Evaluation of the creep–fatigue damage mechanism of Type 316L and Type 316LN stainless steel

Low-cycle fatigue tests with continuous cycling and creep–fatigue tests with 10 min hold times at tensile maximum strain were conducted at 600 °C in air for Type 316L and Type 316LN stainless steels containing nitrogen contents of 0.04% and 0.10%. The creep–fatigue life was less than the fatigue life for both alloys. The fatigue and creep–fatigue life and saturation stress were increased with the addition of nitrogen. The fracture mode was transgranular for fatigue and intergranular for creep-fatigue regardless of the nitrogen content. The dislocation structure was cellular for Type 316L and planar for Type 316LN after fatigue and creep-fatigue tests. Carbides were precipitated at grain boundaries after creep-fatigue tests and nitrogen decreased the precipitation. Creep–fatigue life was well predicted by a model based on cavity nucleation and growth at grain boundaries. The increase of creep–fatigue life with the addition of nitrogen was due to the decrease of precipitation and stress relaxation by the change in dislocation structure.