In order to clarify the mechanisms of intergranular stress corrosion cracking of Alloy X750 in high temperature water, the authors have studied the relationship between the distribution of hydrogen and the microstructure of grain boundary precipitates by using electron micro autoradiographic technique. Two types of Alloy X750 with different heat treatment history were prepared. The one is sensitive to the stress corrosion cracking in high temperature water, which has incoherent precipitates other than M23C6 discretely exist at the grain boundaries. The other has excellent resistance to the stress corrosion cracking and has coherent M23C6 precipitates which semi-continuously exist at the grain boundaries. Tritium and hydrogen were charged into these specimens at room temperature by cathodic charging method. After the charging, specimens attached by the sensitive film (monolayer of fine silver bromide particles film) were kept in a dark box for about 10 day. The distribution of silver particles on the specimen was observed by electron probe microanalyzer and transmission electron microscope. For the specimens which are sensitive to the stress corrosion cracking resistance, the silver particles existed on the grain boundaries, however, for the specimens which have excellent resistance to the stress corrosion cracking, the silver particles were not found at the grain boundaries. From this evidence, it seems that hydrogen trapping behavior at the grain boundaries affects the stress corrosion cracking resistance.
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