the powder decreases during the HIP process and the container becomes joined to the surface of the billet created by the compacted powder.
In addition to the visible changes that have occurred, certain microscopic events also occur during the HIP process. More specifically, during the several hours over which the HIP process occurs, unwanted diffusion effects are created. Elements will migrate from the container to the powder and from the powder to the container during the HIP process. For example, container 101 is conventionally manufactured from low carbon steel or authentic stainless steel such as 304SS. Fe and C (carbon) can diffuse from the container into the metal powder. Conversely, Cr and other elements in the powder can diffuse into the container. Additionally, an unwanted diffusion layer containing e.g., Cr, Ni, and Fe will develop between the container and the billet. Therefore, the cross-diffusion of components creates a region of undesired compositions near the surface of the billet and also represents loss of the substantially expensive, highly alloyed powder used to create the billet.
Unfortunately, depending upon the shape desired for billet 106 (or the shape of the ultimate component to be constructed from billet 106), the above-described diffusion effects for container 101 may require the removal of valuable material from its surface. Again, because of the substantial costs of the original powder, this loss is undesirable. Therefore, an improved device that provides for the reduction or elimination of such diffusion effects and the loss of high value powder materials during HIP treatment would be useful.
The present invention provides an improved method and container for forming billets using hot isostatic pressing. The method and container prevent or control the diffusion of metals between a high value powder alloy and the container used for hot isostatic pressing. Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
In one exemplary embodiment, the present invention provides a container for compaction processing of a powder. The container includes a container top, a container bottom, and an outer wall located between and connecting the container top and the container bottom to define an interior for the receipt of the powder. A diffusion barrier is positioned along the container top, container bottom, and outer wall so as to separate the container from the powder during the compaction processing.
In another exemplary embodiment, a container for compaction processing of a powder is provided. The container includes a container top, a container bottom and an outer wall located between and connecting the container top and the container bottom to define an interior for the receipt of the powder. One or more of the container top, the container bottom, and the outer wall are constructed from the same alloy composition as the powder in order to prevent diffusion between the powder and the container or parts thereof. Alternatively, one or more of the container top, the container bottom, and the outer wall are constructed from an alloy similar to the powder that does not allow for a detrimental alloy phase to form in the can/billet interface during the HIP cycle.
In still another exemplary aspect of the present invention, a method for improving the use of material during hot isostatic pressing is provided. The method includes the step of providing a container for the receipt of a powder intended for hot isostatic pressing. The container includes a top, a bottom, and an outer wall connecting the top and the bottom to define an interior of the container. The method also includes positioning a diffusion barrier along the container so as to separate the powder from the container during the hot isostatic pressing. A powder is inserted into the interior of the container. The container is then submitted to hot isostatic pressing while preventing or minimizing the diffusion of elements between the container and the powder.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:
To provide advantageous improvements as described herein, the present invention provides an improved method and container for forming billets using hot isostatic pressing and, more particularly, to a method and container for preventing diffusion of metals between a high value powder alloy and the container used for hot isostatic pressing. For purposes of describing the invention, reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.
An exemplary embodiment of a container 201 according to the present invention is shown in
Container 201 includes a container top 200, container bottom 235, and outer wall 210. For this exemplary embodiment, container 201 may be constructed of conventional materials as previously mentioned e.g., an authentic stainless steel such as 304SS. As shown in
Container 201 also includes a diffusion barrier 220 separating the high value powder material 205 from the container top 200, bottom 235, and outer wall 210. Diffusion barrier 220 operates to prevent diffusion and is positioned as a layer or inner liner on container 201 located between powder 205 and container 201. Diffusion barrier 220 prevents or minimizes the migration of elements from powder 205 into container 201 or from container 201 into powder 205.
Diffusion barrier 220 is constructed from one or more materials specifically selected to prevent the diffusion process. A variety of materials may be used depending upon the composition of powder 205, container 201, and the conditions of the HIP process. For example, the diffusion barrier 220 could be constructed from various metal nitrides, sulphides, carbides, carbon nitrides or metal oxides. Ceramic material may also be used. In certain applications, diffusion barrier 220 may be constructed from a metal alone such as e.g., tantalum, gold, silver, or copper. Other materials may be applied as well. Again, the objective of material selection for diffusion barrier 220 is to prevent or impede the diffusion of materials between container 201 and powder 205.
A variety of techniques may be used to position diffusion barrier 220 along the inside of container 201. Diffusion barrier 220 may, for example, be constructed of a metal foil that is placed along the inside of the container. The foil could be specifically constructed according to the geometry of container 201 or could be applied as overlapping sheets before placement of powder 205 into container 201. Various plating techniques could also be used to deposit diffusion barrier 220 upon the interior of container 201. For example, electroplating or electroless plating could be used to deposit the desired thickness of barrier material as a layer 220 upon container 201. Chemical vapor deposition can also used to deposit materials of the desired thickness on container 201 to create diffusion barrier 220. Ceramic coating could also be applied through a variety of techniques including e.g., plasma spraying. Using the teachings disclosed herein, one of skill in the art will understand that various other methods may also be used in order to apply diffusion barrier 220.
In the conventional container of
While the present subject matter has been described in detail with respect to specific exemplary embodiments and methods thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.
