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The mechanical properties and corrosion resistance of AM-355, a precipitation-hardening stainless steel, were found to be highly dependent upon carbide morphology. Corrosive attack in salt spray could always be associated with carbide precipitation at matrix discontinuities such as austenite or delta-ferrite grain boundaries. Continuous carbide precipitation at austenite grain boundaries was found to promote stress-corrosion failures in salt spray and to adversely affect certain mechanical properties such as ductility and impact strength.
Heat treatments, based upon the transformation characteristics of the alloy, were developed which produced uniform carbide dispersions. This structure resulted in significant improvements in mechanical properties and resistance to rusting, pitting, and stress corrosion in salt spray.
Carbide morphology was studied by light and electron microscopy as affected by: (1) heat treatment variables such as solutioning temperatures and times, cooling rates, and precipitation temperatures and times; (2) prior history of the alloy; and (3) transformation characteristics of the alloy.
Users of the alloy have accepted the new heat treatment that was developed. This has led to new applications for the alloy in relatively severe marine environments.
Aggen, G. N.
Research associate, Research Laboratory, Allegheny Ludlum Steel Corp., Brackenridge, Pa.
Kaltenhauser, R. H.
Project supervisor, Research Laboratory, Allegheny Ludlum Steel Corp., Brackenridge, Pa.