For purposes of this chapter superalloys are those nickel-, iron–nickel-, and cobalt-base corrosion-resistant alloys generally used above a nominal temperature of 540 C (1000 F).
The iron–nickel-base superalloys are an extension of stainless steel technology and generally are melted and cast to electrode / ingot shapes for subsequent fabrication to components. The iron–nickel-base superalloys usually are wrought, i.e., formed to shape or mostly to shape by hot rolling, forging, etc. On the other hand, after primary production by melting and ingot casting, the cobalt-base and nickel-base superalloys may be used either in wrought or cast form depending on the application or the alloy composition involved. The stainless steels, nickel–chromium alloys, and cobalt dental alloys which evolved into the superalloys
used chromium to provide elevated-temperature corrosion resistance. A Cr2O3 layer on the surface proved very effective in protection against oxidation. Eventually, cast superalloys for the highest temperatures were protected against oxidation by chromium and aluminum. In our opinion, superalloys must contain chromium, probably at the level of 5% (some would argue 8%) or higher for reasonable corrosion resistance.
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright 2006 by John Wiley & Sons, Inc.
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