Creep-Fatigue Interaction in T11 Boiler Tube Steel

This study was undertaken to assess the damage to the mechanical properties of T1l boiler tube steel resulting from about 200 000 h of service. Test results at 1000°F show that the service exposure degrades all strength properties. Tensile strength decreased from 60 to 37 ksi, 1000-h creep strength from 33 to 17 ksi, and fatigue strength (stress ratio = 0.1, life = 200 000 cycles) from 42 to 28 ksi. Also, there was a highly deleterious creep-fatigue interaction, with only 5% fatigue damage reducing creep rupture life by up to a factor of five. Property degradation was accompanied by the formation of voids and a change in the carbide morphology. These findings demonstrate the necessity for accounting for the creep-fatigue interaction in assessing the remaining life of utility components.