Nickel-base superalloys; physical metallurgy of recycling
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I.
INTRODUCTION
THE aircraft
gas turbine engine with its stringent specifications for materials performance and reliability is the best known application for superalloys. Indeed, the American Society for Metals definition of a superalloy: "An alloy developed for very high temperature service where relatively high stresses are encountered and where oxidation resistance is frequently required", aptly describes materials for these applications. ~ However, as this class of alloys approaches its technical maturity, important applications utilizing other characteristics of these remarkable materials have developed (resistance to aqueous corrosion, hydrogen embrittlement, etc.). For this paper, a broader definition of these alloys will be adopted to include all of the nickelchromium and nickel-iron-chromium alloys produced by the traditional manufacturers of superalloys. This broader definition is convenient because of the extensive interchange of scrap and commonality of physical effects of residual elements among these materials. Whatever the application, superalloys are specified for their outstanding physical characteristics which make it possible to exploit extremes of environment. Since component properties and reliability frequently take precedence over price, costly alloying ingredients and sophisticated processing techniques are routinely used by this industry. Base prices for these alloys may range from about $10 to well over $200 per kilogram, yet it is not uncommon for finished components to sell for ten times the base alloy price. In view of this, it may be surprising that there is a significant amount of scrap recycled within the superalloy industry. While direct recycling for some applications such as those for rotating parts in gas turbine engines is effectively precluded, the melt charge for most superalloys includes a J.J. deBARBADILLO is Department Manager, Processing Research and Development, Inco Alloy Products Company, Research Center, Suffern, NY 10901. This paper is based on a presentation made at a symposium on "Metallurgical Implications of Recycling" held at the Louisville meeting of The Metallurgical Society of AIME, October 14, 1981, under the sponsorship of the TMS Non-Ferrous Metals Committee.
METALLURGICAL TRANSACTIONS A
substantial amount of "home scrap",* and often some purchased "prompt industrial scrap",* or "obsolete scrap".* *Home Scrap--Scrap generated by an alloy producer during the conversion of raw materials to primary product. Prompt Industrial Scrap--Scrap generated by a fabricator or manufacturer during conversion of a primary product to a finished product at a location removed from the melting facility. ObsoleteScrap-- Scrap that is generated when used equipment is overhauled or dismantled.
There are a number of reasons v~hy recycling is practiced so extensively by the superalloy industry, perhaps the most important being that the industry itself generates such large amounts of scrap. For example, throughout the industry the weight of semi-finished products shipped is le
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