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TRODUCTION

STAINLESS steels frequently find application at high temperatures, where the Cr content of the alloy is beneficial in resisting oxidation. In particular, applications involving thin gage sheet material, the process of oxide scale growth, and the consequent metal loss may lead to higher effective applied stresses followed by mechanical failure. If heat transfer across the sheet is an important design parameter (e.g., in heat exchangers), then significant oxide scale growth may not be tolerable. In either of these cases, it would be advantageous to be able to predict the useful service life of an alloy at a particular temperature. In addition to embrittlement at ambient temperature, the formation of s phase also has an adverse effect on the corrosion resistance of stainless steels,[1] due to the removal of Cr from solid solution. It might be expected, therefore, that the formation of s phase and the consequent removal of Cr from solid solution in the alloy might have an unfavorable effect on the growth of the chromia surface scale and, therefore, on the resistance of the alloy to high-temperature oxidation. Oxidation and simultaneous s-phase formation in stainless steels has received little attention to date. A brief theoretical discussion exists on the influence of Cr depletion on the width of the sigma precipitate-free zone (PFZ),[2] using an earlier mathematical model, cited by Jost,[3] from original unpublished work by Wagner. The authors did not actually observe any s phase in their thin foil specimens; however, if such a PFZ had existed, it was assumed that the rate of surface oxidation would therefore not have been limited by s-phase dissolution. Sigma-phase has been observed in a 310 stainless steel after oxidation in high pres-

sure CO2,[4] although the authors did not detect a distinct PFZ. It was not clearly established whether the lack of a PFZ was evidence of surface oxidation being limited by sphase formation or whether this might have been explained by surface carburization from the oxidation atmosphere. Clearly, some uncertainty still exists concerning the role of s phase in oxidation. The present study is an attempt to resolve this question. II.

EXPERIMENTAL PROCEDURE

A. Oxidation Testing Oxidation experiments were performed on type 347 austenitic stainless steel (in both sheet and foil form) having the compositions shown in Table I. Both alloys were oxidized in air at 650 7C, 700 7C, 760 7C, and 816 7C, for durations up to 10,000 hours. All specimens were given an identical 600-grit abraded surface finish, before being ultrasonically cleaned for 5 minutes in acetone, followed by 5 minutes in ethanol. Two specimens of each alloy (approximately 20 3 12 mm each) were contained in glazed porcelain crucibles during the oxidation experiments, in order to permit spalled oxide to be collected and weighed with the specimens. All crucibles were thoroughly cleaned with ethanol, and crucibles and specimens were weighed both before and after oxidation. Sufficient numbers of specimens were used to pe