Effect of preload stress on the dynamic response of 9%Cr heat-resistant steels with various microstructures

With a uniaxial oscillation load coupled to different preload stresses, the dynamic responses of 9%Cr heat-resistant steels with different microstructures (martensite and cell structure) were measured on a low-cycle fatigue machine at the elevated temperature of 823 K. The experiment results show that the internal friction of 9%Cr heat-resistant steel is dependent on not only the microstructure, but also the preload stress. The internal friction of 9%Cr heat-resistant steel with martensite reaches the minimum when the preload stress is close to half of its 0.2% offset strength. Within the present range of preload stress, however, the internal friction of cell structure monotonically increases with the preload stress. Based on the analysis of obtained results, it indicates that for the 9%Cr heat-resistant steels with martensite and cell structure, the different behaviors of internal friction with the preload stress are mainly attributed to various dynamic configurations of internal defects.