Industrial electric heating furnaces – Electroslag remelting device
Reexamination Certificate
2000-11-15
2002-12-17
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Electroslag remelting device
C373S021000, C075S010240
Reexamination Certificate
active
06496529
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
The present invention relates to an apparatus and a method for refining and casting metal and metal alloy ingots and other preforms. The present invention more particularly relates to an apparatus and a method useful for refining and casting large diameter ingots and other preforms of metals and metal alloys prone to segregation during casting, and wherein the preforms formed by the apparatus and method may exhibit minimal segregation and lack significant melt-related defects. The apparatus and method of the invention find particular application in, for example, the refinement and casting of complex nickel-based superalloys, such as alloy
706
and alloy
718
, as well as certain titanium alloys, steels, and cobalt-base alloys that are prone to segregation when cast by conventional, state-of-the-art methods. The present invention is also directed to preforms and other articles produced by the method and/or apparatus of the present invention.
DESCRIPTION OF THE INVENTION BACKGROUND
In certain critical applications, components must be manufactured from large diameter metal or metal alloy preforms exhibiting minimal segregation and which are substantially free of melt-related defects such as white spots and freckles. (For ease of reference, the term “metallic material” is used herein to refer collectively to unalloyed metals and to metal alloys.) These critical applications include use of metal components as rotating components in aeronautical or land-based turbines and in other applications in which metallurgical defects may result in catastrophic failure of the component. So that preforms from which these components are produced are free of deleterious non-metallic inclusions, the molten metallic material must be appropriately cleaned or refined before being cast into a preform. If the metallic materials used in such applications are prone to segregation when cast, they are typically refined by a “triple melt” technique which combines, sequentially, vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Metallic materials prone to segregation, however, are difficult to produce in large diameters by VAR melting, the last step in the triple melt sequence, because it is difficult to achieve a cooling rate that is sufficient to minimize segregation. Although solidification microsegregation can be minimized by subjecting cast ingots to lengthy homogenization treatments, such treatments are not totally effective and may be costly. In addition, VAR often will introduce macro-scale defects, such as white spots, freckles, center segregation, etc., into the ingots. In some cases, large diameter ingots are fabricated into single components, so VAR-introduced defects cannot be selectively removed prior to component fabrication. Consequently, the entire ingot or a portion of the ingot may need to be scrapped. Thus, disadvantages of the triple melt technique may include large yield losses, lengthy cycle times, high materials processing costs, and the inability to produce large-sized ingots of segregation-prone metallic materials of acceptable metallurgical quality.
One known method for producing high quality preforms from melts of segregation prone metallic materials is spray forming, which is generally described in, for example, U.S. Pat. Nos. 5,325,906 and 5,348,566. Spray forming is essentially a “moldless” process using gas atomization to create a spray of droplets of liquid metal from a stream of molten metal. The process parameters of the spray forming technique are adjusted such that the average fraction of solid within the atomized droplets at the instant of impact with a collector surface is sufficiently high to yield a high viscosity deposit capable of assuming and maintaining a desired geometry. High gas-to-metal mass ratios (one or greater) are required to maintain the heat balance critical to proper solidification of the preform.
Spray forming suffers from a number of disadvantages that make its application to the formation of large diameter preforms problematic. An unavoidable byproduct of spray forming is overspray, wherein the metal misses the developing preform altogether or solidifies in flight without attaching to the preform. Average yield losses due to overspray in spray forming can be 20-30%. Also, because relatively high gas-to-metal ratios are required to maintain the critical heat balance necessary to produce the appropriate solids fraction within the droplets on impact with the collector or developing preform, the rapid solidification of the material following impact tends to entrap the atomizing gas, resulting in the formation of gas pores within the preform.
A significant limitation of spray forming preforms from segregation prone metallic materials is that preforms of only limited maximum diameter can be formed without adversely affecting microstructure and macrostructure. Producing larger spray formed preforms of acceptable quality requires increasingly greater control of the local temperature of the spray to ensure that a semi-liquid spray surface layer is maintained at all times. For example, a relatively cooler spray may be desirable near the center of the preform, while a progressively warmer spray is desired as the spray approaches the outer, quicker cooling areas of the preform. The effective maximum diameter of the preform is also limited by the physics of the spray forming process. With a single nozzle, the largest preforms possible have a maximum diameter of approximately 12-14 inches. This size limitation has been established empirically due to the fact that as the diameter of the preform increases, the rotational speed of the surface of the preform increases, increasing the centrifugal force experienced at the semi-liquid layer. As the diameter of the preform approaches the 12 inch range, the increased centrifugal force exerted on the semi-liquid layer tends to cause the layer to be thrown from the preform face.
Accordingly, there are significant drawbacks associated with certain known techniques applied in the refining and casting of preforms, particularly large diameter preforms, from segregation prone metallic materials. Thus, a need exists for an improved apparatus and method for refining and casting segregation prone metals and metal alloys.
BRIEF SUMMARY OF THE INVENTION
In order to address the above-described need, the present invention provides a method of refining and casting a preform including the steps of providing a consumable electrode of a metallic material and then melting and refining the electrode to provide a molten refined material. At least a portion of the molten refined material passes through a passage that is protected from contamination by contact with oxygen in the ambient air. The passage preferably is constructed of a material that will not react with the molten refined material. A droplet spray of the molten refined material is formed by impinging a gas on a flow of the molten refined material emerging from the passage. The droplet spray is deposited within a mold and solidified to a preform. The preform may be processed to provide a desired article such as, for example, a component adapted for rotation in an aeronautical or land-based turbine.
The step of melting and refining the consumable electrode may consist of at least one of electroslag remelting the consumable electrode and vacuum arc remelting the consumable electrode to provide the molten refined material. The passage through which the molten refined material then passes may be a passage formed through a cold induction guide. At least a portion of the molten refined alloy passes through the cold induction guide and is inductively heated within the passage. In less demanding applications, e.g., applications in which some small level of oxide contaminants in the alloy can be tolerated, a cold induction guide need not be used. Compo
Forbes Jones Robin M.
Kennedy Richard L.
Minisandram Ramesh S.
ATI Properties Inc.
Hoang Tu Ba
Viccaro Patrick J.
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