Thermo-Mechanical Fatigue Behavior of a Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect

摘要

The purpose of this study is to investigate the thermo-mechanical fatigue behavior of a ferritic stainless steel (Crofer 22 H) for use as an interconnect material in planar solid oxide fuel cells (pSOFCs). Metallic interconnects are subjected to thermal stresses due to mismatch of coefficient of thermal expansion (CTE) between components and temperature gradients during start-up, steady operation, and shutdown stages in a pSOFC stack. Interconnects under mechanical and thermal cycling loading could suffer a thermo- mechanical fatigue (TMF) damage during operation between periodic start-up and shutdown stages. Therefore, TMF tests under various combinations of mechanical loading at a cyclic temperature range are conducted to study the long-term durability of the Crofer 22 H ferritic steel under SOFC operating conditions in the present study. The TMF tests were performed in air at a cyclic temperature range between 25o C and 800o C to simulate the maximum temperature range of pSOFCs between shutdown and steady operation stages. Cyclic mechanical loading was applied under force control with specified yield strength ratios (YSRs) at 25o C and 800o C to simulate various combinations of thermal stresses generated in interconnects of a pSOFC stack. Various combinations of YSRs ranging from 0.2 to 0.6 of at 25o C and 800o C were selected as the applied peak and valley mechanical loads at the temperatures of 25o C and 800o C in TMF tests. Experimental results show the TMF life of Crofer 22 H is mainly dominated by a fatigue mechanism involving cyclic plastic deformation. The relation between TMF life and YSR at 800o C for all given loading combinations is well described by a logarithmic function. Fractographic observation indicates a ductile fracture and fatigue cracking patterns in Crofer 22 H specimens. A fatigue mechanism involving cyclic plastic deformation is the dominant factor in determining the fracture mode of TMF behavior.