Graduation Year
2017
Document Type
Thesis
Degree
M.S.
Degree Name
Master of Science (M.S.)
Degree Granting Department
Engineering
Major Professor
Alex Volinsky, Ph.D.
Committee Member
Wenjun Cai, Ph.D.
Committee Member
Venkat R. Bhethanabotla, Ph.D.
Keywords
Recrystallization, Precipitates, Grain refinement, Hardness, In-situ tensile test
Abstract
The system of upper high nickel alloying steel seal ring and lower high nickel alloying steel seal ring, installed in the grooves of turbine, can extend out and fit with the wall of valve cage, resulting in forming a good seal under the pressure. In the project, the failure steel seal ring is considered. This situation had threatened the safety of the whole steam turbine system. The purpose of this study is to identify the failure cause of the steel seal ring used in nuclear steam turbines. New high nickel steel alloy seal ring was compared with the failed seal ring. The dimensions of macroscopic ring with clearly plastic deformation were measured using calipers. Surface morphology of ring was observed by optical microscopy through metallographic analysis. There is a lot of precipitation in the grain boundaries of used seal ring, along with smaller grain size than the new seal ring. To explore the composition of precipitation, scanning electron microscopy (SEM) with energy-dispersive spectrometer (EDS) were used. The results indicated that the concentration of titanium (Ti) and molybdenum (Mo) was higher in the precipitation of used seal ring. At the same time, the hardness and elastic modulus of used seal ring were reduced, measured by nanoindentation test. In-situ SEM tensile testing were used to record and analyze the generation of crack source and crack development under applied load. The reasons of the seal ring failure can be answered because of these experimental results at both macroscopic and microscopic scales.
The main reason of the seal ring failure is a combination of long-term stress and elevated temperature during turbine operation. Complex work environment caused recrystallization and recovery, resulting in grain refinement and secondary phase precipitation. Further embodiment, recrystallization and recovery caused the elastic modulus and hardness of used seal ring decrease. Moreover, a lot of secondary phase precipitates appeared at grain boundaries during use. The appearance of secondary phase precipitates become the weakest part of used seal ring. The applied load lead to seal ring failure from the formation of microvoids to microvoids aggregated becoming microcracks until to the appearance of cracks at macroscopic scale. These changes of microscopic structure ultimately reflected in critical plastic deformation of used seal ring.
Scholar Commons Citation
Wang, Wenbo, "Failure Analysis of High Nickel Alloy Steel Seal Ring Used in Turbomachinery" (2017). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/6635
