Abstract
309S was examined in the solution treated and quenched condition and after heat treatment at 975 K for either 15 or 300 min. Material was carbide free in the water-quenched condition, but 15 min at 975 K produced a sensitized microstructure of small, discontinuous grain boundary carbides. The 300-min treatment produced larger and essentially continuous carbides; that structure was not sensitized. Depletion of Cr near boundaries in the sensitized material was demonstrated by a stacking fault energy value of about 22 mJ/m 2. The matrix value was about 35mJ/m 2. Such a change implies a reduction in Cr from 23% to about 12%. Smooth and notched bars were tested in all three conditions; tests were made in air, or in high-pressure hydrogen, or after hydrogen charging to supersaturation. Ductility (RA) was decreased only by the hydrogen charging, independent of microstructure, and that decrease was slight. These results could not be explained by a martensite embrittlement mechanism, but were consistent with a rationale based on hydrogen accumulation at inclusion interfaces.
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