Presented by: Tetsuo Shoji (Tohoku University)
Date: Wednesday 8th January 2020
Venue: School of Mechanical Engineering Meeting Room 308
Event Type: Seminars
(Reference: Xiangyu Zhong, Zihao Wang, Fethi Hamdani, Jian Xu, Nishith Kumar Das, Yoichi Takeda, and Tetsuo Shoji, CORROSION, March 2019, Vol. 75, No. 3, pp. 254-266.)
Experimental and theoretical methods were used to focus on certain crucial factors, such as surface condition, oxide film, microstructure, and alloying elements, to assess the long-term reliability of structural components. A hollow cylindrical specimen was designed to evaluate the effect of the surface condition on the initiation and short crack growth behavior of environmentally assisted cracking (EAC) including stress corrosion cracking and environmental fatigue in connection with the structural integrity assessment. In this work, a typical environment of pressurized high-temperature water and flow was considered in the cylindrical specimens, where the inner surface was exposed to the environment. Drilling and honing were used to achieve the surface finish of the specimens. Drilled specimens exhibited a higher EAC initiation and a short crack growth behavior compared with the honed specimens. This reveals that the work-hardened layer is critical in EAC. Special emphasis was placed on the oxidation behavior of the hardened layer and the oxide film with respect to hydrogen-accelerated oxidation (HAO) in order to examine the effect of hydrogen in the metal on the oxidation in high-temperature water. A chemical fatigue test was proposed for the evaluation of oxidation under cyclic exposure to high dissolved hydrogen (DH) and high dissolved oxygen (DO). The result shows that the oxide film was affected by the cyclic exposure to high DH and high DO, suggesting HAO. A mechanical study suggests that a vacancy introduced by deformation, such as by cold work or machining at the surface, can trap a significant amount of hydrogen atoms, which can be important for the HAO and deteriorate the strength of the material. Another typical material degradation mode is microstructural degradation. In this work, a nondestructive detection method of the Laves phase, a detrimental precipitate, was demonstrated, in which a novel electrochemical method was developed to detect and quantify the Laves phase, and the method can be applicable to plant components. Finally, a novel ultra-high purity iron-based alloy was developed for advanced ultra-supercritical (A-USC) applications by adding minor elements, such as Zr, Sc, and Nb, where the Laves phase is critical in increasing the creep resistance. This novel iron-based alloy, developed for A-USC applications, can provide a significant advantage for casting large components for A-USC power plants.