Title: Aging Degradation Characteristics and Long Term Performances of Structural Materials for Energy Conversion Systems

Speaker: Dr. Xiangyu Zhong

Time: 8月17日（周五）上午9:00-11:00

Venue：南区一楼报告厅

Aging Degradation Characteristics and Long Term Performances of Structural Materials for Energy Conversion Systems

Xiangyu Zhong*, Jian Xu, Fethi Hamdani, Nishith Kumar Das, Tetsuo Shoji. 

Degradation of material used in the safety boundary components of energy conversion systems can be a kind of critical phenomena which impacts a structural integrity and safety and also reduces a utilization factor of the power plant resulting in economical burden. Despite regular maintenance and tightly regulated operating procedures, aging related failures, including creep, stress corrosion cracking, corrosion fatigue and high temperature oxidation, do occur in the energy conversion systems.

In the present work, some structural components after long-term service in a supercritical power plant were analyzed by using various analytical techniques. For the 9Cr-1Mo superheater weld-joint, a lot of cavities along the grain boundaries and in grains were also observed in the FGHAZ. It may relate to the grain size in FGHAZ is much smaller than that in other zones. Grain boundary sliding can be a predominant constraint relaxation process in FGHAZ with a greater number of grain boundaries. It showed that this region is most susceptible to creep crack initiation and propagation with prolonged service time at supercritical power plant environments. For the T12 water wall tube, it was found that the oxide scale formed on the inner wall of the tube was a multi-layer structure with a porous outer layer consisted of Fe₃O₄ and Cr-rich ɑ-Fe₂O₃ and a dense inner layer consisted of Cr-rich spinel. The Cr-rich oxide was observed as striations parallel to the oxide/metal interface and the Cr and Mn enrich in the outer layer of the oxide scale were also observed. Some cavities are also observed in the oxide scales. The grain boundaries and voids are the short-path of oxygen diffusion into the metal and the intergranular oxidation was observed. It can be predicted that the possibility for the intergranular stress corrosion cracking (IGSCC) initiation of T12 is higher after long-term service in the supercritical fossil power plant, resulting in the short lifetime of the structural materials.

In order to improve the performance of the structural material used in advanced ultra- supercritical (A-USC) power plant, some iron-based heat resistant alloys were developed with an equivalent creep resistance to nickel-based alloys and also with an equivalent super critical steam oxidation resistance to 310S stainless steel. The alloys show that the addition of Zr and Nb significantly improve the creep and oxidation resistance under A-USC condition. These new alloys can be strengthened during the creep deformation by a formation of very fine Laves phase formation and can enhance the reliability and longevity of the structural component. These new alloys could be promising candidates for the A-USC plant structural component.

Speaker’s Biography:

Dr. Xiangyu Zhong graduated from the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), with a PhD degree in 2013. He joined the New Industry Creation Hatchery Center (NICHe) of Tohoku University, as a post-doctoral researcher after he graduated from the IMR. Now he is the assistant professor of the New Industry Creation Hatchery Center (NICHe) of Tohoku University. His present works focus on the on the high temperature oxidation, stress corrosion cracking and corrosion fatigue of alloys used in energy conversion system. He is also interested in the development and evaluation of new corrosion resistant alloys.