Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2001-03-08
2002-07-09
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C148S677000, C420S590000
Reexamination Certificate
active
06416564
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 improved method for producing large diameter, premium quality ingots of nickel base superalloys. The present invention more particularly relates to a method for producing ingots of nickel base superalloys, including Alloy
718
(UNS N07718) and other nickel base superalloys experiencing significant segregation during casting, and wherein the ingots have a diameter greater than 30 inches (762 mm) and are substantially free of negative segregation, are free of freckles, and are free of other positive segregation. The present invention also is directed to ingots of Alloy
718
having diameters greater than 30 inches (762 mm), as well as to any ingots, regardless of diameter, formed using the method of the invention. The method of the present invention may be applied in, for example, the manufacture of large diameter, premium quality ingots of nickel base superalloys that are fabricated into rotating parts for power generation. Such parts include, for example, wheels and spacers for land-based turbines and rotating components for aeronautical turbines.
DESCRIPTION OF THE INVENTION BACKGROUND
In certain critical applications, components must be manufactured from nickel base superalloys in,the form of large diameter ingots that lack significant segregation. Such ingots must be substantially free of positive and negative segregation, and should be completely free of the manifestation of positive segregation known as “freckles”. Freckles are the most common manifestation of positive segregation and are dark etching regions enriched in solute elements. Freckles result from the flow of solute-rich interdendritic liquid in the mushy zone of the ingot during solidification. Freckles in Alloy
718
, for example, are enriched in niobium compared to the matrix, have a high density of carbides., and usually contain Laves phase. “White spots” are the major type of negative segregation. These light etching regions, which are depleted in hardener solute elements, such as niobium, typically are classified into dendritic, discrete, and solidification white spots. While there can be some tolerance for dendritic and solidification white spots, discrete white spots are of major concern because they frequently are associated with a cluster of oxides and nitrides that can act as a crack initiator.
Ingots substantially lacking positive and negative segregation and that are also free of freckles are referred to herein as “premium quality” ingots. Premium quality nickel base superalloy ingots are required in certain critical applications including, for example, rotating components in aeronautical or land-based power generation turbines and in other applications in which segregation-related metallurgical defects may result in catastrophic failure of the component. As used herein, an ingot “substantially lacks” positive and negative segregation when such types of segregation are wholly absent or are present only to an extent that does not make the ingot unsuitable for use in critical applications, such as use for fabrication into rotating components for aeronautical and land-based turbine applications.
Nickel base superalloys subject to significant, positive and negative segregation during casting include, for example, Alloy
718
and Alloy
706
. In order to minimize segregation when casting these alloys for use in critical applications, and also to better ensure that the cast alloy is free of deleterious non-metallic inclusions, the molten metallic material is appropriately refined before being finally cast. Alloy
718
, as well as certain other-segregation-prone nickel base superalloys such as Alloy
706
(UNS N09706), are typically refined by a “triple melt” technique which combines, sequentially, vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Premium quality ingots of these segregation-prone materials, however, are difficult to produce in large diameters by VAR melting, the last step in the triple melt sequence. In some cases, large diameter ingots are fabricated into single components, so areas of unacceptable segregation in VAR-cast ingots cannot be selectively removed prior to component fabrication. Consequently, the entire ingot or a portion of the ingot may need to be scrapped.
VAR ingots of Alloy
718
, Alloy
706
, and other nickel base superalloys such as Alloy
600
, Alloy
625
, Alloy
720
, and Waspaloy, are increasingly required in larger weights, and correspondingly larger diameters, for emerging applications. Such applications include, for example, rotating components for larger land based and aeronautical turbines under development. Larger ingots are needed not only to achieve the final component weight economically, but also to facilitate sufficient thermomechanical working to adequately break down the ingot structure and achieve all of the final mechanical and structural requirements.
The melting of large superalloy ingots accentuates a number of basic metallurgical and processing related issues. Heat extraction during melting becomes more difficult with increasing ingot diameter, resulting in longer solidification times and deeper molten pools. This increases the tendency towards positive and negative segregation. Larger ingots and electrodes can also generate higher thermal stresses during heating and cooling. While ingots of the size contemplated by this invention have been successfully produced in several nickel base alloys (for example, Alloys
600
,
625
,
706
, and Waspaloy) Alloy
718
is particularly prone to these problems. To allow for the production of large diameter VAR ingots of acceptable metallurgical quality from Alloy
718
and certain other segregation-prone nickel base superalloys, specialized melting and heat treatment sequences have been developed. Despite these efforts, the largest commercially available premium quality VAR ingots of Alloy
718
, for example, are currently 20 inches (508 mm) in diameter, with limited material produced at up to 28-inch (711 mm) diameters. Attempts at casting larger diameter VAR ingots of Alloy
718
material have been unsuccessful due the occurrence of thermal cracking and undesirable segregation. Due to length restrictions, 28-inch VAR ingots of Alloy
718
weigh no more than about 21,500 lbs (9772 kg). Thus, Alloy
718
VAR ingots in the largest commercially available diameters fall far short of the weights needed in emerging applications requiring premium quality nickel base superalloy material.
Accordingly, there is a need for an improved method of producing premium quality, large diameter VAR ingots of Alloy
718
. There also is a need for an improved method of producing ingots of other segregation-prone nickel base superalloys that are substantially free of negative segregation, are free of freckles, and substantially lack other positive segregation.
BRIEF SUMMARY OF THE INVENTION
In order to address the above-described needs, the present invention provides a novel method of producing a nickel base superalloy. The method may be used to cast VAR ingots of premium quality from Alloy
718
in diameters greater than 30 inches (762 mm) and having weights in excess of 21,500 lbs (9772 kg). It is believed that the method of the present invention also may be applied in the production of large diameter VAR ingots from other nickel base superalloys subject to significant segregation during casting, such as, for example, Alloy
706
.
The method of the present invention includes the initial step of casting a nickel base superalloy within a casting mold. This may be accomplished by VIM, argon oxygen decarburization (AOD), vacuum oxygen decarburization (VOD), or any other suitable primary melting and casting technique. The cast ingot is subsequently annealed and overaged by, heating the alloy at a furnace temperature of at least 1200° F. (649° C.) for at least 1
Ballantyne A. Stewart
Bond Betsy J.
Jackman Laurence A.
ATI Properties Inc.
King Roy
McGuthry-Banks Tima
Viccaro P. J.
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