Tearing Resistance Properties of Cr-Mo Steels with Internal Hydrogen Determined by the Potential Drop Method
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MANY pressure vessels are used under high temperature (~725 K (450 C)) and high hydrogen pressure (~15 MPa) in the refining and petrochemical industries. Therefore, it is necessary to determine the degradation in fracture properties of the constituent materials, such as Cr-Mo steels, due to hydrogen. Considerable susceptibility to temper embrittlement has also been reported for Cr-Mo steels. Despite extensive efforts[1–6] over the years to understand hydrogen embrittlement, hydrogen-assisted cracking is still a major problem in the field of hydrogen treatment. The author and colleagues[7,8] have investigated the synergistic effects of hydrogen embrittlement and temper embrittlement on fracture properties. There was no marked indication of a synergistic effect on Charpy impact properties[6] and threshold stress intensity factors at the onset of hydrogen-assisted crack growth,KIH .[8] However, very few studies have addressed the effects of hydrogen and temper embrittlement on the tearing resistance, dJ/da characterized by a J-resistance curve.[9,10] For a structural integrity assessment using a failure assessment diagram (FAD), the characteristics of crack growth due to ductile tearing need to be known in order to predict the remaining life of a reactor with hydrogen-assisted flaws. SHINJI KONOSU, Professor Emeritus, is with Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan. Contact e-mail: [email protected] HIDENORI SHIMAZU, Senior Principal Engineer, is with Techno Staff Co., Ltd., 6-19, Aoki, Kanagawa-ku, Yokohama, Kanagawa 221-0057, Japan. RYOHEI FUKUDA, Engineer, is with Mitsubishi Rayon Co., Ltd., 20-1 Miyuki, Otake, Hiroshima 739-0693, Japan Manuscript submitted August 28, 2014. Article published online September 21, 2015 5626—VOLUME 46A, DECEMBER 2015
In this study, the combined effect of hydrogen and temper embrittlement on dJ/da was investigated for the same three 2.25Cr-1Mo steels and 2.25Cr-1Mo-0.3V steel as were used in our previous studies.[7,8] There are several procedures for measuring the amount of slow-stable crack extension: (1) Direct visual inspection using multiple specimens, (2) the unloading compliance technique,[10], and (3) the electric potential drop (PD) technique.[11–13] The present study employed the PD technique[8] to determine the amount of slow-stable crack extension (Da). A new calibration curve of dc-PD vs Da for CT specimens, incorporating the effect of material removal for machining integral knife edges, was proposed. The proposed calibration curve was used to develop the J-resistance curves. Because some of the original dissolved hydrogen in the specimen dissipates with elapsed time during testing in air at ambient temperature, the hydrogen dissipation characteristics, which depend on the specimen size, were also discussed. A. Experimental Procedure Vacuum-melted laboratory heats of 2.25Cr-1Mo steel were used. The chemical composition and typical mechanical properties of the three conventional steels (hereafter steels A, B, and C) and the V-bearing s
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