Effect of Laves phase strengthening on the mechanical properties of high Cr ferritic steels for solid oxide fuel cell interconnect application

Two types of high chromium ferritic steels envisaged as construction materials for SOFC interconnects, were investigated in respect to microstructure and creep in the proposed application temperature range from 700 to 800 °C. The steel compositions mainly differed in the amounts of the Laves phase forming elements Nb, W and Si. The steel containing these alloying additions exhibited substantially higher creep resistance in the temperature range 700–800 °C than the high purity steel. The Laves phase formation occurred trans- as well as intragranular whereby the extent and size of grain boundary precipitates increased with increasing exposure time. Especially at 800 °C the precipitates inside the grains virtually completely vanished after longer exposure times and only intergranular precipitates remained. This change in precipitate morphology resulted especially at 800 °C in a decrease of creep resistance with increasing exposure time, although the Laves phase containing steel still exhibited higher creep strength than the high purity steel.

► Additions of Nb, W, Si increase creep resistance of 22% Cr ferritic steel.
► Creep rupture life of the strengthened steel two orders of magnitude higher (800 ̊C).
► Combination of solid-solution/precipitation strengthening by Laves phase particles.
► Time/temperature dependant Laves phase precipitation, coarsening and redistribution.
► Resulting in initial increase and subsequent decrease of creep strength at 800 ̊C.