Fatigue behaviour of type 444 stainless steel at elevated temperatures

This paper describes the fatigue behaviour at elevated temperatures in a ferritic stainless steel, type 444. Test temperatures evaluated were ambient temperature, 673K and 773K in laboratory air. Fatigue strength decreased at elevated temperatures compared with at ambient temperature. At all temperatures, cracks were generated at the specimen surface due to cyclic slip deformation, but fractographic analysis revealed a brittle features in fracture surface near the crack initiation site at elevated temperatures. Cracks initiated earlier at elevated temperatures than at ambient temperature and subsequent small cracks grew faster at elevated temperatures even though the difference in elastic modulus was taken into account, indicating the decrease in crack initiation resistance and crack growth resistance. The observed decrease in both resistances was discussed in relation to the 748K(475 degrees C) embrittlement in ferritic stainless steels.