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Stainless steels are widely used in many fields such as chemical, petrochemical, food and nuclear industries and they are characterized by physical, mechanical and corrosion resistance properties that depend on the microstructure and phase transformations: many intermetallic phases, carbides and nitrides precipitate at different tempering temperatures. Sintered stainless steels corrosion resistance and mechanical behavior are worst than that of either cast or rolled or wrought stainless steels: their use is mainly due to their economically attractive production cost and/or to their alternative manufacturing procedure (e.g. duplex stainless steels). In this work, the fatigue crack propagation resistance of two sintered stainless steels, respectively characterized by an austenitic and a ferritic microstructure, is investigated. Fatigue crack propagation tests are performed both in air and under hydrogen charging conditions, investigating the influence of the stress ratio (R= 0.1; 0.5; 0.75). Fatigue crack propagation micromechanisms are investigated by means of a scanning electron microscope (SEM) fracture surface analysis. Although the hydrogen physical behaviour is completely different in fcc and bcc structures, both the investigated austenitic and ferritic sintered stainless steels are susceptible to be embrittled by the hydrogen charging process, for all the investigated stress ratio values. SEM fracture surface analysis allows to identify different fatigue crack propagation mechanisms that are influenced both by the loading conditions (either deltaK and R values), by the steel microstructure and by the test environment.