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Tohru ISHIGURO, Keizo OHNISHI and Juro WATANABE Synopsis : In the heavy oil upgrading and coal liquefaction processes, higher service temperature and higher hydrogen pressure of the reactor vessel have required heavily sectional steel used as vessels for high pressure apprications with enhanced design stress intensity values and improved resistance to hydrogen environment. As a result of a previous alloy modification study, a 3Cr-1 Mo-1/4V-Ti-B steel with enhanced design stress intensity values had been developed. In this study, the hydrogen attack susceptibility of the newly developed steel was examined in the view point of the role of Cr and V-Ti-B additions. The increasing Cr content and the V-Ti-B additions remarkably improved the resistance to hydrogen attack. The species of precipitating carbide were identified by X-ray diffraction on the extracted deposits of Cr-Mo and Cr-Mo-V-Ti-B steels with various Cr contents. Both intensities of Fe3C and Mo2C, which were unstable under high temperature, were decreased as the Cr content was increased. In the microalloyed steels, Fe3C and Mo2C were futher decreased due to the precipitation of VC carbide. The improved resistance to the hydrogen attack in the developed steel was considered to be achieved through changes in the type of precipitating carbides.

Effects of Cr and V on the Hydrogen Attack Susceptibility of B Added Cr-Mo Steels